Investigation of Sewer-Line Failure

Subgrade soils supporting a 0.3 m diameter sewer pipe settled approximately 50 mm, and caused approximately 21% of the 730-m-long pipeline to be removed and replaced. The paper analyzes several reasons for the failure, and presents several lessons learned from the failure. Characterization of the subsurface conditions and incorporation of these results during design and construction could have prevented the failure. In addition, inspection and testing of the subgrade soils followed procedures developed based on the assumption that the subsurface conditions had been adequately characterized. The paper points out several problems relating to the roles during design and construction of the municipal agency, the water district, the geotechnical engineer, and the contractor on the project. The division of construction inspection responsibilities on the project led to a crucial part of the project (the trench bottom) not subject to any inspection. The cost of the pipe replacement was more than 30 times the cost of a detailed subsurface investigation that should have been performed for the project.