Sheng-Qi Yang

Evaluation of creep mechanical behavior of deep-buried marble under triaxial cyclic loading

Triaxial compression experiments were carried out on deep-buried marble specimens to investigate their short-term and creep mechanical behavior under cyclic loading. First, based on the results of short-term triaxial experiments, the elastic, plastic, and strength behaviors of marble were analyzed. The results show that for the same confining pressure, the elastic modulus of marble remains basically constant at the lower axial deviatoric level but decreases slowly after yielding strength; in contrast, the plastic modulus reduces rapidly with the increase of axial deviatoric stress. However, the elastic and plastic moduli of the tested marble were quite independent of the confining pressure. The relationship between axial deviatoric stress and plastic deformation of marble can be described well by the interface model. The peak strength of marble under higher stress increases with the confining pressure, which can be better described in accordance with the Mohr–Coulomb criterion. And then, in accordance with the experimental results of marble creep under triaxial cyclic loading, the instant elastic and plastic strains, and the visco-elastic and visco-plastic strains were all separated successfully, which provided a better foundation for constructing a visco-elasto-plastic creep model of rock. The creep strain rate of marble under different deviatoric stresses is analyzed, which shows that the steady-state creep rate of marble increases nonlinearly with the increase of axial deviatoric stress. In the end, the creep mechanical behavior of marble under cyclic loading is theoretically analyzed using the creep model. The results show that Burgers creep model can describe the creep behavior of marble under the loading condition satisfactorily but is inadequate to describe the creep behavior of marble under the unloading condition. Therefore, by adopting the fundamental hypothesis of visco-plastic mechanics, a visco-elasto-plastic creep model of rock material is constructed, which can describe the unloading creep behavior of marble better than Burgers creep model. The creep model curve agrees very well with the experimental results, which verifies the proposed visco-elasto-plastic creep model.

Particle Flow Modeling of Rock Blocks with Nonpersistent Open Joints under Uniaxial Compression

AbstractIn this study, numerical simulation of rock blocks with nonpersistent open joints under uniaxial compression was undertaken using the particle flow modeling method. First, the micromechanical parameter values of intact material were calibrated through a trial-and-error process using macromechanical laboratory test results. Then, a back-analysis procedure was used to calibrate the joint gap and joint micromechanical parameter values using laboratory test results conducted on jointed rock blocks. Afterward, the effects of joint dip angle, joint persistency, and joint gap on the mechanical behavior of block models having nonpersistent open joints was investigated using the calibrated micromechanical parameter values. The joint dip angle and joint persistency were found to play significant roles in the failure mode, strength, and stress–strain relationship of jointed blocks. The joint gap played a significant to negligible role in the mechanical behavior of jointed block models gradually when the join...

Experimental Study of Mechanical Behavior and X-Ray Micro CT Observations of Sandstone Under Conventional Triaxial Compression

This paper reports a series of triaxial compression experiments and X-ray observations carried out to explore the mechanical behavior and internal damage mechanism of sandstone material. The results show that the Young’s modulus of sandstone increased nonlinearly with increasing confining pressure, but the Poisson’s ratio remained unaffected. The nonlinear Hoek–Brown criterion can better reflect the peak strength properties than the linear Mohr–Coulomb criterion. However, the residual strength of sandstone exhibits a clear linear relationship with the confining pressure, which can be best described by the linear Mohr–Coulomb criterion. The sensitivity of the crack damage threshold on the confining pressure was clearly lower than that for the peak strength. After unaxial and triaxial compression failure, the sandstone specimens were analyzed using a 3D X-ray micro CT scanning system. Based on the horizontal and vertical cross-sections of sandstone specimens, we found that under uniaxial compression and lower confining pressure, the sandstone specimen is dominated mainly by axial splitting tensile cracks; however, under higher confining pressure, the sandstone specimen is mainly dominated by a single shear crack. To quantitatively evaluate the internal damage of sandstone material, crack area and aperture extent for each horizontal cross-section were calculated by analyzing the binarized pictures. The system of crack planes under uniaxial compression is much more complicated than that under triaxial compression, which is also testified by the evolution behavior of crack area and aperture extent. Finally, the fracture mechanism of sandstone during uniaxial and triaxial compression is discussed in detail and simplified models are proposed.

