Deep Rock Mass Research Articles

Damage Behavior Study of Specimens with Double-Prefabricated Cracks under Dynamic–Static Coupling Loads

The surrounding rock of the deep-buried chamber contains high-ground stress and initial cracks. Under a dynamic load, cracks will develop and expand, leading to the fracture and collapse of the confining pressure. Therefore, it is essential to study the failure process of fissured surrounding rock under the joint action of static stress and a dynamic load. In this paper, samples with cracks are used to simulate the defective rock mass. Similar modeling tests and numerical simulation studies were carried out to reveal the damage process of cracked deep rock mass under dynamic disturbance and investigate the impact threshold of rock mass damage under a certain level of hydrostatic pressure. The model test investigated the damage behavior for specimens with double-prefabrication cracks under pressure from a dynamic–static coupling load. The influence of the mechanisms of the angle of a crack, the initial static pressure, and impact capacity on specimen damage was analyzed. It was perceived that, with an increase in the angle of the crack, the omen of specimen damage is less obvious, and the specimen is subjected to sudden damage. On this basis, the damage process of the specimen containing prefabricated cracks under combined dynamic and static loads is realized through numerical simulation, and tests verify the accuracy of the results. The analysis allowed us to come up with a variation rule for the single-disturbance energy threshold for specimens with a prefabricated crack angle and the initial static load level of the specimen containing double-prefabrication cracks. The study lays the foundation for the future analysis of any deep rock mass failure process under dynamic disturbance and the protection of a deeply buried chamber.

The time-dependent deformation and damage constitutive model of rock based on dynamic disturbance tests

Abstract In the deep rock excavation process, the rock will produce deformation and damage under multiple dynamic disturbances, while there is also unrecoverable time-dependent deformation. In order to research this deformation characteristic, a sine wave disturbance triaxial loading test was carried out on red sandstone to simulate the failure process of deep underground rock mass under dynamic disturbance loads with low confining pressure. Based on the results that the deformation of the rock increases suddenly during dynamic disturbances and varies with the number of disturbances, a novel disturbance damage model relating the number of disturbances to the deformation of the disturbances is created to describe the deformation and damage accumulation of the rock under multiple disturbances and an operator is developed to ensure that the model works. In order to describe the time-dependent deformation of rocks, the elasticity model in the traditional Bingham model was improved to a nonlinear elasticity model that varies with time, and its viscous and plasticity models were retained. The time-dependent deformation and damage constitutive model is obtained by combining the improved Bingham model with the disturbance damage model. The model parameters were identified according to the test data, and the finite-element calculation of the model was realized with the secondary development program. The results show the strain of rock increases suddenly under multiple disturbances, and the main reason for rock damage is the action of dynamic disturbances. The fitted curves and finite-element results are consistent with the experimental results. The time-dependent deformation and damage constitutive model of rock not only reflects the decaying rheological and steady-state rheological properties of rock under static loading but describes the characteristics of strain surge and disturbance damage accumulation caused by dynamic disturbances. This article contributes to the characteristics of the deformation of deep rock mass.

Evaluating the structure and mechanical properties of deep rock masses based on drilling process monitoring and borehole televiewer

Evaluating the structure and mechanical properties of deep rock masses based on drilling process monitoring and borehole televiewer

Fracture evolution characteristics of strainburst under different gradient stress

