Multi-scale effects of mechanical property degradation of expansive soils under drying-wetting environments

To explore the multi-scale effects of mechanical properties and structural damage of expansive soils under the action of dry-wet cycles, the remolded expansive soil specimens were subjected to 0-6 dry-wet cycles with recording the cracking process and then, the triaxial strength(CU) test, nuclear magnetic resonance(NMR) test and scanning electron microscope(SEM) test were carried out separately. Furthermore, the macroscope mechanical behavior and micro- and meso-structure damage of the soil were studied. The results indicate that, due to the dry-wet action, the stress-strain curve softens significantly and the shear strength deteriorates. Specifically, the cohesion decreases sharply during the first three dry-wet cycles and tends to be stable after four cycles, while the internal friction angle changes little. Under the action of dry-wet cycles, soil cracks experience the initiation stage, the propagation stage and the equilibrium stage. Both the average crack rate and the average crack width increase at first and then maintain stable. Besides, the average crack width converges slightly at the end of the stable phase. The micro-structure of the soil is significantly affected by the dry-wet cycles, mainly illustrated by the increased total volume of pores and the continuous coarsening and penetration of micropores to form mesopores or macropores. The structure of the soil is broken and loose, and the boundary between particles and pores is blurred. The particle abundance slightly increases, but at a whole in a state of balanced mutual feed. The particle orientation frequency presenting the global isotropic feature has a local optimal orientation. The particle size distribution is characterized by slightly reduced clay particles, moderately increased powder particles and relatively stable sand particles. Based on the above results, the micro- and meso-damage process of the expansive soil under the action of dry-wet cycles was described, and the multi-scale mechanism of soil mechanical performance degradation was obtained. Alternating wet and dry changes first cause the cyclic swelling and shrinkage of hydrophilic clay minerals in the soil, and thus induce the expansion potential and matrix potential. Such potentials repeatedly act on the micro- and meso-structure of the soil, causing fatigue damage and thus a sudden decrease in the cohesion of the soil and a loss of the shear strength. Additionally, the uneven shrinkage of the soil produces tensile stress and drives crack propagation and coalescence, resulting in the degradation of soil integrity and reduction of mechanical properties. The research results may provide a useful guidance for the understanding of mechanical behavior and structural damage evolution of expansive soils in semi-arid areas. © 2020, Science Press. All right reserved.