Observed Performance of Two Adjacent and Concurrently Excavated Deep Foundation Pits in Soft Clay

Abstract

When adjacent foundation pits are excavated concurrently, the stress and deformation of the soil and structures are affected by their interactions. This paper investigates the performance of two adjacent deep foundation pits in Shanghai soft clay. These pits were excavated concurrently with respective excavation depths of 15.45 and 18.5 m and 2-mm-wide confined soil in the middle zone (middle of the pits). The behavior of wall deflections in two adjacent foundation pits was examined on the basis of measured data. Results indicated that (1) greater lateral deflections and deflection rates were observed for walls in the side zone (distant from the middle of the pits) than for those in the middle zone; (2) quantitatively, the maximum lateral deflection of walls in the middle zone had an upper bound of .4% of the excavation depth, which was, coincidently, also the lower bound for walls in the side zone; (3) the diaphragm wall deflections decreased occasionally during the excavation, and the foundation pits tended to move toward the middle zone due to the asymmetrical earth pressure exerted on the diaphragm walls; (4) three-dimensional distribution of the wall deflections was noticed, which was primarily a result of the corner effect and the nonuniform stiffness of the supporting system; (5) the measured maximum ground settlement generally ranged between .5 and .21% of the excavation depth in the first four stages of both foundation pits, whereas in the final stage of the deeper pit, a nonlinear increment of maximum ground settlement was observed, which was closely related to wall deflection and might be attributed to the interactive behavior of the adjacent and concurrent excavations; and (6) no apparent differential heave of columns and walls was found between columns close to the middle zone and those close to the side zone.