Influence of the burial depth of series-connected shallow vertical U-tube ground heat exchangers on rock mass temperature

• Different the burial depth of series-connected shallow vertical U-tube ground heat exchangers were studied. • The burial depth of series-connected shallow vertical U-tube ground heat exchangers significantly changed the temperature field of rock mass. • A model of influence of the burial depth of series-connected shallow vertical U-tube ground heat exchangers on rock mass temperature was proposed. This study investigated the influence of the burial depth ( H ) of series-connected shallow vertical U-tube ground heat exchangers (GHEs) on the rock mass temperature. The 3DEC discrete element program was employed to compute the rock mass temperature field and to quantify the evolution of outlet water temperature at the production well. Simulations considered U-type GHEs with H = 6 m, 9 m, 12 m, and 15 m. The results indicated that temperature superposition between the injection and extraction pipes produced localized temperature increases. Initial rock mass temperature increased with H . The difference in temperature gradients among the H values diminished at the steady-state. Increasing H expanded the vertical extent of the thermal influence zone and reduced its horizontal extent. Larger H yielded clearer thermal plume morphology. During the first 12 years, outlet water temperature decreased slightly as H increased. After 12 years, outlet water temperature rose modestly with increasing H . Time to reach steady state increased with H . Heat exchange efficiency improved with increasing H . The temperature difference between monitoring points A and B increased with H . Larger H produced steeper rock mass temperature gradients and reduced long-term thermal stability. The overall temperature variation across monitoring points reached its maximum (10.2°C) at H = 15 m. Monitoring point E exhibited the largest fluctuation (19.51°C). These findings provide a theoretical basis for optimizing the design of ground source heat pump systems.