Artificial Ground Freezing Technique Research Articles

Experimental research on the cooling effect of a novel two-phase closed thermosyphon with semiconductor refrigeration in permafrost regions

Hybrid two-phase closed thermosyphon (TPCT) artificial ground freezing technique is an effective cooling strategy adaptable to different geological and meteorological conditions in permafrost regions. Previous studies have focused on the feasibility of hybrid TPCT with little attention given to the cooling characteristics and long-term effects of this technique under actual environmental conditions. In this study, a novel structured semiconductor refrigeration device, with simple structure, small size, and wide operating temperature range, was designed and manufactured to act as the active condenser of a hybrid TPCT, and the cooling effect of this device was investigated during a two-year field test under stable power supply. This study showed that: 1) The minimum (maximum) average temperature of the semiconductor refrigeration two-phase closed thermosyphon (SRTPCT) evaporator was 0.5 (2.1) °C lower than the TPCT evaporator. The average ground temperature decreased by 0.3 °C, and the permafrost table rose to 0.2 m at 1 m from the evaporators around SRTPCT. The semiconductor refrigeration device added more than 9.75 MJ of cold storage to the soil around the TPCT evaporator during an operating cycle. 2) The optimal working period for the semiconductor refrigeration device was from June to October, and the cooling effect was better when the ambient temperature was low in the warm season. 3) As active condenser of hybrid TPCT in permafrost regions, semiconductor refrigeration device has a larger heat transfer power enhancement for TPCTs than vapor compression refrigeration device in the warm season, and vice versa in the cold season, as well as a higher heat transfer power enhancement for TPCTs than adsorption refrigeration device. The results of this study provide relevant parameters and references for the design and engineering application of future SRTPCT.

Fuzzy random evaluation of creep model of frozen soft soil in metro tunnel construction using artificial ground freezing technique

Mastering the creep characteristics of artificial frozen soil and scientifically evaluating the creep model is an important guarantee for the safety of subway tunnel freezing construction. Base on the construction of Nantong metro tunnel, the uniaxial compressive strength tests of the artificially frozen soft soil were carried out to obtain the influence law of temperature on the uniaxial compressive strength, and the uniaxial creep tests were carried out to obtain the influence law of temperature and stress grade on creep, at − 5, − 10 and − 15 °C. The experimental results show that the creep characteristics of frozen soft soil specimens have obvious fuzzy randomness. The traditional ant colony algorithm is improved by optimizing the pheromone fuzzification coefficient, which improves the search efficiency and avoids the local optimum effectively. Subsequently, the improved fuzzy ant colony algorithm is used to invert the flexibility parameters of commonly used permafrost creep models. The fuzzy weight of evaluation index and the fuzzy random evaluation matrix were determined to evaluate the optimal creep model under three different stress levels of frozen soft soil. Finally, the reliability of the fuzzy random evaluation method was verified by engineering measured data.

A soil freezing-thawing model based on thermodynamics

The freezing-thawing behavior of soil plays a crucial role in frozen soil engineering. In this study, a soil freezing-thawing model is proposed from the perspective of statistical thermodynamics. First, the phase equilibrium relation for pore ice-water interface is deducted by applying statistical thermodynamics and the theoretical relationship is defined between the freezing temperature (i.e., the temperature at the ice-water interface) and the potential energy (i.e., total suction) of pore-water. This relation reveals that the matric suction of pore-water solely depends on the negative temperature at the pore ice-water interface for a given soil with constant osmotic suction and air pressure. Further, using the aforementioned relation and regarding the ice content of soil as an unknown variable, the thermo-hydraulic field in the soil freezing-thawing process can be solved by the mass and heat conversation. The governing equations related to this model are proposed in detail. Finally, to enhance the applicability of the proposed model, simplified forms in three typical conditions are provided, including the freezing process of saturated soil in artificial ground freezing technique, the capillary water migration coupled with freezing process of unsaturated soil in subgrade engineering and the vapor migration coupled with freezing process of semidry coarse soil under airport runways. Overall, the proposed model can be applied to various conditions and to other types of porous media (such as rock or concrete) that undergo freezing-thawing processes.

