Abstract: The ageing effect of the triaxial undrained shear properties of Nanyang undisturbed expansive soil was studied. Firstly, the undisturbed expansive soil sample was consolidated under the pre-consolidation stress with the conventional triaxial apparatus. The triaxial undrained shear tests were conducted with different unloading ratios (R=0.00, 0.50, 0.75) and unloading time intervals (0, 1, 10, 20, 30 d), respectively. The microstructure features of soil sample was characterized by mercury injection test. Test results show that the mechanical properties of Nanyang expansive soil are closely related to the unloading interval time and unloading amplitude. Under the same axial strain, the deviatoric stress of the soil sample after the unloading process decreases, when the unloading interval (R=0.5 and 0.75) increases. While the deviatoric stress without unloading process increases when R=0. The peak strain of the pore water pressure decreases with the increase of unloading interval. The shear strength of the expansive soil increases linearly with the increase of the intermittent time (logarithmic coordinates) for R=0. However, the strength decreases with the increase of the unloading time interval for R=0.5 and 0.75. Furthermore, it declines fast at the first day (0-1 d) and then becomes slow until stable. Besides, the greater the unloading amplitude is, the smaller the shear strength and modulus are, and the longer it takes to reach a stable state for expansive soil. It is suggested that the slope excavation should be supported timely, and is recommended in the slope stability analysis. The results of mercury test and absorption (discharge) show that the effect of the unloading interval on the shear characteristics of the undisturbed expansive soil is affected by the coupling effect of microfracture development and intergranular cementation.

Key words: undisturbed expansive soil, unloading, ageing effect, shear strength