Conventional Triaxial Tests Research Articles

Experimental Study on Deformation and Strength Characteristics of Interbedded Sandstone with Different Interlayer Thickness under Uniaxial and Triaxial Compression

In order to study the effect of interlayer thickness on the deformation and strength characteristics of interbedded sandstone under load, uniaxial compression and conventional triaxial compression tests were carried out on four interbedded medium-grained sandstone samples with different interlayer thickness, accounting for different proportions of the total height of the sample (5%, 10%, 15%, and 20%). The samples were from the Shendong Mining Area and investigated using an RMT-150C rock mechanics test system. Then the stress–strain curve characteristics, strength characteristics, and failure characteristics of the interbedded medium-grained sandstone with different interlayer thickness were analyzed, and the applicability of Mohr–Coulomb strength criterion and Rocker criterion to interbedded medium-grained sandstone with different interlayer thickness was evaluated. The research results show that, as the thickness of the interlayer gradually increases, the uniaxial compressive strength of the sandstone containing the interlayer first decreases and then increases; as the thickness of the interlayer increases, the elastic modulus of the sandstone containing the interlayer gradually decreases, and the peak strain increases gradually; the thickness of the interlayer has little effect on the triaxial compressive strength of the sandstone containing the interlayer. When the thickness of the interlayer is small, the deviation of the fitted uniaxial compressive strength of the Rocker strength criterion is relatively small, and the applicability is good; the prediction result of the uniaxial compressive strength of the Mohr–Coulomb strength criterion is better; for the tensile strength of the interlayer rock, the Rocker strength criterion has good applicability. The existence of the interlayer hinders the development of medium-grained sandstone cracks, but this effect is weakened with the increasing thickness of the interlayer. The research results are helpful to further reveal the movement law of overlying rocks in Shendong mining area, and provide basic parameters for related research.

A Continuous Test Method for Dynamic Change of Real Mesostructures of Soil

The regularity of dynamic change of real mesostructures of soil is the foundation of the macro- and mesointegrated research. Mesoscale research about continuous image acquisition in real soils, identifying and quantifying mesostructures, is the subject of current interest. Soil mesostructure measurement method is the basis of modeling, and the development of rapid, direct, and lower-cost methods has great practical value for engineering practice. Combined with conventional triaxial loading test equipment, optical measurement, mesoscopic imaging tracking system, and digital image technology, a new method for measuring the dynamic change of the soil mesostructure is designed. This integrated approach to observation and acquisition is realized by direct observation and continuous image capture for dynamic change of real mesostructures of soil. The methods are used to analyze the high compressibility of a certain clay, shear zone development of cement soil, and erosive effects of ecological soil in order to investigate the continuous variation of mesostructures during the shear test. The aim of this paper is to provide a simple, straightforward, continuous, and low-cost test method for revealing the characteristics of consecutive change of real mesostructures of soil in the conventional triaxial test.

Calibrating and validating a soil constitutive model through conventional triaxial tests: an in-depth study on CSUH model

Calibrating and validating a soil constitutive model through conventional triaxial tests: an in-depth study on CSUH model

A Macro–Micro Damage Model for Rock under Compression Loading

Rock damage caused by its microcrack growth has a great influence on the deformation and strength properties of rock under compressive loading. Considering the interaction of wing cracks and the additional stress caused by rock bridge damage, a new calculation model for the mode-I stress intensity factor at wing crack tip was proposed in this study. The proposed calculation model for the stress intensity factor can not only accurately predict the cracking angle of wing crack, but can also simulate the whole range of variation of wing crack length from being extremely short to very long. Based on the modified stress intensity factor, a macro–micro damage model for rock materials was also established by combining the relationship between microcrack growth and macroscopic strain. The proposed damage model was verified with the results from the conventional triaxial compression test of sandstone sample. The results show that the proposed damage model can not only continuously simulate the stress-strain curves under different confining pressures, but also can better predict the peak strength. Furthermore, the sensitivities of initial crack size, crack friction coefficient, fracture toughness, initial damage and parameter m on the stress-strain relationship are discussed. The results can provide a theoretical reference for understanding the effect of microcrack growth on the progressive failure of rock under the compressive loading.

Evolution of physical and mechanical properties of cementing materials during underground energy exploitation and storage

The sealing problem of cement sheath often appears in gas wells for the increasing depth of underground energy exploitation and storage, especially when horizontal multi-stage fracturing technology is used in the shale gas industry. To investigate the effect of high temperature and high pressure (HTHP) on the evolution of the physical and mechanical properties of cement stone, conventional triaxial compression tests of cement stone at 5 different temperatures from 25 °C to 135 °C, with confining pressure varying from 5 to 45 MPa were tested. It was found that with increasing confining pressure and temperature, the mechanical properties of cement stone change from strain softening to ideal plasticity, finally, become strain hardening. The specimens show irregular longitudinal splitting under low confining pressure and room temperature. When the confining stress rises to 30 MPa and the temperature rises to 95 °C, expansive destruction occurs in the radial direction. In addition to the above, the failure mode shows single a shear plane or multiple splitting planes. Peak strength of cement stone is proportional to confining pressure and is inversely related to temperature. There is a negative correlation between elastic modulus and temperature. The effect of temperature on cohesion and friction angle is complex, and with the increasing temperature, the cohesion continuously decreases. The friction angle increases until the temperature reaches 60 °C, but then it starts to drop dramatically. The results of the physical and mechanical properties of cement stone guide the cementing design and construction of horizontal wells and the safety evaluation during underground energy exploitation and storage.

