Field investigations and hydrological studies of two unstable, unsaturated soil slopes in Saint Lucia, West Indies

Abstract

Classical soil mechanics as presented by Karl Terzaghi assumed that the soil is in a fully saturated state. However, in tropical areas of the world residual and colluvial soil slopes exist in an unsaturated condition and the traditional theories and methods of slope stability analysis cannot be readily applied. However, situations may arise when unsaturated soils rapidly become saturated during rainstorms and the traditional theories do apply.

The quick response of pore water pressure to rainfall intensity in a multi-layered residual soil and colluvium has been studied and reported in this research project. The effect of infiltration into a slope of variable saturated hydraulic conductivity values on stability was also examined. It was found that for a multi-layered residual soil the rapid infiltration of water into the more permeable soil layer causing saturation had a relatively quick impact on pore water pressure increase and hence the stability of the slope even though the overlying residual soils of lower saturated hydraulic conductivity remained unsaturated. In tropical areas which experience heavy rainfall periods during the wet season followed by very dry spells for several months of the year the infiltration process particularly in multi-layered unsaturated residual soil slopes is not fully understood.

This thesis attempts to address this problem with the objective of contributing to the understanding of the hydrological processes taking place in a multi-layered unsaturated residual soil slope and a colluvial slope and to demonstrate how quickly the unsaturated soils become saturated and unstable during a 24-hour rainstorm event. The study includes the use of field instrumentation, laboratory testing and finite element numerical analysis to determine the changes in pore water pressure and instability in the two slopes due to infiltration during rainstorms. The sites selected for the study are located on the Caribbean island of Saint Lucia in the West Indies. Both sites experienced land slippage during heavy rainfall events.

A study of the cause of the slope failures of a colluvial soil slope derived from weathered basalt and a residual soil slope from weathered parent andesite rock. The effect of the hydrological process induced by variable rainfall patterns and their effect on the stability of the two slopes unstable are examined. Pore water pressure and matric suction were measured in the field at both sites with tensiometers manufactured by Soil Moisture Equipment Corporation, Santa Barbara, California, and were installed at variable depths at specified locations on the slopes. Slope inclinometers, standpipe piezometers and rainfall gauges were also used for field instrumentation. The combined seepage and limit equilibrium slope stability analyses were conducted using the soil properties from the field and laboratory testing programs. The software Slope/W and Seep/W from Geo-Slope International Ltd, Calgary, Alberta, Canada were used for modeling the slope failures.

The results of the study indicate the sensitivity of the relationship between variable pore water pressures, suction and the degree of saturation as expressed by the Soil Water Characteristic Curve (SWCC).The effect of these stress variables on the stability of the two slopes are also presented. Several areas are highlighted where additional research work is required such as the need to accurately identifying soil parameters for unsaturated residual soils and colluvium both in the field and laboratory. Of particular concern is the accurate measurement of the saturated hydraulic conductivity and suction of these materials which contribute to the frequent occurrence of catastrophic landslides during the rainy season on the Caribbean island of Saint Lucia.