A new type of thermally controlled subgrade is proposed to mitigate persistent frost heave issues of railway subgrades in seasonally frozen regions. A dedicated ground-source heat pump system collects low-grade geothermal energy from the stable soil layer near the subgrade, converts it into high-grade thermal energy, and transfers it to the frigid subgrade for active heating and temperature control, thereby eliminating the adverse effects of frost heave. A 20-metre-long test of thermally controlled subgrade was constructed in a frost heave of the Junggar-Shenchi Railway in Shanxi Province, China. During the winter spanning 2021 and 2022, the heating temperature of the heat pump, the thermal regime of the test subgrade and the natural subgrade, the frost depth, and the track heave were measured. The results indicate that the heat pump temperature could reach a peak of 59.4 degrees C, with the average daily heating temperature during intermittent operation reaching 25.2 degrees C or higher, indicating an efficient heat source that plays a favourable role. The freezing period of the natural subgrade lasted for 141 days, while the subgrade in the test was 20 days shorter. The maximum frost depths at the track centre, shoulder, and embankment slope toe in the test were 88 cm, 75 cm, and 58 cm, respectively. These depths were 60 cm, 122 cm, and 78 cm less than those of the natural subgrade, effectively controlling the frost depth within the threshold that may cause potential structural damage. Under natural conditions, the track heave reached a peak of 9.4 mm, leading to a harmful frost heave scenario. In contrast, the track deformation in the test was less than 3 mm, which did not exceed the regular maintenance threshold. The thermally controlled subgrade proves to be an effective method for preventing and controlling persistent frost heave damage in critical locations such as low embankments, cut subgrades, turnout areas, and culvert roofs.

Lunar in-situ water ice utilization is considered an essential part of the future construction of Lunar Bases. However, the thermal conductivity of lunar regolith without water ice is extremely low, which seriously hinders the thermal mining of lunar water ice. In this study, we proposed a novel approach to optimize the energy ef-ficiency of water ice thermal mining. In this method, a constant temperature heat source with a heating tem-perature selected according to the particle size of water ice was used to slow down the reduction rate of the thermal conductivity of icy soil. Our simulation results showed that the relatively high mining temperature led to the rapid sublimation of water ice near the heat source, reducing the thermal conductivity of the icy soil and the energy efficiency.A relatively low mining temperature decreases the sublimation speed of water ice and reduces energy effi-ciency. The particle size of water ice determined the decreasing rate of thermal conductivity of icy soil, thus affecting its optimum heating temperature. Using a constant temperature heat source at the optimal heating temperature, the energy efficiency of water ice mining could be increased by several orders of magnitude compared with constant power heating.