FORENSIC INVESTIGATION OF THE FAILURE OF AN INTERCEPTOR SEWER, SEA CLIFF AREA, SAN FRANCISCO, CALIFORNIA

On December 11, 1995 a 100-year old brick interceptor sewer ruptured in the exclusive Sea Cliff area of San Francisco. The failure occurred during an intense, but not extraordinary, rainstorm. Storm water was constricted by a partly-lowered gate in an Overflow Structure that had recently been constructed some 1400 feet downstream of the failure. The constrictions caused pressurization of the sewer. At the site of the initial failure, water was forced through cracks in the sewer wall. Disturbance of the sand soil surrounding the brick sewer, caused by earlier excavation of an adit below and the development of a construction sinkhole, relaxed the soil confinement and allowed cracks to widen and water to escape that ultimately resulted in complete rupture of the sewer. Discharge from the sewer eroded a pit over 250 feet wide and 40 feet deep that caused the nationally-televised destruction of a historic home, as well as severe damage to adjacent public and private property. This paper is a summary of the public report of the forensic investigation and subsequent investigations.