AbstractStrainburst often occurs in the loading process of tangential stress concentration from surrounding rocks after excavation and unloading of deep rock mass. The concentrated tangential stress is relatively larger on the tunnel walls, which decreases towards the interior of surrounding rocks with a certain gradient. To explore the fracture evolution characteristics of surrounding rocks under different tangential gradient stresses and understand various phenomena in the strainburst process, a large‐scale true‐triaxial gradient and hydraulic‐pneumatic composite loading rockburst test device was adopted to carry out the strainburst tests under four different gradient stresses. Based on the macroscopic failure phenomena, distribution of rockburst debris, macro‐micro morphology of failure cross section, and monitored data of acoustic emission (AE), the fracture evolution of rockburst specimens was analyzed under different stress gradient loadings. The results indicate that the stress gradient has an obvious influence on the fracture characteristics, resulting in different rockburst phenomena. With the increase of stress gradient, the failure stress of rockburst decreases, while the dynamic failure characteristics of rockburst trends to be significant. The increase of stress gradient contributes to higher fragmentation degree of debris, more uniform distribution and better continuity. The total number of cracks in the model decreases with the rise of the stress gradient, and the proportion of shear failure increases, which is the main reason for the enhancement of the rockburst intensity.

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.

Reactive Transport Modeling of Chemical Stimulation Processes for an Enhanced Geothermal System (EGS)

An enhanced geothermal system is a kind of artificial geothermal system, which can economically exploit geothermal energy from deep thermal rock mass with low permeability by artificially created geothermal reservoirs. Chemical stimulation refers to a reservoir permeability enhancement method that injects a chemical stimulant into the fractured geothermal reservoir to improve the formation permeability by dissolving minerals. In this study, a reactive solute transport model was established based on TOUGHREACT to find out the effect of chemical stimulation on the reconstruction of a granite-hosted enhanced geothermal system reservoir. The results show that chemical stimulation with mud acid as a stimulant can effectively improve the permeability of fractures near the injection well, the effective penetration distance can reach more than 20 m after 5 days. The improvement of porosity and permeability was mainly caused by the dissolution of feldspar and chlorite. The permeability enhancement increased with the injection flow rate and HF concentration in the stimulant, which was weakly affected by the change in injection temperature. The method of chemical enhancement processes can provide a reference for subsequent enhanced geothermal system engineering designs.

Study on influencing factors of radon exhalation from coal measures in the northern margin of Ordos Basin

Coal-uranium synergistic development has become an emerging research area in recent years. Radon gas emitted by uranium decay in the stratum may be used as an indicator to study the uranium-bearing area. Investigating the relationship between radon exhalation law and the depth, lithology, and mineral composition of coal measures is very important. This paper analyses the influencing factors and mechanism of radon release in coal measure strata in the Ordos Basin which is rich in coal and uranium co-occurrence deposits, by drilling a core at a depth of 18.3–84 m in the mining area, and using low-field nuclear magnetic resonance (NMR), polarizing microscope and environmental radon meter tests. The results show that the proportion of micropores decreases with an increase in particle size, and the proportion of macropores and porosity increases. High porosity has a greater effect on the release of radon gas, resulting in a higher exhalation rate in samples with coarse particle sizes. The radon exhalation rate of sandstone is higher than that of coal, and the radon concentration increases with the improved sealing property of deep rock and soil masses. The radon exhalation rate of sandstone is strongly influenced by feldspar and is positively correlated with each other. The radon exhalation was aided by the submicron structure created by feldspar alteration. The research results may be used as a reference for rock radiation risk assessment in mining areas.

Deep Rock Blasting Using Decoupled Charge with Different Coupling Mediums

Blasts using decoupled charge with various coupling mediums are extensively operated in underground excavation, and blasting challenges are always encountered in deep rock mass due to the influence of high in-situ stress. In the present study, the dynamic responses of deep rock mass in blasting with decoupled charge and different coupling mediums (air, dry sand, wet sand, and water) are theoretically and numerically investigated. First, the transmission, propagation, and superposition of stress induced by blasting coupled with different coupling materials, and the effect of static stress distribution around boreholes on the crack initiation and propagation in blasting under conditions of varying hydrostatic pressure and anisotropic in-situ stress are theoretically analyzed with a two-hole planar analytical model. Then, a numerical model is constructed and verified against the rock fracture network obtained from a blasting test. Based on the verified numerical model, two-hole planar computational models with the same configuration as the analytical model are developed and used to perform a series of simulations. The joint effect of the coupling medium and the in-situ stress on the initiation, propagation, and interconnection of blast-induced cracks are numerically analyzed and interpreted by combining with theoretical results. The analytical and numerical investigations indicate that the evolution of blast-induced cracks in deep rock mass is mainly controlled by the stress transmitting from explosive to rock, superposition of explosion stress waves, as well as the magnitude and orientation of in-situ stress. Finally, the implications of current findings for practical blasting in deep rock mass are discussed. This study develops the understanding of rock cracking induced by blasting with different coupling mediums under high in-situ stress and provides some guidance to solve blasting difficulties in deep rock mass.

