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In this study, constant confining pressure (CCP) and reducing confining pressure (RCP) triaxial compression tests were conducted for mechanical characterization of a sandstone material. These triaxial experiments were carried out within a range of confining pressure from 0 to 50 MPa. The experimental results revealed that the sandstone exhibits a significant ductile-semibrittle stress degradation behaviour in the post-peak region. It was found that the plastic flows are mostly perpendicular to the initial, ultimate, fracture, and residual yield surfaces. This demonstrates that the associated flow rule can be adopted to describe the macroscopic plastic deformation of the sandstone. With increasing confining pressure, the brittleness decreases and causes a transformation of the failure mode from tension-splitting failure to mixed tension-shear failure, and then to shear failure. The sandstone exhibits the same deformation and strength behaviours along the CCP and RCP stress paths, however, deformation and failure were more brittle in the RCP tests than in the CCP tests. This experimental investigation can provide an important technical reference for geotechnical engineering projects involving sandstone.

In this study, large-scale triaxial compression tests are conducted on gravelly soil material. The experimental investigation shows that gravelly soil exhibits complicated volume-dilatation/contraction behaviours that are dependent on stress level. To better describe the mechanical behaviours, an elastoplastic constitutive model is proposed within the frameworks of generalised plasticity and the critical state soil mechanics. An evolution equation of the void ratio is incorporated into the model to describe the effect of a relatively dense or loose state on the volume-change behaviours. The model has 10 material constants that could be determined using a few large-scale conventional triaxial tests. The model is then used to simulate the mechanical behaviours of gravelly soil. Comparisons between the numerical simulations and the test data show that the model is capable of capturing the mechanical behaviours of gravelly soil under various confining pressures. This research can be beneficial for understanding the mechanical behaviours of gravelly soils.

The strength behavior and deformation modulus of columnar jointed rock mass have been investigated by conducting a series of uniaxial compressive tests, along with consideration of columnar joint effect. The tests have been performed on the artificial physical model samples, incorporating different angles β (which was the between the loading direction and axial direction of the columns) and heights of specimen. Columnar joint is a special geological structure with self-organized joint network which separates the rock into an assemblage of blocks. Due to the special joint network, the anisotropic characteristics of columnar joint rock mass is very remarkable. The physical model consisted of “intact rock” and “jointed filler”, replaced by cement mortar and white cement, respectively. The angle β were set to 0°, 15°, 30°, 45°, 60°, 75° and 90°, respectively, and the heights of specimen were set to five different values. Through the test, failure strength and failure modes of columnar joint rock mass with different angles β were observed. Based on the analysis of the test results, the uniaxial compression strength and average modulus of columnar joint rock mass showed strong anisotropy, showing a special “U” shape in terms of loading orientation. The size effect of columnar jointed rock mass was very typical, and the failure mode of columnar joint rock mass were analyzed. Furthermore, the relationship between failure strength and joint factor was studied.

In order to study the deformation behavior of fine-grained sandstone subjected to cyclic loading, a series of triaxial cyclic compression tests were performed under different confining pressures in the laboratory. Each of the tests combined constant-amplitude cyclic loading and increasing-amplitude cyclic loading. The experimental results showed that the evolutions of the elastic modulus and Poisson’s ratio with cycles had 3 variation modes according to the applied deviatoric stress. Within a given stress level, the axial and lateral irreversible strain initially increased with a decreasing rate, followed by steady-state increases before failure. When the specimens approached failure, an accelerated increase in the irreversible strain occurred. Under lower stress levels, the volumetric strain was characterized by compaction-dominated behavior. However, it changed from compaction to dilation within each loading cycle under higher stress levels. SEM (scanning electron microscope) tests were conducted on unruptured specimens to study the deformation mechanism of the fine-grained sandstone during cyclic loading. This marks the beginning of a study that aims to provide a reference to the design, construction, and operation of hydropower stations in China.

Triaxial compression tests are conducted on a quasi-brittle rock, limestone. The analyses show that elastoplastic deformation is coupled with damage. Based on the experimental investigation, a coupled elastoplastic damage model is developed within the framework of irreversible thermodynamics. The coupling effects between the plastic and damage dissipations are described by introducing an isotropic damage variable into the elastic stiffness and yield criterion. The novelty of the model is in the description of the thermodynamic force associated with damage, which is formulated as a state function of both elastic and plastic strain energies. The latter gives a full consideration on the comprehensive effects of plastic strain and stress changing processes in rock material on the development of damage. The damage criterion and potential are constructed to determine the onset and evolution of damage variable. The return mapping algorithms of the coupled model are deduced for three different inelastic corrections. Comparisons between test data and numerical simulations show that the coupled elastoplastic damage model is capable of describing the main mechanical behaviours of the quasi-brittle rock.

