Stability Of Rock Engineering Research Articles

Coupled seepage-damage effect in fractured rock masses: model development and a case study

The stability of rock engineering is generally dominated by existing seepage , particularly the seepage evolution due to the rock masses damage. However, the current research generally overlooks the seepage-damage coupling when studying the hydraulic-mechanical coupling effect in rock masses. This study aims to propose a dual-medium model, including equivalent continuous and discrete fracture media to study the coupled seepage-damage effect in fractured rock masses . The dual-medium seepage model considers the substantial water storage of the fracture network and the high conductivity of major large-scale fractures. Also, the seepage evolution is constructed to be a function of stress, seepage pressure and length of crack propagation in the rock mass. To illustrate the new model's application, a case of high-pressure water injection in a coal seam has been investigated to reveal the damage evolution in the coal seam. The results indicate that during the initial stage of the water injection, the seepage pressure in the discrete fracture medium increases faster than that in the equivalent continuous medium. Moreover, the seepage pressure difference between the two media gradually decreases with the increase of the seepage time, eventually forming a stable seepage field in the coal seam. Notably, the high-pressure water injection in the coal seam significantly affects the distribution of the coal seam's stress field, resulting in the effective minimum principle stress changing from compression to tension states. Also, during coal seam water injection, the damage zone and major fracture apertures in the coal seam gradually increase with increasing injection time .

Experimental investigation on rock mechanical properties and infrared radiation characteristics with freeze-thaw cycle treatment

The effect of freeze-thaw weathering cycles caused by diurnal and seasonal temperature changes is a significant disadvantage in the stability of rock engineering in cold regions. To investigate the evolution rule of rock strength and infrared radiation characteristics of igneous, metamorphic, and sedimentary rock subjected to freeze-thaw weathering cycles, freeze-thaw weathering cycle treatment was conducted on coarse-grained granite, fine-grained marble and soft red sandstone from cold regions in Western China. The nondestructive detection technique of nuclear magnetic resonance (NMR) was used for rock damage detection. Infrared thermography was used to monitor the rock debris movement, crack propagation and average infrared radiation temperature (AIRT) during unconfined compressive strength (UCS) testing. The mechanisms of rock damage and change in AIRT induced by freeze-thaw weathering cycles were carefully analyzed. According to the UCS evolution rule, the ranking order (from high to low) of the results of rock resistance to freeze-thaw weathering processes is as follows: marble, granite and sandstone. An indicator called the infrared radiation rate was proposed to measure the rate of increase in infrared radiation temperature with increasing stress during the UCS tests. The rank order (from high to low) of the infrared radiation rate results is as follows: sandstone, marble and granite. Based on the infrared images and photographs of the failed rock specimens, the failure patterns and processes of the different rock types are investigated.

Related Rule Study of Subcritical Crack Growth and Threshold Values in Transversely Isotropic Slates

Elastic parameters and the subcritical crack growth of different bedding angle slate specimens were studied using uniaxial compression testing and the double torsion constant displacement load relaxation method using SANS and MTS Insight machines. To study the relations of the mode-I stress intensity factor K I versus the subcritical crack growth velocity V , the fracture toughness K I C , the stagnation speed, and the threshold values, the double torsion constant displacement load relaxation method was carried out. The related rules between the bedding angles (β) and the uniaxial compressive strength, fracture toughness, and threshold values were investigated. Experimental results show that the uniaxial compression, the fracture toughness, and the threshold value curves move to the bottom then increase with the increase of the β angle. In addition, its fracture toughness is minimal when the β angle of the slates is 45°, and crack initiation and crack propagation are generated under load, which can lead to the failure of the slate. lg K I - lg V relations of transversely isotropic slates measured by this method are in accordance with linear rules, which is in good agreement with the Charles theory. The range of K 0 / K I C for these different bedding angle slates is from 0.511 to 0.789. The test results would provide the basis for studying seepage and time-dependency of rock engineering stability.

