Rate Dependency and Stress Relaxation of Unsaturated Clays

This paper presents the experimental program conducted for evaluation of the rate-dependent and stress-relaxation behaviour of unsaturated reconstituted London Clay. A series of drained constant rate of strain (CRS) compression-relaxation tests with single-staged (SS-CRS) and multi-staged (MS-CRS) loading modes were 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 4.8×10⁻⁷ and 2.4×10⁻⁶s⁻¹ and over a suction range of 0 – 1905 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 pre-relaxation 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 (σp). Furthermore, it was observed that the rate and magnitude of the relaxed stresses increase with increase in pre-relaxation strain, stress, and strain-rate, and decrease with increase in soil suction. At constant suction, an increase in the pre-relaxation strain-rate by a factor of 5 resulted in an increase of the relaxed stresses by a factor of 2.2 – 3.6. Moreover, the coefficient of relaxation (Rₐ) 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 Cₐ/Cc ratio obtained from conventional multi-stage loading (MSL) oedometer test results revealed the validity of Rₐ = Cₐ/C𝒸 correlation for unsaturated reconstituted specimens.