Fractured Rock Mass Research Articles

A semi-theoretical method for determining the permeability tensor of fractured rock masses in three-dimensional space

The permeability tensor is a critical parameter for analyzing the hydraulic behavior of anisotropic permeability in fractured rock masses. However, determining this tensor for three-dimensional (3D) fractured rock masses has proven to be challenging and resource-intensive. Both field tests, requiring numerous costly in situ tests, and numerical experiments, hindered by complex discrete fracture networks with a high fracture density, present difficulties in obtaining accurate results. In response, this study proposes a semi-theoretical method for determining the permeability tensor of 3D fractured rock masses, significantly reducing labor and economic costs. The proposed method focuses on establishing the theoretical relationship of directional permeabilities in a 3D space, with emphasis on the properties of the permeability tensor and the influence of fractures' geometry on the flow rate. To facilitate the construction of the method, anisotropic ellipse and ellipsoid are introduced, providing a description of permeability anisotropy. With this innovative approach, engineers can calculate the permeability tensor even when only one value of permeability is available along any flow direction. The utilization of the anisotropic ellipse and ellipsoid concepts helps simplify the determination process. Through numerical experiments, the method is validated and its accuracy demonstrated, making it a valuable tool for analyzing the hydraulic behavior of 3D fractured rock masses.

The shape function method of nonlinear thermal stress of granite fracture tips in a high-temperature environment

Exposed rock masses in tunnel portals are susceptible to thermal deterioration in southern China, where temperatures are relatively high. The thermal stress field of rock masses is affected by fracture shape and distribution as fractures near the surface are channels for solar radiation energy to be converted into rock thermal energy. In this study, a function expression is developed for triangular heat sources of fractured rock masses in a tunnel portal in a high-temperature environment. By the function expression, the temperature field and thermal stress field are calculated, and the influence of fracture shape parameters and multi-fracture interaction is analyzed. The results are as follows: (1) the temperature field and thermal stress field of exposed rocks are redistributed by fractures. The internal temperature of the fractured rocks is higher than that of non-fractured rocks, and thermal stress near the fracture tip increases. (2) For triangular fractures of the same length, thermal stress increases as the apex angle increases. (3) When the spacing between parallel fractures or coplanar fractures is close, the superposition effect of thermal stress becomes significant. (4) In a high-temperature environment, temperature field and thermal stress field of a fractured rock are both nonlinear as temperature and thermal stress around fractures increase significantly. The results provide effective reference for stability evaluation of fractured rock masses in tunnel portals and offer theoretical foundation for thermal diseases analysis and protection measures of tunnel engineering in high-temperature environments of southern China.

Mathematical modeling of rocks fracture within local structures

Soil massif fracturing has a significant impact on change in engineering and geological conditions and, as a result, on stability of structures. Development of tectonic fracturing of local structures, taking into account the history of the process, its mechanism, resulting stresses in the massif and subsequent deformations of the rocks, led to a change in their structure, composition and strength characteristics, activation of hypergenesis and exogenous processes. The above circumstances require careful attention to identification of areas of increased fracturing, as the most dangerous in terms of risks during the construction of engineering structures. Field methods for assessing the fracturing of rock masses are laborious. It is not always possible to conduct instrumental surveys that allow solving the final problem – establishing patterns and sizes of damaged areas within local structures. The existing mathematical models for assessing fracturing, as a rule, are used to solve local problems: assessing the stability of developed pits, water content of rock masses, degree of fragmentation of individual blocks, etc. This information is not sufficient when assessing the areal distribution of weakened zones and clarifying their boundaries, since it does not take into account the history of the development of the structure, its parameters (dimensions, amplitude of the foundation block uplift, deformation properties of rocks). Aim. To develop a mathematical model of formation of the red-colored strata tectonic fracturing zones based on deformation criterion of destruction and mechanism of development of local structures. Results. The authors have developed a new mathematical model for predicting damage (fracturing) of terrigenous rocks of the red-colored strata that make up local structures, based on the mechanism of formation of local tectonic structures of the 3rd order and the deformation criterion of destruction. The paper introduces the mathematical dependencies that make it possible to predict the size (area) of taxa based on the data on the uplift amplitude of local structures. The results of the research can be used in assessing the fracturing of massifs composed of terrigenous rocks, and make it possible to judge the regularities in distribution of weakened zones within the entire massif being assessed.

