Experimentle Study On The Strength And Deformation Characteristics Of Unsaturated Expansive Soil

In our country, about a quarter of people live in the area where expansive soil widely distributed in. The special destruction which makes our country losses more than$15billion in economic every year frequently and repeatedly happens to the construction and maintaining of surface light project(For example, Highway, Railway, Housing, Water Resources and Hydropower Engineering, etc) because the character of expansive soil is repeatedly expansive-shrinkage, fissured, and over-consolidated. The shear strength indexes are the most important mechanical parameters in the studying on the destruction but the strength characteristics of unsaturated expansive soil is much more complex than general cohesive soil. Although the other scholars have done a lot of research on the strength characteristics of expansive soil, the mainly achievement were focused on how initial water content and compactedness affect the shear strength and nearly no scholar studied on how the expansive potential affect the shear strength of expansive soil.(1) The initial conditions(water content, confining press, expansive potential) has significant influence on stress-strain relation curves and maximum principal stress difference of expansive soils:the maximum principal stress difference was negatively related to the water content and positively related to the confining press and expansive potential; Under the condition of low confining pressure and low water content, expansive potential just influenced the high-low location of stress-strain relation curves, but not the form, there were peak values on the softening stress-strain relation curves and brittle failure occurred; With the increase of confining press and water content, expansive potential not only influenced the high-low location of stress-strain relation curves, but also the form, the stress-strain relation curves changed from softening to sclerotic type with the decrease of expansive potential and plastic failure occurred; When the confining press and water content increased to a certain value, the influence of expansive potential on stress-strain relation curves was the same with the influence of expansive potential when the confining press was low.(2) The shear strength of unsaturated expansive soil was closely related with the water content, expansive potential and drying-watering cycles:the shear strength, cohesive force and the angle of friction decreased when the water content increased, and index relationship exists between cohesive force and water content, linear relationship exists between angle of friction and water content; The shear strength was positively related with the expansive potential; The shear strength decreased when the drying-watering cycles increased, in addition, the reduction of shear strength is big after one time of drying-watering cycle and the reduction is small after two or three times of drying-watering cycle, then the shear strength had the trend to be stable.(3) The transformation of the top district was lower, the bottom district was higher, and the middle district was closed to the integral transformation.(4) The triaxial samples under low water content and low confining press easily emerged shear band(a single shear band and multiple shear bands whose location and form were no rules to follow) in consolidation-undrained triaxial shear test and the form of stress-strain relation curves were softening.(5) Using the pictures taked by the digital image measurement system can track of the growing degree of the shear band in real-time and confirming the formation and development process easily based on the axial strain-time relation curves of each district of triaxial sample and the axial strain cloud pictures of triaxial soil samples at different moment. And in the process, the growth rate of soil which was close by the shear band developed from little to big, the triaxial strain of soil which was distant from the shear band increased slowly and then decreased.