The Rate of Swelling of Overconsolidated Clays Subjected to Unloading

Time-rate of swelling under one-dimensional drainage and either one-dimensional or isotropic deformation conditions was investigated using reconstituted specimens of Duck Creek, Crab Orchard, Cucaracha and Bearpaw shales. Cylindrical specimens were used, drainage was allowed from the top, and pore water pressure was measured at the base of all specimens. Load-deformation-history variables included maximum pressure (80 psi, 500 psi and 1500 psi), overconsolidation ratio (1-07-250), decrement ratio (0-07-15-7), and sustained secondary swelling (up to 3 log time cycles). The swelling index, C_s, is a function of the overconsolidation ratio, OCR. C_s changes with effective stress and the rate of excess negative pore pressure dissipation is very sensitive to the changes. Secondary swelling can be very significant. The ratio of secondary swelling index, C_αs, to C_s can be as high as 0.4. The maximum value of 0.4 for C_αs/C_s contrasts with the maximum observed value of 0.1 for C_αs/C_s for consolidation. However, for any clay, the value of C_αs/C_s is not a constant and it increases with OCR. Terzaghi's theory of swelling correctly predicts the shape of the percent swell-log time curve up to 60% primary swell. Beyond the 60% the shape of the swell curve is a function of the magnitude of C_αs/C_s and decrement ratio, w. Observed excess negative pore pressures dissipate faster than is predicted by the Terzaghi theory and rate of swell data. For decrements that immediately follow sustained secondary swell and for the first decrements from the maximum pressure, 50% of the excess negative pore pressure dissipated as much as 15 times faster than the rate predicted from Terzaghi's theory. A theory of swelling is developed that includes the effects of secondary swelling and takes into account the changes in C_s during a swelling decrement. In one-dimensional swell tests on Cucaracha and Bearpaw, negative pore pressure dissipated very rapidly and eventually the pressure became positive. In these cases 50% of the excess pore pressure dissipated over 30 times faster than predicted from Terzaghi's theory and observed swell data. These observations are interpreted as direct evidence that positive shear-induced pore pressures are superimposed on the excess negative pore pressures during one-dimensional swelling.