A multi-scale study on structure damage of granite residual soil under wetting-drying environments

Abstract

To study the environmental effect with hot and rainy feature to the structural characteristics of granite residual soil under multiple dry-wet cycles，the triaxial consolidation undrained(CU)，computer tomography(CT)，nuclear magnetic resonance(NMR) and scanning electron microscope(SEM) tests were performed. Then，a multi-scale study from microscopic，mesoscopic，and macroscopic perspective about the damage mechanism of the soil. The test results show that the wetting-drying effect shows a change in particle and pore structure at microscopic scale，an expansion of fractures at mesoscopic scale，and a decrease in strength at macroscopic scale. The effective cohesion decreases exponentially with the increase of the wetting-drying cycles and the effective internal friction angle decays in a small range. The development process of meso-fractures can be divided into three phrases，which are the germination stage(0 to 1 cycle)，the development stage(2 to 5 cycles)，and the stable stage(5 to 8 cycles). The pore volume distribution curves of granite residual soil show a bimodal shape. The pores can be divided into four types according to pore size. With the increasing number of wetting-drying cycles，the volumetric proportion of microscopic pores decreased，while that of medium pores and large pores increased drastically. During the wetting-drying process，the clay particles gradually loose and the intergranular pores expand and connect to form connected cracks. Due to the wetting-drying effect，the hydrophilic clay minerals expand during a humidifying process，the microscopic tensile stress increases during a drying process，and the cemented materials decompose and lose. The three factors jointly drive the expansion and connectivity of fractures and cause fatigue damage to the soil structure，leading to the attenuation of mechanical properties of soil. This study provides a useful reference for the understanding of mechanical properties and structural damage rules of granite residual soil in hot and rainy climate.