Failure Mechanism Of Rock Research Articles

NUMERICAL AND EXPERIMENTAL RESEARCH OF ROCK FAILURE MECHANISM AND ITS DEPENDENCE ON mi HOEK-BROWN FAILURE CRITERION

Rock failure mechanism is one of the most important issues in rock mechanics engineering which plays a key role in the stability analysis of various structures. Therefore, different failure criteria have been proposed to understand the failure mechanism of rocks. One of the most commonly used rock failure criteria is the Hoek-Brown criterion, in which there is a parameter called mi, which is very important to the response provided by this criterion. Due to the importance of conducting extensive studies on this parameter, in this current research, by performing a series of experimental triaxial compressive strength test and numerical simulating in PFC-2D code, the effect of the Hoek-Brown constant mi on the failure mechanism and crack growth of different rocks has been studied. Based on the results of this study, it was found that the effect of parameter mi on the failure mechanism of different rocks varied according to the type of rocks, and the greatest effect of this parameter was on the peak strength of rocks. In addition, it was found that under higher lateral pressures, there are less destructive cracks in rocks, and as a result, they show more ductile behaviour.

Surrounding Rock Stresses on a Working Face‐End Roof under Mining Influence

The evolution process of the surrounding rock failure mechanism is studied because of spalling and roof fall accidents at the top corner of longwall top coal caving faces affected by mining and the difficulty of moving the advanced end support. Methods are proposed to improve the stability of surrounding rocks at the top corner of the end including cutting at the top corner of the end, reinforcing the anchor cable, changing the stress distribution of surrounding rocks at the top corner of the end, and transferring the stress concentration area of surrounding rocks to the deeper rock. Field observations of the surrounding rocks at the top corner of the 15107 fully mechanized caving face show that the stress value of the surrounding rocks at the corner between the roof of the return airway and the coal wall of the working face is 28.9 MPa when the surrounding rocks are in a stable state without mining. The stress value of surrounding rocks at the top corner of the end is 32.3 MPa when it is affected by mining, which results in spalling and roof fall. The surrounding rocks are in a stable state when the maximum stress of the surrounding rocks at the top corner of the reinforced anchor cable’s back‐end is 26.1 MPa. The results show that cutting of the surrounding rocks at the top corner of the end and the reinforcement of the anchor cable can avoid the spalling and roof fall when the top corner of the end is affected by mining and can ensure that the end support advances and working face moves forward.

A novel experimental apparatus for single polycrystalline diamond compact cutter tests.

Polycrystalline diamond compact (PDC) bits are increasingly favored in the drilling field due to their high efficiency in rock breaking together with their longevity. To investigate the rock failure mechanism and further improve the performance of PDC bits, innovative experimental equipment is proposed in this paper. With its assistance, we can study the characteristics of rock-breaking using a PDC cutter under different conditions, e.g., high pressure and high temperature (HPHT), confining pressure, and jet impingement. The setup can be grouped into three parts: a rock cutting system, high-pressure jet generation system, and controlling system. A series of experiments was conducted to demonstrate the reliability of the setup. The results demonstrate the improvement in performance of PDC bits in the exploration of HPHT formations.

Optimization of safe gas core retrieval using multivariate numerical simulation, based on different pore rheology and rock lamination

High gas compressibility, facies lamination, and presence of sub-lateral microfractures make shale gas reservoir rocks very sensitive to changing outer pressure and lead to severe damage of the whole core when it is recovered to surface. The approaches currently used for safe core retrieval do not adequately account for gas shale features, therefore, they are not capable of reliable modeling of this process. The article presents an extension of the recently developed optimization of the core tripping algorithm to shale gas formations featuring significant improvements. One, we show the lead role of strong inter-relation between gas flow and rock deformation process in the optimization of the core lifting process and propose a new decompression model of gas saturated shale rocks. Two, we introduce the dynamic permeability concept for shale gases, which considers individual behavior of the pores in organic matter, inorganic matrix, and microcracks. Three, we propose ways for calculation of the stress-strain state in cylindrical domain by applying numerical and semi-analytical solutions. Four, we introduce an interface failure mechanism for lamination rocks. Finally, we discuss the influence of different void types on decompression and dependence of the optimal schedule on microfracturing properties. Eventually, the article outlines several significant conclusions. In the fractured region, the degassing slows down due to crack closure near to the outer core boundary. The interface failure is an overlooked mechanism of rock failure. Frequent and long stoppages are required for preventing interface crack extension.

