Bi-County Water Tunnel, Maryland, United States of America
Bi-County Water Tunnel is a $146m project undertaken by the Washington Suburban Sanitary Commission (WSSC). Located in the vicinity of Washington, DC, US, the tunnel carries clean drinking water from the Potomac water filtration plant to Montgomery and Prince George's County.
It provides additional capacity for the two counties during droughts, emergencies and other instances when there is a high-demand for drinking water.
The 5.3-mile (8.53km) underground water tunnel, which carries 100 million gallons of water a day to 1.8 million households in Montgomery and Prince George's county, became operational in March 2015. It is 10ft in diameter and approximately 200ft underground, with three shafts. Its construction started in 2009. The tunnelling work was carried out by a Robbins tunnel boring machine (TBM) named Miss Colleen.
Water tunnel project background
The idea to build 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 $112.5m contract for the construction of the tunnel was awarded in 2009 to the joint venture of Renda, Southland and SAK, through a bidding process.
Construction of the Bi-County Water Tunnel
The project work comprised excavating one main production work shaft and two recovery shafts, which are used to connect the pipeline to the existing water mains at either end.
The tunnel stretches 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 uses welded pipes with surface valve chambers that are connected to the existing surface main pipelines.
The tunnel is built adjacent to residential areas along the alignment of 1-270 and 1-495 Capital Beltway that surrounds Washington, DC. The construction techniques used, however, minimised the effect on these areas.
The large diameter of the structure posed a number of challenges to the tunnelling work in removing the muck and providing ventilation. Special railroad-style muck cars were designed to solve these problems.
Construction using Robbins' TBM
The tunnelling work was done in two legs. The first leg is 4,124ft (1.25km) long and the second leg is 24,000ft or 4.5 miles-long (7.25km).
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 stretches 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 approximately 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.
Key players involved with Maryland's project
The Washington Suburban Sanitary Commission is the owner of the project, while Jacobs Associates (JA) was responsible for managing the construction process in a joint venture with Louis Berger Water Services.
Contractors for the project were the Renda, Southland and SAK joint venture, while Brierley Associates provided the technical services.
The primary designs were provided by Black and Veatch, while the geotechnical evaluations and interpretations were provided by Haley and Aldrich.
A total of 29,000ft of 84in-diameter steel pipes used for the tunnel were supplied by the Northwest Pipe Company. The TBM was supplied by SAK.
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