Strength characteristics of a modelling silty clay

Abstract

Shear strength is an important soil parameter. Many geotechnical failures have been caused by inadequate shear strength. In order to obtain a high strength modelling silty clay for use in centrifuge model tests of soil-pipeline interaction, direct shear tests, shear vane tests and cone penetration tests were conducted to investigate its shear strength behaviour. -- This thesis first presents some background on the shear strength of soils and measurement techniques. The details of the direct shear tests, the shear vane tests, the cone penetration tests and centrifuge modelling are also discussed. The silty clays used in this study were kaolin and a kaolin-silt mixture; the plasticity indices were 30.6% and 19.5% respectively. The kaolin-silt mixture (K-S) was obtained by mixing equal amounts of kaolin and silt by weight. -- Direct shear and shear vane tests were conducted on both clays at different vertical stresses under various overconsolidation ratios (OCR's). This thesis presents these test procedures and the test results. Cone penetration tests were also conducted in a centrifuge at 50 gravities to verify the results from the direct shear and shear vane tests and to correlate the cone penetration resistance to the undrained shear strength from shear vane and direct shear tests. Centrifuge modelling principles and the current methods to convert cone tip resistance to shear strength are also presented. Test results show that the shear strength of K-S is higher than that of pure kaolin. Shear strengths of heavily overconsolidated clays obtained from the shear vane tests are higher than those obtained from the direct shear tests. A correlation between cone tip resistance and undrained shear strength is presented. The shear strength of K-S interpreted from CPT results was found to lie between the direct shear and shear vane test results and shows a good correlation in the measurement of shear strength.