Triaxial Rheological Test Research Articles

Study on mechanical parameters and creep seepage characteristics of different coal and rock combination samples

Based on the fact that the production of deep coalbed methane (CBM) in Yushe-Wuxiang Block is lower than expected, the paper is carried out uniaxial strength test and triaxial rheological test on coal measure strata reservoirs (mudstone, coal, sandstone and their composite sample) deeper than 1300 m in the block. In the uniaxial strength test, it is found that the stress–strain process of deep coal rock sample was similar to that of shallow one. The brittleness of deep coal rock sample is more obvious. It can also serve as described by the four stages of shallow coal rock sample in the compression process. The compressive strength of the coal in the combined samples will be improved in a small range, while the mudstone will be driven to produce radial tensile failure at the moment of failure. Creep characteristics of the composite sample are similar to those of the single one, and the axial deformation characteristics under lower stress (14 MPa, 18 MPa, 22 MPa) are close to those of coal. The axial deformation characteristics under extreme stress (26 MPa, 30 MPa, 34 MPa) are close to those of sandstone. The established stress-seepage coupled creep damage model can be used to describe the deformation law of each rock sample under different stresses. The permeability characteristics of the composite sample are determined by the poor permeability of the composite formation. There is a great correlation between the volume deformation and permeability evolution of the sample in the test. The permeability model based on the theory of porous media can be well fitted the relationship between volumetric strain and permeability. It is of great significance to predict the influence of creep effect on permeability under different in-situ stresses in engineering.

Creep Behavior and Its Prediction of Saturated Weakly Cemented Medium-Grained Sandstone under Multiaxial Loadings

Weakly cemented medium-grained sandstone in Ordos mining area of China is widely distributed under phreatic water. The creep behavior of saturated rock is important for the stability of water-resisting strata. In this paper, creep mechanical properties of saturated medium-grained sandstone were studied by triaxial rheological test. The results showed that medium-grained sandstone only shows attenuation creep and stable creep under low stress level, and accelerated creep occurs when axial load reaches about 75% of instantaneous compressive strength. The rock samples exhibit volume dilatancy under most loading levels and finally shear fracture. The failure mechanism is the dislocation separation of mineral particles. Based on the energy dissipation theory, the damage evolution equation was modified and introduced into the Burgers model, which can accurately describe the rheological behavior of saturated medium-grained sandstone. The modified model was used to predict the crack initiation time under different deviatoric stresses, which can provide guidance for early warning of water-resisting rock stability.

Study on Rheological Properties of Coal Rock Containing Gas under Disturbance Conditions

In view of the dynamic phenomenon that coal and rock are susceptible to external impact disturbance in the mining process, combined with the rheological hypothesis mechanism of coal and gas outburst, the RLSS-II-type triaxial loading creep test system of gas-containing coal and rock developed by ourselves is used to carry out the conventional triaxial rheological test of gas-containing coal and rock and the rheological disturbance effect test of gas-containing coal and rock under impact disturbance. The strength limit neighborhood of gas-containing coal and rock is determined, and the gas-containing coal and rock entering the strength limit neighborhood are subjected to different impact disturbances. The experimental results show that, (1) under the confining pressure of 0 MPa, 2.5 MPa, and 5 MPa, the longitudinal deformations of gas-bearing coal and rock are 32 mm, 27 mm, and 22 mm, respectively, indicating that the deformation of coal and rock will be affected by confining pressure, and with the increase of confining pressure, the deformation will decrease. (2) In the test, the deformation of coal and rock in the late stage of uniform creep stage can be regarded as a strain threshold. Before this threshold, the strain of coal and rock is not obvious, and the deformation is only 1.1 mm. After exceeding a threshold, the deformation of coal and rock is 9 mm, and the deformation increases significantly. Then, it enters the accelerated creep stage quickly and finally damages. The vicinity of this threshold is called the strength limit neighborhood of coal and rock containing gas. (3) The gas-bearing coal and rock without entering the strength limit neighborhood and entering the strength limit neighborhood are changed by confining pressure and different impact disturbance, respectively. It is found that, whether in the strength limit neighborhood or outside the strength limit neighborhood, the confining pressure has an effect on the strain, but the influence is not large. Under different impact disturbance, the deformation of coal and rock within and outside the strength limit neighborhood is 8 mm and 0.4 mm, respectively, and the deformation changes obviously, indicating that the impact disturbance has a great influence on the deformation of coal and rock within the strength limit neighborhood, and the coal and rock with large impact disturbance are destroyed before the coal and rock with small impact disturbance, indicating that the greater the impact disturbance, the shorter the time required for destruction.

