Weighing the Risks of Installing a Lake Tap with Microtunneling

Abstract

In the summer of 2009 the City of Mercer Island, Washington, began construction on the Sewer Lake Line and Pump Station No. 4 Replacement Project that would replace a portion of the failing sanitary sewers that surround Mercer Island in Lake Washington. The project's original design included the open cut installation of three sewer lines; a 250 mm (10-inch) ductile iron (DI) force main, a 400 mm (16-inch) DI gravity sewer, and a 200 mm (8-inch) DI gravity sewer. The sewer lines were designed to extend from the pump station approximately 41 m (135 feet) into the lake, crossing beneath an active 250 mm (10-inch) asbestos cement sewer line. During construction, the Contractor requested that they be allowed to install the three sewer lines within a single 1500 mm (60-inch) steel casing installed by microtunneling. The City and its' project consultants performed a detailed risk analysis in response to the request, comparing the risks of installing the lake tap with microtunneling versus the 11 m (35-foot) deep open cut construction of the three pipelines that had been specified. This paper discusses how the project team weighed the risks involved with using an alternatively-proposed trenchless method of construction, and the contractual issues that were overcome to successfully execute a change order while managing those risks. The paper also presents the details of the construction of the lake tap in Lake Washington, the challenges of modifying the existing design of the pump station to accommodate the microtunneling equipment, and the details of the successful installation of the three sewer lines.