Conventional Triaxial Compression Research Articles

Experimental study of creep Poisson ratio of hydraulic concrete under multi-age loading-unloading conditions

Elastic creep theory, which is widely used in concrete dam engineering, assumes that the elastic Poisson ratio and the creep Poisson ratio equal the effective creep Poisson ratio. However, differences in creep test conditions and stress state make this assumption controversial. To accurately clarify the Poisson effect of hydraulic concrete, uniaxial compression creep tests with four loading ages (3d, 14d, 28d, and 60d) and conventional triaxial compression creep tests with three loading ages (14d, 28d, and 60d) are conducted under unified test conditions. And high-precision creep data of transverse and longitudinal for multi-age loading and unloading are obtained. We then denoise the creep data by using a Savitzky-Golay filter. Furthermore, based on the denoised creep data, we calculate the Poisson ratio at each stage for uniaxial compression and conventional triaxial compression. Finally, we compare and analyze the Poisson ratio of each stage in different stress states. The results show that the creep tests have the same noise when conducted under the same test conditions, and the optimal denoising by the Savitzky-Golay filter is obtained with parameters W (window width) = 9 and N (polynomial order) = 6. Under different stress states, the elastic Poisson ratio, creep Poisson ratio and effective creep Poisson ratio are not equal to each other, and the elastic Poisson ratio is greater than the creep Poisson ratio. In addition, under the conventional triaxial stress state, the effective creep (creep recovery) Poisson ratio in each direction is different, which indicates that the creep (creep recovery) in each direction is dependent under multi-axial stress state.

Study on Energy Evolution and Damage Constitutive Model of Siltstone

In order to study the energy evolution characteristics and damage constitutive relationship of siltstone, the conventional triaxial compression tests of siltstone under different confining pressures are performed, and the evolution laws of input energy, elastic strain energy and dissipative energy of siltstone with axial strain and confining pressure are analyzed. According to the test results, the judgment criterion of the rock damage threshold is improved, and an improved three-shear energy yield criterion is proposed., The damage constitutive equation of siltstone is established based on the damage mechanics theory through the principle of minimum energy consumption and by considering the residual strength of rock, and lastly, the rationality of the model is verified by experimental data. The results reveal that (1) both the input energy and dissipative energy gradually increase with the increase of axial strain, and the elastic strain energy first increases and then decreases with the increase of axial strain, and reaches its maximum at the peak. (2) The input energy and dissipation energy increase exponentially with the increase of the confining pressure, and the elastic strain energy increases linearly with the increase of confining pressure. (3) According to the linear relationship between the sum of shear strain energy and hydrostatic pressure, an improved three-shear energy yield criterion is established. (4) The model curve can better describe the strain softening stage and the residual strength characteristics of siltstone. The relative standard deviation between the model results and the test results is only 4.35%, which verifies the rationality and feasibility of the statistical damage constitutive model that is established in this paper.

Experimental Study of Size Effects on the Deformation Strength and Failure Characteristics of Hard Rocks under True Triaxial Compression

Size effect has always been the focus of rock mechanics as a bridge between laboratory test and engineering site. Previously, the research conditions and objects of the rock size effect have mostly focused on cylindrical rock samples with different height-to-diameter ratios (H/Ds) under uniaxial or conventional triaxial compression, while there has been little research on the rock size effect under true triaxial compression (TTC), especially rectangular rock samples with different sizes and the same length-to-width-to-height ratio. Based on this, the deformation, strength, and failure characteristics of Beishan (BS) granite and Baihetan (BHT) basalt with different sample sizes under TTC were studied by a comparative analysis method. The size effect of deformation and failure characteristics under TTC are not obvious, including stress-strain curves, Young’s modulus, peak strains, failure angles, and macrofailure mode. However, the damage stress (σcd) and peak strength (σp) have obvious size effect; that is, the smaller the sample size is, the higher the strength is. Additionally, the relationship among the peak strength, sample size, and intermediate principal stress (σ2) is power function. In addition, by comparing the peak strength increment caused by the sample size of the two types of rocks, the σp of the fine-grained BHT basalt is more sensitive to sample size than that of the coarse-grained BS granite. Finally, by analyzing the relationship between the size of the mineral grains or clusters in the two types of hard rocks and the complexity of crack propagation in the fracture surface under TTC, it is suggested that the minimum side length of rock samples should not be less than 10 times the maximum mineral clusters (such as feldspar phenocrysts in BHT basalt). In addition, the method of estimating elastic strain is established by analyzing the relationship between the size of the rock sample σ2 and the elastic strain under TTC.

