Residual Strength Behavior Of Slip Soils Under Different Moisture Contents And Shearing Rates

Slip soil is formed by the birth of landslide and its engineering geological properties are crucial to the deformation and stability of landslide.Therefore,the research on the properties of slip soil is always a hot topic in the field of landslide.But many core issues are not fully understood,especially the identification,origin analysis of slip soil and response law of shear strength to the critical factors such as moisture contents,shearing rates and so on.Accordingly,it is the urgent need to do the fundamental research on slip soil in order to lay a solid foundation for the early identification,stability evaluation and prediction of landslide.In view of regional disaster statistics and principles of geology,Badong formation is a kind of typical slippery strata.Generally,the slip soils in Badong formation used to be the mudstone-type interlayers.So the geological process of slip soils can be revealed by studying the geological evolution of mudstone-type interlayers.As a breakthrough point,the reversal direct shear tests before and after the microcosmic tests are carried out to study the material structure and the mechanical regularity in the evolution process of mudstone-type interlayers,dominated by the groundwater.Based on the theory of disorder degree,an index is used to evaluate the disorder of material structure.Then the torsional ring shear tests are carried out to study the residual strength behavior of slip soils influenced by the critical factors.On this basis,the influences of critical factors on the particle migration and particle directional alignment of shear surfaces are observed by the contrast microcosmic tests.It reveals the mechanism of rate effects on residual strength of slip soils,and the mathematic models which give expression to the rate effects under different initial states are established.At last,taking the typical landslide as an example,the study of deformation and failure process of the landslide is carried out by utilizing numerical simulation technology,with a major consideration of the residual strength behavior of slip soils.The main research contents and achievement as follows:(1)The mudstone-type interlayers of Badong formation are influenced by the exogenic geological processes(including physical and chemical weathering).Their material structure characteristics and engineering geological properties are close to slip soils.Based on the simplification of the natural geological factors,the evolution characteristics of the mudstone-type interlayers are tested with the effects of water.The results show that a part of detrital minerals change into the clay minerals and the content of small size particles increases.Meanwhile,the shear strength indexes of mudstone-type interlayers decrease generally.The peak cohesion,residual cohesion and peak internal friction angle decrease significantly,but the residual internal friction angle decreases slightly.(2)To quantitatively evaluate the material structure(mineral composition and grain composition)variation in the evolution process of mudstone-type interlayers dominated by the groundwater,the index called disorder degree is put forward.With the increasing saturation time,the disorder degree of mineral composition decreases rapidly first and then slowly,whereas the disorder degree of grain composition decreases slowly first and then rapidly.In a summary,the gap of material structure between the mudstone-type interlayers and the slip soils decreases along with the geological evolution of mudstone-type interlayers.(3)The equivalent internal friction angle of slip soils decreases with the increasing moisture content and changes with the variation of consolidation degree.The equivalent internal friction angle with a low shearing rate increases with the increasing consolidation degree,whereas the equivalent internal friction angle with a high shearing rate first increases then decreases with the increasing consolidation degree.The residual strength of slip soils under different initial states displays different types and violent degree of response to the shearing rate.The equivalent internal friction angle of consolidated soils is positively dependent on the shearing rate,but the equivalent internal friction angle of unconsolidated soils is negatively dependent on the shearing rate.(4)To quantitatively evaluate the particle migration of shear surface,the particle migration index M is put forward.In the vertical direction,M increases linearly with the increasing moisture content,whereas decreases linearly with the increasing consolidation degree.Meanwhile,M increases logarithmically with the increasing shearing rate.The increases of moisture content and shearing rate are beneficial to the particle directional alignment of shear surface,but the increase of consolidation degree is not.The above factors govern the particle directional alignment by affecting the physical properties of soils,such as the fine particle content and bound water thickness.(5)The equation which quantitatively assesses three types of rate effects is presented as ?_e=A ln(v)+B,where the coefficients A and B are indexes with certain physical significances.The residual strength of slip soils can display different types and violent degree of rate effects,accompanying the coefficient A.When A is above zero,the residual strength of slip soils shows a positive rate effect.When A is zero,the residual strength of slip soils shows a neutral rate effect.When A is under zero,the residual strength of slip soils shows a negative rate effect.It reveals that the major causes of rate effects on residual strength of slip soils are the variations of fine particle content and bound water thickness.(6)The initial states of soils have a great influence on the rate effect indexes.With the increasing plasticity index of soils,the absolute value of A increases logarithmically,but B decreases logarithmically.On this basis,the mathematic models which give expression to the rate effects under different initial states are established.Furthermore,the statistical analysis clarifies the relationship between the types of rate effects and the deformation and failure process of landslides.(7)The residual strength behavior of slip soils is one of intrinsic factors which govern the deformation and failure process of landslides.Because of the rain infiltration,the heterogeneity of sliding body and sliding zone is more obvious,which provides a foundation for mobilizing the mechanism of residual strength of slip soils and governing the deformation and failure process of landslides.According to different types of rate effects,the residual strength of slip soils has a “boosting effect” or “braking effect” on landslides.The major innovations of the present research include:(1)The mudstone-type interlayers of Badong formation are influenced by the exogenic geological processes(including physical and chemical weathering).Their material structure characteristics and engineering geological properties are close to slip soils.Based on the simplification of the natural geological factors,the evolution characteristics of the mudstone-type interlayers are tested with the effects of water.The results show that with the increasing saturation time,the gap of material structure between the mudstone-type interlayers and the slip soils decreases along with the geological evolution of mudstone-type interlayers.The index called disorder degree is put forward to quantitatively evaluate the material structure(mineral composition and grain composition)variation in the evolution process of mudstone-type interlayers dominated by the groundwater.The disorder degree has a strong correlation with the shear strength index,which may provide the basis for the prediction of shear strength.(2)The equation which quantitatively assesses three types of rate effects is presented as ?_e=A ln(v)+B,where the coefficients A and B are indexes with certain physical significances.The coefficients have a logarithmic relationship with the plasticity index of soils.The residual strength of slip soils can display different types and violent degree of rate effects,accompanying the coefficient A.When A is above zero,the residual strength of slip soils shows a positive rate effect.When A is zero,the residual strength of slip soils shows a neutral rate effect.When A is under zero,the residual strength of slip soils shows a negative rate effect.It reveals that the major causes of rate effects on residual strength of slip soils are the variations of fine particle content and bound water thickness.Furthermore,the mathematic models which give expression to the rate effects under different initial states are established with the aim of enriching the theory of shear strength of slip soil.