Undrained shear strength in clay slopes - Influence of stress conditions. A model and field test study

Doctoral thesis, 2008

To counteract natural hazards such as landslides, current methods for evaluation of soil properties as a mean for determining the safety level of natural slopes need to be refined. The empirical relationships used for estimation of the undrained shear strength from piezocone and field vane test are based on the stress conditions valid for horizontal ground and normally consolidated or slightly overconsolidated soil. Limited studies indicate that the different relations between vertical and horizontal stresses that exist in a slope and for overconsolidated soil have a large influence on these results. The aim of the present study is to document the influence of horizontal stresses on the undrained shear strength, evaluated using piezocone and field vane tests and, if necessary, modify the evaluation accordingly. Model tests with a mini vane and mini piezocone in large triaxial cells were conducted in order to study the effects of the ratio between horizontal and vertical stresses on the undrained shear strength as determined using piezocone and vane tests under controlled conditions. The results were compared with results from direct simple shear tests. The aim was to simulate the stress conditions in the active and the passive zones of a slope as well as for horizontal ground surface. The results from the analysis of the data from the vane model tests indicate that the horizontal preconsolidation pressure and the OCR in the horizontal direction have a major influence on the vane shear strength. Together with empirical data from earlier studies, also the results from the piezocone model tests indicate that the net cone resistance is primarily dependent on the horizontal preconsolidation pressure. Field tests were carried out at two test sites consisting of homogeneous clay and with slopes eroded from originally horizontal ground. The undrained shear strength at the test sites were estimated from piezocone tests, field vane tests and direct simple shear tests. The undrained shear strength from the field vane tests and the piezocone tests at the toe of the test site slopes were also predicted based on the results of the model tests and the stress conditions in the slopes. The undrained shear strength evaluated from the adapted model test results for the field vane test corresponds quite well with the empirical relationship that includes a correction for OCR, for both test sites. This indicates that there is a need for an extra correction of the field vane test results for overconsolidation