Response of a residual soil slope to rainfall

Abstract

Rainfall-induced landslides are a common problem in residual soil slopes of the tropics. It is widely known that rainfall-induced slope failures are mainly caused by infiltration of rainwater; however, the response of a residual soil slope to infiltration is not fully understood. The difficulties lie in the quantification of the flux boundary condition across the slope surface with respect to infiltration and its effect on the pore-water pressure conditions in the slope. Therefore, it is important to understand the response of a slope to different rainfall conditions and the resulting changes in pore-water pressures and water contents. A residual soil slope in Singapore was instrumented with pore-water pressure, water content, and rainfall measuring devices, and studies were carried out under natural and simulated rainfalls. Results indicate that significant infiltration may occur in a residual soil slope during a rainfall. Small total rainfalls can contribute a larger infiltration percentage than large total rainfalls. The percentage of infiltration usually decreases with increasing total rainfalls. The study has indicated the existence of a threshold rainfall of about 10 mm for runoff generation to commence. Infiltration during wet periods may lead to the development of positive pore-water pressures as a consequence of a perched water table condition. Matric suctions are recovered gradually during dry periods due to redistribution. Soil water contents tend to be higher near the toe of the slope than at the crest irrespective of rainfall events, indicating subsurface movement of water in the downslope direction. The study has also indicated a correlation between rainfall amount and relative increase in pore-water pressure. The results can be used to quantify the flux boundary conditions required for the seepage analyses associated with rainfall-induced slope failures.Key words: infiltration, pore-water pressure, water content, residual soil, rainfall-induced slope failures.

Effect of Rainfall Intensity on Infiltration into Partly Saturated Slopes

“…7b), suggests that the shape of the measured infiltration capacity decay curve, and particularly the i f value measured is incorrect (the i f value would be expected to be too high). Rahardjo et al (2005) note that the initial and final volumetric content near the toe of the slope always appeared to be higher than those at the crest, suggesting that some water that infiltrates near the crest of the slope may then flow downslope, parallel to the slope surface, thus resulting in higher water content and lower suction near the toe of the slope than near the crest. The same i f value was therefore adopted for both analyses.…”

“…2 we note that the key assumptions of the Mein-Larson model are that the minimum infiltration capacity of the soil is equal to the saturated permeability and that run-off will not occur unless the rainfall intensity exceeds the saturated permeability of the soil. Rahardjo et al (2005) report measurements of the infiltration response of a residual soil slope subjected to artificial rainfall events. The instrumented slope comprised a 1.5 m deep layer of orange silty clay with a saturated permeability of 5.18 · 10 -6 m/s, overlying a 4-5 m deep layer of purple clayey silt with K s = 1.67 · 10 -7 m/s.…”

“…7a and b. Rahardjo et al (2005) report values for K s and R i which were used in the analysis. The value of i f was taken directly from measured field data for the 47 mm/h rainfall event shown in Fig.…”

Modelling shallow landslide susceptibility by means of a subsurface flow path connectivity index and estimates of soil depth spatial distribution

“…perched water table) at the soil-bedrock interface. Several researchers (McNamara et al, 2005;Rahardjo et al, 2005;D'Odorico et al, 2005) have shown that vertical flow in the unsaturated soil zone is reduced when the infiltration front meets a less permeable layer (for example, the bedrock layer). Under this condition, the infiltrating rainwater collects at the less permeable soil layer, inducing rapid increases of pore-water pressure and unsaturated hydraulic conductivity (according to the relationship between matric suction head and unsaturated hydraulic conductivity).…”

Antecedent rainfall induced shallow landslide-A case study of Yunnan landslide, China

“…Wei in a case study of the Bukit Batok landslides in Singapore found that the failure occurred after a period of heavy rainfall and there was no rainfall at the time of failure [11]. Antecedent rainfall, initial pore-water pressures prior to a signi cant rainfall event, and the magnitude of the rainfall event play a crucial role in the development of the worst pore-water pressure condition in a slope [12].…”