By Jim Rush
The Potomac River has always played an important role in our nation’s history. It was once believed to be the key to accessing the interior of the continent. Its shores play host to our nation’s capital, were the home President Washington, and served as a key divide in the Civil War.
Its waters also serve as the drinking water source for the District of Columbia, as well as a source of recreation and gateway to the Chesapeake Bay. Like many rivers in the United States, however, the water quality in the Potomac had deteriorated badly due to pollution, runoff and sewer overflows by the time the Clean Water Act was passed in the 1972. The act helped focus the attention of the nation on cleaning up waterways focusing primarily on constructing or improvement sewer treatment plants.
As part of ongoing improvements and to address a federally mandated decree, DC Water is implementing the $2.6 billion Clean Rivers Project to clean the Potomac, and its tributaries the Anacostia River and Rock Creek. The projects will reduce CSOs by 96 percent.
The first phase (Anacostia River Projects) is underway and involves constructing a massive underground tunnel system to store and convey captured combined sewage during intense storms, preventing most combined sewer overflows to the Anacostia River. These overflows will be reduced by 98 percent when the tunnel system is completed in 2022. The later phases will address overflows to the Potomac River and Rock Creek.
In total, there are approximately 13 miles of 23-ft ID concrete lined storage and conveyance tunnels, shafts, diversion chambers, overflow facilities, a dewatering pumping station and a replacement sewage pumping station.
Some of the major tunnel projects include:
- Anacostia River Tunnel
- The Anacostia River Tunnel consists of approximately 12,500 ft of 23-ft in diameter and located approximately 100 ft underground. The project includes the following elements: Six drop shafts located along the alignment to deliver captured CSOs to the tunnel and serve as access points for system maintenance (diameters vary from 15 ft up to 75 ft); Installation of three diversion chambers along the tunnel; and ventilation control facilities installed at the shafts.
- A $253 million design-build contract was awarded to Impregilo-Healy-Parsons JV in June 2013. Estimated construction completion is January 2018.
Blue Plains Tunnel
The Blue Plains Tunnel consists of 24,300 lf of 23-ft ID tunnel along with the construction of 132-ft diameter dewatering shaft which will house a 250 mgd pumping station to dewater the tunnel system and a 76-ft diameter screening shaft at the Blue Plains Advanced Wastewater Treatment Plant. The two shafts also serve as main access point for the tunnel construction. Also included are: construction of 55-ft diameter drop shaft at the Main Pumping Station to serve as terminus point for the Blue Plains Tunnel construction; construction of a 50-ft diameter drop/overflow shaft at Joint Base Anacostia Bolling (JBAB); and construction of a 55-ft diameter junction/drop shaft at Poplar Point Pumping Station.
A $330 million design-build contract was awarded to a joint venture of Traylor/Skanska/Jay Dee JV in 2011. Estimated completion date in November 2015.
First Street Tunnel
The construction for the First Street Tunnel includes about 2,700 lf of 18.5-ft ID tunnel approximately 80 to 160 ft deep; three drop shafts, three adit connections, four diversion chambers, and a 6 mgd pumping station housed in a fourth below-grade shaft. The four shafts range from 20 to 65 ft. in diameter with invert depths ranging from 100 to 160 ft below the ground surface. The three adits range from 20 to 400 ft. in length and are 8 to 10 ftID. The invert depths for the diversion chambers are 25 to 35 ft below the ground surface.
A $157 million design-build contract was awarded to a joint venture of Skanska-Jay Dee in October 2013. Completion is scheduled for the spring 2016.
Northeast Boundary Tunnel
The Northeast Boundary Tunnel comprises the construction of a 23-ft diameter tunnel between 40 and 160 ft below the surface. The tunnel alignment encompasses a 27,000-ft long corridor beginning just south of Robert F. Kennedy Stadium and terminating just west of the intersection of Rhode Island Avenue, NW and 6th Street, NW.
Taking A New Approach
DC Water began its tunneling program with the Blue Plains Tunnel, beginning from the Blue Plains Advanced Wastewater Treatment Plant and working its way northerly into the District. The Anacostia River was the next project to be awarded, followed by the First Street Tunnel. Since the water quality of the Anacostia River is significantly impaired by CSOs, the Anacostia River Projects are first in the court ordered schedule and DC Water has prioritized the design and construction of these projects. Additionally, recent flooding caused by the District’s existing, undersized collection system has greatly accelerated the design and construction of the First Street and Northeast Boundary Tunnels.
To help expedite the projects and meet the federally mandated deadlines, DC Water turned to design-build contracting. Although design-build contracts are becoming more prevalent in the North American tunneling market, its use for sewer tunnels is among the first.
