Rock Mass Strength Research Articles

Discrete Element Simulation Analysis of the Bending and Toppling Failure Mechanisms of High Rock Slopes

The high rock slope situated in the Southwest stope of Taiping Mining, Inner Mongolia, is subject to dumping failure due to its instability. The dumping body rock layer of this stope shows obvious bending and lowering of the head. The overturning angle of the rock strata can reach 46°, and tension dislocation along the rock joint can be observed in exposed sections and at the bedding and lithologic interface. The sliding surface also displays a broken line morphology. Through evaluation of regional rock integrity parameters and rock soft and hard parameters, rock-mass strength based on Hoek Brown strength estimation criteria can be developed. Based on the discrete element method, the geological model of layered excavation of the thin layer slope can be constructed. Combined with indoor and outdoor assessments, the characteristics of toppling deformation of the thin layer open-air slope can be studied and summarized. In this study, simulation analysis showed that under first excavation conditions, a crack-, dump-, and antislip zone was formed. The rock in the crack zone formed a “<”-shaped fracture along the slope surface that was squeezed towards the bottom of the slope. In the lower dumping area, the deflection angle gradually increased with excavation, and the deformation range and levels in the antislip area increased with excavation. Following the third excavation, the antisliding zone disappeared and the toppling line changed from a broken line to a straight line. In the final state, the slope collapsed as a whole, with the collapse of the dumping body penetrating the top to the foot of the slope.

Failure Characteristics of Complicated Random Jointed Rock Mass Under Compressive-Shear Loading

Most natural rock masses contain a large number of random joints and fissures, and most of the rock masses at the rock engineering are commonly in both compression and shear stress environment. However, the research on the failure characteristics of complex random jointed rock mass under compressive-shear loading is still limited. To address this gap, this paper uses the particle flow code 2D to establish a discrete fractured rock mass model and carry out a series of numerical tests with different compressive-shear angles (α) and different joint geometric parameters. The effects of compressive-shear angle and joint geometric parameters on the strength and failure characteristics of fractured rock masses are studied. The results indicate that with the increase of α, the peak strength of the specimen decreases gradually, and the failure mode changes from the composite shear failure mode (Mode-I) to a plane shear failure mode (Mode-II) and then to intact shear failure mode (Mode-III). Specifically, the three failure modes occur in the specimens with α = 15°, 30° or 45°, 60°, respectively. The existence of joints affects stress distribution on rock mass during the loading process. Furthermore, the stress at the joint tip is relatively concentrated, while on both sides of the joint is smaller. Three kinds of crack coalescence patterns are observed: tensile, shear, and tensile-shear mixed coalescence. The inclination angle of the rock bridge between adjacent joints affects the specific type of coalescence.

Corrected Rock Fracture Parameters and Other Empirical Considerations for the Rock Mechanics of Rock Masses of Doha, Qatar

The objective of this paper is to provide insights into the intact rock and rock mass properties of the rock formations under the city of Doha, State of Qatar. We also intend to scientifically clarify these properties by presenting and statistically characterizing the ranges of the parameters, and by discussing the correlations between the parameters with respect to their usage and research potential. The rock quality designation corrected (RQDC) parameter is validated and a new parameter, the fracture index corrected (FIC), is proposed. The significantly improved correlation between RQDC and FIC is demonstrated and their derivation is explained. The paper demonstrates the correlation between the rock mass estimation parameters obtained through rock face mapping and discusses the applicability of the Hoek–Brown criterion to the studied rock masses, which is found to be relevant. A discussion about how properly performed triaxial tests can directly provide the rock constant mi values for all geological members is presented. Other estimation approaches for mi are also validated and compared with the existing knowledge base. Data for laboratory and field intact rock and rock mass parameters are combined using equations from various authors to obtain narrow ranges for rock mass strength and rock mass elasticity modulus values. Finally, within the framework of previous studies by other authors on the low-end transition range of rocks toward soils, it is shown that only the Rus formation member is sufficiently soft and can be included in the range.

