Centrifuge modelling of rainfall-induced landslides in unsaturated soil slopes

Matziaris, Vasileios (2019) Centrifuge modelling of rainfall-induced landslides in unsaturated soil slopes. PhD thesis, University of Nottingham.

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Abstract

Landslides and debris flows are geohazards common to countries with mountainous terrains. A significant number of rainfall-induced landslides occurs in regions with tropical and subtropical climates where residual and colluvial soils are widespread. This study is in the context of the Innovative Training Network project MUMOLADE (Multiscale Modelling of Landslides and Debris Flows) which aims to study the multi-scale and multi-phase analysis of landslides and debris flows. The research presented here aims to investigate the initiation conditions of landslides in soil slopes under varied rainfall conditions.

A physical modelling experimental investigation on the initiation of rainfall-induced landslides in unsaturated soil slopes was performed in this study using the geotechnical centrifuge at the University of Nottingham. To accomplish the objectives of the study, a testing apparatus was designed and built. The tests were conducted in a plane-strain centrifuge box with embedded systems for controlling rainfall and groundwater conditions. The container has a transparent Perspex window at one side which provided the ability to measure subsurface ground displacements using digital image analysis. Pore Pressure Transducers (PPTs) were used for measuring pore water pressures within the slope models during the tests.

Initially, the study focused on the behaviour of a fine, uniform sand. Classification tests of the soil were performed in the laboratory, including the Soil Water Retention Curve (SWRC) determined using the tensiometer method. Sand slope models were built at a high inclination angle, about 1.5 times higher than the soil’s angle of friction, with suction providing the necessary cohesive component for stability at 1g. The simulation of two failure mechanisms was attempted in this study. The first mechanism involves the loss of suction due to wetting in a soil slope of an angle higher than the angle of friction due to wetting. The second mechanism

involves the collapse of the saturated voids due to increased pore water pressure which reduces the effective stresses within the soil mass. The second mechanism was not able to be reproduced in the tests conducted in initially unsaturated soil slopes with a low GWT, since the models showed unexpected resistance to failure. On the other hand, soil slope models failed under a condition of an elevated (i.e. shallow) GWT. In order to study further the unsaturated soil conditions under increased gravity tests were performed in which drainage of the soil under these conditions was enabled and measurements of the retained water content and degree of saturation at different g-levels were made. Results

indicated that even medium-grained soils retain an amount of water content in their mass and that the retained water content at each g-level strongly depends on the grain size distribution of the soil. The implication of these tests is that soil slopes retain their stability when gravity increases even at extreme conditions, i.e. at an angle 50% higher than the angle of friction.

In order to overcome the scaling issue of the increased seepage velocity, slope models were built from a slightly finer soil which was scaled down by $\sqrt{N}$. These soil slopes were tested under similar boundary conditions at the sand slopes. The results indicated that the reduction of the permeability caused instability initiation under different combinations of rainfall intensity and duration. Slope centrifuge tests were simulated using limit equilibrium analysis in order to validate the experimental results. The analysis revealed the previous statement, that the elimination of suction was not sufficient to reduce the shear strength of the soil to a level that would cause instability in the slopes. Failure was attributed to the small increase of positive pore water pressures which were generated only to the slopes with reduced seepage velocity (i.e. in slopes made of $M_{90-10}$). Therefore, grain size scaling in soil slope models was found to provide the necessary conditions for the simulation of rainfall-induced landslides in centrifuge tests. Rainfall thresholds for the occurrence of rainfall-induced landslides were obtained and compared to existing data from the literature. The comparison showed that the centrifuge tests conducted in this study provided an over-estimated rainfall threshold.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Marshall, Alec
Heron, Charles M.
Keywords: Centrifuge modelling, slope stability, unsaturated soils, rainfall thresholds
Subjects: T Technology > TA Engineering (General). Civil engineering (General) > TA 703 Engineering geology. Rock and soil mechanics
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 57020
Depositing User: Matziaris, Vasileios
Date Deposited: 17 Dec 2019 15:16
Last Modified: 06 May 2020 12:15
URI: http://eprints.nottingham.ac.uk/id/eprint/57020

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