Rock Mass Characterization Research Articles

Characterization, Modelling and Engineering of Rocks and Rockmasses

Rocks are heterogeneous materials usually associated with geological discontinuities making them weak for sustaining the loads due to civil and mining engineering structures. Characterization details of different isotropic, anisotropic and weathered rocks are presented along with the newly formulated classification system for weathered rockmass. Physical modelling of jointed rockmass under uniaxial, biaxial and polyaxial stress state through newly designed polyaxial system yielded significant results. Critical role of intermediate principal stress on strength and deformation for jointed rockmass is highlighted. New polyaxial failure criteria are proposed and compared. New observations on brittle fracture propagation and interlocking phenomena in crystalline rocks are modelled under polyaxial stress state. Extensive large-scale test results on rock joints under constant normal load and constant normal stiffness in static and cyclic modes were presented along with new predictive models. A new closed-form analytical solution is provided for circular openings in isotropic, homogeneous and elastoplastic rockmass for assessing the wellbore stability and openings. Results of physical modelling tests conducted on shallow tunnels subjected to impact loads and new predictive models developed for squeezing behaviour are also included in this paper. Understanding gained from the extensive characterization and physical modelling of materials in appropriate stress regime used effectively to solve real-time cases. The case studies included are: stability analysis of jointed rock slopes of Chenab bridge abutments, Subansiri power house slope stability, Tindharia landslide in Darjeeling, evaluation of bored tunnelling practices in rocks for Delhi metro and coupled hydromechanical modelling for the slopes in Beas catchment.

Automated 3D Jointed Rock Mass Structural Analysis and Characterization Using LiDAR Terrestrial Laser Scanner for Rockfall Susceptibility Assessment: Perissa Area Case (Santorini)

Rockfalls are one of the most dominant geological hazards in mountainous rocky regions with the potential to turn catastrophic if they occur in an anthropogenic environment. Therefore, the identification of potential rockfall locations is of high importance. Susceptibility is the magnitude that describes these locations and its qualitative and quantitative assessment is necessary for the timely treatment of potential events. Quantitative susceptibility assessment can be conducted using either data-driven methods such as bivariate and multivariate statistics as well as artificial neural networks or numerical methods such as static and dynamic models. In both approaches mathematical assumptions have to be made concerning the predisposing factors distribution and so there is an inherent need to achieve the higher possible confidence level in the input data. Such high-resolution data can be acquired using light detection and ranging (LiDAR) scanners. In the current study, LiDAR technology was implemented, and the data processing technique is analyzed step by step providing the reader with a view of the whole procedure. The results produced by the current methodology are validated and interpreted according to in situ measurements and observations based on unmanned aerial vehicle imagery. Post data processing, joint orientation, joint spacing and potential block volumes were extracted considering both persistent and non-persistent joints. The proposed methodology provides the creation of detailed high-resolution spatial distribution maps of the previously mentioned parameters, considering the variability of their values along the slope. The results can be used in a space-resolved susceptibility assessment providing higher-resolution input data for the subsequent susceptibility analysis.

The impact load cell as a tool to link comminution properties to geomechanical properties of rocks

