Stress In Rock Mass Research Articles

Research on Performance Test of the Optic-Electric Sensors for Reservoir Landslide Temperature Field Monitoring

In recent years, with the superposition of extreme climate, earthquakes, engineering disturbance and other effects, global landslide disasters occur frequently. Due to reservoir landslides being mostly in a multi-field coupling environment, the temperature field will impact the deformation and seepage fields, thereby affecting the stability of the reservoir landslide. The variation in the landslide’s surface temperature also directly affects the stress and deformation of deep rock masses. If hidden dangers are not detected in time, and corresponding measures are implemented, it is easy to cause landslide instability. In order to clarify the temperature measurement performance of different optic-electric sensors and the application characteristics of layout techniques, laboratory calibration tests of temperature sensors under different adhesives and attachment materials are carried out in this paper. It was found that the test data of the iron bar had the best effect among the four attachment materials overall. Therefore, the bar with a high-stiffness material should be preferred when selecting a pipe fitting as the fiber Bragg grating (FBG) temperature attachment in the borehole. However, considering the high requirements for the durability of sensors and layout techniques in on-site monitoring, the long-term stability of the adhesives used in actual monitoring needs to be improved. At the same time, it was found that the platinum 100 (PT100) temperature sensor has relatively higher testing accuracy (A: 0.15 + 0.002 × |t|; B: 0.30 + 0.005 × |t|), a larger temperature measurement range (−200~+850 °C) and better temperature measurement stability when compared to conventional sensors. Moreover, its resistance value has a good linear relationship with temperature. Finally, the Xinpu landslide in the Three Gorges Reservoir area was selected as the research object for on-site monitoring. There was a high correlation between the on-site monitoring results with the laboratory calibration test results. Therefore, through the performance test of optic-electric sensors in reservoir landslide temperature fields, more accurate solutions can be provided for selecting sensors and designing layout techniques to monitor the underground temperature field of landslides under different geological conditions. Thereby, grasping the real-time state information of the reservoir landslide temperature field is achieved accurately, providing an important reference for early warning, prediction, prevention and the control of reservoir landslide disasters.

Back analysis of rock mass parameters in tunnel engineering using machine learning techniques

Efficient determination of the rock mass properties is vitally important for calculating and evaluating tunnel stability in tunnel engineering. The back analysis method has been widely used as an indirect method for determining rock mass parameters based on field measurements. However, most back analysis methods are generally time-consuming for numerical simulation and are merely based on the measured displacement, which leads to the identification of rock mass parameters that cannot fully reflect the characteristics of the surrounding rock. To improve the accuracy of the estimation of rock mass parameters, this paper presents a back analysis method based on multi-output support vector regression (MSVR) and differential evolution (DE) algorithms. Firstly, the global sensitivity analysis of rock mass parameters is analyzed using the elementary effects method. Numerical simulation is then carried out to prepare training samples. DE algorithm is used to determine the optimum hyperparameters of MSVR. Based on the monitoring data, the rock mass properties of the selected sensitive parameters are estimated by the constructed MSVR model. A high-speed railway tunnel is utilized to demonstrate the effectiveness of the MSVR with the DE algorithm (DE-MSVR). The results show that the DE-MSVR with mixed monitoring data of vault settlement, convergence, and rock mass stress has higher forecasting performance than these models with a single type of monitoring data. It is feasible to use the monitoring data at the early stages combined with the numerical simulation for parameter back analysis. Moreover, the comparison results show that the presented method exhibits higher prediction accuracy than the existing back analysis models.

