## 4 Experiment Description and Results Analyses
### Heat-Absorbing Temperatures and Heat-Supplying Temperatures  
Figure 7 shows the temperature on the outer walls of the evaporator and condenser at a heat-supplying temperature of 30 \({}^{\circ}\)C. It was observed that, as the system activated, the supplying and absorbing temperatures increased and decreased, respectively. Due to the gradual heat transfer process of the helically coiled tubes and the distance between the compressor and condenser, the temperatures on the outer walls decreased gradually along the flow direction. When the temperature at the mid-position of the condenser was higher than 30 \({}^{\circ}\)C, the DE-GSHP stopped, while it re-started automatically when the temperature decreased to 5 \({}^{\circ}\)C. Through these start-stop cycling processes, the heat-supplying temperatures varied from 15 to 40 \({}^{\circ}\)C. Furthermore, the heat-absorbing temperature varied from \(-\)5 \({}^{\circ}\)C to \(-\)10 \({}^{\circ}\)C, indicating that the flow temperature was cooler than the surrounding soil. The DE-GSHP efficiently collected heat from the deep stable stratum through the cyclic evaporation and condensation of the refrigerant, driven by the temperature difference between the ground heat exchangers and the surrounding soil. Thus, the temperature remained constant, even when the air temperature fluctuated significantly.  
Figure 6: Test conditions of the experimental facility. (a) Schematic representation (Unit: mm). (b) Experimental conditions.  
Figure 8 shows the average temperature variations on the outer walls of the DE-GSHP. When the preset heat-supplying temperatures were 30, 45, 60, and 75 \({}^{\circ}\)C under the constant temperature operation mode, the actual average temperature ranges were 16.80-30.69 \({}^{\circ}\)C, 28.61-41.78 \({}^{\circ}\)C, 34.40-50.89 \({}^{\circ}\)C, and 46.29-63.45 \({}^{\circ}\)C, respectively. The temperatures at different positions of the GSHP were mainly influenced by the refrigerant flow direction, which could be adjusted by changing the coil direction or the temperature sensor location of the computer-based controller. A stable performance of the GSHP was achieved, despite significant fluctuations in air temperature during the test, as the ground temperature was only affected by these fluctuations up to a few meters below the ground surface.  
Figure 9 depicts the daily average heat-supplying and -absorbing temperatures observed during the tests. The average heat-supplying temperature for the DE-GSHP reached above 50 \({}^{\circ}\)C, while the average heat-absorbing temperature was maintained at less than 0 \({}^{\circ}\)C. Despite the low ambient temperature, the system was able to maintain a desirable output temperature level after being activated. However, the heat-absorbing temperature unfavorably increased with an increase in the preset heat-supplying temperature. This was because the GSHP operated for a long duration in a high preset constant heat-supplying temperature mode, and the heat supplied to the soil surrounding the condenser was transferred to the distant ground. A high proportion of heat around the condenser can deteriorate the condensation effect and increase the circulating temperature of the refrigerant, which, in turn, affects liquefaction [22,23]. During practical applications, soil frost heave can be eliminated if the temperature is increased to above 0 \({}^{\circ}\)C. Hence, fluctua  
Figure 8: Average heat-absorbing and heat-supplying temperatures during different operation modes.  
Figure 7: Instantaneous heat-absorbing and heat-supplying temperatures observed during the tests (constant heat-supplying temperature of 30 \({}^{\circ}\)C).  
tions in the heat-supplying temperature will not affect the anti-frost effect of the DE-GSHP. Conversely, the heat-supplying temperature levels should be determined according to the actual heating demand of the embankments.