Damage Characteristics Of Rock Research Articles

Study on main Frequency precursor characteristics of Acoustic Emission from Deep buried Dali Rock explosion

Rock burst disaster events have become one of the most common disasters in the field of underground engineering, and it is of great practical significance to study the prediction and early warning signs of rock burst. Acoustic emission waveforms generated during a rock burst event, which are often characterized by parameter analysis, that analysis of the waveform spectrum can reflect the characteristics of rock damage and failure. Based on a single-axis strain-type rock burst test of deep buried marble, characteristics of the frequency spectrum, as well as the time-domain dominant frequency of acoustic emissions during the entire rock burst process, were analyzed. We find that the acoustic emission spectrum characteristics also have their own characteristics at different stages of rock burst. In general, The main frequency amplitude value presents an increasing trend, the peak shape of the acoustic emission spectrum shifts between unimodal and multimodal, and the secondary frequency typically appears in the plastic deformation stage. In the high-frequency domain (more than 200 kHz) of the initial loading of stress redistribution, the elastic phase of the energy build-up shows a relatively quiet period, the main frequency distribution of plastic deformation stage shows increasing tendency. Before and after the rock burst event, the dense area of the main frequency migrates from the high-frequency domain to the low-frequency domain, the main frequency in the high frequency field appears dense distribution on the eve of rock burst. These changes were distinguished as characteristic criteria for impending rock burst and can therefore be used for the prediction and early warning of rock burst events.

Numerical investigation into the blasting-induced damage characteristics of rocks considering the role of in-situ stresses and discontinuity persistence

This paper presents a 3D coupled Smoothed Particle Hydrodynamics (SPH) and Finite Element Method (FEM) model, which was developed to investigate the extent of damage zone and fracture patterns in rock due to blasting. The RHT material model was used to simulate the blasting-induced damage in rock. The effects of discontinuity persistence and high in-situ stresses on the evolution of blasting-induced damage were investigated. Results of this study indicate that discontinuity persistence and spatial distribution of rock bridges have a significant influence on the evolution of blasting-induced damage. Furthermore, high in-situ stresses also have a significant influence on the propagation of blasting-induced fractures, as well as the patterns of fracture networks. It is also shown that the blasting-induced cracks are often induced along the direction of the applied high initial stresses. Moreover, additional cracks are normally generated at the edges of the rock bridges probably due to the relatively high stress concentration.

Damage Ratio Based on Statistical Damage Constitutive Model for Rock

The study of damage characteristics of rock mass is of great significance to the analysis of rock mass structure. According to the characteristics of the microscopic unit strength of rock with random distribution, the Weibull distribution is widely used as the statistical functions of the strength of the microunit of rock to establish the damage constitutive model. The concepts of damage ratio De and damage index Cc are proposed. Damage ratio is mainly used to describe the law of damage evolution in rock. Damage index can be used to evaluate the damage degree of rock. The influence of confining pressure on distribution parameters and damage ratio is analyzed through uniaxial and triaxial compression tests of sandstone. The results show that damage ratio is an index of structural characteristics of rock damage, which can reflect the evolution characteristics of microcracks in rock under spatial stress. Critical damage ratio refers to the damage ratio corresponding to the peak stress of rock and can be used as a parameter to characterize the strength of rock for corresponding to the peak strain one to one. The critical damage ratio is linearly related to the logarithmic function of confining pressure. Its relationship is as follows: Der=Ccln⁡σ3+b. With the increase of σ3, the increasing trend of Der slows down and gradually tends to a certain value. The larger the damage index is, the more serious the damage of rock is. The smaller the damage index is, the less serious the damage of rock is. Therefore, the damage index can be used to evaluate the damage degree of rock. It will be an important direction of rock damage mechanics research to distinguish the severity of rock damage by using damage index as the limit value.

Frequency properties of acoustic emissions from the dry and saturated rock

Different frequency bands (FBs) of acoustic emission (AE) signals from rock fractures contain diverse information about the physical and mechanical properties of a rock. This study proposes a frequency division processing method for the standard description expressions of discrete signals and for defining the integrated characteristics of AE signals. The frequency evolution of rock fracture-related AEs was investigated under uniaxial loading conditions, and rocks in dry and saturated states were examined and compared using AE information in FBs. The correlation between the water state and FBs of AE signal was addressed. The FB features (e.g., energy concentration) were analyzed, and their changes in water conditions were also considered by focusing on the FBs with the greatest energy concentrations. During rock fracturing, dry and saturated samples developed various shear-tensile damage patterns or “rock bridge” modes. The moisture property of rock affected the peak strength, plastic deformation, and intensity of the rock destruction, and the statistical relationship observed between rise time/amplitude and average frequency showed that the crack characteristics depended on the presence or absence of moisture. The most suitable FB to distinguish the water state was 31.25–62.5 kHz. The parameter ζ (i.e., variable energy coefficient) exhibited a sudden increase in the FB of 0–31.25 kHz. This rapid change was more remarkable than that in the other FBs. This FB should be considered useful for monitoring to help predict the final rupture. The method developed here provides a new way for examining the rock damage characteristics and discovering the fracturing behavior patterns.

