Boring Yucca Mountain

Yucca Mountain consists of layers of volcanic tuff, with a total thickness of at least 1.8km. Most of the excavation will be in the uppermost and middle Topopah Spring formations, located approximately 300m below the surface. This is the potential subsurface repository horizon, and is more than 100m above the groundwater table.

The 7.8km-long, 7.6m-diameter Exploratory Studies Facility (ESF) tunnel was excavated by Kiewit/Parsons Brinckerhoff using a tunnel boring machine (TBM) supplied by Construction & Tunneling Services, and designed by TRW Environmental Safety Systems.

A large percentage of the ESF tunnel design has been done according to a quality assurance programme ('Q'-standard), similar to that used for nuclear power plants. Ground support is 'Q' classified, based on the assumption that a rock fall can result in release of radiation. This has an impact on ground support design, type of ground support chosen, procurement of ground support products (including lifetime documentation and traceability of materials used in manufacturing), installation of ground support, and verification of the function of the products used.

Tunnelling progress

The TBM was launched from a 60m-long drill/blast starter tunnel. The first part of the ESF tunnel, the North Ramp, was driven at a 2% downgrade against rock beds dipping 2° to 15° to the east. The first 200m of tunnelling was problematic, and steel sets were installed on 1.22m centres.

The Bow Ridge Fault, encountered approximately 200m into the mountain, was filled with a soil-like, weak tuffaceous material, having an unconfined compressive strength as low as 1.4MPa. Even though the fault had slipped approximately 100m, it was only a few metres wide. After crossing the fault, the TBM entered softer material in which steering was difficult. Ground was lost above the TBM, necessitating backfilling and grouting of the void created.

For about 1000m, tunnelling was through the Imbricate Fault Zone, which proved very demanding. Minor faulting events had caused through-going joints, oriented in the same direction, closely spaced and nearly parallel. This, in combination with low stressed rock, led to block fallouts, and steel sets combined with steel lagging had to be used extensively.

Close to the Ghost Dance Fault, two testing alcoves were excavated to gain access to the fault deep inside Yucca Mountain.

After approximately 2700m of tunnelling from the entrance at the North Portal, the TBM reached the Topopah Spring potential repository host rock at approximately 300m depth. In this formation, 3000m of the ESF main tunnel was constructed using Swellex bolts, wire mesh and rolled steel channels for support, and high advance rates were achieved.

Ground support QA

Since the ESF is an underground laboratory where rock characteristics are studied, the US Department of Energy insists that ground support must not interfere with the geotechnical and geological testing. Also, the final ground support system must be installed as the tunnel progresses.

The tunnel must be reinforced in such a way that stored nuclear waste can be retrieved 100 years after it is put in place. Hence, the ground support must ensure long-term stability and maintainability.

Cement grouted rebar bolts cannot be used in areas where scientific investigations will take place, because the grout may penetrate rock fractures. Also, due to the curing time of the grout, this type of bolt cannot be tested immediately after installation.

There is also a ban on the use of epoxy resin based rockbolting systems, since the amount of organic material in the tunnel has to be minimised in order not to pose any threat to nuclear waste packages.

The use of shotcrete is limited, since it can interfere with geological mapping and geochemical tests.

For many such reasons, the only type of rockbolt approved for permanent rock reinforcement in the ESF is Swellex, manufactured by Atlas Copco.

Procurement of ground support materials requires lifetime documentation and traceability, from materials used in the manufacturing, to fully inspected installations. Records are kept in a thorough and precise way, and internal and external audits are carried out to certify that everything is done according to specifications and procedures.

Ground support system

As main support, 3m-long Super Swellex bolts complete with the domed face plate on a 1.5-1m pattern, depending on ground conditions, were used, together with a 250mm rolled steel channel and welded wire fabric (WWF). The steel channel and WWF prevent rocks falling from the roof of the tunnel. In addition to this, steel reinforced concrete inverts were installed as the TBM advanced, providing the surface and track to support the TBM trailing gear.

More than 20,000 Super Swellex rockbolts support about 70% of the tunnel, with steel sets used for the remaining 30%. The bolt is made from a welded circular steel tube that is folded on itself into a 'W' shape to decrease the diameter. Bushings are then pressed onto the collapsed steel tube and the ends sealed by welding, to create a confined space inside the bolt. A hole is then drilled in the lower bushing. When the bolt has been positioned in the borehole, water is injected through the hole drilled in the bushing causing the tube to unfold.

At 30MPa the bolt is fully expanded in the hole and the pump automatically stops. As the pressure inside the bolt reaches 30MPa, steel tubing adapts to the shape of the borehole and may consolidate the material surrounding the borehole and adapts its shape to fit the irregularities of the hole. The resulting frictional and mechanical interlocking reinforces and increases the stability of the rock surrounding the drilled hole.

Rockbolt drilling and installation was carried out at two stations on the TBM. At the first station, four Swellex bolts were installed, together with the WWF and the channel. At the second station three holes were drilled, and the remaining three bolts installed, including the bolt located at the highest point in the tunnel. The nominal spacing was 1500mm, with allowable maximum of 1687mm. Spacing and pressurisation of the bolts was monitored to verify that they were properly installed, with a pressure between 290 and 310 bar.

During tunnelling, five out of every 100 rockbolts installed were tested to check if the proof load was reached.

If the TBM entered a new geological formation, five destructive pull-tests were carried out to verify that the Swellex bolts met the anchoring requirement.

Complementary to this, 20 non-destructive pull-tests were made. Not a single bolt failed in these tests.

Further tunnelling

Further tunnelling since completion of the ESF tunnel has included the 2.681km-long exploratory East-West Cross Drift Tunnel across the potential repository. This employed a 5 m-diameter Robbins hardrock TBM, which started at an intersection with the North Ramp of the ESF and, after an initial curve, followed a tangential alignment, and crossed over the proposed repository block west of the main loop of the ESF tunnel, to terminate in the Topopah Springs geological formation.

Super Swellex 1.8m-long rockbolts were used for ground support on a 1.2x1.2m grid over the full crown, with welded wire mesh and 1.2 m-long steel channels. Twenty steel sets were required in only one area of the tunnel, where the bolts could not provide long-term support. The TBM average advance was 25m/day over 106 mining days, with a best shift of 34.6m, best day of 73.2m, and best week of 266.7m. The TBM was mining for only 25% of the time, due to the concurrent scientific and environmental experiments being carried out.

Although the design of the repository is not yet finalised, a system of tunnels totalling more than 200km is being discussed.