An in-situ test on a borehole provides a way to estimate soil properties, which are needed to design geothermal heat pump systems. Having sufficient testing time becomes an issue in planning, performing, and interpreting the test. This paper develops a quick method to calculate the minimum testing time necessary to estimate soil thermal conductivity within 10% of the estimated value from a very long test. The quick method is based on an analytical model of the in-situ test expressed in a set of graphs with the least number of independent dimensionless groups. The results indicate minimum testing times can vary by over a factor of 100 among different tests. Even among a cluster of boreholes, application of the quick method demonstrates that minimum testing time varies by a factor of four with grout thermal conductivity and whether or not spacers are placed in between U-tube legs. Therefore, no simple rule for the minimum testing time applies to all cases. Instead, the proposed method, based on a set of graphs, offers a quick estimate of the minimum time to help plan, perform, and interpret borehole tests.

Units: Dual