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The freezing and thawing cycles applied to soils can often be causes of irreversible instabilities, uplift, or subsidence. These phenomena can be both natural ( e.g . permafrost regions) or artificially induced ( e.g . artificial ground freezing (AGF), as an excavation earth support technique for tunnelling). When the soil temperature falls below 0°C, the liquid water changes phase turning into ice. The freezing process generally implies an expansion of the soil element due to (i) the lower density of the migration of the ice and (ii) the pore water toward the frozen front. The complex interaction between the solid grains, the water and the ice formation during freezing determines the soil’s overall thermo-hydromechanical (THM) behaviour. Sandy mixtures with different percentages of kaolin were tested on an oedometer developed at the Geotechnical Laboratory of Universita degli Studi di Roma Tor Vergata, working at temperatures below zero. The samples were compressed under five different vertical stresses (50, 100, 200, 400 and 800 kPa, respectively), and then a freezing and thawing cycle was applied by steps to a minimum temperature of -20°C. The experimental results regarding temperature evolution, vertical displacements, and liquid water amount monitored are discussed and interpreted through a micro-to-macro approach. The data analysis revealed interesting outcomes that characterise mechanical and hydraulic hysteresis during freezing and thawing processes. A deeper understanding of the coupled phenomena that occur during the freezing and thawing cycles is supposed to contribute to developing a constitutive model that can reproduce the irreversible volumetric response of frozen soils.

Artificial ground freezing (AGF) has been widely used as a temporary soil stabilization and waterproofing technique in geotechnical practices (e.g., tunnel construction). Many sources of uncertainty exist during AGF. Firstly, groundwater seepage flow can adversely affect the freezing efficacy. Secondly, freeze pipe inclination inevitably occurs during installation, which is likely to yield an unfrozen path and elevate construction risk. Thirdly, as a key soil parameter, the spatial variability in thermal conductivity can also affect the freezing process. In this work, a unit cell model of freeze pipes is established by a coupled thermo-hydraulic finite element method to examine the effects of these sources of uncertainty. The pipe inclination is considered in the unit cell model by prescribing various values of freeze pipe spacing. The thermal conductivity of soil solid is simulated as a three-dimensional lognormal random field to account for the spatial variability of soil. Results are tabulated to evaluate the additional freezing time required in the AGF system due to the existence of these uncertainties. The findings are capable of determining a reasonable range of freeze pipe spacings and the corresponding critical seepage velocity, and can offer practitioners a rule of thumb for estimating freeze pipe spacing.

For decades, artificial ground freezing (AGF) has been used as a temporary soil stabilization and waterproofing technique in multiple geotechnical engineering applications. Experience gained from AGF experiments indicates that the pore water expansion during freezing and the resulting pressure have the potential to induce ground movements in adjacent nonfrozen areas. This process was investigated in this paper using a comprehensive set of in situ temperature and displacement monitoring data collected in the Cigar Lake underground mine, Canada. The data set allowed to investigate the mechanical impact of freezing on a mine tunnel and prompted the need to derive a fully coupled thermo-hydro-mechanical model to predict ground temperature and displacements. Thermodynamically consistent, the model developed for this study is based on a macroscopic continuum approach and uses simplifying assumptions to overcome the computational difficulties associated with the modeling of complex mining environments over a long period of time. This model was used to perform three-dimensional finite-element simulations of the ground freezing and excavation activities in the Cigar Lake mine, showing good agreement with field measurements.

The control of freezing temperatures throughout the artificial ground freezing (AGF) process is always difficult. An overly high temperature of the circulating refrigerant may lead to insufficient frozen soil strength, while an overly low temperature may cause unnecessary energy waste, and even excessive pore ice may damage the soil structure and reduce the frozen soil strength. What's more, overly freezing may damage buildings on the surface. Therefore, it is of great significance to study the optimum freezing temperature (OFT), which is very important for better and more energy-efficient employment of the AGF method. In this paper, we use uniaxial compression and direct shear tests to obtain dynamic mechanical parameters in the soil freezing process. After the analysis of varying mechanical parameters by the entropy weight TOPSIS principal component analysis method, the results show that the interval range of OFT for saturated and unsaturated sandy gravel is [− 10 °C, − 15 °C] and [− 15 °C, − 20 °C], respectively. The findings indicate that, in the AGF method, a lower temperature is not always preferable. According to the results, constructive measures to optimize the temperature field distribution in the AGF method are proposed. The research results will contribute to the assessment of the safety and efficiency of AGF projects.

•AGF as a temporary ground stabilization technique for urban tunnel excavation.•Collection of extensive monitoring data from an AGF field trial Fori Imperiali.•Development and validation of a 3D TH numerical model to back-analyze the field results.•Innovative heat transfer analysis to define the freezing pipes boundary conditions.•Role of thermal and hydraulic properties, and water seepage in the AGF process. This paper describes a field trial of artificial ground freezing (AGF) carried out in connection with the construction of Line C of Roma underground. AGF was one of the options considered for the temporary stabilisation of the ground during the excavation of Colosseo-Fori Imperiali Station. The field trial aimed at assessing the feasibility of AGF in the complex soil profile and groundwater regime of the subsoil of the historical centre of Roma by establishing the response of the subsoil to the imposed freezing loads, the ability to create a continuous frozen wall, and the associated coolant consumption. The extensive monitoring data were exploited to conduct a detailed analysis of the transient freezing process in the stratified subsoil and used to develop and validate a three-dimensional thermo-hydraulic numerical model. Special attention was given to defining and applying appropriate boundary conditions at the freezing pipes. The paper discusses the main factors affecting the time-dependent freezing process and explores the applicability of simplified two-dimensional models for the Fori Imperiali AGF field trial.

