The principle objectives of this research are to investigate the behavior of an unsaturated medium expansive clay and to study the failure mechanism of rain-induced landslips in the unsaturated expansive soil slope. This research consists of two major components, namely full-scale field monitoring of an unsaturated expansive soil slope subjected to artificial rainfall and laboratory study on the behavior of the unsaturated clay. In addition, this research includes a preliminary hydro-mechanical coupled numerical analysis of the research slope....[ Read more ]

The principle objectives of this research are to investigate the behavior of an unsaturated medium expansive clay and to study the failure mechanism of rain-induced landslips in the unsaturated expansive soil slope. This research consists of two major components, namely full-scale field monitoring of an unsaturated expansive soil slope subjected to artificial rainfall and laboratory study on the behavior of the unsaturated clay. In addition, this research includes a preliminary hydro-mechanical coupled numerical analysis of the research slope.

An 11 m high cut slope in the medium-plastic expansive soil in Hubei of China was selected to carry out a well-instrumented field study on the effects of rainfall infiltration. The field study consists of two neighboring monitoring areas (both 16 m wide by about 30 m long): namely area No. 1 with a bare surface and area No. 2 with a well-grassed surface. The instrumentation includes tensiometers, thermal conductivity suction sensors, moisture probes, earth pressure cells, inclinometers, a tipping bucket rain gauge, a vee-notch flow meter and an evaporimeter. Artificial rainfall events were created in both areas separately using a specially-designed sprinkler system. After the simulated rainfalls, a perched water table was deduced at a depth of about 1.5 m below the bare slope surface. The perched water led to the development of significant positive pore-water pressure within the top 2 m soil and a substantial swelling of the unsaturated soil upon wetting. In situ horizontal stress measurements suggest that passive failures might take place within the softened clay at shallow depths after the heavy artificial rainfalls. The abundant cracks and fissures near the ground surface seemed to play a key role in the soil-water interaction. The grass in the grassed area had effects of increasing the infiltrability of the upper soil layer and retarding surface runoff. The influence of rainfall infiltration in the grassed area was significantly deeper than that observed in the bare area.

A comprehensive laboratory test programme was performed on both recompacted and natural specimens (saturated and unsaturated) using a newly-developed double-cell triaxial apparatus and a modified direct shear apparatus. The experimental results were interpreted within the existing elasto-plastic frameworks for unsaturated soils. The unsaturated clay exhibits an elasto-plastic swelling upon wetting at a low confining pressure. Within the suction range considered (0 to 200 kPa), both the yield stress and the post-yield stiffness increase with suction. An anisotropic strain was measured during the wetting and compression under isotropic stress condition. The dilatancy of the clay increases with suction. The contribution of suction to shear strength for the clay is more significant than that of a compacted kaolin. As compared with the re-compacted specimens, the natural specimens generally exhibit a smaller wetting-induced swelling, a larger dilatancy and a higher shear strength. The identified suction-decrease (SD) yield line in (s, p) plane appears to be inclined at an angle greater than 45° to p axis, which is different from the assumption made in Barcelona Expansive Model. The constant-suction yield curve in (q, p) plane exhibits a nearly elliptical and inclined shape with the major axis coinciding with a K₀ line. This major axis appears to rotate with a decrease in suction, suggesting a rotational suction hardening effect.