Abstract
Artificial ground freezing (AGF) technology has been commonly applied in tunnel construction. Its primary goal is to create a frozen wall around the tunnel profile as a hydraulic barrier and temporary support, but it is inevitably affected by two natural factors. Firstly, seepage flows provide large and continuous heat energy to prevent the soil from freezing. Secondly, as a key soil parameter in heat transfer, the soil thermal conductivity shows inherent spatial variability, binging uncertainties in freezing effects and efficiency. However, few studies have explored the influence of spatially varied soil thermal conductivity on AGF. In this study, a coupled hydro-thermal numerical model was developed to examine the effects of seepage on the formation of frozen wall. The soil thermal conductivity is simulated as a lognormal random field and analyzed by groups of Monte-Carlo simulations. The results confirmed the adverse effect of groundwater flow on the formation of frozen wall, including the uneven development of frozen body towards the downstream side and the higher risk of water leakage on the upstream face of the tunnel. Based on random finite element analysis, this study quantitively tabulated the required additional freezing time above the deterministic scenario. Two levels of the additional freezing time are provided, namely the average level and conservative level, which aim to facilitate practitioners in making a rule-of-thumb estimation in the design of comparable situations. The findings can offer practitioners a rule of thumb for estimating the additional freezing times needed in artificial ground freezing, accounting for the seepage flow and spatial variation in soil thermal conductivity.
Highlights
This study developed a coupled hydro-thermal numerical model to investigate the performance of an artificial ground freezing (AGF) system in tunnel excavations
Groundwater flow and spatial variation of soil thermal conductivity were considered as influencing factors
The concept of random field is adopted to characterize the spatial variation of soil thermal conductivity
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Two natural and inevitable factors, namely, groundwater seepage and uncertain soil thermal properties, are likely to affect the freezing evolution and final freezing effects of the AGF technique. Performed a numerical study for the AGF technique applied in a Naples underground project, and they found that the soil thermal conductivity was a key parameter in ground freezing. The spatial variability of soil thermal conductivity brings the uncertainty of freezing efficiency around each freeze pipe, increasing the adverse effects caused by seepage flow. This study combines the effects of seepage flow and uncertain soil thermal conductivity on the soil freezing evolution of a metro tunnel project. The spectral representation method (SRM) is used to generate lognormal random fields of soil thermal conductivity, and groups of Monte-Carlo simulations are carried out for statistical analysis
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