Numerical modeling of logged wellbore breakouts using cohesion-weakening frictional-strengthening models

The wellbore stability analysis based on the simplified models such as the elastic-perfect plastic or the elastic-brittle models results in the unrealistic prediction of the wellbore damage zone characteristics and consequently underestimate or overestimate the required mud pressure respectively. To overcome this weakness, the advanced elastic-plastic models considering the full rock failure mechanisms in the post-peak region are reviewed briefly in this paper. The damage zone surrounding a wellbore located in a carbonate oil field in a southwest of Iran is analyzed based on the different advanced models. The objective is to study how well the numerical simulation results based on the advanced models agree with the logged fallouts with respect to location, depth, and shape of damage zone around the wellbore. An evaluation of which advanced model can best capture the actual rock behavior especially in the post-peak region is carried out by comparing the simulation results with the logged fallouts around the wellbore.The depth of the damage zone and transverse extension angle around the studied wellbore predicted well based on a kind of the cohesion-weakening frictional-strengthening (CWFS) model are 3 cm and 84.3° respectively. The simulation results of this model derived from the tests on the carbonate rocks are in more agreement with the logged breakouts around the studied wellbore drilled in the carbonate rocks like limestone. Moreover, according to the key input parameters sensitivity study carried out in this paper, decrease in the residual cohesion and critical plastic strain parameters result in the larger simulated damage zone. On the contrary, increment of the initial friction angle and drilling mud pressure leads to the smaller simulated breakout around the wellbore. As higher dilation angle is utilized in the numerical model as input parameter, the larger simulated breakout is created around the wellbore.

An experimental investigation on stress-induced cracking mechanisms of a volcanic rock

In underground excavations, different failure features can be induced by complex geological stresses. Therefore, many researchers have investigated rock behaviour and failure mechanisms under high-stress conditions. This study investigated the different failure characteristics to determine the stress-induced cracking mechanisms. Accordingly, a series of tests were performed on a volcanic rock (rhyodacite) under one- to three-dimensional stress states. The results indicated that the samples exhibited different failure characteristics/modes under different stress states. Under 1-D and 2-D compression, the rock exhibited unstable brittle failure induced by tensile cracking (Mode I fracture). Under axisymmetric triaxial stress (σ1 > σ2 = σ3 > 0), the rock exhibited stable failure, and as σ3 increased, the failure mode transitioned from Mode I fracture to Mode II fracture to distributed cataclastic failure. In contrast, under a three-dimensional differential stress state (σ1 > σ2 > σ3 > 0), the risk of unstable failure increased with increasing σ2, and the rock exhibited localized shear failure (Mode II fracture). Based on the acoustic emission (AE)-based cracking classification method, further studies were also conducted on the failure mechanism. Finally, based on the energy release and cracking propagation, the mechanism of the σ2 effect on unstable failure was assessed.

Stability Mechanism and Control Technology for Fully Mechanized Caving Mining of Steeply Inclined Extra-Thick Seams with Variable Angles

Fully mechanized top caving mining of steeply inclined extra-thick seams with variable angles has complex roof rock movement, failure, caving, and filling mechanisms. Moreover, the variable-angle coal seams make rock–equipment stability control challenging. We analyzed rock deformation and failure mechanisms, overburden bearing structures, and working characteristics of supports through physical simulations, field tests, and theoretical analysis of the no. 120210 working face of Zaoquan Coal Mine. Overburden rock movement has distinct regional characteristics, with a large caving height (to the main roof layer) in the upper area, and lower caving height (to the lower main roof) in the lower area. Support loading is regional and unbalanced along the inclined direction. The critical layer of overlying strata migrates, and bearing structures formed by breaking the critical layer exhibit cross-layer generalization along the inclination. The upper region has a cantilever beam structure in the basic roof rock layer; the lower region has an inclined masonry structure with multistage ladders in the breakage basic roof rock layer. Application of a false-inclined working face layout with the tailgate ahead of the headgate and an arc arrangement (radius 28.65 m, arc length 10–20 m) in variable angle areas, and regional control of support working resistance and top-coal caving amounts, control regional failure overburden. Safe and efficient mining was achieved, with the monthly production and recovery rates reaching 344,900 t and 85.86%, respectively. Our results expand the scope of fully mechanized top caving mining and have important theoretical and technological significance.