Experimental Study on Anchorage Mechanical Behavior and Surface Cracking Characteristics of a Non-persistent Jointed Rock Mass

It is significant to conduct anchorage on the non-persistent jointed rock mass by bolts to inhabit the unstable fracture evolution of rock engineering. In this research, a series of experiments are done on 70 specimens to investigate the effect of anchorage method on strength and deformation behavior of a non-persistent jointed rock mass. First, based on the stress–strain curves of anchorage jointed specimens, the effect of anchorage method on the peak strength and elastic modulus of non-persistent jointed rock mass is investigated. The experimental results show that the peak strength and elastic modulus of jointed specimens changes with anchorage method for the same joint angle, while first decreases and then increases from 0° to 90° for the same anchorage method. Second, the effect of pretightening force on the strength and deformation behavior of anchorage jointed specimens is analyzed. The axial stress-axial strain curves of anchorage jointed specimens with various pretightening forces can be characterized into five types: (I) strain softening after the specimen drops to yield platform from peak strength; (II) strain hardening after the specimen drops to yield platform from peak strength; (III) strain hardening after the specimen yields; (IV) stepwise strain softening after the peak strength; (V) single stress drop after the peak strength. The peak strength and elastic modulus of jointed specimens for the same dip angle all increases nonlinearly with the pretightening force. And then, based on a series of surface failure mode of non-persistent jointed rock specimen, it can be seen that the initiation, propagation and coalescence of surface cracks depend on not only the joint angle, but also the magnitude of pretightening force. Twelve crack coalescence types are identified to analyze the surface crack mode of anchorage non-persistent jointed rock specimens. Finally, the effect of pretightening force on brittleness index of non-persistent jointed rock mass is made a discussion.

Ground Support Design of Underground Openings in Seismically Active Mine: A Case Study from a Lead and Zinc Underground Mine

Ground control is multi-tasking and continuous process for the stability of underground openings. Balya underground lead and zinc mine is an important case study area due to its deepest working condition and the highest production amount among Turkish underground metalliferous mines. Mining-induced seismicity needs to be investigated especially for deep as well as tectonically active mine sites. Increasing depth will increase the stress around an underground opening that is a key factor of rock-burst hazard. Rock burst potential can be estimated from analytical approaches based on the relation between rock mass strength and stress around openings; however, one of the best way is to monitor the mining-induced seismicity. The support system is designed based on the actual geotechnical conditions determined from the excavated working face prior to support applications in the mine. Empirical methods are then used to make a decision about the support system. If the location is designated as a seismically active region based on micro-seismic event results, support components are selected as a dynamic support system that must absorb the energy released from the surrounding rock masses. The purpose of this study is to investigate seismically active locations according to mining-induced seismicity and the support system design principles for that locations to protect the openings against seismic loads.

Back-analysis of Donghekou landslide using improved DDA considering joint roughness degradation

Currently, the dynamic behavior of high-speed long-runout landslides triggered by earthquakes remains unclear due to the complexity of such phenomena. Degradation in the shear strength of rock masses is considered a primary factor contributing to the high velocities and long runout distances of these landslides. In this study, the strength degradation induced by changes in rock joint morphology is analyzed considering a numerical model of the Donghekou landslide, by using discontinuous deformation analysis (DDA). First, an exponential joint roughness degradation function (i.e., JRCm) that depends on displacement is incorporated with the nonlinear Barton–Bandis (B–B) shear strength criterion, and this modified B–B criterion is combined with the DDA. Moreover, a virtual joint is introduced in the DDA to model the propagation of cracks in intact rock masses, in order to simulate the dynamic phenomenon of earthquake-induced landslides. Subsequently, this improved DDA (IDDA) is validated via simulations of a shear test and a sliding block over an inclined plane. Finally, the IDDA is employed to simulate the Donghekou landslide, which was induced by the 2008 Wenchuan earthquake in China. Furthermore, the entire dynamic process of the landslide, including crack propagation, instability, initiation, and motion, is depicted. The simulated deposit patterns obtained via the IDDA are in good agreement with the results of field investigations. The simulation results also prove that the IDDA can reflect key movement behaviors of Donghekou landslide and that the degradation of rock joints significantly influences the long runout distance and high velocity of sliding mass.

Stability analysis of rock slopes using strength reduction adaptive finite element limit analysis

The main aim of this study is to propose an effective approach to conduct the stability analysis of rock slopes. Based on the strength reduction technology, the recent developed adaptive finite element limit analysis is adopted to investigate the stability of rock slopes for the first time. The rock mass strength is described by the latest generalized Hoek-Brown failure criterion. To better estimate the rock slope stability, the rock mass disturbance D and slope angle B are also incorporated into the determination of factor of safety by implementing the disturbance weighting factor s and the slope angle weighting factor fB, respectively. The factors of safety obtained from this study are validated by those from existing works based on two real slope cases. For practical use, the factors of safety for the cases of B=45o in undisturbed rock masses are provided in the form of design charts. Finally, a set of design charts for s and a fitting formula for fB are presented and discussed.

