The ageing effect on the shear strength of Gaomiaozi (GMZ07) bentonite with different water contents were investigated in this paper. Water contents of 10%

18% and 24% (saturated) of compacted bentonite specimens were kept constant during curing periods of 0

5

15

30 and 90 days under constant volume conditions. Then

the shear tests on the aged specimens were performed at vertical pressures of 400

800 and 1 600 kPa by using the shear testing apparatus. At the same time

the microstructure features of the aged bentonite specimens were characterized by means of mercury intrusion porosimetry (MIP) test. The test results show that the shear strength of Gaomiaozi bentonite decreases with the ageing time. The shear strength significantly decreases at the early ageing days and then tends to stabilize when the curing time is over certain days. According to the failure envelopes of three specimens with different water contents

the internal friction angles slightly reduce and the cohesion changes little for saturated specimens with the increase in curing time. For the specimens with water contents of 10% and 18%

the cohesion evidently decreases but the friction angles remain almost unchanged. According to the effective stress of unsaturated soils proposed by Alonso et al and the pore-size distribution

it is considered that the increment in the intra-aggregate pore is the reason for the decrease in the shear strength of the unsaturated bentonite with ageing time.