Technical Basis Document No. 4: Mechanical Degradation and Seismic Effects Revision 1

June 2004 

1. INTRODUCTION 
This technical basis document provides a summary of the stability of repository excavations and 
potential mechanical degradation under the action of in situ, thermal, and seismic stresses during 
the preclosure and postclosure time periods. The document further identifies the interactions of 
the mechanical degradation of the emplacement drifts with the engineered barriers, in-drift 
environment, and water seepage into emplacement drifts. This document is one in a series of 
technical basis documents prepared for each component of the Yucca Mountain repository 
system relevant to predicting the likely postclosure performance of the repository. The 
relationship of mechanical degradation and seismic effects to the other components of the 
repository system is illustrated in Figure 1-1. 
The information presented in this document, and the associated references, is part of the ongoing 
development of the postclosure safety analysis that will be included in the license application. 
This information is also used to respond to open Key Technical Issue (KTI) agreements made 
between the U.S. Nuclear Regulatory Commission (NRC) and the U.S. Department of Energy 
(DOE). Placing the DOE responses to individual KTI and NRC Additional Information Needed 
(AIN) requests within the context of the overall mechanical degradation analyses, as it relates to 
1-1 June 2004 No. 4: Mechanical Degradation Revision 1 
postclosure safety analyses, allows for a more direct discussion of the relevance of the 
agreements. 
Appendices to this document are designed to allow for a transparent and direct response to each 
KTI agreement and AIN requests. Each appendix addresses one or more of the agreements. If 
agreements apply to similar aspects of the mechanical drift degradation issue, they were grouped 
in a single appendix. In some cases, appendices provide detailed discussions of data, analyses, 
or information related to the further conceptual understanding presented in this technical basis 
document. In other cases, the appendices provide information that is related to the technical 
basis document information but at a level of detail that relates more to the uncertainty in a 
particular data set or feature, event, or process that is less relevant to the overall technical basis. 
In these cases, the appendices reference the relevant section of the technical basis document to 
put the particular KTI agreement into context, but the technical basis document does not 
reference the appendices. 
This technical basis document provides a summary-level synthesis of many relevant aspects of 
the mechanical drift degradation and ground support design studies. This document describes 
the development of an understanding of the thermal-mechanical behavior of Yucca Mountain 
tuffs and the development of suitable models to represent this behavior when subjected to static 
and transient loading. The analyses described here are applied primarily to estimating the 
stability and rockfall potential of the excavations, and less to the design of ground support, which 
is primarily a preclosure design issue. However, the techniques described here are applicable to 
both the design and performance studies. 
This document presents a summary and synthesis of the detailed technical information presented 
in the analyses and model reports and other technical products that are used as the basis for the 
description of the mechanical degradation analysis and the incorporation of this work into the 
postclosure performance assessment. Several analyses, model reports, and other technical 
products support this summary: 
• Drift Degradation Analysis (BSC 2004a) 
• Subsurface Geotechnical Parameters Report (BSC 2003a) 
• Longevity of Emplacement Drift Ground Support Materials for LA (BSC 2003b) 
• Ground Control for Emplacement Drifts for the LA (BSC 2003c) 
• Scoping Analysis on Sensitivity and Uncertainty of Emplacement Drift Stability 
(BSC 2003d) 
• Ground Control for Non-Emplacement Drifts for LA (BSC 2004b) 
• Development of Earthquake Ground Motion Input for Preclosure Seismic Design and 
Postclosure Performance Assessment of a Geologic Repository at Yucca Mountain, NV 
(BSC 2004c) 
• Geology of the ECRB Cross Drift—Exploratory Studies Facility, Yucca Mountain 
Project, Yucca Mountain, Nevada (Mongano et al. 1999) 
June 2004 1-2 No. 4: Mechanical Degradation Revision 1 
• Resolution Strategy for Geomechanically-Related Repository Design and 
Thermal-Mechanical Effects (RDTME) (Board 2003). 
The basic approach used in this document is to review the relevant portions of these reports to 
provide a comprehensive overview of the data and analyses that describe the process of 
mechanical degradation and to provide a context for the attached KTI resolution documents. The 
details of the data and analyses can be found in the above reports. 
1.1 OBJECTIVE AND SCOPE 
The objectives of this technical basis document are to: 
• Provide an overview of the anticipated mechanical degradation processes in the 
emplacement drifts and the boundary conditions this degradation provides to the 
engineered barrier system and drift seepage 
• Present an overview of the KTI agreements, how they relate to this process, and the 
strategy employed to resolve these issues 
• Present a summary of the relevant geomechanical rock mass properties database 
developed for lithophysal and nonlithophysal rocks 
• Describe the numerical modeling and analysis techniques used for ground support and 
drift degradation analysis 
• Present the analysis of postclosure mechanical degradation under the action of in situ, 
thermal, and seismic stresses, as well as time-dependent strength changes. 
The purpose of the mechanical degradation analyses is to provide an estimate of the temporal 
evolution of the stability of the emplacement drifts under the action of in situ, thermal, and 
seismic loading, as well as time-dependent changes in rock mass strength. In particular, the 
model(s) provide an estimate of: 
• The yield of the rock mass around the emplacement drifts, and the associated rockfall 
once the ground support function is lost due to postclosure corrosion 
• The temporal change in emplacement drift shape and size 
• The temporal evolution of rockfall, including the rockfall particle size distribution and 
total amount 
• The temporal evolution of rockfall loading (both dynamic and quasi-static1) on the drip 
shield. 
1 Quasi-static refers here to the development of loading on the drip shield, which slowly evolves over time but is 
more-or-less in static equilibrium. 
June 2004 1-3 No. 4: Mechanical Degradation Revision 1 
Due to the distinctly different mechanical characteristics of the major rock units that comprise 
the repository host horizon (lithophysal and nonlithophysal rocks), the above estimates will vary 
with location within the repository. 
For the postclosure period, 10 CFR 63.114 requires that DOE conduct a performance assessment 
that considers only events that have at least one chance in 10,000 of occurring over 10,000 years. 
Seismic events that have this probability of occurrence have been analyzed and are discussed in 
this technical basis document. In addition, 10 CFR 63.102(j) provides that, for the postclosure 
period, the event classes analyzed in the performance assessment should consist of all possible 
specific initiating events that are caused by a common natural process (e.g., the event class for 
seismicity includes the range of credible earthquakes for the Yucca Mountain site). Additional 
sensitivity studies are currently being conducted to ascertain the range of credible ground 
motions, and those studies are not discussed in this document. The additional studies do not 
affect the technical bases or conclusions described in this document. The results of those 
sensitivity studies will be included in the license application, as appropriate. 
1.2 SUMMARY OF CURRENT UNDERSTANDING 
1.2.1 General Description of the Repository Layout and Waste Emplacement 
A general description of the design and layout of the repository can be found in Underground 
Layout Configuration (BSC 2003e). The repository is located at approximately 300 m below 
ground surface within the Topopah Spring Tuff—a densely welded tuff unit comprised of a 
number of subunits that dip approximately 15° from west to east. These subunits can be divided 
into two broad, mechanical categories: nonlithophysal and lithophysal2 welded tuffs. The basic 
matrix material of these two subunits is similar in mineralogical, textural, and mechanical 
properties. However, due to varying cooling histories and as a result of position within the 
formation, they are structurally and thermal-mechanically significantly different in character. 
The nonlithophysal rocks are hard, mechanically strong, fine-grained and fractured volcanic 
rocks whose mechanical behavior is strongly controlled by the geometry and surface 
characteristics of its fracturing. The lithophysal rock is composed of the same strong, hard 
matrix material, but has porosity in the form of lithophysal cavities ranging from about 10% to 
30% by volume. The presence of these cavities results in significantly different deformability 
and strength of the rock mass. The current repository layout places approximately 85% of the 
emplacement drifts within the lithophysal rocks (BSC 2003e, Table I-2). 
The repository (Figure 1-2) is accessed from ground surface on the east side of Yucca Mountain 
by three 7.62 m diameter entry ramps (the north and south ramps currently exist and are part of 
the Exploratory Studies Facility (ESF) and are driven at 2.5% grade). Once these ramps achieve 
the depth of the Topopah Spring Tuff, they join a number of subhorizontal, 7.62-m diameter 
access mains. These access mains define the outer perimeter of four panels that are composed of 
5.5 m nominal diameter waste emplacement drifts (BSC 2003e). The emplacement drifts are 
accessed at one end from the fresh air intake main via a turnout (Figure 1-3) and intersect the 
exhaust main at the opposite end. The excavations, with the exception of the turnouts, are driven 
2 Lithophysae: hollow, bubblelike cavities in a volcanic rock surrounded by porous rims formed by fine-grained 
alkali feldspar, quartz, and other minerals. Lithophysae are typically a few centimeters to a few decimeters in 
diameter; however, they can be as small as 1 mm in diameter or less to as large as 1 m or more in diameter. 
1-4 June 2004 No. 4: Mechanical Degradation Revision 1 
by tunnel boring machines (TBMs). The access mains are supported with rock bolts and heavy 
wire mesh, the turnout-access mains intersections with rock bolts and shotcrete, and the 
emplacement drifts with thin (2 to 3 mm), perforated, stainless steel sheeting held tight to the 
drift walls using stainless steel, friction-type rock bolts (Figure 1-4). All of the ground support is 
placed over a 240° arc on roof and walls. The general ventilation method employed is to draw 
fresh air into the repository via the ramps and a number of intake shafts, distribute it down the 
intake mains, through the emplacement drifts, and out through a series of exhaust shafts (BSC 
2003e, Table I-2). 
June 2004 1-5 No. 4: Mechanical Degradation Revision 1 
Source: BSC 2003e. 
NOTE: Footprint of emplacement area boundary is shown as a dashed line. This footprint represents the currently 
characterized area in which emplacement drifts can be located. 
Figure 1-2. Repository Layout in Plan View Showing Ramps, Access Intake and Exhaust Mains, 
Emplacement Drifts, Shafts, and Existing Excavations (as Dashed Lines) 
June 2004 1-6 No. 4: Mechanical Degradation Revision 1 
Figure 1-3. Plan View and Cross-Sectional Views of Primary Repository Excavations 
June 2004 1-7 No. 4: Mechanical Degradation Revision 1 
Source: BSC 2003c and BSC 2004d. 
NOTE: Ground support coverage using Bernold-Type stainless steel sheets (240° of circumference above invert) 
and stainless steel friction bolts. 
June 2004 
Figure 1-4. Ground Support Method for Emplacement Drifts Showing Drip Shield 
A rail-based waste emplacement transporter, drawn by electric locomotives, will leave the 
surface waste handling facilities with a waste package on the emplacement pallet loaded within a 
shielded rail car. The transporter will deliver the waste package, via the access mains, to the 
emplacement drift turnout. In the turnout, the waste package will be transferred, in an automated 
mode, by pushing the waste package and pallet from the transporter onto a docking bay at the 
end of the emplacement drift. The waste package will then be picked up from the loading dock 
and transported into the emplacement drift by an emplacement gantry crane (Figure 1-5). The 
emplacement gantry will deliver the waste package and pallet to their resting location on the drift 
invert structure, and will return to the docking bay. Normal retrieval operations, if required, will 
involve a reversal of the emplacement sequence. At closure, a titanium drip shield will be placed 
over the top of the waste packages (Figure 1-4), and all nonemplacement drifts will be backfilled 
with crushed tuff from the TBM operations. 
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1.2.2.1 Preclosure Effects 
Excavation of the repository drifts will result in concentration of in situ stresses around the 
openings. Because the in situ stresses are relatively small in comparison to the rock mass 
strength, little, if any, yield of the rock mass is expected. Thus, the openings will undergo 
primarily elastic deformation, which equilibrates within a short distance (about two tunnel 
diameters) behind the advancing TBM. Light, temporary ground support is placed directly 
behind the TBM for worker protection purposes. Permanent ground support is placed after the 
emplacement drift is completed and the TBM withdrawn, therefore, it will be subjected only to 
deformation and loading that may occur from transient effects such as thermal and seismic 
loading. 
During the preclosure time period, approximately 100 years from initial excavation of the 
emplacement drifts, forced ventilation will be used to remove approximately 90% of the heat 
generated by the waste packages. This heat removal will keep drift wall temperatures cooler 
1.2.2 General Degradation Processes 
The general process of mechanical degradation of emplacement drifts during the preclosure and 
postclosure time frames involves interactions with several engineered and natural barrier 
systems. A general description of the impact or interaction of the mechanical degradation of the 
emplacement drifts is given here. 
Figure 1-5. Shielded Waste Transporter at Docking Bay Showing Waste Package, Waste 
Emplacement Gantry, and Locomotive 
June 2004 1-9 No. 4: Mechanical Degradation (BSC 2004a, Section 6.2) and will result in small thermally related rock mass stress changes (see 
Section 5.3.1). The emplacement drifts will be supported by rock bolts and slotted stainless steel 
sheeting, thus minimizing, if not eliminating, mechanical degradation of the excavations. 
Although this technical basis document is concerned primarily with the postclosure mechanical 
and seismic degradation effects, a number of the Repository Design and Thermal-Mechanical 
Effects (RDTME) KTI agreements either relate specifically to preclosure ground support 
performance or are concerned with rock mass characterization and modeling issues that are 
common to both preclosure and postclosure performance. Since a detailed review of preclosure 
design issues is not given in this document, Table 1-1 is provided to give a cross-reference 
between the KTI agreement resolution summaries found in the appendices and the specific 
source document that provides greater details of the analyses that support the agreement. 
Appendix 
A 
B 
C 
D 
E 
F 
G 
H 
1.2.2.2 Postclosure Effects 
Repository closure will involve installation of titanium drip shields over the waste packages, 
backfilling of access mains and shafts, and the cessation of forced ventilation. This will result in 
a rapid rise in temperature of the drift walls, reaching approximately 140°C to 165°C within 
NOTE: Document A is BSC 2003d. Document B is BSC 2003c. Document C is BSC 2004b. Document D is BSC 
2003a. 
Data and analysis of rock bridges between joints Sections 3.2, 4.1, and 5.3 
Continuum and discontinuum analyses of ground Section 4.1, 4.2 
support system performance 
Where Addressed 
Sections 3.2 and 4.2 
Sections 3.2, 4.1, 4.2, and 5.3 
Document A 
Sections 4.1, 4.2, and 5.3 
Documents A and B 
Section 3.2 
Document D 
Section 3.2, 4.1, and 5.3 
Documents A, B, C, and D 
Sections 3.2, 4.2, and 5.3 
Document A 
Document A 
June 2004 No. 4: Mechanical Degradation 
Table 1-1. Key Technical Issue Agreements–Relevant Section Supporting Documents 
KTI Agreement 
RDTME 3.05 
RDTME 3.06 
RDTME 3.15 
RDTME 3.16 
RDTME 3.17 
RDTME 3.19 
RDTME 3.02 
RDTME 3.10 
RDTME 3.13 
RDTME 3.04 
RDTME 3.08 
RDTME 3.12 
RDTME 3.09 
RDTME 3.11 
Description 
Technical basis for accounting for effects of 
lithophysae 
Design sensitivity and uncertainty analyses of 
the rock support system 
Modeling joint planes as circular discs, small 
trace length fractures 
Technical basis for effective max rock size 
Determine whether rockfall can be screened 
from PA abstractions 
Critical combinations of in situ, thermal, and 
seismic stresses 
Two-dimensional modeling of emplacement drifts 
Boundary conditions, discontinuum versus 
continuum modeling 
Site-specific properties of the host rock 
Design sensitivity and uncertainty analyses of 
fracture patterns 
Dynamic analyses of ground support 
performance 
Rock movements in the invert 
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Revision 1 Revision 1 
about 20 years after closure. The temperature then slowly decreases over time, with the 
emplacement drift remaining above 96°C for approximately 1,000 years (BSC 2004a). 
