West Bay Builders (WBB) was contracted to construct the new BART Station in Pittsburg, which included the construction of an access tunnel for the station. WBB’s original shoring contractor submitted a design for a 26-foot high temporary shotcrete soil nail wall to support Highway 4 during construction of the concrete wall for the tunnel. Caltrans rejected the shoring plan, stating that tie backs were not allowed to remain under the highway right-of-way.
WBB then elicited the services of Holdrege & Kull to provide an alternative plan. After first determining that conventional soldier beam and wood lagging shoring would be too expensive, H&K developed a plan to use removable Manta Ray mechanical anchors. Boring logs for the project showed that the soil to be supported was dense clayey sand and silty sand. Based on these soil conditions, H&K eliminated the proposed shotcrete and instead recommended a more economical double twist hexagonal mesh facing between the tie-backs, which allowed for quicker installation. The mechanical anchor rods were then to be removed during construction of the permanent concrete wall. H&K’s plan received approval from Caltrans and BART and the plan was successfully implemented.