Frozen Soil Specimens Research Articles

Dynamic impact tests and temperature rise induced damage constitutive model of ice-rich frozen sandy soil

In this study, a soil-ice particle mixing method was used to prepare ice-rich frozen sandy soil specimens. Subsequently, the mechanical behaviors of ice-rich frozen sandy soil with a wide range of total moisture contents (15%–150%) were studied at three negative temperatures (−5, −10, and −15 °C) under the impact pressures from 0.2 MPa to 0.4 MPa. The melting of ice particles caused by the rise in temperature of frozen soil specimens was identified as temperature damage. Moreover, damage mechanism of ice-rich sandy frozen soil specimen during failure process was considered as the combined action of crack propagation and temperature rise. The analysis of the relationship between temperature rise value and dynamic strain was then calculated based on the heat conduction theory. Additionally, the phase transition equilibrium between unfrozen water and effective ice was used to determine the variation in ice volume content as the temperature rises, and the temperature damage variable was defined according to the reduction of dynamic elasticity modulus of ice-rich frozen sandy soil. Lastly, the compound damage variable was deduced and considered in the damage constitutive model. Accordingly, the theoretical and test data were compared to determine the model parameters. The results of the curve comparison indicate that the proposed model is feasible and suitable to be applied to predict the dynamic mechanics of ice-rich frozen sandy soil.

Influence of intermediate principal stress ratio on strength and deformation characteristics of frozen sandy soil under different moisture contents

Frozen ground often contain non-uniform moisture content and are under true triaxial stress state and their practical engineering application are still explored. The present study is carried out to investigate the strength and deformation characteristics of frozen soil with different moisture contents under true triaxial stress state conditions. A series of true triaxial tests were conducted on frozen soil specimens having constant minimum principal stress (σ3) and constant intermediate principal stress ratio (b). The experimental test results showed that the stress-strain curves represent the strain-hardening characteristics under different test conditions. The intermediate principal stress ratio (b) has a dual effect of strengthening and weakening on the strength of frozen sandy soil. When 0 ≤ b ≤ 0.5, the strengthening effect was found to be dominant in the frozen soil specimen, whereas, the weakening effect was dominant when 0.5 < b ≤ 1. The strength of frozen soil under true triaxial stress state is 8.18% to 36.33% higher under different water content conditions, compared to that under generalized triaxial compression stress state. The difference in deformation between the direction of the intermediate principal stress and the minimum principal stress increased with the increment in the intermediate principal stress ratio value. The volumetric strain in the soil sample initially exhibited the characteristics of shear contraction and then shear dilation.

Creep Characteristic Test and Creep Model of Frozen Soil

In order to research the creep deformation characteristics of frozen soil and the effect of various influencing factors on creep, indoor uniaxial creep tests were carried out on frozen soil specimens at temperatures of −5, −4, −3 and −2 °C under loads of 0.25, 0.5, and 0.75 σt, respectively. The creep deformation characteristics of frozen soil under different temperatures and load conditions are analyzed under unconfined conditions. The results show that under the uniaxial creep test conditions, when the load is low, there is no accelerated creep stage in the creep curve, which belongs to the decaying type creep; when the load is 0.75 σt, the creep curve enters the stage of accelerated creep, and the creep turns non-attenuated; temperature is the most important external factor affecting permafrost soil creep, and the proportion of ice is the key internal factor for affecting permafrost soil creep, the temperature is negatively correlated with the proportion of ice and the sensitivity of creep rate to temperature and load increases with the decrease of the proportion of ice; the damage variable D is introduced to modify the creep constitutive equation of the frozen soil, the creep process of frozen soil is well described by the modified creep constitutive equation for frozen soil.

