Study On Deformation Mechanism, Behavior And Control Strategy Of Excavation And Ground Under Dewatering

Numerous studies have been devoted to the performance of excavations and surrounding ground. The land subsidence induced by pressure-relief on confined aquifer also received much attention. In contrast, few studies have focused on retaining wall deflections only induced by dewatering on phreatic aquifer, and meanwhile, the spatial distribution of soil deformation induced by pressure-relief on confined aquifer is still unclear.In this study, six in situ dewatering tests were reported, in which the retaining wall deflections and the spatial distribution of soil deformation induced by dewatering on phreatic aquifer and pressure-relief on confined aquifer were observed, respectively. Based on the test results, two three-dimensional soil-fluid coupled finite element models were established and validated by the test results. Then, the mechanism of retaining wall deflections and soil deformation were studied using the aforementioned models. Besides, a parametric study was performed to investigate the retaining wall deflections and soil deformation under different conventional dewatering conditions. Furthermore, the strategies for controling the dewatering-induced retaining wall deflections and soil deformation were studied based on in situ tests and numerical simulation. The following conclusions can be drawn:(1) The retaining wall can develop considerable inward deflections during the pre-excavation dewatering on phreatic aquifer. The deflection pattern is cantilevered, and the maximum deflection can reach 37.6%-47.6% of the allowable wall deflection for the projects reported in this study. Besides, the wall deflections will be more obvious with the better soil permeability, the larger permeability anisotropy coefficient of soil, the greater excavation width, and the deeper dewatering depth. Moreover, numerical results indicated that the retaining wall deflection induced by dewatering on phreatic aquifer results from three factors:(a) the seepage force around the dewatering well and the soil–wall interaction caused the inward horizontal displacement of the soil inside the excavation;(b) the reduced total earth pressure on the excavated side of the diaphragm wall above approximately 1/2 of the maximum dewatering depth disequilibrated the original earth pressure on both sides of the diaphragm wall; and(c) the different negative friction on the excavated and retained sides of the diaphragm wall led to the rotation of the diaphragm wall into the excavation. However, three strategies can reasonably and effectively control the dewatering-induced retaining wall deflection, which are(a) making excavation braced before dewatering on phreatic water;(b) zoned dewatering and(c) staged dewatering.(2) The soil deformation induced by pressure-relief on confined aquifer shows a three-stage spatial distribution. The soil deformation overlying confined aquifer gradually increases up to down, the soil deformation underlying confined aquifer heaves, and the soil deformation of confined aquifer gradually reduces up to down. Besides, the maximum soil deformation appears at the top of stratum which has drawdown under arbitrary water supply conditions, relieving time and permeability of the overlying and underlying aquitard. Moreover, numerical results indicated that the soil-arch effect will form in soils, which has no drawdown and overlies confined aquifer during its relieving, and in the meantime, the additional tensile stress and tensile deformation appeare there. In this way, the aforementioned soil deformation overlying confined aquifer is formed. The upward seepage force in the soil underlying confined aquifer is the reason why the soils there heave. Whereas, the downward additional stress in the confined aquifer is the reason why the soil deformation there gradually reduces up to down. However, two strategies can reasonably and effectively control the soil deformation induced by pressure-relief on confined aquifer, which are(a) artificial recharge and(b) conbined recharge by twin-well. As to project having a strict requirement for soil deformation, the strategy of conbined recharge by twin-well is recommended. In general, the strategy of “simultaneous recharging and backflushing” works best, then the “recharging before backflushing”, and “recharging after backflushing” should be avoided.