Stress Paths Research Articles

Strength characteristics of different stress spaces of red sandstone under loading and unloading tests

Red sandstone tests were conducted on the rock mechanics test system to clarify the strength and failure characteristics of red sandstone under triaxial loading and different unloading confining pressure rates. Strength change rules of the red sandstone in various stress spaces were analyzed. Results show that a large initial confining pressure indicates a long yield section and large axial strain. A small unloading confining pressure rate indicates a long yield section and large axial strain at failure under the identical initial confining pressure. The confining pressure reduction increases with the unloading confining pressure rate increasing when the red sandstone fails. The stress path of unloading confining pressure weakens the red sandstone cohesion and strengthens its internal friction angle. The general octahedral shear stress formula for judging whether the red sandstone fails under loading and different unloading rates was acquired on the basis of the octahedral shear stress characteristics of the red sandstone. In addition, a 3D empirical criterion for evaluating the red sandstone strength was constructed through the deviator plane function of Eekelen and the strength envelope curve on the meridian plane. A great initial confining pressure indicates serious internal damage of the red sandstone at failure under the identical unloading rate of confining pressure. A low unloading rate of confining pressure indicates internal serious damage of the red sandstone at failure when the initial confining pressure is the same. The analysis of macroscopic failure characteristics revealed that the stress path significantly influences the failure mechanism of the red sandstone. The results are instructive for determining the stability of underground engineering of the red sandstone.

Floor failure behavior and water disaster prevention system of ultra-wide opposite pulling working face mining on confined aquifer

The floor failure depth of the ultra-wide opposite pulling working face (OPWF) mining on the confined aquifer is larger, and the risk of water inrush is higher. Based on the engineering background of Binhu Coal Mine (BHCM) in North China Coalfield, this paper analyzes the distribution rule of floor support pressure in OPWF mining, carries out numerical simulation before and after roof cutting, and microseismic-electrical joint dynamic monitoring. The results show that the failure depth, vertical stress, maximum principal stress, maximum shear stress, and pore water pressure of floor after roof cutting are decreased by 28.6 %, 15.5 %, 12.1 %, 30.9 %, and 41.7 %, respectively, the stress concentration coefficient is also decreased by 19.5 %. Roof cutting blocks the propagation path of mining stress, reduces the mine pressure concentration and controls the floor failure depth. Using the optimal univariate function between the floor failure depth and mining width, mining depth, mining height, roof cutting height, working face stagger distance, a multi-factor prediction model for the floor failure depth of the OPWF mining is established, and the accuracy of the model is more than 90 %. Combined with the methods of exploration, treatment, and monitoring, the water disaster prevention technical guarantee system in the ultra-wide OPWF mining on the confined aquifer is constructed, which provides a new method for the prevention and control of floor water disasters during mining.

The influence of withdrawal-recharging pattern on the deformation characteristics of sand in confined aquifer

Land subsidence caused by excessive groundwater withdrawal has become a global hazard, which demands further researches and the potential measures to control. Using the FlowTrac Ⅱ consolidation test system, six compression tests were designed to investigate the stress state and stress paths of sand within confined aquifers under conditions of withdrawal and recharging. The deformation characteristics of aquifer sand were studied under different withdrawal-recharging patterns. During pumping and recharge processes, sand deformation responses were observed to lag behind changes in applied stress. The characteristics of this hysteresis effect on deformation were summarized. The alternating phenomenon of rebound and compression of sand deformation under the recharging process is analyzed. When the recharging effect was relatively small than withdrawing effect under the stable withdrawal-recharging pattern, the compression deformation was observed in the recharging process. The research results provide a rational explanation for the continuous compression deformation of the aquifer during groundwater level recovery and offer experimental evidence for the rational design of artificial groundwater recharge in engineering construction.