Failure Mechanism and Stability Control of Surrounding Rock of Docking Roadway under Multiple Dynamic Pressures in Extrathick Coal Seam

In view of multiseam mining under goaf, the surrounding rock control problem of lower coal roadway will be affected by concentrated coal pillar left in upper coal seam goaf and dynamic pressure superposition of working face in this coal seam. Under the geological environment of No. 16 extrathick coal seam in the Laoshidan coal mine and taking the working face 031604 as the research background, the reasonable docking position selection of the withdrawal roadway and the docking roadway in the middle mining period and the surrounding rock stability control problems of the withdrawal roadway and the docking roadway during the final mining period were studied by using the methods of field theoretical analysis, numerical simulation, and field measurement. The mechanical mechanism of the nonuniform failure of the retreating roadway and the docking roadway during the final mining period is shown, and the control method of the surrounding rock stability of the roadway is put forward and applied. The results show that (1) through the analysis of the superimposed stress under the concentrated coal pillar and the coal seam in advance, the specific butt joint position is arranged at 860 m away from the open-off cut, which is 10 m away from the goaf of No. 12 coal seam. (2) With the working face 031604 advancing through the process, the deviatoric stress value of the withdrawal roadway gradually increases, the maximum principal stress of the two sides of the roadway deflects clockwise from the vertical direction to the horizontal direction, its angle also gradually increases, and the shape of the plastic zone gradually expands from symmetry to asymmetry. (3) It is revealed that the peak value of deviatoric stress on both sides of the docking position of docking roadway increases gradually under the influence of mining and deflects anticlockwise to the vertical direction with the principal stress angle. The joint action of both is the mechanical mechanism that causes the plastic zone to expand in an asymmetric shape. (4) The coordinated control scheme of support (anchor bolt and anchor cable)—modified (grouting)—is adopted for the withdrawal roadway, and the coordinated control scheme of support (anchor bolt and anchor cable)—changing the cross-section shape of the roadway—is adopted for the docking roadway. The purpose of the smooth connection of working face and rapid and safe withdrawal of equipment is achieved on site.

Post-peak behaviour of rocks under cyclic loading using a double-criteria damage-controlled test method

Cyclic loading–induced hazards are severe instability problems concerning surface and underground geotechnical projects. Therefore, it is crucial to understand the rock failure mechanism under cyclic loading. An innovative double-criteria damage-controlled testing method was proposed in this study to capture the complete stress–strain response of porous limestone, especially the post-peak behaviour, under systematic cyclic loading. The proposed test method was successful in applying the pre-peak cyclic loading and then in controlling the self-sustaining failure of rock during the post-peak cyclic loading. The results showed that the strength of the rock specimens slightly increased with an increase in the fatigue life in the pre-peak region due to cyclic loading–induced hardening. Additionally, a combination of class I and class II behaviours was observed in the post-peak region during the cyclic loading tests; the class II behaviour was more dominant by the increase in fatigue life in the pre-peak region. Damage evolution was assessed based on several parameters, such as the elastic modulus, energy dissipation ratio, damage variable and crack damage threshold stress, both in the pre-peak and post-peak regions. It was found that when the cyclic loading stress is not close to the peak strength, due to a coupled mechanism of dilatant microcracking and grain crushing and pore filling, quasi-elastic behaviour dominates the cyclic loading history, causing more elastic strain energy to accumulate in the specimens.