Discussion on Blasting Vibration Velocity of Deep Rock Mass Considering Thickness of Overlying Soil Layer

The thickness of the overlying soil layer has a certain influence on the blasting vibration response of deep rock mass. The vibration wave velocity of the overlying soil layer during the construction of deep blasting is measured in this paper. Based on the measured data, parameters k and α of the Sadowski equation are used to characterize the influence of the comprehensive geological conditions of the site on the vibration wave propagation. The model of blasting vibration velocity of deep rock mass is established according to the existing blasting theory, and the calculation accuracy of the model is verified according to the field blasting parameters. The new model is used to simulate different overlying soil thicknesses, and the safe allowable distance under different soil thicknesses is calculated. The calculated results show that with the increase of the thickness of the overlying soil layer, the blasting vibration velocity decreases and the attenuation velocity decreases gradually. The research results reveal the reduction effect of overlying soil thickness on blasting vibration to some extent. In the area with overlying soil layer, the safe allowable distance of blasting vibration safety can be appropriately reduced to increase the land utilization rate, which has important reference value for the blasting design and safety prediction of deep rock mass.

Special Issue on Experimental Investigation and Numerical Modeling of Rock Brittle Failure Behavior under High Stress Conditions

To meet the demands of the mining, hydropower, and transportation industries, deep rock mass engineering in China has rapidly developed [...]

Numerical analysis of unstable propagation of three-dimensional parallel hydraulic fractures induced by interferences of adjacent perforation clusters and thermal diffusion

PurposeThe purpose of this study is to investigate the unstable propagation of parallel hydraulic fractures induced by interferences of adjacent perforation clusters and thermal diffusion. Fracture propagation in the process of multistage fracturing of a rock mass is deflected owing to various factors. Hydrofracturing of rock masses in deep tight reservoirs involves thermal diffusion, fluid flow and deformation of rock between the rock matrix and fluid in pores and fractures.Design/methodology/approachTo study the unstable propagation behaviours of three-dimensional (3D) parallel hydraulic fractures induced by the interferences of adjacent perforation clusters and thermal diffusion, a 3D engineering-scale numerical model is established under different fracturing scenarios (sequential, simultaneous and alternate fracturing) and different perforation cluster spacings while considering the thermal-hydro-mechanical coupling effect. Stress disturbance region caused by fracture propagation in a deep tight rock mass is superimposed and overlaid with multiple fractures, resulting in a stress shadow effect and fracture deflection.FindingsThe results show that the size of the stress shadow areas and the interaction between fractures increase with decreasing multiple perforation cluster spacing in horizontal wells. Alternate fracturing can produce more fracture areas and improve the fracturing effect compared with those of sequential and simultaneous fracturing. The larger the temperature gradient between the fracturing fluid and rock matrix, the stronger the thermal diffusion effect, and the effect of thermal diffusion on the fracture propagation is significant.Originality/valueThis study focuses on the behaviours of the unstable dynamic propagation of 3D parallel hydraulic fractures induced by the interferences of adjacent perforation clusters and thermal diffusion. Further, the temperature field affects the fracture deflection requires could be investigated from the mechanisms; this paper is to study the unstable propagation of fractures in single horizontal well, which can provide a basis for fracture propagation and stress field disturbance in multiple horizontal wells.

