Soil moisture and suction measurement and its effect on slope stability

In order to investigate the mechanism of rainfall induced slope failure, a full scale cut slope experiment is being conducted at a being constructed cut slope, Hong Kong. By examining the variations of moisture content and matric suction with daily and hourly precipitation, and the variations of moisture content and matric suction profiles at the crest of the cut slope from June 2001 to December 2002, we observed that the variations of moisture content and matric suction or the movement of “wetting front” during the wet season of 2001 was within the top three meter soil and a transient water table was developed at around one meter to two meter depth during the heaviest rainstorm. We also observed that a “drying front” due to evaporation was developed during the 2001 dry season. Slope stability analysis of the natural slope showed that heavy rainfall infiltration could decrease the factor of safety dramatically while evaporation could increase the factor of safety slowly. The shallow variations of moisture content and matric suction or the shallow movement of the “wetting front” may be useful in the explanation of many shallow slope failures occurred in this region during heavy rainstorms. While the “drying front” due to evaporation during the dry season may illustrate the contribution of the increased matric suction to the increased shear strength of unsaturated soil slope.