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The Seismic Upgrade of Bay Division Pipelines
A Critical Upgrade Beneath A Highway
Innovative sliplining approach
While Mission Blvd., a surface-level street, can be re-aligned in order to work on the complex construction work at Trace B, I-680 cannot be re-aligned or closed to work on Trace A. Thankfully, Caltrans installed a 114-inch diameter, 350-foot segment of corrugated metal pipe (CMP) next to BDPL Nos. 3 & 4 when the highway was constructed in 1969, based on speculation that an additional water line may be needed in the future. In 2013, a new 78-inch diameter BDPL No. 3 pipe was welded and then sliplined into the CMP using a wheel and rail system.
For the parallel BDPL No. 4 pipe beneath the I-680, 400-feet of new 80-inch diameter pipe has recently been installed inside the existing 96-inch pipeline. The designer of record, URS Corp., specified a novel strategy for the sliplining of BDPL No. 4. Workers attached sheets of PTFE (polytetrafluoroethylene) to the bottom of the inside of the existing pipe and the bottom of the outside of the new pipe. PTFE’s low coefficient of friction allowed the pipe to be slipped into place without using rollers or damaging the pipe coating. The PTFE serves a dual purpose in addition to being used for sliplining. Since the annular space was not filled in, the PTFE will allow the new pipe to move more freely within the existing pipe during a seismic event.
Although the team did not identify any precedent on record for this technique, Steve P. Rados completed the sliplining without any issues and maintained a schedule of one 25-foot pipe segment per day. The existing pipe’s profile had minor inconsistencies, so prior to the start of slipline work the existing pipe was re-profiled with shotcrete that continued into the slipline pit. The new consistent profile on the existing pipe allowed for smoother welding fit up and eliminated any curbs or bumps the new pipe might have encountered while being pushed through the existing pipe.