Stress release, undrained storage, and reconsolidation in simulated underwater clay

Abstract

Triaxial testing has examined the influence of storage time and reconsolidation procedures on normally consolidated and overconsolidated samples of reconstituted clay prepared with high pore-water pressures to simulate underwater deposits. "Samples" of illite were consolidated one dimensionally, off-loaded without drainage, stored undrained, reconsolidated, and sheared undrained. The storage times were 15 min, I day, and 7 days. Reconsolidation pressures were (1) isotropic to uO,, (2) isotropic to 0.6 u:,,, and (3) anisotropic to u,',,, 0.53u,',,. The results were compared with "specimens" that had not been off-loaded. Different undrained storage times had no effect on the strengths after reconsolidation, but the strengths were affected by the reconsolidation pressures. Anisotropic reconsolidation produced the best agreement between the control specimens and samples in terms of their strengths andA,-values. The duration of storage time did affect stress-strain parameters such as the undrained compression modulus ESO and the pore-water pressure parameter A,.

Suction effects in deep Boom Clay block samples

“…Finally, the effect of suction release under an isotropic stress close to the estimated sample suction was investigated. A slight swelling (1.7%) was observed and further compression provided a satisfactory value of the overconsolidation ratio confirming the suggestions of taking some precautions before putting a swelling sample in contact with water as suggested by Graham et al (1987). The various experimental data gathered in this study finally evidenced a relatively good state of conservation of the block sample used.…”

“…This swelling is much smaller than that (14%) obtained after saturation at low confining pressure by Coll (2005). Furthermore, the yield stress (p y ') obtained in Figure 8 Horseman et al (1987) cited in Burland (1990) The data of the test of Figure 8 confirm, as suggested by Graham et al (1987), that some precautions have to be taken before releasing the suction of a natural sample. Note however that the slight swelling observed during the test indicate that the initial suction might be higher than 2.5 MPa.…”

“…This saturation procedure and the resulting swelling is likely to alter the initial state of the soil and to lead to unexpected low values of the yield stress, as shown by and Sultan (1997). After Graham et al (1987), during saturation under the in-situ state of stress, suction reduces to zero and no further swelling should occur, allowing to put the sample in contact with water and to apply a back pressure in a standard manner with no significant modification of the initial microstructure.…”

Does long-term storage of clay samples influence their mechanical characteristics?

“…It was found that the discrepancy between UU and CAUC test results increased with increasing storage time. Kirkpatrick et al (1986), Graham et al (1987), Graham and Lau (1988) and Graham et al (1990) extended the above study to overconsolidated kaolin and illite, "underwater" samples of illite, "underwater" samples of grundite and to illite respectively and found more or less the same results, all stressing in particular the importance of anisotropic consolidation prior to shearing. Numerous other studies have demonstrated the inadequacy of the UU test for the measurement of in situ strength.…”

Analysis of the railway heave induced by soil swelling at a site in southern France

“…During the loaded-swell tests, the specimens were reloaded to its overburden stress (path D-C) and then flooded to swell (path C-E-F). When performing triaxial tests on soil specimens taken from great depth, Delage et al (2007), Graham et al (1987) applied the "in-situ" stress prior to re-saturate the soil. This procedure avoids the large soil swelling which may induce a microstructure change, resulting in a modification in mechanical behaviour.…”