Abstract

This research evaluates the thermal properties of the proposed backfill materials (prepacked concrete (PAC)) that function as the heat release medium for the underground power cable system (UPCS). First, a hot disk sensor was used to measure the thermal conductivity of small-scale backfill materials in the laboratory. The horizontal thermal response test (TRT) was then performed with a 5 × 3 × 2 m full-scale sample for the in-situ evaluation. Afterward, the data from the TRT was utilized to couple with the infinite line source model (ILS) to predict the thermal conductivity of the full-scale sample. Finally, the thermal conductivity of the PAC was back-calculated using COMSOL Multiphysics. All these tests were also performed with natural sand (conventional backfill material) to compare with the PAC. Thermal conductivities calculated by numerical simulation and the ILS model were in good agreement (difference < 3%), demonstrating that the proposed ILS model coupled with horizontal TRT data is appropriate for estimating the thermal conductivity of full-scale backfill material in situ. The results of three testing methods indicated that the prepacked concrete has a high thermal conductivity (> 2 W/(mK)), thus satisfying the heat release ability requirement for the UPCS. The laboratory test slightly underestimated the thermal conductivity (less than 8%) compared to the estimated values from the ILS model and numerical model involving prepacked concretes; however, natural sand showed a significant difference (1.365 W/(mK) and 1.8 W/(mK), 32 %) attributed to the influence of the water content change during the TRT. Therefore, it is recommended that the in-situ testing conditions should be considered for the sand (or soils) to avoid the overestimation or underestimation of their thermal conductivity.

Abstract

Earthfills are anthropogenic soil massifs that were originated by different processes than those observed in traditional soil mechanics, thus revealing some deviations to the behaviour of common natural soils. The common characterization of earthfills is based in discontinuous testing such as nuclear densimeter gauge used together with laboratory compaction tests and the stiffness evaluation obtained from plate load tests, which does not give answers in the context of strength evaluation (Cruz et al. 2008; Cruz et al. 2006). The case study presented herein refers to the geotechnical characterization of an earth fill composed by evolutive materials obtained from the de-structuration of schists, which has developed excessive settlements. The performed geotechnical characterization consisted in boreholes and regular SPT tests, Dynamic Probing (DPSH), Piezocone (CPTu) and Marchetti Dilatometer (DMT) tests, as well as triaxial, shear box, consolidation and identification tests. DMT and CPTu tests were selected not only to obtain strength and stiffness parameters, but also because of its ability to access stratigraphy and unit weights. The whole set of obtained results are presented, compared and discussed, revealing a clear convergence between results as well as some interesting particularities that may be useful in fill characterization.