Characterization of energy-driven damage mechanism and gas seepage in coal under mining-induced stress conditions

Gas seepage and progressive failure of coal are common energy-driven mining phenomena. A comprehensive understanding of the energy-driving mechanism behind the catastrophic behavior of mining-induced coal is fundamental to innovating the technology of coal and gas co-mining. Thus, this study simulated three typical mining stress evolution process in protective coal-seam mining (PCM), top-coal caving mining (TCM), and non-pillar mining (NM) to investigate the energy evolution and distribution patterns of coal. The results indicate a strong correlation between energy dissipation and gas seepage. By transitioning from PCM and TCM to NM, the peak elastic strain energy of gas-bearing coal increased by 155.92 %, and the ratio of peak dissipative energy decreased from 51 % to 41 %. Under the PCM stress path, gas seepage decreased the energy storage by 13.52 %, whereas the pre-mining pressure relief and enhanced permeability simulation increased in peak dissipation energy by 49.66 %. Using the cumulative dissipative energy as a damage variable reveals that the degree of coal damage evolution under PCM is higher than other mining methods. Based on the energy-driven damage mechanism, a new coal permeability model was established, and its comparison with classical permeability model demonstrated its excellent fitting effectiveness.

Anisotropic behavior and mechanical characteristics of the Montney Formation

This study investigates the rock mechanics and anisotropic properties of the Montney Formation, Alberta, through two sets of experiments: unconfined compressive strength tests and triaxial compression tests, supplemented by ultrasonic wave velocity measurements. These tests enabled the calculation of dynamic and static stiffness properties and Thomsen anisotropy parameters (ε, δ, γ). Our findings reveal that the Montney Formation exhibits weak anisotropy, with ε values generally below 0.10, contrasting with other strongly anisotropic shale formations. Specimens exhibiting higher clay content and total organic carbon levels show more pronounced anisotropy. Static measurements exhibit a higher degree of anisotropy compared to dynamic measurements. The investigation also explored loading and unloading stiffness parameters, noting a higher E loading-to-unloading ratio for rock specimens with lower elastic properties. This provides important information on the rock's response to stress path effects during stimulation and exploitation. The study also discusses other rock mechanics parameters including Poisson's ratio, tensile strength, and brittleness. The research contributes to a more comprehensive understanding of the geomechanical and petrophysical behavior of the Montney Formation, aiding reservoir characterization and hydrocarbon exploration and production.

Damage evolution in layered rock masses of a mining floor under the influence of fluid–structure coupling

AbstractCurrent research on rock damage in mining floors primarily focuses on the seepage‐stress coupling effect, overlooking the fact that rock masses in coal measure strata are predominantly layered. To address this gap, cyclic loading and unloading triaxial tests were conducted. Additionally, theoretical analysis, mathematical statistics, and other methods were used to investigate the damage evolution law of layered rock masses in coal measures. This investigation was carried out under the coupled effects of a specific stress path, characterized by ‘stress concentration‐stress unloading‐stress recovery’, and a high confined water seepage field. The results show that the compression modulus increases with the increase in confining pressure and osmotic pressure, but its increasing trend gradually slows down. Within a certain range, increasing the confining pressure and osmotic pressure helps to close rock fractures and increase stiffness.

Incremental Viscoelastic Damage Contact Models for Asphalt Mixture Fracture Assessment

Asphalt mixtures are widely used as a surfacing material for pavements due to their several advantages. For this reason, robust numerical models still need to be developed to improve the understanding of their fracture behaviour. Recently, an incremental generalised Kelvin (GK) contact model that relates increments in contact displacements with increments in contact forces was proposed to assess the viscoelastic behaviour of asphalt mixtures within a discrete element method (DEM) framework. In this work, the contact model is extended to allow its application to asphalt mixture fracture studies. Two damage models—a brittle and a bilinear softening—coupled with the GK contact model are proposed to consider damage initiation and propagation. A parametric study is presented that assesses the impact of the GK-Damage parameters, showing a sensitivity to the loading velocity and the Maxwell elements, particularly its viscosity element, on the stress–strain response of a single contact. A reduced-size numerical mastic is initially used to speed up the calibration process of the GK-Damage contact parameters, with subsequent validation on a specimen with real experimental dimensions. It is shown that the proposed calibrated damage models can successfully reproduce the time-dependent behaviour, peak stress, and crack path observed in experimental results, highlighting the benefits of the adopted methodology. For the GK-Bilinear model, the fracture energy and maximum contact tensile stress are shown to adjust both the peak stress and softening response. Uniaxial tensile tests on asphalt mixtures indicate that the GK-Bilinear model provides a more realistic characterisation of fracture development. A higher susceptibility to damage at aggregate-to-mastic contacts compared to contacts within the mastic phase is identified.

