As a trenchless subconsultant to PG&E and Golder, Bennett Trenchless Engineers provided full design assistance and full-time construction inspection services for two trenchless crossings to relocate PG&E’s gas transmission main. During preliminary design, Bennett Trenchless conducted a feasibility analysis to evaluate the use of HDD as an alternative trenchless construction method. However, due to site constraints and limited pipe layout area, it was decided that pipejacking methods from a deep jacking shaft would be required. Microtunneling was used for the 350-foot I-880 crossing and auger boring was used for the 130-foot railroad crossing.
The ground conditions presented unique challenges on this project. The preferred alignment elevation was located between two aquifers. The local flood control district was concerned about cross-contamination between the two aquifer layers, specifically arising from shaft construction. Shaft construction methods were specified that minimized the risk of cross contamination. Significant work area constraints combined with numerous existing utilities made selection of shaft location challenging. The trenchless components of the project were successfully completed in 2017.