Failure Envelope of Cohesive Soils in the Ultra-Low Stress Range

Abstract

ABSTRACT The present investigation aims at defining the normally consolidated drained failure envelope of a cohesive soil in the ultra-low stress range. The effective stress levels at hand are lower by three orders of magnitude or more than in traditional geotechnical practice. Drained shear strength measurements were conducted on freshly deposited Boston Blue Clay sediment beds of varying thickness using a specially designed Tilting Tank. The beds were prepared from a 550% water content slurry. The measured strength values were age dependent, and varied between 19 and 87 Pa for an effective normal stress range of 16.5 to 155 Pa. The failure envelope was linear at a given bed age, and the friction angle increased with bed age. However, all the failure envelopes converged at an effective stress of about 20 Pa, allowing the true cohesion of the soil to be estimated at approximately 10 Pa. For a given bed age and stress level, the drained strength was equal to or higher than the undrained strength measured with an automated fall cone device.