Rock Damage Constitutive Model Based on the Modified Logistic Equation under Freeze–Thaw and Load Conditions

Abstract The stress–strain curves of red sandstone are obtained by performing conventional triaxial compression tests after freeze–thaw cycling. The loss rates of red sandstone under the action of ...

An equivalent-additional-stress-based material point method for the deformation of reinforced soil slopes under supergravity

For a fiber-reinforced soil slope, the constitutive law of the soil-fiber composite is complex, and the failure mechanism for the reinforced slope is not well understood. In the traditional finite-element-method simulations of slope collapse, a large deformation causes problems related to mesh distortion. As a basic calculation framework, the material point method may provide a solution to this problem. Combining the material point method with the concept of the equivalent additional stress expressed in tensor form, an explicit calculation method is developed on the base of the classical iterative method. The conventional triaxial compression tests of the fiber-reinforced soil were simulated using the method. The simulation results were basically consistent with the test results. Numerical simulations of the deformation of fiber-reinforced soil slopes under supergravity using the method were performed. They were compared with laboratory results from centrifugal tests. The main features of the collapse of the reinforced soil slope under supergravity were captured. Modelling the geo-fiber interaction with the equivalent additional stress method proves that the material point method helps in simulating the collapse of reinforced soil slopes.

Investigation on the relationship between CT numbers and marble failure under different confining pressures

Abstract Computed tomography (CT) scanning technology is helpful in investigating rock materials as it can demonstrate the micro structure of rock clearly. Conventional triaxial compression tests and the corresponding graded triaxial loading tests were carried out to investigate the complex failure mechanism of the marble at the Jinping Hydropower Station. After that CT-scanning tests were done on the loaded marble specimens. The test results show that (1) the CT numbers of the specimens have a certain statistical regularity, that is, the CT numbers of the specimens under different confining pressures satisfy the Weibull distribution, as the confining pressure increases, the mean values rise while variances decrease; (2) in the two groups of tests, the average CT numbers corresponding to the conventional triaxial tests are higher than those corresponding to the graded loading tests, but the CT number variances are lower than those of the graded loading tests; and (3) according to meso-damage mechanics, the damage variables of the rock specimens were established based on the definition of CT numbers. The calculation results show that the damage variables decrease with the increase in confining pressure, the damage variables of the rock specimens in the graded loading tests are higher than those in the conventional triaxial test, and the differences between the two loading tests have grown with the increase in confining pressure.

Applicability of single sample method to test strength parameters of sandstone

The single sample method allows the mechanical parameters of rocks to be obtained with very few rock samples; however, the method has not been widely used. This is mainly because the yield point of the single sample method is more difficult to control than the conventional triaxial compressive test and the effect of the different control methods on the measured data is not well understood. The single sample method obtains the strength parameters of the rock by loading a single rock sample with multiple stages of confining pressure. Multistage loading tests are divided into peak strength control and long-term strength control according to yield point control. In this study, multistage loading tests of sandstone were carried out to obtain strength parameters using long-term strength control. The results show that sandstones undergo seriously brittle damage in conventional triaxial compressive tests. Although the sandstones have been rigorously selected, they still vary considerably, and long-term strength points are more difficult to control. The error of strength parameters of sandstone obtained using the single sample method may exceed 20% compared to those obtained by conventional triaxial compressive tests. So this method must be used with caution for sandstones.

Study on Damage Constitutive Model of High-Concentration Cemented Backfill in Coal Mine

In order to construct the damage constitutive model (DCM) of high-concentration cemented backfill (HCCB) in coal mine, the generalized Hoek-Brown strength criterion was used as the failure criterion. For the difference of theoretical derivation of constitutive relation, a new DCM based on residual strength was proposed. Combined with the conventional triaxial compression test, the correctness and rationality of the DCM were verified. The damage evolution characteristics of HCCB were analyzed, and the physical meaning of model parameters was clarified. The results show that (a) the theoretical curves of stress-strain relation are in good agreement with its experimental curves, which means DCM can simulate the deformation and failure process of HCCB. (b) The damage evolution curve of HCCB is S -shaped. To some extent, the confining pressure can inhibit the development of damage. (c) The parameter F 0 reflects the position of the peak point of the DCM, and parameter n is the slope of the straight line segment in the postpeak strain softening stage, which are, respectively, used to characterize the strength level and brittleness of HCCB. The establishment of DCM of HCCB is helpful to reveal its deformation and failure mechanism and provides theoretical basis for its strength design.

