Artificial Freezing Method Research Articles

Influence of Freezing Tunnel Excavation on Foundation Settlement of Buildings

In this paper, the effects of the cement content and excavation speed parameters on the improvement effect of the artificial freezing method and the characteristics of frozen walls are studied by means of a field test, numerical simulation and theoretical model. The optimization effect of the cement content in the artificial freezing method is studied. It is found that 10% is the best content, which can maximize the freezing wall thickness and grouting effect, and promote uniform distribution of the temperature field. At the same time, the influence of the excavation speed on the stress and settlement of the foundation soil is analyzed. It is pointed out that the increase of the excavation speed will aggravate the settlement and stress redistribution, which may threaten the building structure. The evaluation method proposed in this paper verifies that the construction deformation and settlement control are within the safety standards and provides theoretical support and construction guidance for tunnel engineering.

Model test of in-situ remediation for heavy metal-contaminated clayey soil by artificial freezing and shaft washing

The low permeability of clayey soil and the pressure of liquid extraction and injection tend to form preferential flow paths in the soil, diminishing the remediation efficiency of shaft washing. Therefore, a new in-situ repair method combining artificial freezing and shaft washing is proposed. Using Pb and Cd contaminated clayey soil as the research object, in-situ model tests of different eluent types, concentrations, and suction modes were conducted to investigate the removal rate of heavy metals and the distribution of temperature and water field. The results demonstrated that the artificial freezing method can effectively induce the water (eluent) migration from the unfrozen zone to the freezing front. Besides, it effectively resolved the issue of poor washing efficiency caused by preferential flow between the extraction and injection wells during the extraction process. After five freeze-thaw cycles, the removal rate of Pb and Cd reached 46.16 % and 59.04 %, respectively. The combination of artificial freezing and plastic drainage plate technology merges artificial freezing and shaft washing methods to achieve the remediation of heavy metal-contaminated soil, providing a reference for the in-situ remediation of heavy metal-contaminated clayey soil.

Settlement Analysis of Ground Surface and Adjacent Building Caused by Driving and Expansion Excavation of Shield Tunnel Using Artificial Freezing Method

The artificial freezing method can effectively improve the stability of strata and provide favorable conditions for the construction safety of shield tunnel in water-rich strata. Based on the frozen shield tunneling project of a metro station, which is close to important buildings, a reasonable freezing method and parameters are proposed in this paper. The simulation model was established by using Plaxis 3D finite element software (Version 2017). The numerical model was verified based on a large amount of field data. The characteristics of segment deformation, ground surface settlement and vertical displacement of buildings were compared between frozen layer and nonfrozen layer during shield tunnel excavation. It was found that segment deformation in the nonfrozen layer is three times that in the frozen layer. The surface settlement above the frozen layer is less than 2 mm. Expansion excavation of shield tunnel was carried out to meet the space function of subway station using artificial freezing method. The deformation of frozen layer was compared between full section excavation and partial excavation. It was found that the deformation of the former is 4.5 times that of the latter, so the partial excavation was chosen as the main research object. Subsequently, the characteristics of vertical displacement and surface settlement of buildings under partial excavation were studied. It was found that the vertical settlement of buildings away from the frozen layer is greater than that of buildings near the frozen layer. All settlement values meet the requirements, thus ensuring the feasibility of partial excavation and ensuring the safety of construction.

Study on the Acoustic Emission Characteristics and Failure Precursors of Water-Rich Frozen Sandstone under Different Lateral Unloading Rates