Numerical simulation research on impact mechanical properties of frozen soil based on discrete element method

Many engineering activities have been conducted on permafrost. Frozen soil is subjected to impact loading, particularly during blasting and excavation projects. Thus, it is essential to study the impact mechanical properties of frozen soil. A split Hopkinson pressure bar (SHPB) experiment was conducted to investigate the mechanical responses of frozen soil specimens subjected to impact loading under different strain rates at different temperatures. Evident strain rate and temperature effects were also observed. As crack development in the specimens could not be observed experimentally, a two-dimensional particle flow code was utilized to numerically simulate SHPB impact experiments on frozen soil. A contact bonding model was used for the simulation. We assumed that only temperature could change the particle parameters. The parameters and establishment of the geometric model in the simulation were calibrated and validated by comparing them with the experimentally obtained impact stress–strain curves and wave signals. More influences of strain rate and temperature on crack development were presented intuitively in this study, in combination with the propagation of stress waves. The results of the numerical simulation demonstrated that when the frozen soil specimen was subjected to impact loading, shear failure was the primary failure in the specimen. For a given temperature, a lower strain rate decreased the number of cracks generated, increased the duration of crack generation, and delayed the formation of cracks. For a given strain rate, a lower temperature decreased the number of cracks generated and the duration of crack generation; however, the number of tensile cracks was negligibly affected by changes in temperature.

Crack Detection in Frozen Soils Using Infrared Thermographic Camera.

Frozen soils are encountered on construction sites in the polar regions or regions where artificial frozen ground (AFG) methods are used. Thus, efficient ways to monitor the behavior and potential failure of frozen soils are currently in demand. The advancement of thermographic technology presents an alternative solution as deformation occurring in frozen soils generate heat via inter-particle friction, and thus a subsequent increase in temperature. In this research, uniaxial compression tests were conducted on cylindrical frozen soil specimens of three types, namely clay, sand, and gravel. During the tests, surface temperature profiles of the specimens were recorded through an infrared video camera. The thermographic videos were analyzed, and subsequent results showed that temperature increases caused by frictional heat could be observed in all three frozen soil specimens. Therefore, increases in temperature can be deemed as an indicator for the potential failure of frozen soils and this method is applicable for monitoring purposes.

Feasibility study on ice content measurement of frozen soil using actively heated FBG sensors

Ice content is a critical parameter affecting the hydrological, thermal, and mechanical properties of frozen soil. However, it is hard to capture in-situ ice contents due to the limitations of conventional geotechnical monitoring techniques. A new method of ice content measurement for frozen soil has been recently proposed using the actively heated fiber Bragg grating (AH-FBG) sensors. This study aims to examine the feasibility of this measuring technique and investigate some critical factors related to in-situ monitoring. A series of laboratory calibration tests were performed on frozen soil specimens at the initial temperature of −17 °C. The thermal conductivities of the specimens were measured by the proposed method and compared with the results obtained using a heat transfer analyzer. To verify the effectiveness of ice content calculation and estimate the measuring accuracy while considering the presence of unfrozen water, the ice contents recorded by frequency domain reflectometry (FDR) sensors were used to provide the theoretical calculation equation of ice content reference values. Furthermore, a two-dimensional axisymmetric finite element numerical model was built to explore the influencing factors of the AH-FBG method. The experimental results indicate that this method can be used as a reliable tool for measuring ice content but have certain limitations. The series model is found to be the most suitable model for calculating thermal conductivity and ice content based on temperature changes. The numerical simulation results show that this method is applicable to ice content measurement when the initial temperature is below −6 °C and the heating duration is over 110 s. The maximum normalized radius of influence of this measuring technique is 14. To refine the proposed method for field monitoring, further studies are needed to solve these limitations and expand the scope of application.

