Characteristic Strength Of Granite Residual Soil Under Acidic-dry-wet Cycle Conditions

It is considered that the stability of soil structure of granite residual soil slopes subjected to acid rainfall will be degraded by acidity and dry-wet cycles.However,at present,the main research objects that comprehensively consider the dual degradation effects of acidity and dry-wet cycles are concentrated in the field of rock,and there are few related reports on soil.In order to grasp the influence law of acid dry-wet cycle on the strength and strength parameters of granite residual soil,and clarify the microscopic mechanism of the change.In this paper,a certain granite residual soil slope in Xiangxiang City is taken as the research object.In order to solve the above problems,the strength and strength parameters of granite residual soil under saturated acid drywet cycles were studied by repeated drained direct shear test of soils and direct shear creep test in laboratory.Combined with SEM analysis,the microstructure evolution of granite residual soil under the action of acid dry-wet cycle was revealed,and the relationship between macro-and micro-parameters was established.Based on the related theories of physiochemical reaction between water and soil,the microcomponent and structural changes of granite residual soil under the dual action of acidwet and long and short time effects are discussed.The main research conclusions are as follows:(1)Repeated direct shear tests were carried out on the granite residual soil subjected to different acidic dry-wet cycles.The test results show that the shear stress curve of the granite residual soil is a strain hardening type,and the peak and residual strength under dry-wet cycles and acidity all showed a decaying trend,but the peak strength of the normal stress of 300 k Pa showed an increase under the action of acid.By analyzing the attenuation law,it is found that the attenuation trend of strength parameters tends to be stable after two cycles of wetting and drying.With the increase of acidity,combined with the test and fitting results,it is speculated that there is a critical value of change,the residual cohesion changes from increase to decrease at p H=5,and the residual internal friction angle changes from decrease to increase at p H=4.Strong acidity further deteriorates the structure.Through response surface comprehensive analysis,it is concluded that the variation of peak value and residual strength parameters are more sensitive to acidity and dry-wet cycle respectively,and it is preliminarily clear that the unfavorable natural conditions for initial instability and revival of granite residual soil slope are considered comprehensively under acidic drywet cycle.(2)Through the direct shear creep test,the creep characteristics,long-term strength and strength parameters of granite residual soil subjected to acid dry-wet cycle were obtained.The results show that the acid dry-wet cycle reduces the creep properties under low normal stress and increases the creep properties under high normal stress.Long-term strength is attenuated by acidity and dry-wet cycles.The long-term strength parameters were greatly attenuated when the dry-wet cycle was applied twice,and then the changes gradually became gentle.With the increase of acidity,it is speculated that there is a critical value of change based on the test and fitting results.The long-term cohesion decreases slowly when p H=6,and the long-term internal friction Angle increases from decrease to increase when p H=5.Through response surface comprehensive analysis,it is concluded that the changes of long-term cohesion and internal friction Angle are more sensitive to the dry-wet cycle and acid cycle respectively,and the unfavorable conditions of long-term slow movement and instability failure of granite residual soil slope are preliminarily determined considering comprehensively the dry-wet cycle.(3)Through comparative analysis of characteristic strength and strength parameters in different states,the peak strength > residual strength > long-term strength was obtained.Compared with the deterioration critical value of the residual and longterm state,the acidity corresponding to the critical point in the long-term state is weaker,and the decrease in strength indicates that there are more penetrative cracks in the creep process to promote the decay of creep characteristics.The dry-wet cycle always attenuates the cohesion in each state,and the acidic effect attenuates the cohesion of the non-slip soil,and the residual cohesion fluctuates in the sliding soil.Under the longterm effect of dry-wet cycle,the internal friction angle continues to decay.(4)Using the SEM microscopic test,the microscopic parameters in the residual state and the creep state are extracted by image processing.By comparison,it is found that the particle orientation concentration in the residual state is stronger than that in the creep state.Dry-wet cycles and acidity increase the microporosity.Both the acid effect and the dry-wet cycle will make the agglomerates decompose in large quantities.With the strengthening of the acidity,the particles are gradually eroded,and the interface of the hydrolyzed and broken particles is rough.Multiple dry-wet cycles and long-term acidity will make the particles contact smooth.Based on the microscopic parameters,the most relevant microscopic parameters with the macroscopic parameters were explored and the microscopic expressions of the macroscopic parameters were established.