Current tests for characterising rock breakage in comminution do not allow decoupling of rock breakage properties from the machine environment. Furthermore, it is difficult to predict the comminution behaviour of different ores or how they might behave in another breakage device. Consequently, this decoupling process is essential for comminution modelling of variable ores and blends, which interact with different breakage equipment. Previous studies have shown that the Short Impact Load Cell (SILC) is a versatile tool for characterising the primary ore breakage properties such as strength, apparent stiffness and mass-specific fracture energy, regardless of breakage environment. However, its potential as an ore characterisation technique to relate these mechanical properties to the ore characteristics (e.g. mineralogy, texture) has not been investigated in detail. In this first paper, the results from breaking cylindrical and irregular particles from different ore types in the SILC and Slow Compression Testing demonstrate the SILC’s ability to accurately measure strength, stiffness and specific energy of different ore types at different particle sizes which is relevant either for comminution modelling or for rock mechanics testing. The results of breaking cylindrical particles of nine different rock types show that the SILC is capable of matching the results of the slow compression machine for Indirect Tensile Strength from the Brazilian Test and the apparent stiffness results from the Uniaxial Compression test, allowing the accurate calculation of the mass-specific fracture energy distribution for each rock at different sizes. In addition to the cylindrical samples, sets of irregular particles were produced for three rock types and were broken with the SILC. The results show that the median values of strength and stiffness do not change significantly with the shape of the particle, but the standard deviation does increase when irregular particles are tested. If these results are compared with the cylindrical particle results, it can be seen that the geological characteristics of the rock control the median values of the parameters. When the variation around median value is considered it is apparent that particle shape exerts the major control, but geological characteristics still contribute up to 30% of the variation in strength, 25% in stiffness and 25% in the mass-specific fracture energy. The results indicate that the SILC is a suitable device that allows measurement of the primary breakage properties relevant to comminution, which can be easily associated with the geomechanical properties of the rock mass and geological characteristics. The further development of the proposed testing methodology can provide a more comprehensive and less empirical comminution testing for future modelling, incorporating geological and geomechanical properties of the rock.

Damage evolution of hydraulically coupled Jianchuandong dangerous rock mass

Submerged rock masses in reservoir areas can be dangerous when the base rock mass is located in a deterioration zone formed by periodic water level fluctuations. Deformation of the base rock mass is driven by compression due to its own overlying weight and the hydrological conditions of the reservoir; the combination of these two factors represents a complex mechanical environment. This hydraulic coupling accelerates the deterioration and the instability of the base rock mass. To investigate the Jianchuandong dangerous rock mass (JDRM) in the Three Gorges Reservoir area, we devised a new hydraulic coupling mechanism that, when applied to a generalized mechanics model, represents the mechanical environment of the base rock mass in the deterioration zone. In this hydraulic coupling mechanism, when water is in direct contact with the rock mass, the water pressure contributes to the expansion of existing fractures in the JDRM base rock mass. With this hydraulic coupling mechanism, we created a constitutive damage model that reflects the observed stress-strain characteristics of the JDRM base rock mass in these hydrological conditions. After refining our model by incorporating the damage experienced by the submerged dangerous rock mass over time, we determined that the eventual failure of the JDRM can be divided into three stages: the formation of the bottom damage zone, progressive deformation and damage, and sudden rock collapse. Our model indicates that the damage can magnify the effective stress experienced by the base rock mass and lead to the nonlinear acceleration of the deterioration of the dangerous rock mass.

Establishing relationships between structural data from close-range terrestrial digital photogrammetry and measurement while drilling data

Geologists, mine planners, geotechnical, and mining engineers always strive for maximum information to get a better insight of the rock mass before interacting with it. Over the recent decades, close-range terrestrial digital photogrammetry (CRTDP) has been increasingly used for data acquisition and to support the conventional methods for rock mass characterization. It provides a safe, time-saving and contact-free way to gather enough data to minimize user dependent biases. However, it requires an expensive camera, fieldwork and some software to extract the information from images. In addition, it can over-estimate the rock fracturing sometimes due to weathering of the rock face or poor blasting practices. Measurement while drilling (MWD) data include the responses of different drilling parameters to the variations in the rock mass. MWD data are produced in large quantity, as they come from every hole drilled. These data correspond to the inside variations of rock rather than the surface ones counted in photogrammetry.In this paper, structural data are obtained from different bench faces of an open pit mine using a commercial software package, ShapeMetriX3D (by 3GSM). These data are compared to the MWD data of the boreholes that were blasted to produce these bench faces to establish certain relationships between drilling parameters and rock mass structures. Half casts of the boreholes with MWD data were visible on the bench faces of the pre-split wall that allowed a better correlation. The results show abrupt changes in MWD parameters for open joints or cavities with some infilling material and overall increases or decreases in parameters for closely spaced bedding planes, fractures or foliations. The results are promising and suggest the method can be used to characterize the rock mass, modify the charging of explosives in blasting operations and facilitate the geological modeling of the rock mass.

