Experimental study of artificial ground freezing by natural cold gas injection

Abstract

The application of conventional artificial ground freezing (AGF) has two disadvantages: low freezing rate and small frozen range. In this study, a new method with natural cold gas injection was proposed, whereby the shallow soils and water can be frozen rapidly due to the effect of the heat convection. Cold gas from -15 °C to -10 °C, in the winter of northeast China, was injected into the laboratory-scale sand pipe; evolution of the induced frozen front and water migration were studied, and then, the feasibility of the new method was analyzed. According to the evolution of the induced frozen front, the freezing process was divided into an initial cooling stage, phase transition stage, and subcooled stage. The results showed that the increase of initial water content at the beginning of the experiments had little effect on the time required for completing the initial cooling stage, while the time required for the phase transition would increase in nearly the same proportion. In addition, the increase of the cold gas flow rate could not only strengthen the cooling rate of the initial cooling stage but also shorten the phase transition time; thereby, the freezing rate was increased. The freezing rate could reach 0.18-0.61 cm/min in the direction of cold gas flow, and compared to the conventional AGF (months are required for approximately 1 m), the freezing efficiency was greatly improved.