Inertial and turbulent flow in hydro‐mechanically coupled KGD‐like fractures

AbstractFluid flow in deformable fractures is important to many applications including hydraulic stimulation. Fracture flow simulations typically rely on the Poiseuille flow model which presumes laminar quasi‐steady‐state flow conditions with negligible inertia. The high flow rates involved in industrial applications bring these assumptions into question. The GG22 flow model is a new method to capture inertial effects and turbulent flow phenomena with a moderate increase in computational complexity. Here we develop and verify the first hydro‐mechanically coupled finite element – finite volume model for fracture flow which uses the GG22 model. We use the model to examine fracture flow between oscillating elastic plates and show that inertia may elicit phase‐shifts in the fluid response, larger fluid pressures and rock mass stresses, and induce wave‐like behaviour even in a quasi‐static rock mass. We then apply the model to hydraulic stimulation of a cemented fracture in both the viscous and toughness dominant propagation regimes and examine the influence of inertia and turbulence compared to Poiseuille flow. We demonstrate that inertial effects due to the variation of aperture are negligible in two‐dimensional KGD‐like fractures with constant injection rates. If the flow rates are large enough to invoke turbulent flow behaviour, then significantly different fracture propagation behaviour is observed. The modelling established in this article will serve as the basis to examine the role of inertia and turbulence in axisymmetrical radial fractures and dynamic stimulation with pressure pulsing where we expect the role of inertia to be more significant.

Mechanical Behavior and Crack Evolution of Goaf Surrounding Rock with Different Roof-Contacted Filling Rates.

The goaf formed by mining is filled and treated, which greatly improves the safety and stability of the surrounding rock. During the filling process, the roof-contacted filling rates (RCFR) of goaf were closely related to the stability control of the surrounding rock. The influence of the roof-contacted filling rate on the mechanical characteristics and crack propagation of the goaf surrounding rock (GSR) has been studied. Biaxial compression experiments and numerical simulation experiments were conducted on samples under different operating conditions. The results were as follows: (1) The peak stress, peak strain, and elastic modulus of the GSR are closely related to the RCFR and the goaf size; they increase with the increase of the RCFR, and decrease with the increase of the goaf size; (2) In the initial loading stage, a small number of cracks are generated, and the acoustic emission ringing count increases slowly. The mid-loading stage is the crack initiation and rapid expansion, and the cumulative ring count curve shows a "stepwise" growth. In the later loading stage, cracks continue to propagate and form macroscopic fractures, but the number of rings significantly decreases; (3) Shear cracks are prone to occur in the rock part of the GSR; tensile cracks are prone to occur in the backfill; and the crack propagation speed in the rock is faster than in the backfill. Stress concentration is the direct cause of GSR failure. The maximum concentrated stress of rock mass and backfill is 1~2.5 times and 0.17~0.7 times of the peak stress of the GSR, respectively.

Prediction of Grade Classification of Rock Burst Based on PCA-SSA-PNN Architecture

The uncertainty and complexity of rock burst brings great difficulties to the prediction of rock burst grades. In order to estimate the risk grades of rock burst, an integrated method combining principal component analysis (PCA) and sparrow search algorithm (SSA) with probabilistic neural network (PNN) was proposed. Considering that the in situ stress of rock mass, the strength of rock, and the strength of rock mass are the key influencing factors of rock bursts, the maximum in situ stress σ max , maximum tangential stress σ θ , rock strength σ ci , rock mass strength σ cm , and three rock burst evaluation indexes ( σ θ / σ ci , σ ci / σ max , and σ cm / σ max ) were selected to constitute the rock burst grade evaluation index system. Forty-three groups of rock burst engineering data were gathered. After preprocessing the rock burst data using PCA, four of the new linearly independent indexes PCA1, PCA2, PCA3, and PCA4 were obtained for estimating rock burst grades. The SSA was utilized to optimize the smoothing factor in the PNN. Using PCA-SSA-PNN-based architecture, a new multi-index rock burst grade prediction method was proposed. The results from the new multi-index rock burst grade prediction method were compared with those from single- and multi-index prediction methods. It shows that the predictions from the multi-index rock burst prediction methods are closer to the actual rock burst grades than that from the single-index rock burst prediction methods; compared with other multi-index rock burst prediction methods, the prediction accuracy of PCA-SSA-PNN is greater (up to 90%) and more available in the prediction of rock burst grades. The results presented herein may provide reference for the rock burst warning.