Analysis of Seismic Waves Propagating through an In Situ Stressed Rock Mass Using a Nonlinear Model

AbstractIn situ stress is a significant characteristic of underground rock masses. This work extended the time-domain recursive method (TDRM) to study oblique wave attenuation across an in situ str...

Strength Characteristics of Nonpenetrating Joint Rock Mass under Different Shear Conditions

The mechanical property of jointed rock mass is an important factor to be considered in the analysis, evaluation, and design of actual rock engineering. The existence of joints threatens the stability and safety of underground engineering projects built in the rock mass. In order to study the change of mechanical properties and strength characteristics of nonpenetrating jointed rock mass under different test conditions, direct shear tests and triaxial tests were carried out. Direct shear tests under different normal stresses were carried out for nonpenetrating jointed rock mass to prepare specimens for triaxial tests. Then, triaxial tests were carried out to study the change of mechanical properties and strength characteristics of the nonpenetrating jointed rock mass. In the direct shear test part, the greater the normal stress is, the stronger the shear strength and the more serious the shear failure would be. The main conclusions are as follows: (1) the strength of rock mass would increase with the increase of confining pressure for those rock specimens with same degrees of shear after the direct shear test; (2) for rock specimens with different degrees of shear after the direct shear test, if the shearing degree of the rock specimen was greater, the strength of the rock specimen would be lower in the triaxial test; (3) for rock specimens with the same damage degree after direct shear test, the greater the normal stress in direct shear test is, the smaller the peak axial pressure would be in the triaxial test; (4) if the specimen was sheared under higher normal stress in direct shear test, the cohesion of it would be lower and the internal friction angle would be larger. For the specimens under the same normal stress, if the shear failure of one specimen was more serious, the cohesion of it would be smaller and the internal friction angle would be larger.

Permeability characteristics of fractured rock mass: a case study of the Dongjiahe coal mine

The intricate distribution of fractures in rock mass leads to the anisotropy of permeability characteristics of the rock mass, which affects the overall stability of rock mass. Aiming at the water inrush from floor in the deep mining of the Dongjiahe coal mine, the distribution and development law of fractures in coal mine rock mass are determined by combining the acoustic emission information from the RFPA software and the in-site microseismic monitoring data. The representative element volume (REV) of permeability of fractured rock mass is determined by numerical simulation. The results show that: (1) As the increase of surrounding rock pressure, the permeability coefficient is decreasing, as well as the principal values of permeability. (2) Under the same surrounding rock pressure, the surrounding rock pressure has little influence on the principal directions of permeability; when the surrounding rock pressure is different, the principal directions of permeability changes greatly with the change of surrounding rock pressure. (3) With the increase of the trace length, the principal values of permeability dramatically increase and the size of REV gradually decreases. When the trace length reaches a certain size, the influence on the REV can be ignored.

Geotechnical Engineering for Mass Mining

Editor’s note: The Geology and Mining series, edited by Dan Wood and Jeffrey Hedenquist, is designed to introduce early-career professionals and students to a variety of topics in mineral exploration, development, and mining, in order to provide insight into the many ways in which geoscientists contribute to the mineral industry. Abstract The rock mass response to mining is governed by the rock mass characteristics and the mining-induced changes that drive its behavior. To be able to study and accurately predict the response of the rock mass to mining, it is imperative that both the orebody and the enclosing country rocks are well characterized through the collection and analysis of large quantities of good-quality, representative geologic, structural, geotechnical, and hydrogeological data. These are the fundamental constituents of a good geotechnical model whose reliability improves as the mining project matures and moves from exploration and study phases, passes the decision to develop, and proceeds into construction and then operations. Each phase provides greater exposure to the rock mass, reduces uncertainty, and increases reliability in the geotechnical model and in an understanding of the rock mass behavior. The quest of the geotechnical engineer is to understand the rock mass behavior and is no different from that of the geologist who defines the mineral resource, and it warrants (at the very least) the same level of rigor in data collection, analysis, and reporting. Just as the geologist continues to improve the orebody model through grade reconciliation during mining, so the geotechnical engineer must continually revisit and calibrate the geotechnical model during the operational phase of mining through geotechnical monitoring. The increasing demand by investors and stakeholders that the performance of a mine does not deviate from plan due to unforeseen geotechnical surprises warrants a significant shift in the level of geotechnical data collection, analyses, and rock mass monitoring through all stages of study and operations. This demand warrants supporting budgets and assurance processes that are commensurate with the complexity and extent of the geotechnical uncertainties.