An EVP Double‐Yield Nonstationary Flow Surface Model for Soft Clay

According to experimental findings, soft clay has compression and shear creep. The goal of the study is to develop an elastic‐viscoplastic (EVP) model for the viscous behavior of soft clay. The new EVP model is deduced from the associated flow rule, Yin’s double‐yield surface theory, and the critical state soil mechanics. The nonstationary flow surface model with double‐yield surface, compared to other existing models, considers the compression and shear creep properties of soft clay. The model parameters can be identified from two sorts of tests (i.e., the general triaxial tests and triaxial rheological tests). Different types of soft clay, such as Tianjin Coastal clay, Shantou dredger fill, and silted soil in front of dam, were tested to verify the accuracy of the constructed model. All comparisons of computed and experimental findings revealed that the new EVP model accurately reflected the time‐dependent behaviors of soft clay in both shear and compression creep.

Rheology Test and Finite Element Analysis: A Case Study of a Rockfill Dam in China

Rheology often occurs in the dam rockfill deformation. In this paper, we carried out a large-scale triaxial rheological test for the tuff rockfill materials of a concrete faced rockfill dam in China, and then obtained the parameters of the seven parameter rheology model. Finally, we analyzed the dam rheology using the three-dimensional finite element model. The results show that the dam settlement, horizontal displacement, face deflection and the displacement of the joints around the face slab are all obviously increased when considering the rheological behavior of the rockfill. We find that the calculated settlement of the dam body is in good agreement with the filed measured value when the dam is completed, and the maximum settlement during the impoundment period is within the normal range.

Creep damage properties of sandstone under dry-wet cycles

Rock creep properties can be used to predict the long-term stability in rock engineering. In reservoir bank slopes, sandstones which are frequently used in the bank slope undergoing long-term effects of dry-wet (DW) cycles due to periodic water inundation and drainage may gradually accumulate creep deformation, resulting in rock structure’s damage or even geological hazards such as landslides. To fully investigate the effect of DW cycles on the creep damage properties of sandstone, triaxial creep tests were conducted on saturated sandstone with different DW cycles by using a triaxial rheometer apparatus. The experimental results show that both the instantaneous strain and the stabilized strain increase with the DW cycles. In addition, using the Burgers model, four kinds of functions including an exponentially decreasing function, a linearly decreasing function, a linearly increasing function and an exponentially increasing function were proposed to express the relationships between the shear modulus, viscoelastic parameters of the Burgers model and the deviatoric stress under different DW cycles. Through comparative analysis, it is found that the theoretical curves generated using proposed four kinds of functions are in good agreement with the experimental data. Furthermore, macromorphological and microstructural observations were performed on specimens after various triaxial rheological tests. For samples with small number of DW cycles, approximately X-shaped fracture surfaces were observed in shear failure zones, whereas several shear fractures including obvious axial and horizontal tensile cracks, and flaws were found for samples with relatively large DW cycles due to long-term propagation and evolution of micro-fissures and micro-pores. Furthermore, as the DW cycles increases, the variation in micro-structure of samples after creep failure was summarized into three stages, namely, a stage with good and dense structure, a stage with pore and fissure propagation, and a stage with extensive increase of pores, fissures and loose particles. It is concluded that the combination effect of permeation of water molecules through pores and fissures within sandstone, and the propagation of preexisting pores and fissures owing to the dissolution of mineral particles leads to further deterioration of the mechanical properties of sandstone as the number of DW cycles increases. This study provides a fundamental basis for evaluating the long-term stability of reservoir bank slopes under cyclic fluctuations of water level.