Experimental Study of the Failure Mechanism of the Anchorage Interface under Different Surrounding Rock Strengths and Ambient Temperatures

In order to study the anchoring instability mechanism of surrounding rock in deep roadway, the failure mechanism of the bolt‐anchoring agent interface was studied by simulating different strength rock mass and ground temperature environment, using C20, C40, and C60 strength concrete and steel pipe to simulate different surrounding rock strength environments. Indoor pull‐out tests were carried out to study the pull‐out load displacement relationship, ultimate pull‐out force, residual anchoring force, the distribution law of axial stress and tangential stress along the bar, and the energy consumption value of drawing failure at 20, 50, and 70°C. The test results show that, with the decrease of surrounding rock strength or the increase of ambient temperature, the pull‐out force, residual anchoring force, and energy consumption value of anchorage interface gradually decrease; under different axial forces, the axial force distribution of the rod body decreases exponentially from the anchoring end to the opposite end; and the shear stress transfers to the deep part of the anchor body with the increase of the load. According to the failure phenomenon of the specimen, the failure modes of the bolt bolt‐anchorage agent interface can be divided into shear slip mode and shear expansion slip mode. The shear expansion slip formula of anchorage interface is derived. Using high‐strength and temperature‐resistant resin anchoring agent for comparative test, the rationality of the mechanism analysis is proved, which provides more clear guidance for the construction of anchor support.

Engineering geological mapping for empirical design evaluation of Tunnel 10 of Jakarta - Bandung high-speed railway, Indonesia

An engineering geological mapping was carried out at the construction site of the Tunnel 10 of Jakarta Bandung High-Speed Railway to obtain data and information of the engineering geological conditions, particularly the rock masses. This research aims to determine the rock mass classes at the tunnel construction site and recommend the tunnel support system based on the Rock Mass Rating (RMR) and the Japan Society of Civil Engineers (JSCE) systems. This research is expected to better understand the rock mass classes, which were previously determined based on the newly applied Basic Quality (BQ) system for the tunnel support empirical design. The results showed that the research area consisted of young volcanic products, namely moderate to highly weathered tuff breccia and andesitic breccia. The uniaxial Compressive Strength (UCS) of rock mass varies between 1-25 MPa. The RMR value ranges from 21 to 40, indicating disintegrated and poor rock mass quality. The proposed tunnel support system is the combination of shotcrete, steel support for top heading and bench support, arch sidewall, and invert concrete.

Creep Fracture Characteristics of Fractured Rock Mass Strengthened with Toughened Epoxy Resin

For improving the toughness and long‐term strength of the fractured rock mass reinforced with epoxy resin and reducing the opening displacement of the crack tip, three types of converging cracks are established according to the deformation and failure characteristics of the rock mass, structural planes, and epoxy resin composite structure. The stress intensity factor of the type II compound crack and the analytical formula of the crack surface tip opening displacement are obtained. An improved Kelvin nonlinear creep model is proposed. Through the creep isochronous curve, the time and long‐term strength of accelerated creep failure are obtained, as well as creep rupture coupling characteristics. The ductile plastic deformation is used to characterize the toughness, and the toughness of epoxy resin is enhanced by adding mud powder, and the relationship between toughness and long‐term strength is established. Combined with experimental verification, when the drying mud powder content is 20%, the toughness of the epoxy resin has a good correlation with the long‐term strength; when the mud powder content exceeds 30%, its ductility and long‐term strength decrease, and at the same time the toughness effect is significantly weakened. The research results can provide a useful reference for toughened epoxy resin to strengthen fractured rock mass engineering.

Progressive Failure and Acoustic Emission Characteristics of Red Sandstone with Different Geometry Parallel Cracks under Uniaxial Compression Loading

The geometric distribution of initial damages has a great influence on the strength and progressive failure characteristics of the fractured rock mass. Initial damages of the fractured rock were simplified as parallel cracks in different geometric distributions, and then, the progressive failure and acoustic emission (AE) characteristics of specimens under the uniaxial compression loading were analyzed. The red sandstone (brittle materials) specimens with the parallel preexisting cracks by water jet were used in the tests. The energy peak and stress attenuation induced by the energy release of crack initiation were intuitively observed in the test process. Besides, three modes of rock bridge coalescence were obtained, and wing crack was the main crack propagation mode. The wing crack and other cracks were initiated in different loading stages, which were closely related to the energy level of crack initiation. The propagation of wing crack (stable crack) consumed a large amount of energy, and then, the propagation of shear crack, secondary crack, and anti‐wing crack (unstable crack) was inhibited. The relationship between the crack propagation mode and the geometric distribution of existing cracks in the specimen was revealed. Meanwhile, the strength characteristic and failure mode of fractured rock with the different geometric distributions of preexisting crack were also investigated. The energy evolution characteristics and crack propagation were also analyzed by numerical modeling (PFC2D).