The artificial freezing method is used to cross the water-rich soft rock strata in order to exploit deep coal resources. At present, studies that consider both freezing effect and unloading rate are insufficient. To study the influences of the excavation rate using the artificial freezing method on the unloading deformation and failure of the water-rich surrounding rock, we carry out mechanical and synchronous acoustic emission (AE) tests on frozen (−10 °C) sandstone samples under different lateral unloading rates. Combined with the AE signals, the stress, strain and failure process are analysed to determine the mechanical behaviours of frozen rock samples under different lateral unloading rates. The damage difference between normal temperature rock and frozen rock during lateral unloading is studied. According to acoustic emission signals, the damage relationships among acoustic emission amplitude, energy, cumulative acoustic emission energy (CAEE), stress and strain were compared and analyzed. In this paper, acoustic emission 3D positioning system is used to monitor the fracture propagation trajectory in the process of unloading confining pressure of frozen sandstone. The results show that the peak stress of frozen sandstone during lateral unloading is about 2.5 times of that at 20 °C. More than 2 AE amplitudes per second are regarded as the precursor of failure (FP), and point FP is taken as the first level warning. The CAEE of rock samples at 20 °C and frozen rock samples shows the same change law over time, increasing slowly before the FP point and exponentially after the FP point. Peak stress increases and axial strain decreases with the increase of unloading rate of frozen rock sample. The CAEE at point FP and the peak acoustic emission energy (AEE) and the CAEE at the time of failure increase when the unloading rate of frozen rock sample increases. Principal component analysis method was used to extract key characteristic energy to obtain a clearer AEE concentration area, which was defined as second-level early warning. The research results can provide guidance for freezing shaft construction to reduce the occurrence of disasters.

Purification of heavy metal chromium in saturated sand by artificial freezing: Mechanism and method optimization

Heavy metal pollution of soil has become one of the most common hazards in human development. The artificial freezing method, especially the progressive freezing method, can reduce heavy metal pollutants in the soil and promises to be an effective in-situ treatment of contaminated sites. This study analyzes the freezing purification mechanism of heavy metal contaminants in saturated sand and identifies three main factors that impact the effects of purification: freezing rate, initial concentration, and diffusion coefficient. Moreover, one-dimensional freezing tests are carried out by different freezing modes. The experimental results show that the heavy metal chromium could only be removed effectively with a slow freezing rate. By optimizing the freezing mode and freezing rate, a long section of soil was frozen and purified, with the maximum purification rate reaching 65.8%. This study shows that it is feasible to treat contaminated saturated sand by a gradual-cooling freezing method.

An improved model assessing variation characteristics of pore structure of sandy soil thawing from extremely low temperature using NMR technique

During the construction of subway connecting passages in major cities in China, in last decades the artificial freezing method has gradually been becoming one of the most safe and effective methods in excavating saturated sandy soils. This work aims at exploring the changes of pore size and relaxation time induced by temperature change using NMR technology. Sandy soil samples were retrieved from a subway construction site in Beijing. In the laboratory experiments, these samples were loaded by different extremely low temperatures and natural convection thawing. The T2 (i.e., relaxation time) distribution curves and real-time temperature throughout the thawing process were recorded. It was observed that the variation of T2 distribution curves and pore structure changes with temperature may be divided into four stages, i.e., Stage I: spherical and cylindrical pores with radius r = 0.3 nm are formed at temperature lower than −28.0 °C; Stage II: the water adsorbed in pores melts and pore radius increases to the range of 0.3 nm < r < 32.0 nm as temperature increases in the range of −28.0 °C ∼ -0.2 °C; Stage III: capillary water in the pores with radius >32.0 nm melts rapidly as temperature increases in the range of −0.2 °C ∼ 0.7 °C; Stage IV: small amount of pores transform from small-sized absorptive pores and large-sized capillary pores to medium-sized absorptive pores as temperature increases to above 0.7 °C°. The specific threshold temperature and the threshold relaxation time are found as −0.2 °C and 5.17 ms respectively. By quantitatively analyzing the incremental change of T2 distribution curves and the rising temperature, an improved Gibbs-Thomson equation and a Two-Staged model of T2LM-Tmel (the geometric mean of relaxation time - the absolute value of temperature) and T2-Tmel (relaxation time - the absolute value of temperature), which are applicable to NMR technology, were proposed. Based on these results, a new pore structure evolution model of frozen sandy soil thawing from extremely-low temperature was proposed.

Analysis of Damage Constitutive Model for Frozen Red Clay under Uniaxial Compression

At present, it is in the peak period of infrastructure construction in western China, and due to the natural climate, the soil has been frozen for a long time, but there is little research on frozen red clay at present. Therefore, it is necessary to further study the mechanical properties of frozen red clay. In this paper, taking the remolded red clay as the test material, the change characteristics of uniaxial compression and uniaxial creep of frozen soil under different test factors are studied by artificial freezing method, and the damage theory is introduced to analyze the test results. The results of uniaxial compression test of frozen soil show that temperature has a great influence on the uniaxial compressive strength, and the compressive strength increases with the decrease of temperature, and the damage theory is introduced. It is found that the initial damage stress increases with the decrease of temperature, while the initial strain changes little with temperature. A damage constitutive equation which can well reflect the whole damage process of frozen red clay is established. The experimental research results of this paper can provide some theoretical reference for the engineering of soil quality for freezing red clay.

