A study on impacts of groundwater seepage on artificial freezing process of gravel strata

Purpose–This paper aims to study the impacts of groundwater seepage on artificial freezing process of gravel strata,the temperature field characteristics of the strata,and the strata process,closure time and thickness evolution mechanism of the frozen wall.Design/methodology/approach–In this paper several laboratory model tests were conducted,considering different groundwater seepage rate.Findings–The results show that there is a significant coupling effect between the cold diffusion of artificial freezing pipes and groundwater seepage;when there is no seepage,temperature fields upstream and downstream of the gravel strata are symmetrically distributed,and the thickness of the frozen soil column/frozen wall is consistent during artificial freezing;groundwater seepage causes significant asymmetry in the temperature fields upstream and downstream of the gravel strata,and the greater the seepage rate,the more obvious the asymmetry;the frozen wall closure time increases linearly with the increase in the groundwater seepage rate,and specifically,the time length under seepage rate of 5.00 m d
1 is 3.2 times longer than that under no seepage;due to the erosion from groundwater seepage,the thickness of the upstream frozen wall decreases linearly with the seepage velocity,while that of the downstream frozen wall increases linearly,resulting in a saddle-shaped frozen wall.Originality/value–The research results are beneficial to the optimum design and risk control of artificial freezing process in gravel strata.

Influence of artificial freezing onliquefaction characteristics of Nanjing sand

Purpose–This study purposes to study the influence of artificial freezing on the liquefaction characteristics of Nanjing sand,as well as its mechanism.Design/methodology/approach–was studied through dynamic triaxial tests by means of the GDS dynamic triaxial system on Nanjing sand extensively discovered in the middle and lower reaches of the Yangtze River under seismic load and metro train vibration load,respectively,and potential hazards of the two loads to the freezing construction of Nanjing sand were also identified in the tests.Findings–The results show that under both seismic load and metro train vibration load,freeze-thaw cycles will significantly reduce the stiffness and liquefaction resistance of Nanjing sand,especially in the first freezethaw cycle;the more freeze-thaw cycles,the worse structural behaviors of silty-fine sand,and the easier to liquefy;freeze-thaw cycles will increase the sensitivity of Nanjing sand’s dynamic pore pressure to dynamic load response;the lower the freezing temperature and the effective confining pressure,the worse the liquefaction resistance of Nanjing sand after freeze-thaw cycles;compared to the metro train vibration load,the seismic load in Nanjing is potentially less dangerous to freezing construction of Nanjing sand.Originality/value–The research results are helpful to the construction of the artificial ground freezing of the subway crossing passage in the lower reaches of the Yangtze River and to ensure the construction safety of the subway tunnel and its crossing passage.

Combination effect of seasonal freezing and artificial freezing on frost heave of silty clay

To investigate the frost-heave properties of silty clay under the combination action of seasonal freezing and artificial freezing, and verify the feasibility of combined freezing, eight combined freezing experiments were performed on silty clay with water content (mass fraction) of 23.5% and 28.0%, through developed frost-heave test apparatus, in closed or open system. Two sorts of freezing temperature models, namely, constant and sine models, were applied to artificial freezing. The experimental results indicate that the frost-heave degree in seasonal freezing stage accounts for over 90% of the total in open system and it is up to 95% in closed system; the change of artificial sine-freezing temperature has no influence on the frost-heave degree in closed system, however, slight influence in open system. It is found that the variation of temperature gradient of sine-freezing specimen lags behind that of sine-freezing temperature with half phase; sine-freezing temperature model can reduce frost-heave degree of soil. Brand new technology is proposed for the application of artificial ground freezing and new study field of artificial freezing is created.

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.

Coupled thermo-hydro-mechanical modeling of frost heave and water migration during artificial freezing of soils for mineshaft sinking

