The effect of confining pressure, temperature, and suction on the shear strength and stiffness of unsaturated buffer

Abstract

A series of quick-undrained triaxial tests has been conducted on sand-bentonite specimens with degrees of saturation from 50% to 100%, confining pressures from 0.2 MPa to 3.0 MPa, and temperatures of 26$\sp\circ$C, 65$\sp\circ$C, and 100$\sp\circ$C. These test have been described as "undrained-undrained-undrained" (UUU) tests because drainage leads were kept closed through (a) heating, (b) pressuring, and (c) shearing. The sand-bentonite material is to be used as a buffer material in the Canadian concept of nuclear waste disposal. An understanding of the influence of temperature on strength and stiffness of the unsaturated buffer is required, particularly for this application where saturation, pressure, and temperature are expected to vary significantly. Additional tests include effects of wetting up or drying down compacted specimens, effects of air drainage, and effects of lengthening the constant heating time. Specimens obtained from AECL's Buffer/Container Experiment (BCE) are also tested under UUU conditions. Shear strength envelopes in q,p,S space show increases in strength with increasing pressure, and suction; but only small decreases in strength with increasing temperature. Initial slopes of these envelopes are pressure and temperature dependent. Saturation of pores during shearing at high pressure produces maximum shear strengths. These strengths can be related to the saturated specific clay volume. Altering water contents in compacted specimens affects strength and compressibility by changing soil structure and suction. Air drainage results in only slightly higher strengths and stiffnesses than in UUU tests. The BCE specimens have broadly similar strengths and stiffnesses to those of the laboratory-compacted UUU and wetted or dried specimens in the range of water contents investigated.