| Wuxi is the area which mostly suffers land subsidence hazard seriously in Su-Xi-Chang area of China.Although the rate of subsidence has been controlled due to the prohibition of groundwater exploitation in Su-Xi-Chang area,there is some area where land subsidence happened still.Because,for most of time in the past,the perspectives of the researches of land subsidence of most scholars are almost from macro-mechanics of soil,with the shortage of monitoring tools,there are still some questions of the mechanisms of land subsidence needing to be resolved.To have a better understanding of the mechanisms of subsidence and rebound of aquitards and aquifers,this paper used the samples of a drill hole in Guangming village Wuxi as the objects of study,conducted some soil test to comprehensively analyze the physical and mechanical properties of soil samples of different layers,investigated the pore structure of clay in aquitard and the relationship between creep characteristics and micro-structure,combined with the monitoring result of distributed fiber optical sensors of the continuous deformation of Quaternary layers,systematically analyzed the mechanisms of subsidence of subsidence area,concluded that:1.Comprehensive analysis of layers of subsidence area is very important for determining the stage of life of land subsidence and understanding the mechanisms of subsidence and rebound.The main content included that the investigation of physical and mechanical properties including water content,porosity ratio and compression factor etc.and the micro-response of different soil like clay and sand to stress.2.The first pressure aquifer including aquitard(Adl)and aquifer(Afl)which is not the main pumping layer of Wuxi,has higher water content,porosity ratio and compressibility;the deformation of Ad1 and Afl are dominated by reversible deformation at present,that is why the subsidence of Ad1 and Afl are in the early stage of life,which lead to the higher subsidence potential of Ad1 and Afl.The water content,porosity ratio and compressibility of the second pressure aquifer including aquitard(Ad2)and aquifer(Af2)has declined at some degree,and the rate of declination has been controlled due to the prohibition of water pumping.However because of the minimal permeability of aquitard,the subsidence will continue with minimal rate,and also the subsidence and rebound of layers shows some correlation with seasons;Af2 still has higher water content and porosity which means the second pressure aquifer still has some subsidence potential.The second aquitard(Ad3)has the lowest water content,porosity ratio and compressibility,also is dominated by irreversible deformation,which means the deformation of Ad3 comes from the residual deformation of aquitard.3.For aquitards,the number of compressible pores of clay has certain correlation with the compressibility of soil.The compressibility index of clay which is obtained through the plane compressible porosity and formal factor of pores that are calculated from SEM,has a positive correlation with the compressibility of soil:the more of compressible pores are,the more compressibility of soil is,through which the subsidence potential of different layers could be predicted combine with the depth of layers.4.For aquifers,the creep characteristics of sands are closely related with the particle size and grading of sands:when the stress is high enough,the worse the grading of sands is,the more the creep deformation is;for well graded sands,the more the number of large particles of sands is,the more the creep deformation is.Through the analysis of broken law of sands in creep from micro perspective,found when particles contacted by means of point to point and point to surface,the particles of sand have higher possibility to be broken.5.The deformation of subsidence area includes reversible deformation and irreversible deformation.For irreversible deformation of subsidence layers,when the pore water pressure is reduced,the effective stress is going to raise,and then the stress which is put on the pore water is transferred to soil structure,which lead to the compression of aquifers;on the contrary,when the pore water pressure is raised,like the recharge of underground water,the pressure water is recharged,then the pressure which is put on the soil structure in the past is shifted to pore water to some degree,which lead to the rebound of layer.The discharge of aquitards water is minimal due to the poor permeability of clay which lead to the lag of aquitards deformation,when the water level of adjacent aquifer reduced and then the subsidence stabilized,the deformation of aquitards could last several decades or even hundreds of years. |
