THE MICRO-MECHANISM OF STRENGTH CHANGE OF BRECCIA SOIL IN OUTWASH DEPOSIT UNDER RAINFALL AT HUANXIPO

The micro-mechanism of strength change of breccia soil in outwash deposit at Huanxipo under rainfall is examined using field shear test, X-ray powder diffraction, scanning electron microscope and soil-water chemical effect test. Rainfall has a significant effect on the shear strength of breccia soil in Huanxipo outwash deposit. When it is saturated, the cohesion and internal friction angle are 64.3% and 22.5%lower than those at natural water content. After soaked by the rain, the microstructure of soil become loose and its cohesion decreases. The micro-mechanism of strength attenuation can be summarized as the swelling of clay minerals, the dissolution of calcium-magnesium cement, the ion's exchange and adsorption and the microscopic action of the breccia and water. Their mean performances are as follow. First, the clay minerals swell after absorbing water, causing its intergranular pores to increase and its form to turn from flocculent to layered or dispersed. Then, the dissolution of calcium-magnesium cement causes a significant increase in the concentration of calcium and magnesium ions in the solution. Then, ion exchange leads to an increase of sodium ion concentration, the thickening of diffusion layer in clay particle surface and the reduction of intergranular gravitational force. Lastly, the micro-physicochemical action of the breccia and water leads to a decrease in the interfacial contact surface roughness.