After closure, the ground support will corrode and lose its function over the period of perhaps 
decades to centuries, resulting in unsupported emplacement drifts through the large majority of 
the postclosure period. 
During the postclosure period, the emplacement drifts will be subjected to the following loading 
conditions (Figure 1-6): the in situ gravitational/tectonic stress state, transient thermally induced 
stresses due to expansion of the rock mass3, and seismic stresses due to potential earthquake 
shaking. Additionally, the rock mass strength, particularly in lithophysal rock, will exhibit a 
degree of time-dependency as a result of typical stress corrosion mechanisms4. The impact of 
these in situ and induced stress components is the potential for rock mass yield in the immediate 
region around the tunnels and some degree of rockfall. The rockfall has potential performance 
impacts on the following systems (BSC 2004e): 
• Mechanical Effects on Engineered Barriers 
- Effects include possible rock particle impacts and dynamic loading of the drip shield, 
either under gravitational or earthquake-induced accelerations. 
- Quasi-static loading and contact from rock particles resting on the drip shield 
following rockfall. 
- Dynamic loading of the drip shield could occur from seismic ground motions applied 
to a previously collapsed drift. 
• Mechanical Effects on In-Drift Environment 
- Rockfall of sufficient volume could result in an “insulating” blanket surrounding drip 
shields, impairing heat transfer to the rock mass and increasing waste package 
temperatures. 
- Rock particles within the drift will potentially alter the in-drift moisture and chemical 
environment around and upon the engineered barrier system components, and 
therefore need to be accounted for in drip shield and waste package corrosion. 
• Mechanical Effects on Seepage of Groundwater Into the Drift 
- Rockfall will result in changes to the size and shape of the emplacement drifts as a 
function of time, thus altering the seepage flow to the drift. 
3 
4 
A large portion of the thermally induced strains and stress are recoverable as the rock mass cools over time. 
“Stress corrosion” is a term commonly used in the field of rock mechanics (and metals) that represents timedependent, 
subcritical crack growth that occurs when existing material flaws in the rock are subjected to stresses 
that are near the failure state of the material. This process, which occurs at a more rapid rate in the presence of 
moisture, may result in damage and yield at applied stresses that are less than the short-term strength. Corrosion 
here does not refer to corrosion of metals. 
June 2004 1-11 No. 4: Mechanical Degradation Revision 1 
- Rock particles within the drift will have impacts on the drift capillary strength and 
seepage transport mechanisms and travel paths within the drift depending on particle 
size distribution. 
- Damage around the drift itself will affect the rock mass hydraulic characteristics that 
affect the capillary barrier in the periphery of the drift. 
Figure 1-6. Potential Postclosure Performance Impacts of Rockfall on Engineered Barriers, In-Drift 
Environment, and Groundwater Seepage into Drifts 
A series of NRC KTI agreements deal specifically with the methodology for analysis of and 
estimation of rockfall size distribution and volume as a function of the variability of rock mass 
properties, loading conditions and strength time-dependency. A number of KTI agreements in 
other areas (waste package, drip shield, seepage) address the impacts of rockfall as these 
performance issues. 
June 2004 1-12 No. 4: Mechanical Degradation Revision 1 
1.2.3 Summary of Approach to Addressing Preclosure and Postclosure Issues 
The primary technical issues that have formed the basis for the mechanical degradation 
resolution strategy can be summarized in three categories: (1) geotechnical characterization and 
rock mass property definition, (2) development and validation of numerical modeling tools, and 
(3) design and performance analyses. 
1. Geotechnical Characterization and Rock Mass Property Definition 
• Development of laboratory and in situ testing database of thermal-mechanical and 
time-dependent material properties for intact rock and fractures 
• Development of a relationship between geologic structure (fractures in the 
nonlithophysal rocks and lithophysae in the lithophysal rocks) and rock mass 
material response 
• Determination of the impact of the variability of geologic structure on rock mass 
property size effect, variability, and uncertainty. 
2. Development and Validation of Numerical Modeling Tools (determination or 
development of appropriate models for sensitivity studies of excavation stability and 
rockfall under in situ, thermal, and seismic loading) 
• Continuum versus discontinuum modeling 
- Discontinuum modeling required for representing rockfall 
- Continuum modeling suitable for parameter studies 
• Two- versus three-dimensional models 
- Three-dimensional models required in joint-controlled nonlithophysal rock, 
where response to seismic load is generally anisotropic 
- Two-dimensional models sufficient in generally isotropic lithophysal rock 
3. Design and Performance Analyses 
• Design studies 
- Verification, via empirical and numerical analyses, of the functional and 
operational requirements and specification of ground support 
- Develop initial ground support designs and specifications based on 
requirements 
- Verify design concepts via analysis of rock mass stability in lithophysal and 
nonlithophysal rock for in situ stress and preclosure thermal and seismic 
loading 
1-13 June 2004 No. 4: Mechanical Degradation Revision 1 
- Development of observation and maintenance plans for ground support 
• Performance analyses 
- Stability analysis and rockfall estimate under in situ, thermal, and seismic 
loading 
- Assessment of long-term stability of tunnels under quasi-static loading and 
time-dependent rock mass strength degradation 
The approach taken to address these issues (Figure 1-7) is based on development of a 
progressively more detailed understanding of the mechanical behavior of the lithophysal and 
nonlithophysal rock masses, starting with basic geologic characterization to understand rock 
mass variability, followed by laboratory and in situ testing with closely coupled numerical model 
development and validation. The process was developed from laboratory to field scale, allowing 
testing of the models and development of confidence in their ability to predict complex 
degradation modes. Next, the validated site-specific models were used to conduct sensitivity 
studies to examine thermal and mechanical drift degradation for variation in rock mass structure 
(and properties) under static and transient loading. The KTI agreements that have been 
addressed at each stage in this process are also given in this figure. 
The process was composed of six major program work elements. The approach initially 
involved developing a detailed understanding of the thermal-mechanical properties and 
variability of lithophysal and nonlithophysal rocks, and developing validated numerical models 
that can be used for design and performance assessment. The outcome of this process was a set 
of material models and properties and their ranges that were used as input to sensitivity studies of 
drift degradation in response to seismic events and to time-dependent processes. 
As described in Section 3, the mechanical and thermal properties of the lithophysal rocks are 
sensitive primarily to the lithophysal porosity. Because the lithophysal cavities range in size 
from millimeters to over a meter, and are of widely different shape and distribution within the 
rock mass, sampling and mechanical testing of sufficiently large samples presents a challenge, 
and thus characterization of properties and property variability is an issue. 
To overcome this issue, mechanical testing at increasing scales was used to understand porosity 
and size effects. Calibrated numerical modeling using discontinuum methods were used as a 
means of exploring the variability of lithophysal porosity, size, shape, and distribution on failure 
mechanisms and mechanical property variations. Thus, testing was used to define the basic 
lithophysal rock mass properties and was supplemented by modeling to explore variability 
ranges. 
The sensitivity modeling was aimed at estimating tunnel stability in the preclosure and 
postclosure time frames. In the preclosure period, the primary issue was the verification of 
ground support methods. In the postclosure period, the primary issues were estimation of 
mechanical degradation of emplacement drifts from thermal and seismic loading, or timedependent 
degradation response of the rock mass. 
June 2004 1-14 No. 4: Mechanical Degradation Revision 1 
studies. PSHA = probabilistic seismic hazard analysis. 
NOTE: Process starts with compilation and analysis of basic geotechnical mapping, followed by laboratory and field 
testing and model validation to develop rock mass property estimates for design and performance sensitivity 
Figure 1-7. General Approach to Resolution of the Repository Design and Thermal-Mechanical Effects 
Key Technical Issues 
A review of the six major program work elements is as follows: 
1. Geotechnical Characterization–Further analysis of the extensive, existing rock mass 
geologic and geotechnical characterization data from the ESF and the Enhanced 
June 2004 1-15 No. 4: Mechanical Degradation Revision 1 
Characterization of the Repository Block (ECRB) Cross-Drift, as well as surface 
outcrops and boreholes, to estimate the geometrical variability of rock mass fracturing 
and lithophysae. These data are supplemented by new detailed panel mapping of 
lithophysae in the ECRB Cross-Drift. Detailed statistical analysis of the fracture 
geometry in the middle nonlithophysal unit of the Topopah Spring Tuff (Tptpmn) and 
the lithophysae geometry in lower lithophysal unit of the Topopah Spring Tuff (Tptpll) 
is performed to provide the basic rock mass structural input and its variability to the 
modeling and analysis activities (see Section 2.3). 
2. Laboratory Testing and Model Calibration–A large amount of data for thermal and 
mechanical rock properties exists for nonlithophysal rocks. These data have been 
supplemented with laboratory direct shear testing of representative fracture samples. 
Additional compression tests on nonlithophysal samples with varying size and 
saturation levels, as well as static fatigue testing to derive time-dependent strength 
properties, have been conducted. Laboratory tests on lithophysal rock cores with a 
large diameter (290 mm) have been conducted to determine mechanical and thermal 
properties as a function of porosity, temperature, and saturation level. These 
laboratory tests, combined with in situ testing and previous large diameter (267-mm) 
cores, are used to establish the basic engineering properties of the lithophysal and 
nonlithophysal rock masses and their variability with porosity (see Sections 3.2.1, 
4.2.2, and 4.2.3). 
The lithophysal laboratory data (i.e., strength, modulus, and variation with porosity) 
and observations of failure mechanisms are used as the basis for initial calibration of 
discontinuum numerical models5. Two discontinuum numerical approaches are used 
to back-analyze laboratory data and to provide an understanding of the impact of 
lithophysal cavities on deformability and yield mechanisms. These approaches can 
represent the presence of voids as well as the physical reality of complex failure 
mechanisms involving interlithophysae fracturing and compaction of void space. 
PFC2D and PFC3D, which use a “micromechanical” discontinuum approach for 
representing rock, and UDEC, a standard discontinuum modeling program, are used to 
reproduce stress-strain behavior and failure mechanisms observed from the laboratory 
experiments. The models are also calibrated against static fatigue testing, which 
determines a relationship between the “time-to-failure” and applied stress level for 
nonlithophysal cores. These data are used to develop a basic mechanistic approach for 
predicting time dependency that is based on a standard stress corrosion crack growth 
model. 
5 
3. In Situ Material Properties Testing and Model Validation–In situ mechanical and 
thermal testing of lithophysal rocks is performed to determine the size effect (porosity 
and fracture) on rock mass constitutive behavior. These tests are further used for 
Discontinuum numerical models simulate rock masses as actual blocks of solid material separated by fracture 
surfaces that have cohesive tensile and frictional strength. When the rock mass yields, the fractures may 
physically break in shear or tension, creating individual blocks that are free to detach from the surrounding rock 
mass. Continuum numerical models represent the rock mass as a continuous body in which the stiffness and 
yielding effects of fractures are represented via material stress-strain relationships. The rock mass cannot 
represent detachment of individual blocks from the parent rock mass. 
June 2004 1-16 No. 4: Mechanical Degradation Revision 1 
validation of the numerical modeling approaches at increasing size scales. This results 
in numerical modeling approaches that can be used with confidence for extrapolating 
the mechanical response of lithophysal rocks (see Sections 3.2.3 and 4.2.7). 
4. Model Extrapolation for Establishing Property Variability–Due to the size effect 
on thermal and mechanical properties introduced by lithophysae, it is necessary to test 
large sample sizes (for lithophysal units) to obtain realistic in situ rock mass 
properties. It is impractical to perform a statistically large number of in situ tests to 
determine strength and deformability variability. However, the numerical models, 
suitably calibrated against the laboratory and field data, provide the capability of 
exploring the impact of lithophysal variations such as porosity, lithophysal shape, size 
distribution, and spacing on strength and deformability variation. These models have 
been used to provide an understanding of the basic mechanics of the lithophysal rock 
mass and to define the resulting input property ranges for subsequent design and 
performance calculations. The models are also used as a means of understanding the 
impact of lithophysal porosity on stress redistribution within the rock mass and thus its 
impact on stress-related time dependency (see Section 4.2.3.3). 
5. Best-Estimate Constitutive Models and Property Ranges for Sensitivity Studies– 
The testing data and numerical extrapolations are used to define the constitutive6 
models and property ranges for the lithophysal rock mass. The lithophysal rock mass 
properties are subdivided into five categories. These categories are based on 
lithophysal porosity level and are indicative of the in situ rock mass strength and 
moduli variation. Laboratory data from direct shear testing and field characterization 
of joints are used to define property ranges for modeling of nonlithophysal rocks (see 
Section 4.2.4). 
6 
6. Design and Performance Assessment Sensitivity Studies–Sensitivity studies of 
excavation stability under in situ, thermal, and seismic loads in the preclosure and 
postclosure time frames have been performed. Numerical model sensitivity studies are 
performed using the range and distribution of rock mass properties determined from 
testing and model extrapolation. The deformation and yield of the rock mass around 
the openings in preclosure time are used to define ground support requirements and 
appropriate support methods. These models are supplemented with practical empirical 
ground support specification methods, where applicable, to determine ground support 
methods and materials that will require minimal or no maintenance over the preclosure 
period. Extensive discontinuum modeling studies are used to examine thermal and 
seismic loading effects on mechanical degradation of the emplacement drifts in the 
postclosure. In nonlithophysal rocks, analysis of the jointing is used to generate a 
large number of three-dimensional, stochastically defined fracture geometries that 
realistically represent the rock mass fracturing documented from field mapping. These 
jointing realizations and associated emplacement drifts are subjected to postclosure 
A constitutive model in this case is a set of mathematical equations that describe the relationship of applied stress 
to strain for the rock mass. These equations include a description of the elastic and failure characteristics of the 
rock mass. Typically, rock is described by yield criteria that account for the strength of the fracture fabric of the 
rock mass. A typical form of yield criteria for rock is the Mohr-Coulomb or Hoek-Brown criteria (e.g., 
Hoek 2000). 
June 2004 1-17 No. 4: Mechanical Degradation Revision 1 
rock mass temperature levels and ground motions representative of seismic events 
with 10-5, 10-6, and 10-7 annual exceedance frequencies. The rock mass is also 
subjected to predicted postclosure temperature distribution. Two-dimensional models 
representing the range of strength categories of the lithophysal rocks are subjected to 
similar thermal and seismic loading. The rockfall generated by these simulations is 
presented in a number of ways, including the size distribution of the particles, the peak 
velocities of each particle and the total mass of rock dislodged from the surrounding 
mass. This information is used to define dynamic and quasi-static loads applied to the 
drip shield as well as the resulting shape of the tunnels (see Section 5.3). 
