Influence of Soil Structure on the Stress-Strain Behavior of Sand-Bentonite Mixtures

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Courtesy of ASTM International

The influence of soil structure on the behavior of clay-rich mixtures has been examined. Mixtures of sand and bentonite, known as buffer, have been studied to determine their suitability for nuclear fuel waste containment. Triaxial tests have been performed on compacted specimens to determine the strength and constitutive behavior of the buffer. Two techniques were followed in preparing specimens. In the first method, specimens were compacted to dry densities of 1.5 Mg/m3 and 28% moisture content and were allowed to equilibrate to their confining pressures prior to test. The second method, which aimed at shortening the test durations, involved forming specimens at dry densities corresponding to equilibrium states at the test pressure levels. This required compacting sand-bentonite mixtures at different densities and moisture contents.

The study indicates that the compressibility and stress-strain behavior of the buffer are strongly influenced by specimen preparation techniques. Because of the different degrees of soil manipulation and moulding moisture contents, the specimens had different macrostructures. This affected the swelling and shear behavior of the buffer. However, preyield porewater pressure response and ultimate shear resistance appeared to be unaffected by preparation techniques. Shear test results suggest that the buffer is anisotropic and slightly strain softening in shear. An ultimate strength envelope of c´ = 0 kPa and f´ = 14° is determined in the pressure range of 0 to 1.5 MPa.

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