Frozen Sandy Soil Research Articles

Mechanical properties and constitutive model of artificial frozen sandy soils under true triaxial stress state conditions

Triaxial compression tests were conducted on frozen sandy soils under a constant minimum principal stress (σ3 = 1.6 MPa) and various intermediate principal stresses (σ2 = 1.6, 3.4, 5.2, 7.0, 8.8, 9.8 MPa). The purpose of the research was to investigate the influence of intermediate principal stress on mechanical properties of frozen soil, and to establish a constitutive model to predict the stress-strain relationship of frozen soil under complex stress state. The test results obtained demonstrated that the crack damage stress and failure stress initially increase and then decrease with an increase in the intermediate principal stress. However, the crack initiation stress exhibits an initial increase up to a specific value, after which it stabilizes. From the perspective of crack propagation, the influence mechanism of intermediate principal stress on the strength was discussed. With the variation of stress state, the difference between intermediate principal strain and minimum principal strain gradually increases. The deformation difference was revealed using the stress superposition principle and Poisson's effect. Finally, the constitutive model based on the Weibull distribution and Drucker-Prager strength criterion can accurately represent the stress-strain relationships of frozen sandy soils under true triaxial stress states.

A nonlinear elastic-strain hardening model for frozen improved sandy soil under uniaxial compression loading condition

Artificial freezing is one of the most effective methods in the excavation of water-rich soils. This work aims at investigating the influence of cement‑sodium silicate grout (C-S grout) and organic polymer stabilizer (OPS) on the uniaxial compressive strength (UCS), stress-strain curve, and unfrozen water content of frozen sandy soil. A series of uniaxial compression tests and nuclear magnetic resonance (NMR) tests were conducted on the saturated frozen sandy soils improved by C-S grout and OPS (“C-S grout-improved soil” and “OPS-improved soil”) under different negative temperatures (i.e., −5 °C, −10 °C, −15 °C, and −20 °C). Based upon the experiment results and existing stress-strain models, including improved Duncan-Chang model and elastic-strain hardening model, a nonlinear elastic-strain hardening constitutive model for improved soils was proposed, in which each parameter has well-defined physical meaning. The results showed that, as the temperature decreases, the strengths of frozen improved soils gradually increase. The strength of OPS-improved soil first increases and then decreases with the increase of OPS dosage. Contrary to the UCS, the unfrozen water content of two improved soils was observed to gradually decrease with the decrease of temperature. As the OPS dosage increases, the unfrozen water content of improved soils decreases first and then increases. When the strain is <0.2, the stress-strain curves of frozen C-S grout-improved soil exhibits a behavior of yielding first and then hardening after the nonlinear elastic stage, while OPS-improved soil exhibits continuous strain hardening behavior. With temperature and unfrozen water content given, the new nonlinear elastic-strain hardening model can accurately predict the stress-strain behavior of frozen improved soils. This study is helpful to the stability analysis of artificial frozen walls and pre-control of environmental deformation during the excavation of water-rich sandy soils.

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.

Micro-macroscopic mechanical behavior of frozen sand based on a large-scale direct shear test

The mechanical properties of frozen soil are of utmost importance for infrastructure development in cold regions, and the microscopic characteristics play a vital role in revealing the mechanism of strength and deformation evolution. In this study, the mechanical behaviors of frozen sand at various negative temperatures are investigated by a self-developed temperature-controlled large-scale direct shear apparatus. The shear stress-shear displacement curve, dilation curve and shear strength characteristics at different negative temperatures and normal pressures are analyzed. According to the test results, as the cooling temperature decreases, the cohesion and internal friction angle of frozen sandy sample both increase significantly, and the shear dilatation tends to be larger. The brittle behavior of frozen sand becomes more obvious at lower temperatures. Then, a discrete element method (DEM) model is established to investigate the microscopic mechanical properties of frozen sand in the direct shear test. The particle movement, force chain evolution, and shear surface development during the shearing process are analyzed. The simulations of the DEM model can well explain the microscopic mechanism of the macroscopic strength and deformation law of frozen sandy soil. It is hoped that the research results can provide a reference for the DEM application in frozen soil mechanics.

Effect of clay content on the thermal conductivity of unfrozen and frozen sandy soils

Thermal conductivities of five sandy soils subjected to freezing-thawing and drying-wetting cycles were determined. Specimens were prepared at five different clay contents of 0, 5, 10, 15, 20% prior to compaction at the Proctor maximum dry density. Soil specimens were saturated prior to freezing-thawing and drying-wetting experiments. During a freezing-thawing cycle, soil's thermal conductivity and unfrozen water content were measured at different temperatures ranging from -3 °C to 0 °C. During a drying-wetting cycle, soil's thermal conductivity was measured at different water contents. The experimental results were then compared with predictions by two of the most appropriate existing models, namely De Vries's and Johansen's models, in order to provide insights into the effects of clay content on soil thermal conductivity at (saturated and unsaturated) unfrozen and frozen states.

