Numerical Investigations of Weathered Leda Clay Slope Stability Due to Rainfall Infiltration Extending the Mechanics of Unsaturated Soils

Abstract

The slope stability analyses in Leda clays deposits of Ottawa and Quebec region which are sensitive in nature are based on conventional soil mechanics assuming saturated soil conditions. The gradual weathering associated with environmental factors contribute to cracks in the surface layer of Leda clay slopes that significantly escalate the susceptibility of slope failures. The Leda weathered clay surface slope and a certain depth of intact soil immediately below it is typically in a state of unsaturated condition. The hydro-mechanical behaviour of the unsaturated weathered crust of Leda clay is highly sensitive to snow melt and rainfall water infiltrates to greater depths. Due to this reason, the conventional slope stability analyses in geotechnical engineering practice based on the principles of saturated soil mechanics may not be reliable. During the last two decades, slope stability analyses based on numerical simulation techniques extending the principles of unsaturated soil mechanics proposed in the literature are being implemented in practice for conventional soils. In these numerical techniques, the hydro-mechanical soil property functions that are required are predicted using the saturated coefficient of permeability, effective shear strength parameters and the soil water characteristics curve (SWCC). The SWCC used in these numerical techniques of conventional soils behavior is unimodal in nature that has only a single air-entry value (AEV), which are representative of a typical intact soil behavior. There are limited slope stability analyses reported in the literature of Leda clay deposits considering the principles of unsaturated soil mechanics. In addition, information about the SWCC of Leda clay is scarce. For rigorous analyses, it is important to consider the influence of soil properties of in-situ weathered crust and the soil below it. The SWCC features for considering such a scenario should consider the influence of bi-modal nature of the SWCC. For this reason, the focus of this thesis has been directed towards developing simple techniques for estimating both the mono and bimodal SWCC of Leda clay and use them in the slope stability extending numerical techniques. As a part of this study, models have been developed for estimating the SWCC using the grain size distribution curve and soil properties collected from geotechnical reports of various projects in the Ottawa region. A range of AEVs were estimated to reasonably represent considering the typical in-situ weathered Leda clay crust behavior. Three unimodal SWCCs were estimated utilizing the lower, upper, and median AEV (i.e., 5 kPa, 80 kPa, 15 kPa). Using this information, a bimodal SWCC is approximated by superimposing two unimodal SWCCs to conduct slope stability analysis. In this thesis, a numerical model has been established to examine the slope stability of weathered Leda clay considering the influence of different rainfall infiltration scenarios. This model utilizes a coupled hydro-mechanical stress analysis to perform numerical analysis using commercial GeoStudio software that includes SEEP/W, SIGMA/W and SLOPE/W. To ensure accuracy, the soil properties used in the numerical modelling are derived through an extensive literature review of Leda clay slopes. Extensive numerical studies were undertaken to understand the impact of various soil properties on the factor of safety (FS). Pore water pressure (PWP) profiles variation demonstrate influence of rainfall infiltration in the surficial layer. Coupled analysis shows a faster downward movement of the wetting front than uncoupled analysis due to the influence of hydraulic and mechanical stresses, resulting in soil softening and volume change upon wetting. The faster rate of wetting also contributes to a rapid loss of shear strength, consequently, resulting in a lower FS than uncoupled analysis. The numerical technique proposed in this thesis is useful for the geotechnical engineers who routinely assess the stability of Leda clay slopes for rationally assessing the potential landslide hazards triggered by snow melt and rainfall events.