Energy absorption characteristics and theoretical analysis of frozen clay with pre-existing cracks under uniaxial compressive impact load

Two important engineering design parameters, namely, dynamic energy dissipation and fragment size distribution, are closely associated with the crushing efficiency of the frozen soil in the cold regions. The crack distribution inside the frozen soil exerted a notable effect on its mechanical properties and energy dissipation characteristics under dynamic loads. In this study, a conventional split Hopkinson pressure bar with 50 mm diameter was used to conduct uniaxial impact compressive test in the frozen soil under different pre-existing crack conditions. The effects of pre-existing cracks on the energy dissipation and fractal dimension of the frozen soil were investigated based on the relationship among the energy absorbency rate, fractal dimension, and dynamic peak strength. The experimental results showed that energy absorbency rate and fractal dimension decreased as the growth of the number of pre-existing cracks, while both the energy absorbency rate and fractal dimension of frozen soil specimen with pre-existing cracks at the end surface remained less than those at the side surface. The effect of crack length on the energy absorbency rate and fractal dimension was larger than compared to the number and obliquity of cracks. In addition, the energy–time curves under different test conditions were also compared. Finally, the adaptability of the constitutive model was verified by comparing the theoretical and experimental curves based on the energy dissipation results.

Evaluation of dynamic characteristics of silt in Yellow River Flood Field after freeze-thaw cycles

Frothing is a main disease of highways in Yellow River Flood Field, due to the loss of dynamic strength of roadbed soils under the couple effects of temperature, salt, and vehicle traffic load. This is strongly linked to the dynamic characteristics of silt in this region. To analyze these couple effects on the dynamic characteristics of silt, a series of tests (i.e., freeze-thaw cycling tests, vibration triaxial tests and ultrasonic wave velocity tests) were conducted and two kinds of silt (i.e., salt-free and 3%-salt silt) were designed. The results indicate that the dynamic shear strength and dynamic modulus decrease with increasing freeze-thaw cycles, while the damping ratio simultaneously increases. Furthermore, compared to salt-free silt, the decrement of dynamic shear strength and dynamic modulus of silt with 3% salt is more significant, but the damping ratio of 3%-salt silt is larger. In ultrasonic wave velocity tests, ultrasonic wave velocity of frozen soil specimens decreases as the number of freeze-thaw cycles increases. Based on the results of ultrasonic wave velocity tests, a preliminary model is proposed to evaluate damage of silt through field measurement ultrasonic data. The study could provide a theoretical basis for the treatment of silty soil highway.

A damage and elastic-viscoplastic constitutive model of frozen soil under uniaxial impact loading and its numerical implementation

We analyzed the uniaxial impact compression due to different strain rates on frozen soil specimens at different temperatures using split-Hopkinson pressure bar (SHPB) and obtained the dynamic stress–strain curves under the corresponding conditions. The experimental results showed that the dynamic mechanical behavior of frozen soil under impact loading shows an obvious strain-rate effect and temperature effect, as well as obvious elastic-viscoplastic deformation characteristics. Furthermore, the dynamic stress–strain curves of frozen soil showed instability with the evolution of internal micro-cracks, micro-voids, and shear bands in frozen soil. The dynamic mechanical behavior of frozen soil was described by introducing an advanced rate-dependent continuous damage evolution model and an elastic-viscoplastic constitutive model based on the Chaboche unified viscoplastic constitutive theory and using the Drucker–Prager yield criterion. To obtain a numerical solution of the elastic-viscoplastic constitutive equation, the constitutive equation was discretized into an incremental form by Euler's method, and the closest point projection algorithm was used as the numerical integral to update the stress state during the deformation process of frozen soil. The constitutive model was numerically solved, and the theoretically calculated curves agreed well with the experimentally measured curves. Thus, we verified the rationality and applicability of the uniaxial dynamic constitutive model constructed for this study and developed its numerical integral algorithm.