Finally, the models are used to predict the time-related degradation in the lithophysal 
rocks resulting from quasi-static loading and time-dependent strength reduction. 
Sensitivity studies for the range of rock mass strength categories are used to predict 
the change in shape of the emplacement drifts and the total dislodged rock volume as a 
function of time. This information provides a basis for the estimation of in-drift and 
seepage effects due to mechanical degradation (see Section 5.3.2.2.4). 
A detailed discussion of the methodology for inclusion of drift degradation estimates into the 
total system performance assessment is given in Seismic Consequence Abstraction (BSC 2004e). 
The drift degradation estimates, including rockfall and drift size and shape, are considered in 
both “nominal” and “seismic” performance scenario classes7. The nominal scenario class 
considers the estimated time-dependant change in drift shape, and the volume of associated 
rockfall on and around the drip shield is considered. The mechanical damage to the drip shield 
resulting from quasi-static loading of dislodged rock is considered, as well as the impacts of the 
rockfall on heat transfer mechanisms. The seismic scenario class takes into account the rockfall 
induced by possible seismic events. Here, the estimates of rockfall onto the drip shield as a 
function of the annual exceedance frequencies are used to estimate the drip shield damage level 
and the subsequent effects on seepage of ground water through it. Additionally, dynamic loading 
of the drip shield due to seismic shaking of a previously failed, rubble-filled drift are also 
considered. Although not specifically part of the drift degradation effects, the seismic scenario 
also includes estimation of performance consequences of vibratory motion on waste package 
damage resulting from waste package impacts with the pallet, adjacent waste packages, and the 
drip shield. 
1.3 ORGANIZATION OF THE REPORT 
The report format is as follows: 
• Introduction (Section 1)–The objectives and scope of this technical basis document, 
and a summary of the mechanical degradation issues and issue resolution strategy. 
7 The TSPA model takes into account “nominal” and “disruptive events” scenario classes. The disruptive events 
scenario class includes both seismic and igneous scenarios. The nominal scenario class includes FEPs expected 
to occur over the life of the repository, including seismic events with annual probabilities in the range of 10-4 and 
10-5 per year. The seismic scenario class includes higher consequence, lower probability events with annual 
probabilities in the range of 10-6 to 10-8 per year. 
1-18 June 2004 No. 4: Mechanical Degradation Revision 1 
• Geologic Characteristics of the Repository Host Horizon Relevant to Mechanical 
Degradation (Section 2)–The geology of the Topopah Spring Tuff as it relates to 
understanding of the thermal-mechanical rock mass properties and the variability 
introduced by fracturing and lithophysae. 
• Thermal-Mechanic Rock Properties Database—Review of Laboratory and In Situ 
Testing for the Nonlithophysal and Lithophysal Rock Masses (Section 3)–A review 
of the results from nearly 20 years of laboratory and field testing programs for 
lithophysal and nonlithophysal rocks. 
• Development of Rock Mass Material Modeling Approaches for Nonlithophysal and 
Lithophysal Rocks (Section 4)–The development of appropriate mechanical material 
models for lithophysal and nonlithophysal rocks. The generalization of these data into 
modeling approaches for mechanical degradation studies in lithophysal rocks. 
Calibration and validation of discontinuum numerical models against these data and 
extrapolation studies using these models to establish variability in mechanical properties 
of the lithophysal rocks. 
• Analysis of Preclosure and Postclosure Drift Mechanical Degradation under 
Gravitational, Thermal, and Seismic Loading (Section 5)–The details of numerical 
­parameter 
studies of mechanical degradation under various loading conditions and time 
dependent strength loss. The output from these studies to other disciplines. 
• Summary and Interactions with Engineered and Natural Systems (Section 6)–A 
summary of the conclusions and information feeds to other postclosure performance 
systems. 
• References (Section 7)–Sources of information used in this document. 
• Appendices–Address specific RDTME KTI agreements. 
1.4 NOTE REGARDING THE STATUS OF SUPPORTING TECHNICAL 
INFORMATION 
This document was prepared using the most current information available at the time of its 
development. This technical basis document and its appendices providing KTI agreement 
responses that were prepared using preliminary or draft information reflect the status of the 
Yucca Mountain Project’s scientific and design bases at the time of submittal. In some cases this 
involved the use of draft analysis and model reports and other draft references whose contents 
may change with time. Information that evolves through subsequent revisions of the analysis 
and model reports and other references will be reflected in the license application as the 
approved analyses of record at the time of license application submittal. Consequently, the 
Yucca Mountain Project will not routinely update either this technical basis document or its KTI 
agreement appendices to reflect changes in the supporting references prior to submittal of the 
license application. 
June 2004 1-19 No. 4: Mechanical Degradation INTENTIONALLY LEFT BLANK 
1-20 No. 4: Mechanical Degradation 
Revision 1 
June 2004 Revision 1 
2. GEOLOGIC CHARACTERISTICS OF THE REPOSITORY HOST HORIZON 
RELEVANT TO MECHANICAL DEGRADATION 
June 2004 
2.1 GENERAL GEOLOGY OF REPOSITORY HOST HORIZON 
The lithostratigraphy and geologic evolution of the Yucca Mountain site is described in Yucca 
Mountain Site Description (BSC 2004f). This section gives a summary of the geologic 
characteristics of the repository host horizon relevant to the analysis of mechanical degradation 
response of the repository excavations. 
Site-specific characteristics of the rock units of the Topopah Spring Tuff that constitute the host 
rock at the repository horizon are found in the geologic mapping of those units in both the main 
drift and ramps of the ESF and the ECRB Cross-Drift. 
The locations of the ESF and ECRB Cross-Drift, and the lithostratigraphic units exposed by the 
tunnels, are illustrated in the geologic cross section (Figure 2-1). The units that comprise the 
host rocks of the repository horizon are zones of the crystal-poor member (Tptp) of the Topopah 
Spring Tuff. The host rocks are shown schematically in Figure 2-2. In descending order (by 
depth), the host rocks consist of the lower part of the upper lithophysal zone (Tptpul), the middle 
nonlithophysal zone (Tptpmn), the lower lithophysal zone (Tptpll), and the lower nonlithophysal 
zone (Tptpln). 
The repository host rock units can be categorized into two general engineering classifications: 
nonlithophysal units (Tptpmn and Tptpln) and lithophysal units (Tptpul and Tptpll), based on the 
relative proportion of lithophysal cavities. 
The nonlithophysal units are generally hard, strong, fractured rocks with matrix porosities of 
10% or less. Fractures that formed during the cooling process are the primary structural features 
found in these units. In contrast, the lithophysal units have significantly fewer fractures of 
significant continuous length (i.e., trace length greater than 1 m), but have relatively uniformly 
distributed porosity in the form of lithophysal cavities. Lithophysal porosity in the Tptpul and 
Tptpll is generally on the order of less than 10% to about 30% by volume. The groundmass that 
makes up the rock matrix in the lithophysal units is mineralogically the same as the matrix of the 
nonlithophysal units, but is heavily fractured with small scale (lengths of less than 1 m) 
interlithophysal fractures in the Tptpll; however, it is relatively fracture-free in the Tptpul. 
2-1 No. 4: Mechanical Degradation nonlithophysal), Tptpll (lower lithophysal), and Tptpln (lower nonlithophysal). 
Revision 1 
June 2004 
Source: Mongano et al. 1999, Drawing 06-46-345 and 0A-46-345. 
NOTE: Repository host horizon includes the major subunits of the Topopah Spring Formation, designated the Tptpul (upper lithophysal), Tptpmn (middle 
Figure 2-1. Geologic Cross Section at the Location of the Enhanced Characterization of the Repository Block Cross-Drift 
2-2 No. 4: Mechanical Degradation Revision 1 
Source: Buesch et al. 1996, Appendix 2; Mongano et al. 1999, pp. 12 to 43. 
Figure 2-2. Structure of the Topopah Spring Tuff Showing the Relative Relationship between 
Fracturing and Lithophysae in the Major Flow Subunits 
June 2004 
2.2 UNDERGROUND REPOSITORY LAYOUT AND LITHOLOGIC 
INTERSECTIONS 
The repository consists of four panels that will cover about 5 km2 within the Topopah Spring 
Tuff (Figure 2-3). The repository layout extends about 5 km in length (north–south) with the 
widest part being about 2 km (east–west). The total length of all excavated openings including 
the drifts, turnouts, exhaust mains, exhaust shafts and raises and other miscellaneous openings is 
about 110 km. The emplacement drifts comprise about 66 km of tunnels contained primarily 
within the Tptpll (less than or equal to 81%) and the Tptpmn (less than or equal to 12%). The 
remaining geologic units comprise roughly 7% (Tptpul about 4% and Tptpln about 3%) of the 
emplacement drift area. Overall, the nonlithophysal rocks comprise roughly 15% of the 
2-3 No. 4: Mechanical Degradation Revision 1 
emplacement area, whereas the lithophysal rocks comprise approximately 85% (BSC 2003e, 
Table I-2). 
Source: BSC 2004g. 
Figure 2-3. Overlay of the Lithostratigraphic Units on the Planned Repository Layout 
2.3 ENGINEERING CHARACTERISTICS OF THE ROCK MASS IMPORTANT TO 
MECHANICAL PERFORMANCE OF REPOSITORY EXCAVATIONS 
The rock matrix material is a typical fine-grained volcanic rock of silica content. The structure 
of the rock mass defines the properties and overall mechanical response of the rock mass to 
thermal and mechanical loading. The fracture geometry and properties in the nonlithophysal 
rocks and the degree of porosity (total and lithophysal) in the lithophysal subunits are the 
primary geologic structural features that impact rock mass behavior. 
Geotechnical mapping of fractures has been performed in the ESF and the ECRB Cross-Drift 
(CRWMS M&O 1998a; Mongano et al. 1999). Figure 2-2 presents a schematic of the Topopah 
Spring Formation illustrating the general occurrence of fracturing and lithophysae in the various 
subunits of the repository host horizon. The occurrence of fractures and lithophysae are roughly 
inversely proportional as demonstrated quantitatively in Figure 2-4, where the fracture density 
(fractures with trace length greater than 1 m), determined from detailed line mapping, and the 
approximate percentage of lithophysal porosity in the ECRB Cross-Drift are shown. The density 
June 2004 2-4 No. 4: Mechanical Degradation Revision 1 
of fractures with trace length greater than 1 m is significantly larger in the Tptpmn and Tptpln 
(20 to 35 fractures/10 m), as compared to 5 fractures/10 m or less in the Tptpul and Tptpll. 
Lithophysae, on the other hand, are sparse in the Tptpmn and Tptpln. 
Source: Mongano et al. 1999, Figure 13. 
NOTE: There is an inverse relationship between fracture density and lithophysal porosity. 
2.3.1 Characterization of Fractures 
Fracturing in the Tptpmn–The field fracture database, constructed from mapping in the ESF 
and ECRB Cross-Drift during tunneling operations, consists of full-periphery maps and detailed 
line surveys of all fractures with length of one meter or greater. The full periphery maps consist 
of traced fracture lengths drawn directly on invert-to-invert surface maps of the tunnels and 
include the dip and dip direction of the feature and any intersections with other fractures. 
Detailed line surveys consist of mapping the detail geometry and surface characteristics of every 
fracture crossing a line hung along the springline of the tunnel. The details of this mapping are 
provided by Mongano et al. (1999). In total, a database of more than 35,000 fracture 
descriptions is available. Additionally, a small-scale fracture-mapping program was conducted 
to document the characteristics of fractures of less than 1-m length in the nonlithophysal and 
lithophysal rocks. 
In summary, there are four sets of fractures in the Tptpmn with the geometrical and surface 
characteristics provided in Table 2-1. 
Figure 2-4. Composite Plot of Fracture Frequency and Lithophysal Porosity as a Function of Distance 
along the Enhanced Characterization of the Repository Block Cross-Drift 
June 2004 2-5 No. 4: Mechanical Degradation Revision 1 
Table 2-1. General Characteristics of Fracture Sets in the Middle Nonlithophysal Unit 
Set 
1 
Mean 
Azimuth/Dip 
120°/84° 
2 215°/88° 
3 302°/38° 
Comment 
Rough to smooth, planar 
Smooth but curved 
Random fractures with generally 
flat to moderate dip 
Trace Length Median 
from Full Periphery 
Geologic Maps (m) 
3.3 
3.1 
3.6 
3.4 
Mean 
Spacing (m) 
0.48 
1.08 
3.40 
2.46 VPP 329°/14° Random fractures with generally 
flat to moderate dip 
Source: DTNs for tunnel mapping include: GS960908314224.020, GS000608314224.006, 
GS960908314224.015, GS960908314224.016, GS971108314224.025, 
GS960708314224.008, GS000608314224.004, and GS960708314224.010. 
NOTE: Trace length medians are taken from a compilation of tunnel mapping and synthetic tunnel 
samples from FracMan. VPP = vapor phase parting. 
The fractures, particularly the high angle sets (sets 1 and 2), have mean trace lengths less than 
the diameter of the emplacement drifts (Figure 2-5), with ends that sometimes terminate either 
against other fractures or in solid rock. Thus, rather than having continuous joint sets, there is 
often a solid rock bridge between joint tracks. A photograph of a typical wall in the ECRB 
Cross-Drift (Figure 2-6), demonstrates an important aspect of the fracturing in the Tptpmn. The 
fracture traces were painted during the detailed line survey, as seen in this photo, and each 
fracture termination was logged as being against another fracture, within solid rock, or 
continuous. The photo shows the common occurrence of fractures that terminate in solid rock 
(T-junctions) as opposed to continuous structures (arrowheads). The subvertical fractures have 
smooth surfaces but often have curved surfaces with large-amplitude (dozens of centimeters) 
asperities and wavelength of meters (Mongano et al. 1999). 
Figure 2-7 shows that low angle vapor phase partings are relatively continuous structures seen 
throughout the Tptpmn. These continuous, but anastomosing fractures are subparallel to the dip 
of the rock unit, and are filled with concentrations of vapor-phase mineralization (primarily 
tridymite and cristobalite). The surfaces are rough on a small scale and, unlike the subvertical 
fractures, have cohesion as a result of the mineral filling. 
The nature of the fracture geometry governs the estimates of the stability and degradation of the 
nonlithophysal rock mass, particularly under the action of seismic shaking, as well as estimates 
of the support function and level of required ground support. Most rock mass classification 
schemes are based on experience of rock masses with continuous joint sets that create regular, 
blocky masses (e.g., Hoek 2000). In the Tptpmn, the relatively short trace lengths and 
nonpersistent joints create relatively few kinematically removable blocks. This is evidenced by 
the fact that only a small number of rock blocks have actually been dislodged in the ECRB 
Cross-Drift (BSC 2004a, Appendix F). They were either dislodged under the action of the TBM 
or were scaled out of the drift back and walls immediately after mining. There have been no 
reported keyblock failures in the ECRB Cross-Drift since excavation, even though only light 
bolting and meshing is used for ground support. 
June 2004 2-6 No. 4: Mechanical Degradation Revision 1 
Source: Mongano et al. 1999, Figure 14. 
Figure 2-5. Fracture Trace Length as a Function of Depth in the Enhanced Characterization of the 
Repository Block Cross-Drift and by Subunit of the Tptp from Detailed Line Surveys 
NOTE: T-junctions on fractures indicate terminations; arrowheads show continuous features. 
