Bi-County Water Tunnel, Maryland, United States of America
Bi-County Water Tunnel is a $160m project undertaken by the Washington Suburban Sanitary Commission (WSSC). Located in the vicinity of Washington, D.C., the tunnel will carry clean drinking water from the Potomac Water Filtration Plant to Montgomery and Prince George's County.
It will provide additional capacity for the two counties during droughts, emergencies and other instances when there is a high demand for drinking water.
The 5.3mi (8.53km) underground water tunnel is expected to be operational by the middle of 2014. It will be 10ft in diameter and approximately 200ft underground, with three shafts. Its construction was commenced in 2009. The tunnelling work is being carried out by a Robbins tunnel boring machine (TBM) named 'Miss Colleen'.
Water tunnel project background
The idea to construct the tunnel to serve the water needs of Montgomery and Prince George's County was conceptualised in the 1960s. The planning and community outreach works were undertaken in 2004 and 2005 respectively.
The contract for the construction of the tunnel was awarded in 2009 to the joint venture of Renda, Southland and SAK, through a bidding process. The JV won the contract for $112.5m.
Construction of the Bi-County Water Tunnel
The project work will comprise of excavating three shafts: one main production work shaft and two recovery shafts, which will be used for connecting the pipeline to the existing water mains at either end.
The tunnel will stretch from the main access shaft near Connecticut Avenue to the Stoneybrook / Beach Drive shaft, and from the Connecticut Avenue shaft to the shaft near Tuckerman Lane.
The final lining will use welded pipes with surface valve chambers that will be connected to the existing surface main pipelines.
The tunnel is being built along the alignment of 1-270 and 1-495 Capital Beltway that surrounds Washington, D.C. The tunnel is located adjacent to residential areas. The construction techniques being used, however, minimise the effect on these areas.
The large diameter of the structure poses several challenges to the tunnelling work with regards to the removal of muck and providing ventilation. Special railroad-style muck cars have been designed to solve these problems.
Construction using Robbins' TBM
The tunnelling work is being done in two legs. The first leg is 4,124ft (1.25km) long and the second leg is 24,000ft or 4.5mi (7.25km) long.
The first leg stretches east to the Stoneybrook shaft, where Beach and Stoneybrook Drives meet in Kensington. It was completed in November 2011.
The second leg will stretch from the Connecticut Shaft west to the Tuckerman Shaft, where Tuckerman Lane passes under I-270 in Rockville. As of October 2012 the second leg was about 78% complete.
The construction was delayed for some time when the TBM broke down due to the unfavourable conditions underground, such as the presence of fractured rocks. The tunnel is scheduled to be in service by the middle of 2014.
Key players involved with Maryland's project
The Washington Suburban Sanitary Commission is the owner of the project, while Jacobs Associates (JA) is responsible for managing the construction process in a joint venture with Louis Berger Water Services.
Contractors for the project are the Renda, Southland and SAK joint venture, with Brierley Associates providing the technical services.
The primary designs are being provided by Black and Veatch, while the geotechnical evaluations and interpretations are being provided by Haley and Aldrich.
A total of 29,000ft of 84" diameter steel pipes used for the tunnel are being supplied by the Northwest Pipe Company. The TBM was supplied by SAK.
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