Mechanical responses and fracturing behaviors of coal under complex normal and shear stresses, Part I: Experimental results

This work presents experimental tests based on coal collected from a coal mine based underground water reservoir (CMUWR). The mechanical responses of dry and water-soaked coal samples under the complex normal and shear stresses under multi-amplitude and variable frequency is investigated. The experimental results reveal the effects of stress path, water soaking and frequency on deformation, energy dissipation, secant modulus and shear failure surface roughness. The experimental results show that when normal and shear stresses are applied simultaneously, there is a significant competitive relationship between them. On the dominant side, the strain rate will be significantly increased. The sample under a loading frequency of 0.2 Hz exhibits a longer fatigue life. During the cyclic shear test, the shear strain of the water-soaked sample is higher than that of the dry samples. The average roughness coefficient of failure surface exhibits an increasing pattern with increase in shear strength, the elevated roughness of a shear surface is advantageous in constraining shear displacements of specimens, thereby lowering the energy dissipation. This study can provide theoretical and practical implications for a long-term safety evaluation of CMUWR.

Threshold strain for the accumulation of excess pore-water pressure in saturated coral sands under complex cyclic loading patterns

The liquefaction of coral sands caused by the accumulation of excess pore-water pressure is a major factor contributing to catastrophic events on coral reefs, and accurately estimating this excess pore-water pressure accumulation holds significant importance. High-quality laboratory test results are essential for analytical or numerical calculations. In this study, a new test method is employed to conduct a series of undrained, multi-staged, stress-controlled multidirectional hollow cylinder tests on saturated coral sand under complex loading conditions. The concept of threshold strain (γt) and the method for determining γt of saturated coral sand specimens under complex loading conditions are proposed. The test results demonstrate that γt of saturated coral sand remains insensitive to cyclic loading conditions (including frequency, stress path, and mode) but increases with increasing relative density. The range of volumetric threshold strain, degradation threshold strain, and flow threshold for saturated coral sand under different initial states and cyclic loading conditions are 0.0183%–0.0341%, 0.0242%–0.0454%, and 1.006%–1.614%, respectively. This research provides a novel approach for accurately determining input parameters required for resolving and implementing coupled models in numerical modeling.

Stress path-based compressive strength model of FRP-confined recycled aggregate concrete

The compressive strength model of fibre-reinforced polymer (FRP) confined recycled aggregate concrete (RAC) is important in practical engineering designs. The existing research has showed that the compressive strength of FRP-confined RAC can be predicted by the models for FRP confined natural aggregate concrete (NAC). However the accuracy of existing compressive strength models for FRP-confined NAC vary greatly because of the unclear understanding of the confinement mechanisms. Previous research has shown that the axial stress of confined concrete tends to be path-dependent. In this study, a confining stress path-based compressive strength model of FRP-confined RAC is proposed based on theoretical analyses of the confining stress path of FRP-confined RAC. In the proposed model, an effect index is determined to express the influence of confining stress path on the compressive strength. The relationship between the effect index and confinement ratio and that between lateral stress dominant index and confinement ratio are obtained. The proposed model is verified by comparison with previous experimental test results and those predicted by existing representative models. It is found that the compressive strength of FRP-confined RAC is path-dependent for small confinement ratios (less than 0.43) and path-independent for large confinement ratios (larger than 0.43).

