On a slow moving slope in Hong Kong

Abstract

This thesis investigates the mechanism of a slow moving landslide through a case study based on a full-scale field monitoring conducted in a slow moving natural slope. The slope is gentle and located within a southwest-trending valley which is about 500 meters long with an average slope angle of 17°. The slope is mainly consisted of a thin colluvium top layer and a thick completely decomposed andesite layer underneath. The slope was intersected by a natural drainage channel at its toe. Previous records indicate that the slope moved slowly during or after rainfall but there is still lack of sufficient information allowing one to fully understand the mechanism.

To characterize soil properties of the slope, a detailed site investigation and laboratory tests were carried out prior to the instrumentation. The full-scale comprehensive field instrumentation had been completed at the end of 2007. The real-time monitoring system was consisted of a rain gauge, vibrating wire piezometers, moisture probes, WTDRs (water time-domain reflectometry), jet-fill tensiometers, in-place inclinometers and an automatic data acquisition system with wireless transmission ability.

Field monitoring of the slope was successfully conducted for two years between 2008 and 2009. With the monitored data, spatial distribution of groundwater levels in two years was obtained. The variations of groundwater levels were analyzed with hourly rainfall; groundwater responses during selected rainfall events were also analyzed. Moisture contents and matric suctions within the shallow soils at the site were monitored and analyzed. Field measured Soil-water Characteristic Curves (SWCCs) were obtained and compared with the laboratory measured SWCCs. Variations of moisture contents measured in deep soils were also analyzed. Based on the monitored results of the hydrogeological responses; a hypothetical conceptual hydrogeological model was proposed. The subsurface deformations recorded by the inclinometers were analyzed and the deformation mode of the site was also discussed.

A coupled seepage and deformation analysis was conducted based on the finite element program ABAQUS. Then, the numerical simulation results were compared with field monitored data. Consequently, the simulation results verified the hypothetical conceptual hydrogeological model. It is believed that the deformation of the slope is mainly associated with the quick rise of the groundwater table during or after the heavy rainfalls.

Finally, conclusions and recommendations for future research on natural terrain landslide were given. It is hoped that the study of the slow moving slope can provide a useful reference for the future research.