Observed Performance of a Deep Excavation in Clay

A test section was established next to a 40‐ft‐deep, braced excavation in soft to medium‐stiff, saturated clays in Chicago, Ill. Surface and subsurface three‐dimensional ground movements, pore water pressures, sheet‐pile deformations and strut loads were measured. Results of these observations were correlated with construction activities at the test section. Larger than expected ground‐surface settlements adjacent to the excavation were observed. The large movements occurred as a result of overexcavation during construction. The largest incremental ground movements occurred when the excavation was approximately half completed. Base stability computations indicated that the factor of safety against basal heave at that point in construction was 1.1. Two distinct shear zones developed in the soil mass; their initiation corresponded to the times when the largest incremental movements occurred. Soil displacements were always directed towards the excavation with magnitudes, at later stages of construction, larger than those measured on the sheet pile. Pore water response was markedly influenced by sheetpile installation and strut preloading; as a result, little net change in pore pressures was observed at the end of construction. Magnitudes of measured strut loads were within the levels expected, based on standard design procedures.