In order to investigate the effects of hydration state on the stress-strain-strength behaviour of compacted expansive soil

a series of mechanical property tests under one-dimensional consolidation and triaxial compression are carried out under two typical hydration states which are both saturated. The conventional saturation method is adopted for the first hydration state

while the second state permits samples swelling to steady state. The results show that there is a significant difference of volume expansion between the two hydration states; and the volume expansion rate is influenced by both the deformation constraint condition and the length of seepage. Compared with the first hydration state

the soil sample under the second hydration state has greater compression index λ

swelling index κ

and lower elastic shear modulus. The effective internal friction angle is 77.2% of the corresponding value of the first hydration state

which reflects the variability of saturated shear strength of the compacted expansive soil. The yield phenomena of consolidation curves can be observed with the yield stresses 123.2 kPa for the first hydration state and 94.5 kPa for the second hydration state. Under the first hydration state

the samples present strain softening and shear dilatancy at lower confining pressures

while they present strain hardening and shear contraction at higher confining pressures. Under the second hydration state

the samples present shear contraction and weak strain softening. The samples under the two hydration states show overconsolidation in the process of consolidation and shear loading. The overconsolidation is not due to preconsolidation pressure only; however

it is partially caused by that swelling of the soil is constrained in the process of loading. Under the different hydration states

the expansive soil has both different initial void ratios and different swelling potentials

while the interaction of the swelling potentials with external constraint conditions

drainage conditions and stress states leads to the complex stress-strain-strength behaviour.