Static And Dynamic Characteristics Of Cement-Clay Around Metro Tunnel Under Freezing And Thawing Condition

Under extremely complicated engineering geological and hydrogeological conditions,the combined method of cement soil reinforcement and artificial freezing is more and more widely used in tunnel shield origin,receiving end construction and rescue of operated subway tunnels.There is little comparative study on the static and dynamic characteristics of clay before and after cement improvement around the tunnel.Therefore,through the indoor static triaxial test,dynamic triaxial test,SEM test and MIP test,this paper compares the static and dynamic char-acteristics of clay before and after cement modification around the subway tunnel and the var-iation of microscopic pore structure parameters.The main research results are as follows:?1?The static triaxial test focuses on the effects of mixing ratio,confining pressure and freezing temperature on the static characteristics of plain soil and cement soil.The test results show that the mixing ratio is the main factor determining the type,strength and stiffness of the soil.When the confining pressure is 170 kPa and the freezing temperature is-30°C,the stress-strain curve of the soil and the low-content cement soil(M_br?5%)shows a significant strain-hardening type,which is characterized by plastic deformation.While the stress-strain curve of the 10%and 15%cement soil samples shows a significant peak with the increase of shear strain.After the peak,the intensity is significantly reduced.And it is strain-softening type,which is characterized by brittle failure.The type of failure of the 7%sample is between brittle failure and shape failure.The static strength of the soil increases with the decrease of the mixing ratio,the confining pressure and the freezing temperature under the action of freezing and thawing.Under the action of freezing and thawing,for samples with a confining pressure of 170 kPa,the final failure strength values of 3%,5%,7%and 10%cement soil samples increased by 0.41times,1.05 times,2.03 times and 2.26 times,respectively.The static triaxial test also obtained the shear strength index of cement soil with plain soil and different mixing ratios.In the initial stage of loading,the soil and low-content cement soil(M_br?5%)showed plastic strain with the increase of axial strain.And the secant modulus decreased gradually,which showed stiffness softening.However,as the axial strain continues to increase,the curve gradually becomes gen-tle and the attenuation phenomenon is less obvious.For cemented soil with 7%and 10%blend-ing,the secant modulus increases with axial strain at the initial stage of loading,showing stiff-ness hardening.However,when the strain reaches a certain value,the secant modulus decreases as the axial strain increases.?2?In the dynamic triaxial test,the critical dynamic stress exists in the soil and different mixing ratio cement soil.When the dynamic stress is less than the critical dynamic stress,the cumulative plastic strain tends to be stable with the increase of the vibration frequency.When the critical dynamic stress is greater than or equal to the cumulative dynamic stress,the cumu-lative plastic strain increases rapidly and the sample quickly breaks.Under the action of vibra-tion load,the cumulative plastic strain with the mixing ratio of 7%cement soil quickly reached a stable value,indicating that it has a good reinforcement effect.The effective average principal stress and dynamic stress amplitude are two indispensable factors for the development of cu-mulative plastic strain in soil.When the dynamic stress amplitude is the same,the cumulative plastic strain of the soil will decrease with the increase of the effective average principal stress.And when the effective average principal stress is the same,the accumulation of the soil will increase with the increase of the dynamic stress amplitude.In order to comprehensively con-sider the above two factors,CSR is defined to measure their influence on the development of cumulative plastic strain in the soil,that is,the plastic strain of the soil gradually accumulates as the CSR value increases.The parameters were fitted to the experimental results based on the Monismith calculation model.The secant modulus of the sample increases with the increase of the blending ratio.For the stiffness development law of the specimen,it can be divided into three stages:when the cumulative strain is small,the curve appears in a fast rising phase,and after reaching the peak value,With the increase of axial strain,the secant modulus decreases rapidly,and the final decline tends to be slow.And the stiffness softening curve can be used for fitting.The dynamic stress-dynamic strain curves of plain soil and cement soil under vibration load have obvious nonlinear characteristics under freezing and thawing.Hardin-Drnevich's hy-perbolic model fits the dynamic stress-dynamic strain curve with significant effects.?3?In the SEM image,the structural unit body of the plain soil contacts more in the form of edge-plane or edge-edge,forming an overhead structure with a high porosity.And a small number of granular bodies are relatively independent and suspended in the cross-section of the sample.The fibrous or acicular gel silicate hydrate formed by the hydration of the cement wraps the structural unit of the clay and fills the pores of the clay to form agglomerates and a network of cement.The smaller the effective average principal stress,the larger the dynamic stress am-plitude,the more compact the soil structural unit bodies are,and the pores between the particles are gradually compressed,which is equivalent to the“adjustment”and“reorganization”of the entire microstructure.The shape factor of the pores of the sample tends to decrease as the pore diameter increases,indicating that the larger the pore diameter of the sample,the narrower the pore shape.The pore sizes of plain soil and cement soil samples are mainly distributed in 0¹.The larger the cyclic stress ratio CSR is,the more concentrated the pore size of the sample pores are.The pore area cumulative distribution curve is steeper in the pore area less than 0.5,indi-cating that the pores of plain soil and cement soil are dominated by tiny pores.For the plain soil sample,as the cyclic stress ratio increases,the curve slopes to the lower left,indicating that the proportion of tiny pores increases.The pore structure of the sample has obvious fractal charac-teristics.And the obtained fractal dimension is between 1.25 and 1.35.The fractal dimension of the pores of the plain soil and cement soil sample shows a good quadratic function relation-ship with the cyclic stress ratio CSR and the cumulative shaping strain.For the plain soil sam-ples,the fractal dimension D of the pores decreases first and then increases with the increase of the cyclic stress ratio CSR,while the pore fractal dimension of the cement soil sample gradually increases with the increase of the cyclic stress ratio CSR.The fractal dimension of the pores of the plain soil and cement soil sample increases with the increase of the cumulative shaping strain.?4?In the MIP test,there is a threshold for the mercury intrusion in the soil during the attack.When the mercury pressure is less than the threshold,the mercury inflow increases slowly.And when the mercury pressure reaches the threshold,as the mercury inlet pressure increases,the pores of the sample are quickly filled with mercury.The pore radius of the soil is in one-to-one correspondence with the mercury intrusion pressure.As the pressure of the mer-cury intrusion increases,the pore size of the mercury in the sample gradually decreases.The pore size of the sample is mainly distributed between 10 nm and 1000 nm.Under different effective average principal stresses,the pore volume percentage curve of plain soil and cement soil larger than a certain pore diameter has little difference.The growth rate of pore percentage curve of plain soil sample decreases with the increase of dynamic stress amplitude.Under dif-ferent effective average principal stresses,the cumulative specific surface area of the pores of plain soil and cement soil does not change much.Under different dynamic stress amplitudes,the larger the dynamic stress amplitude,the higher the final value of the cumulative specific surface area of the plain soil sample.And the final value of the cumulative specific surface area of the cement soil sample pores is not much different.For plain soil,the larger the dynamic stress amplitude is,the smaller the pore size of the soil pores is.The pore distribution curve of the cement soil sample under different dynamic stress amplitudes is approximately coincident,indicating that the cement soil reinforcement effect is significant.Under some cyclic stress loads,the pore characteristics of the sample are not obvious,so the curves overlap.The fractal geomechanical fractal model of plain soil and cement soil has uniform fractal characteristics,and the obtained fractal dimension is between 2.5 and 2.9.