Trenchless Technologies Conquer Design, Planning, and Construction Challenges on a Washington, D.C. Area Conveyance Project

摘要

Trenchless technologies are an effective tool for efficient project delivery of water and wastewater conveyance systems. Trenchless methods are used to reduce risk and avoid obstacles on projects. From reducing environmental impacts in green-field conditions to reducing traffic congestion in urban environments to conquering exceedingly large depths where slope must be maintained, trenchless technology can be a cost-effective way to successfully deliver challenging conveyance system projects. The Broad Creek Augmentation Project in Maryland consists of 10 such trenchless installations - 8 constructed by microtunneling and 2 by conventional tunneling machine. The Project was implemented by the Washington Suburban Sanitary Commission (WSSC), the water and wastewater utility servicing Prince George's and Montgomery Counties in Maryland. The Broad Creek Augmentation Project consists of 4.8 miles of new sewer conveyance pipeline, an upgraded and expanded pump station, and wastewater treatment plant upgrades at the discharge end of the pipeline. The pipeline portions of the Project are being constructed under three separate Contracts. The conveyance system improvements consist of the construction of a new parallel conveyance system adjacent to the existing system. Both the existing and new conveyance systems will be used together to attain maximum capacity, but the implementation of valves and junction structures will allow for each system to operate independently if needed. The new conveyance system uses three pipe designs: force main, gravity sewer, and pressure sewer. During the planning and design phases of the Project, several obstacles, design constraints, and a need to reduce impacts to third parties led to the implementation of these trenchless installations. Obstacles included crossings of Broad Creek and Piscataway Creek, two crossings of the highway SR 210, and the crossing of the heavily trafficked Fort Washington Road and Livingston Road intersection. Because the parallel conveyance system is designed to operate independently to permit maintenance, the new pipeline required a continuous downslope to allow for drainage. This resulted in some sections with cover depth so large that the conventional cut and cover methods became impractical and trenchless methods became the more cost-effective alternative. At another location on the Project, the alignment crosses National Park Service property and regulated wetlands. Trenchless methods were used here to reduce a potentially lengthy permitting process by tunneling below the environmentally sensitive area. Trenchless technology is not without its risk, and an appropriate risk assessment, trenchless design, and implementation of mitigation measures are required during the design phase. During design, a risk register was maintained to identify and help mitigate tunneling risks. This information was used in the preparation of Geotechnical Baseline Reports (GBRs) for the Project. WSSC implemented three GBRs, one for each of the three Contracts. The GBRs establish ground conditions and behavior for bidding purposes, provide information on past tunnel construction in the area, and delineate geotechnical risk between the Owner and the Contractor. The Project consists of highly variable and challenging ground conditions ranging from the Aquia Formation consisting of sands and silts, cemented sands, and sandstone corestones, to alluvium containing significant amounts of timber up to 100% of the tunnel face. The GBR baselined expected conditions and stated requirements for all trenchless installations and associated shaft construction. Though the Project is currently in construction, all 10 trenchless installations have been completed, and some sections of the pipeline are in active use. This paper will discuss the following topics: 1. Definition of trenchless technologies and summary of available methods 2. Key features of the Broad Creek Augmentation Project 3. Design constraints and obstacles that led to the selection of 10 trenchless solutions 4. Assessment of trenchless methods and ultimate selection of microtunneling and jack and bore methods 5. Strategy for design and construction of shafts 6. Development of the Broad Creek GBRs, their implementation during construction, and successes for risk mitigation 7. Construction issues, solutions, and lessons learned.