Impact of groundwater level and soil air entry suction on energy tunnels’ thermal performance

The use of tunnels for exploiting shallow geothermal energy has gained significant attention in recent years. Consequently, understanding the influence of various ground conditions is essential for accurately estimating heat exchange rates. In practice, tunnels may locate above groundwater level, within a partially saturated zone. Since thermal properties of ground are moisture dependent, heat exchange rates vary according to soil type, groundwater level, and air entry suction. A numerical model was developed and validated to investigate influences of groundwater level and air entry suction in sand, silt, and clay soils. The results show that the heat exchange in sand improves in the groundwater presence, and this positive effect disappears when the groundwater level falls more than half a tunnel diameter below the invert. The known influence of groundwater flow becomes significant when groundwater level is above the invert. In silt, the presence of groundwater level between ground surface and five tunnel diameters below the tunnel centreline has a moderate effect, while in clay, this effect is negligible. Finally, parametric analyses were employed to create design charts that enable preliminary estimation of heat exchange rates in sand based on groundwater level.