Abstract
Rainfall triggered landslides are one of the most common natural hazards in many countries all around the world, as well as in Turkey. This study investigates the effect of unsaturated soil properties on landslide-triggering rainfall intensity-duration threshold. In addition to the time to failure, the suction (negative pore water pressure) distribution in the slope, the shape and depth of the failure surface are also evaluated. Properties of soil-water characteristic curve which affect the distribution of suction in the soil in response to changes in water content are varied in a parametric study. Effects of air entry value (relates to particle size), de-saturation rate (relates to uniformity of particle size distribution), saturated volumetric water content (relates to void ratio) and residual volumetric water content (relates to fines content and characteristics) are evaluated by determining the intensity-duration threshold numerically by carrying out infiltration and slope stability calculations. The variation of unsaturated soil properties is found to significantly alter the landslide mechanism (ranging from mere surface erosion to groundwater level rise in response to complete infiltration) and consequent intensity-duration thresholds for the same slope geometry. Among the parameters considered, air entry value appears to be the most influential parameter. The effects of the slope angle and initial moisture condition on threshold rainfall intensity-duration are also investigated. This study could be useful for creating the basis and mechanical understanding for future research on early warning systems.
Highlights
Slope instability in unsaturated soils is very common in many parts of the world
This study investigates the effect of unsaturated soil properties on the distribution of suction in the slope, the shape of the failure surface, failure time, and rainfall intensity-duration threshold
We considered SWCC of a sand (Edosaki sand from [4, 18] as starting point and we generated SWCCs with different air entry values (AEV - the suction corresponding to the border of saturated and unsaturated states of the soil); saturated volumetric water contents; desaturation rates (DSR - defined as the rate of change of volumetric water content (θ) with matric suction (Ψ)) and residual volumetric water contents as defined in Fig. 2 [19]
Summary
Slope instability in unsaturated soils is very common in many parts of the world. The rainfall is the major triggering factor, the slope topography, geology, hydrology and material characteristics all contribute to development of such instability. Shape of the slip surface, time to failure and slope instability-triggering rainfall intensityduration threshold were investigated by performing staged seepage (equalization and rainfalling) and limit equilibrium slope stability calculations This general topic has been studied in the literature, the novelties in this study are (i) modeling seepage and slope stability numerically without the assumption of infinite slope, (ii) considering the equalization and rainfall stages using the drying and wetting unsaturated properties of the soils separately (i.e. considering hysteresis in SWCC); (iii) characterizing SWCC through independent physical soil properties, rather than curve fitting parameters, (iv) demonstrating the effects of unsaturated soil properties on the I-D threshold
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