Influences of spatial variability of hydrothermal properties on the freezing process in artificial ground freezing technique

Artificial ground freezing (AGF) has been extensively applied in underground engineering construction as a popular ground-support technique. In recent decades, a series of investigations on AGF have been conducted through analytical, experimental and numerical methods. However, as critical parameters in AGF, the inherent spatial variabilities of hydrothermal properties of soils have been merely considered in previous studies. Therefore, this work aims to explore the influences of spatial variability of the hydrothermal properties (i.e., thermal conductivity and permeability) on AGF by stochastic finite element method combined with Monte Carlo simulation (MCS). Two-dimensional lognormal random fields are generated to account for the spatial variability of hydrothermal properties. The closure time is adopted as an indicator to evaluate the impact of uncertainty in hydrothermal properties on the freezing efficiency of AGF. In addition, the effect of MCS number on the statistics of the closure time in AGF is determined. Results demonstrate that spatial variability of the hydrothermal properties considerably affects the estimation of closure time. This study reveals the importance of random analysis in coping with the variation in hydrothermal properties and provides references for the design and construction of AGF.

Three-Dimensional Numerical Modeling of Artificially Freezing Ground in Metro Station Construction

In this study, the engineering background of No. 2 complex connecting passage of Binhu Road Station/Jinhu Square Station of Nanning Metro Line 3 is investigated, where the artificial ground freezing technique is adopted. A three-dimensional finite element model is established to investigate the temperature development of the frozen soil curtain, with a simulation of the dynamic evolution of the frosted soil curtain. The finite element model is validated by comparing the overall trend of the measured temperature value and the resulting temperature value, which are roughly the same. According to the design scheme, the weakest part of the whole frozen soil curtain is the top of the bell mouth where the downhole tunnel intersects the connecting passage. It is recommended to make a row of smaller freezing holes to enhance the freezing effect in this area. The thickness of the frozen soil curtain reached 1.75 m or more, indicating that the whole frozen soil curtain meets the design requirements and shows the right features for excavation construction. After freezing for 40 days, the average thickness of the frozen soil curtain is 2.4 m, indicating that the freezing effect meets the design requirements. The project can be successfully carried out, which suggests that the underneath passage construction is feasible. As a result, the results of the numerical model are applicable for comparable projects using artificially freezing ground in metro station construction.

Coupled Geothermal Energy Simulations

This paper presents the numerical solution of thermal field of a set of geothermal coaxial probes coupled with the surrounding soil. These probes, used for the excavation of Naples underground’s tunnels with the artificial ground freezing technique, are converted for the heat transfer fluid transport for a geothermal plant. One-dimensional energy equation has been used with a single unknown in order to extract the solution of the temperature inside the probe, the temperature of the fluid, which is a boundary condition for calculating the temperature range of the soil surrounding the probes. In turn, the latter is used as a source term for the solution of the energy equation for the fluid. This paper also presents a comparison between two different probe materials i) internal and external stainless-steel crown; ii) internal rigid PVC crown and external stainless-steel crown. Such a numerical scheme allows to take into account the 3D effects of the fluid dynamics in porous media as is the soil, and, at the same time, considering a “reduced order model” inside the geothermal probes, as is the 1D model, for considering the thermal coupling effects.

Sparce Subspace Learning and Characteristic Based Split for Modelling Artificial Ground Freezing

Artificial Ground Freezing technique is often used to build underground structures, such as urban tunnels. modeling activities are necessary to optimize the design of the freezing process. The paper introduces a numerical solution of the phenomenon of water freezing for various soil types, using the finite element method combined with the model order reduction technique. In particular, the characteristic based split method is used as a base to build a model reduction scheme by means of Sparse Subspace Learning. The aim of this study is to develop a mathematical model able to carry out numerical results with negligible CPU time, by varying three parameters reproducing the characteristics of the soil, i.e. the porosity, the Darcy number and the Rayleigh number. The obtained results show how the soil parameters influence the freezing process and the dimension of ice wall.