Friction-damage coupled models and macroscopic strength criteria for ice-saturated frozen silt with crack asperity variation by a micromechanical approach

This paper aims to present physical friction-damage coupled models and derive macroscopic nonlinear strength criteria for ice-saturated frozen silt. Two material scales are considered here. The material at the mesoscale is composed of a frozen silt matrix and embedded mesocracks. At the microscale, the frozen silt matrix is composed of elastic mineral grains and elastic ice crystals. Considering this microstructure, for the ice-saturated frozen silt, the plastic deformation is related to frictional sliding along the mesocracks, and damage is due to the propagation of mesocracks. We obtain the effective elastic properties with a two-step linear Mori-Tanaka homogenization procedure. For the second homogenization step, we deduce the macroscopic stress-strain relations and local thermodynamic forces associated with damage and plasticity. Then, an energy release-based damage criterion is developed to capture the evolution of damage, and a friction sliding criterion with an associated/nonassociated local plastic flow rule is introduced to describe the rate of plasticity. The friction sliding criterion considers the variation of surface asperities of mesocracks as a result of pressure melting and damage evolution. In this context, associated/nonassociated multiscale friction-damage models are established. Furthermore, associated/nonassociated macroscopic nonlinear strength criteria as inherent parts of the corresponding multiscale models are derived under a large range of compressive stresses. For application, a semi-implicit local numerical algorithm of the multiscale models is developed. The accuracy of associated/nonassociated multiscale models is assessed by comparing their numerical simulations with conventional triaxial compression (CTC) tests. Although the associated multiscale model correctly predicts the nonlinear strength behavior and axial deformation of the Lanzhou frozen silt under CTC tests with different temperatures, it fails to quantitatively reproduce volumetric deformation. Nonassociated multiscale model predictions with experimental data of volumetric deformation are significantly improved compared to the associated multiscale model.

Constraints of sedimentary structures on key parameters of shale gas reservoirs: A case study of the Wufeng and Longmaxi formations of the Ordovician‐Silurian transition in Luzhou area, Sichuan Basin, southwestern China

There are various sedimentary structures developed in the black shales of the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation in Luzhou area, southern Sichuan Basin, southwestern China. In order to evaluate the controlling effects of sedimentary structures on the key parameters of shale reservoir properties, collected classification of sedimentary structures, scanning electron microscope observation, analysis of shale fabric and porosity characteristics, low‐pressure nitrogen adsorption, methane isothermal adsorption, and conventional triaxial compression test were carried out based on shale samples from Wells A‐H1, B‐H1, C‐H1 and D‐H1 in the study area. The sedimentary structures recorded in these shales are categorized into five types: homogeneous bedding, weak horizontal bedding, strong horizontal bedding, discontinuous ripple bedding, and gravel‐bearing massive bedding. In general, the palaeo‐hydrodynamics and oxygen content of water columns were weak when homogeneous bedding shale was deposited. The organic matter is the most abundant and the total organic carbon content is up to 4.01% on average. The average quartz content is 43.5%, and the brittleness index is the highest. The porosity and adsorption capacity were the highest, with an average of 4.57% and 79.5 scf/ton, respectively. Combined with its mechanical properties of low Poisson's ratio and high Young's modulus, homogeneous bedding sedimentary structure is associated with the shale units possessing the highest organic matter abundance, the strongest reservoir property, and the best fracability, which is the priority target for exploration and development of the Wufeng to lower Longmaxi Formation in the Sichuan Basin, southwestern China.