Experimental Test and Prediction Model of Soil Thermal Conductivity in Permafrost Regions

Soil thermal conductivity is a dominant parameter of an unsteady heat-transfer process, which further influences the stability and sustainability of engineering applications in permafrost regions. In this work, a laboratory test for massive specimens is performed to reveal the distribution characteristics and the parameter-influencing mechanisms of soil thermal conductivity along the Qinghai–Tibet Engineering Corridor (QTEC). Based on the measurement data of 638 unfrozen and 860 frozen soil specimens, binary fitting, radial basis function (RBF) neural network and ternary fitting (for frozen soils) prediction models of soil thermal conductivity have been developed and compared. The results demonstrate that, (1) particle size and intrinsic heat-conducting capacity of the soil skeleton have a significant influence on the soil thermal conductivity, and the typical specimens in the QTEC can be classified as three clusters according to their thermal conductivity probability distribution and water-holding capacity; (2) dry density as well as water content sometimes does not have a strong positive correlation with thermal conductivity of natural soil samples, especially for multiple soil types and complex compositions; (3) both the RBF neural network method and ternary fitting method have favorable prediction accuracy and a wide application range. The maximum determination coefficient (R2) and quantitative proportion of relative error within ±10% ( P ± 10 % ) of each prediction model reaches up to 0.82, 0.88, 81.4% and 74.5%, respectively. Furthermore, because the ternary fitting method can only be used for frozen soils, the RBF neural network method is considered the optimal approach among all three prediction methods. This study can contribute to the construction and maintenance of engineering applications in permafrost regions.

A micromechanics-based elastoplastic constitutive model for frozen sands based on homogenization theory

A micromechanics-based constitutive model is formulated to describe the nonlinear elastoplastic behaviors of frozen sands. In the model, frozen soils are conceptualized as two-phase materials, in which the first phase is regarded as virgin/undamaged frozen soil specimen including the components of soil particles, ice crystals, and partial water with elastic–brittle behaviors (bonded elements), and the second phase is treated as fully damaged soil sample including soil particles and unfrozen water with elastoplastic behaviors (frictional elements). The interactions between bonded elements and frictional elements are well considered in the representative volume element within the framework of continuum micromechanics and homogenization theory. Moreover, the non-uniform distribution of strain is taken into account and the breakage process is considered due to the structural degradation/loss of ice crystals in the representative volume element. It is found that the ice crystals are gradually breaking up and then melting into unfrozen water with the increase of external loads, so at the moment the bonded elements are considered to transform into frictional elements and then both elements bear the external loads collectively. A novel binary-medium constitutive model is established by the Mori–Tanaka technique and then validated by two groups of laboratory tests about frozen soils and unfrozen sands under conventional triaxial compression conditions. Eventually, the predictions demonstrate that the theoretical calibrations are well in agreement with the available experimental stress–strain responses.

Triaxial stress path tests on artificially prepared analogue alpine permafrost soil

Some degrading rock glaciers have been exhibiting deepening depressions, accelerating strain rates, and, in some rare cases, sudden release of mass movements. Warming permafrost already mobilizes lower strength as temperatures rise; however, unusual stress paths with lateral stresses greater than vertical stresses, instead of vice versa, could exacerbate this condition with lower strength at failure, and hence higher vulnerability. This paper investigates the mechanical behaviour of artificially frozen soil specimens at temperatures between –3.0 and –0.3 °C under various stress paths: axial (A) and lateral (L), compression (C), and extension (E), for total stress paths AC, AE, LE, LC. Acoustic emissions were detected during shearing to expose how the deformation mechanisms develop from a microstructural point of view. Deviatoric stress mobilized in the stress path tests was linearly dependent on the temperature (within the ranges tested): a temperature increase resulted in a decrease in residual deviatoric stress. Comparison between the residual deviatoric stresses obtained from the different stress path tests indicates that (i) values mobilized with radial stress greater than axial stress were lower than vice versa, (ii) more strength was mobilized when changing lateral stress paths than axial, with (iii) the lowest strength mobilized in AE beneath a depression.

