At 2.1m finished diameter, tunneling at Maryland’s Bi-County Water Tunnel is a tight operation. The water transfer pipeline, for owner Washington Suburban Sanitary Commission (WSSC), travels across a densely urban area and links up two existing water supply mains in Montgomery County.
“The length of the second drive, approximately 7km, without any intermediate access shafts, is what makes this project difficult. This, in combination with a small tunnel cross section, makes it logistically challenging to develop muck handling and ventilation plans,” said Steven Pinault, Project Manager for WSSC. The entire tunnel stretches 8.5km, with the shorter 1.5km long section having already been excavated by spring 2011.
Work is being carried out by the Oscar Renda, Southland, and SAK joint venture, with Black & Veatch as the primary designer. A joint venture of Jacobs Associates and EA Engineering Science and Technology is providing construction management services. The 3m diameter, contractor-owned Robbins Main Beam machine being used is one of the world’s longest running TBMs in operation. The machine has been used on at least 10 different projects between 1973 and 2011, totaling nearly 50km of hard rock tunnels.
Supplying water to Washington
Though located in Maryland, the Bi-County Water Tunnel will increase supplies of clean drinking water to eastern areas of two counties in both Maryland and Washington D.C. The increased water supply will alleviate current capacity limitations from an existing 1.37m diameter main. The tunnel will also serve as an additional high-pressure water source during peak-use periods, droughts and emergencies.
Long-term benefits
Plans for the Bi-County Water Tunnel were drawn from a long-term water supply project in the area, dating back to the 1970s. Original water tunnels under the same name were built in 1980 to 1981 using Jarva Kelly-Type TBMs boring 3m diameter tunnels.
“The tunnels were built in solid rock, and lined with shotcrete. Due to leakage rates and rock competency issues discovered during testing, the tunnel was not placed into service until it was lined with a 2.4m steel pipe in the early 1990s,” says John Mitchell, project manager for WSSC.
These original tunnels, bored through quartz, gneiss, schist, and granite, have provided good capacity over several decades. However, the 1.37m diameter water main has continued to act as a bottleneck, particularly in recent years. Without additional capacity, water pressures are predicted to fall below two bar, affecting firefighting and emergency capabilities, as well as reducing performance when multiple appliances are used on one water line by consumers.
To prevent the problems, the next phase of the plan called for a final tunnel to tie two systems together, providing reliable water supply from the Potomac River to WSSC customers in Montgomery and Prince George’s Counties. WSSC took into account average per capita consumption, along with a user increase of about 5,000 new accounts per year, to determine that the new tunnel would need to be in operation in 2013.
The alignment of the Bi-County project was decided upon following studies in 2004. The tunnel route follows the I-270 and I-495 ring roads around Washington D.C. Much of the area is urban and residential, resulting in limited access and a deep alignment between 27 and 84m below the surface. The depth also ensures relatively competent bedrock with a lessened risk of mixed face tunneling.
Because of the limited access, a 10.6m diameter, 50m deep shaft at Connecticut Avenue was identified as the only place from which to launch and remove a TBM. Smaller shafts would be constructed at either end of the line, at Tuckerman Lane and Stoneybrook Drive. “A tunnel boring machine was really the only option for us, in order to lessen environmental impacts, particularly in residential areas near the Tuckerman site,” says Mitchell.
Geologic testing
Extensive geological testing was performed along the tunnel alignment prior to excavation. WSSC took 21 core samples, from both vertical and inclined positions. “We performed acoustic televiewing inside some of the bore holes. If we saw an interesting feature, we would lower equipment into the hole itself to look at potential rock foliation or shear zones,” says Mitchell. An acoustic televiewer is a type of technology that combines an ultrasonic transducer and a downhole inclinometer to provide a high resolution, sonic image of the borehole wall. While several areas of concern were identified in the mainly granitic rock, designer Black & Veatch determined that rock bolts would be sufficient, and recommended this to the contractor as the primary method of support in the tunnel.
Boring with restricted surface access
The 3m diameter TBM, dubbed ‘Miss Colleen’, was launched in July 2010 from the main Connecticut Avenue shaft located in the middle of the tunnel alignment. Starter and tail tunnels, totaling some 120m in length, were required for the machine launch. Work was done prior to the launch to boost the machine power from its previous project—an increase from 600 to 900kW in order to accommodate hard granite averaging 140MPa UCS. Contractor Renda/Southland also took measures to simplify the operator’s cab and increase the bearing size to 2.5m in anticipation of the hard rock.