A Modified Cubic Law for Rough‐Walled Marble Fracture by Embedding Peak Density

The property of water flow through a single rock fracture is the base of describing the seepage characteristics of jointed rock mass. Five artificial tensile fractures of coarse‐grained cylinder marble samples were made at about the midpoint of the long axis by using a self‐made splitting mold. The upper and lower surfaces of the tensile fractures were scanned by a 3D laser scanner (OKIO) to obtain their 3D coordinates. Then, the Geomagic Studio Software and rock surface topography scan test software were used to obtain peak density values of each single fracture surface. To study the seepage characteristics of open fracture, 4 rectangular plastic spacers with the size of about 3 mm × 2 mm × 0.2 mm were put into the fracture when water flowed through the single rough fracture tests were conducted under different normal stresses using the self‐developed radial flow system. According to the testing data, the relationships between the seepage characteristics of single rough rock fracture and the peak density of fracture surface were studied. It is discovered that the 3D fracture morphology had great influences on the seepage characteristics of the single rock fracture. A modified cubic law was put forward to present the relationship between the seepage characteristics of a rough rock fracture and peak density of two fracture surfaces. Comparison between the modified cubic law and the experimental data showed a relatively good agreement.

Sensitivity analysis of bursting liability for different coal-rock combinations based on their inhomogeneous characteristics

Because of the close relationship between rock burst and structural characteristics of coal and rock mass, it is important to study failure and bursting liability of different coal-rock combinations to predict rock burst. Hence, based on the structure and interaction of roof-coal-floor system, and the inhomogeneous characteristics of different media, the progressive failure process and acoustic emission (AE) characteristics of different coal-rock combinations (i.e. rock-coal (R-C) model and rock-coal-floor (R-C-F) model) are studied using realistic failure process analysis (RFPA2D), and the impact of different coal-rock combinations on bursting liability is revealed. Further, the bursting sensitivity for different combinations is compared and determined. The results show that the indices of bursting liability for rock-coal combinations are greater than those of single coal, while those of the R-C-F model is also higher than those of the R-C model. The uniaxial compressive strength of the R-C-F model is the highest, the elastic strain energy before stress peak is accumulated, and the softening characteristic after stress peak is significant with the energy rapidly releasing, indicating that the duration of dynamic fracture DT is small and bursting energy index KE is high, and the AE characteristics are more significant. Hence, it is more sensitive and reasonable to evaluate the bursting liability considering the interaction of the roof-coal-floor structure, which can reflect the burst risk in actual field. The results can be used to evaluate the burst risk more scientifically and comprehensively and provide strong guidance for the prediction and prevention of rock burst.

Study on Impact Tendency of Coal and Rock Mass Based on Different Stress Paths

With the increased mining depth, the dynamic disaster of rock burst in coal mines has become increasingly prominent, and the impact tendency of coal and rock mass in deep coal seam mining is a necessary condition for the occurrence of rock burst and an important index to measure the failure of coal and rock mass. Laboratory tests and numerical tests were used to study the impact tendency of coal and roof strata, including the deformation characteristics, failure characteristics, and bending energy index of the coal and rock mass of different sizes, the failure law and energy evolution characteristics of tlhe coal and rock mass under the same size, and the unloading characteristics of the coal and rock mass under the same size and different confining pressures. The results are shown as follows: (1) The rock roof was determined to have a weak impact tendency through the mechanical test. (2) With the increased size, the microcracks in the rock samples increased correspondingly, and the increased meso‐defect leading to the increased heterogeneity was an essential reason for the size effect. The strength of the rock mass decreased with the increased specimen size. The larger the specimen size was, the lower the bending energy index was. (3) Triaxial loading and unloading were tested for the same size under different surrounding rocks. Under the same loading conditions, with the increased confining pressure, the strength and bending energy index of rock mass increased correspondingly, and the failure of rock mass transformed from tensile to shear failure. The failure form and strength characteristic of rock under the unloading condition are different from those under the loading condition. The failure degree was intense, with a high bending energy index. Compared with the loading situation, the impact tendency caused by unloading was higher, and the dynamic impact disaster was more likely to occur.