Study on the Mechanical Behavior and Acoustic Emission Properties of Granite under Triaxial Compression

To study the rock mechanical behaviors and damage process mechanism of granite samples under triaxial stress, conventional triaxial compression tests were carried out on an RMT-150B rock mechanics testing machine and acoustic emission detector. The test results show that the strength of the granite sample has a good linear relationship with the confining pressure, the cohesion force c of the granite samples is 29.37 MPa, and the internal friction angle is 54.23° by calculation based on the Mohr-Coulomb strength criterion. The larger the initial confining pressure of the rock sample is, the larger the crack initiation stress ( σ ci ) and dilatancy stress ( σ cd ) of the granite specimen are, the larger the energy values at the crack initiation point and dilatancy point are, and the larger the peak energy storage and energy release rate at the failure are. In the case of a small initial confining pressure, the AE ringdowning counts and the cumulative AE ringing counts increase to their maximum instantaneously at the peak stress point, and the damage of the sample develops rapidly. While the initial confining pressure is high, the AE ringing counts and the cumulative AE ringing counts of the granite specimens increase evenly, and the deformation damage of the granite specimens is slow. Before the crack initiation point, AE signals are mainly low-energy and low-frequency friction-type AE events, while after the dilatation point, AE signals of samples are mainly high-frequency and high-energy fracture-type AE events. The failure mode of granite samples judged by acoustic emission parameters according to the distribution of characteristic values of AE parameters RA and AF is consistent with the reality. The AE b value of the granite sample is large when the confining pressure is low, and there will be a sudden drop, the decrease time is late, and the decrease rate is large. Under the same stress level, the larger the confining pressure is, the larger the damage variable D is.

CONSTITUTIVE RELATIONS OF GRANULAR MATERIALS BY INTEGRATING MICROMECHANICAL KNOWLEDGE WITH DEEP LEARNING

Constitutive relations of granular materials are of great significance to many fields, such as geotechnical engineering. Different from traditional phenomenological constitutive theory, this study explores a micromechanics-informed data-driven constitutive modelling approach for granular materials via machine learning models. On the basis of Vogit’s homogenization assumption, an analytical small-strain stress-strain relation is established. This relation uniquely determines a group of micromechanical fabric variables associated with the constitutive behavior of granular materials. These recognized variables, together with principal strain and stress sequence pairs reflecting macroscopic properties of granular materials, are obtained via a series of discrete element models of triaxial compression tests. Considering the fact that these microscopic fabric tensors are internal variables, which cannot be directly used as inputs of a material constitutive model, a directed graph is introduced to incorporate microstructural information implicitly in the prediction of stress-strain responses. The gated recurrent unit (GRU) based recurrent neural networks are used as basic deep learning models to describe the mapping relation between nodes in the designed directed graph. In this study, the entire stress-strain prediction model can be assembled with two neural networks that are trained separately, after unfolding the directed graph from the target node to the source node. By testing the trained deep learning model based on brand new datasets, the results demonstrate that the proposed training approach can satisfactorily capture the multi-directional stress-strain responses with reversal loadings, such as conventional triaxial compression with unloading-reloading cycles, true-triaxial compression with constant intermediate principal stress (constant-b), and constant mean effective effective stress (constant-p) conditions with unloading-reloading cycles. The prediction results also show that the trained model possesses satisfactory interpolation and extrapolation capability. Considering the excellent ability of deep learning in terms of capturing the mechanical responses of granular materials and the unique features of open learning when new data is available, integrating a data-driven paradigm with theoretical constitutive models may be one of the important directions for constitutive research of granular materials.

Experimental Investigation of Early-Warning Critical Energy for Strainbursts

This research aimed to establish an early-warning critical energy for coal instability based on the energy theory and acoustic emission characteristics of coal under triaxial compression. To obtain an early-warning critical strain energy indicating the increase in the risk of coal instability, conventional triaxial compression and acoustic emission (AE) tests were carried out on coal specimens taken from a 980-m-deep mine with initial confining pressures of 10, 15, 20, 25, 30 and 35 MPa. Stress–strain relations, AE features, and energy evolution characteristics during triaxial compression were analyzed. It was found that the energy evolution and AE event count changes across different loading stages. With increasing axial stress, most of the input energy stored in the coal specimens was in the form of elastic strain energy and the AE event count was close to zero, indicating that the coal grains reach a state of balance. After the elastic deformation stage, a portion of the input energy was consumed by inelastic deformation. Once the stress level exceeded the volumetric compressibility–dilatancy transition stress, the AE event entered a period of relative quiet, and the rate of energy dissipation abruptly accelerated, indicating that the coal grains achieved another state of balance before THE instability or failure. The balance of the rock grains is broken again (AE event count and the rate of energy dissipation both increased dramatically), coal achieved the peak strength and instability soon. The point at which the dissipated energy ratio α increased rapidly or the starting point of a quiet period, indicates an increase in the risk of coal instability. The corresponding elastic strain energy accumulated within the coal can be regarded as a precursor to instability or strainburst. Accordingly, a fitting formula is presented to predict the early-warning critical energy for brittle coal subject to different minimum principal stress. The analysis results in this paper can be helpful in the assessment of coal instability risk.