Large underground structures geometry evaluation based on point cloud data analysis

As a result of underground exploitation performed by blasting technique or mechanical excavation large underground structures are created. Usually, due to the mining and geological conditions of the exploitation, obtained excavations geometry may be far from this assumed at the design stage. The geometry of the excavations is of key importance in several important stages of the technological process. Taking into account economic issues, deviations from the planned cross-section of the excavation generate additional, often considerable operating costs. If the cross-sectional area is greater than the assumed one, the ore may be impoverished, since the dimensions of mining faces are usually following the dimensions of the deposit zone. On the other hand, when the cross-sectional area is smaller than assumed, to maintain the proper operation of the mining plant, it is necessary to re-cut the excavation. It is particularly important from a mine ventilation point of view, as it generates problems in supplying adequate amounts of air to work areas, as well as transport, causing a significant problem for moving machines. Underground mine workings tend to change over time as a result of processes taking place in the rock mass aimed at establishing a new stress and strain state of rock mass after the excavation has been made. Considering the extent of underground mining structures, there is a need to develop a method that would allow the assessment of the geometry of workings, both newly created and existing ones, with the assumption of quick acquisition of measurement data, their processing, and analysis. Addressing those needs, in this paper, the authors are presenting a method based on a 3D point cloud of excavation geometry obtained thanks to measurements with the use of Terrestrial Laser Scanner/LiDAR, the procedure used to convert the point cloud into a spatial model of excavations, as well as data processing procedures used to extract informative parameters describing the geometry. Several statistical parameters are proposed for feature analysis along the tunnel length.

Numerical simulations on shear behaviour of rock joint network under constant normal stiffness conditions.

In this study, the numerical direct shear tests were conducted to investigate the shear mechanical properties of joint networks under constant normal stiffness (CNS) boundary conditions. The influence of random joint number on shear stress (τ), dilation (normal displacement, δv) and normal stress (σn) of rock mass were studied quantitatively with fixed main slip surface. At the same time, the internal stress evolution process and failure process were analyzed. The results reveal that the number of random joints (γ) has little effect on the shear and normal stresses. The normal displacement of the sample generally decreases as the number of random joints increases. In addition, the normal displacement of the specimen is absorbed by the random joints when the number of random joints in the specimen increases to a certain level: when γ is greater than 6 and the shear displacement (μ) reaching 10 mm, the specimen exhibits shear contraction. Therefore, the internal random joints mainly control the failure mode and dilatancy performance of the specimen, while the main joint of the rock controls the shear stress of the specimen.

Геомеханическое обоснование формирования зон хрупкого разрушения пород в окрестности сопряжений горных выработок рудников АО «Апатит»

Introduction. Prediction of excavations stability is an important step towards ensuring geo-mechanical safety during their construction. A reliable prediction of the stability of excavations includes such factors as the initial stress state, physical and mechanical characteristics of rocks, rock mass fracturing, and a number of others. These factors should be considered when performing calculations. The geo-mechanical conditions of the Apatite deposits are different, rock pressure can manifest itself in the form of brittle fracture of rocks, limited displacement of the rock contour, the formation of local rock failures or arching. In this paper, we consider only areas of the rock mass, where the loss of stability of the excavation contour is associated with the formation of brittle fracture zones. An analysis of methods for predicting brittle fracture zones suggests that they are devoted to their formation in the vicinity of single excavation and do not consider the formation of brittle fracture zones in the areas of excavation junctions. Despite the fact that a significant number of research papers are devoted to the formation of plastic deformation zones and the assessment of the excavation contour stability, they mainly consider two forms of geo-mechanical processes - rock failure along fractures and rock contour deformations. At the same time, little attention is paid to the formation of brittle fracture zones in the areas of excavation junctions in rocks predisposed to the development of such geo-mechanical processes. Considering the conditions of the Apatite deposits, it can be noted that the values of deformational and strength characteristics of rocks are very high. The rock mass is different in fracturing characteristics, however, the volume of rocks with excavations, which can be classified as weakly fractured, is very significant. The initial stress state of rock mass is characterized by the tectonic nature, where the magnitudes of the largest horizontal stresses are several times greater than the vertical stress component. All this makes it possible to classify the conditions of Apatite deposits as very difficult and rock burst hazardous. Methods and materials. To assess the correctness of the numerical modeling using model of brittle fracture of rocks, the following typical problems are considered: forecasting the development of brittle fracture zones in the vicinity of excavations of research laboratory in Canada; forecast of zones of brittle fracture of rocks for mining-geological and geo-mechanical conditions of the Apatite enterprises. Numerical modeling and the stress-strain state prediction in the vicinity of excavations was carried out in a generalized plane-strain formulation (three-dimensional formulation with the assignment of appropriate boundary conditions). An analysis of numerical simulation results suggests that the increase in the size of the brittle fracture zone is that greater, the larger the junction span. This effect was obtained for all four variants of excavation junction. The following results of the stress-strain state formation in rock mass in the vicinity of excavation junctions were obtained: the size of the brittle fracture zone as a whole tends to increase in the areas of excavation junctions, while the type of junction plays a significant role in its size; the higher the value of the σmax index, the more clearly the influence of the junction type on the size of the brittle fracture zone is manifested, while at the ratio σmax/σc < 0.8, the difference in the sizes of the zones in the junction and the extended section of the excavation is not significant. It was noted that the excavation junction has an impact not only on the size of the brittle fracture zone, but also on the nature of this zone formation. There is a “flow” of the brittle fracture zone from the junction area directly to the single excavation, the direction of which coincides with the direction of the maximum principal stresses. Thus, it is possible to form rock failure in the excavation located in a moderately stressed rock mass; The proposed model for predicting brittle fracture zones based on setting the functions of hardening/softening of the medium, makes it possible to effectively predict brittle fracture zones in the vicinity of excavation junctions. In general, the research results confirm the patterns of brittle fracture zones development in the vicinity of excavation junctions obtained by the authors, however, an improved model for describing the development of geo-mechanical processes in rocks prone to brittle fracture made it possible to significantly increase the reliability of predicting the shape of the brittle fracture zone near the excavation.