In Situ Rock Bolt Pull Tests Performance in an Underground Powerhouse Complex: A Case Study in Sri Lanka

This paper presents results of 71 in situ nondestructive rock bolt and tendon pull tests conducted in the Powerhouse Complex of the Uma Oya Multipurpose Development Project, located in Sri Lanka. The pull tests are performed on 6 and 8 m Double Corrosion Protected fully and partially grouted rock bolts (high strength threadbar) as well as 4 m normal fully grouted rock bolts and 26 m pre-stressed tendons, which are installed in different types of Gneiss. Based on the recorded load–displacement curve of the rock bolt head in each test, stiffness of fully and partially grouted rock bolt is measured in the bonded section. Effect of some parameters on the stiffness such as rock mass types and characteristics, bond length as well as rock bolt diameter and steel type are investigated. Primarily, the pull tests are done to approve the working load capacity of rock bolts and tendons and quality control of the installation process. Besides, the stiffness of bonded rock bolt is also calculated. The results show that the stiffness declines by decreasing the rock mass mechanical characteristics, rock bolt bond length, and rock bolt diameter as well as steel bar grade.

Deformation Rules for Surrounding Rock in Directional Weakening of End Roofs of Thin Bedrocks and Ultrathick Seams

With the deployment of China’s energy strategy in the western regions, complex geological mining conditions such as thin bedrock and ultrathick seams in western China have caused a series of problems such as serious deformation of the surrounding rock at the ends of the working face and the increase in the lead abutment pressure of the roadways; the research on end roof deformation in the resource exploitation in western China has become one of the great demands of the industry. Based on the failure characteristics of rock mass, relying on the actual mining geological conditions of a coal mine in Inner Mongolia, the failure characteristics of the overlying rock strata under the influence of mining were simulated and analyzed using similar material simulation experiment, which intuitively reproduced the failure and deformation processes of the immediate roof, main roof, and key strata and revealed the mechanical mechanism of the directional weakening of the end roof. It is of great significance for the stability control of the surrounding rock at the end of the fully mechanized caving face in the thin bedrocks and ultrathick seams, reducing the abutment pressure of gate roadway and controlling the spontaneous combustion of residual coal in the goaf.

Experimental study of nonlinear damping characteristics on granite and red sandstone under the multi-level cyclic loading-unloading triaxial compression

Damping is an important dynamic characteristic of rock masses and has an important impact on seismic engineering. In this paper, a multi-level cyclic loading-unloading triaxial compression experiment was conducted on granite and red sandstone under different confining pressures with two stress paths: one with different amplitudes and the other with the same amplitude. The effects of the two stress paths on the damping ratio, damping coefficient, and elastic modulus evolution characteristics under different confining pressures were studied. The damping parameters and dynamic elastic modulus of the two types of rock under different confining pressures were calculated, and the variation in the damping parameters with the stress level and strain level was determined. The linear interrelation of the damping ratio, damping coefficient, and dynamic elastic modulus was explored. The results show that the damping coefficients of the red sandstone and granite can be expressed as a linear function of the dynamic elastic modulus. The dynamic elastic moduli of the red sandstone and granite increased with increasing strain ratios and stress ratios. Under the variable-amplitude stress path, the dynamic elastic modulus growth rate of the red sandstone was approximately 60% of that of the granite. Under the fixed-amplitude stress path, the dynamic elastic modulus growth rate of the red sandstone was approximately 20% of that of the granite.

