Cofferdam for BARTD Embarcadero Subway Station

Inward deformation of cofferdam walls during deep excavation in soft soils are inevitable but steps can be taken to minimize these movements. An excavation 1,160 ft (354 m) long by 55 ft (17 m) wide by 70 ft (21 m) deep in a busy downtown San Francisco street, flanked by major buildings, was performed for the BARTD Embarcadero station successfully in soil strata that contained varying depths of soft clay locally known as recent Bay mud. To provide an impervious rigid cofferdam wall of adequate strength and reasonable thickness a soldier pile and tremie concrete (SPTC) system of sheeting was selected. Successive excavation cuts below prescribed bracing levels were held to a practical minimum. Struts were preloaded to reduce compression deformations. Inclinometers recorded deflections of the walls while strain gages enabled determination of strut loads. The thicker the layer of soft clay the greater the inward movement of the walls notwithstanding the corresponding increases in wall sizes.