Application of artificial neural networks for stress state analysis based on the photoelastic method

The present article proposes an evolutionary development of the photoelasticity method for measuring stresses based on annular photoelastic sensors application along with stress pattern recording with the aid of a digital camera and its recognition using artificial neural networks. The analysis of the modern application of the photo-elasticity method for various problems within the theory of strength is presented. The principle of operation of photoelastic sensors based on the photoelasticity effect is considered. Optical patterns in an annular photoelastic sensor are presented for various values of the horizontal stress. The calculation of the stress state of the sensor for the following full-scale experiment has been performed, the estimate of the threshold conditions under which the sensor can be applied has been performed. As a result of a laboratory experiment, a dataset of 1500 isochromatic images has been assembled. A subspecies of a neural network, namely a convolutional neural network, has been applied as a machine learning algorithm. Different combination of models and optimizers have been employed. The application of downhole sensors for continuous monitoring of alterations in the rock mass stress state and the integration of this data into a digital field model based on Internet of Things technologies has been proposed.

Effects of intermediate stress on deep rock strainbursts under true triaxial stresses

The effect of intermediate stress (in situ tunnel axial) on a strainburst is studied with a three-dimensional (3D) bonded block distinct element method (DEM). A series of simulations of strainbursts under true triaxial in situ stress conditions (i.e. high tangential stress, moderate intermediate stress and low radial stress) of near-boundary rock masses are performed. Compared with the experimental results, the DEM model is able to capture the stress-strain response, failure pattern and energy balance of strainbursts. The fracturing processes of strainbursts are also numerically reproduced. Numerical results show that, as the intermediate stress increases: (1) The peak strain of strainbursts increases, the yield stress increases, the rock strength increases linearly, and the ratio of yield stress to rock strength decreases, indicating that the precursory information on strainbursts is enhanced; (2) Tensile and shear cracks increase significantly, and slabbing and bending of rock plates are more pronounced; and (3) The stored elastic strain energy and dissipated energy increase linearly, whereas the kinetic energy of the ejected rock fragments increases approximately exponentially, implying an increase in strainburst intensity. By comparing the experimental and numerical results, the effect of intermediate stress on the rock strength of strainbursts is discussed in order to address three key issues. Then, the Mogi criterion is applied to construct new strength criteria for strainbursts by converting the one-face free true triaxial stress state of a strainburst to its equivalent true triaxial stress state. In summary, the effect of intermediate stress on strainbursts is a double-edged sword that can enhance the rock strength and the precursory information of a strainburst, but also increase its intensity. ©2022 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Quantitative assessment of rock mass stress–strain behavior at Severomuysky Tunnel