The accidents caused by the instability of bedding rock slope are often encountered in the expressway construction and operation in China. The shear strength parameters of the structural plane play a major role in controlling the stability of the bedding rock slope. In this paper, in-situ direct shear tests were performed on the structural planes containing the thick muddy interlayer , which were widely distributed between the layers of muddy slate in Guizhou Expressway. A numerical model considering the thickness of the muddy interlayer and the shape of the structural plane is established using the UDEC program. Numerical analysis has been performed to reveal the shear mechanical behaviors and the progressive failure mechanism of the muddy structural plane from a mesoscopic perspective. The thickness of the muddy interlayer has a great influence on the shear mechanical behaviors of the structural planes containing the weak layer. With the increase of the thickness of the interlayer, the peak shear strength of the structural plane decreases, and the stress-displacement curve presents a strong nonlinear characteristic. Due to the uneven spatial distribution of the thickness and morphology of the muddy interlayers in actual engineering practices, it should not only adopt the results from in-situ direct shear tests, but also consider the interlayer thickness for the stability analysis of the bedding rock slope.

In this study, we propose a digital image processing technology for estimating the macro permeability property of heterogeneous geomaterials. The technology is based on a connected-component labeling algorithm and provides a novel and effective method for geometry vectorization and microstructure identification. A color photo of a soil and rock mixture (SRM) is taken as an example. Information about the distribution of aggregate and a vectorgraph, which can be used in numerical analysis, are obtained automatically. A numerical permeability test is carried out to estimate the macro permeability coefficient of the heterogeneous medium. The effects on macro permeability of three parameters, scale dependency, material heterogeneity and the rock fraction, are discussed. The results indicate that the SRM has a scale dependent property and the representative element volume (REV) length is about six times the maximum major axis of the aggregate. The heterogeneity parameter has a major effect on macro permeability characteristics within a certain range. There is a weak tendency for the macro permeability to decrease as the rock fraction increases. Although the rock fraction is not the only factor, it does have an influence on the macro permeability. We conclude that the novel method developed in this study has good prospects for widespread application in macro parameter estimation and related research fields.

This study develops a coupled elastoplastic damage model for brittle rocks within the framework of irreversible thermodynamics. In the model, degradations in the deformation and strength of rocks induced by damage are considered in the elastic stiffness matrix and plastic yield criterion. The thermodynamic force associated with damage is expressed as a state function of elastic strain and the internal plastic variable. The damage potential and criterion, as functions of the thermodynamic force, are constructed to describe the damage evolution of rocks, and to determine whether they are damaged, respectively. Constitutive relationships, and their numerical formulations of a return mapping algorithm for finite element method implementation, are deduced under three different loading conditions: damage, plastic and coupled plastic damage. For engineering application, the developed model is implemented in the finite element programme Abaqus as a user-defined material model (UMAT). The coupled model is then used for simulating the re-distributed stress fields, displacement and EDZs of the sidewall rock mass around the underground powerhouse of the Wudongde hydropower plant.

In order to understand the mechanical behaviours of the surrounding rocks in the underground caverns of the Wudongde hydropower plant, triaxial tests are performed on a type of dolomite. It is revealed that damage induced by crack development is the main factor controlling the nonlinear plastic deformation and failure behaviour of the dolomite in both pre- and post-peak regimes. Based on this understanding, a coupled elastoplastic damage model is developed for capturing the dolomite’s mechanical behaviours. In the model, the effects of plasticity and damage on rocks is described by introducing plastic hardening and damage softening commonly in the plastic yield surface. Which are both derived from a suitable Helmholtz free energy function. The model is used to simulate the triaxial tests. Comparisons between test results and the numerical modelling show that the developed model is capable of describing the macro mechanical behaviours of the Wudongde dolomite.