Figure 2-6. Fractures in Wall of the Enhanced Characterization of the Repository Block Cross-Drift in 
the Tptpmn 
June 2004 2-7 No. 4: Mechanical Degradation Revision 1 
Figure 2-7. Low-Angle Vapor-Phase Partings in Tptpmn 
Fracture geometry and surface characteristics are required for numerical analysis of mechanical 
degradation of emplacement drifts. Three-dimensional discontinuum numerical methods are 
used to simulate the degradation of the drifts under the action of in situ, thermal, and seismic 
stressing. These models require geometric input of fracture distributions that are statistically 
similar to those observed in the ESF and ECRB Cross-Drift. The mapping data (both full 
periphery and detail line surveys) are used as the basis for generation of a synthetic rock mass 
fracture distribution developed stochastically using the FracMan fracture generation program 
(USGS 1999). Section 4.1.1 discusses the use of FracMan for generation of a statistically 
equivalent fracture domain and the subsequent discontinuum modeling. 
Fracturing in the Tptpll–Short-length fractures (less than 1-m trace length), coupled with the 
lithophysae, are the most important features that govern stability in the Tptpll. Whereas the 
Tptpul tends to have sparse, small-scale, interlithophysal fracturing (Figure 2-8a), the Tptpll has 
abundant fracturing (Figure 2-8b). The fractures, existing throughout the Tptpll, have a primary 
vertical orientation and have spacing of a few centimeters. 
Thin-section analyses of the fracturing in the Tptpll and the Tptpmn show vapor phase 
alterations on most of the fracture surfaces within the rock mass away from the tunnel wall, 
indicating they are natural fractures (i.e., not mining-induced) that were formed during the 
cooling process (BSC 2004a, Section 6.1.4.1). Near the tunnel wall, it is clear that some of the 
fractures have been disturbed by mining, and that at a small number of locations, new, stressinduced, 
wall-parallel fractures have been created in the immediate springline of the tunnel. 
These stress-induced fractures are observed to a depth of about 0.5 m in some of the large 
diameter (290 mm) core holes drilled in the springline area for rock mechanics testing purposes. 
June 2004 2-8 No. 4: Mechanical Degradation Revision 1 
No matter the origin of the fracturing, it is clear that the Tptpll has a ubiquitous fracture fabric 
that is evident in panel mapping or when large diameter core is removed from boreholes 
(Figure 2-9). The matrix material, although strong and similar to the Tptpmn, has numerous 
fracture surfaces that tend to separate during the drilling process. The result is breakage into 
small blocks, making removal of large lengths of core (and thus laboratory testing of sufficiently 
large samples) very difficult. These fractures, which interconnect the lithophysae, tend to create 
blocks with dimensions on the order of about 10 cm or less on a side. Longer length fractures 
that cut the entire drift are widely separated and have been found to be incapable of producing 
kinematically possible wedges (BSC 2004a). Therefore, the potential mode of failure within the 
Tptpll under seismic or time-dependent yield will be in a raveling mode that creates small block 
sizes. 
Emplacement drift ground support for preclosure in this type of rock mass requires a more-orless 
continuous surface confinement that prevents any loosening of these small blocks. The 
design solution developed for support of this rock mass is a thin, continuous, perforated steel 
sheeting that is bolted directly to the emplacement drift surface (see Figure 1-4). Calculations 
indicate that large blocks do not form in the Tptpll (BSC 2004a, Section 6.4.3). The combination 
of lithophysae and fractures in this zone tend to create small blocks with dimensions on the order 
of about 10 cm or less on a side (BSC 2004a, Section 6.1.4.1). Blocks of this size are not 
capable of breaching the drip shield or waste package under dynamic loading. 
June 2004 2-9 No. 4: Mechanical Degradation NOTE: The Tptpul (a) is characterized by a relatively unfractured matrix between lithophysae, whereas the Tptpll (b) 
is abundant in natural, short-length fractures that interconnect lithophysae. Spacing of the fractures is 
generally less than 5 cm. Hackly surface in (a) is a result of TBM cutting process. 
Figure 2-8. Comparison of Lithophysae and Fracturing in the Tptpul and Tptpll 
2-10 No. 4: Mechanical Degradation 
Revision 1 
June 2004 Revision 1 
Source: BSC 2004a, Figure O-3. 
NOTE: Lithophysae have red “L” identifiers with cavities outlined in red and rims in green. Spots have blue “S” 
identifiers with cyan outlines. Lithic clasts have orange “C” identifiers with gold outlines. The panel shown is 
1 × 3 m. Geometry of lithophysae, rims, and spots are scaled from the photographs and used to determine 
porosity, size, and shape distributions. Lithophysal porosity averages about 20% within the Tptpll. 
Figure 2-9. Lithophysae, Spots, and Clasts of Tptpll (as Discussed Below) in Panel Map 1493 Located 
on the Right Rib from Station 14+93 to 14+96 
2.3.2 Characteristics of Lithophysae 
Although the character of the lithophysae varies between the Tptpul and Tptpll, the mineralogy 
of the matrix material within both of these units is the same as in the nonlithophysal units. 
The lithophysae in the Tptpul (BSC 2004a): 
• Tend to be smaller (roughly 1 to 10 cm in diameter) 
• Are more uniform in size and distribution within the unit 
• Vary in infilling and rim thicknesses 
• Have a volume percentage that varies consistently with stratigraphic position 
• Are stratigraphically predictable. 
June 2004 2-11 No. 4: Mechanical Degradation Revision 1 
In contrast, the lithophysae in the Tptpll tend to be highly variable in size, ranging from roughly 
1 cm to 1.8 m in diameter (BSC 2004a). They also: 
• Have shapes that are highly variable from smooth and spherical to irregular and sharp 
boundaries 
• Have infilling and rim thickness that vary widely with vertical and horizontal spacing 
• Have volume percentages that vary consistently with stratigraphic position 
• Are stratigraphically predictable. 
A detailed study of the lithostratigraphic features in the lower lithophysal zone exposed in the 
ECRB Cross-Drift has recently been completed. These data are summarized in Drift 
Degradation Analysis (BSC 2004a), and a representative plot is given in Figure 2-9. The data 
package documents the distributions of size, shape, and abundance of lithophysal cavities, rims, 
spots, and lithic clasts, and these data can be displayed and analyzed as (1) local variations, (2) 
along the tunnel (a critical type of variation), and (3) as values for the total zone. 
In addition to the variation in the abundance of features, such as lithophysae along the ECRB 
Cross-Drift, there are variations in the size, shape, and distance between features. These types of 
variations are most easily observed with panel map data8 (Figure 2-9), which have been 
converted into porosity variations as functions of distance along the ECRB Cross-Drift, through 
the entire Tptpll subunit (Figure 2-10). This information provides direct input to mechanical 
degradation studies in the following ways: 
• The panel maps and porosity, size, and shape variations of lithophysae provide the basis 
for numerical estimation of impact of lithophysae on rock mass properties. 
• Rock mass properties in the lithophysal rocks are primarily a function of porosity, and 
the variation in porosity provided by the direct panel mapping allow the variation in rock 
mass properties to be estimated. 
8 A total of 18 1-by-3-m panel maps were developed in the ECRB Cross-Drift from top to bottom of the Tptpll. 
June 2004 2-12 No. 4: Mechanical Degradation Revision 1 
Source: BSC 2004a, Figure O-15. 
NOTE: Porosity of the 5-m averaged large-lithophysae inventory is not included in the total. 
Figure 2-10. Calculated Porosity of Lithophysal Cavities, Rims, Spots, Matrix-Groundmass, and the 
Total Porosity in the Tptpll Exposed along the ECRB Cross-Drift 
This information has been used to develop a model for simulation of the spatial variability of 
lithophysal porosity within the Tptpll (BSC 2004a, Appendix T). The lithophysal porosity 
within the Tptpll follows a general stratiform geometry in a fashion similar to the occurrence of 
lithostratigraphic contacts within the overall Topopah Spring Tuff (i.e., the Tptpmn, Tptpll, etc.). 
The ECRB Cross-Drift transects the entire Tptpll unit, allowing determination of the stratiform 
nature of laterally continuous subzones of lithophysal porosity within the Tptpll. The lateral 
continuity of lithostratigraphic features and the projection of these features along the apparent 
dip of the ECRB Cross-Drift has been used to develop a vertical cross section of the distribution 
of lithophysal porosity within the Tptpll. Figure 2-11 presents two simulated vertical projections 
of lithophysal porosity for 50-m-tall vertical cross sections through the top and lower portions of 
the Tptpll. Each of the “cells” of these cross sections represent a volume of the Tptpll and have a 
lithophysal porosity associated with it. As seen in these cross sections, the lithophysal porosity 
occurs as stratiform subzones, with the highest values (i.e., 20% or higher) occurring in thin 
bands near the top of the subzone. The lowest lithophysal porosities occur in the lower portions 
of the subzone near the contact with the Tptpln. These simulated cross sections are used in 
Section 5.3 of this report as a basis for examination of the impact of spatial variability on rock 
mass mechanical response. 
June 2004 2-13 No. 4: Mechanical Degradation Revision 1 
No. 4: Mechanical Degradation 2-14 
Source: BSC 2004a, Figure T-5. 
NOTE: Cross section A is a 50 x 200-cell table representing a 1 x 1-m grid, and cross section B is a 20 x 80-cell table representing a 2.5 x 2.5-m grid for the 
simulated section at 17+56 in the ECRB Cross-Drift. Cross sections C and D represent simulated section at 20+14 in the ECRB Cross-Drift. Two 50 x 
200-m Simulated Cross Sections (Aspect Ratio is not 1:1) at Upper (A/B) and Lower (C/D) Sections of the Tptpll. 
June 2004 
Figure 2-11 Illustration of the Process of Sampling and Modeling Spatial Variability Using Lithophysal Porosity Revision 1 
Stress Component 
2.3.3 In Situ Stress State 
The in situ stress state at and in the vicinity of the Yucca Mountain site has been determined by 
hydraulic fracturing by Sandia National Laboratories (CRWMS M&O 1997a) and by Stock et al. 
(1985). A summary of the measurements is given in Table 2-2. 
Table 2-2. In Situ Stress Estimates at Yucca Mountain Site 
ƒÐ1 
Magnitude (MPa) 
~0.023 ~ depth (m) 
ƒÐ2 0.617 ~ ƒÐ1 
Direction 
Vertical 
N15‹E 
N105‹E ƒÐ3 
Source: DTN: SNF37100195002.001. 
2.4 SUMMARY 
The rock mass that comprises the repository host horizon consists of a number of subunits of the 
Topopah Spring Tuff. From an engineering and mechanical degradation standpoint, these 
subunits can be divided into two broad classifications: lithophysal and nonlithophysal rocks. The 
matrix material of both is similar mineralogically and mechanically, with the distinguishing 
characteristics, from a geomechanics standpoint, being their structural features (i.e., the fractures 
and lithophysae). Due to the importance of these structural features in the mechanical response 
of the rock, it has been necessary to perform detailed geologic and statistical descriptions of the 
geometric characteristics of the fractures and lithophysae. This information has been used to 
provide the basis for input to engineering design and performance modeling of these units, as 
described in Section 5.3. 
0.362 ~ ƒÐ1 
2-15 June 2004 No. 4: Mechanical Degradation INTENTIONALLY LEFT BLANK 
2-16 No. 4: Mechanical Degradation 
Revision 1 
June 2004 Revision 1 
3. THERMAL-MECHANICAL ROCK PROPERTIES DATABASE— 
REVIEW OF LABORATORY AND IN SITU TESTING FOR THE 
NONLITHOPHYSAL AND LITHOPHYSAL ROCK MASSES 
3-1 
3.1 INTRODUCTION 
This section of the report reviews the rock mass properties database for repository host horizon 
units. Two documents provide a detailed review and analysis of this information: Yucca 
Mountain Site Geotechnical Report (CRWMS M&O 1997b) and Subsurface Geotechnical 
Parameters Report (BSC 2003a). The former presents field and laboratory test data available 
through June 1996, while the latter presents the geomechanical data acquired since that time. 
The original report presents data primarily from small-diameter cores and thus provided little 
information on more representatively sized samples of lithophysal rock. A major laboratory and 
field testing effort to obtain data on lithophysal rocks was conducted in 2002 and 2003. This 
section presents an overview of the geomechanical database, and is organized in terms of rock 
type. 
To perform estimates of the mechanical degradation of emplacement drifts subject to stresses 
and time dependent material property changes, it is necessary to determine the basic thermal and 
mechanical properties of intact rock and the rock mass and to estimate their variability within the 
repository host horizon. The properties and geotechnical characteristics that need to be 
determined are: 
. Strength properties (T 
• Mechanical Properties of Intact Rock 
. Elastic moduli (E) and Poisson’s ratio (v) 
o) (from uniaxial, triaxial compression, and tensile testing). 
• Mechanical Properties of Joints 
. Normal (Kn) and Shear (Ks) stiffness 
. Shear strength (ós) 
. Surface roughness (Jr) and dilation angle. 
• Thermal Properties 
. Conductivity (k) 
. Heat capacity (Cp) 
. Expansion coefficient (á). 
• Geotechnical Characterization 
. Fracture geometry statistics (dip, dip direction, spacing, length) 
. Fracture surface characteristics (roughness, planarity) 
. Classification indices (Q’, RMR, GSI). 
Figures 3-1 and 3-2 illustrate the approaches taken to defining rock mass thermal-mechanical 
constitutive models and properties for design and performance assessment studies. Laboratory 
testing of small cores from surface-based boreholes (diameter of 50 mm) and large cores from 
drilling within the ESF and ECRB Cross-Drift and Busted Butte (diameter of 305 mm and 267 
mm, respectively), as well as field geotechnical characterization, form the bases for initial 
June 2004 No. 4: Mechanical Degradation Revision 1 
properties estimates and materials model definition. For nonlithophysal rocks (Figure 3-1), 
which are typically strong and fractured volcanic materials, a standard geomechanical approach 
to rock properties estimation and numerical model definition is taken (e.g., Hoek 2000). 
Laboratory testing and ESF and ECRB Cross-Drift field mapping, using geotechnical 
characterization methods, are used to perform estimates of the in situ rock mass properties for 
use in ground support design parameter studies. Rockfall analyses require that the rock mass be 
modeled as a discontinuum material with explicitly defined fractures; therefore, for these studies, 
a detailed, statistical description of the rock mass fracturing and fracture properties is required 
for direct input to three-dimensional numerical models. The FracMan program (USGS 1999) is 
used for the fracture geometric modeling and field estimates, and direct shear testing of large 
joints in the laboratory is used for estimating surface properties. 
Definition of the material properties of, and mechanical constitutive modeling approach for, 
lithophysal rocks requires a different approach (Figure 3-2). The use of empirical techniques for 
estimating rock mass properties is not particularly applicable to these rock masses since there is 
little available experience in excavating similar rock types. Due to the presence of the 
lithophysal cavities, the rock mass properties are also both porosity- and size-dependent. In 
other words, the properties of the material are a function of both the size of the sample being 
tested, and the size, shape and degree of lithophysal cavities that the sample contains. 
