Field and laboratory study of stress relief due to unloading in block samples of sensitive clay

Abstract

A block sample goes through stress changes during and after sampling. Sensitive clays are affected by sample disturbance and stress changes have a great effect on sample quality. Reduction of the in-situ total stresses to zero causes a soil sample to develop a negative pore pressure. In an ideal situation, a block sample retains its negative pore pressure during sampling and storage, preventing it from swelling. To study this, five in-situ tests were conducted in which the pore pressure variations inside block samples were monitored during and after sampling. The pore pressure was measured continuously during a storage period of several days and the results were compared with tests on reconstituted clay in an attempt to simulate the same stress conditions as those in situ. Similar to the in-situ tests, the pore pressure inside the reconstituted samples was measured throughout the test. The findings suggest that the negative pore pressure in mini-block samples may be reduced in a matter of minutes or hours after sampling. Parallel testing on mini-block and reconstituted clay samples indicated that pore pressure dissipation had a negative effect on the preconsolidation pressure, constrained modulus and sample quality.