Applicability of liquid air as novel cryogenic refrigerant for subsea tunnelling construction

The artificial ground freezing technique has been widely adopted in tunnel construction in order to impede heavy water flow and to reinforce weak sections during excavation. While liquid nitrogen is one of common cryogenic refrigerants particularly for rapid freezing, it has a serious potential risk of suffocation due to an abrupt increase in nitrogen content in the atmosphere after being vaporized. This paper introduces a novel cryogenic refrigerant, liquid air, and addresses the applicability of it by performing a series of laboratory chamber experiments. The key parameters for the application of artificial freezing using liquid air in subsea tunnel construction are freezing time and energy consumption, which were evaluated and discussed in this paper. The comparative study of these parameters between the use of liquid air and liquid nitrogen demonstrates that liquid air with no risk of suffocation can be a potential substitute for liquid nitrogen delivering the equivalent performance. In addition, the theoretical model was adopted to evaluate the chamber experiments in an effort to estimate the freezing time and the energy consumption ratio (energy consumption for maintaining the frozen state to the energy consumption for freezing soil specimens).

Data-driven framework for predicting ground temperature during ground freezing of a silty deposit

Predicting the frozen zone near the freezing pipe in artificial ground freezing (AGF) is critical in estimating the efficiency of the AGF technique. However, the complexity and uncertainty of many factors affecting the ground temperature cause difficulty in developing a reliable physical model for predicting the ground temperature. This study proposed a data-driven framework to accurately predict the ground temperature during the operation of AGF. Random forest (RF) and extreme gradient boosting (XGB) techniques were employed to develop the prediction model using the dataset of a field experiment in the silty deposit. The developed ensemble models showed relatively good performance (R2 > 0.96), yet the XGB model showed higher accuracy than the RF model. In addition, the evaluated mutual information and importance score revealed that the environmental attributes (ambient temperature, surface temperature, humidity, and wind speed) can be critical in predicting ground temperature during the AFG operation. The prediction models presented in this study can be utilized in evaluating freezing efficiency at the range of geotechnical and environmental attributes.

Modeling Artificial Ground Freezing for Construction of Two Tunnels of a Metro Station in Napoli (Italy)

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.

Thermal and hydraulic analysis of selective artificial ground freezing using air insulation: Experiment and modeling

Abstract In some artificial ground freezing (AGF) applications of civil or mining projects, only particular parts of the ground need to be frozen. Selective artificial ground freezing (S-AGF) is an AGF technique where a specific portion of the freeze pipe is insulated, usually by an air gap, to prevent any undesirable ground freezing. In this study, a laboratory scale experimental setup that mimics actual S-AGF systems has been established. The experimental rig is built in a fully controlled environment and equipped with advanced instrumentation. Additionally, a mathematical model coupling the flow of the coolant, air, and porous ground is developed by solving the conservation equations of mass, momentum, and energy. The mathematical model is then validated against the experimental measurements of the ground and outlet coolant temperatures. Moreover, the natural convection inside the air gap is further validated at higher Rayleigh numbers with an experimental study from the literature. The results indicate that the energy consumption of AGF plants can be significantly reduced by applying the S-AGF concept, as compared to convectional AGF systems. Also, it is found that the optimum air gap thickness, L opt , relates to the cavity height, H, and Rayleigh number, Ra H , as L opt ~ 2 HRa H - 1 / 4 .

Accumulative plastic strain behaviors and microscopic structural characters of artificially freeze-thaw soft clay under dynamic cyclic loading

The artificial ground freezing (AGF) technique has been extensively employed in the construction of underground structures. Due to the existence of freeze-thaw cycle in the AGF technique, the properties of soft clay deteriorate and thus larger settlement is induced during its later operation phase. To investigate the strain behaviors of the soft clay undergoing freeze-thaw cycle and discuss the differences between the soft clay undergoing and without undergoing freeze-thaw cycle under dynamic cyclic loading, a series of dynamic triaxial tests with consideration of different influencing factors as dynamic stress amplitudes, freezing temperatures, and freezing temperatures were conducted. Based on the experimental data, an empirical model for predicting the accumulative plastic strain was proposed and validated. Besides, the microscopic structural characters of the artificially freeze-thaw soft clay were explored via mercury intrusion porosimetry (MIP) tests. The results show that the total accumulative plastic strain of specimens increases with the increase of the dynamic stress amplitude, while decrease with the increase of the loading frequency. The specimens undergoing freeze-thaw cycle produces larger plastic strain, and the lower the freezing temperature is, the larger the total accumulative plastic strain is. The empirical accumulative plastic strain model, which can synthetically reflect the effects of dynamic stress amplitudes, loading frequencies, freezing temperatures and number of cyclic loading is proposed and validated. The freeze-thaw cycle has a significant effect on the strain behavior of soft clay in a way decreasing the pore volume and increasing the pore size of the specimens. Results obtained in this paper may provide meaningful references for studying the deformation characteristics of the soft clay in engineering applications.

