Experimental Study of Artificial Ground Freezing by Natural Cold Gas Injection

Zhao Liu,Youhong Sun,Bingge Wang,Qiang Li

doi:10.3390/app10176055

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.

Highlights

Artificial ground freezing (AGF) is used to temporarily freeze the formation matrix and water together, which can improve the strength of soil and redirect the flow paths of groundwater
The evolution of temperature in the freezing model is the main thermal characteristic during natural cold gas injection, which reflects the development of the frozen range
Before the temperature in the freezing model dropped below 0 ◦ C, the water had not been frozen yet, and the thermal conductivity of the water, 0.613 W/(m·K), is much lower than that of ice, which is 2.31 W/(m·K)

Summary

Introduction

Artificial ground freezing (AGF) is used to temporarily freeze the formation matrix and water together, which can improve the strength of soil and redirect the flow paths of groundwater. This method is widely used for deep excavation, tunnels construction, and strengthening building foundations, especially in soft rocks and shallow soils [1,2]. The construction of Nanjing Metro Line 2 in China used AGF to treat the inrush of water during excavation, in which brine at −20 ◦ C, circulating in double-walled vertical pipes with well spacing of 1 m, was used to freeze formations, and construction conditions were realized successfully after 40 days [6]. The Fürth subway and undercrossing of the Limmat River in Zurich were constructed using brine at −40 ◦ C, circulating in horizontal pipes

Results

Discussion

Conclusion