Mechanism And Law Of Ground Subsidence Induced By Groundwater Level Fluctuation

In recent years,with the rapid development of China’s economy,the acceleration of urbanization and the strengthening of construction activities,surface collapse accidents are becoming more and more frequent,and the harm is huge,which has become a serious social problem.In this paper,the internal mechanism and law of surface collapse induced by groundwater level change are studied by using the research methods of model test,theoretical research,numerical simulation and engineering example analysis.Firstly,by designing and making an indoor model test device,the breeding and development process of seepage collapse under the conditions of different groundwater level variation frequency,groundwater level variation amplitude,soil moisture content,overburden thickness and karst opening width are simulated.The scale of soil cavity,internal earth pressure of soil,pore water pressure,change of soil surface settlement,critical state of collapse and final form of collapse in different stages are compared and analyzed,and the effects of different factors on collapse caused by groundwater level change are discussed.The test results show that the collapse of the soil can be judged by monitoring the change of earth pressure and pore water pressure in the soil.When the change trend of earth pressure changes suddenly and tends to be stable,it means that the soil has entered the failure limit state.And each small change of pore water pressure indicates that a large collapse failure has occurred in the soil and the soil hole is expanding unsteadily.With the acceleration of the frequency and amplitude of groundwater level rise,the faster the soil disintegration and failure speed,the shorter the incubation duration of collapse,and the larger the area of the final collapse pit.However,the critical overburden thickness of soil collapse is almost not affected by the frequency and amplitude of groundwater level change.It is proved that there is a limit equilibrium thickness of overburden soil during soil collapse,which is only related to the physical characteristics of soil and has nothing to do with external factors.The different moisture content of soil has a great influence on the final collapse form of soil.There are three failure modes: straight tube collapse,funnel collapse and altar collapse.The smaller the thickness of overburden soil,the stronger the anti collapse ability of soil,and with the decrease of overburden thickness,the rise span ratio of developed soil cave decreases obviously.The larger the width of karst opening is,the larger the initial diameter of soil cave is.Under the action of seepage,the arch foot of soil cave is more prone to traceability failure,resulting in poor overall stability of soil mass.Secondly,based on the arching effect in soil,the collapse balance method is improved by using the theory of "rational arch axis",and the formula of critical overburden thickness when collapse occurs is deduced.The experimental results are used to verify the rationality of the improved collapse equilibrium method and it is found that the error of the formula is within the acceptable range.Then,on the basis of model test,several typical collapse stages are simulated and analyzed.It is found that the shear stress concentration area is generally located at the arch foot of the soil cave and moves to both sides with the development and expansion of the soil cave;The maximum vertical stress is also distributed at the arch foot of the soil cave,and there is a low value area of vertical stress at the top of the soil cave.When the low value area of stress at the top of the cave disappears,it indicates that the development of the soil cave has entered an unstable state.Finally,according to the causes of a collapse accident case and combined with the actual situation of the disaster,the measures and suggestions for the prevention and treatment of the collapse accident are put forward,such as exploring the stratum structure of the underlying karst,controlling the dewatering and drainage of the foundation pit in the urban area,regularly maintaining the underground pressure pipeline,and strengthening the soil layer in the high-risk area in advance.