Artificial freezing of water-bearing soil layers composing a sedimentary deposit can induce frost heave and water migration that affect the natural stress-strain state of the soil layers and freezing process.In the present paper,a thermo-hydro-mechanical(THM)model for freezing of water-saturated soil is proposed to study the effects of frost heave and water migration in frozen soils on the formation of a frozen wall and subsequent excavation activity for sinking a vertical shaft.The governing equations of the model are formulated relative to porosity,temperature,and displacement which are considered as primary variables.The relationship between temperature,pore water,and ice pressure in frozen soil is established by the Clausius-Clapeyron equation,whereas the interaction between the stress-strain behavior and changes in porosity and pore pressure is described with the poromechanics theory.Moreover,constitutive relations for additional mechanical deformation are incorporated to describe volumetric expansion of soil during freezing as well as creep strain of soil in the frozen state.The ability of the proposed model to capture the frost heave of frozen soil is demonstrated by a comparison between numerical results and experimental data given by a one-sided freezing test.Also to validate the model in other freezing conditions,a radial freezing experiment is performed.After the validation procedure,the model is applied to numerical simulation of artificial freezing of silt and sand layers for shaft sinking at Petrikov potash mine in Belarus.Comparison of calculated temperature with thermal monitoring data during active freezing stage is presented.Numerical analysis of deformation of unsupported sidewall of a shaft inside the frozen wall is conducted to account for the change in natural stress-strain state of soil layers induced by artificial freezing.

Artificial ground freezing of underground mines in cold regions using thermosyphons with air insulation

Current practice of underground artificial ground freezing(AGF)typically involves huge refrigeration systems of large economic and environmental costs.In this study,a novel AGF technique is proposed deploying available cold wind in cold regions.This is achieved by a static heat transfer device called thermosyphon equipped with an air insulation layer.A refrigeration unit can be optionally integrated to meet additional cooling requirements.The introduction of air insulation isolates the thermosyphon from ground zones where freezing is not needed,resulting in:(1)steering the cooling resources(cold wind or refrigeration)towards zones of interest;and(2)minimizing refrigeration load.This design is demonstrated using well-validated mathematical models from our previous work based on two-phase enthalpy method of the ground coupled with a thermal resistance network for the thermosyphon.Two Canadian mines are considered:the Cigar Lake Mine and the Giant Mine.The results show that our proposed design can speed the freezing time by 30%at the Giant Mine and by two months at the Cigar Lake Mine.Further,a cooling load of 2.4 GWh can be saved at the Cigar Lake Mine.Overall,this study provides mining practitioners with sustainable solutions of underground AGF.

Thermal performance of cast-in-place piles with artificial ground freezing in permafrost regions

During the construction of cast-in-place piles in warm permafrost,the heat carried by concrete and the cement hydration reaction can cause strong thermal disturbance to the surrounding permafrost.Since the bearing capacity of the pile is quite small before the full freeze-back,the quick refreezing of the native soils surrounding the cast-in-place pile has become the focus of the infrastructure construction in permafrost.To solve this problem,this paper innovatively puts forward the application of the artificial ground freezing(AGF)method at the end of the curing period of cast-in-place piles in permafrost.A field test on the AGF was conducted at the Beiluhe Observation and Research Station of Frozen Soil Engineering and Environment(34°51.2'N,92°56.4'E)in the Qinghai Tibet Plateau(QTP),and then a 3-D numerical model was established to investigate the thermal performance of piles using AGF under different engineering conditions.Additionally,the long-term thermal performance of piles after the completion of AGF under different conditions was estimated.Field experiment results demonstrate that AGF is an effective method to reduce the refreezing time of the soil surrounding the piles constructed in permafrost terrain,with the ability to reduce the pile-soil interface temperatures to below the natural ground temperature within 3 days.Numerical results further prove that AGF still has a good cooling effect even under unfavorable engineering conditions such as high pouring temperature,large pile diameter,and large pile length.Consequently,the application of this method is meaningful to save the subsequent latency time and solve the problem of thermal disturbance in pile construction in permafrost.The research results are highly relevant for the spread of AGF technology and the rapid building of pile foundations in permafrost.

Macro-micro behaviors and failure mechanism of frozen weakly cemented mudstone

Understanding the mechanical properties and multiscale failure mechanism of frozen soft rock is an important prerequisite for the construction safety of tunnels,artificially frozen ground and other infrastructure in cold regions.In this study,the triaxial compression test are performed on mudstone in the weakly cemented soft rock strata in the mining area of western China,and the mechanical characteristics and failure mechanism of weakly cemented mudstone are systematically investigated under the combined action of freezing and loading.Furthermore,the quantitative relationship between the microstructural parameters and the macroscopic strength and deformation parameters is established based on fractal theory.Thus,the failure mechanism of frozen weakly cemented mudstone is revealed on both micro- and macro-scales.The results show that temperature and confining pressure significantly affects the elastic modulus and peak strength of weakly cemented mudstone.With decreasing temperature,the compressive strength increases,while the corresponding peak strain decreases gradually.On the deformation curve,the plastic deformation stage is shortened,and the brittle fracture feature at the post-peak stage is more prominent,and the elastic modulus correspondingly increases with decreasing temperature.Under low-temperature conditions,most of the weakly cemented mudstone undergoes microscopic shear failure along the main fracture surface.The micro-fracture morphology characteristics of weakly cemented mudstone under different temperatures are quantified via the fractal dimension,and an approximately exponential relationship can be obtained among the fractal dimension and the temperature,compressive strength and elastic modulus.