An elastoplastic model of frost deformation for the porous rock under freeze-thaw

Freeze-thaw action has caused many engineering geology disasters in cold regions, such as frost cracking of tunnel and pavement, rockfall of rock slope, falling off of building materials. The freeze-thaw damage of porous rock is mainly induced by the frost heaving pressure due to pore water freezing. Although some elastic crystallization theories have been proposed to model the freezing process of pore water, the long-term frost heave characteristics under cyclic freeze-thaw are rarely studied. The ice crystallization and frost heave theory under freeze-thaw have been developed in the elastoplastic regime in this study. A novel elastoplastic model has been proposed to estimate the long-term frost heave and frost damage under freeze-thaw. This model has considered the influence of the pore size, yield stress of matrix and degree of water saturation on the development of the frost heaving pressure in pores. During freezing, a plastic region will produce inside the matrix around the pore. This plastic region gradually expands outwards with the increase of this pressure. During thawing, the elastic deformation will be recovered but a considerable residual plastic strain is accumulated. However, the growth rate of the residual plastic strain will decrease under freeze-thaw due to the production of new empty voids. Therefore, the freezing of pore water in porous rock is an elastoplastic problem. Besides, the water saturation and mechanical properties of the porous rock are very important to quantify the frost heave. This study provides a better understanding of the frost damage of the porous rock and a reference for the stability of rock engineering under freeze-thaw in cold regions.

Cracking behavior of rock containing non-persistent joints with various joints inclinations

Non-persistent joints are commonly observed in the rock mass, and play a significant role in the stability of rock engineering. A detailed understanding of the cracking process is critical for the evaluation and control of the stability of surrounding rock. Using a three-dimensional Particle Flow Code (PFC), the influence of the joints inclination on the mechanical behavior and the associated cracking process and displacement field of specimens containing non-persistent joints were investigated in this study. First, the micro parameters used in the PFC3D model were calibrated based on the mechanical properties of the experimental material. Next, cubical specimens containing 9 non-persistent joints were created using the particle deletion method. The results of the simulated compressive experiment show that different joints inclinations result in variations in the mechanical properties and failure modes of specimens containing non-persistent joints. The internal fracture patterns in PFC3D were compared with that of the reconstructed model from X-ray CT scanning. Finally, further detailed research on the cracking process and displacement field mode were conducted to reveal the failure mechanism of specimens containing non-persistent joints with various joints inclinations.

Shear Failure of Bolted Joints considering Mesoscopic Deformation Characteristics of Rock

In rock engineering of the cold region, there are a lot of rock joints. The shear characteristics of joints play a decisive role in the stability of rock engineering in the cold area. In this paper, based on the numerical simulation method of particle flow, reasonable microscopic parameters are selected for the numerical simulation of the direct shear test of bolted joints. The results show that the shear stiffness and contact modulus are linearly and positively correlated. The greater the contact modulus, the greater the residual stress, the better the synergetic effect between rock and bolt, and the more developed the microcrack. The smaller the contact stiffness ratio, the greater the residual stress. The shear stiffness decreases with the increase in the contact stiffness ratio, and the larger the contact stiffness ratio, the slower the shear stiffness decreases, while the shear strength does not change with the contact stiffness ratio. The contact stiffness ratio has a weak effect on the number of cracks in the model. The shear stiffness increases with the increase in the parallel bond modulus, and the shear strength decreases with the increase in the parallel bond modulus. The binding stiffness is independent of the shear stiffness, and the peak shear stress decreases with the increase in the binding stiffness ratio. The greater the bond stiffness ratio, the greater the number of cracks.

Experimental study on fracture behaviour of three‐flawed sandstone specimens after high‐temperature treatments

AbstractThe crack coalescence of rocks significantly affects the stability of rock engineering, and extensive studies have been performed on preflawed rock specimens without thermal treatment. However, the fracturing behaviour of preflawed specimens after thermal treatment has not been investigated comprehensively. In this study, three‐flawed sandstone specimens with different flaw inclinations after high‐temperature treatments were tested under uniaxial compression. Photographic, acoustic emission and digital image correlation techniques were used to investigate the crack initiation, propagation and coalescence behaviour. Experimental results show that the peak strength, elastic modulus and peak strain of the three‐flawed specimens were lower than those of intact specimens and that they gradually recovered with increasing flaw angle. The peak strength and elastic modulus first increased and then decreased, whereas the peak strain increased with temperature. Noncoalescence, indirect coalescence and direct coalescence were three patterns observed between the two adjacent pre‐existing flaws. Finally, the mechanism of high temperature in alteration of the mechanical properties of sandstone was revealed through microobservations.