A coupled hydro-thermo-mechanical model based on TLF-SPH for simulating crack propagation in fractured rock mass

In this paper, a hydro-thermo-mechanical coupling model based on the smoothed particle hydrodynamics with total Lagrangian formula (HTM-TLF-SPH) is proposed to simulate the crack propagation and instability process of fractured rock mass. TLF-SPH uses the Lagrangian kernel approximation, that is, the kernel function and its gradient need only be calculated once in the initial configuration, which is much more efficient than the smoothed particle hydrodynamics (SPH) based on the Euler kernel approximation. In TLF-SPH, particles interact with each other through virtual link, and the crack propagation path of rock mass is tracked dynamically by capturing the fracture of virtual link. Firstly, the accuracy and robustness of the HTM-TLF-SPH coupling model are verified by a reference example of drilling cold shock, and the simulation results agree well with the analytical solutions. Then, the crack propagation law of surrounding rock and the evolution characteristics of physical fields (displacement, seepage and temperature fields) after excavation and unloading of deep roadway under the coupling condition of hydro-thermo-mechanical are investigated. In addition, the seepage and heat transfer processes of the surrounding rock of Daqiang coal mine under different coupling conditions are successfully simulated. Meanwhile, the effect of the boundary water pressure difference on the temperature and seepage fields under the hydro-thermal coupling condition is quantitatively analyzed.

Damaged creep constitutive models of rock-like materials containing multiple flaws and their relationships to those of the intact materials

A number of pre-flawed specimens made of sandstone-like material with four different numbers and five different angles were prepared and tested in three various loading paths to investigate the time-dependent mechanical behaviors. It was determined that the deformations and strengths increase and decrease with increasing flaw number and decreasing flaw angle, respectively. The ratio of the long-term strength to the uniaxial compressive strength of specimens with the same pre-existing flaws varies within a small range (0.637–0.729). The specimens in the creep tests exhibited more shear failures than those in the uniaxial compression tests. A modified Burgers model was then developed. By using different parameter values, it can well describe the entire creep phases of all the pre-flawed and intact specimens, which is in accordance with the principles of damage mechanics. In view of this, pre-existing flaws were regarded as a type of damage to the intact material, and a damage creep model was developed that can derive the creep curves of fractured materials from those of the intact material. This idea provides a reference for the study of the damage creep model of fractured rock masses.

Dynamic Coupled Hydromechanical Approach for Fractured Rock Mass Seepage Using Two-Dimensional Discontinuous Deformation Analysis

Dynamic Coupled Hydromechanical Approach for Fractured Rock Mass Seepage Using Two-Dimensional Discontinuous Deformation Analysis

Damage and failure characteristics of single fractured cyan sandstone subjected to freeze–thaw cycles under uniaxial compression

