Influence Of Wetting-Drying Cycles On Shear Properties Of Unsaturated Compacted Kaolin

In recent decades, due to the climate deterioration, the heavy rainfall and prolonged dry season alternate in most parts of the world appear alternately. The soil subjected to cyclic wetting-drying in the proceedure of water absorbing and draining causes a series of disasters such as landslides, which result in severe threat to livehood and enconomic properties. However, the influence of cyclic wetting-drying on shear behavior is usually not taken into account in current design of slope stability and a potential danger may exist. Therefore, it is of great importance to study the effect of cyclic drying-wetting on unsaturated soil. Two serious of tests were planned, including the path and magnitude of cyclic wetting-drying tests. Shear properties of compacted kaolin subjected to cyclic wetting-drying was explored,then the mechanism of cyclic wetting-drying was investigated.Firstly, a system of measurement of water volume change was modified according to the requirement of tests. Then, 24 direct shear tests were carried out in the laboratory investigation, including 16 tests on path of cyclic wetting-drying and 8 tests on magnitude of cyclic wetting-drying. Applying net normal stress of 100kPa and 200kPa, samples were sheared at suction levels of 1kPa, 50kPa, 100kPa, 200kPa respectively prior to equalization. After shearing, a series of experimental data were obtained, including void ratio, shear strength, total volunm, gravimetric water content, dilatance. The difference of shear properties was investigated and analyzed at different suction level and stress level, trying to get the regularity of unsaturated shear properties and comprehensively study the influence of cyclic wetting-drying on unsaturated shear properties.In the procedure of cyclic wetting-drying, the variation of void ratio at drying is greater than that at wetting at the same net normal stress level. At the same suction level, the greater the net normal stress is, the less the variation of void ratio is. At cyclic wetting-drying, the variation of soil frame is reflected by variation of void ratio. And shear strength, total volume, gravimetric water content, dilatancy change. Shear properties also changes as hydraulic and mechanical behavior change. The analyses of results are presented as follows:(1)At the same suction level and net normal stress level, the peak shear stress, and peak dilatancy in the drying stage are all greater than that in the wetting stage.(2)At the same suction level, cohesion at wetting is greater than that at drying.(3)At the same suction level and net normal stress level, gravimetric water content at drying is greater than that at wetting and the soil sample is more stiff and brittle. (4)The influence of magnitude on internal friction and cohesion increases as suction increases, that is to say, the influence of magnitude of cyclic wetting-drying on shear strength increases. At the same time, the trend of dilatancy is not affected so much by magnitude of cyclic wetting-drying, but the variation of dilatancy is affected by magnitude of cyclic wetting-drying.