Experimental study on mechanical properties of multi-layered rock mass and statistical damage constitutive model under hydraulic-mechanical coupling

The triaxial compression tests of multi-layered rock mass samples prepared from the laboratory were carried out by using the rock servo-controlled triaxial rheological test system, and the mechanical properties of multi-layered rock mass are studied. According to the test results, the deformation characteristics and failure modes of multi-layered rock mass samples are analysed. The relationship between layers and mechanical parameters of rock mass samples is discussed. The results show that the peak strength and the peak strain increases along with the increase of confining pressure. Failure forms of multi-layered rock mass samples include monoclinic shear failure, tension crack failure, and interfacial shear slip failure. The more layers, the more broken the samples are. The more layers there are, the smaller the peak strength and the cohesive force of multi-layered rock mass samples are. Moreover, there is an exponential relationship between the internal friction angle and the number of layers. Based on the test results, considering the weakening of the mechanical properties of rock mass by the number of layers, the statistical damage constitutive model of the rock mass under hydraulic-mechanical coupling is established and verified. The results indicate that the model fits the experimental data well.

Mathematical methods to unloading creep constitutive model of rock mass under high stress and hydraulic pressure

The main purpose of this paper is to study the unloading mechanical characteristics of rock mass unloading under high stress and high water pressure. An unloading creep test of deep sandstone was conducted under different initial stresses and hydraulic pressures by using the TFD-2000 rock triaxial rheological test system. Based on the experimental results, we can explore unloading creep properties and time-dependent deformation characteristics of rock mass. Deformation characteristics of rock masses during the attenuation and stable creep stages under different stresses and hydraulic pressure conditions were analyzed, and the variation laws of creep parameters in different creep stages were studied and fitted by a mathematical model. The applicability of the fitting results was discussed. A nonlinear unloading creep constitutive model and mathematicalmethods of rock mass under high stress and high hydraulic pressure was proposed based on element model theory. Parameters of this nonlinear creep constitutive model were recognized. Results demonstrated that the nonlinear deformation characteristics run through the entire creep stage. During the attenuation and stable creep stages, elastic deformation parameters generally present exponential functional variations with the increase in time and hydraulic pressure. In the accelerating creep stage, viscous deformation parameters also present an exponential functional variation. The test curve fits with the model curve obtained by the optimal numerical analysis software 1stOpt, which verifies the accuracy and applicability of the model. This study can provide theoretical references for accurate prediction of time-dependent deformation of deep rocks under high stresses and high hydraulic pressures.

Experimental Study on Mechanical Property of Highly Weathered Tuff for High-altitude Tunnel

In the context of Milashan Tunnel in predominantly tuff whose mechanical property quickly deteriorates when in contact with water, it is necessary to study through lab test the creep property of highly weathered tuff surrounding the high altitude tunnel. This paper covers reconstruction of standard specimen with measurements of tuff dry density, natural water content and particle grading curve; analysis of axial stress and strain curves based on triaxial compression test; plotting failure stress circle to obtain tuff strength parameters c and φ; obtaining tuff rheological parameters from triaxial rheological test; deriving creep process curve by multi-specimen method; simulation of its rheological characteristics using Burgers model; and lastly identifying rheological parameters using MATLAB. It provides basic data to study deformation mode and monitoring measurement in actual construction.

Indoor Rheological Test and Creep Model Analysis of Soft Soil in Qingyi River region in Wuhu Anhui

The shear strength and the failure deviatoric stress of the Wuhu soft soil under different confining pressures were determined by indoor undrained consolidation triaxial rheological tests. Selecting 20%, 40%, 60% and 80% of the failure deviatoric stress as deviatoric stress and 100kPa, 200kPa and 300kPa as confining pressures, the rheological tests were conducted by separate loading method. Based on the creep curves obtained from the rheological tests, the related model parameters were determined after modifying the stressstrain and strain-time relation of Singh-Mitchell and Mesri creep model. And the empirical creep model was obtained, of which the stress-strain and strain-time relation were expressed in the form of power function. The analysis indicated that the creep characteristics of Wuhu soft soil was described more accurately by the new empirical creep model than Singh-Mitchell and Mesri creep model.