In order to explore the freeze–thaw (FT) damage characteristics of fractured rock masses, FT cycle tests and uniaxial compression tests were conducted on intact and pre-cracked cyan sandstone samples, respectively. The degradation laws of physical and mechanical properties of samples under FT cycle conditions were analyzed, and the FT damage mechanism and failure mechanism were explored accordingly. The results indicate that as the number of FT cycles (N) increases, the saturation weight change rate and porosity of the sample continue to increase, while the P-wave velocity continues to decrease. The closure stress, initiation stress, damage stress, and peak stress decrease linearly with increasing FT cycles, and increase linearly with increasing crack inclination angle (β). Moreover, as FT cycles increase, the geological strength index GSI decreases linearly, while the damage factor (D) increases linearly. A Hoek Brown strength degradation model for fractured rocks subjected to FT damage under uniaxial compression was obtained, and its high reliability was verified using test results. The FT damage of cyan sandstone is mainly dominated by the frost heave force inside the rock, and the degree of FT damage of fractured samples is much higher than that of intact ones. The concentration of frost heave force at the crack tip leading to cracking is the key to exacerbating the FT damage in fractured rock masses. However, the crack initiation mechanism is still controlled by β. Whenβ = 90°, it exhibits tensile initiation mode, and whenβ = 45°, it mainly exhibits shear initiation mode. Moreover, the fracture of the single crack sample conforms to the compression-shear fracture criterion. Taking the sample with pre-crack inclination angle of 45° as an example, the variation of KI and KII, and the relationship between them are discussed accordingly. The results of this paper are beneficial to clarify the influence of FT cycles on the mechanical properties of cyan sandstone, and can provide reference for the study of related issues.

Application of Bayesian method for mining-induced tremors: A case study of the Xinjulong coal mine in China

Mining-induced tremor is a dynamic phenomenon occurring during the fracturing or slipping of rock and coal mass in coal mines, and the tremor mechanism is one of the key problems in the field of rockburst. The Bayesian inversion method can more accurately analyse the source rupture type under the premise of considering waveform data and model errors. First, we added different noise levels to the waveform and determined filtering parameters based on the spectrum. The rationality of the Bayesian inversion was verified by observing the focal mechanism solution. Finally, a total of fourteen mining-induced tremors were selected, and the parameters of the moment tensor (MT), type of rupture, and fault plane solutions were obtained. The results demonstrate that (1) the tremor events in the roof are mainly tensile/shear failure, and compression failure mainly occurs in the coal seam. (2) Events on the floor are closely related to gob and mining activities. (3) The rupture form of the events near the tectonic region is more complex, and the fault plane solutions (strike, dip, and slip) were obtained by MT conversion. (4) The relation between rockbursts and mining-induced tremors is discussed by analysing events 1# and 14# that occurred in the Xinjulong coal mine. These research findings can provide a reference for the analysis of source parameters and rupture forms of mining-induced tremors.

Study on surface deformation pattern in mine closure area of complex karst mountainous region based on SBAS-InSAR technology

Liupanshui City in Guizhou Province represents a karst mountainous region with a delicate geological environment. The area has a long history of coal mining, and several coal mines have been progressively closed in recent years. However, even after closure of coal mines, the fractured rock mass in the mined-out areas continues to undergo deformation over a specific period. In karst mountainous regions, this deformation is affected by various factors including elevation, slope, precipitation, and vegetation. In this study, we employed SBAS-InSAR technology to construct a time series of surface deformation data from January 2019 to May 2022 within Liupanshui City’s LuJiaZhai-DaPingDi Minefield. Subsequently, this data was comprehensively analyzed in conjunction with time series vegetation cover, monthly precipitation, elevation, and slope data from the identical period. The key findings of this research are as follows: 1) After the closure of the mine, the subsidence area gradually stabilized, yet the volume of subsidence continues to increase. The subsidence area primarily occurs near the MaiZiGou Coal Mine air-mining area, which had a relatively short closure time, as well as in higher elevation areas in the southern and eastern parts of the minefield. Specifically, the maximum cumulative subsidence and the subsidence rate during the study period reached −60.3 mm and −21.83 mm/a, respectively. 2) Surface deformation is closely linked to slope, vegetation type, and rainfall, with subsidence rates and amounts noticeably higher during the rainy season than in the dry season. 3) Surface vegetation cover types display varied effects on deformation, with grassland or shrub areas being more sensitive to precipitation than forests. Forests, especially pine and fir, show a delayed subsidence response to rain, typically 2–3 months post-rainy season onset. This inhibitory effect lessens with increasing slope, particularly beyond a 25° threshold, where responsiveness to precipitation and associated subsidence significantly increase. The findings of this study hold substantial scientific implications for the restoration and management of closed mining areas, as well as the prevention of geological hazards in karst complex mountainous regions.

