Matrix Suction in Silt and Sand Slopes. Significance and Practical Use in Stability Analysis

Doctoral thesis, 1997

Conventional stability analysis in unsaturated silt and sand slopes often results in unrealistically low factors of safety. The purpose of the study was to increase the knowledge and the understanding of the unsaturated zone and its matrix suction in order to facilitate more elaborate stability analyses of silt and sand slopes. Great effort was made to prepare a method for determination of the shear strength in unsaturated silts and sands by the use of water retention curves. The method facilitates determination of the shear strength for practical purposes without the need to perform elaborate, time and cost-consuming laboratory tests. Measurements of matrix suction were performed in situ at different test sites. Grain size distributions and water retention curves were determined in the laboratory in order to identify any relationship between the composition of the soil and the corresponding matrix suction. Infiltration tests were made in the field and in the laboratory in order to study the effect of precipitation on matrix suction. Some of these tests were simulated numerically. The proposed method for determination of shear strength in unsaturated silts and sands taking matrix suction into account resulted in very good agreement with known laboratory test results. Based on the results from the field and laboratory tests, recommendations have been prepared for how to use the proposed method in engineering practice and for how matrix suction can be taken into account in stability analyses in unsaturated silts and sands. Stability analysis taking matrix suction was found to give results which were more in agreement with existing geometry and observed sliding activity compared to conventional stability analysis.