Experimental Study On Complex Loading And Unloading Mechanical Behavior Of Sandstone After Drying-Wetting Cycle

Rock masses in reservoir slope are prone to deterioration of physical and mechanical properties after the water level rise and fall and rainfall.These dry-wet cycles weaken the stability and strengthen the deformability of rock masses,causing disasters such as collapse and landslide of the reservoir slope rock masses.Therefore,it is necessary to investigate the effects of different dry and wet cycles on mechanical properties of rock masses in reservoir slopes.At the same time,the rock mass is generally under its self-weight and additional stress conditions caused by excavation,different loading and unloading stress history should also be considered in the research.The results are helpful to provide theoretical guidance for the stability analysis of rock masses in practical reservoir slope.This paper studies the mechanical properties of the rock under complex stresses under different dry-wet cycles.Triaxial tests under loading and unloading stress paths on the sandstone after the dry-wet cycles were carried out.In order to reveal the differences deformation and failure mechanism of sandstone under dry-wet cycles and complex stress,the mechanical behavior,energy evolution and microscopic failure characteristics were comprehensively analyzed.The main research contents are as follows.(1)The sandstone was subjected to 0,1,3,and 6 wet and dry cycles,respectively,which were then subjected to triaxial tests under loading and unloading with different confining pressures.The test results show that with increasing the wet and dry cycles,both the elastic modulus and peak strength continued to decrease and gradually approached to a stable level.The overall peak strength shown an increasing trend with increasing the confining pressure.In the unloading test,the peak strength occurred earlier than that in loading test,and with increasing the dry and wet cycles the peak strength also occurred earlier with a lower residual strength.These results indicate that the failure of the unloaded sample was mainly caused by dilatancy.The increase in the number of dry and wet cycles leaded to deterioration effects that resulted in structural damage in the tested rock samples.(2)The energy evolution of different tested samples was studied.The total energy absorbed by the rock sample at the stress peak point,the elastic energy stored inside,and the change law of the dissipation energy with the number of dry and wet cycles and the confining pressure were investigated.The results showed that with increasing the number of dry and wet cycles,the total energy,elastic energy,and dissipated energy density of the rock decreased accordingly.The absorbed energy increased with a larger confining pressure,indicating that the confining pressure can increase the rock resistance ability to destroy.During the loading process,the energy was basically stored in the rock in the form of elastic energy,until the energy was quickly released after failure,and subsequently the elastic energy was transformed into dissipated energy.During the unloading process,the elastic energy was lower than that in loading process,and the dissipated energy increase was higher than that in loading process,which was more remarkably after reaching the peak strength.(3)The macroscopic and microscopic failure characteristics of all rock samples under various test conditions were analyzed.The morphology feature of the main failure surface was observed by a scanning electron microscope,and the degradation mechanism of water-rock interaction was explored from a microscopic point of view.From a macro point of view,it was found that in different loading and unloading tests,the confining pressure can inhibit the hoop expansion;as the number of dry and wet cycles increased,the damage degree became more severe and complex.The failure angle in rock sample under unloading was higher than that under loading test.Under unloading,there were fragments fallen from the failure surface,which is significantly different from results under loading.From a microscopic point of view,as increased the confining pressure,the internal loose porous structures decreased.As the number of dry and wet cycles increased,the failure surface changed from tightly arranged to porously with many secondary colloidal particles attached,showing a “honeycomb”shape with the overall structural changes on the failure surface.