Experimental study and numerical simulation of water-sand two-phase flow in fracture network

ABSTRACT The flow characteristics of water-sand two-phase in fractured rock mass has an important influence on underground engineering. A series of laboratory experiments are carried out by using a self-made fracture network test system. Based on the Euler-Lagrange method, a numerical simulation study of water-sand two-phase flow is conducted. The effects of sand mass fraction w, sand particle size Φ and combination of inlet and outlet on the flow characteristics of water-sand are systematically analyzed. The results show that the water-sand outflow is directly proportional to the fracture openings, and the growth rate of water-sand outflow is significantly different under multiple inlet-outlets configurations. The final volume proportions of water-sand are 14.1%, 35.9% and 50% respectively. Fluidity I exhibits an inverse correlation with fracture opening, w and Φ. Under the condition of the single inlet and outlet, the minimum fluidity I for 0.6 mm opening is 3.28E–7 m2+ns2- n/kg, the maximum fluidity I for 2.0 mm opening, reaching 3.88E–6 m2+ns2- n/kg. Based on the response surface regression method, a multivariate regression model for three fracture openings was established, boasting exceptional predictive accuracy and reliability. Particle residence time correlates positively with w and Φ. The turbulent kinetic energy k is significantly affected by the fracture opening and decreases with the increase of w and Φ. The variation range of k is 0.38 m2/s2 ~0.63 m2/s2. The results deepen our understanding of the migration behavior of water-sand in the fracture network, and provide a theoretical guidance for groundwater resource protection and other underground engineering.

Study on the Damage Characteristics and Internal Variable Modeling of Single-Fracture Sandstone under the Coupling Effect of Freeze–Thaw and Fatigue Load

The fractured rock mass in the western cold region is affected by freezing and thawing disasters and is prone to local damage and fracture along the fissures’ ends. The fatigue damage induced by repeated frost heave and traffic loads seriously endangers the stability of cold region roadbed. This paper selects sandstone as the research object. Firstly, 20 freeze–thaw cycles were performed on fractured sandstone samples with different inclination angles of 30°, 45°, 60°, and 90°. Subsequently, triaxial compression and triaxial fatigue loading tests were conducted to explore the mechanical properties and fracture morphology evolution mechanism during the compression process of freeze–thaw fractured sandstone. Nuclear magnetic resonance technology (NMR) was used to measure the H-containing fluid inside rock pores. The microscopic damage characteristics inside the rock were analyzed from the NMR T2 relaxation spectrum signal and pore size distribution characteristics. Based on the internal variable theory of continuum mechanics, a fatigue model of freeze–thaw fractured sandstone with different inclination angles was established. The results show that sandstone strength was negatively correlated with the fracture dip angle, and the axial deformation and shear failure angle were positively correlated with the fracture dip angle. The mechanical properties of the sandstone were deteriorated by fatigue loading. When the crack angle was 90°, the fatigue failure strength of the rock sample was the lowest. The T2 spectrum distribution of the fractured sandstone mainly had three peaks and the pore size was mainly medium and small pores. There was a small leftward shift after freeze–thaw cycles and fatigue loading. The T2 spectrum area was significantly affected by fatigue loading, with the highest rate of change at a crack angle of 30°. Through the fine correspondence between the axial residual deformation and the deformation modulus, a fatigue model with different crack inclination angles was established using the axial residual deformation as the internal variable, and the rationality of the model was verified by fatigue loading tests.