Strength, Deformability and X-ray Micro-CT Observations of Deeply Buried Marble Under Different Confining Pressures

In this research, a series of triaxial compression experiments and X-ray observations were conducted to explore the strength, deformability and internal damage mechanism of deeply buried marble. The results show that an increase in confining pressure results in obvious brittle–ductile transition characteristics of deeply buried marble. The Young’s modulus of the marble increased nonlinearly with increasing confining pressure. The peak and residual strength of the marble exhibit a clear linear relationship with the confining pressure, which can be described by the linear Mohr–Coulomb criterion. The sensitivity of the residual strength on the confining pressure was clearly higher than that of the peak strength. After uniaxial and triaxial compression failure, marble specimens were analyzed using a three-dimensional X-ray micro-CT scanning system. Based on horizontal and vertical cross-sections, the marble specimen is mainly dominated by axial splitting tensile cracks under uniaxial compression, but under confining pressure, the marble specimen is mainly dominated by a single shear crack. To quantitatively evaluate the internal damage of the marble material, the crack area and aperture extent for each horizontal cross-section were calculated by analyzing the binarized pictures. The system of crack planes under uniaxial compression is more complicated than that under triaxial compression, which is also supported by the evolution of the crack area and aperture extent. Finally, the brittle–ductile transition mechanism of the marble is discussed and interpreted according to the proposed conceptual models.

Numerical Modeling of Jointed Rock Under Compressive Loading Using X-ray Computerized Tomography

As jointed rocks consist of joints embedded within intact rock blocks, the presence and geometrical fabric of joints have a great influence on the mechanical behavior of rock. With consideration of the actual spatial shape of joints, a numerical model is proposed to investigate the fracture evolution mechanism of jointed rocks. In the proposed model, computerized tomography (CT) scanning is first used to capture the microstructure of a jointed sandstone specimen, which is artificially fabricated by loading the intact sample until the residual strength, and then digital image processing (DIP) techniques are applied to characterize the geometrical fabric of joints from the CT images. A simple vectorization method is used to convert the microstructure based on a cross-sectional image into a layer of 3-D vectorized microstructure and the overall 3-D model of the jointed sandstone including the real spatial shape of the joints is established by stacking the layers in a specific sequence. The 3-D model is then integrated into a well-established code [three-dimensional Rock Failure Process Analysis, (RFPA3D)]. Using the proposed model, a uniaxial compression test of the jointed sandstone is simulated. The results show that the presence of joints can produce tensile stress zones surrounding them, which result in the fracture of jointed rocks under a relatively small external load. In addition, the spatial shape of the joints has a great influence on the fracture process of jointed rocks.

The Modeling of Time-Dependent Deformation and Fracturing of Brittle Rocks Under Varying Confining and Pore Pressures

A numerical hydro-mechanical model for brittle creep is proposed to describe the time-dependent deformation of heterogeneous brittle rock under constant confining and pore pressures. Material heterogeneity and a local material degradation law are incorporated into the model at the mesoscale which affects the mechanical behavior of rocks to capture the co-operative interaction between microcracks in the transition from distributed to localized damage. The model also describes the spatiotemporal acoustic emissions in the rock during the progressive damage process. The approach presented in this contribution differs from macroscopic approaches based on constitutive laws and microscopic approaches focused on fracture propagation. The model is first validated using experimental data for porous sandstone and is then used to simulate brittle creep tests under varying constant confining and pore pressures and applied differential stresses. We further explore the influence of sample homogeneity on brittle creep. The model accurately replicates the classic creep behavior observed in laboratory brittle creep experiments. In agreement with experimental observations, our model shows that decreasing effective pressure, increasing the applied differential stress, and decreasing sample homogeneity increase the creep strain rate and decrease the time-to-failure, respectively. The model shows that complex macroscopic time-dependent behavior can be explained by the microscale interaction of elements. The fact that the simulations are able to capture a similar hydro-mechanical time-dependent response to that of laboratory experiments implies that the model is an appropriate tool to investigate the complex time-dependent behavior of heterogeneous brittle rocks under coupled hydro-mechanical loading.