Development and Application of a New Triaxial Testing System for Subzero Rocks

Abstract To face the challenges of coal production today, the mechanical behaviors of rock in subzero and disturbed environments are critical in the geomechanics of coal mining and artificial freezing engineering. Disturbances significantly affect the long-term mechanical properties and nonlinear viscoelastic-plastic deformation characteristics of frozen soft rock. This article summarizes current rock triaxial test systems, which cannot concurrently solve the problems of subzero temperatures, creep, disturbance, and high sampling rates. Based on existing equipment limitations and the current research aims, an innovative, applicable, and scalable measurement/control system based on National Instruments hardware is integrated into LabVIEW to design and construct a new triaxial testing system capable of achieving both dynamic disturbances and subzero temperatures. The testing system can produce a basic environment with confining pressures up to 30 MPa, axial forces up to 500 kN, and lowest temperatures of −30°C. The system has a 51.2 kS/s sampling using 5-channel closed-loop control. By changing the transfer valve between the supercharger and the servo valve, both conventional triaxial compression and creep tests can be carried out. The triaxial creep tests are performed on frozen rock specimens under step loading and graded disturbances to analyze the creep deformation characteristics and failure modes of the specimens. The results show that at low stress levels, the disturbance load closes and reorganizes microcracks in the rock specimens, reducing the specimen deformation rate. With increasing stress levels or disturbance intensities, the specimen cracks expand rapidly, the creep rate increases, the disturbance energy stored in the rock specimen releases, and the specimens rapidly fail. Therefore, this disturbance creep phenomenon must be considered, and timely support should be provided for exposed frozen rock wall to guarantee construction safety. Moreover, the test results also verify that the design of the new testing system is reliable, feasible, and useful.

Compaction of the Groningen Gas Reservoir Sandstone: Discrete Element Modeling Using Microphysically Based Grain‐Scale Interaction Laws

AbstractReservoir compaction, surface subsidence, and induced seismicity are often associated with prolonged hydrocarbon production. Recent experiments conducted on the Groningen gas field's Slochteren sandstone reservoir rock, at in‐situ conditions, have shown that compaction involves both poroelastic strain and time independent, permanent strain, caused by consolidation and shear of clay films coating the sandstone grains, with grain failure occurring at higher stresses. To model compaction of the reservoir in space and time, numerical approaches, such as the Discrete Element Method (DEM), populated with realistic grain‐scale mechanisms are needed. We developed a new particle‐interaction law (contact model) for classic DEM to explicitly account for the experimentally observed mechanisms of nonlinear elasticity, intergranular clay film deformation, and grain breakage. It was calibrated against both hydrostatic and conventional triaxial compression experiments and validated against an independent set of pore pressure depletion experiments conducted under uniaxial strain conditions, using a range of sample porosities, grain size distributions, and clay contents. The model obtained was used to predict compaction of the Groningen reservoir. These results were compared with field measurements of in‐situ compaction and matched favorably, within field measurement uncertainties. The new model allows systematic investigation of the effects of mineralogy, microstructure, boundary conditions, and loading path on compaction behavior of the reservoir. It also offers a means of generating a data bank suitable for developing generalized constitutive models and for predicting reservoir response to different scenarios of gas extraction, or of fluid injection for stabilization or storage purposes.

Experimental Investigation on Anisotropic Strength and Deformation Behaviors of Columnar Jointed Rock Mass in Confined State

Columnar jointed rock mass (CJRM) has a special joint structure formed during the cooling of lava. The self-oriented joint structure can induce mechanical anisotropy of the rock mass, which poses a challenge for geotechnical engineering. To understand the mechanical properties of CJRM, conventional triaxial compression tests were conducted on the artificial CJRM specimens. It can be found from the experiment results that the orientation of columns has a distinct impact on the strength and deformation modulus. Four failure modes of CJRM specimens at different dip angles of columns under different confining pressures can be observed. The anisotropy degree in strength and deformation modulus is estimated, where it is found that the increase of confining pressure decreases the anisotropy in strength and increases the anisotropy in deformation modulus.

