Shallow landslides triggered by rainfall in nsaturated soils

Ahmadi-Adli, Mohammad
Rainfall triggered landslides are common natural hazards with significant consequences all over the world, including Turkey. Majority of available methods for predicting rainfall-induced slope instability are based on regional statistical data of past slope failures and rainfall records rather than a physically-based model that takes the mechanism of the problem into account. Current study aims to define a numerical model for typical slopes in the region (Northern Turkey), use unsaturated soil properties and obtain rainfall intensity-duration (I-D) thresholds for later use in early warning systems. In order to verify the findings from numerical simulation of seepage and slope stability of unsaturated finite slopes (at 44 to 60 degrees) subjected to infiltration (SEEP/W and SLOPE/W), 16 laboratory flume tests on a fine sand soil at three relative densities (34, 48 and 61%) subjected to different rainfall intensities (4 to 67 mm/hr) are carried out. To study infinite slopes subjected to rainfall, a MATLAB code is developed and Laminar Box setup is designed and manufactured to verify the results in future works. This study achieves several feats for the first time in the literature (to the author’s knowledge): (i) Necessity of considering hysteresis effects (using wetting and drying soil water characteristic curves and hydraulic conductivity functions) in numerical simulation of infiltration and evaporation/drainage through unsaturated slopes is assessed. (ii) Rainfall intensity duration thresholds that would trigger a landslide (I-D plots) are obtained physically in the laboratory and by numerical simulations. The obtained I-D plots are linked to landslide mechanism rather than statistical data. (iii) The shape of the I-D threshold is demonstrated to be a linear relation in log-log plot for the soil used in this study. (iv) Below a certain rainfall intensity (15 mm/hr in this study) landslides are not triggered in unsaturated soil used in this study (i.e. the I-D plot seems to be asymptotic to the rainfall duration axis). (v) The effect of density of the soil on the I-D threshold is demonstrated by physical laboratory tests and numerical simulations, and it is observed that shallow landslides are not triggered by any rainfall in dense soils used in this study. (vi) Sensitivity analyses show that soil particle size seems to be the most influential parameter effecting I-D thresholds.
Citation Formats
M. Ahmadi-Adli, “Shallow landslides triggered by rainfall in nsaturated soils,” Ph.D. - Doctoral Program, Middle East Technical University, 2014.