Heat transfer performance of HDD‑installed horizontal ground heat exchangers: influence of subsurface stratigraphic structure under low-groundwater-flow conditions

Horizontal ground heat exchangers (GHEs) installed using the Horizontal Directional Drilling (HDD) method offer a cost‑effective alternative to vertical systems in regions with fragile geological formations such as Japan. When placed within permeable aquifer layers, these systems may benefit from groundwater‑induced convective heat transfer; however, the actual influence of groundwater flow on horizontally installed GHEs remains insufficiently understood. This study experimentally investigates the heat transfer performance of two HDD‑installed horizontal GHEs of different lengths under both heating and cooling conditions in Saga City, Japan. The subsurface at the installation depth consists of alternating clay and coarse sand layers, and a nearby boring survey indicates that the heat exchangers traverse the boundary between these strata. Measured heat transfer rates were compared with theoretical predictions based on the Kelvin line‑source model to estimate the effective thermal conductivity of the surrounding soil. The north heat exchanger exhibited thermal conductivities of 1.0–1.2 W/(m·K), whereas the south exchanger showed higher values of 1.6–2.9 W/(m·K), reflecting strong geological heterogeneity along the horizontal installation path. Analysis of Nusselt and Fourier numbers revealed no clear enhancement due to groundwater flow, consistent with previously reported low groundwater velocities in the region. These findings indicate that subsurface stratigraphic heterogeneity plays a dominant role in the thermal performance of horizontal GHEs, while the influence of groundwater flow remains limited under low-velocity conditions. The results provide practical guidance for the design and installation of HDD-based horizontal ground heat exchangers in heterogeneous subsurface environments.