Study on the Permeability Evolution Model of Mining-Disturbed Coal

Coal permeability plays an important role in the simultaneous exploitation of coal and coal-bed methane (CBM). The stress of mining-disturbed coal changes significantly during coal mining activities, causing damage and destruction of the coal mass, ultimately resulting in a sharp increase in permeability. Conventional triaxial compression and permeability tests were conducted on a triaxial creep-seepage-adsorption and desorption experimental device to investigate the permeability evolution of mining-disturbed coal. The permeability evolution models considering the influence of the stress state and stress path on the fracture propagation characteristics were established based on the permeability difference in the deformation stages of the coal mass. The stress-strain curve of the coal was divided into an elastic stage, yield stage, and plastic flow stage. As the axial stress increased, the permeability decreased and then increased, and the curve’s inflection point corresponded to the yield point. The permeability models exhibited a good agreement with the experimental data and accurately reflected the overall trends of the test results. The results of this study provide a theoretical basis for coal mine disaster prevention and the simultaneous exploitation of coal and CBM.

Statistical Damage Constitutive Model for High-Strength Concrete Based on Dissipation Energy Density

To study the energy evolution law and damage constitutive behavior of high-strength concrete based on the conventional triaxial compression tests of C60 and C70 high-strength concrete subjected to five different confining pressures, the failure characteristics of high-strength concrete are analyzed at different confining pressures, and the evolution of the input energy density, elastic strain energy density, and dissipation energy density with axial strain and confining pressure are quantified. Combined with a continuous damage theory and non-equilibrium statistical method, the ratio of dissipation energy density of concrete to dissipation energy density corresponding to peak stress is used as the mechanical parameter. Assuming that the mechanical parameter obeys the Weibull distribution laws, the statistical damage variable describing the damage characteristics of concrete were derived. According to the Lemaitre strain equivalent principle, the damage variable is introduced to the generalized Hooke law to establish the statistical damage constitutive model for high-strength concrete. The results show that: (1) the input energy density and dissipation energy density increases with the increase of axial strain, while the elastic strain energy density increases first and then decreases as a function of the axial strain and reaches the maximum value at the peak stress; (2) the input, elastic strain, and dissipated energy densities corresponding to the peak stress of the two high-strength concretes all increase as a function of confining pressure, and the elastic strain energy density corresponding to the peak stress increases linearly as a function of the confining pressure; (3) the statistical damage constitutive model results of C60 and C70 high-strength concrete are in good agreement with the test results, and the average relative standard deviations are only 3.64% and 3.99%. These outcomes verify the rationality and accuracy of the model.

Determination of Shear Strength Parameters by Multistage Triaxial Tests in the Long-Term Analysis of Slopes

Accurate determination of soil parameters is of great importance in the analysis of landslides, which is a common type of natural disaster. In the solution of landslide problems, the direct shear box test is generally preferred as a laboratory test type for the determination of the shear strength parameters of soils. However, the need for drained parameters in long-term problems reveals a disadvantageous aspect of this experiment, especially in clayey soils, because pore water pressures cannot be measured in the direct shear box test, which is widely used in the laboratory to determine the shear strength of the soils.Multistage triaxial compression tests used to obtain shear strength parameters in the laboratory are advantageous compared to conventional triaxial compression tests because of the time and financial concerns, as well as the avoidance of differences between the samples to be tested. In this study, a new method was tried in addition to the methods used to obtain shear strength parameters with consolidated-drained multistage tests in normally consolidated clayey soils and the results were promising. Thus, since it increases the usability of multistage tests, it will be helpful in terms of preventing possible disasters by enabling analysis in many more sites.

