Research On Macro-Mesoscopic Evolution Mechanism Of Whole Process And Trend Prediction Of Land Subsidence Caused By Groundwater Exploitation

Water,the source of life.Since the reform and open policy,China's annual average groundwater exploitation exceeds 2.5 billion m3,more than 400 cities across the country are currently exploiting groundwater,accounting for more than 30%of the total amount of freshwater used in cities,it is as high as more than 70%in some areas such as northwest China and North China.The unreasonable development and utilization of groundwater leads to the formation of a worldwide land subsidence and its derived series of ecological environment problems.Strengthening the research on the mechanism and law of land subsidence caused by groundwater exploitation is helpful to improve and promote the theoretical development in the field of land subsidence.Grasping the response law of soil deformation and land subsidence to different groundwater exploitation conditions,so as to put forward a scientific and reasonable groundwater exploitation scheme.This is of great practical significance to ensure the healthy and stable development of national economy and society.Supported by monitoring and prevention of land subsidence projects of the Territorial Resources Department of Shandong,this paper carried out a theoretical analysis of the whole process of land subsidence caused by groundwater exploitation in stages,studied the similarity criteria of soil deformation and land subsidence under groundwater exploitation.A series of physical model tests under different conditions were carried out through the design of a large physical simulation test system for land subsidence caused by extraction of confined water considering the lateral groundwater recharge and structure of pumping well.The microstructural evolution law tests of aquifer sandy soil under pumping condition and the quantitative analysis tests of microstructural characteristics and parameters of clay under consolidation condition were carried out respectively,and the correlation of macro-and micro-parameter is analyzed.Based on the fluid-solid coupling theory of porous media,the spatiotemporal evolution characteristics and distribution rules of soil deformation and land subsidence under the groundwater exploitation were studied,the sensitivity analysis of the main soil parameters was also carried out.Based on the fluid-solid coupling theory of particle discrete element,the mesoscopic transport characteristics and laws of soil particles under the condition of groundwater exploitation were analyzed,and the essential mechanism of land subsidence was discussed.Taking North Shandong area,which is a part of North China Plain,as the research background,the characteristics of regional land subsidence caused by groundwater overexploitation were studied,and the evolution trend of land subsidence under different groundwater exploitation conditions was analyzed and predicted.The main research work and results are as follows(1)The theoretical analysis of the whole process of land subsidence caused by groundwater exploitation in stages.The whole process of land subsidence caused by groundwater exploitation was divided into three stages:change of seepage field,soil interlayer coupling effect in the exploited horizon,and subsidence transfer in the non-exploited horizons.The theoretical analysis of each stage was carried out respectively,and the calculation formula of land subsidence transfer law in the whole process was established taking into account the groundwater exploitation and the physical and mechanical properties of each soil layer.Meanwhile,based on the similarity theory,the whole process of land subsidence caused by groundwater exploitation was analyzed by similarity criterion,so similar scale that should be followed when the model test is similar to the prototype test was obtained.(2)Development of physical model test system for land subsidence caused by groundwater exploitation and model test.Adopting modularized design idea,a physical model test system to study the macro-and micro-mechanism of land subsidence caused by groundwater exploitation,which fully considers the structure of the pumping well and the three-dimensional recharge of groundwater,was designed and developed.Based on this test system,model tests under different water production conditions were carried out,the law of land subsidence affected by water supply or not was obtained,the proportion relation and hysteresis of deformation between aquifer and aquifuge were revealed,the time-variant permeability coefficient of aquifer and the evolution of negative air pressure in soil under the groundwater exploitation were clarified.(3)Study on microstructure evolution of sand clay and correlation of macro-and micro-parameter.Under pumping condition,the deformation model test of sand was carried out,the mesoscopic structure evolution rule of sand was analyzed by PIV technique.Under consolidation condition,the clay was tested by SEM and its parameters were quantitatively analyzed.The mesoscopic structure evolution process and law of sand deformation under the groundwater exploitation were revealed,and the influence of consolidation load on the evolution law of microstructure parameters under different scales of clay was compared and analyzed.The results indicate that the microstructures at smaller scales have greater load influence range and stress sensitivity,the relationship between apparent void ratio and macro void ratio of clay is linear regression,the relationship between apparent void ratio and compressibility shows a good exponential growth.(4)Macroscopic and mesoscopic numerical simulation analysis of land subsidence caused by groundwater exploitation.A macroscopic numerical analysis model was established by adopting the Biot's fluid-solid coupling theory of porous media.The spatiotemporal evolution characteristics of sand-clay interbed when the same groundwater amount is pumping from different aquifers were analyzed,and the sensitivity and response degree of land subsidence to the main parameters of soil were discussed.The results show that the sensitivity of soil parameters within the aquifer can be ranked as:permeability coefficient>elastic modulus>Biot-Willis coefficient>Poisson ratio,within the aquifuge can be ranked as:elastic modulus>permeability coefficient>Biot-Willis coefficient>Poisson ratio.A mesoscopic numerical analysis model was established based on the fluid-solid coupling theory of particle discrete element.The migration rule of soil particles and the adjustment process of particle structure under the condition of groundwater exploitation were studied.The evolution law of contact force chain of particles and flow field were analyzed.(5)Analysis on the characteristics of regional land subsidence caused by groundwater exploitation and prediction of subsidence evolution.Boxing county was taken as the engineering background,the characteristics of dynamic distribution of groundwater and evolution of land subsidence in the study area was systematically analyzed.A three-dimensional fluid-solid coupling numerical model of land subsidence was established to reflect the development process of land subsidence and predict the subsidence evolution law under different groundwater extraction schemes.The results show that a 20%reduction in existing groundwater exploitation is a reasonable and effective way to slow down land subsidence hazards.At the same time,in order to realize reasonable and effective prevention and control of land subsidence,the step-down groundwater exploitation should be carried out in different regions,measures should be taken according to local conditions...