Effects Of Particle Size Distribution On Shear Strength Of Weathered Granitic Rocks And Soils

Collapsing gully is a unique soil erosion form in granite red soil hilly area of South China.Collapsing gully occurs on the weathering crust of granite,and the weathering crust soil has significant vertical differentiation,which directly leads to the non-uniformity of the shear strength in the vertical direction,and is also an important factor for the collapse and instability of the weathering crust.Among all properties of weathered granitic soil,the vertical differentiation of particle composition on the weathered crust is considered of the most important factors affecting the strength characteristics of soil.We therefore selected a typical collapsing gully in Tongcheng,Hubei Province,to collected the weathering crust layer A and C granite weathered soils.Through field survey and in-situ shear experiment,the relationship between the particle composition and shear strength of weathered granitic soil were investigated.According to the parameter range obtained in the previous studies,we arranged the remolded samples with different particle composition characteristics to explore the influence of particle composition characteristics on the characteristic parameters of shear strength and stress-strain characteristics of weathered granitic soil by using direct shear tests under saturated conditions were carried out to.The empirical model of the relationship between shear strength and various influencing factors can provide quantitative support and theoretical basis for the comprehensive discussion of particle composition on the strength characteristics of granite weathering crust,collapse instability and the estimation of strength of granite weathering rock and soil mass,and provide scientific basis for the treatment of collapsing gully.The conclusions are as follows:(1)Through in-situ survey and test,the results showed that the clay content of weathered granitic soil was 5%-40%,the silt content and gravel content were 20%-40%and 0%-20%respectively,and the sand content was the largest,20%-70%.In the same study site,the soil particle composition of different occurrence layers(A,B,BC,C)showed significant differences;but there was no significant difference between the same occurrence layers.There were five types of weathered granitic soil in the study area:clay,clay loam,loam,sandy loam and loamy sand.Its in-situ shear strength was under 70 k Pa,and has a significant positive correlation with clay content(R~2=0.72,p<0.05),a significant negative correlation with sand content(R~2=0.74,p<0.05),and a certain positive correlation with gravel content.(2)Through the simulated shear test of the remolded granite weathered soil samples,the strain characteristics of the remolded samples were all stress hardening.For weathered granitic soil without gravel,the shear strength of remolded samples increased first and then decreased with the clay content increased.When the dry density was small(1.25-1.35 g/cm~3),the shear strength reaches the maximum value when the clay content was 26.28%;when the dry density was large(1.40-1.45 g/cm~3),the shear strength reached the maximum value when the clay content was 34.75%.Soil cohesion varied in a coordinated way with shear strength,both shown the positive relationships with clay content,while the internal friction angle decreased with clay content increased.The results of variance analysis showed that the dry density,clay content and their interaction had a significant effect on the shear strength index of weathered granitic soil.On the whole,the effects of dry density and clay content on shear strength index was less than that of single factor,and the most significant effect was when the vertical pressure was256 k Pa.From a single factor point of view,dry density and clay content had a greater impact on the internal friction angle and cohesion respectively,while when they worked together,the effect on cohesion and internal friction angle was similar.(3)Importantly,for the weathered granitic soil with gravel,the influence of gravel on different texture soil was different.With respect to clay,with the increasing of gravel content,the shear strength increased first and then decreased,and the maximum value was obtained when the gravel content was 21%.The change trend of cohesion was consistent with the shear strength,and the internal friction angle was continuously increasing.For sandy loam,with the increase of gravel content,the cohesion decreased,but the shear strength and internal friction angle continued to increase,and the growth rate decreased when the gravel content exceeded 21%.The results of variance analysis showed that texture type,gravel content and their interaction had a significant impact on the shear strength index of granite weathered soil.On the whole,the joint effect of texture type and gravel content on the shear strength index of soil was less than that of a single factor,and the influence of each factor on the shear strength showed a more significant trend with the increase of vertical pressure.Texture type,gravel content and their interaction all had great influence on internal friction angle.(4)For the influencing factors of shear strength of weathered granitic soil,binomial function could better describe the relationship between grading parameters and shear strength index.With the increasing of the nonuniform coefficient,the shear strength increased first and then decreased,and the maximum value of the nonuniform coefficient was between 600 and 800,and the change trend of cohesion was consistent with it.With the increase of the curvature coefficient,the cohesion decreased first and then increased,while the internal friction angle was the opposite.The minimum and maximum values of the cohesion were obtained when the curvature coefficient was between 1 and 2.The coefficient of non-uniformity increased from 2.63 to 2.86,the degree of soil irregularity increased,the angle of internal friction continued to decreased,the shear strength and cohesion showed a trend of increasing first and then decreasing.(5)Through the multiple linear regression analysis of the shear strength index and its influencing factors of the weathered granitic soil,we found that for the weathered granitic soil without gravel,the primary and secondary order of the influence of each factor on the remolded granite soil was:dry density>curvature coefficient>clay content.For the gravelly granite weathered soil,the order of influence of each factor on the remolded sample was curvature coefficient>gravel content>nonuniformity coefficient.The shear strength of weathered granitic soil was fitted by nonlinear method,and the result was good.The model can be used to predict the shear strength of weathered granitic soil under different vertical pressures.