Improved analytical prediction of ground frost heave during tunnel construction using artificial ground freezing technique

The reasonable prediction of ground frost heave provides guiding significance for tunnel construction that uses artificial ground freezing technique. Single-pipe freezing theory, which considers the freezing point of soil and assumes the constant surface temperature of a freezing pipe, is suggested for solving the radius of the freezing front before the closure of the frozen wall. By contrast, the radius of the freezing front after the closure of the frozen wall can be calculated using flat-panel freezing theory, which considers the freezing point of soil and determines the average temperature of the freezing pipe circle. On the basis of these suggestions and the analytical prediction proposed by Cai et al. (2014), an improved analytical prediction of ground frost heave was established by the stochastic medium theory according to the formation process of the frozen wall. The improved analytical prediction was applied to an actual tunnel freezing project. Then, the heaving and horizontal displacements of the ground surface are obtained, which agree well with the field-measured data. The effectiveness and practicality of the improved analytical prediction are verified in this study.

Model test and numerical simulation of frost heave during twin-tunnel construction using artificial ground-freezing technique

Based on the actual twin-tunnel freezing project of the Shanghai Metro in China, this study aims to establish a model test system that can simulate horizontal ground freezing, and the temperature distribution and frost heave ‘displacement are investigated. The model test results show that the frost heaving displacement of the soil was closely related to the closure time of the frozen wall. When the sequential freezing mode was adopted in the model test, the frost heaving displacement during the freezing period of the downlink tunnel was smaller than that during the freezing period of the uplink tunnel under the influence of water migration. Considering two different freezing modes, namely, sequential freezing and simultaneous freezing of uplink and downlink tunnels, we conducted a numerical simulation of the horizontal ground freezing of the twin tunnels. Compared with simultaneous freezing, sequential freezing can reduce frost heave displacement to a larger extent. This study can provide a reference for the design and construction of twin tunnels using artificial ground-freezing technique.

Numerical simulation of frozen wall formation in water-saturated rock mass by solving the Darcy-Stefan problem

Artificial ground freezing (AGF) is a common technology of shaft sinking through water-bearing strata. The AGF technique is used to create a frozen wall preventing shaft flooding. An additional factor that makes shaft sinking more complicated is associated with external groundwater flows occurred due to hydrostatic pressure gradients. In this paper, we study the influence of groundwater seepage on the frozen wall formation in fluid-saturated rock mass in the framework of the two-dimensional two-phase Darcy-Stefan problem. The results of numerical simulation of the thermal and hydraulic properties of the sandstone layer at the site of Petrikov Mining and Processing Plant are presented. It has been found that the external groundwater flow has a significant effect on the growth of a frozen wall in the case when the groundwater velocity magnitude is greater than or equal to 50 mm/day. This critical seepage velocity strongly depends on how quickly the water content and rock mass permeability decrease with decreasing temperature, or on the parameters of the rock mass freezing characteristic curve and permeability versus temperature curve. The proper setting of these parameters is a sine qua non for creating adequate mathematical models of heat and mass processes in the artificially frozen water-saturated rock mass.

