Rate Dependency and Stress Relaxation of Unsaturated Clays

This paper presents the experimental program conducted for evaluation of the rate-dependent and stress-relaxation behavior of unsaturated reconstituted London clay. A series of drained constant rate of strain (CRS) compression-relaxation tests with single-staged (SS-CRS) and multistaged (MS-CRS) loading modes was performed in an innovative CRS oedometer cell where soil suction evolutions were monitored using two high-capacity tensiometers (HCTs). Specimens were tested at two strain rates of $4.8\times {10}^{-7}$ and $2.4\times {10}^{-6}{\mathrm{s}}^{-1}$ and over a suction range of 0–1,905 kPa. The coupled and independent effects of strain rate and soil suction on one-dimensional stress–strain and stress-relaxation responses, including the effects of prerelaxation strain, stress, and strain rate under both saturated and unsaturated conditions, were evaluated. An increase in suction and strain rate resulted in an increase of the yield vertical net stress (${\sigma }_p$). Furthermore, it was observed that the rate and magnitude of the relaxed stresses increased with increases in prerelaxation strain, stress, and strain rate, and decreased with an increase in soil suction. At constant suction, an increase in the prerelaxation strain rate by a factor of five resulted in an increase of the relaxed stresses by a factor of 2.2–3.6. Moreover, the coefficient of relaxation ($R_{\alpha }$) was found to be suction dependent, falling within a range of 0.011–0.019 and 0.017–0.029, respectively, for slow and fast strain rates during MS-CRS tests. Comparing these results with the $C_{\alpha }/C_c$ ratio obtained from conventional multistage loading (MSL) oedometer test results revealed the validity of $R_{\alpha }=C_{\alpha }/C_c$ correlation for unsaturated reconstituted specimens.