The article presents the selected geotechnical engineering studies accomplished in 2021–2022 by the Research Center for Applied Geomechanics and Convergent Technologies in Mining, College of Mining, NUST MISIS for the reconstruction project of Severomuysky Tunnel (ST) of the Baikal–Amur Mainline. The studies included the quantitative and qualitative geotechnical assessment of rock mass, its stress–strain analysis and susceptibility to rock bursts. Within the rock mass quality rating of ST, using the geological logs on 441 sample boreholes, the lab-scale test data on the physical and mechanical properties of 1820 rock samples were processed. The average values of the rock mass quality rating are presented for three geotechnical elements. The stress–strain analysis of ST rock mass and the lab-scale tests of rock brittleness made it possible to detect sites susceptible to rock bursts along the transportation tunnel, and to develop appropriate geotechnical monitoring activities. The recommendations are given to implement the described geotechnical surveys in construction and reconstruction of analogous objects and facilities, and in solid mineral mining.  The geotechnical research and other operations were carried out under guidance of RSRS GmbH Railway Infrastructure Projects CEO E. V. Dorot and the Company’s experts. The authors appreciate participation of the Project Engineers of the Research Center for Applied Geomechanics and Convergent Technologies in Mining, College of Mining, NUST MISIS N. G. Vysotin, A. M. Yanbekov, M. A. Kosyreva and S. S. Shermatova.

Influence of complicated faults on the differentiation and accumulation of in-situ stress in deep rock mass

Influence of complicated faults on the differentiation and accumulation of in-situ stress in deep rock mass

Development and application of a borehole stress meter in rocks surrounding the roadway, based on optical-fiber sensing technology

Stress in rock masses is an important parameter in the design and construction of underground engineering, such as the design and maintenance of mine roadways and the design of mining working faces. It is also a fundamental force causing the deformation and failure of geotechnical engineering excavation. At present, the abutment-pressure monitoring technology of the surrounding rocks of the coal mine roadway in China is not intelligent and systematic and lacks some high-precision sensing instruments and multi-functional monitoring systems. The mechanical model of the rocks surrounding the borehole was constructed by theoretical analysis of problems in the stress monitoring technology for underground rock masses in coal mines. Additionally, the interaction between the surrounding rocks and the borehole stress meter was analyzed. The borehole stress meters for tubular-structure fiber Bragg grating (TS-FBG) and cystic-structure fiber Bragg grating (CS-FBG) were designed by combining the sensing principle and sensing characteristics of fiber Bragg grating, and the performance of the two kinds of fiber Bragg grating borehole stress meters was compared by laboratory test. The track roadway of the 14,301 tested working faces in the Shaqu Coal Mine was taken as an example, and the stress of the rocks surrounding the 14,301-track roadway was monitored in real time by CS-FBG borehole stress meter during the mining of the working face. The following conclusions are drawn from the field application. The rig-site utilization results revealed obvious stress growth and stress peak zones in the mining-stress change curves of each measuring point on the two sides of the 14,301-track roadway in the process of mining the tested working face. Additionally, there were four stages: rapid rise, uniform growth, rapid rise to the peak, and rapid decline. Maximum stress monitored by the second station was 18.5 MPa, and the influence range of stress was over 140 m. Maximum stress monitored by the first measuring station was 19 MPa, the influence range of stress was about 80 m, and the peak stress position was about 20 m in front of the coal wall. Rig-site utilization proved the design of the CS-FBG borehole stress meter to be reasonable. Performance was stable and reliable, and the successful operation of field monitoring achieved the expected effect.

