Lessons Learned from Ground Improvement on BART Fremont Central Park Subway Project in Fremont, California

Abstract

The San Francisco Bay Area Rapid Transit (BART) is a heavy rail transit system connecting San Francisco with other cities in the Bay Area, California. Currently, BART is extending the existing Fremont line approximately 8.7 km south to a new Warm Springs Station. The project is divided into two major contracts, Fremont Central Park Subway (FCPS) and Line, Track, Station and Systems (LTSS). The FCPS contract primarily consisted of the construction of a 1.9 km long cut-and-cover subway box tunnel through challenging ground conditions and under a lake within a major recreational park. As part of the cut-and-cover subway box construction, ground improvements including installation of large diameter jet grout columns and cement deep soil mixing (CDSM) walls. Jet grout columns were installed using the conventional double-fluid system (grout and air) and a new method called the “jetting down” method. To support the subway excavation and to cut off the groundwater, contiguous walls consisting of overlapping CDSM columns were installed. To resist uplift from excess hydrostatic pressure and significant water inflow, a 3 to 7.3-m thick soil cement grout plug was constructed under the subway box. This paper will discuss the challenges during construction of ground improvement for the FCPS project and lessons learned during installation of large diameter jet grout columns and CDSM cut off wall.