Prediction Model of Dilatancy Stress Based on Brittle Rock: A Case Study of Sandstone

In this paper, the dilatancy stress and mechanical characterization of sandstone were evaluated under uniaxial loading at different elastic modulus and porosity conditions. The prediction model of dilatancy stress was established using a regression equation and an artificial neural network based on a multilayer perceptron (ANN–MLP). The results indicate that: (1) The rock crack initiation stress, dilatancy stress and its elastic modulus are a power function relationship, while porosity is linearly negatively correlated. (2) σci/σmax hardly changes with the change of elastic modulus (E) and porosity (n); its value is about 0.443. σcd/σmax increase with the increase in the elastic modulus, but decrease with the increase in the porosity. (3) Most of the rock samples are observed as a tensile failure when the porosity is low, while they are a shear failure at medium porosity and tensile shear composite failure at high porosity. (4) The optimum value from the ANN–MLP model for dilatancy stress with architecture 6-5-1 having coefficient correlation (R2, 0.96%) was obtained at mean absolute error (MAE, 0.18981) and root mean square error (RMSE, 0.17016). It is worth mentioning that the research results will help and provide a reference for the related to rock mechanics test, rock engineering deformation and failure mechanism, and will also give specific guidelines significance for the efficient design of excavation and support in deep rock engineering.

Experimental and numerical research on fracture behaviors of sandstone under different loading rates

The macroscopic deformation process of rock is the external performance of microscopic damage in essence, and research from the macro and micro perspectives can effectively reveal the failure mechanism of rock. To further investigate the fracture behaviors of yellow sandstone disc specimens under different loading rates, seven groups of Brazilian disc splitting tests with different loading rates from 1 × 10–6 to 1 × 10–4 s–1 were carried out. A systematical investigation of loading rate effect on macroscopic mechanical responses and microscopic damage characteristics has been conducted from the stress–strain relationship obtained from material testing system, from damage three-dimensional localization based on acoustic emission techniques, and from fracture morphology by scanning electron microscope. In addition to laboratory experiment, parallel bond models of different grain size distribution indexes based on particle flow code were established. The coupling effect of loading rate and grain size distribution index was further analyzed. The results interpret that the effect of loading rate on rock fracture behaviors is determined by the process of crack initiation, propagation and coalescence, and it is also affected by distribution of different size grains. The differences of rock fracture behaviors under different loading rates were discussed from the mechanism of energy input and dissipation. The research have actual significance for stability design and evaluation of rock excavation engineering.

Debris flows originating in the mountain cryosphere under a changing climate: A review

Debris flows originating in the mountain cryosphere (DFMC) are one of the most globally important, widely distributed mass flows (and natural geohazards) in mountain areas with a high altitude and/or high latitude. This is particularly the case in high mountain areas that have been undergoing rapid glacier retreat, permafrost degradation, and other melt/thaw related processes. As a consequence, the actual hazards and potential risks of DFMC have drawn increasing attention in the context of global climate change (i.e. a rising temperature and higher occurrence of strong precipitation events). Unlike debris flows at low elevations, where their occurrence is closely related to precipitation (intensity and duration), the breach of a DFMC event depends on precipitation and/or air temperature, which in turn influence melt/thaw processes, rending the formation mechanism much more complicated. Although research has been widely carried out on DFMC in past decades, there is still a long way to go before we have reached a complete understanding of the formation mechanism and triggering conditions. This review summarizes recent progress in the study of DFMC, including typical DFMC events and their causes, the failure mechanisms of rock (or ice-rock joints), the characteristics of moraine deposits, initiation through hydraulic erosion (entrainment), the relationship between DFMC initiation and meteorological conditions, and the slope stability of the mountain cryosphere under a changing climate. Several issues that should be addressed in future research are also discussed.

Study of failure mechanism of bottom-hole rock mass approaching a gas-fractured zone during nitrogen drilling

In order to restore the normal development of nitrogen drilling technology, it is significant to gain a deep and comprehensive insight into the bottom-hole rock failure mechanism induced by gas-induced fracture. In this paper, the mechanism of bottom-hole rock failure induced by gas-induced fracture was analyzed by means of conventional analysis method of wellbore instability. The research suggests when the bottom-hole is gradually approaching the gas-induced fracture, the difference between circumferential stress and radial stress of the crack face gradually increases. The increasing shear stress caused by the difference between principal stresses leads to the compression-shear failure; then, the failure mode evolved from single-shear failure mode to combined mode of shear and tensile failure with the failure zone extending. If the wellbore is connected with the fracture, then the high-pressure gas in fracture carrying plenty of debris is injected into the wellbore, releasing lots of energy. The mechanism analysis of bottom-hole rock failure can explain the abnormal behaviors of logging data in well QL1 located in Baimamiao structural belt, Dayi County, Sichuan Province. The results not only provide a theoretical basis for the prevention and control of bottom-hole rock failure, but also provide a new perspective for studying the dynamic instability of deep rock in the field of petroleum engineering.

