Study On Mechanical Properties And Constitutive Modeling Of Saturated-unsaturated Completely Decomposed Granitic Soils

Granite residual soil is common in Guangdong,Fujian and other places in China,and is often encountered in engineering construction.Since the construction process of the various characteristics of the residual soil inadequate understanding,leading to the construction deviation,bearing design errors,lack of preparation work.Led to landslides,collapse pit,uneven settlement and other issues,resulting in greater economic losses and even casualties.Therefore,this article uses weathered granite residual soil in Deqing,Guangdong Province as the test material to test the main characteristics of soil samples under saturated-unsaturated conditions.Provide a certain reference for the actual project construction.After completing the water content test,particle size analysis test and the mineral composition analysis test.The results show that the granite residual soil in the Zhaoqing area of Guangzhou in the natural state has a water content of 21.39%.It shows that the main mineral components of the residual soil composition are kaolinite and quartz.After measuring the particle size distribution of the soil sample through the particle size analysis test,it can be classified as sandy cohesive soil.The results of the dry-wet cycle test of unsaturated soil show that the volumetric water content of the undisturbed soil increases with the matric suction during the dehumidification process.SWCC presents a typical three-stage distribution,namely boundary effect zone,transition zone and residual zone.The air intake value is 40 k Pa,and the residual water content is 6.12%.During the dry-wet cycle,the characteristic curve of the residual soil hysteresis is obvious;After the dehumidification and humidification process,when the sample matric suction is back to 0 k Pa,the volumetric water content drops significantly compared to the initial state.Based on the relevant fitting parameters of the soil-water characteristic curve,the fitting of the relative permeability coefficient of residual soil is completed.It is found that the change trend of the relative permeability coefficient of the soil sample is highly consistent regardless of the dehumidification process or the humidification process.That is,the size of the matric suction of the soil sample,the level of water content,or whether the pores inside the soil sample are connected,has a considerable impact on permeability.When the water content in the soil sample is high,the permeability is good,and when the water content is reduced,the permeability will also decrease rapidly;By controlling the triaxial tests and matric suction net confining pressure,it is not difficult to find that the following conclusion.Whether it is saturated soil or unsaturated soil,the shear strength is growing with the confining pressure.And whether it is in saturated or unsaturated state,its partial stress-strain curve shows strain softening type.Combining the test results of 9 samples,it can be known that the internal friction angle of the residual soil is 27.5°.The cohesion of the sample increases linearly with the increase of the matric suction.When the matric suction force is 0kpa,the cohesive force is 21.23 k Pa,and the correlation growth coefficient of the suction force and cohesive force is 0.4038;Combining the results of the dry-wet cycle and the triaxial test of unsaturated soil under controlled net stress,the parameters of the BBM model are obtained.Compression coefficientl（0）and rebound coefficientkof saturated soil,early consolidation pressure p₀^*and reference stress p^c of soil,related parameters of soil stiffness and suction growthbandg,compression coefficient l_s and rebound coefficient k_s when suction is loaded,early maximum suction s₀,critical state slope M.It also demonstrates the uniformity of the BBM model for saturated-unsaturated soil.