Mechanical responses and fracturing behaviors of coal under complex normal and shear stresses, Part II: Numerical study using DEM

This work presents particle-based numerical simulations on coal pillars in a coal mine based underground water reservoir (CMUWR). We aim to replicate the stress–strain characteristics and present the acoustic emission behavior of the coal under complex dynamic stress paths. The study reveals failure characteristics of coal exposed to monotonic/cyclic shear load under constant/cyclic normal loads. Based on the evolution of stress-time-dependent bond diameter implemented in particle model, different damage paths are established for dry and water-immersed samples under two loading frequencies. Furthermore, the numerical Gutenberg–Richter’s b-value was calculated from the released energy emanating from bond failure, and this work presents the evolution of numerical Gutenberg–Richter’s b-value. The numerical simulation contributes to a micromechanical understanding of the failure mechanisms of coal under water-immersion and cyclic stress, providing valuable insights for strength prediction of CMUWR.

Dynamic response of a permafrost railway subgrade with 3D train-track-subgrade-ground model simulations

Studying train-induced response characteristics is essential for safely operating permafrost railway subgrades. A three-dimensional thermal-mechanical coupling nonlinear dynamic model of train-track-subgrade-ground relationships was established to analyse the train-induced dynamic stress, acceleration and stress path characteristics of a permafrost railway subgrade, and field monitoring data were used to verify this model. The differences between the 2D and 3D models are also discussed, along with the seasonal changes, train speed, axle load, and train type affecting permafrost subgrades. The main results are as follows. First, the vibration load significantly impacts the subgrade 6 m below the sleeper, producing distinct vertical dynamic stress waves due to the wheels and bogies. Dynamic compression stress dominates the subgrade and is influenced by the train structure, speed, and sleeper spacing. While the 2D model tends to underestimate the dynamic stress in shallower layers, it concurs with the 3D model in deeper subgrade dynamics within a 10% margin of error. Then, the principal stress axis of the subgrade soil rotates synchronously with train movements, exhibiting regular stress paths in the YZ plane (longitudinal section) with depth-dependent variations in the stress cycles and deviatoric stress. Finally, predominantly originating from sleeper-induced vibrations, the subgrade vibration acceleration varies with the train speed, sleeper spacing, and season and is most pronounced in the vertical direction. This study provides theoretical guidance for the vibration response of permafrost subgrades on the Qinghai-Tibet Railway (QTR).

Experimental investigation on mechanical behavior of natural soft clay by true triaxial tests

Laboratory tests on undisturbed and remolded Shanghai clay were conducted with a novel mixed boundary true triaxial apparatus. The strength and deformation behaviors of the structured soft clay at different Lode angles were carefully investigated. The relationship between the shear stress ratio and the dilatancy ratio of the clay in three different stress spaces, that is p–q space, TS stress space and tij modified stress space, are discussed in detail. In particular, the influences of the stress path and the structure of non–disturbed clay on the strength, deformation and failure criterion of Shanghai clay were quantitatively evaluated. The shear strength and dilatancy of undisturbed /remolded clay decreases with the increase of the Lode angle. It is also found that the shear strength and dilatancy of the undisturbed clay are larger than those of the remolded clay; while the influence of the structure on the dilatancy decreases with an increase in the Lode angle. In the π–plane, the maximum strength of Shanghai clay under different stress paths generally obeys the SMP failure criterion. The coincidence of the stress–dilatancy relationship given by the TS and tij stress spaces with the test results is much better than that given by p–q stress space. Furthermore, the linearity of the plastic potential curves of the undisturbed/remolded clay is more evident in tij stress space.