Direct shear experiment on salt rock incorporating loading rate effect

To investigate the shear behavior of salt rock under different loading rates, direct shear testing was conducted on high-purity salt rock obtained from Pakistan using a YZW-100 apparatus. The experimental results of the shear stress, deformation, and failure characteristics of the salt rock showed that the loading rate is an essential factor influencing the shear strength parameters of the rock. When the shear loading rate was increased, the cohesion of the salt rock significantly increased; however, the internal friction angle slightly decreased, nearly remaining unchanged. The shear failure mode of the salt rock manifested by shear stress-displacement curves tended to be ductile failure with increased normal force. The residual stress of the salt rock was large, indicating its high friction-bearing capacity. The dilatancy deformation was also subjected to shear loading. The fracture section of the salt rock exhibited visible local scratch marks. These experimental results are helpful for further understanding the failure mechanism of salt rock and the stability of underground gas storage under different gas injection and production scenarios.

Numerical simulation on effect of heterogeneity on mode I fracture characteristics of rock

Rock is more sensitive to tensile loading than compressive loading, since the tensile strength of rock is much lower than compressive strength. The fracture characteristics of rock in the tensile state are of great significance to the understanding of rock failure mechanisms. To this end, we have conducted numerical simulation researches on mode I cracking process of rock with varying homogeneity, using the Realistic Failure Process Analysis program. With the increase of homogeneity, cracks are concentrating to the ligament area with a decreasing number of crack bifurcations, and the main crack path is becoming smooth. Crack behaviors and mechanical properties are influenced significantly when the homogeneity index is in the range of 1.5 to 5. When the homogeneity index is greater than 30, they are not affected by rock homogeneity and the rock can be considered as essentially homogeneous. It is considered that the global and local strengths are affected by the distribution of rock mechanical properties at mesoscale, which influence the crack behaviors and mechanical characteristics.

Study on the Stage Failure Mechanism and Stability Control of Surrounding Rock of Repeated Mining Roadway

China is one of the leading countries in the mining and utilization of coal resources, and the problems of coal-mining technology and safety have been concerned by the world, while the serious deformation and destruction of surrounding rock and the difficulty of support have brought inconvenience to the mining of coal resources due to repeated mining. This paper takes the actual engineering 22205 mining roadway in Buertai mine as the research background, through the combination of numerical simulation and field measurement. In this paper, the stress environment, plastic zone, and surrounding rock deformation in the advancing process of coal-mining face are studied, and the stress evolution law of surrounding rock in repeated mining roadway is obtained. It is clarified that the surrounding rock deformation is the failure mechanism under the combined action of principal stress difference and stress direction deflection. As a result, the surrounding rock of the roadway is asymmetrically deformed and destroyed, and the corresponding surrounding rock control scheme is put forward. The results show that the influence of repeated mining on roadway stress environment can be divided into four stages with the mining process: the stability stage of mining influence, the expansion stage of primary mining, the stable stage after primary mining, and the expansion stage of second mining. At the same time, the shape changes of the plastic zone and the displacement monitoring results of the monitoring are analyzed, and the results are obtained; the stage of stress change is suitable, and combined with the failure characteristics of surrounding rock in each stage, it is put forward that reinforcement measures should be taken in the stable stage after mining; the specific reinforcement scheme is determined according to the expansion form of plastic zone and field measurement. The on-site monitoring shows that there is no roof fall accident during the use of the roadway, which ensures the safety in production.

Preliminary Study on High-Energy and Low-Energy Microfracture Event Evolution Characteristics in the Development Process of Rock Failure