Macro- and micromechanical characteristics and energy evolution law of micro/nano carbon fibre grouting samples

This study comprehensively explores the macro- and micromechanical attributes and energy evolution dynamics of micro/nano carbon fiber (CF) specimens. The investigation centers on a graded sand gravel grouting specimen, aiming to determine the optimal CF content through uniaxial compression tests conducted on samples containing 0.0 %, 0.5 %, 1.0 %, and 1.5 % CF. Utilizing CT and PFC2D simulation analyses, we establish the interrelation between micro- and macromechanics. The findings reveal similar failure modes across CF samples with varying contents, predominantly marked by shear failure in the principal crack of the inclined section, accompanied by proximal secondary cracks. The post-peak stress–strain curves exhibit notable divergences. Specifically, with increasing CF content, the total strain energy, elastic strain energy, and dissipated energy of the CF samples initially decline, subsequently rise, and then diminish once more. Furthermore, the 1.0%CF sample demonstrates heightened dissipated energy, uniaxial compressive strength, and elastic modulus in contrast to the 0%CF sample, showcasing superior macroscopic mechanical properties and enhanced toughness. A staged microcrack evolution unfolds, delineating defect initiation, pore evolution, crack development, and crack penetration stages. The non-uniform distribution of CF and sand gravel profoundly influences the propagation trajectory of minute cracks. Additionally, the failure modes and energy evolution trends from both models closely mirror experimental outcomes. The reinforcing characteristics and crack-resistant attributes of CF usher the transition from brittle failure (0%CF) to ductile failure (1.0%CF). PFC simulations highlight hysteresis in the model's damage and failure, underscoring CF's role in restraining grouting sample failure and shifting it from brittle (0%CF) to ductile (1.0%CF) failure, thus accentuating CF's pivotal role in impeding crack propagation. These findings furnish invaluable theoretical insights for bolstering grouting reinforcement in fractured rock masses.

Assessing Stress Variability in Fractured Rock Masses with Frictional Properties and Power Law Fracture Size Distributions

AbstractThe presence of fractures in rock masses plays a major role in its stress state and its variability. Each fracture potentially induces a stress perturbation, which is correlated to its geometrical and mechanical properties. This work aims to understand and quantitatively predict the relationship between fractured systems and the associated stress fluctuations distribution, considering any regional stress conditions. The approach considers the rock mass as an elastic rock matrix into which a population of discrete fractures is embedded—known as a Discrete Fracture Network (DFN) modeling approach. We develop relevant indicators and analytical solutions to quantify stress perturbations at the fracture network scale, supported by 3D numerical simulations, using various fracture size distributions. We show that stress fluctuations increase with fracture density and decrease as a function of the so-called stiffness length, a characteristic length that can be defined as the ratio between Young’s modulus of the matrix and fracture stiffness. Based on these considerations we discuss, depending on DFN parameters, which range of fractures should be modeled explicitly to account for major stress perturbations in fractured rock masses.

Efficiency and seismic safety of constructing underground structures in complex rock masses

Purpose. To develop new resource-saving method of underground construction and evaluate how effective it is, to set thresholds for safe seismic ground vibrations which accompany explosions during breaking in mine workings. Methodology. The work used method of analysing mining and geological conditions of workings, field surveys of rock conditions in face, experiments are conducted on rock samples taken from blasting sites, more detailed data are obtained on rock properties, type and direction of development of crack systems along the workings cross-section by funneling method and approved research methods in accordance with current State Standards. Findings. Research has been carried out to determine main features of physical and mechanical properties of rocks, fracture and tectonic structure of rock mass and development of fracture systems. According to the results of ejection funnel parameters, the anisotropy coefficient was calculated, and according to data on identification of crack systems and their density, fracture coefficient was calculated. The experimental data obtained were used to adjust rational distances between contour boreholes and along the entire cross-section of working face. Based on corrected drilling and blasting operations (DBO), experimental explosions were carried out in workings. It was established that the borehole utilisation rate (BUR) was 0.95–0.97, uniformity of rock mass crushing was achieved, and explosive material consumption was reduced by 10–15 %. Instrumental measurements of explosion impact in workings proved seismic ground vibrations at protected facilities amounted to 0.4 cm/s with a duration of 0.05 s, which did not exceed the State Standard. Originality. Optimal DBO parameters are substantiated based on changes in numerical parameters of anisotropy and fracture coefficient, as well as radius of fracture zone along the cross-section of working face. The idea of forming a shielding zone along the contour of workings with explosive charges having an elongated symmetrical cut was confirmed and technically implemented. Practical value. Laboratory and field research results are fundamental for designing borehole layouts along workings and refer to major initial data used to justify design parameters of blasting chart.