Development of a DEM-Based Method for Modeling the Water-Induced Failure Process of Rock from Laboratory- to Engineering-Scale

Abstract The time-dependent hydromechanical behavior of rocks is crucial to assessing the long-term stability of rock engineering, such as reservoir rock slopes and underground waste storage facili...

Numerical Investigations on Shear Behavior and Failure Mechanism of Non-persistent Jointed Rocks

The shear behavior of the discontinuities has a significant influence on the stability of rock engineering. In this paper, the numerical and experimental direct shear tests were performed to investigate the shear behavior of non-persistent jointed rocks. Through a comprehensive calibration of shear stress-displacement curves and failure mode, good agreement was successfully achieved. Further numerical analysis was conducted to study the macro–micro failure mechanism. The results indicate that the shear failure of fractured rock mass begins at the end of crack in the rock mass where the crack initiation appears and as the crack expands inward of rock a macroscopic shear fracture zone is formed along the fracture plane finally. The rotation radian of particles appears obvious zoned phenomenon. The particles with larger rotation radian are mainly distributed at the location where the cracks occurred. The process of specimen failure is the process of unceasing redistribution of the rotation radians of particles inside the specimen. The distribution of contact force is obviously regional and directional and the distribution area of crack is always consistent with that of compressive stress concentration. The failure process of rock is the process of energy dissipation within the specimen, and the failure of the specimen is a kind of instability phenomenon driven by energy.

The creep behaviour and permeability evolution of breccia lava in triaxial creep tests

Breccia lava is a comparatively weaken rock material that has been widely distributed in the Southwest China. Its creep behaviour and permeability evolution are significantly important to the long-term safety and stability of rock engineering. This paper presents the experimental results of breccia lava under triaxial creep tests. The microstructure of the breccia lava before and after the creep tests was studied by scanning electron microscopy and optical microscopy. The results suggested that the erosion of calcite induced by pore-water pressure contributes greatly in increasing the permeability in creep stage.

Experimental Investigation on the Shear Properties of Heterogeneous Discontinuities

The shear behaviour of rock discontinuities is a key factor affecting the stability of rock engineering. To study the effect of heterogeneity on the shear behavior of rock joints, laboratory tests were performed on heterogeneous rock joints with different joint roughness coefficient (JRC). As a comparison, two groups of tests were also conducted on the corresponding homogeneous rock joints. Test results showed that, the shear stress–displacement curves of low strength rock joints, heterogeneous rock joints and high strength rock joints with low roughness are all in line with the trend of the typical single peak value curve, while the curves of high strength rock joints with JRC greater than 12–14 accord with the trend of strain hardening curve. The shear displacement corresponding to the peak shear strength of heterogeneous rock joints is smaller than that of the homogeneous rock joints. The shear strengths of the three types rock joints all grow exponentially with the increase of roughness. The shear strength and its growth rate of heterogeneous rock joints were between those of two corresponding homogeneous rock joints. The surface damage of rock joints increases with the increase of JRC and the damage degree of high strength rock joints is more serious than that of low strength rock joints with the same JRC. The surface damage of heterogeneous rock joints presents obvious heterogeneous characteristics.

Development of a damage rheological model and its application in the analysis of mechanical properties of jointed rock masses

AbstractFossil fuel resources (eg, coal, oil, and natural gas) exist in underground reservoirs, where discontinuities are widespread and influence the stability of rock structures. The mechanical behavior of jointed rock masses significantly affects the long‐term stability of rock engineering. A major challenge in this area is to link the time‐dependent deformation with damage influence induced by distribution of joints in rock masses. In this paper, a damage mechanical theory is adopted which deals with several groups of joints distributed in rock masses. Based on the geometrical distribution of joints, a damage model considering the influence of normal vector and area density of joints is used to describe the discontinuities. A damage rheological model for jointed rock masses is developed and programmed in the finite difference software FLAC3D, and an example of jointed rock masses is used to verify the validity of the model. The damage rheological model could predict the visco‐elastic strain which corresponds to primary creep property, visco‐plastic strain which corresponds to steady state creep property, and damage strain which reflects the damage effect of joints to rock masses. Finally, we use this model to predict the displacements and damage zones in the rock masses surrounding an underground opening.

