Identification Of Critical Rainfall Duration Periods For Individual Natural And Excavated Slopes

A major dilemma in any slope stability exercise is to decide what the design water level should be and recent assessments of failures of new ‘engineered slopes’ has indicated that incorrect modelling of groundwater levels was a critical contributing factor in 70% of reported (major and minor) incidents, and accounted for 100% of all major failures of engineered slopes. In Hong Kong, it is generally recommended to design to a minimum Factor of Safety (FoS) of 1.2 for a 1 in 10 year return period groundwater condition, and a FoS of 1.0 for 1 in 1,000 years amongst other criteria (GEO, 1984). This would seem reasonable as under normal conditions, groundwater level is wholly governed, in some way, by antecedent rainfall. In adopting this approach, a first step must be to determine the rainfall duration period that influences a particular slope. This has been attempted by comparing moving average curves of different durations of rainfall, with the shape of the groundwater level/time curve. It is considered that if a moving average rainfall curve can be found that matches the groundwater level curve in terms of corresponding peaks and troughs, then this is likely to be the period for which antecedent rainfall affects the groundwater level at any given time. It is not considered that the vertical scales should necessarily be similar. One might expect the moving-average rainfall curve to have a greater vertical exaggeration than the groundwater, as increasing intensity rainfall will give an increasing ratio of run-off to infiltration beyond a certain threshold value (Premchitt et al, 1992). Similarly, one might expect the moving-average rainfall curve to advance slightly ahead of the groundwater level curve, as time for descent of the wetting band may need to be allowed for.