Three-Dimensional Numerical Simulation Of Land Subsidence Considering Dewatering And Excavation Of Tunnel

Cut and cover method is a common method in tunnel construction,but it inevitably causes land subsidence.To prevent an adverse impact on the surrounding environment,the ground surface subsidence is essential for evaluating the rationality of construction supporting design.In the tunnel construction,many factors can cause disturbance and land subsidence,including supporting,dewatering,and excavation work.However,previous simulations mostly focused on the influence of a single factor.To get the more practical results,all of the three factors must be taken into consideration,thus the whole construction process can be simulated in three-dimension.In order to evaluate land subsidence resulting from dewatering and excavation of a tunnel construction,three-dimensional numerical models were built with ABAQUS software for three construction sections of a tunnel under the lake in Wuxi in this thesis,and each model was for one representative section which had different excavation and supports plans.In the simulation,dewatering,excavation and supporting were considered,and the construction of waterproof curtain,retaining wall and pile foundation was taken into consideration too.Firstly,the field data of surface subsidence at the top of the foundation pit on the shore where construction had begun and that of the surrounding area were analyzed.The section was numerically simulated,and the simulation results were compared with the field data.The simulation results were in good agreement with the field ones.Secondly,two sections under the lake with different supporting and excavation methods were simulated: one was for slope opening excavation construction and the other was for bored pile enclosure construction.Based on the simulation results of the two representative sections,many concerns were investigated,including the surface displacement in three directions,the ultimate subsidence curve of the ground surface,the impact of pile foundation on the surface displacement in the whole process,and the uplift at the bottom of the foundation pit after each layer was excavated.In addition,the influence of each construction step on other steps was studied separately,and the difference of surface subsidence in the two sections was compared while the proportion of each construction step in ultimate subsidence was analyzed.With the increasing distance from the foundation pit,the ground surface displacement changes from uplift to subsidence and then gradually decreases.The proportion of excavation and dewatering in the ultimate settlement is 30%-40% and 60%-70%,respectively.By comparing the two sections,it is found that the ground subsidence of the vertical excavation section protected by bored cast-in-place piles is greater than that of the sloping excavation section with the same buried depth,and the latter has a narrower and deeper subsidence trough.In the entire process of construction,the bottom uplift of foundation pits was impeded through the fraction between piles and the nearby soils,and the subsidence was controlled accordingly.The simulation results are helpful for designing foundation pit and choosing construction methods in tunnel construction.