Evolution Law Of Cracks&Slope Stability Analysis Of Expansive Soil

Expansive soil is a kind of special clays featured with the swell-shrinking characteristic and crack character.The effect of cracks on the unsaturated expansive soil in aspect of seepage and strehgth has attracted much interest.It is well recognized that cracks can induce slope instability.One of the main effects of cracks is that they destroy the soil structural integrity and then provide preferential pathways for water flow,which significantly increases the hydraulic conductivity of the soil mass and,in turn,pore-water pressures in the soil mass.As a result,the shear strength of the soil decreases.Complexity of cracks development progress and limited observation methods prevented research progress in the evolution law of cracks.Related theoretical research results are relatively scarce.With the booming economy and progressively development of urbanization,the hazards and damages of expansive soils to the geotechnical projects like highways and slope stability become increasingly prominent.Focused on the crack characteristic,the expansive soil is analyzed through laboratory test and numerical simulation.Experimental researches are carried out on crack observation,quantificational description of cracks,seepage characteristic of crack networks and strength characteristic of cracked soil.Based on the test result,a method to forecast the SWCC and permeability function for cracks considering crack volume changes during drying-wetting processes are developed.The SWCC and permeability function are applied to a finite element analysis of an unsaturated cracked expansive soil slope during infiltration with a crack-pore dual medium model.The main work & conclusions are as follow:(1)Cracks developing laboratory tests were carried out.Taking the expansive soil of Nanjing as the research object,laboratory model test for cyclic rainfall-evaporation was carried out for the produce of cracks in the expansive soil sample.Digital camera connected with computer was used for the real-time tracking photography to collect the dynamic development progress of cracks.(2)The image processing technique based on the MATLAB software was employed to quantitatively extract the cracks feature parameters,such as crack ratio,average aperture and fractal dimension.According to the result,the relationship between the crack morphology feature parameters and the water content was used to reveal the crack evolution law.(3)Different types of strength tests were carried out for recognizing the effect of drying-wetting cycles on the expansive soil.A series of direct shear tests and unsaturated triaxial tests have been carried out for the expansive soil sample after repeated drying-wetting cycles.The smaller-sized direct shear test soil samples have obvious cracks.The direct shear test results showed relationship between the shear strength parameters with the crack fractal dimension D and the number of drying-wetting cycles.The unsaturated triaxial test results revealed the drying-wetting cycle effect on the SWCC,shrinkage behaviors,and mechanical properties of expansive soil.The decrease of the soil shear strength is a macroscopic reflection of the change in the soil microstructure.(4)A new method to predict the SWCC is proposed.The fractal theory is used to describe the SWCC and permeability function of cracks and soil.Then,the SWCC and permeability function for cracked soil at different states along a crack development path can be obtained considering crack volume changes.(5)Crack-pore dual porosity medium seepage model was presented.The cracked expansive soil is viewed as an overlapping continuum of a crack network system and a soil matrix system.One kind of crack-pore dual porosity medium model for the dual-porosity media,in which the different hydraulic characters can be considered,is established.A rainfall infiltration in a simple cracked porous expansive soil slope is simulated as an illustration.The results show that the wetting front in the crack distribution area drops much faster than that in the soil area without crack.That leads to the rapid attenuation of suction in the closed area formed by fracture,and the affects the stability of the slope obviously.(6)The effect of some parameters,such as crack depth,crack saturated permeability coefficient,soil fractal dimension,and rainfall intensity,was revealed.The results show that the crack depth and the crack saturated permeability coefficient have a significant impact on the pore water distribution and stability of slope.The soil fractal dimension,soil saturated permeability coefficient and air entry value are used to describe the pore size distribution quantitatively,and they are particularly associated with the stability of unsaturated expansive soil slope.The MMF model can be used to predict the effect of rainfall intensity on the slope stability.