Study on the freezing–thawing deformation of consolidated soils under high pressure

The freezing-thawing deformation behaviors of consolidated soils under high pressure have been investigated in a high-pressure-low-temperature （HPLT） Kq consolidation apparatus with a small strain sensor. The tests cover a variety of frozen soil temperatures ranging from -2℃ to -10 ℃, and a series of applied pressures ranging from 1 MPa to 5 MPa. The test results show that, for the consolidated soils under high pressure, their freezing-thawing deformation was caused by the realignment and the deformation of soil particles, the phase change of water, and the water redistribution in the soil. As for the deformation produced by thermal expansion and contraction,it is about 0.04-0.05 mm, accounting for only about 7%~9% of the total deformation. Taking the freezing-thawing deformation produced by temperature disturbance as a creep deformation, the creep models of the developing soil deformation will be determined by the soil＇s final temperature, i.e., the desired temperature. For the soils under a desired temperature between -2℃ and -5℃, the freezing-thawing de-formation develops according to a non-attenuation creep model; but for the soils with a desired temperature lower than -5℃, a full attenuation creep model is followed. The applied pressure and soil type also have a significant influence on the maximum freezing deformation. Generally, the greater the desired pressure applied, the less the maximum deformation is; and the loess freezing deformation is larger than that of sand.

Artificially frozen ground and related engineering technology in Japan

Since the 1970's, frozen ground has been developing near the Tokyo Bay area around liquefied natural gas(LNG) inground storage tanks. For disaster prevention purposes, the tanks are constructed below the ground surface. Since the temperature of the liquid stored in the tanks is -162℃ the soil surrounding the tanks freezes. Since this frozen ground has existed for almost half a century, we have permafrost near Tokyo. The development of artificial frozen ground may cause frost heaving, resulting in frost heave forces that may cause structural damage of adjacent LNG in-ground storage tanks.Therefore, the demand for frozen ground engineering increased and consequently we now have advanced technology in this area. Fortunately, we use this engineering technology and artificial ground freezing for civil engineering, especially in big and crowded cities like Tokyo. This paper provides a summary of the testing apparatus, test methods, and assessment methods for frost heaving.

Average temperature calculation for straight single-row-piped frozen soil wall

The average temperature of frozen soil wall is an essential parameter in the process of design, construction, and safety manage- ment of artificial ground freezing engineering. It is the basis of calculating frozen soil＇s mechanical parameters, fiarther prediction of bearing capacity and, ultimately, safety evaluation of the frozen soil wall. Regarding the average temperature of sin- gle-row-piped frozen soil wall, this paper summarizes several current calculation methods and their shortcomings. Furthermore, on the basis of Bakholdin＇s analytical solution for the temperature field under straight single-row-piped freezing, two new calcula- tion models, namely, the equivalent trapezoid model and the equivalent triangle model, are proposed. These two approaches are used to calculate the average temperature of a certain cross section which indicates the condition of the whole frozen soil wall. Considering the possible parameter range according to the freezing pipe layout that might be applied in actual construction, this paper compares the average temperatures of frozen soil walls obtained by the equivalent trapezoid method and the equivalent tri- angle method with that obtained by numerical integration of Bakholdin＇s analytical solution. The results show that the discrepancies are extremely small and these two new approaches are better than currently prevailing methods. However, the equivalent triangle method boasts higher accuracy and a simpler formula compared with the equivalent trapezoid method.

Analytic computation on the forcible thawing temperature field formed by a single heat transfer pipe with unsteady outer surface temperature

A comprehensive and systematic research on the forcible thawing temperature field formed by a single heat transfer pipe with unsteady outer surface temperature was carried out by analytic computation according to the theory of similitude. The distribution law of thawing temperature field, calculation formulas of thawing radius b, heat flux density q and average thawing temperature T were obtained. It theoretically explains that the main influential factors of thawing radius b, heat flux density q and thawing average temperature T are K,f, Lx and co（l）, but Lc affects little. Finally, based on the forcible thawing project of Hulusu air shaft lining, the field data indicate that the analytical formulas of this article are comparatively accurate.

