Microtunneling Beneath Historic Alexandria, Virginia

摘要

In March 2001, Crescent Resources, LLC (Crescent) acquired a 1.2 million m~2 (300-acre) former railroad-switching yard site, which bridges the City of Alexandria (City) and Arlington County, Virginia known as Potomac Yard. Crescent plans to develop the site into a high-quality, mixed-use community with hotels, retail stores, offices, town homes, apartments, parks and play fields. As a condition of the development, Crescent was required to plan, design and construct the required infrastructure to support the planned development prior to the issuance of building permits for the Potomac Yard site. Jacobs Civil Inc. (Jacobs) was engaged by Crescent to determine the most feasible approach to convey the projected wastewater flow from the Potomac Yard site to the Alexandria Sanitation Authority's (ASA) Wastewater Treatment Plant located 2.6 km (8,600 ft) to the south of the development. The area between the Potomac Yard site and the treatment plant consisted of the historic Old Town District in the City of Alexandria, Virginia, and the City had earlier indicated that an open cut installation through the area would meet with objection. Jacobs prepared a feasibility report, which evaluated the technical and economic feasibility of installing the proposed 750-mm (30-in) sanitary trunk sewer by means of trenchless construction methods. Microtunneling was the preferred construction method due to the ground conditions, the urban setting, the preference for a gravity pipeline and the need to reduce community disruption and third-party impact. After acceptance of the proposed installation method, Jacobs provided final design and construction phase services for the trunk sewer, which was routed beneath active rail lines consisting of the Washington Metropolitan Area Transit Authority's (WMATA) Blue Line and CSX Transportation (CSXT) as well as through the historic streets of the City of Alexandria, Virginia. This paper discusses the aspects of the design development, including the selection of alignment, geotechnical investigation program and settlement monitoring criteria, coordination with WMATA and CSXT, the contractor selection process, community outreach program, coordination/impact mitigation with existing utilities, and innovative construction methods.