Energy Criterion for Fracture of Rocks and Rock-like Materials on the Descending Branch of the Load-Displacement Curve.

This article deals with the problem of predicting the brittle fracture of rocks and similar materials, which can also include frozen sandy soils. Such materials, due to the diversity of their conditions of origin, are characterized by natural heterogeneity at the micro-, meso-, and macro-levels, which makes it difficult to develop sufficiently universal criteria for their strength. Despite a number of known models and criteria of strength and fracture, the search for such criteria remains an urgent problem. In this paper, using the energy approach to the mathematical modeling of mechanical systems, the fracture criterion is justified, which differs from the known criteria that do not require integration to calculate the strain energy We and dissipation energy Wd. The well-known relation for the input energy W=We+Wd is used. The object of the study was the ratio of dW=dWe+dWd. The main research question concerned what the ratio of dWe and dWd would be at the point of brittle failure. The search for an answer to the question led to the justification of a differential energy criterion for the failure of brittle materials on the descending branch of the full stress−strain curve. It was found that the point of predicted fracture is determined by the equality σ=0.5 εEtangential (if there is an inflection point on the ascending branch) or σ=0.5 εEsecant_secant. The main result of the work was ascertaining the differential strength and fracture criteria of brittle materials in the form of inequalities and equations, which were oriented for application in engineering calculations. Examples of application of the developed criteria are given; their consistency with the experimental data known from the literature confirmed.

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.

Study on Energy Dissipation Characteristic of Ice-Rich Frozen Soil in SHPB Compression Tests

Frozen soil will inevitably bear dynamic loads (impact and blasting) in the construction process in cold regions; the investigation of dynamic energy dissipation characteristic of ice-rich frozen soil is beneficial to optimize blasting parameters and the design of underground explosion-proof structure. In this study, the dynamic impact laboratory tests were conducted for frozen sandy soil with various water contents (from 15% to 110%) and strain rates (from 450 s-1 to 728 s-1) based on the split Hopkinson pressure bar (SHPB) system. The influences of strain rate, temperature, and water content on the energy parameters (i.e., absorption energy, reflected energy, and absorption energy rate) were systematically analyzed. Moreover, the energy dissipation characteristics of frozen sandy soil under different deformation stages were studied. Test results revealed that under impact load, the proportion of reflected energy to incident energy was the largest for frozen soil materials. Both the absorption energy and reflected energy were associated with strain rate. However, compared with the water content and temperature, the sensitivities of strain rate on the absorption energy rate were not obvious. At -10°C, the average absorption energy rate decreases from 17.6% to 14.5% when the water content increases from 15% to 37.5%, with a reduction of 18%. However, it substantially decreases to 6.9% at 45% water content, with a large-scale reduction of 61% compared with that at 15% water content. The energy dissipation parameters (i.e., absorption energy, releasable elastic strain energy, and dissipation energy) were closely associated with the water content, temperature, and strain rate.

Characteristics and Prediction of the Thermal Diffusivity of Sandy Soil

Revealing the variation law of thermal diffusivity of sandy soil can provide a theoretical basis for the engineering design and construction in cold and arid regions. Based on experimental data of sandy soil samples, the distribution characteristics and influence of dry density and moisture content on thermal diffusivity are analyzed in this work. Then, the prediction model based on the empirical fitting formula and RBF neural network method for thermal diffusivity of frozen and unfrozen sandy soil is established, and the prediction accuracy of different prediction methods is compared. The results show that (1) thermal diffusivity of sandy soil is positively correlated with the particle size. With the increase of sand size, thermal diffusivity of sandy soil increases significantly. (2) Partial correlation among natural moisture content, dry density, and thermal diffusivity varies with different frozen and unfrozen conditions. (3) For unfrozen sandy soil, the binary RBF neural network prediction model is obviously better than that of the binary empirical fitting formula model. (4) The ternary prediction model has significantly higher prediction accuracy than that of the binary prediction model for frozen sandy soil, and the ternary RBF neural network model has the best prediction effect among the four methods.