Freezing Effect of Enhancing Tubes in a Freeze-Sealing Pipe Roof Method Based on the Unsteady-State Conjugate Heat Transfer Model

The freeze-sealing pipe roof (FSPR) method was applied as an innovative construction technology to the Gongbei Tunnel of the Hong Kong–Zhuhai–Macau Bridge. A freezing scheme involving master freezing tubes, enhancing freezing tubes, and limiting freezing tubes is the key component of the freezing effect of the FSPR method during the construction process under various working conditions. This is related to whether the thickness and temperature of the frozen soil meet the design requirements under various complex working conditions, and it is also related to frost heave control and energy saving. Based on the unsteady-state conjugate heat transfer model, different freezing schemes of enhancing freezing tubes—that is, the shape, layout, operating duration, and heat preservation—were simulated to analyze the freezing effect, which can be measured by the thickness of frozen soil around the steel pipes and the average temperature of the frozen soil curtain. The results show that the greater the contact area between the enhancing tube and the inner wall of the steel pipe, the better the freezing effect, and that the semicircle enhancing freezing tube scheme is superior to the other three shapes of freezing tubes. The arrangement of enhancing freezing tubes far away from the excavation surface, without heat preservation measures, has a better freezing effect due to the function of the hollow pipe as a freezing pipe. Moreover, the enhancing freezing tube can be operated intermittently to control frost heave. Our research simulated the temperature fields of different media—such as steel pipes, frozen soil, and air—providing a design basis for similar projects, such as the combination of the pipe-roofing method and artificial freezing method.

Application of Artificial Freezing Method in Deformation Control of Subway Tunnel

The metro tunnel construction mainly passes through the downtown area, including ground buildings, infrastructure, underground pipelines, existing subway lines, and station floors. However, it is very difficult to control the ground deformation, especially in the water-rich soft ground or in the tunnel engineering construction with strict requirements for surface settlement control. The traditional stratum reinforcement method may not meet the engineering needs. In this case, the artificial freezing method (AFM) provides a solution for such engineering problems. This paper summarizes the basic principle and characteristics and application scope of AFM, and compared with the other three stratum reinforcement methods, AFM has better effect in controlling surface displacement and vault settlement, while grouting method, pipe shed method, and pipe curtain method have absolute advantages in controlling surface uplift. Then, through engineering examples, the reinforcement effect of AFM in the four aspects of subway connecting passage, shield inlet and outlet, interval tunnel, and damage repair is introduced. Through analysis, it is found that the reinforcement effect of AFM in water-rich weak stratum is very good. Finally, the future development of AFM is prospected in order to provide some experience and guidance for the follow-up research.

Research on Creep Characteristics and the Model of Artificial Frozen Soil

To study the long-term stability of the large-scale freezing project, the longest subway connecting channel constructed by the artificial freezing method in China is taken as the background. The uniaxial compressive strength test, triaxial shear test, and triaxial creep test of artificial frozen soil under different temperatures were carried out by using the MTS 370.25 mechanical testing machine. A series of triaxial creep tests for artificial frozen soil under different stresses and temperatures were carried out. Based on the creep test results, a fractional-order constitutive model of frozen silt with sand was proposed. Comparing the calculated results with the tested ones, it is found that the proposed fractional-order model can simulate the properties of frozen silt with sand very well. Compared with the other creep models of frozen soil, the proposed model here has higher accuracy and strong stress sensitivity. Through the simulation of the connecting channel construction and comparison with the observed data in site, the effectiveness of the model is verified, which further verifies that the model has important guiding significance for the freezing project.

