## 4 Discussion
### Analysis of Feasibility  
There are two main factors affecting the feasibility of the method proposed in this study--the freezing rate and the injectability of cold gas. According to the above results, there is no doubt that cold gas injection can ensure a high freezing rate. At the same time, the decrease in the pressure drop is also conducive to the injection of cold gas; therefore, the injectability of cold gas can also be guaranteed.  
In this study, the influences of soil type and gas injection direction were ignored, but the influences of these two factors can be roughly inferred based on the above results. First of all, the frost-heaving characteristics and particle size distribution of soil primarily affect the flow of fluids, and then they affect the injectability of cold gas. The injectability mainly depends on the opening of gas flow channels; based on the above results, gas flooding can quickly open the gas flow channels before the pore water is frozen. Therefore, we can infer that for homogeneous soils, the existence of gas flooding can ensure the opening of gas flow channels, so as to guarantee the injectability of cold gas. For heterogeneous soil, the permeability difference is the main characteristic. In general, cold gas will flow along the direction of high permeability; this area will be frozen first. Then, cold gas flow turns to the low permeability matrix. In addition, existence of the firstly frozen area will freeze the surrounding low-permeability area by heat conduction. Therefore, cold gas flow has the potential to freeze heterogeneous media successively. However, there are several application limitations of this new method on the fractured formation; for example, cold gas may flow away from the fractures. In addition, the injection direction of gas was set as upward in this study, under the action of buoyancy, the flow of gas will undoubtedly speed up, and then, the cooling rates were improved. However, compared to the driving force of gas injection, the effect of buoyancy appears very small. It should be noted that the cold gas will gather to the upper part of the formation due to buoyancy in the actual construction process, but in the near source area, gas will tend to flow laterally under the driving force of injection. Therefore, the \"cylindrical\" frozen range around the injection well can be achieved. In conclusion, the method proposed in this study has a high freezing rate with strong applicability. However, considering the low heat capacity of gas, the further away from the gas injection point, the lower the freezing rate. Therefore, in the actual construction process, the frozen range of a single well should be designed reasonably. Perhaps a range of approximately 3 m is appropriate.  
In addition, the impacts of gas injection and displacement of water on structural instabilities need to be considered. Gas injection, performing various roles in porous media, is involved in several domains; in this work, we adopt cold gas injection to freeze the soil. Without a doubt, local variationsof underground pressure and structural disturbance will be brought about by cold gas injection, but according to the previous research of gas injection (e.g., air sparging, new Austrian Method of tunnel construction), no obvious negative effect will be caused [30; 31; 32]. In addition, the flow rate can also be controlled to reduce the impact on the formation. Therefore, the impact of gas injection and displacement of water can be controlled to ensure safe construction.