Elastoplastic constitutive model for overconsolidated clays with an advanced dilatancy relation

AbstractThe dilatancy behavior of overconsolidated (OC) clays is a key factor in determining their strength and deformation characteristics. Recognizing the limitations of previous dilatancy relations for OC clays, a novel dilatancy relation is proposed that can effectively capture the changes in dilatancy point, volume dilatancy and contraction with the overconsolidation ratio (OCR). As OC clays revert to the normally consolidated (NC) state, the proposed dilatancy relation smoothly transitions to that of the modified Cam‐clay (MCC) model, ensuring a unified description of the dilatancy relation between OC and NC clays. The dilatancy relation can be easily incorporated into the constitutive model of different theoretical frameworks. Subsequently, this advanced dilatancy relation is integrated into a new elastoplastic constitutive model for OC clays within the framework of bounding surface and generalized plasticity theory. The validity of the proposed model is confirmed through drained triaxial compression and extension, undrained triaxial compression and extension, as well as complex stress path tests for clays with various OCRs, and the simulation results of the proposed model are compared with those of the SANICLAY model. The comparative analysis demonstrate that the model performs well in simulating the behavior of OC clays.

Experimental investigation of borehole breakout formation in Gosford sandstone

This study investigates the in-situ borehole breakout formation using true-triaxial laboratory experiments on cubic Gosford sandstone. Four series of tests have been conducted under varying stress conditions following two approaches: PD method (loading onto specimens with pre-drilled boreholes) and PS method (loading onto intact specimens and then conducting drilling). Borehole camera observations show that a large amount of rock debris remains within the breakout zone under the PD conditions, while clear V-shaped notches are observed in the PS tests. The breakout geometries are extracted based on point cloud data obtained from 3D scanning. Both breakout width and normalised depth under both testing approaches exhibit positive correlations with the maximum horizontal stress while decreasing with minimum horizontal stress. The PS tests tend to yield wider and deeper breakouts compared to the PD tests. The difference in breakout width is generally consistent across various stress conditions at approximately 10° – 15°, whereas the variation in the normalised breakout depth increases with the severity of the borehole failure. The discrepancies are believed to be attributed to the excavation unloading induced strength-weakening and removal of rock debris by circulating fluid and drilling vibration. The outcomes of this study demonstrate that the stress path has critical impacts on the rock failure behaviours around openings, and the PS tests are suggested for investigating the borehole breakout phenomenon.

Effect of true triaxial principal stress unloading rate on strain energy density of sandstone

Deep rock are often in a true triaxial stress state. Studying the impacts of varying unloading speeds on their strain energy (SE) density is highly significant for predicting rock stability. Through true triaxial unloading principal stress experiments and true triaxial stress equilibrium unloading experiments on sandstone, this paper proposes a method to compute the SE density in a true triaxial compressive unloading principal stress test. This method aims to analyze the SE variation in rocks under the action of true triaxial unloading principal stresses. Acoustic emission is used to verify the correctness of the SE density calculation method in this paper. This study found that: (1) Unloading in one principal stress direction causes the SE density to rise in the other principal stress directions. This rise in SE, depending on its reversibility, can be categorized into elastic and dissipated SE. (2)When unloading principal stresses, the released elastic SE density in the unloading direction is influence by the stress path and rate. (3) The higher the unloading speed will leads to greater increases in the input SE density, elastic SE density, and dissipative SE density in the other principal stress directions. (4) The dissipated SE generated under true triaxial compression by unloading the principal stress is positively correlated with the damage to the rock; with an increase in unloading rate, there is a corresponding increase in the formation of cracks after unloading. (5) Utilizing the stress balance unloading test, we propose a calculation method for SE density in true triaxial unloading principal stress tests.

Contribution to the experimental and theoretical study of the behavior of soil-structure interfaces

Designing civil engineering works often requires taking into consideration the contact and interface conditions between the soil and structure. These may refer to the discontinuity surfaces between the soil and a structure such as foundations, geotechnical works, and others. Current research aims to improve the calculation methods for structures through a better understanding of the behavior of materials and interfaces. The present work falls into this context. It involves two parts. The main purpose of the first part is to present the results of the monotonic and cyclic soil-structure interface tests which were carried out using a modified direct shear box apparatus. The tests were conducted using a local material, namely the sand from the town of Messâad in the Wilaya of Djelfa (Algeria), in accordance with a constant normal stress path. In addition, a parametric study, dealing with the effect of density, interface roughness, and type of adhesive used on the behavior of the interfaces, was carried out as well. As for the second part, it primarily aims to validate the Modjoin interface model, which was developed at the Lille Mechanical Laboratory, based on the tests that were carried out in the first part. It is important to know that this model includes the main concepts used in soil modeling, such as isotropic hardening, characteristic state, critical state, softening, and others. Moreover, several validation tests are also presented in this work in order to describe the behavior of the soil-structure interface under monotonic loading to assess the performance of the model.