Stress solution for an arbitrarily shaped tunnel with a neighboring circular cavity

This paper gives an analytical stress solution for the plane strain problem that the deep rock mass contains an arbitrarily shaped tunnel and a circular cavity subjected to in situ stresses. Based on the discovery that the mapping function for an arbitrarily shaped hole can also manage a tunnel and a circular hole mapping into two circles, the plate bounded by the two holes is conformally mapped onto an annulus. The complex variable method is used to establish the stress boundary condition, and then the two analytical functions are solved by the power series method. Comparison with ANSYS solution is made to validate the correctness of deductions and solving process. To consider practical underground engineering, the cases of different lateral stresses, distances of the two holes, sizes, and positions of the cavity are discussed.

Determination of geo-stress in deep strata incorporating borehole diametral deformation measurement and overcoring

In deep energy engineering such as coal mining, oil & gas extraction, underground hydropower projects and deep buried water tunnels, rock failure and large deformation hazards often occur due to the complicated geological conditions and high geo-stress. Knowledge of the geo-stress state is an essential content in deep rock engineering. The complex environment of high temperature and high hydraulic pressure in deep rock mass makes geo-stress measurement more difficult than shallow strata. In this study, a novel method for geo-stress measurement based on the cross-sectional borehole diametral deformation is proposed. A borehole diametral deformation gage (BDD gage) is developed based on the measurement method. The scheme design of multi-contacts and micro-optical imaging avoid the influence of the high temperature and high hydraulic pressure environment. The results of the calibration experiment show that the measurement accuracy of BDD gage reaches 3 μm. Incorporating with the wireline coring tool for rapid overcoring, geo-stress measurement in deep vertical borehole via stress relief is realized. Field test procedure is summarized and application is carried out in a deep borehole of limestone stratum in Guizhou Province, China. Measurement process is verified to be rapid and simple. Measurement results including magnitude and direction of principal stresses are consistent with the results from hydraulic fracturing method. Special discussions were conducted on the measurement error of BDD gage and the improvement design of wireline coring tools. This article provides new ideas for geo-stress measurement in deep borehole.

Effect of prestress on propagation of blasting-induced main crack in specimens with empty hole

In-situ stresses have a significant effect on blast results in deep rock mass. In this paper, firstly, the effect of a prestress (similar to an in-situ stress) on the propagation of blasting-induced main crack in the specimens with an empty hole was studied by dynamic caustic method, considering reflected stress wave. Then, the effect of prestress on the stresses around the empty hole during blasting was studied using numerical analysis. The results show that the prestress can significantly increase (1) the hoop tensile stresses on the upper and lower side of the empty hole, (2) the length of the main crack, (3) the dynamic stress intensity factor (DSIF), (4) the propagation velocity of the main crack, and (5) the crack arrest time when the prestress is parallel to the co-axis direction of the empty hole and the blasthole. Moreover, the prestress promotes the attenuation of DSIFs in the early stage of blasting and limits the increase of DSIFs under vertical reflected stress wave. The lower side of the empty hole is easier to be fractured than that of the upper side. Meanwhile, the peak compressive hoop stress on the upper side of the empty hole does not appear closest to the empty hole.