Experimental investigation on mining-induced strain and failure characteristics of rock masses of mine floor

Underground mining is known to cause the deformation and failure of the surrounding rocks and frequently associated with water-inrush through the mine floor. In this study, in-situ strain tests along with borehole imaging measurements were utilized to investigate the deformation and failure characteristics of the rock mass of the mine floor caused by mining. We directly measured the strain and borehole images of the rocks in the mine floor at different depths in five coal mines in the Yanzhou mining area, China. We observed from plotting the increments of strain that mining caused and then led to the deformation and failure of the rock mass of the mine floor. The zone in the floor that was affected by stress from mining was over 170 m in length and decreases with depth. We also found that the depth of failure in floor of the five different coal mines varied between 8.4 to 20 m.

The influence of permeability characteristics of rock mass on hydrogeological engineering

Rock mass; Permeability; Hydrogeology

Numerical simulation study on rheological failure characteristics of rock mass under high stress

This paper uses the RFPA numerical simulation software to establish a numerical model of the rheological failure of the rock mass under stress. Rheological failure characteristics of the body was researched, and the results shows: (1) The rupture sequence of rock rupture is from the corner to the middle. When the rock loses stability under pressure, the rock often ruptures from the corner. The corner gradually collapses and cracks. Then the cracks spread to the middle of the rock. Many cracks extending from the corners are in the rock. The central part intersects each other and eventually causes the rock to break. (2) Rock samples of different lithologies have different stress values when they break under the same confining pressure. From the experimental process, we know that granite>sandstone>mudstone. Therefore, the higher the strength of the rock, the harder the rock will be broken. (3) The weaker the plasticity at rupture, the stronger the brittleness and the stronger the sudden change of rupture. In the deep mining process, the greater the confining pressure, the more obvious the rheological characteristics of the rock, and the greater the total energy released during the rock failure process.

Stability Analysis of Surrounding Rock in Circular Tunnels Based on Critical Support Pressure

Accurate calculation for the critical support pressure of tunnels plays an important role in tunnel stability evaluation and support design. In this study, a mechanical model for circular tunnels is developed. Considering the intermediate principal stress and strain‐softening characteristic of rock mass, the critical support pressure when the plastic zone and damage zone begin to occur is determined based on the unified strength criterion and strain‐softening model. Through the example study, the critical support pressure under different intermediate principal stress coefficient is solved. Furthermore, the effect of initial field stress, softening coefficient, and maximum damage variable on the critical support pressure are also discussed. The results show that the critical support pressure and radii of plastic and damage zones all decrease with the increase of the intermediate principal stress coefficient. The larger the initial field stress, the larger the critical support pressure. The softening coefficient and maximum damage variable of rock mass has no influence on the critical support pressure when the plastic zone begins to form, but has a significant effect on the critical support pressure when the damage zone begins to form. As softening coefficient increases and maximum damage variable decreases, the critical support pressure when the damage zone which begins to form increases. Data presented in this contribution provide significant theoretical insights into evaluating tunnel stability and support system reliability.

Rock Slope Stability Analysis by Using Integrated Approach

Slope stability assessment is an essential aspect of mining and civil engineering. In this study, Songwe open-pit mine in Malawi was investigated to establish possible pit slope instability. In performing the analysis, an integrated approach entailing rock mass characterisation, kinematic and numerical methods were applied. Based on rock mass classification system, Songwe Hill carbonatite rock mass is characterised as a good rock but still it possesses numerous random discontinuities that present a complex challenge in geotechnical engineering. Dip 6.0 software was used in carrying out kinematic analysis based on the attributes of discontinuities. The results show that there is a 16% likelihood of planar failure in the divided slope sections of the planned pit. Thus, slope angle optimisation to 41° has been proposed as a counter-measure to minimise the potential risk of planar failure. At the optimised angle, the risk of planar failure could be reduced by 44%. On the other hand, wedge failure was found to be improbable since no joint intersections were found in the critical zone of potential failure. For numerical analysis, finite element code was applied using FLAC3D 5.0 application. The results demonstrate that overall slope angle of 41° would offer a favourable balance between safety and mining economics as mining operations progress to deeper horizons thereby avoiding a costly push back solution due to instability.