Study On Dynamic Characteristic And Microstructure Of Loess Under Subway Loading

In recent years,due to the rapid development of the subway construction in China,the soil settlement deformation under the subway loading is quite obvious.As active faults are developed and loess is widely distributed in China,the subway engineering in loess area inevitably crosses the development area of active faults.Accordingly,the problem of loess settlement deformation in active fault site caused by subway loading becomes more and more prominent.Combined with cyclic dynamic triaxial test,scanning electron microscope test and image processing technology,the subsidence deformation characteristics of loess in near-fault region under subway loading were investigated.The effects of dynamic stress and loading frequency on the dynamic characteristics and microstructure of loess in near-fault region were studied.And the calculation model of micro parameter and macro parameter was established by grey relational degree,the relational degree between micro parameter and macro parameter was analyzed.The major researches and achievements are as follows:(1)The cyclic dynamic triaxial tests were carried out on the loess in near-fault region under the control of dynamic stress and loading frequency using the British GDS dynamic triaxial test system.The cumulative plastic strain and stiffness degradation index of the undisturbed loess and remolded loess under different dynamic stress and loading frequency were analyzed.(2)The cumulative plastic strain test result and the stiffness degradation index test result of the undisturbed loess in near-fault region were fitted by the cumulative deformation model and the stiffness degradation model respectively.The results show that the empirical formula and the second-order logarithm model have high fitting accuracy for the cumulative plastic strain and stiffness degradation curves,respectively.The empirical formula and the secondorder logarithm model can be used to predict the settlement deformation of loess in near-fault region under subway loading.(3)The microstructural characteristics of loess in near-fault region before and after subway loading were analyzed by scanning electron microscopy,and the microstructures of undisturbed loess and remolded loess were compared.The results show that the morphology,arrangement and scale characteristics of the structural units such as particles and pores are different between the undisturbed loess and remolded loess before the subway loading.The microstructural characteristics of undisturbed loess and remolded loess under subway loading are as follows:larger particles are crushed and gradually become smaller,smaller particles are squeezed to form larger aggregate particles,the number of large pores is reduced,the number of small pores is relatively increased,and the particles gradually become dense.(4)Image processing software was used to obtain pore micro-parameters of loess in nearfault region under subway loading,and the variation rules of pore micro-parameters of undisturbed loess and remolded loess under different dynamic stress and loading frequency were analyzed.The results show that the pore micro-parameters change under the subway loading,and the pore morphology characteristics(roundness and fractal dimension)and pore scale characteristics(max radius and abundance)change most obviously,while the pore arrangement characteristics(directional probability entropy and directional frequency)change little.The pore shape gradually tends to be smooth and equiaxial circular and the soil compaction degree increases.(5)Based on the grey relational analysis method,the influence degree of pore microparameters on the stiffness degradation index of undisturbed loess and remolded loess was studied.The results show that the influence degree of pore micro-parameters on the stiffness degradation index of undisturbed loess and remolded loess from strong to weak is as follows:average max radius,average roundness,average fractal dimension,directional probability entropy,average abundance.And the influence degree of each pore microscopic parameter on the stiffness degradation index of undisturbed loess is greater.