Influence of freeze tube deviation on the development of frozen wall during long cross-passage construction

This paper investigates the influence of the deviation in freeze pipe installation on the development of the frozen wall in long cross passages by numerical simulation with ANSYS software.The study case is from the artificial ground freezing project along the Fuzhou Metro Line 2 between Ziyang Station and Wuliting Station.Two freezepipe configurations,i.e.,one with perfectly aligned pipes without any deviation from design and another with randomly distributed deviation,are included for comparison.The effect of the random deviation in the freeze pipes on frozen wall interconnection time,the thickness of the frozen wall and the development of the temperature field is explored.For the characteristic section of the numerical model at a depth of 25 m,it is found that the frozen wall interconnection time under the random deviation case and no deviation case is 24 days and 18 days,respectively.The difference in the thickness of the thinnest frozen wall segment between the random deviation and no deviation cases is the largest in the early freezing stage(up to 0.75 m),which decreases with time to 0.31 m in the late freezing stage.The effects of random deviation are more significant in the early freezing stage and diminish as the freezing time increases.

A Frost Heaving Prediction Approach for Ground Uplift Simulation Due to Freeze-Sealing Pipe Roof Method

Freeze-sealing pipe roof method is applied in the Gongbei tunnel,which causes the ground surface uplift induced by frost heave.A frost heaving prediction approach based on the coefficient of cold expansion is proposed to simulate the ground deformation of the Gongbei tunnel.The coefficient of cold expansion in the model and the frost heaving rate from the frost heave test under the hydration condition can achieve a good correspondence making the calculation result closer to the actual engineering.The ground surface uplift along the lateral and longitudinal direction are respectively analyzed and compared with the field measured data to validate the model.The results show that a good agreement between the frost heaving prediction model and the field measured data verifies the rationality and applicability of the proposed model.The maximum uplift of the Gongbei tunnel appears at the center of the model,gradually decreasing along with the lateral and longitudinal directions.The curve in the lateral direction presents a normal distribution due to the influence of the constraint of two sides,while the one along the lateral direction shapes like a parabola with the opening downward due to the temperature field distribution.The model provides a reference for frost heaving engineering calculation.

Revision of thickness design of cylindrical frozen walls considering frost heave

This paper outlines development of the thickness design of cylindrical frozen walls in artificial ground freezing （AFG）. A plain strain mechanical model coupled with infinite surrounding soil and rock takes into account the frost heave ratio to investigate the influence of frost heave on the thickness design of frozen wall, and superposition method is used to solve the complicated problem of frozen wall swelling. A revised formula referred to as ＂Baoshen＂ formula has been proposed. This formula provides a convenient analytic solution for any AGF problem involving not only frost heave but also the action of surrounding soil.

Mathematical models of steady-state temperature fields produced by multi-piped freezing

The multi-piped freezing method is usually applied in artificial ground freezing （AGF） projects to fulfill special construction requirements, such as two-, three-, or four-piped freezing. Based on potential superposition theory, this paper gives analytical solutions to steady-state frozen temperature for two, three, and four freezing pipes with different temperatures and arranged at random. Specific solutions are derived for some particular arrangements, such as three freezing pipes in a linear arrangement with equal or unequal spacing, right and isosceles triangle arrangements, four freezing pipes in a linear arrangement with equal spacing, and rhombus and rectangle arrangements. A comparison between the analytical solutions and numerical thermal analysis shows that the analytical solutions are sufficiently precise. As a part of the theory of AGF, the analytical solutions of temperature fields for multi-piped freezing with arbitrary layouts and different temperatures of freezing pipes are approached for the first time.

Analytical Solution to Steady-State Temperature Field of Two Freezing Pipes with Diferent Temperatures

The existing analytical formulas to calculate the temperature field distribution of artificial frozen soil walls are all based on the conditions that the surface temperatures of all freezing pipes are equivalent. In this paper, analytical solution of steady state temperature field of two freezing pipes with diferent temperatures is deduced based on thermal potential superposition method. The correctness of the analytical formulas is verified by comparing the temperature field distributions of the analytical formulas and those of the numerical simulations in the same conditions. And discussions are made to analyze the influence of some parameters on temperature field distribution of this condition.