Strength Of Fractured Rock Mass Research Articles

Triaxial Compression Strength Prediction of Fissured Rocks in Deep-Buried Coal Mines Based on an Improved Back Propagation Neural Network Model

In deep coal mine strata, characterized by high ground stress and extensive fracturing, predicting the strength of fractured rock masses is crucial for stability analysis of the surrounding rock in coal mine strata. In this study, rock samples were obtained from construction sites in deep coal mine strata and intact, as well as fissured, rock specimens were prepared and subjected to triaxial compression tests. A numerical model based on the discrete element method was then established and the micro-parameters were calibrated. A total of 288 triaxial compression tests on the rock specimens under different conditions of confining pressure, loading rate, fissure dip angle, and fissure length, were conducted to obtain the triaxial compressive strength of the fractured rock specimens under different conditions. To address the limitations of traditional back propagation (BP) neural networks in solving stochastic problems, a modified BP neural network model was developed using a random factor and an interlayer mean square error corrected network model evaluation function. The traditional and modified BP neural network models were then employed to predict the triaxial compressive strength of the fractured rock specimens. Through comparative analysis, it was found that the modified BP neural network prediction model exhibited smaller errors and significantly reduced overfitting, making it an effective tool for predicting the strength of fractured rocks in deep coal mine strata.

Real-time estimating method on rock strength via MWD of roofbolter and its application to in-situ grouting quality evaluation

Grouting reinforcement is an effective method for controlling fractured rock in the deeply buried roadways of coal mines. However, quantitatively evaluating the strength of grouted rock masses remains a challenge. The design of grouting reinforcement is primarily based on empirical knowledge. Consequently, this study proposes a real-time in-situ method for evaluating grouting quality using measure while drilling (MWD) based on a roofbolter. Initially, we validated the method through a numerical study using the discrete element method. Several 3D rock samples with varying crack densities were established and drilled numerically, demonstrating that the strength of fractured rock masses can be better estimated using the rock drillability index rather than drilling specific energy. Therefore, a model, known as the Pd–Rc model, was established to estimate rock strength based on the rock drillability index. Subsequently, an in-situ validation was conducted using a self-developed MWD testing system to assess the grouting quality in fractured rock masses. The results show that the strength of the rock mass increases by 38%, 56.0%, and 49.7%, respectively, after grouting, as observed in three boreholes. This method allows for quantitative estimation of grouting quality.

Influence analysis on the shear behaviour and failure mode of grout-filled jointed rock using 3D DEM coupled with the cohesive zone model

Grouting is one of the primary methods used to reinforce the strength of fractured rock masses. In this study, a three-dimensional discrete element method (3D Co-DEM) grouted joint shear model, which consists of upper and lower intact rock blocks, 3D real joint surfaces and a grout-filled layer, was developed with the cohesive zone model. The 3D real joint surface was imported into the model using 3D scanning point cloud data, and the grout-filled layer was described by a cohesive zone model. Then, a numerical simulation algorithm was developed to implement the numerical direct shear test and grout-filled layer cracking simulation. Also, the parameters of the proposed method were calibrated, and the effectiveness of the cohesive zone model and direct shear numerical test were verified by comparison with analytical and experimental results. Finally, grouted jointed specimens with regular sawtooth were used to analyse the influencing factors of shear characteristics. Numerical tests showed that shear peak strength and residual strength decrease with increasing grout water-cement ratio but increase with increasing JRC. Specifically, with increasing sawtooth inclination angle, the shear peak strength first decreases and then increases, while the peak shear strength increases as the sawtooth height increases.

Modelling slurry flowing and analyzing grouting efficiency under hydro-mechanical coupling using numerical manifold method

Grouting is a typical HM (hydro-mechanical) coupling process between slurry and rock and widely used in underground engineering to reinforce the strength of fractured rock mass. In present work, based on numerical manifold method (NMM), an NMM-HM grouting model is proposed to investigate slurry flowing and analyze grouting efficiency. In this NMM-HM grouting model, the slurry flows in fractures and grouting pressure can cause fracture aperture variation and fracture propagation. Then, two numerical examples for central embedded crack and hydraulic fracturing are adopted to validate the presented model. Furthermore, grouting simulations are conducted on a multi-crack and a roadway with fracture networks. Results show that Grouting pressure in fractures can increase fracture aperture, which is beneficial for grouting efficiency. However, fluid pressure could also cause closure of neighboring fractures or even fracture propagation and failure of rock mass, which will weaken grouting effectiveness. Therefore, the coupled HM impacts on grouting efficiency and grouting pressure should be seriously considered and optimized to achieve the expected grouting performance in practice.