Energy Evolution Law of Marble Failure Process Under Different Confining Pressures Based on Particle Discrete Element Method

The energy dissipation usually occurs during rock failure, which can demonstrate the meso failure process of rock in a relatively accurate way. Based on the results of conventional triaxial compression experiments on the Jinping marble, a numerical biaxial compression model was established by PFC2D to observe the development of the micro-cracks and energy evolution during the test, and then the laws of crack propagation, energy dissipation and damage evolution were analyzed. The numerical simulation results indicate that both the crack number and the total energy dissipated during the loading process increase with confining pressures, which is basically consistent with the experiment results. Two damage variables were presented in terms of the density from other researchers’ results and energy dissipation from numerical simulation, respectively. The energy-based damage variable varies with axial strain in the shape of “S,” and approaches one more closely than that based on density at the final failure period. The research in the rock failure from the perspective of energy may further understand the mechanical behavior of rocks.

Study on Mechanical and Permeability Characteristics of Containing Gas Coal-Rock Under Conventional Triaxial Compression

Study on Mechanical and Permeability Characteristics of Containing Gas Coal-Rock Under Conventional Triaxial Compression

Experimental Study on the Mechanical Properties and Seepage Characteristics of Red Sandstone with a Single Persistent Joint under Triaxial Compression

In this research, the conventional triaxial compression experiments for intact red sandstone specimens and the specimens with a single persistent joint at different inclination angles, i.e., 0°, 30°, 45°, and 90°, were conducted at first. Based on the results of the conventional tests, the effects of the confining pressure and the joint inclination angle on the mechanical properties including deformation behavior and strength parameters were summarized and analyzed, respectively. We find that the strength and deformation of jointed red sandstone are enlarged due to the increment of confining pressure, and the mechanical parameters of specimens show a U-shaped development with the rise of the joint angle. Besides, to investigate the effects of the pore pressure on seepage characteristics of rocks with joint angles at 0°, 45°, and 90°, a series of triaxial compression drainage tests on the jointed red sandstone were performed. The results show that the pore pressure has a weakening effect on the strength of jointed specimens, which can reduce the strengthening effect induced by confining pressure. Meanwhile, the tested specimens mostly present shear failure modes. As a result, the mechanical responses, seepage characteristics, and cracking modes in red sandstone containing a single persistent joint under triaxial compression are revealed.

A new nonlinear empirical strength criterion for rocks under conventional triaxial compression

A new nonlinear empirical strength criterion for rocks under conventional triaxial compression

Influence of Water Saturation and Real-Time Testing Temperature on Mechanical Behavior of Sandstone Under Conventional Triaxial Compression

Influence of Water Saturation and Real-Time Testing Temperature on Mechanical Behavior of Sandstone Under Conventional Triaxial Compression

Experimental investigation on the mechanical behavior of hybrid steel-polypropylene fiber reinforced concrete under conventional triaxial cyclic compression

Hybrid fiber reinforcing technology has gained a wide recognition in concrete built infrastructure. The cyclic stress–strain relation of hybrid fiber reinforced concrete (HFRC) plays a crucial role in the elastoplastic analysis of HFRC that further determines the structural damage progression and hysteretic behaviors. This paper investigates the mechanical behavior of HFRC subjected to conventional triaxial cyclic compression. A total of 48 HFRC cylindrical specimens are tested for different fiber volume fraction, fiber aspect ratio and confining pressure. The acoustic emission behavior of HFRC is analyzed with an elaboration of failure pattern transition as well as the mechanism analysis of fiber effects, and the macroscopic mechanical properties evolutions in terms of triaxial compressive strength, plastic behavior and stiffness deterioration were also investigated. It is clearly indicated from the test results that the incorporation of hybrid fibers together with the applying confinement can significantly enhance the mechanical property of concrete with the failure pattern transformed from a tensile failure to a shear failure mode. An increment in the strength of HFRC is observed as the steel fiber dosage increases. The polypropylene fiber contributes largely on improving the plastic behavior of HFRC at an expense of a slight decrease in the compressive strength. It is noted that the effect of confinement outweigh the fiber inclusion in restriction of damage degradation whilst the fiber effect is more pronounced as the confinement is low. Thereafter, the stiffness deterioration in terms of Young’s modulus, shear modulus, bulk modulus and Poisson’s ratio are investigated. The results show that the Young’s modulus and shear modulus of HFRC monotonously reduce during the damage process, whilst the Poisson’s ratio would monotonously increase and the bulk modulus exhibits a hyperbolic evolution form. Applying confinement and adding fibers can restrain the deterioration process of the stiffness, and the influences of fibers on stiffness deterioration are more significant under a low confinement level than that under a high level.