The role of time in the hydrological behavior of residual soil slopes during rainfall events

This study established an in-situ instrumentation program with an automatic logging system to monitor the variation of soil moisture content with time at different elevations in a residual soil slope. The role of time in the hydrological behavior of a residual soil slope during a rainfall event was investigated in terms of soil moisture content. A typical relationship between soil moisture content and time is identified for the soil in the slope during a rainfall event, and two key times are defined in the relationship: the time when the wetting front in the soil at a given depth develops during rainfall (Ts) and the time when the contribution of the unsaturated soil strength to the stability of slopes during rainfall reaches the lowest condition (Te). The distributions of Ts and Te with depth are analyzed and discussed. In addition, an increasing rate of soil moisture content with respect to time (Rm) is also defined as the variation of soil moisture content with time at a given depth during a rainfall event. Several notable findings drawn from the research are as follows: (1) in a single rainfall event, Ts and Te values increase with depth; (2) the distribution of the rainfall intensity in a rainfall event and topography of the slope affect Ts and Te values; and (3) the majority of the Rm values measured in the residual soil slope ranged from 3 to 10%/h; Rm values are mainly governed by soil constituents and permeability.