Bringing Tolt Pipeline No. 1 Back to Life: Sliplining a 66-in. Diameter Water Supply Main through an Active Landslide Area

Seattle Public Utilities (SPU) selected HDR Engineering, Inc. (HDR) to rehabilitate approximately 2,600 linear feet (LF) of the 66-in. diameter concrete pipeline known as Tolt Pipeline No. 1 (TPL1), which traverses an active landslide area that has been slowly moving approximately 1 in. per year for the last 20 years. HDR initially performed a detailed condition assessment of TPL1 to determine if sliplining would be feasible to minimize open-cut installations within the slide area. The pipeline alignment and profile, pipe material design parameters, and slipline pipe pull forces were evaluated to determine the number of pit locations and maximum slipline lengths, resulting in a total of three slipline segments of a 54-in.-diameter high-density polyethylene (HDPE) DR-17 pipeline using four launch and retrieval shaft locations. The slipline segments included a pull length of 170 LF limited by appurtenances, a pull length of 1,100 LF, to a maximum pull length of 1,125 LF, resulting in the total slipline rehabilitation of 2,395 LF.