Development of Grouting Test System for Rough Fissure Rock Body and Research on Slurry Diffusion Law

The surface roughness of grout in fractured rock masses has a significant impact on the diffusion characteristics of grout, especially in millimeter-scale fractures. In this study, a self-constructed experimental system for grouting in rough fractured rock masses was used to conduct grout diffusion tests with varying fracture roughness, fracture aperture, and grouting pressure. A theoretical model was developed to account for the combined effects of fracture roughness, aperture, and grouting pressure on grout diffusion, and its validity was verified. The results showed that the theoretical calculations and experimental results had an error rate of around 12%, indicating the high reliability of the theory. Fracture aperture, grouting pressure, and fracture roughness all exhibited nonlinear relationships with the grout pressure distribution. With increasing diffusion distance, the grout pressure decreased, and the rate of decrease gradually slowed down. Fracture roughness and aperture had a relatively small impact on grout pressure, while grouting pressure had a significant influence on grout pressure distribution. The difference in grout pressure between the initial and final stages of diffusion was small, whereas in the middle stage of diffusion, the difference was more pronounced. This research provides a valuable reference for the selection of grouting techniques in the roadway surrounding rock projects.

Diffusion mechanism of cement-based slurry in frozen and thawed fractured rock mass in alpine region

The problem of slope stability after freeze-thaw damage in cold regions has become increasingly prominent. To explore the grouting diffusion mechanisms of different grouting materials in frozen and thawed rock masses, a true triaxial stress-state frozen and thawed rock mass fracture grouting experiment was conducted. Computed tomography (CT) and scanning electron microscopy (SEM) were performed on the grouted specimens to obtain the grouting diffusion patterns and three-dimensional reconstruction of the grout veins in different frozen and thawed rock masses under fracture grouting. The results show that Portland cement slurry with low viscosity has better diffusion performance in unfrozen rock mass and rock mass after 10 freeze-thaw cycles. In the case of cement slurry containing graphene oxide and microsilica powder additives, the reinforced area exceeded that of ordinary cement after 10 freeze-thaw cycles, with the reinforced area reaching its maximum after 25 cycles. The “slurry-rock” interface primarily forms three modes: contact, embedding, and covering. Graphene oxide and microsilica effectively fill the larger pores in the cement particles, with microsilica being particularly effective in promoting cement hydration and reducing microdefects in the transition zone between the slurry and rock interfaces.

Mechanical properties of fracture-grouted prefabricated sandstone after thermal-acid coupling treatment: An experimental study

Grouting reinforcement is an efficient approach to increase the stability of engineered fractured rock masses that may be subjected to high temperatures and acidic groundwater in complex geological environments. To reveal the mechanical properties and failure characteristics of the grout-reinforced fractured rock masses under thermal-acid coupling conditions, prefabricated fractured red sandstone specimens with different inclination angles (IAs) were firstly grouted using a new high-strength fast anchoring agent (HSFAA). Then, the grout-reinforced specimens (GRSs) were then immersed in hydrochloric acid (HCl) solutions with different temperatures (varying from 25 to 300 °C). Finally, the macroscopic mechanical properties, microscopic morphology, and crack evolution characteristics of the GRSs after thermal and acid solution treatments were investigated by the uniaxial compression test, X-ray computed tomography (CT) and scanning electron microscopes (SEM). The results confirmed that the HSFAA featured with high fluidity, rapid setting, excellent early strength, and micro-expansion, was an ideal grouted reinforcement material for fractured rock masses. The compressive and tensile strengths of the HSFAA material at a 1-day curing were 2.01 and 1.64 times those of the ordinary Portland cement 42.5 (P.O 42.5), respectively. The fractured IA, thermal-acid coupling method and temperature affected the mass loss, strength parameters, and failure modes of the GRSs to different degrees. The thermal-acid coupling treatment affected the initiation location and extension direction of cracks within the GRSs. The increasing temperature led to the larger and more varied internal cracks, as well as the more complex 3D spatial morphology within the GRSs.