Figure 3-1. Development Strategy for Rock Properties Database and Modeling Strategy for 
Nonlithophysal Rocks 
June 2004 3-2 No. 4: Mechanical Degradation Revision 1 
Figure 3-2. Development Strategy for Rock Properties Database and Modeling Strategy for Lithophysal 
Rocks 
To address these issues, an approach is taken based on laboratory and field-scale testing at 
increasingly larger scales to define the size effect and to provide data for model calibration and 
validation. An integral aspect of this approach is the comparison and validation of discontinuum 
numerical models that can simulate the basic mechanical response of the lithophysal tuff to 
applied stresses. 
Once it is demonstrated that the model can account for the observed response, it is used for 
conducting parametric compression testing studies on simulated “samples” of lithophysal tuff to 
investigate the impact of variability of lithophysae shape, size, and porosity on the range of 
expected mechanical properties. In this manner, the numerical models are used to extend the 
field and laboratory testing and establish the range of material properties for design and 
performance studies. This technique, combined with the laboratory testing, is used to establish 
the upper and lower bounds of lithophysal rock properties. In addition, the rock mass properties 
of large-scale samples of Tptpll, taking into account the impact of spatial variability of 
lithophysal porosity, are examined numerically. Comparison of the mechanical properties ranges 
to in situ ESF and ECRB Cross-Drift fracturing observations and thermal stress-induced spalling 
in the Drift Scale Test (DST) are used as validation tests of the model and properties ranges (see 
Section 4.2.7). All of these studies are used to define the base set and range of rock mass 
properties for use in performance assessment. 
3.2 GEOMECHANICAL PROPERTIES OF YUCCA MOUNTAIN ROCK 
3.2.1 Mechanical Intact Rock Properties of Nonlithophysal and Lithophysal Rocks 
In the late 1970s and through the mid-1980s, many samples were tested, from units extending 
from the upper-most parts of the Paintbrush Tuff down through the lower regions of the Crater 
Flat Tuff. From the beginning, an approach was adopted to assign a baseline set of conditions 
June 2004 3-3 No. 4: Mechanical Degradation Revision 1 
and then study the effects of other conditions (i.e., sample related, environmental, and inherent 
rock characteristics) relative to that baseline. 
Initially, samples were difficult to obtain, so the baseline conditions were defined as: each test 
specimen was machined as a right-circular-cylinder, with a nominal diameter of 25 mm (1 in) 
and a 2:1 length: diameter ratio, and tested in a water-saturated state at room temperature, 
atmospheric pressure, and a nominal axial strain rate of 10-5/s. The results from these test series 
(Olsson and Jones 1980; Nimick et al. 1985; Price and Jones 1982; Price, Jones, and 
Nimick 1982; Price and Nimick 1982; Price, Nimick, and Zirzow 1982; Price, Spence, and 
Jones 1984) revealed that there is some lateral (i.e., within a unit) and vertical (i.e., unit to unit) 
variability. However, the variabilities in the elastic and strength properties of the tuffs (all 
having similar chemical constituents) are predominantly a function of the tuff porosity 
(Figure 3-3; Price 1983; Price and Bauer 1985). Figure 3-3 shows a compilation of the 
unconfined compressive strength and Young’s modulus as a function of total porosity for test 
samples with a diameter of 50.8 mm. To account for the impact of lithophysae on strength and 
deformability, cores with a diameter of 254 mm (Figure 3-4) were sampled (Price et al. 1985). 
These cores were extracted from the Tptpul from Busted Butte, which is adjacent to Yucca 
Mountain. These results, combined with testing of Tptpul and Tptpll cores with diameters of 
305 and 267 mm, are discussed further in Section 4.2. 
Confined (triaxial) compression testing has been conducted on 25.4 and 50.8 mm samples taken 
from surface boreholes from the various welded and nonwelded tuff units (Figure 3-5). 
Environmental parameters used during the testing, in addition to confining pressure, include 
temperature and saturation (either room dry, heated dry, or vacuum saturated). A detailed 
description of the test data can be found in Subsurface Geotechnical Parameters Report (BSC 
2003a). The results show significant variability, presumably due to the variability in porosity of 
samples within the Tptpmn. Sample temperatures of 150°C resulted in a slight decrease in 
strength compared to those samples tested at room temperature. 
Indirect tensile strength testing was conducted on 50.8 mm diameter core samples from the 
Tptpmn and Tptpll under saturated conditions. The results show mean values of 
10.8 MPa ±4.02 MPa (14 samples) in the Tptpmn and 8.33 MPa ±2.93 MPa (24 samples) in the 
Tptpll (BSC 2003a, Table 8-35). 
The next key study (Price 1986) examined the effect of sample size on the elastic and strength 
properties of the middle nonlithophysal zone of the Topopah Spring tuff (Figure 3-6).9 
Sometime later, a model relating strength, sample size, and functional porosity was developed 
from these data (Price et al. 1993) in combination with the early fits of strength versus functional 
porosity. 
To further explore the effect of sample size for nonlithophysal material, a single large block of 
material from the nonlithophysal section of the Tptpll near its lower boundary with the Tptpln 
was obtained from Busted Butte (outcropping adjacent to Yucca Mountain, Figure 3-7). 
9 As seen in Figure 3-8, this original work on sample size influence on properties of the Tptpmn has been recently 
supplemented by additional size-effect studies on nonlithophysal rock from the base of the Tptpll subunit. 
June 2004 3-4 No. 4: Mechanical Degradation Revision 1 
Source: BSC 2003a, Figures 8-22 and 8-11. 
NOTE: Porosity is composed of matrix and lithophysal porosity. All measurements are from a 50.8 mm diameter 
core that is saturated and at room temperature with a length to diameter ratio of 2:1 and a strain rate of 10-5. 
Small cores from lithophysal zones generally contain only small amounts of lithophysal porosity, and thus 
the above tests are not indicative, in general, of properties of the lower and upper lithophysal units. 
Figure 3-3. Intact Unconfined Compressive Strength (a) and Young’s Modulus (b) for Topopah Spring 
Subunits as a Function of Effective Porosity 
June 2004 3-5 No. 4: Mechanical Degradation Revision 1 
Figure 3-4. Photograph of Busted Butte Sample from the Upper Lithophysal Zone 
June 2004 3-6 No. 4: Mechanical Degradation Source: BSC 2003a, pp. 8-106 and 8-107. 
Figure 3-5. Plot of Axial (ó1) Versus Confining (ó3) Stress for 50.8 mm Diameter Specimens of the 
Tptpmn: Room Dry and Room Temperature (top), Saturated at 150°C (bottom). 
No. 4: Mechanical Degradation 
Revision 1 
June 2004 3-7 Revision 1 
Source: Price 1986, Figure 11. 
Figure 3-6. Effect of Sample Size on the Unconfined Compressive Strength of Welded Tuff from the 
Middle Nonlithophysal Zone 
NOTE: The block shown was obtained from the nonlithophysal portion of the Tptpll near its lower boundary with the 
Tptpln, Busted Butte. 
Figure 3-7. Development of Rectangular Specimens for Matrix Size Effect and Anisotropy Study 
June 2004 3-8 No. 4: Mechanical Degradation Revision 1 
Several studies (Martin et al. 1993a; Martin et al. 1993b; Martin et al. 1995; Martin et al. 1997a; 
Martin et al. 1997b) have produced indications that the strength properties of the tuffs are 
somewhat time-dependent. A total of nine nonlithophysal rock samples obtained from the 
Tptpmn at Busted Butte were used to conduct static fatigue experiments with saturated rock 
conditions at 150°C. Except at very fast rates of deformation (i.e., an axial strain rate of 10-3/s), 
the strengths of the tuffs were found to decrease with decreasing strain rate (from 10-5/s to 
10-9/s), although the average change was relatively minor (about a 10% decrease in strength per 
decade change in strain rate). In addition, constant stress (creep) experiments at fairly high 
stresses (most tests at 100 MPa and higher) resulted in very little strain accumulating after 
several million seconds. The existing test data are described in Section 5.3.3.2.4 (Table 5-8) and 
in detail in Drift Degradation Analysis (BSC 2004a, Appendix S). 
A total of 110 samples with size ranging from 26 to 223 mm diameter were cut and tested to 
examine both size effect and mechanical anisotropy. The mechanical anisotropy studies 
included testing of 62 to 51 mm samples drilled at three mutually perpendicular directions from 
the same block of material. 
The results of the sample size on unconfined compressive strength are shown in Figure 3-8, 
where the unconfined compressive strength is plotted as a function of the sample volume (as a 
log-log plot), and is compared to the data given in Figure 3-6 for the Tptpmn. The vertical offset 
of the two lines is indicative of the slightly different average strength of the Tptpll and Tptpmn 
matrix material, although the size effects for the two are virtually identical. The mechanical 
anisotropy is demonstrated in terms of the average values for the Young’s moduli from each of 
the perpendicular orientations. As seen in Figure 3-9, there is a maximum anisotropy of 
approximately 10.5% in the average matrix moduli, which is considered to be a second order 
effect in comparison to lithophysal and fracturing effects. 
A series of tests were run on 50.8-mm nonlithophysal Tptpll samples from the same outcrop 
boulder to examine the impact of saturation level on unconfined compressive strength. It is 
impossible to accurately control moisture content at specific levels of saturation for a rock 
sample, so a number of tests were performed to fully dry and saturate the samples before 
allowing them to equilibrate at room humidity conditions (Table 3-1). As seen in this table, the 
presence of moisture has a significant effect on unconfined compressive strength, particularly 
whether the samples are under heated-dry or exposed to humid air conditions. Complete drying 
of samples increases the mean strength of the samples tested by approximately 20%. 
June 2004 3-9 No. 4: Mechanical Degradation Revision 1 
Source: BSC 2004a, Figure E-20. 
NOTE: Results from the 2003 testing of Tptpln/Tptpll samples (DTN: SN0306L0207502.008) are compared to 
previous testing of samples from the Tptpmn (Price 1986). The 2003 data, although from the Tptpll, are 
composed of matrix material and contain no observed lithophysae. 
Figure 3-8. Results of Size Effect Study Showing Variation in Sample Unconfined Compressive 
Strength as a Function of Sample Volume 
Source: BSC 2004a, Figure E-21. 
NOTE: Averages of values from 62 samples shows maximum of 10.5% anisotropy in the average modulus. 
Figure 3-9. Anisotropy in Young’s Modulus of Nonlithophysal Tptpll Matrix for Three Mutually 
Perpendicular Coring Directions 
June 2004 3-10 No. 4: Mechanical Degradation Revision 1 
Table 3-1. Impact of Moisture Conditions on Unconfined Compressive Strength of Nonlithophysal Tptpll 
Samples 
Test 
Condition 
1 
2 
3 
4 
Mean Strength 
(MPa) 
213 
176 
158 
149 
Moisture Condition 
Samples dried by slow heating to 200°C, tested at 200°C 
Samples dried by slow heating to 200°C, then slowly cooled in dry environment, 
exposed to room humidity for about 30 minutes, and tested at room temperature 
Samples allowed to equilibrate with room humidity, tested at room temperature 
Samples water saturated, tested at room temperature 
Source: BSC 2004a, Table E-12. 
NOTE: Strengths are mean values from testing of 51-mm diameter samples at each moisture condition. 
This strength decrease in the presence of moisture is consistent with other testing of silicic rocks 
and is a typical stress-corrosion mechanism involving chemical alterations due to moisture in 
flaws within the samples. The compression test data reported here is, unless otherwise noted, all 
at room humidity conditions. 
Following a conservative design approach, performance calculations performed for ground 
support or postclosure effects assume average strength conditions from room temperature testing, 
with data ranges to cover fully saturated conditions. 
Large Core Laboratory Testing of Lithophysal Rocks–In an attempt to obtain more 
representative samples of lithophysal rocks, a series of large core (305-mm diameter) samplings 
of the Tptpul and Tptpll were taken in the ESF and ECRB Cross-Drift. The objective of the 
drilling was to obtain samples that had a number (approximately 3 to 5) of lithophysae across a 
sample diameter. Drilling of such large core in this material is quite difficult due to the tendency 
of the core to break in shear when a large or poorly placed lithophysae is encountered. 
Figure 3-10 shows a series of these samples as prepared for laboratory compression testing. 
Approximately 20 samples were tested at varying environmental conditions, including air dry, 
saturated under vacuum, and at 200°C. Plots of the unconfined compressive strength and 
Young’s modulus as functions of the approximate levels of lithophysal porosity10 are given in 
Figure 3-11. In these results, the unconfined compressive strength and Young’s Modulus are 
relatively insensitive to lithophysal porosity above approximately 20%, with rapid increase in 
both for lithophysal porosities below about 15%. This trend fits the general exponential 
relationship between these parameters and porosity as seen in Figure 3-3, and can generally be 
viewed as an extension of this response at higher porosity levels. As was shown previously via 
field measurement in the ECRB Cross-Drift (Figure 2-10), lithophysal porosity through the 
Tptpll averages approximately 15%, with a range from about 10% to 25%. The laboratory-scale 
unconfined compressive strength of the Tptpll can vary from as high as about 25 to 30 MPa to as 
low as about 10 MPa, while the Young’s modulus can vary from 20 to 5 GPa. 
10 Lithophysal porosity is approximate, as it was estimated from core surface measurements. 
June 2004 3-11 No. 4: Mechanical Degradation Revision 1 
Figure 3-10. Photographs of Large Lithophysal Core Samples (290 mm/11.5 diameter) from the Tptpll 
and Tptpul (a) and a Sample in Unconfined Compression (b) 
June 2004 3-12 No. 4: Mechanical Degradation Revision 1 
Source: DTNs: SN0208L0207502.001; SN0211L0207502.002; SN0305L0207502.006 (for laboratory data). 
Lithophysal Rocks 
A convenient way of presenting these data for design purposes is in terms of the relationship 
between the unconfined compressive strength and Young’s modulus (Figure 3-12). Some of the 
primary mechanical properties that are input data to numerical models are the unconfined 
compressive strength and Young’s modulus. Figure 3-12 illustrates the interrelationship of these 
parameters, the total range of their variation, and that they vary in a generally linear fashion, 
Figure 3-11. Unconfined Compressive Strength (a) and Young’s Modulus (b) as Functions of Estimated 
Lithophysal Porosity from Large Core (290 mm diameter) Compression Tests from 
June 2004 3-13 No. 4: Mechanical Degradation Revision 1 
irrespective of the lithophysal porosity values. As will be discussed later, this entire range of 
data is subdivided into a series of categories for design parameter studies. Discontinuum 
numerical models, calibrated to this data, are used to examine the impact of variability of 
lithophysal porosity, shape, size and distribution on the variability of these parameters for design 
purposes. 
Source: DTNs: SN0208L0207502.001; SN0211L0207502.002; SN0305L0207502.006 (for laboratory data). 
NOTE: Linear fits made to 11.5-in. and 10.5-in. core tests. 
Figure 3-12. Unconfined Compressive Strength as a Function of Young’s Modulus from Large Core 
(290 mm and 267 mm diameter) Compression Tests from Lithophysal Rocks 
June 2004 
3.2.2 Summary of Laboratory Testing of Lithophysal and Nonlithophysal Cores 
In summary, a large amount of data has been collected to date on tuffaceous samples from Yucca 
Mountain at a baseline set of conditions. These data have shown that the variabilities in elastic 
and strength properties are not random functions of lateral or vertical position, but primarily a 
function of porosity and its spatial distribution. 