The evolution characteristics of high-energy and low-energy microfracture events play an important role in the brittle failure mechanism of rock and reasonable microseismic (MS) monitoring and acoustic emission (AE) monitoring. The bimodal distribution (BMD) model is commonly used to observe the evolution characteristics of high-energy and low-energy MS events; however, its precise mechanism remains unclear. The evolution characteristics of high-energy and low-energy microfracture events are assessed in this study based on a BMD model. MS monitoring results from the No. 22517 working face of the Dongjiahe Coal Mine are studied, and AE monitoring results of a biaxial compression experiment of a granite specimen are analyzed. High-energy MS events in the No. 22517 working face are found to be generated by an increase in the failure scale of the overlying rock mass upon exiting the insufficient mining stage and entering the sufficient mining stage. The change characteristics of the high-energy AE hits are positively correlated with crack evolution characteristics in the granite specimen and negatively correlated with changes in the Gutenberg-Richter b value. A precise high-energy and low-energy AE hit evolution mechanism is analyzed based on the microscopic structure of the granite specimen. Similarities and differences between high-energy MS events and low-energy AE hits are determined based on these results. Both are found to have bimodal characteristics; an increase in the failure scale is identified as the root cause of the high-energy component. The bimodal distribution of AE hits is far less obvious than that of MS events.

Rock Breaking Performance of TBM Disc Cutter Assisted by High-Pressure Water Jet

A high-pressure water jet can break rock efficiently, which is of great potential to overcome the problems of a tunnel boring machine (TBM) in full-face hard rock tunnel digging, such as low digging efficiency and high disc cutter wear rate. Therefore, this paper presented a new tunneling method that is a TBM coupled with a high-pressure water jet. The rock failure mechanism under the coupled forces of a disc cutter and water jet was analyzed at first. Then, the finite element method (FEM) and smoothed particle hydrodynamics (SPH) method were used to establish a numerical model of rock broken by the disc cutter and water jet. Effects of parameters on rock breaking performance were studied based on the numerical model. Moreover, an experiment of the water jet cutting marble was carried out to verify the reliability of the numerical simulation. Results showed that the high-pressure water jet can increase the TBM digging efficiency and decrease the forces and wear rate of the disc cutter. The optimum nozzle diameter is 1.5 mm, while the optimum jet velocity is 224.5 m/s in this simulation. The results can provide theoretical guidance and data support for designing the most efficient system of a TBM with a water jet for digging a full-face hard rock tunnel.

On the Failure Mechanism of Brittle Granite in 2-D Rock Indentation

Abstract Rock indentation is one of the main mechanical rock breaking approaches, in which the indenter moves perpendicular to the rock surface. A detailed understanding of the failure mechanism of brittle granite is of great importance to achieve high efficiency in rock breaking and to optimize the cutting parameters during deep hard formation drilling. In the present study, a rock indentation simulation model of Ya’an granite is carried out, using the grain-based model (GBM), which is based on the discrete element method of Particle Flow Code (PFC2D), to reproduce the crack initiation, propagation, and coalescence processes in intact rock. This study also includes an experimental program to examine the crack initiation, propagation, and the acoustic emissions in rock specimens. The results show that the microcracks generate gradually in intact rock with respect to penetration depth, and some of the microcracks coalesce with the crack number increasing, eventually resulting in the macrofracture formation. Four types of microcracks will generate during the indentation: intragrain tensile crack, intragrain shear crack, intergrain tensile crack, and intergrain shear crack. Among these four types of cracks, the intergrain tensile crack and intragrain shear crack are the two main crack types generated in indentation, especially the intergrain tensile crack. The rock failure mechanisms are different with the variation of grain size, and the lateral fragmentation of rock is restricted because of the chip hold-down effect of hydrostatic pressure. This study leads to an enhanced understanding of rock breaking mechanisms of Ya’an granite and provides the basis for improving the rock excavation machine design.

A Fast Method to Determine the Critical Depth of Cut for Various Rock Types

Knowing correct values of the rock mechanical properties is crucial for many engineering applications in subsurface. Rocks may show two failure modes during cutting: ductile and brittle. In the ductile mode, rock deforms plastically, and the debris is powdered ahead of the cutting face. On the other hand, chips are the major cutting characteristics for the brittle failure during rock cutting. The critical depth of cut represents the transition point between these two models, so knowing this value helps better predict the failure mechanism of rock. In this paper, a new method is introduced based on measuring the roughness of the groove for determining the transition point of failure modes for every rock sample after the scratch test. The graph depicting the average change in the surface roughness (Rt) versus the scratched surface roughness (ΔR) can be used to identify the rock failure mode and determine the transition point for the cutting process. The value of this slope increases until the depth of cut reaches the transition point, and then the slope reaches a constant value. The main purpose of this paper is to estimate the critical depth of cut of different rock specimens employing the new surface roughness model.