INFLUENCE OF CYCLING PORE-WATER PRESSURES AND PRINCIPAL STRESS RATIOS ON DRAINED DEFORMATIONS IN CLAY

Seasonal changes in groundwater levels affect the rate of downhill creep movements in slopes. This process has been studied in triaxial tests on undisturbed specimens of a natural clay from Bluefish Lake, 50 km north of Yellowknife, N.W.T. Specimens were first anisotropically consolidated to low stresses that correspond to conditions at shallow depths in creeping slopes. Pore-water pressures (back pressures) in the specimens were then cycled systematically (over periods lasting 4-48 h) with the drainage leads open. Resulting axial and volumetric strains were measured, and shear and lateral strains deduced from them. Strain rates decreased with increasing total times of testing. They increased with increasing values of the ratio DELTA-u/DELTA-u(f), with increasing values of deviator stress q, and with decreasing values of K = sigma-3c'/sigma-1c'. Systematically increasing the pore-water pressures in the specimens produced clear estimates of failure at low stresses.