Though there are obvious trends in the Young’s modulus and strength data when plotted against 
porosity, the data have a significant scatter, based on sample porosity variability. The secondary 
effect that creates the scatter is the distribution of the porosity within the sample. Other 
investigations have examined the effects of many other conditions (i.e., sample related, 
environmental and inherent rock characteristics); for example, sample size, saturation, pressure, 
temperature, deformation rate, attenuation, anisotropy have all been studied. 
3-14 No. 4: Mechanical Degradation Revision 1 
In Section 4, the development of mechanical material models and numerical modeling 
techniques are described for both lithophysal and nonlithophysal units. In lithophysal rocks, the 
trends observed in the relationship of mechanical properties to porosity are used as a basis for 
development of the models. In situ measurements of the variability of lithophysal porosity are 
used as the basis for establishing a linkage between the measured mechanical properties and the 
estimated field properties ranges. For nonlithophysal rocks, discontinuum modeling approaches 
are used in which intact blocks and fractures are modeled explicitly. The intact core testing data 
described in this section is used to assign the mechanical properties of intact blocks, whereas 
fracture shear strength properties (described in Section 3.2.4) define the fracture response. In 
conclusion, the intact and large core rock mechanical property information collected over the last 
two decades has provided adequate characterization and property values for design and analysis 
of the behavior of Yucca Mountain tuffs. 
3.2.3 In Situ Mechanical Testing of Nonlithophysal and Lithophysal Rocks 
In situ testing provides data on the mechanical characteristics of the rock mass at a scale 
commensurate with the excavation dimension. Field compression testing has been performed in 
both the nonlithophysal and lithophysal rocks. For nonlithophysal rocks (Tptpmn), field testing 
is restricted to measuring the deformation modulus, as the rock mass strength is too large to 
conduct strength measurements. However, in the lithophysal rocks, the in situ rock mass 
strength is sufficiently low that both the deformation modulus and unconfined compressive 
strength can be measured. 
The Plate Loading Test, conducted as part of the larger DST, was used to apply load to the drift 
wall adjacent to the DST in the Tptpmn unit. The results of the testing show an estimated rock 
mass modulus from 11.4 GPa to 29.5 GPa for ambient and thermally perturbed fractured tuffs, 
respectively (George et al. 1999). 
A series of three pressurized slot tests (PSTs) were conducted to perform deformation modulus 
and strength measurements in lithophysal rock units (Table 3-2). PST#1 was conducted in the 
poorest quality Tptpll material characterized by large lithophysae and heavily fractured matrix at 
the transition between the Tptpmn and the Tptpll. PST#2 was conducted in good quality Tptpul 
in the ESF and PST#3 was conducted in what was considered to be average Tptpll repository 
conditions in the ECRB Cross-Drift. All three tests included compressions at ambient 
temperature and one test included compressions at elevated temperature. 
3-15 June 2004 No. 4: Mechanical Degradation Revision 1 
Table 3-2. Summary of In Situ Slot Compression Tests in Lithophysal Rock Units 
Location 
ESF 57+77 
Test 
PST#1 
Thermal- 
Mechanical Unit 
Tptpll 
PST#2 Tptpul 
Tptpul 
ESF 63+83 
ESF 63+83 
Location 
Wall 
Wall 
Wall 
Floor 
Temperature 
ambient 
ambient 
temperature–90°C 
ambient ECRB 21+25 PST#3 Tptpll 
Source: Sobolik 2002a, Table 1; Sobolik 2002b, Table 1; Schuhen and Sobolik 2003, Table 1. 
NOTE: Metric stationing is used throughout the ESF, so that Station 57+77 is located 5,777 m 
from the start of the tunnel. 
The flatjack slot test first involves placing steel flatjack bladders into parallel, thin, sawcut 
excavations in the sidewall or floor of a tunnel (Figure 3-13). The parallel sawcuts isolate an 
approximately 1-m3 rock specimen between them, which is subsequently compressed by 
pressurizing the flatjacks within the slots. Typically, an instrumentation borehole (305 mm in 
this case) is drilled midway between the slots to allow observation of the interior of the rock 
sample and to monitor deformations during the flatjack pressurization. The flatjack slots are 
excavated larger than the flatjacks to minimize end effects due to block attachment to the 
surrounding solid rock. The flatjacks’ pressure is raised in a series of pressure cycles to monitor 
hysteresis effects and time-dependent strain at increasing levels of applied stress. 
(b) Slot Test 2 Showing Central Instrumentation Hole and Parallel Slots 
Figure 3-13. Photographs of (a) Preparation of Slot Test 3 in the Floor of the Enhanced Characterization 
of the Repository Block, Tptpll Unit, Lowering of Flatjack into Place in Sawcut Slot and 
Figure 3-14 shows flatjack pressure versus instrumentation hole diametral deformation for the 
tests. The loading results show a typical elastic-plastic response in which a linear loading slope 
is followed by yield and plastic deformation. Yielding of the rock was typically in shear, 
emanating from the central borehole, and resulting in rockfall in the form of small rock particles 
in the central borehole during yield. The results, summarized in Table 3-3, show that the rock 
mass has deformation modulus and strength that lie at the lower end of the design range given in 
June 2004 3-16 No. 4: Mechanical Degradation Figure 3-12, but consistent with the same general relationship of strength to modulus as observed 
in the large core laboratory tests. However, the results illustrate the difficulties with performing 
large in situ slot tests. PST#1 was located in the poorest quality lithophysal rock immediately 
adjacent to the contact of the Tptpmn. The low value of modulus obtained indicates that the skin 
of rock surrounding the tunnels at this particular sidewall locations, is likely in a preyielded state 
due to mining-induced stress and excavation effects induced by the TBM. PST#2 resulted in 
shear along a preexisting fracture outside the block (without failing the block itself), and PST#3 
resulted in spalling of the tunnel floor prior to actually failing the rock block between the 
flatjacks. 
Source: Sobolik 2002a, Figure 7; Sobolik 2002b, Figures 7 and 9; Schuhen and Sobolik 2003, Figure 7. 
Figure 3-14. Composite of Flatjack Pressure versus Central Hole Diametral Strain for the Three 
Pressurized Slot Tests 
Table 3-3. Summary of Mechanical Properties Results from the Pressurized Slot Tests 
Test 
PST#1 
PST#2 
PST#2 
PST#3 
Source: Sobolik 2002a, Table 1; Sobolik 2002b, Table 1; Schuhen and Sobolik 2003, Table 1. 
NOTE: PST#1 in poorest quality Tptpll, PST#2 in good quality Tptpul, and PST#3 in typical repository Tptpll. 
Strength here is the peak flatjack pressure reached during the test. PST#2 failed on the fracture 
Strength (MPa) 
6a 
NA 
11a 
7a 
Location 
ESF 
ESF 
ESF 
ECRB Cross-Drift 
outside block, and PST#3 spalled the tunnel floor above flatjacks. 
a Results do not account for presence of central hole in failure load. 
Tuff Unit 
Tptpll 
Tptpul 
Tptpul 
Tptpll 
Condition 
Ambient 
Ambient 
Heated, >80°C 
Ambient 
E (GPa) 
0.5 ±0.3 
3.0 ±0.5 
1.5 ±0.5 
1.0 ±0.3 
No. 4: Mechanical Degradation 3-17 
Revision 1 
June 2004 Revision 1 
3.2.4 Mechanical Properties of Fractures 
3.2.4.1 Direct Shear Test Results 
Five direct shear tests were performed on core samples with a diameter of 305 mm and with 
internal fracture surfaces from the Tptpmn unit. Photographs of the direct shear samples are 
shown in Figure 3-15. Two tests were performed on subvertical cooling joints, and three tests 
were conducted on more or less horizontal vapor-phase partings (VPPs). The two fracture types 
have a physically distinct appearance, and testing of these fractures resulted in equally distinct 
fracture behaviors (Table 3-4). 
Figure 3-15. Photographs of Direct Shear Samples from Rough Vapor Phase Partings (a) and Smooth 
Cooling Joints (b) 
Cohesion and friction angle parameters were determined from repeated tests on the same fracture 
surface (Table 3-4). The advantage of this is that the same fracture is being tested in the same 
equipment setup. The disadvantage is that because of repeated tests on the same fracture surface, 
degradation of the asperities changes the fracture behavior on subsequent loading tests. 
June 2004 3-18 No. 4: Mechanical Degradation Table 3-4. Direct Shear Test Summary of Tptpmn Fractures 
Joint Type Test ID 
65A-643 Cooling 
65A-657 Cooling 
65A-642 VPP 
65A-646 VPP 
65A-647 VPP 
Source: DTN: GS030283114222.001. 
Length 
(mm) 
238.76 
142.24 
241.30 
226.06 
246.38 
NOTE: JRC = the measured value of joint roughness coefficient; CC = the correlation coefficient of a 
linear fit through the data. 
Cohesion 
Area (m2) JRC (MPa) 
2.00 0.04 
1.00 0.02 
15.00 0.06 
16.00 0.03 
10.00 0.05 
-0.01 
0.08 
0.72 
0.66 
0.84 
Table 3-5 illustrates that cooling fractures have lower cohesion, lower peak friction angle, and 
much lower peak dilation angle than the VPP fractures. 
Table 3-5. Summary Statistics of Direct Shear Tests of Fracture Peak Strength 
Count Mean 
2 
3 
2 
3 
2 
3 
Unit 
Cooling Tptpmn 
Tptpmn 
Cooling Tptpmn 
Tptpmn 
Cooling Tptpmn 
Tptpmn 
Joint 
VPP 
VPP 
VPP 
Direct Shear Rock Joint Dilation Angle at Peak Stress 
1.6 
13.7 
2.33 
0.15 
3.61 
2.12 
2.55 
1.22 
1.6 
14.0 
Source: DTN: GS030283114222.001. 
Error 
Deviation / 
Mean 
Direct Shear Rock Peak Cohesion (MPa) 
Standard Standard 
Deviation 
1.97 
0.12 
0.06 
0.09 
0.04 
0.05 
0.32 
0.74 
Direct Shear Rock Peak Friction Angle (°) 
0.01 
0.04 
0.4 
1.9 
0.3 
1.1 
33.4 
44.0 
Median Minimum Maximum 
0.03 
0.72 
33.0 
44.5 
0.08 
0.84 
-0.01 
0.66 
33.7 
45.7 
33.1 
41.9 
4.1 
16.3 
-1.0 
12.1 
The final normal stress and shear stress measured in the last shearing for each fracture was used 
to determine the degraded friction angle values assuming the cohesion was zero. These degraded 
shear results are shown in Table 3-6. The peak and degraded friction angles are roughly the 
same for the smooth subvertical cooling fracture, whereas the degraded friction angle for VPP 
fractures was slightly higher than the peak value. 
Table 3-6. Summary Statistics of Direct Shear Fracture Degraded Strength 
Unit 
Tptpmn 
Tptpmn 
Joint 
Cooling 
VPP 
Direct Shear Rock Degraded Friction Angle (°) 
Count Mean 
2 
3 
Source: DTN: GS030283114222.001. 
33.4 
46.9 
No. 4: Mechanical Degradation 
Error 
Standard Standard Deviation / 
Deviation Mean 
2.7 
3.0 
1.9 
1.7 
0.08 
0.06 
3-19 
Friction 
Angle (°) 
33.7 
33.1 
45.7 
41.9 
44.5 
Median Minimum Maximum 
33.4 
46.5 
35.3 
50.0 
31.5 
44.1 
Revision 1 
CC 
1.00 
1.00 
0.99 
1.00 
0.99 
June 2004 3.2.4.2 Rotary Shear Tests 
A series of 22 rotary shear tests have been conducted on natural rock fractures from core samples 
in the repository host units. The samples ranged in size to 76 mm and tests were conducted at 
room dry, room temperature and at 175°C. Rotary tests involve application of constant normal 
stress and torque to undercored samples (undercoring creates a pipe-like sample with an inner 
and outer radius). The shear stress induced on the surfaces of the joint result in slip and dilation 
of the surfaces can be determined with continued displacement. Table 3-7 provides summary 
statistics of the results of this testing; further discussion can be found in Subsurface Geotechnical 
Table 3-7. Summary Statistics of Fracture Strength Using Rotary Shear 
Parameters Report (BSC 2003a). 
Rock Unit 
Tptpul/ll 
Peak CC* 
0.92 
Count 
6 
Peak 
Friction 
Angle 
40° 
Peak 
Cohesion 
(MPa) 
1.48 
Temperature 
(° C) 
Room 
0.85 0.79 8 Tptpmn/ln Room 42° 
0.96 1.38 8 Tptpmn/ln 175 44° 
NOTE: CC* refers to the correlation coefficient of a linear fit through the data. 
Source: DTNs: SNL02112293001.001; SNL02112293001.003; SNL02112293001.005; SNL02112293001.006; 
SNL02112293001.007. 
(Eq. 3-1) 
June 2004 
3.2.4.3 Empirical Estimate of Peak Dilation Angle 
Barton (in Duan 2003, p. 40) used the following equation to empirically estimate peak dilation 
angles11, ø peak, for Yucca Mountain joint sets: 
øpeak = 1 JRC log. . . 
JCS .. . 
2 
11 
. ó n . 
where JRC is joint roughness coefficient, JCS is joint wall compressive strength, and ón 
is the 
effective normal stress. A range of laboratory scale JRC values for joint sets is estimated 
(Table 3-4) by making JRC measurements from roughness traces of the rotary shear and direct 
shear laboratory fractures, making field JRC measurements of fractures in the ESF, and 
correlating JRC values to U.S. Bureau of Reclamation field roughness statistics (R1 to R6). 
Adopting a “common” value of joint wall normal strength (JCS) of 100 MPa, a normal stress 
(ón) range of 4 to 8 MPa, and the joint roughness coefficient (JRC) range shown in Table 3-8, the 
range of peak dilation angles is derived. The empirically derived field dilation angle estimates 
are consistent with those determined from the laboratory testing (Table 3-5). 
Dilation angle is defined as the ratio of the displacements perpendicular and parallel to the fracture surface when 
subjected to shear. This value provides an indication of the roughness of the joint surface. 
3-20 No. 4: Mechanical Degradation 
Revision 1 
Ultimate 
Cohesion Ultimate 
CC* 
0.76 
Ultimate 
Friction 
Angle 
32° 
(MPa) 
1.51 
0.90 0.60 39° 
0.91 0.24 44° Revision 1 
Nonlithophysal 
Rock 
Table 3-8. Estimate of Peak Dilation Angles for Topopah Spring Formation Fracture Sets 
Cooling Joint Sets 1 and 4 
Cooling Joint Set 2 
VPP Joint Set 3 
Lithophysal Fractures 
Joint Roughness 
Coefficient Range 
2 to 4 
4 to 8 
12 to 16 
12 to 20 Lithophysal 
Rock 
NOTE: Peak dilation angles calculated from Equation 3-1 for JCS = 100 MPa and ón = 4 to 8 MPa. 
3.2.5.1 Thermal Conductivity 
Thermal conductivity is a proportionality constant that relates the heat transfer (conduction) rate 
per unit area in a material to the normal temperature gradient. Thermal conductivity of rock is 
used in predicting temperature changes in the rock mass after waste emplacement. 