Rock Mechanics Characteristics Test and Optimization of High-Efficiency Mining in Dajishan Tungsten Mine

Rock mechanics test is not only the basis for obtaining the mechanical parameters of rock but also an important means for studying rock mechanics and engineering. In this paper, the uniaxial compression deformation test, Brazilian splitting test, and cornea pressure shear test are carried out for rocks in the Dajishan tungsten mine. The basic mechanical parameters such as uniaxial compressive strength, tensile strength, elastic modulus, Poisson’s ratio, and internal friction angle of ore rock and surrounding rock are obtained. Meanwhile, damage characteristics of rock are deeply studied and analyzed under different experimental conditions. According to rock mechanics parameters which are obtained from indoor rock mechanics tests, three design schemes of stope structure parameters are optimized by using the FLAC3D numerical simulation software. On the premise of ensuring the stability of the stope structure, the recovery rate of ore and the production capacity of the stope are taken into consideration. It is suggested that the second scheme should be adopted for mines (18 m for ore room and 7 m for ore pillar), which provides scientific guidance for the safe and efficient mining of mines.

Experimental and Numerical Study on the Damage Evolution Behaviour of Granitic Rock during Loading and Unloading

Theoretical and experimental studies have revealed that the damage evolution plays an important role in stability of rock structures. To investigate the damage characteristics of rocks during loading and unloading, a series of conventional triaxial tests and numerical simulations were conducted on granitic rock specimens under different confining pressures. The stress-strain characteristics and fracture patterns of tested specimens were first analyzed. It was found that the failure strain in unloading is smaller than the failure strain in loading. And the difference between the two strains is growing with increasing confining pressure. The failure patterns of specimens displayed two different failure mechanisms: a single distinct failure and a “X” failure. Based on the law of energy conservation, the energy evolution was analyzed. The results indicated that absorbed strain energy converted into elastic strain energy and dissipation energy. For evaluating and predicting damage, two damage degrees were proposed considering increase of dissipation energy and decrease of tangential modulus, respectively. The results show that before the reversal point of volumetric strain, the damage degrees were almost unchanged. During the process of unloading the damage degrees increases fast. For the same strain, lower confining pressure shows more damage. It indicates that the confining pressure has negative effects on increase of the damage degree. Then, the discrete element model based on elastic and unbreakable voronoi blocks was set-up for tri-axial tests. The energy evolution and damage process were simulated. And the ratio of failed contacts was used to simulate the damage degree. It shows that stress-strain behavior as well as micro- and macro-mechanical damage evolution can be reproduced by the DEM model.

Evaluation of Gas Migration and Rock Damage Characteristics for Underground Nuclear Waste Storage Based on a Coupled Model

In the deep geological repository of nuclear waste, the corrosion of waste generates gas, which increases the storage pressure, changes the properties of the rock strata, and affects the stability of nuclear waste repository. Therefore, it is of great importance to understand the gas migration in the engineering barrier and the potential impact on its integrity for the safety assessment of nuclear waste repository. A hydro-mechanical-damage model for analyzing gas migration in sedimentary rocks is established in this paper. On the basis of which, a set of coupled formulas for the coupling of gas migration in rock mass is established. The model considers the characteristics of gas migration in sedimentary rock, especially the microcracks caused by the degradation of elastic modulus and damage, and the coupling between the rock deformation and failure of fractures. The numerical simulation of gas injection test is beneficial to understand the mechanism of gas migration process in sedimentary rock.

3D Numerical Simulation and Experiment Validation of Dynamic Damage Characteristics of Anisotropic Shale for Percussive-Rotary Drilling with a Full-Scale PDC Bit

