DESIGNING A COMBINED SEWER INTERCEPTOR FOR INACCESSIBILITY – SEATTLE'S ALASKAN WAY VIADUCT AND SEAWALL REPLACEMENT PROJECT

Seattle's central business district is bordered on the west side by Elliott Bay. A seawall holds the soil in place along Seattle's waterfront which contains the Alaskan Way surface street, many utilities and the foundations for the elevated Alaskan Way Viaduct. The seawall is being eaten by gribbles and is at the end of its useful life. The 1950s-era viaduct has also reach the end of its useful life as is evident by crumbling concrete, exposed rebar, cracking concrete, weakening column connections, and deteriorating railings. The 2001 6.8 Nisqually earthquake further damaged the viaduct and led engineers to recommend replacement of the structure. A study completed in 2001 found that there is a 1-in-20 chance that in the next 10 years an earthquake of sufficient magnitude could cause the viaduct to fail. Replacement or rehabilitation of the viaduct and seawall is moving forward with an integrated project team supported by WSDOT, the FHA, the City of Seattle, and the Port of Seattle.One of the challenges to the project is the lack of space to relocate existing utilities to accommodate the transportation work. Over twenty utilities are currently located within the right-of-way. Relocated utilities can not be designed with industry standard horizontal separation because of the lack of space. This tight spacing results in future challenges if the owner needs to excavate the utility for maintenance. Additionally, surface features such as street cars, street car loading platforms, mechanically retained fill, and pedestrian activity centers will restrict the ability for utility owners to complete surface excavation to access their utilities. Therefore, future maintenance of the buried utilities may be challenging.A new combined sewer interceptor will be located within the project corridor with restricted access from the surface. This restriction has led Seattle Public Utilities and the design team to investigate a pipe design that goes beyond what is customary to protect against pipe failure and to provide maintenance access. The design assesses potential pipe failure methods, how the pipe can be designed to avoid failure, and repair methods if failure does occur. Potential failure methods include: loading, differential settlement, seismic, and internal and external corrosion. Pipe material and joints assess have included none traditional pipe materials such as:Welded steel pipe with cement mortar lining and coatingFiberglass reinforced polymer pipeReinforced concrete pipe with interior liningHigh density polyethylene structural wall pipeBecause of the inaccessibility of some manholes, maintenance access for pipe cleaning and video inspection was reviewed. Locating dead end manholes off-line in accessible locations was accessed. These off-set, off-line manholes would be used only to insert equipment and not contain sewer flows.