Evaluation of bentonite-based thermal backfill materials

The knowledge of soil thermal resistivity is required for projects such as thermal backfills for underground power cables, high-level nuclear waste disposal, pipelines carrying hot fluid and so on. The role of the backfill is to dissipate heat easily with minimal loss of moisture and low shrinkage. Clayey soils such as bentonite have high water retention capacity but low strength, high shrinkage and high thermal resistivity. Cohesionless geomaterials such as sand and fly ash exhibit low shrinkage and water retention. These materials have lower thermal resistivity than bentonite. The mix of sandfly ash with bentonite contributes to higher strength. Therefore, it is possible to mix bentonite with sand or fly ash to obtain an optimal thermal backfill mix with requisite thermal and engineering properties. There are few studies that deal with the thermal resistivity of bentonite-based backfill mixes. The present study deals with the development of the thermal resistivity variation curve (TRVC) for such mixes, which is the variation of thermal resistivity with water content. The critical moisture content has been determined from the TRVC and has been correlated with geotechnical properties such as plastic limit and optimum moisture content.