The lower rate of penetration (ROP) is one of the key technical difficulties during drilling of shale reservoirs. Percussive-rotary drilling (PRD) is crucial for increasing ROP. One of the core problems of ROP optimization for PRD are the dynamic damage characteristics of rock fragmentation. By considering the dynamic drilling parameters, a new model for estimating the PRD with a full-scale polycrystalline diamond compact (PDC) bit is established. The mechanical parameters of shale are measured by a wave velocity method. Rock damage characteristics are simulated by using the finite element method. The numerical simulation model is verified by the actual drilling case in LMX shale reservoir in Sichuan (China). The results indicate that rock element damage occurs along the direction of maximum principal stress. The order of decreasing rock damage rate is impact-static load, static load and impact load. When the impact load has the same peak value, and the rock elements in contact with the cutters obtain more energy with load frequency increasing. The rock fragmentation efficiency under a sine wave is higher than rectangular and pulse waves. The rock can obtain more energy to be broken with the increasing impact load duration and peak values. When the impact-static load goes over the rock damage threshold value, the higher the peak value of the impact load is, the more energy the rock will obtain. The higher the lateral vibration amplitude of the drill bit, the lower the efficiency of rock fragmentation. Repetitions of drill bit axial vibration at one indentation point will reduce the ROP, and the axial vibration energy of the drill bit is consumed. Therefore, a small lateral movement and reasonable axial vibration frequency increase the rock breaking efficiency. The ROP was increased through the suppression of drill string and the application of vibration. The study results can be used in the optimization designs of bit trajectory and ROP for PRD tools.

Damage Evolution of Rocks under Triaxial Compressions: An NMR Investigation

To investigate the damage characteristics of rocks subjected to triaxial compression, marble specimens were prepared and subjected to varying levels of axial compressions with identical confining of 20 MPa. The loaded specimens were then tested using Nuclear Magnetic Resonance (NMR) technique to obtain their micro NMR parameters. The changes in transverse surface relaxation time (T2) distribution, peak area, porosity and microcracks structure distribution of the specimens due to varying compression ratios were systematically analysed. It is found that with the increase of the axial compression, the size and the number of the microcracks, porosity and the peak area increase, indicating that severer damage is induced at high compression ratio. The proportion of the large microcracks area to the total T2 area exceeds 96% in spite of the increase in compression ratio, indicating that the large microcracks dominate the degradation of the specimen. Based on the Magnetic Resonance (MR) images, the variation of the microcracks distribution can be visually presented under the different compression loadings. The NMR method presented in this study shows a good way of identifying meso-damage evolution characteristics of rock.

Analysis of Rock Damage Characteristics Based on Particle Discrete Element Model

Rock is a heterogeneous medium that is composed of minerals of various sizes. Under the action of external loads, the generation, propagation and coalescence of microdefects in the rock mass determines the macroscopic deformation and fracture of rock. To understand the damage law of rock and to reveal the evolution of dynamic failure, a uniaxial compression model was established based on particle flow code. Acoustic emission and energy characteristics of rock damage were analyzed, and the damage constitutive models of rock were discussed. During rock uniaxial compression, acoustic-emission events undergo a relatively quiet, sudden increase and sharp decrease for three periods, which corresponds to the compaction and elastic deformation stage, yield stage and post-peak stage in the stress–strain curve. Before the yield stress is reached, the proportion of bond and strain energies is larger. Friction energy accounts for a small proportion of the total energy, and a reciprocal relationship exists between them. The constitutive model that is based on friction energy can better reflect the variation in stress and strain, then the constitutive model based on acoustic-emission parameters.

Experimental Study on the Ultrasonic Testing including Porosity of Rock Damage Characteristics

Cement mortar specimens are used to simulate the fracture of rock material under uniaxial compression test, using the ultrasonic instrument measurement test process, the waveform and wave velocity results along with the change of specimen fracture extension are obtained. Experimental results show that at 0 MPa, there are micro-defects in the specimen, leading to the irregular change of the waveform of the measuring points 4# and 5#; At 1 - 4 MPa, there is no crack on the surface of the specimen, the waveform of ultrasonic wave is stable, and the velocity of each point increases with the increase of the strain, but the measuring point increases the wave velocity when the load is applied; When the specimen surface cracks, expect the waveform of measuring point 4# and 5# change in irregularities, the measuring point 2# waveform also changes irregularly and measuring point of the wave velocity has decreased; At the crack propagation stage, ultrasonic wave also changes irregularly along with the expansion of crack, wave velocity with the increase of the strain is also obvious downward trend; When the crack penetrates the whole specimen, the waveform of each measuring point changes abnormally, the continuity is bad, the waveform is irregular and the measured wave velocity is low.

Investigation into the characteristics of rock damage caused by ultrasonic vibration

• The idea of the limited incident energy is put forward for analysis of the rules of the formation of rock crack instead of the spall stress in the critical normal stress criterion.