Study on the whole process application of advanced grouting pipe shed support under urban complex stratum conditions

The stability control of surrounding rock is always one of the important problems in tunnel construction. As a representative of tunnel pre-construction technology, advance grouting pipe roof support method is widely used in underground space engineering under various complex geological conditions, especially in strata with many fractured rock masses and rich groundwater content. Due to the complexity of the application of advance grouting pipe roof method, few studies have been conducted on the whole process of pipe roof application in actual projects. Taking the shield departure project of Guangzhou Metro Line 13 as the background, this paper uses the modified BQ method to grade the surrounding rock environment of the tunnel from the perspective of the stability evaluation of the surrounding rock of the tunnel. The result shows that the revised surrounding rock is grade v. further combining with the geological exploration report, it is found that the rock mass joints and fissures are large and may further develop under the change of the surrounding rock mass stress environment, and the formation water content is large. The surrounding rock is prone to large deformation under the action of groundwater, and even lead to surface collapse in serious cases. Therefore, it is necessary to adopt the advance grouting pipe roof support method. Subsequently, the influence of different pipe roof design parameters such as pipe roof diameter, number of pipe roofs, reinforcement thickness and layout range on its support effect is analyzed by using finite element software, and appropriate parameter values are determined and applied to the background project. Finally, considering the measurement error existing in actual monitoring, a dynamic safety evaluation method of tunnel structure based on reliability theory is proposed, and the background project is analyzed and evaluated by this method. The analysis shows that the tunnel structure with advance grouting pipe roof support meets the safety requirements, confirms the effectiveness of the design parameters, and can provide reference for tunnel construction under the same geological conditions.

Study on the dynamic response and the hazard of rock burst under the influence of fault slip

In the process of coal mine production, rock burst accidents near faults occur frequently. In order to further reveal the induced mechanism of fault slip rock burst, a numerical simulation method for dynamic calculation of fault slip under the influence of mining is proposed, and the law of fault slip and dynamic response characteristics of the stope are studied. The study shows that with the working face getting closer to the fault, the fault activity and the number of high-energy microseismic events increase significantly, and the fault slip dynamic load has a significant impact on the vibration velocity and abutment stress of coal and rock mass near the stope. Under the effect of a strong fault slip dynamic load, the coal and rock mass in the area with peak abutment stress undergo plastic failure and release a large amount of energy, and the potential of fault slip rock burst is high. The evaluation index Icd of rock burst hazard under fault slip dynamic load is proposed, and the rock burst hazard of working face passing through the fault is analyzed. The research results provide a theoretical basis for further revealing the induced mechanism and hazard assessment of fault slip rock bursts.

Research on Mesoscopic Characteristics of Limestone Grouting Diffusion Based on Discrete Fracture Network

The limestone stratum has well-developed rock fissures and abundant groundwater. Under extreme rainstorm conditions, tunnel surrounding rock seepage is serious especially. In order to reveal the grouting diffusion mechanism of karst fractured rock mass and realize effective water plugging, in this paper, the basic mechanical parameters of limestone are obtained by laboratory triaxial test, and the porosity distribution of rock mass is obtained by nuclear magnetic resonance test. The PFC2D discrete element software is used to simulate the mechanical characteristics of compressive deformation and fracture propagation pattern of the limestone and compared with the triaxial test results. Discrete fracture network theory is used to establish PFC2D numerical simulation of grouting diffusion model of fractured rock mass. And the filling diffusion mechanism and stress variation rule of cement slurry in fractured limestone under different grouting pressure are studied. The results show that (1) During the grouting process, the micro-cracks expand along the tip of the existing cracks, and extend to the surrounding cracks to finally form through cracks, so as to achieve the effect of grouting reinforcement. (2) With the increase of grouting pressure, the maximum displacement of particles increases continuously, but with the prolongation of grouting time, the maximum displacement of particles reaches a stable state and no longer increases. (3) During the grouting process of the fractured rock mass, with the increase of grouting time, the stress of the fractured rock mass finally reaches a stable state. This study provides an important reference for the setting of grouting parameters of limestone rock mass.