The strength of stratified rocks is closely related to their inherent structural anisotropy, which cannot be accurately represented by the isotropic failure criteria. In this study, the Pietruszczak-Mroz (PM) anisotropic failure criterion was used to describe the directional dependency of strength of stratified rocks. In the meridian plane, the frictional coefficient was established by incorporating an anisotropic variable defined as a joint invariant of fabric and stress tensors. In the deviatoric plane, the anisotropic failure surface was characterized by a smooth shape function of Lode's angle. The material parameters for the failure criterion can be determined using the triaxial test. The PM anisotropic criterion was utilized to predict the anisotropic strength of specific stratified rocks. Analysis of the experimental data validated the assertion that PM anisotropic criterion has excellent ability to model the anisotropic strength features of stratified rocks.

A novel robust method for estimating equivalent tunnelling Mohr–Coulomb strength parameters in elastic–plastic or elasto–brittle–plastic media satisfying Hoek–Brown failure criterion is proposed. Based on the best uniform approximation technique, this method consists of explicit and closed-form formulae in terms of elementary functions, making the estimation of equivalent parameters more convenient than other methods. A simple code is provided and a series of benchmark test cases are performed for validation. When the tunnel support is accurately known, the performance estimates of this method are better than estimates of best fitting in an artificial stress range and best fitting in the existing range methods, but not as accurate as the performance of equating model responses method and that proposed by Jimenez et al. However, when the estimation of support pressure is poor, this method is the most robust among all methods. In elasto–brittle–plastic rock mass, the advantage of robustness of this method i...

Landslides are one of the major natural disasters that are frequently occurring in southwestern China. The main objective of this study is to investigate the relationship between failure of landslide and fractal dimension using displacement time-series. Taking two slopes in southwestern China as examples, we estimate the fractal parameters of displacement time-series and analyze the relationship between fractal dimension and the stability of the slope during the evolution of landslides. The result shows that the fractal dimension increases when a landslide undergoes a transition from stability to failure, but decreases during the evolution from active to stable. Fractal dimension has a critical value of 1.10 in the transition between active and stable, which indicates that fractal dimension is an important nonlinear parameter in the evaluation of the stability of landslides, although more studies and in situ data are required for validation.

The Wudongde Reservoir, now being constructed on the lower reaches of the Jinsha River, will be the forth largest hydropower plant in terms of capacity in China. This project includes a concrete arch dam and two large-span underground caverns in both abutments. For their design and construction, in situ stress measurements and geological survey were conducted. In this study, following an introduction of the geological conditions and topography, an interpretation on the test results of in situ stresses is presented for clarifying the distribution features of natural stresses in the engineering area. It is interesting to find that the vertical stress is predominant in the project area and that the horizontal stress reduces largely in the relaxation zone near the bank slopes, while concentrates at the valley bottom and becomes normal in the depth of both abutments. Also analyzed are the effects of valley topography, geological conditions in the project area as well as the macro-regional tectonics on the distribution of in situ stresses at the dam site. A noteworthy finding is that topography and thickness of overburden are the main factors for the intermediate–low stress levels, that regional tectonics governs the stress orientation at the regional scale, and that river erosion, caprock structure and karstic processes have also contributed to the evolution of the valley and in situ stress state.

Many experiments conducted under the three-dimensional stress condition reveal that damage of brittle rocks shows obvious deformational and time-dependent effect. Based on triaxial rheological test results, the creep behaviours and creep rupture characteristics of metamorphic volcanic fine conglomerate were investigated. It is found that the test results agree well with numerical curve, and the relationship between creep damage variable and loading time can be fitted by an exponential function. By analysing the tertiary creep, a new nonlinear viscoplasticity model on the basis of creep damage was proposed, and then, the three-dimensional nonlinear creep damage constitutive equations were deduced and validated by tests. Comparisons between numerical predication and test data show that this nonlinear creep damage model is valid and reasonable. Therefore, it can better describe the complete creep phases of the metamorphic volcanic fine conglomerate, which include primary creep, secondary creep and tertiary c...

This paper reports a comprehensive investigation on the deformation characteristics, failure mechanism and stabilisation treatment of Baitieba slope on the left abutment of Xiluodu hydropower plant on the downstream reach of Jinsha River. In the first two years following the excavation, the development of the slope movement after rainfall became a threat to the water inlet and spillway tunnels constructed beneath it, as well as to the construction of the plant. On the basis of field investigation and real-time monitoring, the deformation characteristics and the failure mechanisms of the slope were investigated. There are two sliding surfaces in the slope: a shallow one which is a rotational slide; a deep one as a result of the reactivation of the ancient slip zone induced by excavation which is a translational slide. The deep sliding surface is dominant in translating the global soil mass to move. Based on the analysis of the movement tendency and stabilities, the efficacies of the used stabilisation meas...

