Measured Behavior of Braced Wall in Very Soft Clay

During 1977–1978 a series of 25 ft wide (8 m) by 30 ft deep (9 m) sewer culverts were built in the Islais Creek Basin near San Francisco Bay. The sheetpile wall supported excavations for the culverts were made through rubble fill into a soft clay whose depth and strength varied along the culvert alinement. In the area of the thickest clay deposit, the soil was the weakest, apparently because the consolidation pressures were very low. There were a number of possible reasons for this situation, the most likely being the presence of artesian pressures. In the weakest clay, where basal heave factors of safety of the excavation were at or possibly below one, measured lateral movements during excavation were large, in sharp contrast to the smaller movements in adjacent areas where the clay was slightly stronger. One near failure occurred in the weak clay when a problem with the structural system developed. In general, the measured movements, strut loads and pore pressures are consistent with data from other projects when relative basal heave factors of safety are considered.