Study On The Consolidation And Subsidence Mechanism Of The Clay Layer Induced By Pumping From The Confined Aquifer

Land subsidence has occurred in more than 150 countries and regions around the world.The loss caused by land subsidence in Shanghai alone is as high as 290 billion yuan,and the economic loss caused by an average of 1 mm is about 153 million yuan.Based on the research on the ground subsidence in Shanghai,this paper uses theoretical analysis,numerical simulation and model test to study the ground subsidence and the consolidation caused by the dewatering in the soft soil area,and there are the achievements as follows.(1)Based on the one-dimensional consolidation theory,the transfer matrix method is used to obtain semi-analytical solution about the soft soil consolidation caused by overlying load,dewatering in the confined aquifer.After comparing with the analytical solution obtained by the traditional solution,the correctness and applicability of the method are tested.The change of effective stress and settlement at different depths with time are studied.The closer to the surface,the faster the settlement develops and the greater the settlement finally.The settlement caused by the cyclic deepening is obviously different with the settlement induced by the constant water head.In the settlement mode,the ground subsidence caused by the constant water head drawdown is gradually accumulated,while the ground subsidence caused by the cyclic drawdown.On the settlement value,the ground subsidence caused by the constant water head is greater than ground subsidence caused by cyclical drawdown.The ground subsidence fluctuation caused by the cyclic drawdown is significantly behind the cyclic stress fluctuation caused by the drawdown,and this lag time does not change with the extension of time.For different peaks of cyclical deepening,the period of ground subsidence caused by it is not synchronized.The larger the peak,the longer the period.At the first peak,the ground subsidence corresponding to each peak of the cycle depth will not reach its maximum value,but will be reached after the peak;(2)In order to reflect the objective fact that the permeability coefficient and the compression coefficient change continuously during the consolidation process of soft soil,the relationship between the permeability coefficient and the compression coefficient with the change of effective stress is introduced into the one-dimensional consolidation equation,and the transfer matrix method is used to obtain its semi-analytical solution.The settlement of single-layer and doublelayer foundations in dewatering is discussed.It is found that after the introduction of nonlinearity,the consolidation time of the clay layer is relatively longer,and the final settlement is smaller.In order to consider the anisotropy of the permeability coefficient in practice,the Biot consolidation theory considering the permeability anisotropy is used to investigate the ground subsidence induced by dewatering in the confined layer.The results show that the consideration of anisotropy is reasonable and necessary for the confined aquifer,but for the soft clay layer,whether or not the permeability coefficient anisotropy is considered does not have much influence on the result.And it is also found in the process of soil deformation induced by dewatering,stretching and shear deformation in the overlying clay layer,making it necessary to consider the deformation inside the soil;(3)The influence of different soil parameters on land subsidence and horizontal displacement of the ground in the system consisting of overlying clay layer,the contained aquifer and underlying clay layer during the dewatering process of the confined layer is discussed by using the finite element software.For the overlying aquitard,among the five parameters of rebound coefficient,compressibility coefficient,soil thickness,vertical and horizontal permeability coefficient,the most influential factor on land subsidence and ground horizontal displacement is its thickness.For the underlying aquitard,the soil layer thickness is the most influential to the ground subsidence,and the compressive coefficient is the most influential to the horizontal displacement of the ground.For confined aquifers,among the five parameters of vertical permeability coefficient,horizontal permeability coefficient,elastic modulus,friction angle and cohesion,the factor that has the greatest influence on land subsidence and ground horizontal displacement is the horizontal permeability coefficient.(4)The laboratory model test was carried out,and the ground subsidence induced by dewatering in the confined aquifer was studied by using the sensor uniformly arranged on the soil surface and particle image velocimetry technology.The results show that the closer to the pumping well,the more serious the settlement after pumping,and the pore pressure in the soil will undergo a process of decreasing first and then increasing,which is the falling funnel on the pore pressure.In the three-layer soil flow system,the stratified sedimentation law of the soil layer is studied,the overlying and the underlying aquitard have a large amount of compression,occupying most of the ground settlement,and the confined aquifer hardly compresses.