Monitoring and Evaluation of Artificial Ground Freezing in Metro Tunnel Construction-A Case Study

The turn-back tunnel of Guangzhou Metro Line 3 Tianhe Station with a large span was excavated in sandy clay which may easily break and disintegrate. The artificial ground freezing (AGF) technique was used to stabilize the soil and to prevent its collapse during excavations. Most of the existing theoretical analysis and numerical simulation on the AGF technique are purely based on a couple of assumptions, which are not able to produce accurate predictions. It would be more accurate for the AGF analysis to include the field monitoring. In this study, the coupled method of the field monitoring, the analytical formula, and the numerical method is used to evaluate the thickness and average temperature of the frozen zone. Field monitoring was conducted to measure the temperature of the brine and the ground. Analytical formula was used to compute the thickness and the average temperature of the frozen zone. Numerical simulation is also carried out to predict the thickness and the temperature field of the frozen zone. According to the analytical and numerical analysis, the computed thickness and average temperature of the frozen zone meet the designed requirements of the project, which are further confirmed by the successful excavation of the tunnel. This indicates that the coupled method used in this paper is reliable and would be helpful for the AGF application in practical engineering.

Analytical solution for the soil freezing process induced by an infinite line sink

One-dimensional soil freezing process induced by an infinite line sink is investigated, a problem that is generally encountered during the application of the artificial ground freezing technique. The special feature of the freezing process of soil is that the amount of liquid water in the soil pores decreases gradually with the soil temperature decreasing, and thus latent heat is continuously released. After approximating the continuous phase change process of the soil with a multi-step phase change process of a polymorphous material, the original problem is transformed to a cylindrical multiphase Stefan problem. Using the similarity transformation technique, an analytical solution for the problem is developed, in which there are coefficients that need to be determined by a group of non-linear equations. A theoretical proof is presented showing that these coefficients can be appropriately determined by the non-linear equations. The computational results of the analytical solution are compared with those of the numerical solution, and the two solutions are in good agreement. The effects of the variation of different parameters on the movement of freezing front are studied. The results show that neglecting the existence of unfrozen water will underestimate the freezing front velocity. The relative error of this underestimation under different conditions is also investigated and discussed. In situations that the unfrozen water content is given as a discrete function of the temperature, the analytical solution can be applied directly, and an illustrative example is presented.

Artificial Ground Freezing In Tunnelling Through Aquifer Soil Layers: a Case Study in Nanjing Metro Line 2

In this paper, the construction of a section of Nanjing Metro Line 2, a shallow underground tunnel constructed with an underground mining method, was analysed. Due to the existence of two aquifer soil layers near the tunnel, water and sand inrush occurred during the tunnelling process. This disrupted tunnelling project was covered until the artificial ground freezing technique was used to achieve a waterless environment. After the 40-day active freezing period, the ground temperature conditions satisfied the design requirements, thereby ensuring the construction schedule. The ground freezing technique and other supplementary measurements were introduced in this study, which could be useful for other similar projects.

Creep behavior of ice-soil retaining structure during shaft sinking

The article is devoted to analysis of deformation of an ice-soil retaining structure during a vertical mine shaft sinking by applying the artificial ground freezing technique. The analysis was performed by finite element numerical simulation. Since frozen soils possess rheological properties, creep deformation of the ice-soil structure was considered. To determine the creep deformation, the Vaylov’s constitutive relations was used. In the relations the creep behavior is described by the Norton-Bailey creep law and the volumetric creep strain is restricted to be zero. The wall thickness of the ice-soil structure was estimated by the Vaylov’s formula that is widely used in structural design of potash mines. As a result of the study, it was established that for time required for the lining installation at large depths of the shaft sinking the creep deformation of the ice-soil structure exceeds admissible value guaranteeing safety of the excavation process.

Exact solution for a two-phase Stefan problem with power-type latent heat

A two-phase Stefan problem with latent heat a general power function of position is investigated. The background of the problem can be found in the soil-freezing process during the application of the artificial ground-freezing technique. After introducing the specific engineering condition, the governing equations of a two-phase Stefan problem are developed. An exact solution for the problem is established using the similarity transformation technique and the theory of the Kummer functions. It is proved that the coefficients in the solution can be appropriately determined if certain inequality is satisfied. Special cases of the solution are discussed, and several solutions reported in the literature are recovered. A similar two-phase Stefan problem involving first type of boundary condition is also introduced and solved. The coefficients in the solution can always be properly determined with no additional requirement. In the end, computational examples of the solution are presented and discussed. The exact solution provides a useful benchmark that can be used for verifying general numerical algorithms of Stefan problems, and it is also advantageous in the context of the inverse problem analysis.