Contractors working for the Washington Suburban Sanitary Commission (WSSC) on the bi-county water main project have had a challenging couple of weeks. The worst snow fall in over 100 years hit the region in early February with more than 24 in. (60cm) recorded in a single event. So severe were the conditions that airports were closed, and the roads were so icy that snow ploughs were not allowed to be used.
“It has impacted on us in the last week but I don’t anticipate many weather delays once mining starts,” says Tim Winn, project manager for the Renda Southland contracting joint venture. The bi-county water main is named after the two areas served by the WSSC; Montgomery and Prince George’s County. The area has over 1.8 million customers, spread over 1000 sq miles (2600 service might experience low water pressure under very specific conditions in the future,” explains WSSC project manager for design John Mitchell.
Plans to install a new larger main and boost supply capacity have been in progress for over 30 years. “The project went into a number of planning studies. It was originally envisaged in the 1970s but we continued to postpone construction until we have reached this point of need,” says Mitchell.
Once WSSC had decided on the route of the new main they were then left with two options; either to use the traditional water main construction technique of cut and cover, or go for a tunnelled option. “We chose the tunnel because over time tunnel technology has improved and bought the cost closer [to cutand- cover] and also this particular alternative minimizes disruption to WSSC customers and is more environmentally friendly,” says Mitchell.
Building the new main involves construction of an 84 in. (213 mm) diameter steel water pipe, running for 5.3 miles (8.5 km), that will boost water supplies to the catchment area by up to 100 million gallons/day. Site preparation began in August 2009 and the team is aiming for completion in August 2013. Three contractors are working together on the US$ 113M project; pipeline specialist Oscar Render Contracting from Roanoke, Texas; tunnel specialist Southland Contracting from Fort Worth, Texas and pipeline and tunneling contractor SAK Construction from head east for 4100 ft (1250m) before returning to the main shaft and heading west for most of the bore, which is 21 000 ft (6400m).
“Our main working shaft is about 75 per cent complete. We have also begun the first receiving shaft which is about a third of the way complete,” says WSSC project manager for construction Steve Pinault.
The main working shaft situated at Conneticut Avenue will be 35 ft (10.6m) in diameter and extend down to 160 ft (48.8m) below ground level. The site was previously used by the Maryland State Highway Administration for construction work on the nearby I-495 and so an area of six acres had previously been cleared, providing WSSC’s contractors with working space.
The receiving shaft, called the Stoneybrook shaft will be 110 ft (33m) deep with a 20 ft (6m) diameter, and a final retrieval shaft – the Tuckerman shaft – sits at the end of the bore. “We are excavating with drill-and-blast for the tail and starter tunnel to install the TBM and we will begin mining towards the Stoneybrook shaft from Connecticut Avenue. We hope to average approximately 80 ft (24.4m) per day,” says Winn.
To use drill-and-blast in a suburban area, WSSC has had to work hard with the local community to inform them as to the impact of the works. Any residents within 1000 ft of the blast site were offered pre-blast surveys of the property. Peak particle velocities are to be limited to below 1 in./s for any surrounding structures, and blasting hours are 7am to 7pm. Consultant McKown Associates has been appointed to advise the contractor on blasting.
Ground conditions along the route of the main are classed as good to very good and at the depth of 200ft there are just two types of rock formation present: “There are two types of rock. Syskesville formation of metamorphic gneiss and schist, and then the Georgetown intrusive suite of metamorphized quartz, tonelite schist and amphibolite,” explains Mitchell.
The rock analysis was performed using 21 borings taken along the formation and data from three previous boreholes were also used. “We also reviewed existing information from a previous WSSC tunnel and metro construction in the area,” says Mitchell. It was the location of the hard rock that dictated the depth of the tunnel. “We wanted to construct deep enough that we stay in solid rock the whole time and avoid mixed face conditions,” says Mitchell.
Rock bolts will support the tunnel and a series of ribs will be installed in areas of poor ground. “These are to be confirmed on site,” says Winn.
The bore itself is scheduled to take 14- 15 months and a major concern for the site team is the small diameter versus the length. “To move enough air into the face and remove the muck in an efficient manner is going to be pretty difficult as we get into the longer portion of the project, further away from the access shaft,” says Pinault “To manage this we are going to use a continuous ventilation pipe, which is to be rolled on site. We also have reversible fans and back up systems.”
Muck removal will be a straightforward cart locomotive system with the material being used as fill locally. If contractors achieve the planned 80 ft/day (24.4 m/day) there will be 233 yd3/day (178 m3/day) of muck to remove and with swell this could increase to 350 yd3/day (268 m3/day). “Our peak truck volume is up to 480 yd3/day just to be conservative,” says Mitchell.
More challenging for the team is the pipe installation which follows on after the bore. Each steel section is 160 ft (50m) long and must be welded to the preceding pipe before the team grout the surrounding area. “The biggest risk section is the tunnelling but once this is complete there is the phase where the steel pipe is inserted and that is a significant amount of work,” says Pinault.
However according to WSSC the biggest challenge has been in communicating the project to the local community and getting its support. “The trickiest thing was the community outreach and getting ‘buy in’,” says Mitchell. “Two of our working shafts are located within the property of Maryland National Park and Planning Commission, so we needed permits from agencies for work within the park and that took a lot of coordination and planning to obtain.”
In addition due to the urban setting work areas at the shafts are generally small, particularly at the receiving shaft, Stoneybrook where space is constrained by a wetland, a major road, a national park and private property.
“We are making the most out of every inch of space. The road and wetland come together at that point, and the property owner adjacent did not want us to take any space from them. Everything we could get we got from the park,” says Mitchell.
The proximity of the working shafts to private properties means that the team are being extra careful to keep the neighbours happy. “We are within a few hundred feet of residents so we are trying to be good neighbours. We are in close quarters for quite some time so that is my biggest concern,” says Winn.
Despite the space constraints, proximity of neighbours and terrible weather, the team remain upbeat about meeting their construction timetable. Over the next few weeks the TBM will arrive at the site from another project in St Louis and assembly is expected to take three weeks. At the same time the team continues to excavate the shafts and prepare for the drill-and-blast of the tail and starter tunnels. By the summer of 2011 boring will be complete and construction of the steel water main can begin. By 2013 residents of Prince George’s County and Montgomery will benefit from increased water supply.
The main working shaft is 75 per cent complete and will be 160ft (48.8m) deep Working space is tight around the working shafts but Conneticut Avenue has six acres for contractors