Numerical simulation of particle migration from crushed sandstones during groundwater inrush

Groundwater inrush through fault fracture zones is caused by small particle migration from fractured rocks of the faults. To investigate particle migration with the water flow, a 3D model was established for the solid-water two-phase flow. First, the simulated crushed sandstone was represented by certain different-sized particles with a novel cohesive force. The discrete element method (DEM) was applied for particles considering the cohesive force, the collisions, the friction, and other conventional forces. Second, the process of particle migrating from the crushed sandstone was simulated under multiple effects accompanied by some experiments. The results indicate that the migration characteristics vary with different-sized particles, and the mass loss for different-sized particles are high at the beginning leading to stabilized conditions at different times. It can be also found that the total mass loss rate and the final mass loss all increase with the increases of initial water velocity, while the final mass loss decrease with the increases of the axial force. Moreover, selected stimulation results were compared with the experimental results and the previous simulated results, and reasonable agreements could be obtained, which would provide consults for particle migration during groundwater inrush through fault fracture zones in underground engineering.

Deformation and Damage Failure Behavior of Mudstone Specimens Under Single-Stage and Multi-stage Triaxial Compression

In tunnel engineering, due to the effect of excavation disturbance, the surrounding rock mass can produce an excavation damage zone with different damage extents. Therefore, knowledge of rock deformation and damage behavior is especially significant for the design of deep tunnel support. However to date, a few experiments and numerical simulations have been conducted to investigate the deformation and mechanical failure behavior of damaged rocks. Therefore, in this research, multi-stage triaxial compression test was used to investigate the mechanical behavior of mudstone specimens with different damage extents by experiment and two-dimensional particle flow code. First, a group of micro-parameters was calibrated by single-stage triaxial compression experiments of mudstone, and the numerical results agree very well with the experimental results. Then, multi-stage triaxial compression experiment and discrete element modeling of mudstone specimens were carried out. The more axial strain the specimens sustained, the less strength they had (because the degree of damage increased). A damage variable was defined by the ratio of the area of micro-cracks to the total area of the specimen. As the post-stress reducing ratio increases, the damage variable increases rapidly until the post-stress reducing ratio reaches 0.4; then, it remains constant. The force field were analyzed to reveal the damage evolution mechanism in the mudstone specimens under multi-stage triaxial compression.

Experimental Study on the Shear Behavior of Bolted Concrete Blocks with Oblique Shear Test

The shear behavior of concrete blocks reinforced by fully grouted bolts with different diameters was studied in this paper. More than 90 intact cubic samples (100 mm × 100 mm × 100 mm) with bolts ranging from 2 mm to 5 mm in diameter were tested at a constant stain rate of 0.5 mm/min. An oblique shear apparatus, which could simultaneously apply shear and normal force on tested samples at three slope angles (53°, 58°, and 63°) of a predetermined shear plane, was employed. The results indicate that the bolt has no evident influence on the shear behavior of intact concrete blocks at the prepeak shear strength stage. The bolt could significantly reduce the shear strength drop in the peak shear strength of the concrete block and contribute to reserving the residual shear strength of concrete blocks, especially at steep slope angles of the shear failure plane. The shear resistance provided by the bolt to the concrete block at the residual shear slip stage has a positive relationship with the diameter. The bolt with a larger diameter inflected in the vicinity of the shear failure plane of concrete block at the postpeak shear strength stage; additional normal force and direct shear resistance could still be persistently provided. Two empirical equations of the apparent cohesion and apparent internal angle of the bolted concrete block were obtained by linear regression considering rb, which is the ratio of the cross-sectional area of the bolt to that of the bolted concrete block.