Field determination of shallow soil thermal conductivity using a short-duration needle probe test

The thermal conductivity of shallow subsoil, one of the important input parameters in the design of a horizontal closed-loop ground heat exchange system, can be derived by means of a thermal needle probe survey. The probe is mounted on a 1.5 m handle, allowing short-duration (10 min) thermal response tests to be carried out at various locations and depths within a land area designated for such a heat exchanger. At two study sites in Central England, a minimum of 12-16 such determinations were required to obtain a stable estimate of bulk thermal conductivity (based on the geometric mean of single measurements), although one cannot necessarily assume that this minimum requirement would be generally valid at all sites. At the two sites, the method proved relatively rapid and able to deliver a value of geometric mean thermal conductivity with a typical uncertainty of the order of 15%.