Experimental Investigation on Permeability Evolution of Granite Samples Containing a Grout Infilled Fracture under Triaxial Compression

Grouting has been widely used in rock engineering to improve the stability and strength of fractured rock mass, as well as to decrease the permeability of fractured rock mass. Fractures infilled with grout may endure different in-situ stress conditions during the construction and operation life of tunnels and other underground spaces in fractured rocks. To study the stress induced changes in permeability of fractures infilled with grout, we carried out triaxial compression tests on the fractured rock samples infilled with grout, during which the permeability of fractured rock samples was measured. Both planar and rough fractures with different apertures were tested. The results showed that the permeability of infilled fracture samples increases with increasing grout width, but the strength of infilled fracture samples decreases with increasing grout width. The evolution of permeability is highly related with the volumetric strain. The permeability decreases slightly with increasing deviatoric stress from 0 to crack damage stress, followed by drastic enhancement in permeability with further increasing deviatoric stress. The sample’s permeability is enhanced by about 5~10 times at the peak deviatoric stress compared with the initial value at 5MPa deviatoric stress.

A study of tensile strength of fractured rock mass by phase field method in DEAL.II with local refinement technique

Cracking propagation in elastic and porous media is still challenge topics in mechanical, energy, and environmental engineering. In this paper, the phase field method will be used to model the cracking propagation at the small scale for elastic media. This method is doing well in DEAL.II with the help of local refinement technique which allows studying the tensile strength of fractured rock mass behavior without prior knowledge of cracking propagation path and reduction of computational consumption. This implementation is applied to model a fractured rock mass in which a plenty of explicit fractures are distributed though total energy released by Griffith's criterion. Through these applications, we demonstrate and highlight the performance of the phase field method with local refinement technique in modeling crack propagation as well as investigate the tensile strength of fractured rock mass dependency its crack orientation

A study of tensile strength of fractured rock mass by phase field method in DEAL.II with local refinement technique

Cracking propagation in elastic and porous media is still challenge topics in mechanical, energy, and environmental engineering. In this paper, the phase field method will be used to model the cracking propagation at the small scale for elastic media. This method is doing well in DEAL.II with the help of local refinement technique which allows studying the tensile strength of fractured rock mass behavior without prior knowledge of cracking propagation path and reduction of computational consumption. This implementation is applied to model a fractured rock mass in which a plenty of explicit fractures are distributed though total energy released by Griffith's criterion. Through these applications, we demonstrate and highlight the performance of the phase field method with local refinement technique in modeling crack propagation as well as investigate the tensile strength of fractured rock mass dependency its crack orientation

Estimation of the shear strength of fractured Gosford sandstone based on fractal theory and numerical modelling

The effects of joint roughness on the peak shear strength of fractured rock mass has been extensively investigated. Among these investigations, Barton's JRC-JCS model has been well recognized for its capability to characterize surface geometry. However, shear strength estimation is still a major challenge because of the difficulties involved in accurately calculating the joint roughness coefficient (JRC). Thus, alternative methods such as statistic methods based on fractal theory have been proposed to evaluate the impact of surface morphology on rock joints. Extensive evidences show that, the application of the fractal dimension alone cannot produce a satisfactory estimation of either the JRC or the peak shear strength. Therefore, this paper aims to investigate the peak shear strength of rock joints by adopting the fractal theory and via a numerical simulation. Gosford sandstone taken from Sydney Basin is adopted as the rock materials for both numerical validation and shear strength investigation. Along with numerical simulations on shear strength, a linear function has been proposed to describe the correlations among the root-mean-square of the first derivative of fracture profile (Z2), fractal dimension and the standard deviation. The peak shear strength is then accurately interpreted as a function of joint roughness indices: fractal dimension (D), standard deviation (SD) and Z2. Predictions by the proposed equation are compared with both the numerical simulation results and that reproduced by Barton's equation. Good agreement has been achieved even when the joint roughness coefficients (JRC) is varied which proves that the proposed equation is highly accurate and can be reused in other related applications.

Casing Shear Deformation Analysis of Shale Gas well Based on Shale Formation Strength Reduction Method

The problem of casing deformation caused by fault slippage due to the hydraulic fracturing process in Changning-Weiyuan area, Sichuan Province, has seriously affected the economic benefits of shale gas wells. In order to clarify the relationship between the strength of fractured rock mass, the size of fractured rock mass, and the parameters of cement sheath, based on the characteristics of artificial shale formation during hydraulic fracturing, the shale strength is reduced. The finite element model of the casing, cement sheath and formation is analyzed, and the deformation law of the casing during the shale strength reduction process is analyzed. The results show that the greater the reduction of the strength of the shale, the greater the risk of shear deformation of the casing. The fracturing height has a great influence on the casing deformation, which increases substantially linearly, and the crack width has little effect on the casing deformation. 2 The Young’s modulus of the cement sheath has little effect on the deformation of the casing, and reducing the Young’s modulus of the cement sheath cannot effectively alleviate the deformation of the casing. Increasing the wall thickness of the cement sheath increases the contact area between the cement sheath and the formation, and the degree of deformation increases. 3 increasing the wall thickness of the casing can alleviate the deformation degree of the casing, and the wall thickness of the casing should be optimized during the fracturing process. The research results have a certain guiding role for the prevention and treatment of casing deformation.