Statistical characteristics of the thermal conductivity of frozen clay at different water contents

Abstract Over 50% of the global land area suffers cyclic freeze-thaw actions, resulting in extensive distribution of frozen soil. Frozen soil contains lenses of ice, and thus, it exhibits temperature-related properties and behaviors. The thermal conductivity of frozen soil is a key property and an essential parameter for performing thermal analyses on frozen soil. However, the thermal conductivity is variable and cannot be determined accurately based on small-scale specimen tests because no frozen soil specimens are identical in nature. Thus, little is known about the variability and statistical characteristics of frozen soil. To address this lack of knowledge, we conducted large-scale tests on numerous samples ( ≥ 50 specimens under each set of conditions) and measured the thermal conductivities of frozen clay from the Qinghai-Tibet Plateau (QTP) at 3 water contents (10%, 14% and 18%). The measured data show that the thermal conductivities of the 50 clay specimens differ remarkably and exhibit obvious variability, even at the same soil temperature. Next, we selected 4 potential probability distributions for comparison with the measured curves. The normal distribution, lognormal distribution and gamma distribution all demonstrate good agreement with the measured data and support the hypothesis. Based on goodness-of-fit analysis, we recommend using a normal distribution to describe the thermal conductivity of frozen clay. The measured data and conclusions provide a theoretical basis and reference for thermal analyses on cold regions engineering.

A field condition-based test method for artificially frozen soils

The formation process of artificially frozen soils in the field, which significantly affects the behaviour of frozen walls, can be divided into three stages – that is, K0 drained consolidation (K0DC), drained freezing under applied stress (F) and the formation of lateral thermal gradient (G). The K0DCGF method was thus developed and its reliability was comprehensively evaluated by testing on hollow cylindrical specimens. It was found that the measured K0 at the inner and outer surfaces of the hollow cylindrical specimens was identical to that of the cylindrical specimens. Unlike the widely observed frost heave during freezing prior to stress application, drained freezing under applied stress resulted in a slight shrinkage of the specimen's volume. The weakening of the lateral thermal gradient on the K0DCGF-based strength was smaller than that observed in previous studies of frozen soil specimens with a vertical thermal gradient due to the parallel structures of the frozen zones with different temperatures in the frozen hollow cylinder. The proposed K0DCGF method was validated using multiple tests and can be used for assessing the strength behaviour and associated mechanism of various artificially frozen soils.

Residual deformation and stiffness changes of frozen soils subjected to high- and low-amplitude cyclic loading

The present paper reports the results of a laboratory experiment that aimed to investigate the residual deformation and stiffness changes of frozen silty clays under cyclic triaxial loading. Two sets of cyclic stress magnitude — namely, low and high level — were applied to the reconstituted artificial frozen soil specimens with identical properties, and the effect of six levels of confining pressure was examined. The results indicate that two distinct development categories of residual axial strain with number of loading cycles are induced by the low- and high-level cyclic stress. The dynamic Young’s modulus derived from each test experiences a significant increase with the number of loading cycles, which is partly attributed to the cyclic densification. A special observation from the tests under high-level cyclic stress along with a confining pressure of 0.3 MPa also indicates that the dynamic Young’s modulus keeps as a relatively constant value when the change of residual volumetric deformation transfers from compression to dilation. The empirical relationships were proposed to properly reflect the development of the residual axial strain and dynamic Young’s modulus with the number of loading cycles.

Dynamic Mechanical Properties and Failure Mode of Artificial Frozen Silty Clay Subject to One‐Dimensional Coupled Static and Dynamic Loads

The dynamic stress‐strain relationship of artificial frozen silty clay under one‐dimensional coupled static and dynamic loads is obtained using modified split Hopkinson pressure bar (SHPB) equipment. The variation in dynamic compressive strength, dynamic deformation modulus, energy dissipation, and failure mode of artificial frozen silty clay with axial precompressive stress ratio are studied in this research. Experimental results indicate that the dynamic stress‐strain curves under uniaxial state and one‐dimensional coupled static and dynamic loads can be divided into four stages, i.e., compaction stage, elastic stage, plastic stage, and failure stage. The dynamic compressive strength, first‐stage deformation modulus, second‐stage deformation modulus, and absorbed energy density of artificial frozen silty clay present a trend of first increase and then decrease with the increase of axial compressive stress ratio, and the axial compressive stress ratio corresponding to the peak value is 0.7 in this test. In addition, there is a very similar effect of axial precompressive stress ratio on dynamic compressive strength and second‐stage deformation modulus of artificial frozen silty clay. At 0.4 axial compressive stress ratio, spall phenomenon appears at circumferential direction and center position of the frozen soil specimen has no obvious failure. Shear failure appears at 0.7 to 0.9 axial compressive stress ratio, and the larger the axial compressive stress ratio applies, the more obvious the shearing surface appears; moreover, comminution failure mode appears at 1.0 axial compressive stress ratio.