Peak Dilation Angle Range 
1.4 to 2.2 
2.8 to 4.4 
8.4 to 8.8 
8.4 to 11.0 
June 2004 
3.2.5 Thermal Properties of the Repository Host Rocks 
Thermal properties (i.e., thermal conductivity, heat capacity or specific heat, and coefficient of 
thermal expansion) of lithostratigraphic rock units at the repository host horizon are important 
parameters used in the design and performance assessment because they are used in calculating 
the transient rock mass temperature and thermally induced stress. Their values are estimated 
primarily based on laboratory and field measurements. These measurements are essential in 
providing not only the site-specific values of thermal properties but also the information of their 
spatial variability and dependencies on temperature, porosity and fracture, and moisture content. 
Additionally, these testing efforts have also assisted in development of theoretical models that 
describe spatial variation of thermal properties and correlation between rock mass thermal 
properties, intact rock thermal properties, and other rock properties such as porosity. 
Numerous laboratory tests using small specimens containing few voids and/or fractures show 
that intact rock thermal characteristics of lithophysal and nonlithophysal of rocks are similar 
(CRWMS M&O 1997b, Tables 5-11, 5-13, 5-15, and 5-16), indicating that similar methods may 
be used for acquiring intact rock thermal properties. However, rock mass characteristics of these 
rocks are quite different due to their different dominant features. These impacting factors are 
reflected in the difference in their rock mass thermal properties, suggesting, for example, the use 
of different methods of acquiring rock mass thermal properties for the lithophysal rocks versus 
those for the nonlithophysal rocks. Due to the associated relatively high cost and logistics 
involved, only a limited number of field thermal tests have been conducted (BSC 2002a, 
Sections 6.2.3.5, 6.3.1.4, and 6.3.3.6.5). Thus, estimation of rock mass thermal properties and 
their spatial variations based solely on the available field testing data may not be sufficient. This 
points to the desirability of using theoretical models to aid in describing the correlation between 
intact rock and rock mass thermal properties and the spatial variations. 
Factors that may affect thermal properties include temperature, porosity, fracture, moisture 
content, specimen size or scale, mineral content, and loading condition. The effects of these 
factors on thermal properties have been a primary focus of investigation in the characterization 
of Yucca Mountain host rocks. 
3-21 No. 4: Mechanical Degradation Revision 1 
Due to the heterogeneity and discontinuities in the rock units that will host the repository, 
thermal conductivity of the rock of interest is both scale- and direction-dependent. When 
increasing the size of testing specimens, the degree of heterogeneity and the impact of 
discontinuities increase, thus affecting the thermal conductivity of the rock specimen. It is 
important to understand the effect of scale on the rock thermal conductivity and the difference 
between the intact rock and the rock mass, so a correct value of thermal conductivity can be used 
in the design. 
Intact Rock Thermal Conductivity–Thermal conductivity of intact rock was estimated based 
on laboratory thermal conductivity measurements using small specimens. These specimens had 
nominal dimensions of 50.8 mm in diameter and 12.7 mm in length (Brodsky et al. 1997, Section 
2.1, Table 2-1), and contained few voids or fractures. The effects of these discontinuities on the 
measurements were considered minimal. A large number of laboratory thermal conductivity 
measurements have been conducted since the late 1990s. The results are summarized in Table 39 
for the four rock units at the repository host horizon. 
Table 3-9. Intact Rock Thermal Conductivities for Repository Units 
Below 100°C (W/m·K) Above 100°C (W/m·K) 
Dry Dry Air Dry Saturated Rock 
Units 
Tptpul 
Mean 
1.97 
Standard 
Deviation 
0.11 
Standard 
Deviation 
0.21 
Standard 
Deviation 
0.22 
0.11 0.12 0.45 2.33 Tptpmn 
0.04 0.08 0.13 2.13 Tptpll 
Mean 
1.20 
1.68 
1.65 
N/A 
Mean 
1.07 
1.51 
1.45 
N/A 
Mean 
1.06 
1.60 
1.54 
N/A 
Standard 
Deviation 
0.12 
0.49 
0.03 
N/A N/A N/A N/A Tptpln N/A 
June 2004 
Source: CRWMS M&O 1997b, Tables 5-11 and 5-13; DTN: SNL01A05059301.005. 
Rock Mass Thermal Conductivity–Thermal conductivity of the rock mass is the effective value 
of thermal conductivity that relates the heat conduction rate to the normal temperature gradient in 
a rock mass. It accounts for the effects of voids, fractures, and any heterogeneity or 
discontinuity on thermal conductivity. Correlations between intact rock or rock matrix thermal 
conductivity, porosity (both matrix and lithophysae), and rock mass thermal conductivity are 
developed in Subsurface Geotechnical Parameters Report (BSC 2003a, Section 8.3.3.2). These 
efforts included both conducting experimental tests and developing theoretical approaches. 
A limited number of experimental tests have been conducted to estimate rock mass thermal 
conductivity. They included laboratory measurements using large specimens and field 
measurements in the DST and the ECRB Cross-Drift. The DST measurements were of the 
Tptpmn unit, while the ECRB Cross-Drift measurements were of the Tptpll unit. Table 3-10 
provides a range of rock mass thermal conductivity for the Tptpmn and Tptpll units obtained 
from the field measurements. Compared to those listed in Table 3-9, it is seen that the in situ 
values are within the ranges observed in the laboratory measurements on small specimens. 
3-22 No. 4: Mechanical Degradation Revision 1 
Table 3-10. Rock Mass Thermal Conductivities from Field Measurements 
Tptpmna 
Range of Thermal Conductivity (W/m·K) 
1.69 to 1.95 
1.73 to 2.18 Tptpllb 
Rock Unit 
a
b 
Source: DTNs: LL980411104244.061; LL980902104244.070; 
UN0106SPA013GD.004; UN0201SPA013GD.007. 
DTNs: SN0206F3504502.012; SN0206F3504502.013; 
SN0208F3504502.019. 
Alternative analytical approaches have also been developed to estimate rock mass thermal 
conductivity (BSC 2002b). The rock mass thermal conductivity, estimated from analytical 
correlations for the various repository host horizons, is summarized in Table 3-11. The 
analytical approaches, which account for matrix void volume, compares well to field 
measurements in Table 3-10. Compared to those listed in Table 3-9 for the intact rock, it is 
obvious that porosity and moisture content have significant effect on the rock mass thermal 
conductivity. For conservatism, ranges of thermal properties that encompass the rock mass 
thermal conductivity are used in design and performance assessment. 
Table 3-11. Rock Mass Thermal Conductivities for Repository Units 
Saturated (W/m·K) Dry (W/m·K) 
Rock Units 
Tptpul 
Standard 
Deviation 
0.24 
0.27 Tptpmn 
0.25 Tptpll 
Mean 
1.18 
1.42 
1.28 
1.49 
Mean 
1.77 
2.07 
1.89 
2.13 
Standard 
Deviation 
0.25 
0.25 
0.25 
0.27 0.28 Tptpln 
3.2.5.2 Heat Capacity 
Heat capacity of a substance is defined as the amount of energy required to raise the temperature 
of a unit mass of the substance by one-degree (Nimick and Connolly 1991, p. 5). It is an 
important parameter used in thermal analysis to evaluate temperature changes in rock after waste 
emplacement. For solid materials, heat capacity is strongly dependent on temperature. For the 
temperature range of interest in the design and performance assessment, heat capacity for the 
repository rock units is estimated for three temperature ranges, 25°C to 94°C, 95°C to 114°C, 
and 115°C to 325°C, corresponding to the preboiling, transboiling, and postboiling regimes, 
respectively. 
Only a limited number of laboratory and field measurements have been made to estimate rock 
heat capacitance (product of heat capacity and density). These measurements covered only a few 
rock units. Instead, the heat capacity values used in the design and performance assessment are 
Source: DTN: SN0208T0503102.007. 
The thermal conductivity ranges provided in Table 3-11 are used as the basis for parametric 
analysis of rock mass temperature distributions in the postclosure period (see Section 5.3.1). 
3-23 June 2004 No. 4: Mechanical Degradation largely based on the calculated values obtained from analytical methods. The estimates have 
been compared with laboratory and field measurements and correlate sufficiently well as to 
validate the estimate model and the resulting values. These methods are presented in Heat 
Table 3-12. Rock Grain Heat Capacities for Repository Units 
Average Average Standard 
Deviation 
Capacity and Thermal Expansion Coefficients Analysis Report (BSC 2003f). 
Rock Grain Heat Capacity–The calculated average values of rock grain heat capacity for the 
four repository rock units are presented in Table 3-12. These values were estimated based on 
available data on mineral abundance and mineral heat capacity. 
Rock 
Units 
Tptpul 
Tptpmn 
Tptpll 
Tptpln 
T = 25°C to 94°C T = 95°C to 114°C 
Standard 
Deviation 
90 
110 
100 
70 
90 
110 
100 
70 
870 
870 
870 
870 
780 
780 
780 
780 
Source: DTN: SN0307T0510902.003. 
NOTE: All measurements are in (J/kg·K). 
Rock Mass Heat Capacity–Rock mass heat capacity is the effective value of heat capacity that 
accounts for the effect of air-filled voids and of water that exists in the voids. 
Efforts to measure the rock mass heat capacity were made. Rock mass volumetric heat capacity 
or heat capacitance of the Tptpll unit was estimated from the thermal measurements in the ECRB 
Cross-Drift. The estimated rock mass volumetric heat capacity values range from 1.96 × 106 to 
2.30 × 106 J/m3·K (DTNs: SN0206F3504502.012; SN0206F3504502.013; 
SN0208F3504502.019). Given a bulk density value of 2,360 kg/m3 for the Tptpll unit, the rock 
mass heat capacity is estimated to range from 831 to 975 J/kg·K. 
The calculated values of rock mass heat capacity for the four repository rock units using the 
analytical methods are summarized in Table 3-13. These values were estimated for the 
preboiling, transboiling, and postboiling regimes, based on the available data on rock matrix 
porosity and saturation, lithophysal porosity, rock grain heat capacity, and density. 
Table 3-13. Rock Mass Heat Capacities for Repository Units 
Average Average Standard 
Deviation 
Rock 
Units 
Tptpul 
Tptpmn 
Tptpll 
Tptpln 
NOTE: All measurements in (J/kg·K). 
T = 25°C to 94°C T = 95°C to 114°C 
Standard 
Deviation 
1000 
900 
1000 
800 
300 
300 
300 
300 
940 
910 
930 
900 
Source: DTN: SN0307T0510902.003. 
3600 
3000 
3300 
2800 
3-24 No. 4: Mechanical Degradation 
Revision 1 
Average 
T = 114°C to 325°C 
Standard 
Deviation 
110 
130 
120 
90 
990 
990 
990 
990 
Average 
T = 114°C to 325°C 
Standard 
Deviation 
300 
300 
300 
300 
990 
990 
990 
990 
June 2004 Revision 1 
It is appropriate to use the rock mass heat capacity in the design and performance assessment if 
the phase change over the transboiling regime cannot be accounted for in the analysis. 
Otherwise, the rock grain heat capacity should be used because the modeling accounts for the 
heat capacity effects in boiling of pore water. The estimates have been compared with laboratory 
and field measurements and sufficient correlation is found to validate the estimate model and the 
resulting values. 
3.2.5.3 Coefficient of Thermal Expansion 
Thermal expansion is a mechanical response in the form of strain because of the change of 
temperature. The coefficient of thermal expansion (CTE) of rock is strongly dependent on 
temperature. It is an important parameter in thermal-mechanical analysis to predict thermally 
induced rock displacements and stresses and to evaluate stability of repository openings and 
performance of installed ground support during heating. 
Intact Rock CTE–Intact rock CTE was estimated based on laboratory thermal expansion 
measurements using small specimens. A large number of thermal expansion measurements have 
been made on specimens taken from the rock units at the repository host horizon. Most of the 
measurements were conducted on dry or saturated specimens over a temperature range of 25°C 
to over 300°C. Table 3-14 summarizes the measured intact rock CTE for the four repository 
rock units. 
Rock Mass CTE–Rock mass CTE is the effective thermal expansion that rock mass experiences 
when subjected to a change in temperature. It accounts for the effects of voids, fractures, 
moisture content, and any heterogeneity or discontinuity that affect the thermal expansion. 
Estimation of rock mass CTE is based on field or large core (diameter of 305 mm) thermal 
expansion measurements. Two major field tests, which involved the measurements of rock mass 
CTE, are the Single Heater Test and the DST. Both tests are located in the Tptpmn unit. The 
results from these measurements are summarized in Table 3-15. 
There are no field thermal expansion measurements available in the Tptpll unit. The best data 
available on rock mass CTE for this rock unit are those from laboratory thermal expansion 
measurements on specimens with a nominal diameter of 305 mm (12 in.). The results from these 
laboratory measurements are also presented in Table 3-15. 
After comparing the rock mass CTEs presented in Table 3-15 to those listed in Table 3-14 for the 
intact rock, it is apparent that the former are lower than the latter. The difference decreases as 
temperature increases, which indicates that the effect of fractures or voids on CTEs diminishes as 
more fractures or voids are closed by rock deformation as a result of temperature increase. From 
the perspective of ground support design, use of the intact rock CTE is conservative. 
3-25 June 2004 No. 4: Mechanical Degradation Mean 
Mean 
Dry 
Std. Dev. 
Mean 
Saturated 
Std. Dev. 
Tptpmn 
Mean 
Dry 
Std. Dev. 
Mean 
June 2004 
Temperature 
Range 
Mean 
Saturated 
Std. Dev. 
Mean 
Dry 
Std. Dev. 
Mean 
Saturated 
Std. Dev. 
Mean 
Dry 
Std. Dev. 
Mean 
Saturated 
Std. Dev. 
Mean 
Dry 
Std. Dev. 
Mean 
Saturated 
Std. Dev. 
Mean 
Dry 
Std. Dev. 
Temperature 
Range 
Rock 
Units 
Tptpul 
Tptpmn 
Tptpll 
Tptpln 
Rock 
Units 
Saturated 
Std. Dev. 
Tptpul 
Saturated 
Std. Dev. 
Tptpll 
Mean 
Dry 
Std. Dev. 
Mean 
Saturated 
Std. Dev. 
Tptpln 
Mean 
Dry 
Std. Dev. 
Source: CRWMS M&O 1997b, Tables 5-15 and 5-16; DTN: SNL01B05059301.006. 