Quantitative Analysis of the Microstructures of Deep Silty Clay Subjected to Two Freezing–Thawing Cycles under Subway Vibration Loading

The bypass of subway tunnels in soft soil areas is usually constructed by the artificial freezing method due to the waterproof property and high strength of frozen soil. The thawing settlement and deformation of silty clay under subway vibration loading is becoming a major issue. The Shanghai subway lines cross vertically and horizontally, of which the deepest station reaches 33.1 m. With the rapid development and utilization of underground space, the depth of subway excavation will become increasingly deeper. Under subway vibration loading, the dynamic characteristics and microstructures of deep clay should be investigated. In this paper, a series of scanning electron microscopy (SEM) tests and mercury intrusion porosimetry (MIP) tests were carried out to study the microscopic parameters of deep silty clay subjected to two freezing–thawing cycles under cyclic loading. The freezing–thawing soil produced higher axial strain than that of the unfrozen soil with the same loading. After freeze–thaw cycling, the soil particles show the phenomenon of agglomeration. As the pore diameter increases, the pore shape coefficient decreases gradually from 0.45 to 0.85. Under similar vibration loading, the anisotropy of the frozen–thawed clay increases as the freezing temperature decreases. The fractal dimension obtained by SEM tests is between 1.25 and 1.33. There is an approximate linear relationship between the axial strain and the fractal dimension. In MIP tests, there exists a bottleneck effect in the process of mercury intrusion. The pore volume and pore diameter of the freezing–thawing soil increase with decreasing freezing temperatures and freezing–thawing cycles, respectively. The thermal fractal model is applicable to the fractal characteristics of silty clay. The results can provide a reference for the design and construction of deep-buried subway tunnels in soft soil areas.

Dynamic Splitting Behavior and the Constitutive Relationship of Frozen Sandstone Containing a Single Fissure

Fractured sandstone is widely distributed in mining areas throughout western China where the artificial freezing method is extensively adopted to construct vertical shafts. Blasting and excavation generate stress waves and break frozen fractured sandstone. Among the failure modes of frozen fractured rocks, tensile failure is very common. In this study, the dynamic tensile strength of fractured sandstone samples with four crack inclination angles (0°, 30°, 60°, and 90°) is tested by using a split Hopkinson pressure bar at four subzero temperatures (−5, −10, −15, and −20°C). Accordingly, a damage constitutive relationship that considers the effect of fissure angle and freezing temperature is established. The results show the following: (1) the fissure angle does not significantly affect the dynamic tensile strength of frozen fractured sandstone but mainly affects the failure mode of the sample. (2) The dynamic tensile strength of fractured sandstone has a negative linear correlation with the freezing temperature. (3) When the fissure angle is small, only tensile cracking occurs; when the fissure angle is large, tensile cracking occurs along both the loading direction and the fissure; and shear cracking occurs along the fissure as well. (4) Regardless of the fissure angle, tensile cracking is initiated at the stress-concentration zone and then propagates towards the loading end. Fissure ice provides both resistance to deformation and resistance to crack propagation which affects the crack propagation and coalescence mode. A dynamic constitutive relationship is established by considering the effects of fissure angle and freezing temperature on the dynamic properties of frozen fractured sandstone, which is proven to be highly reliable and provides a reference and basis to study the dynamic mechanical properties of similar rock types.

Study of frost resistance of generative buds of scion/ rootstock combinations of sweet cherry by freezing in controlled conditions

The paper presents the results of two-year research on frost resistance study in 18 scion/rootstock combinations of sweet cherry: Fatezh and Chermashnaya varieties grafted on 9 rootstocks (Izmailovsky, AVCH-2, V-5-88, VC-13, Moskoviya, VSL-2, Stepnoy Rodnik, Colt and seedling rootstock Prunus magaleb L.). The research was carried out in 2019-2020 in the laboratory of physiology and biochemistry of FSBSI «Federal Horticultural Research Center for Breeding, Agrotechnology and Nursery» by artificial freezing method in the climate chamber TH-6 JEIO TECH -1000. It was established that after freezing at t = - 29ºC, the floral rudiments' preservation in the Fatezh variety was more than 80%, and in the Chermashnaya variety — about 40%. Limits of frost resistance change of varieties' reproductive buds depending on the rootstock were shown: Fatezh variety — from 62.2% (VSL-2) to 81.7% (Izmailovsky), and Chermashnaya variety — from 25.5% (P. magaleb L) to 65% (Izmailovsky). When using Izmailovsky and AVCH-2 clone rootstock, the preservation of flower rudiments increased by 15 -20% compared to P. magaleb L. The most frost-resistant scion/rootstock combinations were revealed: Fatezh/Izmailovsky and Fatezh/AVCH-2, which preserved more than 87% of live rudiments. A positive correlation between floral buds' frost resistance and chlorogenic acid content has been established.

