Experimental Study On Expansive Soil Residual Strength

Expansive soil is a kind of high plastic clay which is easy to water-swelling to soften and dehydration-shrinking,whose shear strength is an important parameter to calculate slope stability of oblique cuttings,embankments,channels,earth dams,and so on.However,due to its characteristics of swell-shrink,fissure and over-consolidated,it is more complex than general clay’s mechanical properties,and brings great harm to the engineering.Evolution of expansive soil slope deformation and failure is the decaying process of the shear strength of the slope soil from the peak intensity to the residual value.Therefore,expansive soil slope stability analysis should be combined with practical engineering problems considering the selection of soil strength indicators.In this thesis,having a series of research about strength characteristics and attenuation rule of expansive soil of C ixian County Hebei of South-to-North Water Diversion as the research object by electric strain controlled direct shear apparatus,direct shear / residual shear apparatus,triaxial tests,"pre-section" triaxial tests and numerical simulation software.The main contents and results are as follows:(1)The research of shear strength characteristics of expansive soil of different clay contents in the areas under different stress and different states are made by using electric strain controlled direct shear apparatus.The results showed that: In the direct shear test the shear strength of expansive soil significantly reduced after the soil was saturated,and increased after consolidated.And the effect of saturation on the strength of expansive soil which has larger clay content is more significant,and that the "cure" effect of consolidation on the strength of weak expansive soil has lesser cla y content is more significant;With the increase of clay content,the cohesion gradually decrease;internal friction angle first decreases and then increases with the increase of clay content,and the critical value is about 32%.(2)The residual strength characteristic of expansive soil under a slow shear rate is studied by using direct shear / residual shear test system.The results show that: the shear strength of expansive soil sample after the peak strength significantly increases with the increase of clay content;Residual strength ηr with the increase of clay content decreases exponentially;While residual strength’s internal friction angle θ_r is logarithmic with clay content;cohesion c_r is irregular.Clay content mainly by affecting the residual strength θ_r value to influence the shear strength of soil.The peak strength and residual strength of expansive soil are improved to some extent under a large vertical pressure,but it is ineffective for the clay of larger clay content.(3)In order to study expansive soil residual strength characteristics under complex stress condition,this section proposes triaxial test and the "pre-section" triaxial tests about expansive soil,and analyzed and compared with direct shear / residual shear test results,it is found that residual strength measured by the triaxial specimen with(45°+ θ / 2)pre-found section inclination is closest to repeate direct shear test results."Pre-section" triaxial test can better simulate the stress state,is reasonable and feasible.Finally,establish a complete strain softening curve of expansive soil by integrating triaxial test results,provide certain theoretical value and practical value for revealing the soil strength characteristics.(4)Consider the effect of atmosphere influence layer of expansive soil slope,research and analysis the influence of material strength parameters selected on the results in the process of slope stability calculations at the same time.It finds that the safety factor of expansive soil slope decrease with the decline of strength index of atmosphere influence layer,the slope gradually tend to be unstable state;The expansive soil slope safety factor decreases with the increase of the atmosphere influence layer depth.The shear failure of expansive soil slope general y occurs in the range of atmosphere influence layer.