Damage model of unsaturated frozen soil while considering the influence of temperature rise under impact loading

Constitutive models and damage laws of frozen soil under high strain rates are crucial for the design of underground soil structures, and they lay an essential foundation for the research of instability phenomena in cold region engineering. The expansion and evolution of micro-defects and temperature damage are two basic mechanisms that control the macro and micro damage behavior of frozen soil. Considering frozen soil as a composite material, this study analyzes the damage mechanism of brittle materials subjected to impact loading. Finally, a dynamic model of unsaturated frozen soil under high pressure is established based on the complementary energy equivalence principle and Eshelby's equivalent inclusion theory. To analyze the effect of high-speed compression on the meso behavior of frozen soil, according to the Hugoniot energy theory, the evolution of temperature rise in the impact process is formulated. The results for frozen sandy soil confirm that the rate of temperature rise increases with the strain rate and has a significant weakening effect on the bearing capacity. Thus, the damage behavior of frozen soil changes according to different loading histories. Finally, based on the corresponding theoretical results, the mechanical response of frozen soil under different loading histories and the corresponding performance indicators are compared and analyzed.

Experimental study and modelling of the thermal conductivity of frozen sandy soil at different water contents

In a wide range of earthworks and engineering applications such as ground heat exchanger piles and energy piles, radioactive waste disposal in deep underground locations, installation of underground power cables, etc., a better understanding of soil thermal conductivity is required in cold regions. The thermal conductivity (λ) of frozen sandy soils in eastern Turkey was measured for five volumetric water contents (θ = 0.219, 0.230, 0.246, 0.260, and 0.320 m3/ m3) at six frozen temperatures (Tf = 4, 0, −7, −12, −20 and −25 °C). Moreover, six proposed models were analyzed to determine the λ. The results demonstrated that a decrease in the Tf increased the λ and reached a plateau at a very low Tf for all θ. Increasing the θ increases the λ at the same Tf. Also, the de Vries model provided for the highest precision, followed by the Chen, the Parallel, the Johansen, the Cote and Konrad, and the serial models, respectively. Further, the changes of volume in the soil were incorporated into the parallel and series models, and a new model was developed to estimate the λ of the frozen soil. The new model can be effectively used to estimate the λ.

TWO-SIDED ESTIMATIONS OF RESISTANCE TO PENETRATION OF A CONE INTO FROZEN GROUND

This work presents experimental data pertaining to the determination of the ultimate strength of frozen soil under uniaxial compression in the range of strain rates of 400–2700 s-1. Finite expressions are obtained for quadratic approximation coefficients that depend on the impact velocity of a stress normal to the impactor surface and the experimentally determined physical and mechanical parameters of the soil, namely shock adiabat and the dynamic strength in compression. The resulting expressions are verified by comparing them with known experimental data related to the penetration of a steel impactor into frozen sandy soil. It is shown that a difference between the results of two-sided estimates and the experiments does not exceed 15%.

An elastoplastic damage constitutive model for frozen soil based on the super/subloading yield surfaces

Soil particles and ice form bonded structure in frozen soil. The modulus and bond strength of frozen soil decrease with the damage of the bonded structure during deformation. An elastoplastic damage constitutive model, based on the super/subloading yield surfaces, is proposed for frozen soil, in which the damage variable and bond strength are used to consider the elastic and plastic damage respectively. The model is validated with the observed mechanical behaviors of frozen sandy soil and frozen saline soil in triaxial compression tests with different confining pressures. The comparisons illustrate that the model can describe the hardening/softening and dilatancy of frozen soils with the change of confining pressures. The results also show that the expansion of frozen soil during shear is related to the loss of bond strength induced by the development of damage of the bonded structure.

ОЦЕНКА КОНТАКТНЫХ НАПРЯЖЕНИЙ ПРИ ВНЕДРЕНИИ УДАРНИКА В ПРОЧНЫЙ ГРУНТ

Finite formulas have been derived for evaluating contact stresses in a rigid impactor penetrating a soil, taking into account the friction in the framework of the local interaction model. In analyzing dynamic deformation of the soil, its volumetric compressibility, shear resistance and initial strength are accounted for. The obtained evaluations of resistance to penetration of an impactor into the soil are based on a quadratic relation between the stress normal to the impactor surface and impact velocity. The authors have pioneered in deriving finite expressions for coefficients of a trinomial approximation as a function of experimentally determined physical-mechanical parameters of the soil - a dynamic compressibility diagram (a shock adiabat) and a yield strength - pressure diagram. Impact compressibility of soils is described based on Hugoniot's adiabat - a linear relation between shock wave velocity and mass velocity of the medium particles behind the shockwave front. Plastic deformation obeys the Mohr - Coulomb yield criterion with a constraint on the limiting value of maximal tangential stresses according to Tresca's criterion - the Mohr - Coulomb - Tresca plasticity condition. An earlier obtained analytical solution of a one-dimensional problem of a spherical cavity expanding at a constant velocity from a point in a half-space occupied by a plastic soil medium is used. A formula for determining critical pressure (a minimal pressure required for the formation of a cavity, accounting for internal pressure in the framework of Mohr - Coulomb's yield criterion) is also used, which generalizes a known solution for an elastic ideally plastic medium with Tresca's criterion. The derived formulas have been verified by comparing their results with the available data from experiments on the penetration of a steel conical impactor into a frozen sandy soil. It is shown that the disagreement between the numerical and experimental results is within 10%.