Analysis on damage mechanical characteristics of marble exposed to high temperature

Study on the mechanical response of rock under the combined effects of high temperature and mechanical load using the method of damage mechanics is a relatively new research direction of rock mechanics. The thermal damage and mechanical damage are indicated by the uniaxial compression mechanical test of marble after exposure to high temperature. The overall damage evolution equation of rock is deduced and high-temperature damage constitutive equation is established on the basis of the macroscopic phenomenological damage mechanics and nonequilibrium statistical methods. The evolution law of the damage characteristics of rock after exposure to high temperature, taking the temperature and mechanical load as the control variable, is discussed. The results show that the effect of temperature on the mechanical properties of marble can be accurately described through the definition of thermal damage using elastic modulus. The high-temperature damage constitutive model curves are similar to the experimental curves, reflecting well the brittleness of marble. Additionally, the constitutive relations of rock after exposure to high temperature have close relationships with the elastic modulus, compressive strength, and peak strain. The overall damage of rock changes along two evolutionary pathways of high temperature and strain, reflecting the mutual combined influences of high temperature and strain on the material damage extension, can accurately reveal the damage mechanical behavior and the damage propagation law of rock material after exposure to high temperature.

Support technologies for deep and complex roadways in underground coal mines: a review

Based on geological and mining characteristics, coal mine roadways under complex conditions were divided into five types, for each type the deformation and damage characteristics of rocks surrounding roadways were analyzed. The recent developments of roadway support technologies were introduced abroad, based on the experiences of supports for deep and complex roadways from Germany, the United States and Australia. The history and achievements of roadway support technologies in China were detailed, including rock bolting, steel supports, grouting reinforcement and combined supports. Four typical support and reinforcement case studies were analyzed, including a high stressed roadway 1,000 m below the surface, a roadway surrounded by severely weak and broken rocks, a chamber surrounded by weak and broken rocks, and a roadway with very soft and swelling rocks. Based on studies and practices in many years, rock bolting has become the mainstream roadway support form in China coal mines, and steel supports, grouting reinforcement and combined supports have also been applied at proper occasions, which have provided reliable technical measures for the safe and high effective construction and mining of underground coal mines.

Study on Smooth Blasting of Tunnel in Fault Formation

From the characteristics of rock damage, the mechanism of rock blasting into seam was discussed. Based on the joint mechanism of stress waves and explosive gases, the blasting method adopting slotted pipe charge structure in fault formation was studied, and the test results were used in Guanjiao tunnel of Qinghai-Tibet railway. The field tests show that the perimeter hole adopting slotted pipe charge structure improves the phenomenon of over-excavation in fault formation obviously.

Study on Smooth Blasting of Tunnel in Fault Formation

From the characteristics of rock damage, the mechanism of rock blasting into seam was discussed. Based on the joint mechanism of stress waves and explosive gases, the blasting method adopting slotted pipe charge structure in fault formation was studied, and the test results were used in Guanjiao tunnel of Qinghai-Tibet railway. The field tests show that the perimeter hole adopting slotted pipe charge structure improves the phenomenon of over-excavation in fault formation obviously.

Rock damage control in bedrock blasting excavation for a nuclear power plant

Drill and blast is a major method for bedrock excavation in nuclear power plant engineering. Under blasting excavation, blast-induced vibration propagates in rock mass and induces damage to surrounding rock. Studying the characteristics of wave propagation and blast-induced damage to rock mass is most important to restrict the damage extension. In the present paper, in support of the bedrock blasting excavation for the Linao Nuclear Power Plant project (LNPP), the site seismic survey and sonic wave test are presented to study the characteristics of wave propagation and the blast-induced damage for rock mass. The explosive load, rock damage characteristics and peak particle velocity (PPV) attenuation law are also investigated by numerical simulation using commercial software. It can be seen that the numerical simulation results agree well with that obtained by site monitoring. Based on the site monitoring and the numerical simulation, the relationship between PPV at 30 m away from the charge hole and the damage depth of rock mass is established. A simple method based on safety threshold of vibration velocity for rock mass to restrict the damage depth of rock mass under blasting excavation is presented. According to the damage controlled method and the presented safety threshold of vibration velocity for rock mass, the remained bedrock can be simply and effectively protected during blasting excavation.

An Experimental Damage Model and Its Application to the Evaluation of the Excavation Damage Zone

An experimental damage model, which can simulate both post-peak and pre-peak damage characteristics of rock by assuming distinct elastic constants in each damage level, was proposed. Each damage level was identified by two successive threshold values of major principal stresses, which could be determined from uniaxial and triaxial compression tests and measurement of acoustic emissions. Elastic constants pertaining to each damage level were expressed in terms of confining pressure (minor principal stress) by regression equations of the experimental data. Post-peak behavior was simulated by selecting suitable values of the Hoek-Brown constants mr and sr for post-failure. The proposed experimental model was implemented into FLAC by writing a FISH function. The parametric studies on Hoek-Brown constants for post-peak behavior revealed that peak strength depends greatly more on sr than mr, and post-peak slopes of stress–strain curves depend mainly on mr.