Strength of undisturbed and reconstituted frozen soil at temperatures close to 0

The strength of warm frozen soils in permafrost is fundamentally significant to estimate and predict the ground settlements from construction activities. A study was therefore initiated to assess the strength and its behaviors of undisturbed and reconstituted frozen soils at temperatures close to 0 °C. A series of triaxial compression tests (TCT) were performed by using a developed testing apparatus and a matching specimen-preparation method. The confinement was applied from air pressure, the temperature in the specimen was maintained using two-end refrigeration, and multi-stage loading on a single specimen was adopted to determine the strength. The test results showed that the strength, both for the undisturbed and reconstituted frozen-soil specimens, was significantly dependent on the temperatures and independent of the applied confining pressures. Additionally, the strength of undisturbed frozen soils was about 1.6 times more than that for reconstituted frozen soils. These observations were closely associated with the structures existing between pore-ice and gravels with large diameters.

Evaluation of one-dimensional freezing behavior for ice-rich sandy soil

This paper evaluates a methodology that enables one-dimensional freezing to make a homogeneous frozen soil specimen. Numerical analyses that simulate one-dimensional freezing are conducted, considering the phase change of pore water. After evaluating the applicability of the numerical analysis model, the effects of geotechnical characteristics on the freezing behavior of ice-rich sandy soil are evaluated through parametric studies. In addition, a nonlinear regression model is proposed that can predict the time required for complete freezing of saturated soil. According to the results from nonlinear regression analysis, the model demonstrates reliable accuracy (R2 = 0.998) in predicting the amount of time required for complete freezing, even with data from testing datasets not used in the developing stage.

Development of a novel plane strain test apparatus for frozen soils

In this paper, a novel apparatus enable of high controlling accuracy of temperature was developed for plane strain test of frozen soils. A cryostat was used to control test temperature. To improve temperature distribution uniformity of frozen soil specimens, two guiding plates were installed into the cryostat to force cooling air distribute uniformly surround specimen. According to test results, the temperature difference between different monitored points and average temperature can be controlled at ±0.2 °C. By taking Chinese standard sand as study object, compression tests under different axial strain rates were conducted. The images on the surface with strain restrained were captured during loading process, and strain localization evolution was analyzed with PIV technique. It was indicated that, with decrease of strain rate, shear band inclination and width do not change notably, while the strain field distributes more uniformly and concentration of which was decreased.

Dynamic mechanical experiments and microstructure constitutive model of frozen soil with different particle sizes

To reveal the influences of soil particle size on the dynamic impact mechanical properties of frozen soil, four groups of frozen soil specimens composed of different particle sizes are tested using a split-Hopkinson pressure bar. Based on the Druger–Prager failure criterion and coupled damage-plasticity, a dynamic micro-constitutive model is established for describing the dynamic mechanical behavior of the frozen soil. Macroscopically, frozen soil is assumed to be homogeneous and continuous, although a large number of micro-cracks and micro-voids are distributed randomly throughout the volume. When a frozen soil specimen is subjected to a substantial shock, the propagation of micro-cracks and the collapse of micro-voids can induce damage. The evolution equations of the two damage mechanisms are proposed. Finally, through a comparison, it was shown that simulation results agreed well with the experimental results, thus validating the suitability of the developed model.