Temperature compacted bentonite

Villar, M.V., and Lloret, A., Temperature influence on the hydromechanical behavior of a compacted bentonite. Proc. Int. Meeting on Clays in Natural and Engineered Barriers for Radioactive Waste Confinement. Reims 2002. 

TEMPERATURE INFLUENCE ON THE MECHANICAL BEHAVIOUR OF A COMPACTED BENTONITE... 

The space between the drift and the heaters is fdled by blocks of compacted bentonite with a smectite content in the range of 88%-96%. The test is heavily instrumented with measurements of temperatures, relative humidity (equivalent to total suction), pore pressures, displacements, and stresses. 

This design was adapted by Nagra to suit local conditions, resulting in the concept shown in Fig. 2. The repository would be situated 800-1200 m deep in the crystalline basement of northern Switzerland and, because the original design specifications called for temperatures of less than 100°C in significant parts of the backfill, an option with a very thick compacted bentonite backfill (or buffer) was selected. 

Three tests on bentonite compacted at an initial dry density of 1.7 g/cm with its hygroscopic water content have been performed at different temperatures (20, 40 and 60 °C), following the suction and stress paths indicated in Figure 4. In the first step, under a vertical load of 0.1 MPa, the suction of the sample was equalised to 14 MPa (due to mechanical limitations of the cell, this is the maximum suction that can be controlled). Afterwards, keeping this suction constant, the vertical load was increased up to 5 MPa. Under this vertical load, suction was decreased by steps until... 

Figure 5 shows the volume change measured in the samples loaded at different temperatures keeping their water content constant. As it has been observed in saturated materials, the temperature increases the compressibility of the bentonite. A certain reduction in the size of the elastic domain with the temperature can be observed (Hueckel Borsetto 1990), despite the fact that the vertical stresses applied are smaller than the compaction load. 

Many of the lining materials commonly used for liquid storage cannot be used in salt gradient solar ponds. Compacted soils, native clays, soil additives or soil cement, swelling clays such as Bentonite, are not Impermeable to high temperature saturated salt brine solutions. Only the flexible membrane lining materials offer the potential for impermeability (zero leakage)... 

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