Too Much Excitement under Highway 87

摘要

In early 2005, Santa Clara Valley Water District (District) staff discovered low pressure (～5 psi, or 35 kPa) groundwater leaking into a RCCP section of 66" (168 cm) high pressure pipeline under a major freeway embankment in downtown San Jose. The leak was discovered during an internal eddy current inspection on adjacent PCCP sections of the pipe. Although it had not been detected externally or deemed obvious from modest changes in system flows, leakage from the pipeline during normal operation was a big concern as the pipe operates at 177 psi (1220 kPa). If the leak worsened, it could have undermined the Highway 87 freeway embankment and impacted use of the freeway, caused a major outage in the District water supply system, and posed a major threat to public safety. Investigation showed that the leak occurred in an area under a surcharge embankment, at a joint located between a horizontal and vertical thrust block, where consolidation had been expected and factored into the design. The situation was complicated by construction changes which impacted joint performance; these changes were not adequately recorded on the as-built drawings. Localized deflection/rotation exceeded the design tolerances, leading to a leak at a gasketed joint. External geotechnical investigations showed modest void-space below but not next to the pipeline in this area; it's not clear if these voids were caused solely by consolidation and local pipe movement or by the leakage from the pipe, or a combination of factors. Settlement records strongly suggested that primary settlement due to embankment construction had stabilized. With a basic understanding of the mechanism involved in this failure, staff was able to confidently complete the repairs and put the pipe back into service. Repairs included: installation of flexible Weko-Seals at the two joints of concern; welding of patches on the holes drilled in the 66" (168 cm) pipe to investigate the voids; grouting of the voids below the pipe; patching of cement-mortar lining; and installation of a new precast vault for improved access. This experience produced a number of major "lessons learned" which will help District staff and others in the future.