Comparative study on the hydro-mechanical behaviour of natural and compacted expansive stiff clay at saturated and unsaturated States

This study compares the hydro-mechanical behaviour of intact and compacted Nanning stiff clay specimens under varying suction conditions using suction-controlled oedometer tests. All specimens were equilibrated at different suctions (0–38 MPa) and subjected to stepwise loading and unloading. Results showed that intact specimens exhibited insignificant suction-induced axial strain due to constraining effect of original diagenetic bonds on clay mineral activity. Both types of specimens displayed reduced compressibility with increasing suction. Upon unloading, rebound curves transitioned from bi-linear to linear as suction increased, indicating the competition between physico-chemical and mechanical effects in soils. The yield stress of the intact specimen was higher than that of the compacted one at suctions below 4.2 MPa, but lower at higher suctions (9, 21 and 38 MPa). Such phenomena were associated with the remoulding effect on the microstructure configuration of soil. Further comparison of rebound curves within the sensitive suction range (0–9 MPa) revealed that, diagenesis bonds in the natural stiff clay can be damaged by hydro-mechanical loads, leading to structural degradation. As a result, physico-chemical effects are activated during unloading, causing distinct rebound. However, residual bonds still impose certain mechanical constraints to resist the activated physico-chemical effect, resulting in a concave-down rebound curve.