Deep excavations may fail after rainfall because high-stress unloading can create an unsaturated state with appreciable suction, and subsequent wetting may trigger either swelling or collapse depending on soil type and stress path. Herein, a series of high-stress oedometer tests was conducted to investigate the evolutions of suction, volume, K0 coefficient, and small-strain shear modulus. Two clayey soils were employed: nonexpansive kaolinite and expansive clay. Samples were tested under three loading–1st unloading–soaking–2nd unloading stress paths, covering two maximum vertical stresses (1.6 and 3.2 MPa) and two unloading ratios at soaking (0.25 and 0.50). Results show: (1) unloading-induced suction increments up to 408 kPa (kaolinite) and 610 kPa (expansive clay) as vertical stress decreases from 3.2 MPa to ~5 kPa under K0 condition; suction change increases linearly with the logarithm of the vertical-stress change. (2) During soaking, volumetric changes reach 47.2% and 55.8% of the prior rebound for kaolinite and expansive clay, while K0 and small-strain shear modulus vary by <3%. (3) An effective stress-based model is proposed for the volume evolution upon high-stress unloading and subsequent soaking, incorporating the effect of unloading-induced suction. This model effectively captures the volume change in the unloading and soaking stages with R2 ≥ 0.86.
Yang et al. (Wed,) studied this question.