In Situ Stress Conditions Across Clay Slopes. A Study Comprising Seven Test Sites

Doctoral thesis, 1994

The in situ stress conditions in clay slopes differ mainly depending on the stability of the slope, the soil properties, the pore pressure situation and also on which part of the slope is studied. The objective of the study is to investigate the in situ stress conditions in slopes and how they vary with seasonal variations in pore pressures and external loading conditions, such as the excavation and filling of earth material. The main part of the study includes a literature survey, measurements and calculations of horizontal stresses at seven test sites and an analysis of the results. The Limit Equilibrium Method, Janbu's GPS, was used to calculate horizontal stresses at six of the test sites. Two different numerical programs, GEOnac and CRISP, were utilised to study horizontal stresses and displacements at one of the test sites. Another numerical program, FLAC, was utilised to study stresses at one of the test sites. For two slopes the mobilisation of shear stresses across the slopes were calculated with the numerical programs. The results are compared with calculations according to Janbu's GPS along the critical shear surface. The study includes natural slopes only affected by seasonal variations in pore pressures and a fluctuating water level, natural slopes where the stability was increased and one slope formed by excavation in the passive zone from an originally horizontal ground surface. Measurements were made by means of total earth pressure cells, the dilatometer, piezometers and by inclinometers. Cells and piezometers were placed at different depths and at different positions in the slopes studied. The measurements showed that the effective horizontal stresses at a given height above the sea level were almost the same independent of in which part of the slope the measurements were made. Furthermore, it was found that the changes in pore pressures and total horizontal stresses with time follow each other relatively well. However, close to a stream with a changing water level, the horizontal stresses do not change as much as the pore pressures during rapid pore pressure changes. Janbu's GPS in general gave results in good agreement with the measured total horizontal stresses. When the stability in two slopes was increased it was found that the Janbu's GPS gave changes in the total horizontal stresses throughout the slope, which the measurements did not. Calculations according to the numerical programs gave total horizontal stresses in good agreement with the measured stresses. For those two programs which simulated displacements due to an excavation the calculated displacements were larger and extended to greater depths than those measured.