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.

Mechanical properties and stability analysis of surrounding rock of underground cavern under various stress loading paths

Under the influence of geological tectonic movement and human engineering activities, the geological environment of rock mass is often very complex, and its stress state is also complex and constantly changing. During the excavation of underground caverns, the stress of surrounding rock will be redistributed, and its radial stress and tangential stress are constantly changing and adjusting. From this point of view, the excavation of underground chamber causes strong stress differentiation in surrounding rock, and the experimental study under simple stress path cannot fully reflect the actual state of rock engineering. In order to comprehensively understand the damage mechanism of rock under complex stress paths, this paper takes the granite of an underground powerhouse of a power plant as the research object, designs and carries out a variety of laboratory tests such as conventional triaxial test, unloading test and creep test, and obtains the deformation and failure characteristics of granite under different stress loading paths. Based on the laboratory results, it puts forward specific surrounding rock support recommendations. This study can provide more targeted supporting measures for surrounding rock under different stress states, which is of practical significance to maintain stability of surrounding rock in the factory area.

TRIPLE-SHEAR ELASTO-PLASTIC CONSTITUTIVE MODELS OF UNSATURATED CLAYS IN NORMAL CONSOLIDATION

In the framework of the critical state soil mechanics and adopting the triple-shear unified strength criteria with the methods of the single-stress variable and the double-stress variable for unsaturated soils, the yield stress and the failure stress ratio in the yield function of saturated soils are replaced respectively with the yield stress that can reflect the influence on soil saturation and the triple-shear failure stress ratio. Four triple-shear elastoplastic models of unsaturated clays are established with the methods of the equivalent substitution and the coordinate translation. These models can reflect the soil strength interval effects, the tension and compression strength differences and the cohesions for unsaturated soils under full stress states. The Nanchang unsaturated clays are taken as an example to determine the parameters of the four models, and to verify the models with the conventional triaxial consolidation drainage tests (CD tests) and true triaxial numerical simulations. The CD tests show that all the four models can well reflect the mechanical characteristics of the unsaturated clays. The models of the double-stress variable gave better approximations to the tests than that of the single-stress variable. The models with the equivalent substitution method fit better with the tests than that with the relative coordinate translation method. The true triaxial numerical simulation shows that the shear stresses and the bulk strains obtained by the double-stress variable method are larger than those obtained by the single-stress variable method. With the increase of the intermediate principal stress influence coefficient b, the shear stresses and the bulk strains from the four models will also increase. If other conditions remain unchanged, the shear stress increases with the increase of matrix suctions.

Instability of Compacted Residual Soil

Static liquefaction of loose sands has been observed to initiate at stress ratios far less than the steady-state stress ratio. Different collapse surface concepts largely based on undrained triaxial test results have been proposed in the literature to explain the above instability phenomenon of loose sands. Studies of the instability behavior of fill material derived from residual soils remain limited. The present study investigated the instability behavior of a compacted residual soil using the conventional undrained triaxial tests and specially equipped constant shear triaxial tests. The test results were characterized in the p’: q: v space using the current state parameter with respect to the steady-state line for the residual soil. A modified collapse surface that has gradients varying with p’ and v was proposed for the loose residual soil to represent the instability states of undrained loading. Under constant shear stress conditions, the soil can mobilize stress ratios higher than those defined by the modified collapse surface. An instability surface was therefore presented for the instability states reached in static loading. Further, an alternative method of deducing the instability surface from the undrained stress paths was introduced.

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.

Creep Properties of Expansive Soils under Triaxial Drained Conditions and its Nonlinear Constitutive Model

The creep behaviors of expansive soils play an important role in landslide prediction and long-term stability analysis. In this paper, triaxial drained compression creep tests of expansive soils were conducted on the improved stress-controlled triaxial apparatus. The test results show that only transient deformation and attenuation creep occur with low deviator stress, and the increment of axial strain increases exponentially with deviator stress increasing; while deviator stress reaches a certain value, attenuation creep, steady creep and accelerated creep all occur in a creep curve. Meanwhile, the volumetric strain presents the shear shrinkage characteristic at the initial stage of loading, and the shear shrinkage is small. With the extension of loading time, the volumetric strain gradually varies from shear contraction to dilatancy. When entering the accelerated creep stage, the development rate of volumetric strain increases sharply. Besides, isochronous stress-strain curves of expansive soils indicate that their creep process possesses nonlinear characteristics, and the nonlinear degree is related to creep time and stress level. Imitating the empirical formula of cyclic cumulative deformation of clay, a new nonlinear creep model is presented, which may well describe the creep property of expansive soils. Furthermore, critical failure stress could be obtained based on the proposed creep model. The ratio of the critical failure stress to conventional shear failure stress ranges from 70% to 80%, with average of 75.56%, therefore, critical failure stress may be estimated by conventional triaxial tests with the margin of error 5.5% within.

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.