An elastoplastic constitutive model for frozen sandy soil considering particle breakage

A series of triaxial compression tests of frozen sandy soil are carried out under five confining pressures (1 MPa, 4 MPa, 6 MPa, 8 MPa and 10 MPa) at –6 °C. By comparing the grain size distribution curves of frozen sandy soil before and after shearing, it is found that significant particle breakage can occur during triaxial shearing. Particle breakage changes internal structure of geomaterials and has a significant effect on their stress–strain relationships. In order to accurately describe the effect of particle breakage of frozen sandy soil on the stress–strain relationships, an elastoplastic constitutive model for frozen sandy soil considering particle breakage is proposed. Based on the energy balance equation established by Indraratna and Salim, the constant critical state stress ratio (Mcr ) in the energy balance equation is modified to the stress ratio (M) which changed with shear strain during the shearing process. A yield function, considering particle breakage, is proposed using the modified energy balance equation. The hardening law is determined based on the rebound test results of frozen sandy soil, and the associated flow rule is adopted in the model. Compared with experimental data, the model can well simulate the stress–strain relationships under different confining pressures for frozen sandy soil. Highlights The particle breakage characteristics of frozen sandy soil are studied. The energy balance equation considering particle breakage is modified to calculate the energy dissipation of frozen sandy soil. An elastoplastic constitutive model for frozen sandy soil considering particle breakage is proposed.

Simulating the dynamic behavior and energy consumption characteristics of frozen sandy soil under impact loading

The dynamic mechanical properties of frozen sandy soil (initial moisture content of 20%) are investigated using a 30-mm-diameter split Hopkinson pressure bar system. The results demonstrate that the dynamic compressive strength and final strain increase with the strain rate, while the dynamic compressive strength decreases with increasing freezing temperature. To characterize the energy consumption properties of frozen sandy soil under impact loading, the relationship between the energy density and experimental variables is studied and energy consumption regimes are analyzed. Finally, a constitutive model is developed for predicting the dynamic strength and damage of frozen sandy soil subjected to impact loading. The proposed model is based on the continuum fracture mechanics of microcrack nucleation, growth, and coalescence in order to formulate the evolution of damage. The results reveal that the model effectively describes the relationships between the stress and strain of frozen sandy soil.

Effect of the Mass Fraction of Ice on the Strain Rate Dependence of Strength under Dynamic Fracture of Frozen Soil

Experimental dependences of the strength of frozen sandy soil on strain rate are analyzed using a structural–temporal approach. Results of dynamic uniaxial compression tests at a temperature of −18°C and strain rates of 400 to 2600 s−1 of frozen sandy soil specimens of two types with an ice mass fraction of 10 and 18% measured at room temperature are presented. The strain rate dependence at different freezing temperatures of frozen sand with an ice mass fraction of 30% is studied using known experimental data.

Energy Consumption Analysis of Frozen Sandy Soil and an Improved Double Yield Surface Elastoplastic Model considering the Particle Breakage

The stress‐strain relationship of frozen soil is a hot research topic in the field of frozen soil mechanics. In order to study the effect of particle crushing on the stress‐strain relationship, a series of triaxial compression tests for frozen sandy soil are performed under confining pressures from 1 to 8 MPa at the temperatures of −3 and −5°C, and the energy consumption caused by particle breakage is analyzed during the triaxial shear process based on the energy principle. It is found that the energy consumption caused by the particle breakage presents a hyperbolic trend with axial strain. In view of the obvious advantages of the double yield surface elastoplastic model in describing soil dilatancy, stress path effect, and stress history influence, a modified double yield surface elastoplastic model for frozen sandy soil is proposed based on the energy principle. The validity of the model is verified by comparing its modeling results with test results. As a result, it is found that the stress‐strain curves predicted by this model agree well with the corresponding experimental results under different confining pressures and temperatures.

Прочность и модуль упругости мерзлых песчаных грунтов как материала лесозаготовительной дороги

Прочность и модуль упругости мерзлых песчаных грунтов как материала лесозаготовительной дороги