Tim Winn, principal in charge for the Renda/Southland JV, cites several reasons for the machine’s long-running and consistent success: “The most important factor is probably the main structure of the TBM, which was built very rigid. It was definitely built for severe rock conditions. We also maintain the machine on a regular basis, checking the lube system, changing belts and hoses, and monitoring cutter wear.”
The machine excavated its first 1.2km of tunnel, exiting into the Stoneybrook shaft in November 2010. Ground support consisted of rock bolts, steel straps, and wire mesh being added in less competent ground. “We have had a variable amount of cutter changes, depending on the material. At times, the granite is inlaid with quartz and iron pyrite, which is very abrasive, dense material and can add significantly to our push times,” says Winn. Despite the challenges, the contractor averaged 12m per 10-hour shift.
Due to the small size of the Stonybrook shaft, it was not possible to resume tunneling from the location. Once the TBM broke through into the site, Renda/Southland began operations to remove TBM components from the Stonybrook shaft, and back the rest of the machine to the Connecticut Avenue launching area.
“We removed the cutterhead and lifted it out of the Stonybrook shaft. The machine was then backed out, using a specially built frame to make the trailing gear rigid. Once the TBM was at the main shaft, we essentially turned all the components 180 degrees and then pieced them back together,” says Winn.
Turning the pieces around, particularly larger bolted-together components, required some maneuvering with a crane in the 10.6m wide shaft. Rebuilding of worn sections of the cutterhead, including the addition of hardened steel, was also done prior to launch. The entire process took approximately two months.
Launch of the TBM was accomplished the first week of February 2011 from the Connecticut Avenue shaft. By March 2011, the TBM had advanced more than 300m into its second 7.2km long drive, in the opposite direction towards Tuckerman Lane. Advance rates were averaging about 12m per 10-hour shift in ground conditions including granitic rock of 140MPa UCS, similar to that found in the first tunnel section.
Limited Space, Urban Area
The 7.2km length of tunnel with no intermediate shaft is the project’s current challenge. “The diameter makes necessities such as adequate air movement and mucking very complex,” says Winn. To remove an appropriate volume of air, 900mm diameter duct is run along the top of the tunnel. Scavenger fans also push air back from the boring face and towards the ducting that trails the machine. Air is removed by two 100 hp fans, then put through a silencer—a noise reduction device for work inside residential areas.
Muck is being removed from the tunnel using single track muck cars with California switches. A new switch is placed every 3km in the tunnel to allow multiple muck trains in a relatively small space. Full muck cars are then hauled out of the shaft for dumping on the surface. Proximity to the highways has resulted in restricted trucking and hauling hours, preventing muck haulage from 6am to 9am and from 3pm to 6pm daily. The excavated material is being stored at a firm nearby, to be reprocessed and used as fill on private property.
As of March 2011, work at the Tuckerman Lane Shaft was ongoing, with residential areas within 140m of the site requiring pre-blast surveys and surface monitors for noise and vibration. The 10.6m diameter, 42m deep shaft is being excavated using drill and blast charges, which have to date been well within specified limits.
Crews are also readying surface piping at Tuckerman Lane to tie into an existing 2.4m diameter line at the shaft site. At the Stonybrook site, crews are tying in the other end of the tunnel to the pre-existing 2.4m diameter Bi-County Water Tunnels, thus bridging the gap between the two systems.
All excavation is expected to be complete in 2012 and the pipeline will become operational in 2013. While no other tunnels are being planned by WSSC at the moment, Mitchell speculates that more are likely: “As this area continues to develop, there will be continuing concern about community and environmental impacts. Much of the infrastructure was built in the 1950s and1960s, and these aging lines will eventually need to be replaced with new utility lines at larger diameters. All these trends are making TBM tunneling a very attractive option.”
At 7km in length, a section of the Bi-County Water Tunnel being bored by a 3m diameter Robbins TBM is logistically challenging for both muck removal and ventilation. The TBM was launched on Maryland’s Bi-County Water Tunnel in July 2010, from a main shaft at Connecticut Avenue. WSSC Bi-County Water Supply Main The Renda/Southland JV broke through into an intermediate shaft in November 2010, and then removed some TBM components for launch in the opposite direction.