An artificial ground freezing (AGF) technique in the horizontal direction has been employed in Naples (Italy), in order to ensure the stability and waterproofing of soil during the excavation of two tunnels in a real underground station. The artificial freezing technique consists of letting a coolant fluid, with a temperature lower than the surrounding ground, circulate inside probes positioned along the perimeter of the gallery. In this paper, the authors propose an efficient numerical model to analyze heat transfer during the whole excavation process for which this AGF technique was used. The model takes into account the water phase change process, and has been employed to analyze phenomena occurring in three cross sections of the galleries. The aim of the work is to analyze the thermal behavior of the ground during the freezing phases, to optimize the freezing process, and to evaluate the thickness of frozen wall obtained. The steps to realize the entire excavation of the tunnels, and the evolution of the frozen wall during the working phases, have been considered. In particular, the present model has allowed us to calculate the thickness of the frozen wall equal to 2.1 m after fourteen days of nitrogen feeding.

The mechanical properties of frozen ground are key parameters in design and implementation of artificial ground freezing (AGF) in underground projects. Soil samples were obtained from the urban underground railway project site in Tabriz, Iran. The specimens were classified as SP and CL according to the USCS. The specimens were remolded in accordance with the site conditions. Over 120 triaxial compression tests were conducted on the frozen samples at different temperatures, confining pressures and strain rates. The results show that the frozen SP and CL soils exhibit strain-softening and strain-hardening behaviour, respectively. In all cases, Young’s modulus increases with decreasing temperature and increasing strain rate and confining pressure. Also, the shear strength increases with decreasing temperature and increasing strain rate. In all tests, the Young’s modulus and shear strength of the SP soil are greater than the CL soil. Based on the results of this research, the application of artificial ground freezing was recommended for coarse-grained and non-cohesive soils like SP in the Tabriz underground railway project. 

Artificial ground freezing technology empowering the construction of urban subway tunnels has been recognized as one of the most environmentally, friendly and efficient construction methods. However, ground frost heave and thaw settlement are the primary issues to be addressed in engineering practice, and anticipating these issues in advance will bring tremendous assistance to the construction of subway tunnels. Therefore, a three–dimensional thermodynamic coupling method is derived considering the phase transition process and the anisotropic characteristics of freeze–thaw soil. By calling the compiled incremental matrix equation in ABAQUS, the whole process simulation of the freezing construction of a plane skew connecting channel of Fuzhou Metro Line 5 is realized. The numerical simulation results indicate that the evolution process of the freezing temperature field and thawing temperature field in numerical simulation is consistent with the theoretical design, and the natural thawing time is about 1.5 times of the positive freezing time. Besides, the evolution law of ground surface displacement in numerical simulation is consistent with the field measurement, and their displacement–time curves conform to the power function fitting relationship, and the correlation coefficients are all greater than 0.9. After freezing for 45 days, the ground surface frost heave displacement at the midpoint of the connecting channel in numerical simulation is 52.43 mm, while the measured value on site is 49.58 mm, with an error of only 2.85 mm. After thawing for 68 days, the ground surface thaw settlement displacement at the midpoint of the connecting channel in numerical simulation is − 23.77 mm, while the measured value on site is − 24.02 mm, with an error of only 0.25 mm. All these indicate the accuracy of the established numerical simulation prediction method.

Liquid nitrogen is the most common refrigerant adopted in the artificial ground freezing (AGF) method for the rapid freezing of soil. However, the relatively high price of liquid nitrogen demands the reuse of liquid nitrogen in AGF, which utilizes partially gasified liquid nitrogen after an initial injection. This study investigated the reusability of liquid nitrogen in AGF by performing a field experiment. Temperatures of the ground were monitored near the sub-freezing pipes installed 1 m away from the main freezing pipes, where liquid nitrogen was initially injected. A frozen wall having a thickness of 1 m was formed between two sub-freezing pipes after 5 days of injecting liquid nitrogen into the main freezing pipes. Furthermore, the lowest temperature of  − 12 °C measured in the sub-freezing pipe implied that the temperature of nitrogen after circulating through the main freezing pipe was sufficiently low to freeze the surrounding soil formation. The freezing rate, elapsed time for freezing, and freezing duration evaluated from the monitored temperature data also demonstrated the promising potential of reusing liquid nitrogen in AGF for saturated silty deposits.

This research work presents a comprehensive experimental study of frost heave in a fine-grained material to investigate the effects of top freezing (TF) and bottom freezing (BF) mechanisms with ice lens formation. A novel test device was built to investigate artificial ground freezing (AGF)-related temperature and load boundary conditions. This paper includes 62 frost heave experiments and test observations up to 10 days. The long test duration allows a precise examination of ice lens growth during thermal steady state when the frost line remains largely stable and the ice lens grows. This state corresponds to the holding phase of a practical in situ AGF implementation where the cooling is used to maintain the frozen body thickness. The freezing observations show that BF heaving is larger than TF heaving in most cases. This is caused by the more favorable hydraulic conditions caused by gravitational effects and vertical cracking that occurs during ice lens formation due to suction. This facilitates water accumulation at the ice lens. An applied load reduces the differences between BF and TF conditions beyond a certain value which corresponds to an overburden capable of preventing the formation of the longitudinal cracks.