3-26 No. 4: Mechanical Degradation 
Table 3-14. Intact Rock Coefficients of Thermal Expansion for Repository Units 
200– 
225°C 
25.60 
7.08 
29.34 
10.73 
15.53 
1.02 
14.57 
2.04 
15.42 
2.22 
15.14 
3.26 
N/A 
N/A 
12.78 
1.53 
100– 
125°C 
CTE on Heat-Up (10-6/°C) 
CTE on Cool-Down (10-6/°C) 
150– 
175°C 
12.95 
1.76 
13.51 
2.57 
11.74 
0.47 
10.95 
0.52 
11.73 
1.76 
10.75 
1.01 
N/A 
N/A 
11.56 
2.75 
150– 
175°C 
100– 
125°C 
10.22 
0.69 
9.52 
0.52 
8.73 
2.04 
9.50 
0.27 
9.37 
2.78 
9.12 
0.57 
N/A 
N/A 
9.58 
1.07 
200– 
225°C 
75– 
100°C 
7.91 
0.65 
8.89 
0.39 
7.93 
0.94 
8.95 
0.24 
7.03 
1.31 
8.77 
0.54 
N/A 
N/A 
8.79 
0.47 
225– 
250°C 
50– 
75°C 
7.00 
0.33 
8.43 
0.36 
7.78 
1.90 
8.45 
0.30 
7.20 
1.09 
8.15 
0.47 
N/A 
N/A 
8.24 
0.57 
250– 
275°C 
25– 
50°C 
7.59 
0.01 
7.41 
0.42 
7.20 
0.84 
6.89 
1.45 
7.09 
0.45 
6.41 
0.75 
N/A 
N/A 
6.55 
1.29 
275– 
300°C 
33.46 34.81 28.46 19.87 
2.90 3.38 1.92 2.44 
30.06 34.20 29.24 21.56 
8.76 15.44 9.61 3.32 
25.84 36.20 38.28 27.79 
4.41 5.05 2.14 1.45 
22.55 28.39 30.08 24.82 
4.27 6.30 5.33 2.25 
17.91 19.05 19.71 17.30 
3.92 4.90 5.31 3.93 
21.69 22.11 20.16 17.15 
8.17 8.25 4.78 2.71 
N/A N/A N/A N/A 
N/A N/A N/A N/A 
15.07 15.38 15.52 15.02 
1.63 2.28 2.89 2.70 
125– 
150°C 
10.76 
0.32 
10.86 
1.34 
10.11 
0.87 
10.12 
0.36 
9.87 
0.69 
9.87 
0.68 
N/A 
N/A 
10.65 
2.17 
175– 
200°C 
175– 
200°C 
16.73 
3.19 
19.38 
6.89 
12.96 
0.70 
12.09 
1.01 
13.20 
1.85 
12.55 
1.80 
N/A 
N/A 
11.90 
2.35 
125– 
150°C 
17.14 23.61 35.28 
2.61 5.79 5.21 
16.63 22.27 30.33 
4.29 7.57 11.03 
12.75 14.51 17.93 
0.84 1.32 3.02 
11.88 13.72 17.20 
2.78 3.42 5.10 
12.26 13.66 16.75 
1.65 1.38 3.16 
11.47 15.80 22.06 
3.63 5.54 14.24 
N/A N/A N/A 
N/A N/A N/A 
11.98 13.03 14.08 
2.56 2.61 2.06 
225– 
250°C 
32.83 
3.35 
32.35 
8.56 
20.60 
2.04 
19.45 
3.47 
17.80 
3.29 
25.19 
27.61 
N/A 
N/A 
13.87 
1.11 
75–100°C 
11.91 13.93 
0.41 1.30 
10.82 13.40 
0.91 2.41 
10.65 11.48 
0.47 0.63 
9.93 10.73 
1.07 1.86 
9.92 11.56 
0.54 2.77 
8.88 10.10 
2.50 2.87 
N/A N/A 
N/A N/A 
9.95 10.85 
1.09 1.87 
9.84 
6.95 9.68 
1.80 0.81 
9.14 9.83 
0.52 0.40 
8.38 9.34 
1.25 
Revision 1 
275–300°C 
53.94 
3.49 
48.83 
18.41 
50.39 
7.55 
41.56 
7.92 
26.93 
8.26 
33.40 
17.99 
N/A 
N/A 
17.78 
4.38 
35–50°C 
250– 
275°C 
43.98 
8.99 
40.16 
17.22 
31.23 
3.75 
27.24 
6.23 
20.65 
4.80 
26.15 
13.65 
N/A 
N/A 
15.28 
1.94 
50–75°C 
10.84 
0.01 0.26 
0.45 
8.50 9.16 
0.57 0.64 
7.03 8.12 
2.39 2.33 
N/A N/A 
N/A N/A 
5.24 9.20 
0.23 0.63 Table 3-15. Rock Mass Coefficients of Thermal Expansion for Repository Units 
Specimen Source 
Single Heater Testa 
Drift Scale Testb 
12” Specimensc 
Rock Unit 
Tptpmn 
Tptpll 
b BSC 2002a, Table 6.3.3.6-5. 
c DTNs: SN0208L01B8102.001; SN0211L01B8102.002. 
Mean 
4.14 
2.36 
5.88 
2.03 
2.41 
4.19 
4.40 
7.44 
9.81 
12.55 
6.50 
6.60 
10.04 
15.34 
Source: a BSC 2002a, Table 6.2.3.5-1. 
Temperature 
70°C 
117°C 
160°C 
50°C 
75°C 
100°C 
125°C 
150°C 
175°C 
200°C 
80°C 
120°C 
160°C 
200°C 
No. 4: Mechanical Degradation 3-27 
Revision 1 
Standard 
Deviation 
N/A 
N/A 
N/A 
1.29 
0.93 
2.07 
1.96 
0.45 
0.80 
N/A 
1.49 
1.73 
1.69 
5.58 
June 2004 INTENTIONALLY LEFT BLANK 
3-28 No. 4: Mechanical Degradation 
Revision 1 
June 2004 Revision 1 
4. DEVELOPMENT OF ROCK MASS MATERIAL MODELING APPROACHES 
FOR NONLITHOPHYSAL AND LITHOPHYSAL ROCKS 
Section 3 reviewed the basic laboratory and in situ thermal and mechanical testing data that have 
been generated by the Yucca Mountain Project for the repository host horizon units. These data, 
typically on small-scale samples, need to be generated to provide input properties and property 
ranges for design and performance analyses. This section describes the integration of geologic 
mapping and geotechnical characterization studies with the laboratory and field testing to 
produce field-scale rock mass properties. Constitutive modeling approaches are also described. 
June 2004 
4.1 MECHANICAL DEGRADATION MODELING APPROACH FOR 
NONLITHOPHYSAL ROCK 
The nonlithophysal rocks are strong, hard materials. The degradation behavior of tunnels in 
these rock units is controlled by the occurrence of “keyblocks,” or kinematically removable 
wedges, which can dislodge and fall under the action of external loading. Dislodging of these 
keyblocks does not necessarily lead to extensive failure and may simply result in isolated rock 
falls. Thus, isolated blocks may become dislodged, yet the excavation remains stable. 
Keyblocks in the 5-m-diameter ECRB Cross-Drift are first evident in the crown at about 
Station 10+50 in the Tptpmn unit. Most of the keyblocks in this region are of minor size and 
have typically been forcibly removed by scaling operations immediately after excavation, but 
prior to ground support installation. Keyblocks are possible in this area because of the increased 
presence of planes of weakness (i.e., a vapor-phase parting) in the near horizontal orientation that 
intersects with two opposing near vertical joint planes. The largest resultant void is 
approximately 0.5 m3 at approximately Station 11+55 as shown in Figure 4-1. No unstable 
keyblocks (i.e., those that have fallen out at a later time due to gravity) have been observed in the 
field (BSC 2004a). 
The approach taken here to represent the degradation response of nonlithophysal rocks is to 
explicitly model the fractured, blocky response of the material to allow a direct calculation of 
rockfall and opening shape change as a function of loading. This approach requires that the 
stochastic nature of the fracturing be captured in the modeling. Two items are required to 
successfully implement this approach: a tool for producing representative fractured volumes of 
rock, and numerical models that can simulate the physical, three-dimensional collapse modes of 
a blocky rock mass subjected to seismic and other loadings. A sufficient number of mechanical 
simulations12 using representative fracture realizations are necessary to adequately describe the 
full range of stochastic response of the rock mass. Uncertainty in the estimate of rockfall arises 
from three sources: 
1. The uncertainty in the knowledge of the fracture geometry 
2. The uncertainty in mechanical properties of the fractures 
3. Uncertainty in the applied loadings. 
12 A sufficient number of fracture realizations is described in Section 5.3. 
4-1 No. 4: Mechanical Degradation Revision 1 
Figure 4-1. Evidence of Key-Block Occurrence in the Enhanced Characterization of the Repository 
Block Cross-Drift, Station 11+55 
June 2004 4-2 No. 4: Mechanical Degradation Revision 1 
Uncertainty in the fracture geometry is inherently accounted for through stochastic 
representation of fracture geometries using the FracMan fracture simulation model and by 
conducting a sufficiently large number of analyses with randomly selected fracture patterns. 
Uncertainty in fracture mechanical properties is investigated by varying the properties over their 
expected ranges. Uncertainty in the applied loading is accounted for by the use of measured in 
situ stresses, inclusion of thermal loading histories, and use of 15 sets of probabilistically defined 
ground motions that account for the range of uncertainties in the ground motion (BSC 2004c). 
This section describes the development of the stochastic fracture geometry model that provides 
the input for the three-dimensional discontinuum stability model. The model and modeling 
approach is described in Section 5.3. 
4.1.1 Development of Fracture Geometries for Nonlithophysal Rock 
Analysis of seismic response and rockfall in emplacement drifts in fractured, nonlithophysal rock 
is, in general, a three-dimensional problem requiring the rock mass to be represented as an 
explicitly fractured assemblage. To achieve this objective, the 3DEC, three-dimensional 
discontinuum program (BSC 2002c) is used to model the mechanical response of a rock block 
assemblage subjected to in situ, thermal, and seismic loads. The 3DEC program allows direct 
input of the fracture geometry in creation of a “synthetic” rock mass composed of an assemblage 
of blocks within which emplacement drifts may be simulated. The details of the 3DEC model 
are described in Section 5.3. 
The blocks of nonlithophysal rock are significantly stronger than the in situ and thermally 
induced stresses, and thus the problem of modeling this material is essentially one of elastic 
blocks separated by fracture surfaces. Therefore, in modeling of the stability of the tunnels and 
the rockfall that may occur from the applied load, the fracture geometry and surface properties 
become of primary importance. A methodology for defining statistically representative fractures 
is therefore required as a direct input to the 3DEC program. In particular, the input fracture 
geometry must provide an adequate representation of the orientation, length, spacing and 
continuity of fractures and their variability, as this controls the size and number of removable 
blocks that surround the tunnel. Additionally, the surface characteristics, including roughness, 
planarity, and alteration/infilling define the shearing and tensile resistance of the fractures under 
load. 
The development of a stochastically defined fracture system, representative of the actual rock 
mass is accomplished using the FracMan program (USGS 1999). The existing fracture mapping 
database, described in Section 2.3.1, provides the basic input to the FracMan program, which 
develops sets of planar, circular fractures that conform to the statistical variability of the 
geometric characteristics of the input data. Statistical models are fitted to the various geometric 
characteristics of each fracture set in the database, followed by generation of representative 
fracture sets. These representative fractures are then back-checked against the statistical 
variability and geologic realism of the original sets (i.e., field data) to achieve an acceptable 
facsimile. Details of this process are described in Drift Degradation Analysis (BSC 2004a). 
A three-dimensional representative rock mass cube, 100 m on a side, is generated using FracMan 
for each Topopah Spring subunit. Each fracture is described by its centroid coordinate, dip, dip 
4-3 June 2004 No. 4: Mechanical Degradation direction, and radius. These geometric properties are used as direct inputs to the 3DEC program 
for development of a block geometry within which emplacement drifts can be randomly 
excavated. 
4.1.2 Example—Fracture Geometry Generation for the Middle Nonlithophysal Unit 
Because the large-scale fracture control of block geometry is most prevalent in the 
nonlithophysal rock, and in the Tptpmn in particular, an example of the FracMan methodology 
for construction of an equivalent fracture model for this unit is given. The analysis for the 
Tptpmn uses a classical approach to identify sets based on orientation only (Mongano et al. 
1999; CRWMS M&O 2000). 
The detailed line survey data are used to condition FracMan to develop representative fracture 
trace lengths and spacings. Table 4-1 displays the mean orientation of the sets, a comparison of 
average fracture radius converted to diameter and average trace length, and intensity (average 
spacing) from FracMan and average spacing from the detailed line surveys. 
Table 4-1. Comparison of Data from Detailed Line Survey, Full-Periphery Geologic Maps, and FracMan 
Output for the Tptpmn 
Inter-Fracture 
Distance (m) 
Set Number 
Set 1 
Observed 
Orientation 
(Strike/Dip) 
120/84 
(210/06) 
Set 2 215/88 
(305/02) 
Set 3 302/38 
(212/52) 
Observed FracMan 
0.48 
1.08 
3.40 
2.46 
0.79 
1.29 
3.16 
1.48 
FracMan 
Orientation 
(Strike/Dip) 
125/84 
214/86 
299/43 
327/08 329/14 
(239/76) 
Vapor-Phase 
Parting 
Source: DTNs for tunnel mapping include GS960908314224.020, GS000608314224.006, GS960908314224.015, 
GS960908314224.016, GS971108314224.025, GS960708314224.008, GS000608314224.004, and 
GS960708314224.010. 
NOTE: Trace length medians are taken from a compilation of tunnel mapping and synthetic tunnel samples from 
FracMan. 
A direct comparison between actual full periphery geologic maps from the ESF to synthetic full 
periphery geologic maps from FracMan is given in Figure 4-2. This comparison ensures that the 
synthetic fracture geometries are not only quantitatively validated, but similar from a geological 
perspective as well. Details of the quantitative comparison of FracMan results to the full 
periphery and detailed line surveys is given in Drift Degradation Analysis (BSC 2004a). 
4-4 June 2004 No. 4: Mechanical Degradation 
Trace Length 
Median from Full 
Periphery Geologic 
3.3 
3.1 
3.6 
3.4 
Maps (m) 
Trace Length 
Median from 
FracMan (m) 
2.8 
2.9 
3.7 
3.5 
Revision 1 Revision 1 
Source: (a) DTNs: GS990408314224.004; GS000608314224.006; GS960908314224.015; GS960908314224.016; 
(b) BSC 2004a. 
NOTE: The purpose of this figure is to illustrate the geologic structure contained on a full periphery geologic map. 
The annotated information on this figure is not intended to be legible. 
Figure 4-2. Comparison of Full Periphery Geologic Maps from the Tptpmn in the Exploratory Studies 
Facility (a) with Simulated Full Periphery Geologic Maps from the FracMan Cube (b) 
June 2004 4-5 No. 4: Mechanical Degradation Revision 1 
4.2 MECHANICAL DEGRADATION MODELING APPROACH FOR LITHOPHYSAL 
ROCK 
June 2004 4-6 
4.2.1 Material Model Requirements 
The lower lithophysal unit (Tptpll)13 is characterized by intense, small-scale fracturing. Joint 
sets are not as clearly defined as in the middle nonlithophysal (Tptpmn) unit. Average joint 
spacing is less than 1 m, and at certain locations this spacing is much smaller, on the order of 
0.1 m. In addition to fracturing on different scales, the lithophysal rock mass is characterized by 
the presence of almost uniformly distributed holes (lithophysae) of varying size (from less than 
1 cm to greater than 1 m in diameter). The lithophysae account for up to 30% of the rock mass 
volume. The size of the internal lithophysae structure and fracture spacing is much smaller than 
the drift size (i.e., 5.5-m diameter). Since the lithophysae size is small in comparison to the drift