This study proposes a practical method for determining active earth pressure of confined cohesionless soil. A trapezoidal thrust wedge model confined by two vertical rigid retaining walls is developed to derive the governing equations of earth pressure distribution and the resultant force. The resultant force depends on the unknown incline angle of the sliding plane and must be maximized with respect to the angle. The active earth pressure can be determined once the incline angle is obtained by solving the governing equations with the finite-difference method. Based on these equations, parametric studies are conducted to investigate the effects of the confined width on the active earth pressure and the inclination of the sliding plane. As expected, the active earth pressure decreases with the confined width due to the arching effect. In contrast, the incline angle of the slip plane in the confined cohesionless soil is in close proximity to that of Coulomb’s solution. Moreover, the incline angle of...

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 18.50 m and 20-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 0.40% 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 0.05 and 0.21% 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.

Dewatering must be conducted for shallow confined aquifers located below deep excavations. However, its effects on ground settlement remain unclear, due to the complicated seepage and stress condition of the strata. In this study, a coupled numerical method was adopted to study the effects of dewatering in a confined aquifer on ground settlement during excavation. A hypothetical excavation model depicting the soil layers that are typical in Shanghai was developed in a finite difference software. Excavation and dewatering in a confined aquifer were simulated. The numerical results were analyzed in association with those of an identical excavation in which the confined aquifer was removed through modification of its hydraulic conductivity. Comparative results showed that dewatering in the confined aquifer during excavation aggravates ground surface settlements, especially for the ground surface twice the maximum excavation depth (2 H_e) behind the wall, where H_e was the excavation depth. The dewatering-induced settlement beyond 2 H_e showed a similar profile as groundwater drawdown in the confined aquifer owing to the insignificant deformations of the strata above the confined aquifer. However, the settlement profile within 2 H_e was inconsistent with the drawdown profile, due to the effects of unloading inside the excavation and soil-wall interaction. At the end of excavation, the confined aquifer was significantly compressed due to dewatering. However, it was found that the aquitard above the confined aquifer expanded dramatically with its maximum expansion located near 0.5 H_e. This may be attributed to the local soil arch and insignificant consolidation in the aquitard. Additionally, postexcavation pumping resulted in significant compression of the aquitard and further aggravate the ground surface settlement.

Excavation-induced ground settlement causes adverse impact or damage on adjacent facilities. If a foundation pit is connected to the facilities, the responses of the facilities can be influenced by soil inside the pit. Through an extensive long-term monitoring program, this study investigates the responses of a newly built metro line to the excavations of foundation pits that shared structures with a metro station. The vertical displacement and the relative deflection of the rail tracks are analyzed in association with the excavation processes. The rotation of the metro station due to asymmetric soil unloading in each pit is studied. Moreover, a comparison of displacement development between the rail tracks and the supporting structures is carried out. Results indicated that (1) the rail tracks inside the metro station moved upward significantly, whereas the rail tracks in the tunnels settled slightly; (2) significant relative deflection of rail tracks was observed in the longitudinal direction and the maximum relative deflection was located at the connections between the metro station and shield tunnels; (3) the asymmetric soil unloading resulted in distortions of the metro station and the displacement of the rail track close to the pit was larger than the track located farther away from the pit; (4) the displacement developments of the rail tracks were concurrent with those of the interior steel columns and the common walls (the walls shared by the metro station and the foundation pits in this study) and the maximum displacements of the metro line were close to the vertical displacement of the common walls throughout each excavation; and (5) the superimposed effect caused by the deep excavations resulted in excessive movements of the rail tracks and noticeable dislocations were observed at the connection between the metro station and shield tunnels.