Analysis Of Settlement Due To Withdrawal Of Groundwater In And Around Foundation Pit

The soft soil deposit generally has abundance of groundwater with high groundwater table. When a deep foundation pit is excavated in this deposit, in order to ensure the safety, it is necessary to pump groundwater during excavation. The groundwater pumping causes the transient seepage of groundwater to form a drawdown cone around the pit. Drawdown of the groundwater level will cause the increase of effective stress of soil outside the pit. Consequently, the soft clay will consolidate and settlement will occur around the pit. In this study, the settlement around pit due to the withdrawal of groundwater will be analyzed. The main content and new findings of the work are as follows:(1) A finite element model, in which, groundwater seepage is analyzed three dimensionally and settlement is calculated vertically by using Terzhaghi's one dimensional consolidation theory. The proposed model was employed to a practical project of dewatering before the excavation of foundation pit in Shanghai. The maximum calculated dewatering-induced-settlement in the pit is 0.235 m. The measured settlement in the pit is 0.25 m. This indicates that this model can simulate the field case fairly well.(2) Determination of parameters: The parameters including hydraulic conductivity, the porosity, and the coefficient of specific storage of soil layer are determined from field and laboratory tests. The hydraulic conductivity was determined by the field permeability test following the procedure established by the Shanghai Foundation Design code. The porosity was calculated from void ratio, which was determined from physical property test in laboratory. The coefficient of specific storage was calculated from the coefficient of volumetric compressibility. For cohesive soil, the coefficient of volumetric compressibility was determined from laboratory odometers test and for cohesiveness soil, it was determined from Young's modulus and Poisson's ratio. The analytical results show that the parameters determined on the base of the foundation design code of Shanghai can adopted to simulate the settlement due to withdrawal of groundwater in and around foundation pit.(3) During dewatering, the groundwater seepage field from the established model shows that flows are always to the pit, where the wells were allocated. The analytical results showed that when pumping in the artesian aquifer, the groundwater will replenish from other confined aquifers, where groundwater is rich. The analytical results also showed that the settlement in long-term is mainly contributed from aquitard with clayey soil and the contribution to settlement from aquifer is very small.(4) Effect of pit conditions on the calculation results: The effects of variation of pit conditions on the calculation results are conducted. The pit conditions include soil parameters, the depth of pumping wells, and the type of retaining wall of the pit. The results indicate that vertical hydraulic conductivity has stronger influence on settlement than horizontal hydraulic conductivity; specific storage has only a little influence on settlement; the settlement increases with the increase of the depth of pumping wells; and the pumping rate has direct effect on the settlement, i.e. settlement increases with the increaase of pumping rate. In the analysis, the pumping effect of phreatic water and aquifer water are also discussed separately. The results show that in the short-term, the range of groundwater drawdown when pumping confined aquifer water is larger than that in pumping phreatic water, whereas the settlement caused by pumping confined aquifer water is smaller than that in pumping phreatic water.