Effect of stress evolution in destressed borehole on gas desorption in deep coal seam

Hydraulic flushing is a crucial technology for enhancing deep coal seam gas extraction, while the gas desorption characteristic is a critical factor impacting efficient gas extraction. This paper first analyzed stress variation around borehole during flushing and subsequently conducted experimental research to explore the effect of stress evolution on gas desorption. The results indicated that stress variation had a relatively insignificant effect on gas desorption during the early stage. When stress variation peaked in coal samples, instability and damage occurred, leading to a surge in the gas desorption rate. The ultrasonic wave velocity and dynamic elastic modulus of coal under different stress paths exhibited significant changes. With an increase in desorption time, the coal dynamic elastic modulus decreased, and the damage coefficient gradually increased. When comparing gas desorption behavior during constant stress and stress relief, Pingdingshan Mine (PDS) and Jiaozishan Mine (JZS) coal samples exhibited desorption amounts 2.85 and 1.41 times higher after stress relief, respectively. Based on the influence of stress relief on gas desorption rate and structure, a theoretical matrix scale change model was deduced. The mechanism controlling the rate of gas desorption by stress relief was illustrated. These research results offer theoretical guidance for implementing stress relief techniques to optimize coal seam gas extraction in practical field applications.

A theoretical model and verification of soil column deformation under impact load based on the Duncan-Chang model

The dynamic compaction method has been widely adopted in foundation treatment to densify the soil fillers. However, for the complexity of the impact behavior and soil mechanical properties, the theoretical research of dynamic compaction lags behind its practice for complex soil properties and stress paths. This paper presents a theoretical model applied to describe soil column plastic deformation under impact load. The relationship among stress increment, strain increment, and plastic wave velocity was derived from the aspect of propagation characteristics of stress waves in soil first. Combined with the Duncan-Chang Model, a one-dimensional theoretical model was established then. A numerical model was developed further to check the performance of the model. It showed that the deformation at the end of the soil column was mushroom-shaped. Both the axial and lateral deformation increased with the impact velocity. While some particles located at the side of the soil column end may splash under repeated impact. The theoretical deformations of the soil column were consistent with the experimental results both in the direction of axial and lateral.

Residual stress distribution and relaxation in U-rib full penetration welds by ultrasonic impact treatment

Ultrasonic impact treatment (UIT) as an effective method for alleviating welding residual stresses (WRS) is used to improve the fatigue performance of welded structures. However, its application to single- and double-sided U-rib welds in orthotropic decks has been limited. To address this, an experimental and numerical investigation on WRS and UIT was carried out for different deck thicknesses. A 3D thermo-elastic-plastic approach was employed to model the temperature and stress fields by the ABAQUS software. Additionally, a dynamic explicit calculation method was developed to simulate the UIT process, and its accuracy was verified by comparing the actual and numerical grooves and WRS after UIT. Ultrasonic stress measurement was employed to determine the distribution and magnitude of WRS before and after UIT. The results demonstrate that most relative errors of residual stress numerical and test results under different stress paths after UIT are within 10 %, confirming the reliability and effectiveness of this method. For single-sided U-rib welds after UIT, the reduction in WRS at the weld seam ranges from approximately 25.6 % to 87.4 %, while for double-sided U-rib welds, the reduction varies from about 22.2 % to 65.5 %. The impact of UIT on longitudinal residual stresses is more significant compared to its effect on transverse residual stresses. However, UIT results in a relatively smaller reduction in WRS away from the weld seam. Furthermore, it is observed that altering the deck thickness results in different disparities in WRS outcomes after UIT.