Field measurement and numerical investigation of artificial ground freezing for the construction of a subway cross passage under groundwater flow

Abstract

Artificial ground freezing is an environmentally friendly and reliable method to provide temporary ground improvement and groundwater isolation under adverse geological and hydrological conditions. Groundwater flow supplies a substantial source of heat which has an undesirable impact on the closure of frozen soil body. Furthermore, the phenomenon of frost heave and thaw settlement may threaten the surrounding structures. In terms of a ground freezing case in Beijing Metro Line 19, field measurement of ground temperature and deformation were conducted for the prediction of the formation of frozen curtain and the site stability. A hydro-thermal coupling model considering the effect of groundwater flow and phase transition was proposed to quantify the temperature distribution. The numerical model considers the soil particles, crystal ice and liquid water as separate phases, and thermal parameters as variable values related to temperature. Through two basic physical laws and corresponding governing equations, the model captures the relevant coupled relationship in heat and mass transfer. The proposed model is validated by means of the field measured data. Furthermore, the influence of groundwater velocities on the closure of frozen curtain was investigated. The results indicate groundwater flow has an adverse influence on the formation of freezing curtain, even leading to a failure to the closure. The thickness and temperature of frozen curtain with a groundwater velocity of 2 m/d can attain the design values after freezing for 50 days. Moreover, the ground and tunnel deformation in the process of freezing construction were well controlled.