Study on temporal and spatial variation of temperature field in horizontal freezing and thawing settlement

Artificial freezing method is commonly used in underground engineering excavation, such as mine, foundation pit and subway. The problem of post-construction thaw settlement has attracted more and more attention, but it is difficult to calculate the development law of thaw temperature field of complex frozen curtain by analytical method. Therefore, this paper adopts numerical method to study this problem. In this paper, taking the horizontal freezing construction of a subway connecting passage as an example, the development law of temperature field in the melting process is analyzed, and the results are basically consistent with the measured data. As a general means, numerical simulation is feasible to simulate the development law of temperature field in horizontal melting of complex curtain. The results of this paper can be used to guide the design and construction of artificial freezing.

Evolution of temperature field and frozen wall in sandy cobble stratum using LN2 freezing method

There is high risk of water inrush and ground collapse when tunneling in sandy cobble stratum in confined aquifer. Among various methods, artificial freezing method is an effective method to pre-reinforce the soft ground to prevent water inrush and ground collapse during tunneling. This paper mainly focuses on application of artificial freezing method using liquid nitrogen in sandy cobble stratum in confined aquifer. The R1 metro tunnel in China’s Jinan city, a well-known City of Springs, is used as a case study. In order to prevent water inrush and ground collapse accidents in the shield machine reception process, liquid nitrogen is applied as a freezing method in the sandy cobble stratum near the reception shaft. First, the heat transition of water in soil associated with liquid nitrogen vaporization is analyzed. Then, a three-dimensional (3D) model is built and the freezing process is simulated. Considering the temperature change of gas nitrogen along the freezing pipe when flowing, the distribution of temperature field and the growth of frozen wall associated with freezing time is obtained. The numerical simulation results show that the freezing method using liquid nitrogen in the sandy cobble stratum can meet the requirement of shield machine reception in sandy cobble stratum rich in groundwater.

The coupled moisture-heat process of a water-conveyance tunnel constructed by artificial ground freezing method

Artificial freezing ground method is widely applied in adverse geotechnical engineering conditions to improve the strength and reduce permeability of soil. It is a challenge to ascertain the progression of frozen wall and analyze the arrangement of freezing pipes due to the complexity of freezing process involving ice-water phase change and water migration. To analyze the progression of frozen wall and its influencing factors, a hydro-thermo coupling model is established, and a water-conveyance tunnel constructed by artificial freezing method is carried out. The results show that: (1) Water migration and ice-water phase transition have significant influences on temperature distribution, and their effects should not be neglected. (2) The thickness of frozen wall with temperature lower than −10 °C has attained the required design value at the end of active freezing. Meantime, under drive of temperature gradient, water migrates and redistributes. (3) The interval angle for two adjacent pipes is the main influencing factor on progression of frozen wall, followed by the freezing pipe diameter and horizontal spacing. Additionally, for coarse-grained soil, the influence of seepage on development of frozen wall is significant. This study will provide a theoretical reference for frozen wall design.

Application of Artificial Freezing of Water-rich Sand Stratum in Metro Communication Passage

In view of difficult excavation of the water-rich sand stratum, the layout of freezing holes, the selection of freezing parameters, the thaw settlement control of the soil after natural thawing, and the grouting parameters optimization and other construction techniques in the freezing process of water-rich sand stratum in Fuzhou metro communication passage are introduced. For the active freezing construction process, analysis is made on the brine loop temperature, soil temperature, and pore pressure of pressure relief hole; the laws of soil temperature changes and freezing wall formation are summarized to accurately judge the timing of passage excavation. Relevant specific measures are proposed for existing problems in freezing construction of communication passage. Successful application of artificial freezing method in water-rich sand stratum is confirmed, thus providing reference for other similar projects.

