Mitigating Geohazards along Aging Infrastructure—The Tolt Pipeline

Seattle Public Utilities (SPU) in Washington State owns and operates two major water supply pipelines extending from the Tolt Reservoir in the foothills of the Cascade Mountain Range to the City of Seattle. These pipelines measure about 56 kilometers (km) in length and supply the City with about 30 percent of its water supply. The first pipe, designated Tolt Pipeline 1 (TPL1), is a 1.7-meter (m) diameter, concrete cylinder pipeline that was constructed in 1960. The second pipe, designated Tolt Pipeline 2 (TPL2), is a 2.3-m diameter, steel pipeline that was constructed in 1998. TPL2 was field mortar lined and is tape coated. Together they are referred to as the Tolt Pipeline. Since 2000, SPU has documented slope movements starting with ground surface tension cracks along a 150-m long stretch of the Tolt Pipeline. Within this stretch, the pipelines are buried about 1 to 3 m below a paved roadway surface. According to geologic and LiDAR (Light Detection and Ranging) mapping, the pipelines are located within a large, ancient landslide complex. Various studies and mitigation measures have been implemented since 2000, and currently SPU is evaluating additional mitigation measures due to continuing slope movements.