Numerical determination of deformability and strength of 3D fractured rock mass

We used a systematic numerical method to determine deformability and strength parameters of a fractured rock mass with irregular and stochastic fracture systems in 3D. In this case, different DFN realizations were generated based on Monte Carlo simulation and then compliance tensors and Representative Elementary Volumes (REV) for rock mass deformability and strength were determined in 3D. Stress-dependent strength of fractured rock mass was examined and different theoretical and empirical failure criteria were adopted. The numerical modeling results show anisotropy in deformability parameters in different directions. The obtained rock mass deformability moduli and peak strength of rock mass are reduced expectedly compared to intact rock. The results were compared to the analysis in 2D which had been performed in the previous stages of this research work. The approximated REV of rock mass deformability and strength properties in three-dimensions were found to be smaller than those in 2D numerical simulations. The results of rock mass behavior under different sets of confinement stresses indicate that characteristic parameters are completely stress-path-dependent. The approximated characteristic parameters from Empirical Hoek & Brown (EHB) criterion shows that rock engineering design based on EHB generally is more conservative. The Mogi-Coulomb criterion, considering the intermediate principal stress, estimates different values of cohesion and friction angle of rock mass compared to the conventional and extensional tri-axial tests. Therefore, care should be taken when such parameters are used in rock engineering projects.

Determination of RVE size based on the 3D fracture persistence

This paper presents an application of a 3D fracture network model and 3D persistence to confirm the representative volume element (RVE) in a fractured rock mass. Based on field fracture data collected from the Songta dam site, a large 3D fracture network is generated to analyse the RVE. The 3D persistence, which is governed by the comprehensive characteristics of fracture location, orientation, size and density, strongly affects the shear strength of fractured rock masses. Hence, it is valuable to confirm the RVE size considering the persistence, which combines the geometric and mechanical properties of a rock mass. The persistence values of the entire model and of cubes of different sizes, which are located in the centre of the model, are determined. A relative error of 10% is selected as the benchmark to evaluate the similarity between the cubes and the model. Finally, 15 m is confirmed as the size of the RVE in the study area.

Shear behaviour of rock joints with unsaturated infill

Behaviour of soil-infilled rock joints has significant importance with respect to the strength of fractured rock mass. The presence of even a small amount of fine-grained infill material within a joint can reduce its shear strength considerably, depending on the degree of saturation of infill. Therefore, it is crucial to examine how the infill material can adversely affect the joint shear strength. Previous studies of infilled joints have mainly been focused on idealised regular joint patterns owing to the simplicity and reproducibility in laboratory testing. Current literature on infilled rock joints has also neglected the effect of the degree of saturation of infill on the shear behaviour. In most instances, fully saturated infill has been used or assumed, and the contribution of matric suction on the shear strength of joints having unsaturated infill has not been studied. In this study, a series of triaxial tests on natural joint profiles having joint roughness coefficient (JRC) of 10–12 is carried out at constant moisture content. A semi-empirical model is proposed and validated on the basis of laboratory data.

Numerical evaluation of strength and deformability of fractured rocks

Knowledge of the strength and deformability of fractured rocks is important for design, construction and stability evaluation of slopes, foundations and underground excavations in civil and mining engineering. However, laboratory tests of intact rock samples cannot provide information about the strength and deformation behaviors of fractured rock masses that include many fractures of varying sizes, orientations and locations. On the other hand, large-scale in situ tests of fractured rock masses are economically costly and often not practical in reality at present. Therefore, numerical modeling becomes necessary. Numerical predicting using discrete element methods (DEM) is a suitable approach for such modeling because of their advantages of explicit representations of both fractures system geometry and their constitutive behaviors of fractures, besides that of intact rock matrix. In this study, to generically determine the compressive strength of fractured rock masses, a series of numerical experiments were performed on two-dimensional discrete fracture network models based on the realistic geometrical and mechanical data of fracture systems from field mapping. We used the UDEC code and a numerical servo-controlled program for controlling the progressive compressive loading process to avoid sudden violent failure of the models. The two loading conditions applied are similar to the standard laboratory testing for intact rock samples in order to check possible differences caused by such loading conditions. Numerical results show that the strength of fractured rocks increases with the increasing confining pressure, and that deformation behavior of fractured rocks follows elasto-plastic model with a trend of strain hardening. The stresses and strains obtained from these numerical experiments were used to fit the well-known Mohr-Coulomb (M-C) and Hoek-Brown (H-B) failure criteria, represented by equivalent material properties defining these two criteria. The results show that both criteria can provide fair estimates of the compressive strengths for all tested numerical models. Parameters of the elastic deformability of fractured models during elastic deformation stages were also evaluated, and represented as equivalent Young's modulus and Poisson's ratio as functions of lateral confining pressure. It is the first time that such systematic numerical predicting for strength of fractured rocks was performed considering different loading conditions, with important findings for different behaviors of fractured rock masses, compared with testing intact rock samples under similar loading conditions.

