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The TBM and its role in creating an underground laboratory

Scientists on the Yucca Mountain Project used a tunnel boring machine, to create an underground laboratory within Yucca Mountain, called the Exploratory Studies Facility. The tests completed within this facility have given scientists and engineers much-needed information about the mountain s interior.

Impressive in size and capability, the tunnel boring machine and all its trailing equipment weighs 860 tons and measures 140 meters (460 feet) in length. It has 13 trailing decks, or work platforms, that are towed behind the machine in the form of a train.

The 7.6-meter (25-foot)-diameter machine began its journey in September 1994 from a starter tunnel extending 60 meters (196 feet) into Yucca Mountain.

To work properly, the machine was positioned inside a tunnel so it could grip the sides of the tunnel and push the cutters into the rock to advance the tunnel. Its hardy mechanical cutters chip and grind their way through rock and soil. The cutter head, with 48 cutter discs 43 centimeters (17 inches) in diameter, chips and flakes the rock. The chips (known as muck) move out through the back of the machine on a series of conveyor belts to a designated area outside the tunnel.

Why use a Tunnel Boring Machine?

Tunnel boring was the most efficient and ultimately the most environmentally sound and physically safe method for gaining access to the underground environment found at Yucca Mountain. The machine consumed less money and less time than drill-and-blast excavation, and performed with minimal environmental disruption.

Tunnel boring machines are built to cope with varying underground conditions. The same machine generally can adapt to both hard rock and loose ground. Sometimes, the machine would encounter both conditions in a single day. Often, the machine's operators did not know what conditions they would face only a few meters ahead. The only thing they could count on was that conditions would vary.

Assembled on a rail line that would carry it into Yucca Mountain, the TBM had been described as an automated tunneling system.

Tunnel boring machines also act as locomotives for cars carrying special equipment or facilities. One of these cars provided a platform for geologists taking rock samples. The same car carried cameras that made detailed geological maps of the tunnels. These maps provided a permanent record of the machine's underground passage and yielded information about the geology of the area through which it passed. This data is used by engineers to design the proposed repository.

By the spring of 1997, the Tunnel Boring Machine had advanced the full course of the eight-kilometer (five-mile) tunnel and emerged at the facility's south portal.

As long as a football field, this tunnel boring machine has given scientists direct access to an underground laboratory.

Maximizing the opportunity afforded by the tunnel and the entire facility, scientists conducted studies to:

• Determine the rate of water seepage into the proposed repository
  
  
• Test the groundwater flow above and below the water table, including how rain water and other fluids might move from the surface through fractures and faults
  
  
• Determine how the distinct geologic units govern the flow of gases and fluids within the mountain
  
  
• Establish, with thermal tests, the effects of waste-generated heat upon the rock, water, and installed ground support systems in a repository.

The TBM cutterhead can be seen after it reached daylight in April 1997. The TBM made a 2.5-year journey excavating through Yucca Mountain.

TBM facts and acronyms

Conveyor systems

• From the cutterhead to the rear of the TBM.
• From the rear of the TBM to the outside of the tunnel.
• From the outside of the tunnel to the muck pile.

Cutterhead
The rotating head at the front of the TBM that cuts the rock.

Cutterhead bearing
Two-row tapered roller. High capacity for hard rock.

Decks (also trailing decks)
Work platforms that are towed behind the TBM in the form of a train. There are 13 decks on this TBM.

Design
Gripper Shield TBM, model 760, custom-built for Yucca Mountain.

Diameter
7.62 meters (25 feet).

ESF
Exploratory Studies Facility. An underground laboratory for scientists to help determine if Yucca Mountain is suitable for the disposal of high-level nuclear waste. The ESF is approximately a 10-kilometer (6.2-mile) network of tunnels.

Forward shield
The metal parts of the TBM next to the rotary head that provide temporary support and prevent rock material from falling and interfering with the TBM cutting action.

Gantry
An elevated platform that moves with the TBM to all the roof of the tunnel to be mapped.

Gripper shield
The mechanical protection that prevents individuals from access to the grippers while they are engaged to the tunnel rib.

Gripper shoes
Large curved pressure plates that push against the rock wall, holding the TBM steady, so that the cutterhead can push forward into the rock.

Launch chamber
The initial 60 meters (196 feet) of the ESF where the TBM began tunneling operations. This initial portion of the ESF was completed using conventional drill-and-blast techniques.

Leak mitigation system
The devices installed on the TBM that minimize the spillage of fluids used in the operation of the TBM, thereby reducing environmental impacts.

Length
140 meters (460 feet).

Manufacturer
Construction & Tunneling Services, Inc. (CTS), Kent Washington. It took approximately 10 months to manufacture the TBM.

Maximum advance rate
The maximum amount of distance the TBM will travel in a given amount of time is 5.3 meters (18 feet) per hour.

Minimum turn radius
The measure of how tight a turn the TBM can make. The TBM’s minimum turn radius is 151 meters (500 feet).

The 5-meter (16.5-foot) Tunnel Boring Machine (TBM) is prepared for the cross-drift excavation. The 225-ton machine was used to excavate portions of the Texas Super Collider.

Muck
The earth and rock that are excavated during TBM operation.

North Portal
North opening to the ESF. The ESF also has a South Portal.

Operating voltage
12.47 KV.

Operations
Team of companies contracted to the Department of Energy, including Kiewit/Parsons Brinckerhoff, Fluor Daniel, Morrison-Knudsen, and Duke Engineering Services. Kiewit/Parsons Brinckerhoff was the company contracted to operate the TBM.

Pad
The areas near the portal entrances, that accommodated all the facilities used to construct the ESF. Both North Portal and South Portal pads were constructed.

Power
12 electric motors generating 3,800 horsepower.

Tail shield
The portion of the tunnel boring machine located behind the cutterhead that protects the TBM crew from falling rocks when installing ground support. This overhead section is constructed of two-inch-thick steel and is 12 feet long.

Weight
720 tons (1,440,000) lbs.).

Note: From March through October 1998, miners used a smaller, 5-meter- (16.5-foot) diameter tunnel boring machine to excavate a tunnel called the Enhanced Characterization of the Repository Block (ECRB). This 2.7-kilometer (1.7-mile) ECRB tunnel, or cross drift extends from the north portion of the Exploratory Studies Facility and crosses above the actual area being studied for a repository.

Scientists will collect data on geology and the behavior of water within that area to verify models and predictions about performance of the natural features of Yucca Mountain.