Assessment of self-boring pressuremeter tests in sensitive clay

Abstract

The present paper analyzes field test results obtained by means of the Cambridge self-boring pressuremeter (SBPM) in a sensitive Champlain clay of Quebec. It is shown that, although the undrained shear strengths deduced from the SBPM tests are higher than the values obtained with the field vane, the observed discrepancy is reduced by taking into account progressive failure effects. It is not the SBPM test that overestimates the undrained shear strength, but rather the vane test that underestimates it in this sensitive clay. Total and effective field stress paths are inferred from the SBPM tests, and it is shown that the effective stress paths approach the tension zone of the Mohr–Coulomb failure diagram before heading towards the large-strain failure envelope of the clay. The pressuremeter interpretation has been thoroughly worked through for various uncertainties like undrained test conditions, plane-strain conditions, and disturbance, basing the final judgment on a comparison of the effective stress adjacent to the probe with that inferred from laboratory testing. It is also shown that the pore-water pressure, which is generated at the wall of the cavity during the plastic phase of the expansion, is practically equivalent to the increase in total radial pressure. The iterative forward modelling (IFM) technique is used to obtain linearly elastic – perfectly plastic soil parameters by constraining the fit to match the initial horizontal pressure.Key words: self-boring pressuremeter, sensitive clay, stress paths, strain rate, progressive failure, pore pressures, vane tests.