Bay Area Transit (BART) - Warm Springs Extension, Walnut Avenue

Bay Area Transit (BART) - Warm Springs Extension, Walnut Avenue

Construction of Abutment 2 Reinforced Earth Walls
Precast MSE Retaining Wall
Fremont, CA
Bay Area Rapid Transit
Warm Springs Constructors (Kiewit / Mass. Electric, JV)
Harper Precast

Major construction on the Warm Springs Extension in Fremont, CA began in August of 2009 with the commencement of the Fremont Central Park Subway Contract. Work on the design-build Line, Track, Station and Systems (LTSS) contract, which began in October of 2011, will continue until the fall of 2015. This project, when complete, will add 5.4-miles of new track from the existing Fremont Station south to a new station in the Warm Springs District of the City of Fremont.

In 2011 the BART (Bay Area Rapid Transit) voted to authorize the award of the Line, Track, Station and Systems Design-Build Contract to Warm Springs Constructors (Kiewit/Mass. Electric, JV). The Reinforced Earth Company (RECo) was selected to design and supply the four MSE walls for this project that included two technically complex structures.

The two walls supporting Walnut Avenue Bridge are the first RECo MSE walls supporting a true abutment in California. In a true Reinforced Earth abutment, the bridge beams are supported on a spread footing bearing directly on the MSE structure. One of these two walls (Abutment 2 wall) is also the first MSE wall in the U.S. that is constructed directly above a seismic fault line. A pile supported abutment cannot be used at this location because the abutment is located directly above Hayward Fault. The use of piles, which behave like anchors for the abutment, would have adverse effects on the bridge superstructure during a seismic event due to the relative movement between the two tectonic plates at this fault. Seismic differential movements at the ground level above the fault are estimated to be more than 6 inches vertically and 24 inches horizontally. Therefore, to protect the MSE wall and the abutment from these potential extreme differential ground movements, a two-foot thick concrete slab is constructed below the wall as a "fuse" that will level up the potential sharp differential movements along the abutment.

In order to better accommodate differential settlement requirements slip joint panels were utilized at the fault lines. The two abutment walls were designed to accommodate a horizontal seismic acceleration coefficient of 0.75g with a maximum allowable seismic lateral deformation of 2 inches. Due to these extremely high seismic design parameters both the reinforcing strip lengths and densities (number of strips per panel) are over 2.5 times the norm for a wall of this scale.

This project required approximately 1700 panels that were cast by Harper Precast in Salt Lake City. Harper did an excellent job at casting the high strip density panels which required custom tie strip placement. The reinforcing strip installation was labor intensive and extreme care was taken to adjust the layout pattern to avoid excessive overlapping. Warm Springs Constructors’ crew was very meticulous with their installation procedures which resulted in superb finished structures and a very successful project.