Study of Influence of Environmental Factors on Deep Shale Creep Properties

In order to study the influence of environmental factors on creep properties of deep shale, a series of creep tests of deep black shales are performed under different environmental conditions, including stress deviation, temperature, and chemical pH value. The influence of these conditions on creep properties of deep shale is studied. The results show that the creep and creep rate of shale will grow with the increase of temperature, stress difference, and acidity-alkalinity. We get nonlinear creep model of deep shale when pH = 4 and T = 30°C. The critical stress difference of deep shale is no larger than 31.04 MPa when the chemical pH value is 4 and the temperature is 30°C. By scanning shale after corrosion, we know that the effects of chemical pH value on the creep characteristics are mainly determined by the feldspar dissolution and corrosion caused by chemical action. Our work has important theoretical significance and practical value for evaluating rock engineering stability.

Failure Mechanism and Acoustic Emission Characteristics of Rock Specimen with Edge Crack Under Uniaxial Compression

There are many cracks in rock mass which have a significant influence on the stability of rock engineering. Most of current researches on the failure mechanism of cracked rock are mainly focused on the internal cracks. However, edge cracks around underground openings are common in many engineering fields. Understanding the failure mechanism of rock with edge cracks is essential to assess the stability of underground structures. Rock failure is closely related to the cracking processes and acoustic emission characteristics is deemed as an effective means of monitoring the cracking processes of rock. In this study, a series of numerical simulations utilizing particle flow software PFC2D were performed to investigate the influence of the length and position of edge crack on the failure mechanism and acoustic emission characteristics of rock specimens with edge cracks under uniaxial compression. The results show that: the strength of specimens is sensitive to the change of crack length and position; the directions of crack propagation in each specimen is in good agreement, the different crack tip position and the specimen boundary lead to different failure modes; the influence of crack length on acoustic emission is greater than that of crack position.

Mesoscopic Weakening Feature of Marble During Water Rock Interaction

Research on the water–rock interaction has become an important subject in rock mechanics field, the link between the mineral particles is weakened and the microstructure of rock mass is changed because of chemical corrosion effect after water–rock interaction. In this paper, double torsion testing of marble specimens has been adopt to determine subcritical crack propagation parameters during rock interaction, the morphological characteristics in this process were also quantified while the surface was scanned by a three-dimensional high-accuracy non-contact laser morphology instrument Talysurf CLI 2000. It shows the discreteness and deviation degree of datum plane on the surface of the rock specimens are increased after water–rock interaction, the highness difference decreased and become more coordinated. Meanwhile, the corresponding relationship between stress intensity factor KI and subcritical crack growth velocity v shows that rock interaction speeds up the subcritical crack growth. Those testing results provide a basis for the micro damage mechanism studying for rock engineering stability which are related to hydrochemistry.

Study on Macro–Meso Failure Mechanism of Pre-fractured Rock Specimens Under Uniaxial Compression

There are many fractures in natural rock mass which have a significant influence on the stability of rock engineering. Rock is a kind of aggregate made up of various particles, the research on the meso level can fundamentally reveal the failure mechanism. In order to comprehensively research the effect mechanism of fractures on the failure characteristics of fractured rock, uniaxial compression numerical experiments were conducted based on discrete element method. The macroscopic and mesoscopic mechanical response of pre-fractured rock specimens with varying fracture length and dip angle were studied. The evolution process of macroscopic and mesoscopic physical quantities was obtained. The results show that the macroscopic failure process of fractured rock was an evolution process of rock from the microscopic level to macroscopic level that resulted from the interaction of internal particles under compression loading. In this process, the energy dissipation was also evolving. There was an obvious relativity between the particle rotation radian region-related feature and the distribution location and number features of cracks. The distribution pattern of prefabricated fractures had a great influence on the failure mode and plays an important role in the macroscopic characteristics and the evolution of internal components of pre-fractured rock specimens. Research result is full of important theory significance and engineering value to evaluate the stability of rock engineering.