A Rheological Model of Sandstones considering Response to Thermal Treatment

Time‐dependent rheological response of geomaterials to thermal treatment is a crucial issue in geothermal energy utilization and deep mineral mining. This response, however, has not yet been fully considered in the existing rheological constitutive models for sandstones. In order to experimentally investigate such responses and establish the associated rheological constitutive model, this study considers the sandstone specimens which have been thermally treated under different temperatures. The triaxial rheological test in conjunction with the scanning electron microscope is employed in the investigation to observe the mechanically and macro‐/micromorphologically rheological response. Investigation results show that the thermal treatment induces microcracks and microdefects, and subsequently, they propagate during the creep. As a consequence, the heterogeneous deformation occurs, and macrocracks are present, leading to the irregular fluctuation and mutation in strain over time. A higher temperature contributes to a more severe structure damage and in turn reduces the intactness of sandstones and elevates the rheological response. The investigation allows successful establishment of a three‐dimensional constitutive equation considering the instantaneous elastic response to thermal treatment. Based on the equation, a nonlinear visco‐elastoplastic rheological constitutive model is developed for sandstones. Comparison with three existing rheological models shows that the model developed in this study could well represent the rheological process of the thermally treated sandstones.

Acoustic emission characteristics of creep fracture evolution in double-fracture fine sandstone under uniaxial compression

Acoustic emission (AE) experiments were carried out to study short-time creep behavior under uniaxial compression for cuboid-shaped fine sandstone specimens with two pre-existing cracks (the sample scale was 70 mm × 70 mm × 140 mm and the lengths of cracks were 2a = 20 mm and 2a = 30 mm) using the RLJW-2000 servo-controlled triaxial rheological test system of rock and PCI-II AE system. Based on the experimental results of creep curves, the mechanical properties of specimens with different pre-existing crack lengths were analyzed. The experimental results showed that compared to the intact specimen as the standard, the specimens with pre-existing crack lengths of 20 and 30 mm exhibited a decrease in the creep damage strength from 160.4 to 100 and 55 MPa, respectively. The pre-existing crack length significantly affected the strength of the specimen. Based on the experimental results of the spatial evolution of AE events, the three-dimensional evolution law of micro-cracks in the specimens was studied. The experimental results showed that the AE events could well reflect the crack evolution process of specimens with cracks. The occurrence of AE event points was mainly due to the initiation and propagation of micro-cracks within the sample; and with the increase of stress levels, the distribution of event points exhibited different characteristics. In the first two stages of creep loading, AE events were produced to a lesser extent and were evenly distributed inside the sample. With the increase of the creep stress level, the AE activity increased, in particular at the four ends of pre-existing cracks, and evolved from the ends of the fissures, thereby enabling the prediction of the penetration position of the micro-cracks within the fractured rock mass. Moreover, AE location results also directly reflected the interior stress field propagation process. AE location results directly reflected the spatial position, direction, and spatial curved face of crack propagation in the rock sample with pre-existing cracks, which are extremely significant to study the mechanism of rock failure.

HKCV Rheological Constitutive Model of Mudstone under Dry and Saturated Conditions

Triaxial rheological tests are performed on mudstones collected from the soft interlayer in the Three Gorges Reservoir Area. In the tests, both dry and saturated conditions are considered, and a complete rheological process is then observed. Based on such laboratory observations of stress-strain, a four-element rheological constitutive model is developed, which is composed of (i) the Hook element, (ii) the Kelvin element, (iii) the viscoelastic-plastic body, and (iv) the nonlinear viscous body (HKCV model). The HKCV model adopts the one-dimensional and three-dimensional equations that are derived. The rheological parameters required are identified, allowing the successful development of the HKCV model. A comparison with the laboratory test observations and the existing model estimates shows that the estimates of the HKCV model are relatively consistent with the observations of the triaxial rheological test. The HKCV model better characterizes the rheological process than the three existing models. However, the HKCV model has the limitation of requiring more parameters than the existing models.

Application and Research of Triaxial Rheological Test System

Application and Research of Triaxial Rheological Test System

Rheological mechanical properties of altered-rock

In order to investigate the rheological properties of altered-rock in Xiaowan Hydropower Project, triaxial creep tests were carried out on altered-rock specimens using a rock rheology testing machine. The rheological test results show that the time-dependent rheological properties of altered-rock are very significant. Based on the triaxial rheological test results, the relationship between axial strain and time under different confining pressures is investigated. The rheological effect on the stress–strain curve of altered-rock is discussed. Moreover, the relationship between axial strain rates and deviatoric stress under different stress levels is discussed in detail. The rheological failure mechanism under different confining pressures is also analysed. The triaxial rheological law of altered-rock specimen is achieved. The Burgers rheological model is used to fit the creep curves, and its parameters are obtained. The result shows that Burgers model can exactly describe the rheological properties of altered-rocks in Xiaowan Hydropower Project.

