Borehole heat exchanger backfilling control-Evaluation of a magnetic method

摘要

When installing borehole heat exchangers, it is essential to use proper backfilling to avoid hydrogeological failure. In this study, a magnetic susceptibility system for backfilling control was investigated at the laboratory scale. The sensor penetration depth, its sensitivity and the unknown values were assessed by using magnetite-containing backfilling material samples. In addition, the capability to detect slurry levels and cavities was investigated, as well as measurement disturbances. Measured values correspond to volume susceptibilities in the SI system multiplied by a factor of 350 for translation into clear integers in the range from 0 to 11 [-]. This approach was evaluated by comparative measurements using a second susceptibility sensor. The reason for the translation is to provide an easier system for handling at construction sites. The 20-35 mm penetration depth and a sensitivity of only 10% for penetration depths > 20 mm is sufficient for slurry level detection; however, it is not sufficient for detecting all critical cavities. Differentiation between susceptibility weakening as caused by tubes, spacers and cavities is difficu in addition, specific types of rocks can create disturbances. Optimization of penetration depth, measurement procedure as well as material susceptibility can improve quality control to avoid failure events and possibly ease future installation approval in currently forbidden hydrogeological areas.