Triaxial cyclic loading tests have been performed to assess the influence of plastic deformation on inelastic deformational properties of anisotropic argillite with bedding planes which is regarded as a kind of transversely isotropic media. Considering argillite’s anisotropy and inelastic deformational properties, theoretical formulae for calculating oriented elastic parameters were deduced by the unloading curves, which can be better fitted for the description of its elasticity than loading curves. Test results indicate that with the growth of accumulated plastic, strain, the apparent elastic modulus of argillite decreases in a form of exponential decay function, whereas the apparent Poisson ratio increase in a form of power equation. A ratio of unloading recoverable strain to the total strain increment occurred during a loading cycle is defined to illustrate the characteristic relations between anisotropic coupled elasto-plastic deformation and plastic strain. It is significant to observe that high stress level and plastic history have an inhibiting effect on argillite anisotropy.

In order to reflect the tertiary rheological characteristics of hard rocks at the high stress states, a new nonlinear visco-elastic-plastic model is proposed on the basis of linear visco-elastic-plastic model and nonlinear visco-elastic-plasticity. And then the corresponding constitutive model are deduced, which can be used for describing rocks’ long-term strength characteristics and their creep deformational behavior and time-dependent damage under interaction of coupled seepage-stress field in rock engineering. At last, considering the time effect of rock damage in the process of tertiary creep, a coupled seepage -stress creep damage model for investigating the whole creep deformation behavior, including tertiary creep failure process is established, and the related equations governing the evolution of stress, creep damage and rock permeability along with the creep deformation of rock is introduced.

Triaxial creep tests were carried out under seepage pressure by using rock servo-controlled triaxial rheology testing equipment. Based on experimental results, rock rheological properties influenced by seepage−stress coupling were studied, and variations of seepage rate with time in complete creep processes of rock were analyzed. It is shown that, when the applied stress is less than failure stress level, the creep deformation is not obvious, and its main form is steady-state creep. When applied stress level is greater than or less than but close to fracture stress, it is easier to see the increase of creep deformation and the more obvious accelerative creep characteristics. The circumferential creep deformation is obviously higher than the axial creep deformation. At the stage of steady-state creep, the average of seepage flow rate is about 4.7×10^(−9) m/s at confining pressure (σ3) of 2 MPa, and is about 3.9×10^(−9) m/s at σ3 of 6 MPa. It is seen that the seepage flow rate at σ3 of 2 MPa in this case is obviously larger than that at σ3 of 6 MPa. At the stage of creep acceleration, the seepage flow rate is markedly increased with the increase of time. The variation of rock permeability is directly connected to the growth and evolution of creep crack. It is suggested that the permeability coefficient in complete creep processes of rock is not a constant, but is a function of rock creep strain, confining pressure, damage variable and pore water pressure. The results can be considered to provide a reliable reference for the establishment of rock rheological model and parameter identification.

Based on an anisotropic failure criterion that includes a scalar parameter in terms of the mixed invariants of the stress and micro-structure tensors, an elasto-plastic model is suggested for the description of mechanical properties of transversely isotropic rocks. And on the basic principles of return mapping, we deduce the formulations of return mapping suit for transversely isotropic rocks, especially the second derivative of plastic potential function and unsymmetrical consistent constitutive matrix. For application, an experimental program is conducted and several tests of argillite specimens are simulated with various confining pressures and loading orientations. The comparison reveals that the proposed model and its return mapping scheme are suitable for describing the mechanical properties of argillite.

In order to know about the accelerated creep properties of volcanic breccias, triaxial compression rheological experiments are carried out on rock servo-controlled triaxial rheology equipment. Based on the experimental results, the accelerated creep phase of creep curve of volcanic breccias is analyzed. It is shown that the strain rate increases sharply, the rock damage rapidly increases; the acoustic emission dramatic increases with the increase of time during the stage of accelerated creep, and the accelerated growth of rock damage leads to the sharp increase of creep rate. Then based on the accelerated creep deformation, the relation of damage variable versus time is obtained, and and the creep damage constitutive equation of volcanic breccias is established. It is shown that the established nonlinear creep damage models based on time function is correct and reasonable.