Experiment Study on Rheological Model of Soft Clay

This paper adopts the conventional tri-axial apparatus to respectively conduct tri-axial rheological tests on soft clay of different confining pressures by multi-stage loading vertical stress, specifically on the rheological behavior of the soft clay in Beijing area. The experimental results show that the deformation of this kind of clay has obvious time effects and nonlinear characteristics. According to the results, the test is to propose generalized Kelvin three unit and five unit model. Power function creep model and time hardening experiential models are used to describe the clay rheological behavior. The results show that, time hardening creep model is an advanced model with high accuracy, simple equations and fewer parameters and establishes a typical clay rheological model that fits Beijing area.

Creep behaviour and permeability evolution of cataclastic sandstone in triaxial rheological tests

This article reports the results of triaxial creep tests of cataclastic sandstone cored from the dam foundation of a hydropower station. Meanwhile, the permeability measurements are performed in course of creep tests under hydro-mechanical coupling. The creep behaviours of studied rock and their effects on the permeability evolution have been analysed in detail. The results show that the studied rock exhibits pronounced irreversible time-dependent deformations. The strain rate increases by exponential functions as the deviatoric stress increased and the confining pressure decreased. The long-term strength can be defined at which the critical point of expansion strain rate is greater than the compression strain rate. In addition, important plastic deformation, obvious dilation and large strain rate are observed in the last stage of experimental tests. The permeability change shows some mutative correlation with the material porosity during the time-dependent deformation. But the trend of change is decreasing with the increase of deviatoric stress for all specimens. During the stage of steady creep, the permeability variation slowly decreases quasi-linearly. It indicates that the fluctuation has no significant effect on permeability evolution. These results may provide the basis for the establishment of hydro-mechanical coupling creep model and the long-term stability analysis of structures.

Study on Rheological Behavior of Undisturbed Soft Clay in Wenzhou Considering Structural Property

Based on boundary surface elastoplastic theory, a boundary surface elastoplastic constitutive model is established which is considered both structural property and rheological behavior of soft clay with the method of taking Cam-Clay yield surface as boundary surface. The structural parameters are introduced to calculate structure-damage strain. Plastic volumetric strain is regarded as a function timetso that a rheological parameter is introduced. The constitutive model is validated by numerical analysis of triaxial rheological tests.

The Application of Cubic Spline Interpolation Function in Determining Rock Rheological Long-Term Strength

The rheological long-term strength is determined according to the triaxial rheological test data of diabase at the dam area of Dagangshan Hydropower Station. Firstly, based on the stress-strain isochronous curve method and connected the test points with cubic spline interpolation function, the maximum deviation point in the long-term interval is determined as the turning point and established the long-term strength by nonlinear least square method. The results show that this method is consistent with the other methods. Finally, the advantage and disadvantage of this method is analyzed. This method can overcome the randomness of artificial selecting the turning points. Therefore, maximum deviation point method is relatively a reasonable and effective method to determine the rheological long-term strength of rock.

An Elastic-Viscoplastic Model for Time-Dependent Behavior of Soft Clay and Its Application on Rheological Consolidation

To describe the time-dependent behavior of soft clay, this paper extended one-dimensional Nishihara model to three-dimensional stress state based on the framework of Perzyna’s overstress theory and modified cam-clay model. The yield criterion of modified cam-clay model was used to describe the plastic properties of soft clay, and the overstress theory was used to describe the strain rate effect. Triaxial rheological tests were carried out on Ningbo soft clay and the rheological characteristics were studied. Based on laboratory results, the parameters of proposed model were determined by curve fitting, which show that this model is suitable for the rheological characteristics of Ningbo soft clay. The analysis of parameters shows that, the value of parameters changes slightly with different deviatoric stress when the confining pressure was constant, but changes notably with the increase of confining pressure. A user material subroutine of the proposed constitutive mode was coded on the platform of the FEM software ABAQUS and verified by triaxial compression of soil column. A plain strain problem was computed to analyze the rheological consolidation properties of soft clay, in which the rheological effect and the finite strain effect were considered.