DESIGN AND CONSTRUCTION OF UNDERGROUND BASIN USING COMPOSITE SLURRY WALL 500 FEET AWAY FROM THE PACIFIC OCEAN

The wet weather equalization basin (EB) planned for Pacifica, CA is a 45 feet deep rectangular shaped underground water tank containing over 2.1 million gallons of water, and located only 500 feet away from the Pacific Ocean. The location and size of the water tank, and the geotechnical characteristics of the selected site presented many engineering and construction challenges. The engineering had to consider a structural system which can fit into the limited area of the site and yet need to meet the storage volume requirement. In addition, the system construction should meet the city’s tight budget and schedule. The proposed solution was to design and construct a composite slurry wall system consisting with 3 feet slurry wall and 2 feet cast-in-place(CIP) wall. The slurry wall would serve as a support of excavation system with temporary bracings during excavation and later integrate into the permanent structure system of the water tank, with an additional CIP wall to resist earth pressures and earthquake loadings. The slurry wall construction had to consider the shallow ground water table for the slurry trench excavation and tremie concrete pour. Also, the limited site space brought challenges in coordination and logistics.