Shear failure properties of compacted bentonite under unsaturated and saturated conditions

Abstract

Compacted bentonite is planned for use as a buffer material in the geological disposal of the high-level radioactive waste. In the present paper, in order to study the failure behavior of the compacted bentonite under the unsaturated condition, a series of constant volume direct shear tests was performed using a newly developed high confining pressure direct shear testing apparatus. At the same time, shear bands were observed during direct shearing using an image processing system, which consists of a microscope and PIV digital image analysis. In addition, the insides of the sheared specimens were observed with a micro-focus X-ray CT scanner.
The over-consolidated unsaturated compacted bentonite specimen shows brittle failure behavior from an early stage of shearing. A shear band with a number of large cracks developed during shearing. By injecting water into the shear band, the cracks are gradually sealed by the swelling and expanding bentonite. On the other hand, according to the results of direct shear test for the saturated compacted benonite specimen, the shear resistance of the compacted bentonite decreases with increasing the swelling pressure and the pore water pressure. In addition, no shear bands or cracks were generated in the saturated specimen.