“Design-build was chosen to improve the schedule and tap into the contractor’s technical experience. It has been quite successful. All of our projects are on schedule and budget,” said DC Water’s Christopher Allen, Assistant Director for the Clean Rivers Project. “We highly recommend a multiphase procurement, consisting of a Request for Qualifications (RFQ) open to any interested design-build contractor team. A short list of the three most qualified teams is chosen and a subsequent ‘collaboration period’ occurs, where the three prospective design-builders analyze the project to develop conceptual design and construction strategies in confidence with the owner’s team. Based on acceptance of these strategies, the design-builders submit a separate detailed Technical Proposal and a Price Proposal. The Technical Proposal is generally weighted at 30 to 40 percent with the price component weighted at 60 to 70 percent. The Technical Proposals are opened first and scored. Then the Price Proposals are opened. The team with the highest combined score is awarded the job. The two unsuccessful Proposers are paid a stipend for their efforts. In the three major tunnel contracts DCCR has awarded, two of the best technical proposers have also submitted the lowest price.”
The First Street Tunnel weighted the score more heavily toward the Technical Proposal. In response to severe flood events that occurred in the summer 2012 in the Bloomingdale and LeDroit Park neighborhoods of the District, a multi-agency taskforce was assembled by Mayor Vincent C. Gray to evaluate a range of potential actions to address combined sewer flooding. As a result of Task Force recommendations, the First Street Tunnel design and construction schedule was significantly accelerated to mitigate flooding in the area. In this instance, cost was less of a driver.
“To encourage innovation, we changed the weighting of the technical vs. the cost to encourage the bidding teams to bring something innovative to the table,” said DC Water’s Bill Levy, program manager for tunnel design. “We actively engaged the firms and rewarded them if they brought us innovation.”
Blue Plains Tunnel Update
The Blue Plains Tunnel is gearing up now for full production. The TBM “Lady Bird” advanced 1,200 ft and was shut down in October to allow crews to switch over from a temporary mucking system to the permanent mucking system. The permanent mucking system consists of a horizontal belt that carries muck to a carousel consisting of seven muck cars, which are then hoisted out by a gantry crane. Crews have also installed a vertical storage unit in the shaft for the belt magazine. The Traylor/Skanska/Jay Dee JV opted for this mucking configuration due to the sticky nature of the clay soils expected to be encountered for most of the alignment.
One of the most unique aspects of the Blue Plains Tunnel is the Figure 8 shaft from which the TBM was launched. The shafts were connected at the bottom, allowing ample room for TBM assembly. The dual shafts came about as a result of DC Water’s need for a separate pumping station and screening facility at the location. Additionally, the construction of the Figure 8 allowed a smaller footprint within the confines of the active treatment plant.
To date, mining has progressed well and the contractor is hopeful that will continue as mining progresses further. “We are very optimistic with the way the machine is running,” said Traylor/Skanska/Jay Dee’s Mike Jatczak. “The ground is very conducive to mining fast and we don’t expect that to change.”
First Street Tunnel Update
The First Street Tunnel is just getting under way, but it involved unique challenges due to the built out environment in which it is being constructed. In particular, challenges including working in close proximity to area residents and businesses. In some cases, structures are as close as 10 ft to existing buildings and within areas with limited traffic access. DC Water collaborated with potential design-builders and selected Skanska/Jay Dee, in part due to their proposal to utilize ground freezing for shaft and adit construction.
“With ground freezing, we are able to create excavation support with significantly less construction material at the site, and reduce traffic noise and dust – all while reducing our construction footprint,” said Scott Hoffman, project manager for Skanska/Jay Dee.
Additionally, Skanska/Jay Dee was able to site the refrigeration plant at a more remote location, piping utilities to the shaft locations. Skanska/Jay Dee subcontracted the ground freezing work to Moretrench.
Justin Carl, First Street Tunnel project manager for DC Water, said that collaboration with the public was a key to this project. “We had extensive public outreach meetings with the community and were able to incorporate mitigation requirements to lessen the impact to the residents into the contract,” he said. He added that the DC Water was able to work with the District in expediting permitting to allow the fast-track nature of the project.
In the case of the First Street Tunnel, design-build has allowed it proceed at an accelerated schedule, Levy said. “This project was probably what design-build was made for,” he said. “We would not have been able to make the schedule without it. While collaborating with the JV, we were able to continue working on the design. And with the feedback from the design-builder we were able to gain confidence that we were able to build within the constraints the community was asking us to work within.”