Three-dimensional partition-of-unity generalized node method for the simulation of fractured rock mass

A three-dimensional (3D) partition-of-unity (PU) generalized node method (GNM) is developed for the simulation of fractured rock mass. In the 3D PUGNM, generalized nodes with independent deformations are directly defined on the isolated blocks to simplify the construction of PU approximations in 3D problems. The explicitly representing of polyhedral blocks with complex geometric shapes by a proper topological structure, the computational geometric algorithms for block cutting, and the subdivision of integration tetrahedrons adapting to physical boundaries are totally avoided in the present 3D PUGNM, and thus the bottleneck problems encountered in the existing PU methods for three dimensions have been well solved. The present 3D PUGNM has the advantages of simple data structure, strong robustness, and convenient numerical implementation, and paves the way for promising application in various engineering problems. Several numerical examples are tested to demonstrate the accuracy, correctness and robustness of the proposed method.

A Novel Deformation Analytical Solution and Constitutive Model for Fractured Rock Masses

In order to study the deformation and stability of a fractured rock mass, existing research suggests that fracture deformation, which is usually obtained by evaluating the equivalent deformation modulus, dominates the deformation of a fractured rock mass. However, this parameter is difficult to obtain in theory and practice, which limits the application of rock mass deformation analysis methods. In order to calculate the deformation of fractured rock masses, the mass of the rock is regarded as a sponge-like material, and it is assumed that the deformation of a water-saturated fractured rock mass under external force load is approximately equal to the net flow of fracture water. Based on this assumption, firstly, the relationship between rock mass deformation and fracture flow is studied through a single-fracture rock mass model. The hydraulic properties of the fracture are characterized by the permeability coefficient, and the fracture deformation in the rock mass is equivalent to the fracture flow. The fracture deformation calculation formula is derived from the fracture hydraulics calculation formula, and this is compared with the measured data. The rationality of the calculation formula was verified. On this basis, the calculation formula for rock mass deformation, including multiple groups of fracture surfaces, is proposed, and the stress-strain constitutive relationship of a complex rock mass is established. The correctness of the calculation method was verified by comparing it with other theoretical calculation results.

Pressure-Shear Crack Initiation and Expansion Mechanism of Complex Cracked Rock Mass under the Seepage Stress

This work was to analyze and discuss the propagation mechanism of compressive shear initiation of complex fractured rock mass with seepage stress. The dense marble of Daye Iron mine with bulk density of 26.6 kN/m3 and uniaxial compressive strength of 52.4 MPa was selected as the material, and the upper and lower fracture surfaces were polished smoothly. The crack initiation criterion under compressive shear stress state is analyzed by taking the theory of fracture mechanics and classical mechanics. The coupling equation in the extended finite element simulation is established. The influence of lateral pressure on the crack propagation law, the relationship between lateral pressure and fracture, the initial expansion angle and pressure change law, and the effect of working face length on the crack expansion are analyzed. Results. The initial expansion angle of cracks increases with the increase of lateral pressure, and that of a single crack decreases with the increase of pressure. When other conditions are constant, the crack angle of the crevice also shows a trend of increasing with the increase of lateral pressure. When the lateral pressure becomes smaller, the initial expansion angle is relatively small. With the progress of the step size, the expansion angle shows a gradually decreasing trend, that is, the initial expansion angle gradually decreases with the increase of water pressure. The smaller the working face length, the smaller the expansion length of the floor crack. Conclusion. The expansion of the floor cracks is mainly formed by the tensile shear failure, and the fracture water pressure will reduce the initiation stress, which makes the rock mass more prone to the fracture failure.