Strength characterization of rock masses, using a coupled DEM-DFN model

A discrete element model (DEM) is used to study the mechanical behaviour of a pre-fractured brittle medium subjected to triaxial loadings where progressive failure can occur. The initial discrete fracture network (DFN) is based on a fractal distribution and it is used to characterize the influence of clustering and size distribution of pre-existing fractures, on the strength of fractured rock masses. The proposed approach captures the progressive failure process occurring in the rock matrix as shearing displacement takes place along pre-existing discontinuities. The results show that the mechanical behaviour of fractured rock masses is mainly dependent on the fracture intensity. However, for a given fracture intensity, the strength can exhibit a 50 per cent variability depending on the size distribution of the pre-existing fractures. This difference can be attributed to the different mechanisms that involve sliding and crushing of blocks in the case of large interconnected fractures or progressive failure of the rock matrix through coalescence of cracks in the case of small unconnected fractures. Clustering of fractures was found to influence the spatial variability of the mechanical properties and therefore to have a scale effect on strength. The results outline the relevance of three parameters, the power-law exponent of the fracture size distribution, the clustering fractal dimension which fixes the fracture-to-fracture correlation number and the fracture intensity, to characterize the mechanical behaviour of rock masses.

Study on mechanical parameters of fractured rock masses

The equivalent strength parameters of fractured rock masses are prerequisite for stability analysis of geotechnical engineering projects constructed in fractured rock masses which are encountered frequently in western china. Based on generated mesh of fractured rock masses, combined with statistic damage constitutive model of intact rock and damage model of structural plane, progressive failure of fractured rock masses is studied using finite element method (FEM). Furthermore, Scale effect and anisotropy of compressive strength of fractured rock masses are studied. Study results show that the strength decreases and tend towards stability rapidly from intact rock to fractured rock masses, and the anisotropy of strength of fractured rock masses is not significant. At last, based on numerical simulation conducted on 10 m scale rock masses under different confining pressures, the equivalent strength parameters of fractured rock masses are gained and the results are compared with Hoek-Brown criteria. The method developed is helpful for determination of strength parameters of fractured rock masses.

Effect of fissure water on mechanical characteristics of rock mass

In view of the effect of fissure water in fractured rock mass on the strength of rock mass in engineering projects, we prepared specimens of cement mortar to simulate saturated rock mass with continuous fractures of different slope angles. By exerting static and dynamic loads on the specimens, the mechanical characteristics of rock mass with fissure water under these loads can be analyzed. Our experimental results indicate that the static compressive strength of saturated fractured rock mass is related to the slope angle. The lowest compressive strength of fractured rock mass occurs when the slope angle is 45°, while the highest strength occurs when the specimen has no fractures. Fissure water can weaken the strength of rock mass. The softening coefficient does not vary with the slope angle and type of load. The hydrodynamic pressure of fractured rock mass gradually increases with an increase in dynamic load. For a 0° slope angle, the hydrodynamic pressure reaches its highest level. When the slope angle is 90°, the hydrodynamic pressure is the lowest.

Investigating the scale effects in strength of fractured rock mass

Based on the method of fractal geometry, the relationship between quantity and size of fracture in a rock mass can be studied. It was found that this relationship is power: N δ = N 0 δ - D . Based on this result and theoretical analysis, the nonlinear relationship between rock mass strength and fracture quantity is found: Rc = Rc m + Δ Rc Exp [ - BN δ ] .the formula includes the residual strength of fractured rock mass Rc m, the difference in value ( Δ Rc ) between the intact rock strength Rc 0 and the residual strength, and fracture quantity. Then, combining the relationship between fracture quantity and the size of rock mass, the formula of rock mass strength is found: Rc = Rc m + Δ Rc Exp ( - BN 1 m 2 ( i - 1 ) D l E 2 - D ) . The formula includes fracture quantity, the fractal dimension of fracture quantity distribution and rock mass size. The results show that the attenuation index of rock mass strength is related to the fractal dimension of fracture quantity distribution: α = 2 - D .