A statistical damage constitutive model under freeze-thaw and loading for rock and its engineering application

The freeze-thaw damage of rock is a critical problem to study rock engineering in cold regions. When water is frozen in pores, 9% volume expansion will induce damage in rock for a huge ice pressure. Thus, rock will be deteriorated and softened after freeze-thaw. Besides, loading damage produces when the inner stress exceeds the bearing capacity of rock. So it's crucial to establish a damage constitutive model under freeze-thaw and loading in order to evaluate the stability of rock engineering in cold regions. The freeze-thaw damage of rock is expressed by static elastic modulus in this paper, which can be accurately and simply predicted by damage evolution equation using P-wave velocity or compression strength proposed by Liu et al., 2015. Loading damage can be obtained by statistical theory assuming that micro-unit strength satisfies the Weibull distribution and the maximum-tensile-strain yield criterion. Then the final statistical damage constitutive equation under freeze-thaw and loading is derived, in which unknown parameters can be determined through carrying out compression experiment on rock after different freeze-thaw cycles. This developed model is validated by a previous experiment and applied to analyze the stability of a tunnel under coupled thermo-hydro-mechanical condition in cold regions. The results show that the proposed constitutive model is very suitable for rock suffering freeze-thaw cycles and it is of high accuracy and good practicality.

Effect of Drying - Wetting Cycles on Triaxial Compression Mechanical Properties of Sandstone

To investigate the geological hazards caused by the action of drying-wetting cycle on rocks, we conducted a conventional triaxial compression test on sandstone under a high number of repeated drying-wetting cycles. By using the TAW-2000D microcomputer-controlled electro-hydraulic servo triaxial rock testing machine, this study investigated the influences of repeated drying-wetting cycles on the deformation and strength characteristics of sandstone. At the same time, the relationships among the strength, elastic modulus, and confining pressure of sandstone were analyzed. The results show that, when the drying-wetting cycles are relatively low, both the compressive strength and elastic modulus of the sandstone increase with the increase of the confining pressure, and the more drying-wetting cycles are, the more they increase. When the confining pressure imposed on the sandstone is constant and the drying-wetting cycles amount to 10- 15 times, all the compressive strength, elastic modulus, cohesion, and internal friction angle significantly decrease. When the drying-wetting cycles exceed 15 times, the internal friction angle of the sandstone increases first and then gradually decreases. The fractures of the sandstone exhibit a transformation from brittle failure to ductile failure. Under the action of drying-wetting cycles, the sandstone is greatly influenced by the softening action of water soak at the initial stage. The conclusions provide favorable evidence for the analyses of the long-term stability of rock engineering.

Testing Study of Subcritical Crack Growth Mechanism During Water Rock Interaction

Subcritical crack growth plays an important role in evaluating the long-term stability of structures in rocks. By applying the constant-displacement-relax method, two groups of test specimens that one immersed in groundwater and the other in air were tested to get the parameters of subcritical crack growth in double torsion test. The relations of the stress intensity factor K I versus the subcritical crack growth velocity V were obtained under the two different environments, and the behavior of subcritical crack growth was also analyzed. The results showed: the relations of lg K I − lg V accorded with linear rules, which is in good agreement with Charles theory; Compared with specimens in nature state, the lg K I − lg V curves of saturated water specimens locate top left comer of those of air specimens. The slope of curve is smaller, and the intercept is bigger, which shows that the water–rock interaction speeds up the subcritical crack growth. And A increases 2.9 × 1018 folds but n decreases from 85.12 to 40.83 because of the water–rock interaction. Meanwhile, the fracture toughness K IC also decreases obviously from 2.55 in air to 2.26 in water due to water rock interaction. The testing results provide a basis for time-dependence of rock engineering stability.

Experimental Researches on Long-Term Strength of Granite Gneiss

It is important to confirm the long-term strength of rock materials for the purpose of evaluating the long-term stability of rock engineering. In this study, a series of triaxial creep tests were conducted on granite gneiss under different pore pressures. Based on the test data, we proposed two new quantitative methods, tangent method and intersection method, to confirm the long-term strength of rock. Meanwhile, the isochronous stress-strain curve method was adopted to make sure of the accuracy and operability of the two new methods. It is concluded that the new methods are suitable for the study of the long-term strength of rock. The effect of pore pressure on the long-term strength of rock in triaxial creep tests is also discussed.