Pore-water pressure in residual soil slope

Residual soil slopes in Singapore usually have a permanent groundwater table at or below the toe of the slope. The soil above the groundwater table can be divided into three zones: (1) the capillary fringe where the soil is fully saturated (2) a zone where the degree of saturation is less than 100% but with a continuous water phase, and (3) a zone nearest to the ground surface where the degree of saturation is much less than 100% and the water phase is discontinuous. The zone nearest to the ground surface experiences the greatest interaction with the atmospheric conditions and is in a state of constant flux exchange. Several residual soil slopes in Singapore have been instrumented to investigate the changes in pore-water pressure conditions in the slope. In this paper, the monitoring results from one such slope with tensiometers and thermal conductivity sensors installed in zones 2 and 3 are presented. This paper focuses mainly on the pore-water pressure measurements from the thermal conductivity sensor. The pore-water pressure conditions are interpreted with respect to climatic conditions. The rate of reduction in negative pore-water pressure during rainfall and the rate of negative pore-water pressure recovery after rainfall are examined in detail. The results show that there is a critical depth of rainfall influence and that the rate of recovery of negative pore-water pressure decreases with depth from the ground surface.