Research on initiation stress of jointed rock mass under triaxial compression test

Research on initiation stress of jointed rock mass under triaxial compression test

Equivalent discontinuum analysis of stress and displacement in underground rock masses

The complexity and high frequency of fractures in rock masses make discrete fracture network (DFN) models suffer from burdensome computational consumption when every fracture is explicitly simulated. Meanwhile, the uncertainty in the spatial distribution of fractures also leads to the unnecessary explicit simulation of small fractures to achieve more accurate results. In this study, an equivalent discontinuum analysis (EDA) method is proposed to deal with complexly fractured rock masses in the deep underground. An equivalent discrete fracture network (E-DFN) with simple geometric configurations is extracted based on a defined significance index of fractures. The influence of fractures that are neglected in E-DFNs is superposed on the remaining fractures by reassigning their material properties. Using the proposed EDA method, the anisotropy of the deformation modulus of the original DFN model is well represented by the E-DFN models. Compared with the equivalent continuum and discontinuum models, the E-DFN models with reassigned fracture properties have a better potential for maintaining stress variability and displacement discontinuity in rock masses.

A control method for hydraulic fracturing of the hard roof with long and short boreholes

During the mining of the working face, the hard roof can lead to a series of problems, such as air leakage in goafs, gas emission and concentration, casualties, and damages to buildings and ecological environment. The treatment of the hard roof has always been a difficult problem in major coal-producing countries. Although many scholars have conducted some theoretical research and field practice in this field in recent years, the aforementioned safety problems still occur frequently. In order to understand the mechanism of crack propagation caused by hydraulic fracturing of the roof, based on the theoretical analysis of linear elastic fracture mechanics and coal rock mechanics, the shear fracture criterion was determined for the formation of horizontal fractures in the natural cracks. The anisotropy of the crack tip, the adhesion forces in the coal and rock fracture surface and the conditions for hydraulic-induced crack propagation were also considered. The formation of cracks involves a vertical main crack and multiple extended-airfoil branch cracks which interconnect transversely and longitudinally. Under the effect of hydrodynamic water wetting and hydraulic fracturing, the stress of coal and mass changed. Meanwhile, the mechanical parameeters, such as compression, tensile strength and elasticity modulus and etc., were weakened, thus achieving the goal of structural transformation and slight weakening of coal and rock mass. According to the bidirectional stress of coal and rock mass, the maximum principal compressive stress of crack opening was calculated, and the propagation mechanism of hydraulic fracturing cracks in the hard roof was obtained. The hydraulic fracturing technology for controlling the hard roof was illustrated and has been applied in No. 2 Well of Sihe Coal Mine for many years. The research results could provide reference for the control of the hard roof in other similar mines.

Research on Rheological Energy Characteristics of Fractured Sandstone Strengthened with CFRP

The rheological energy characteristics and evolution law of fractured sandstone strengthened with CFRP (carbon fiber reinforced plastic) were studied to solve the problem of rheological failure of rock after engineering excavation disturbance. In this paper, the graded loading rheological tests of fractured sandstone after reinforcement are carried out. The results show that the failure rheological stress of sandstone increases with the increase of the area strengthened by CFRP. When the reinforcement area is from 3140 mm2 to 4710 mm2, the failure rheological stress of rock mass is increased from 65 MPa to 75 MPa, approximately 15.4%. Except for the initial rheological stage, the elastic energy is decreasing and the dissipative energy is increasing. The elastic energy is fully released, and the dissipative energy is provided by the total energy when the rock is destroyed. The energy dissipation ratio (Ud/U) of sandstone under the two reinforcement areas reached the minimum value at the stable rheological stage, which was 0.26 (A = 3140 mm2) and 0.42 (A = 4710 mm2) respectively. The energy mechanism of CFRP is that CFRP stores energy mainly and consumes energy secondly before the energy inflexion. However, CFRP switches to consuming energy mainly and storing energy secondly after the energy inflexion. The energy storage coefficient of CFRP can directly describe the function of elastic energy or dissipative energy of CFRP under arbitrary stress. When the energy storage coefficient T > 1, the damage of CFRP is small. Further, the energy storage coefficient reaches the maximum value of 5 at the energy inflection point (55 MPa). When the energy storage coefficient T < 1, the damage of CFRP is large, and the energy storage coefficient reaches the minimum value of 0.005 at the stress of 40 MPa. During the rheological process of fractured sandstone strengthened with CFRP, the energy evolution shows the relationship between elastic energy and dissipated energy. The energy mechanism of CFRP explains the working mechanism of the reinforced structure in the rheological process and plays a guiding role in the analysis of the rheological failure of the reinforced rock in the practical engineering.