Infrared radiation constitutive model of sandstone during loading fracture

The significant increase in energy demand will inevitably boost the exploitation of natural resources. The exploitation of resources including coal, oil, geothermal, natural gas, shale gas, and other energy has to be extracted with deep mining to fulfill the industry energy demand. With the increase of mining depth and the utilization of deep rock mass, the problem of high ground stress has become increasingly prominent. During high ground stress conditions, the mechanical behavior of hard rock will change significantly. Constructing a reasonable stress–strain damage constitutive model of hard rock is the basis for the design of a resource mining construction scheme, numerical simulation analysis, more accurate acquisition of rock mechanical characteristics, and evaluation of engineering rock stability. This is very crucial that needs to be addressed in a better way to solve the scientific problems of rock engineering. In this research, the infrared radiation observation experiments of sandstone biaxial under different lateral stresses are carried out. It is found that the infrared radiation energy has a near power function relationship with the effective stress in loading and fracture process. The quantitative characterization method of infrared radiation of the first invariant of stress and the second invariant of deviator stress is established. The cumulative high-temperature point scale factor amplitude is proposed to characterize the plastic volumetric strain of sandstone. Based on the effective stress and plastic strain, the plastic and damage models are established respectively. The three-dimensional plastic damage constitutive model of loaded sandstone based on infrared radiation is constructed. The model has clear input parameters having physical significance, and the compaction stage of sandstone is considered. The stress prediction of sandstone during biaxial loading is realized through the secondary development of a finite element software subroutine. The research results can lay a theoretical foundation for the analysis and evaluation of the mechanical characteristics of hard rock in the process of deep energy exploitation.

Segmentary Damage Constitutive Model and Evolution Law of Rock under Water-Force Coupling Action of Pumped Storage in Deep Mine

The deformation and failure of surrounding rock mass under different water environments is a basic mechanical problem encountered in the safe operation of ground pumped storage power station and abandoned mine pumped storage power station. According to the influence of different water environments on the failure characteristics of deep surrounding rock mass, it is necessary to summarize the damage evolution law of deep rock mass under different water environments and construct the constitutive model. In this paper, the loading mechanical test is carried out after the natural immersion of the rock in different water environments. The influence of the change of the geological water environment on the damage evolution characteristics of the rock is analyzed from the perspective of the deterioration of the mechanical parameters. On this basis, the damage statistical constitutive model is constructed, and the damage evolution analysis is carried out. The results show that the degradation degree of mechanical parameters such as compressive strength and elastic modulus of sandstone is in the order of distilled water immersion, simulated groundwater immersion and natural state. The damage evolution of sandstone under water–rock interaction is divided into four stages: no damage, rapid damage, deceleration damage and failure. The theoretical curve of the model is in good agreement with the uniaxial test curve of rock under different water environments. The segmented damage constitutive model based on the long compaction stage of sandstone under water–rock interaction reasonably reflects the change of stress–strain relationship of damage failure, and the physical meaning of parameters is clear.

Study on the crack propagation behaviour of eccentric uncoupled blasting in a deep-level rock mass

ABSTRACT To explore the dynamic crack propagation behaviour in the fracture zone and the damage characteristics of the rock mass around the blasthole during eccentric uncoupled blasting of a deep-level rock mass, theoretical analysis, model experiments and numerical simulation studies have been described in this paper. Firstly, the stress state of the crack and the stress distribution state and failure mechanism of the rock mass around the blasthole under the influence of the in-situ stress, and the pressure distribution of blast shock wave on the blasthole were analysed. Secondly, with the help of a new digital laser caustics experimental system, the eccentric uncoupled blasting model experiment under the action of the in-situ stress was carried out. The fractal damage theory is used to quantitatively evaluate the damage degree of the rock mass in different areas around the blasthole, and the blast-induced crack propagation mechanical behaviour characteristics in the fracture zone are analyzed from the perspective of dynamics. Then, based on the LS-DYNA software, a numerical simulation study was conducted on the eccentric blasting of deep rock mass. Finally, some discussions and suggestions are provided for the analysis of blasting mechanisms in a deep-level rock mass.