Accumulated deformation and microstructure of deep silty clay subjected to two freezing-thawing cycles under cyclic loading

The problem of the settlement and deformation of soft clay is becoming more and more prominent under the subway vibration loading. Due to the complexity of the construction environment in the soft soil areas, the artificial freezing method is often used to reinforce the soil, and sometimes it is also subjected to twice freezing. The Shanghai subway line crosses vertically and horizontally. The deepest excavation depth reaches 33.1 m, so the construction of the subway tunnel will develop to a deeper stratum. In this paper, the GDS dynamic triaxial tests were conducted to study the accumulated deformation of the gray silty clay of layer no.8 in Shanghai, 60 m in depth. Considering the influence of vibration time, frequencies, freezing temperatures, and freezing-thawing cycles, the axial strain of the deep silty clay before and after freezing-thawing was analyzed. The results show that the strain of the soil subjected to one freezing-thawing cycle and two freezing-thawing cycles increases by 39.6% and 72.8% comparing with the unfrozen soil, respectively. With the increase of vibration time, the rates of increasing of the axial strain gradually decrease. The higher the frequency and the lower the freezing temperature, the more deformation produces to the soil. The microstructures of the soil before and after freezing-thawing were obtained by mercury intrusion porosimetry (MIP) tests. There is a bottleneck with the pressure about 100 psi in the mercury intrusion process. Under the same cyclic loading, the lower the freezing temperatures and the more the freezing-thawing cycles, the more the mercury intrusion volume and the cumulative pore area. The most probable pore diameter of the unfrozen soil is about 524 nm smaller than that of the thawing soil, which is about 1050 nm. The pores 10–100 nm in diameter account for a large part. This research is of great guiding significance for the construction of deep subway tunnels.

Creep behavior of soft clay subjected to artificial freeze–thaw from multiple-scale perspectives

Relying on the application of the artificial freezing method on subway tunnel construction, a series of triaxial creep tests were carried out to study the creep behavior changes of Shanghai soft clay subjected to artificial freeze–thaw action. On this basis, MIP tests were conducted with the soil samples before and after creep for comparison to investigate the microstructure changes. The results indicate that freeze–thawed soil produces smaller creep deformation and instantaneous deformation than the unfrozen soil. On a micro-level, during the creep process, the soil skeleton reaches a new structure balance with smaller pore volume and pore area. But the diameter of the maximum pore increases. The change rate of total intrusion volume is a pivotal micro-parameter to evaluate creep strain as there is a good linear relationship between them.

Experimental study on the strength, deformation and crack evolution behaviour of red sandstone samples containing two ice-filled fissures under triaxial compression

The artificial freezing method is an effective method of shaft construction in water-rich soft rock beds. Engineering frozen walls involves the use of intact rock material, flaws and ice. To better understand the mechanical properties of frozen walls, explore the failure laws of frozen fractured rock, and solve the problem of complex ice-rock coupling, it is necessary to carry out laboratory triaxial tests on frozen fractured rock. Herein, X-ray diffraction and mesostructure observations are used to analyse the mineral composition and microstructural characteristics of red sandstone specimens taken from the Shilawusu coal mine. Then, the strength, deformation and crack evolution behaviour of red sandstone containing two pre-existing ice-filled fissures are obtained under triaxial compression using the self-developed DRTS−500 subzero rock triaxial testing system. The experimental results show that the peak strength and elastic modulus of the specimens with pre-existing flaws are all much lower than those of the intact specimens, and the strength parameters are distinctly related to the flaw combinations. For the frozen red sandstone containing two intermittent ice-filled fissures, the peak strength, elastic modulus and shear strength parameters all have a linear relationship with the confining pressure and temperature, whereas the shear strength (σ1 − σ3) has a nonlinear relationship with the confining pressure. The deviatoric stress-strain curve of the samples is divided by four characteristic stresses (compacting stress, crack initiation stress, dilatancy stress and peak stress) into five stages from loading to failure. The lower the confining pressure or temperature is, the faster the curve falls after reaching the stress-strain peak value, indicating that the sample exhibits increasingly brittle characteristics. The ultimate failure modes of the specimens containing two pre-existing ice-filled flaws include mixtures of several types of cracks (shear cracks, main cracks, secondary cracks, tensile cracks, coplanar cracks, slip cracks and wing cracks), which depend on the flaw combinations. An obvious crack coalescence failure mode can be observed in the GG and GS specimens, but no obvious crack coalescence phenomenon can be observed in the SS specimens.