Model Test And Mechanism Study Of Ground Subsidence Induced By Deep-water Level Seepage

In order to study the formation mechanism of subsidence caused by seepage erosion at deep water level,a constant-head plane strain test device is designed,an indoor model test is carried out,the material properties of different soil samples are tested,the formation process of the subsidence is observed.According to the different factors that affect the formation of collapse,the test is divided into internal cause test and external cause test.In the internal factor test,the univariate test is carried out based on the controlled variable method,and the different effects of water content,clay content,salt gradation on cavity size,model surface settlement and collapse state are discussed.The test results show that the mechanical properties and permeability coefficient of the soil sample will fluctuate greatly depending on the composition of the soil sample.The cavity expansion process contain several gradual soil drops,eventually forming collapse pits and loose areas;water infiltration and cavity expansion will cause surface settlement of the model,the sudden increase of settlement indicates that the collapse will happen soon;The expansion rate of cavities,the growth rate of subsidence and the state of subsidence will show a big difference with the change of water content;The total subsidence,the total test time and the clay content are positively correlated,and the collapse pit changes from the funnel type to the vertical type with the increase of the clay content;The addition of salt particles can improve the stability of the soil around the cavity,increase the critical cavity area,and change the collapse mode;The growth rate of the sedimentation volume is positively correlated with the proportion of the fine-grained salt mass,but the different salt gradation has no significant influence on the cavity expansion process and collapse state.In the external factor test,the orthogonal test is carried out with the width of the discharge opening,the height of the water level,and the difference in the water head as independent variables;the influence of different boundary conditions on the difference of responses in the test is discussed,and the sensitivity of each factor is analyzed.The test results show that the increase of head difference,water level and width of discharge outlet can induce collapse;the increase of the test duration and the width of the discharge opening will promote the growth of loose zone area,the area and shape of the collapse pit are directly affected by the loose zone;under the action of water infiltration and cavity expansion,the internal earth pressure of the model is constantly adjusted during the test,and the earth pressure on both sides of the model and the earth pressure in the middle part of the model interact with each other;the influence of the width of the discharge opening,the height of the water level,and the difference of the water head on the amount of surface subsidence is successively weakened,and the width of the discharge opening has a significant influence on the height.Based on Dupuit formula and critical gradient formula,the expression of cavity critical width is obtained by taking the critical gradient of seepage point of saturation line and seepage point of foundation streamline as the benchmark;Then the parameters of slope formula and Dupuit formula are analyzed,and it is found that the calculation results based on the critical gradient of seepage point of foundation streamline are generally larger.The collapse mode can be divided into vertical type and funnel type;Based on the limit equilibrium method,the expression of critical thickness of overlying soil layer in vertical collapse is derived;Based on the calculation method of soil pressure in shallow tunnel,the critical state of the overburden layer is the state when the vertical resultant force acting on the collapse body is zero,and the expressions of the critical thickness of the overburden layer and the inclination angle of the slip surface in funnel collapse are derived;At the same time,the working conditions when the ground is inclined are discussed,it is found that the critical thickness of overlying soil is the most sensitive to cohesion.