Influence of water on rockburst of surrounding rock in deep circular tunnels under triaxial internal unloading conditions

Rockburst is one of the most serious engineering geological hazards encountered in deep rock mass. In this study, we investigate the influence of water on the rockburst mechanism in circular tunnels considering the whole stress path of high in situ stress + internal unloading + stress adjustment, based on external loading-internal unloading (ELIU) tests on intact air-dried and saturated sandstone cubic specimens (100 × 100 × 100 mm3). The proposed ELIU method uses true triaxial equipment to first load each intact cubic specimen with two-dimensional high stresses, followed by an internal drilling device for internal unloading to form circular holes. For comparison, cubic specimens with predrilled holes 25 mm in diameter were subjected to internal drilling – external loading (IDEL) tests, i.e., drilling first and then loading. The results show that water reduces the uniaxial compressive strength, Young's and initial moduli, and elastic energy storage capacity of sandstone materials, and increases the significance of shear failure. Furthermore, saturated tunnel structures induce static spalling failure, while dry tunnels lead to dynamic rockburst failure. The influence of water on the stability of the rock mass surrounding a tunnel is analyzed, including the induced weakening of surrounding rock strength and the amplification of the unloading effect. In summary, water prevents rockburst through the following mechanism: (i) water reduces the brittleness of the surrounding rock and enhances its plasticity; (ii) water reduces the elastic energy storage capacity and increases the energy dissipation capacity of the plastic zone; and (iii) water induces the redistribution of stress throughout the surrounding rock, which reduces the stress concentration near the free surface and transfers the maximum stress to the deeper surrounding rock.

Ground Movement in the Hanging Wall of Underground Iron Mines with Steeply Dipping Discontinuities: A Case Study.

To ensure the safety of underground mining activities and effectively protect the surface production facilities and houses of the nearby residents, the ground movement caused by the sublevel caving method needs to be studied. In this work, the failure behaviors of the surface and drift of the surrounding rock were investigated based on the results of in situ failure investigations, monitoring data, and engineering geological conditions. The results were then combined with theoretical analysis to reveal the mechanism responsible for the movement of the hanging wall. Driven by the in situ horizontal ground stress, horizontal displacement plays an imperative role in both the movement of the ground surface and underground drifts. Accelerated movement is found to occur in the ground surface which coincides with the occurrence of drift failure. Failure occurs in the deep rock masses and then gradually propagates to the surface. The steeply dipping discontinuities are the main reason for the unique ground movement mechanism in the hanging wall. As steeply dipping joints cut through the rock mass, the rock surrounding the hanging wall can be modeled as cantilever beams subjected to in situ horizontal ground stress and lateral stress due to caved rock. This model can be used to obtain a modified formula for toppling failure. Also, a mechanism of fault slipping was proposed, and the condition required for fault slipping was obtained. Based on the failure mechanism of steeply dipping discontinuities, the ground movement mechanism was proposed considering the horizontal in situ ground stress and caved rock mass: slippage of fault F3, slippage of fault F4, and toppling of rock columns. Based on the unique ground movement mechanism, the goaf surrounding rock mass could be divided into six zones: a caved zone, a failure zone, a toppling-slipping zone, a toppling-deformation zone, a fault-slipping zone, and a movement-deformation zone.

Model test on deformation and failure mechanism of tunnel support with layered rock mass under high ground stress

To control the large deformation of the Changning Tunnel, based on the constant resistance and energy absorption characteristics of the NPR anchor cable, first, through the physical model test, the mechanics of the NPR anchor cable controlling the deformation of the floor heave was studied, including the temperature field, strain field, and displacement field during the tunnel excavation and loading process. Then, through field tests, the control effect of NPR anchors was studied. The results of the model test show that under the support of NPR anchor cables, with the increase of geostress, the asymmetric deformation of the vault and floor is obvious. During the loading process, the shallow rock mass pressure strain area at both ends of the floor is larger, squeezing the floor and causing strong temperature changes and floor heave failures of the rock mass, while the deep rock mass strain value changes smoothly and is relatively stable. After the axial force of the NPR anchor cable transitions from the growth stage to the stable state, the rock mass deformation converges and stops growing. In the field test, the NPR anchor cable can reduce the primary support deformation from more than 2000 mm to 108 mm, and reduce the stress on the support structure, which is helpful in avoiding the risk of initial support deformation and secondary lining damage in the deep-buried, layered rock mass tunnels.