Experiment and Failure Analysis of Rock‐Like Material Affected by Different Excavation Depths

In order to increase the understanding of the strength and failure mechanism of rock mass during tunnel excavation, a series of uniaxial compression tests were conducted on mortar specimen with cracks and holes by using a rock mechanics servo‐controlled testing system. And by monitoring the experimental process, the initiation, propagation, and coalescence process of cracks were observed and characterized. According to the experimental results, the influences of the excavation depth on the mechanical parameters and fracture characteristics of mortar specimens with single hole and the ones with single‐hole crack were analyzed in detail. In the specimens with single hole, the peak strength decreases with the increase of hole depth, but the peak strain and elastic modulus have no obvious linear correlation with the hole depth. And the position and angle of initial crack change differently with the increase of the hole depth. The position of initial crack moves from the side of the hole to the top of the hole. When the hole depth exceeds 50%, the crack initiation angle is no longer inclined to the axial stress direction, but parallel to the axial stress direction. In the specimens with single prefabricated crack, the wing‐shaped secondary cracks are generated at the tip of the precrack, and the antiwing‐shaped secondary cracks are generated at the tip when approaching the peak stress. However, in the specimens with single‐hole crack, no antiwing‐shaped crack appears. And when the hole depth reaches 80%, two wing‐shaped cracks appear at the precrack tip. One of the new wing‐shaped cracks appears in the direction of the extension line of the precrack.

Compressive strength of flawed cylindrical specimens subjected to axial loading

Discontinuities are known to have a significant impact on the engineering characteristics of the rock masses, governing their potential failure pattern, increasing their deformation, and reducing their strength. In particular, the impact of non-persistent joints on the strength and failure mechanism of rock mass needs to be investigated further. The impact of different flaw geometrical characteristics such as flaw inclination, flaw length, flaw aperture, and flaw filling on uniaxial compressive strength of specimens has not been investigated thoroughly. In this paper, a series of uniaxial compression tests were conducted on cylindrical specimens containing an open central flaw. The effect of different parameters such as flaw inclination, flaw length, flaw aperture, and filling on the uniaxial compressive strength of specimens have been investigated through laboratory experiments. Response Surface Methodology (RSM) is adopted to analyze the impact of flaw parameters on the compressive strength of the constructed samples. The results of the experiments show that flaw inclination and flaw length have a significant impact on the peak strength of the samples, meaning that strength increases by growing of flaw angle and decreases by increasing of flaw length. In addition, at a low flaw length, aperture affects the UCS significantly, while by increasing flaw length, its effect decreases dramatically, and strength drops at a flaw inclination of 45 degrees. Conversely, at a higher flaw length, by increasing flaw inclination, the UCS increases constantly. It also has been observed that changing the flaw aperture had no important effect on the peak strength.

Quantifying Roof Falling Potential based on CMRR Method by Incorporating DEMATEL-MABAC Method; A Case Study

This work incorporates the DEMATEL-MABAC method for quantifying the potential of roof fall in coal mines by means of the coal mine roof rating (CMRR) parameters. For this purpose, considering the roof weighting interval as a quantitative criterion for the stability of the roof, the immediate roof falling potential was quantified and ranked in 15 stopes of Eastern Alborz Coal Mines Company. In this regard, on the basis of the experts’ judgments, the fuzzy DEMATEL method was used for designation weights of the parameters, and the MABAC method was incorporated to quantify and rank the stopes (alternatives). “UCS of roof” and “joint spacing” in the immediate roof were found to be the most important parameters that controlled roof falling in stopes; and “joint persistence” was also found to be a quite significant parameter. Finding confirms that overall strength of rood rock mass plays a main role in the falling potential. Comparison of the coefficients of determination (R2) between the weighting interval and proposed model with that and original CMRR indicated more than 15% increase, which represented that the new proposed model was more accurate to quantify roof quality. The findings of this work show that using this combined method and specializing the CMRR method for a given mine geo-condition to assess the quality of the roof and its potential of collapse possesses a higher performance when compared with the original CMRR method.

Effect of Joint Density on Rockburst Proneness of the Elastic‐Brittle‐Plastic Rock Mass

The prediction of rockburst proneness is the basis of preventing and controlling rockburst disasters in rock engineering. Based on energy theory and damage mechanics, the quantitative functional relationship between joint density and energy density was derived. Then, the theoretical results were verified by numerical simulation and uniaxial compression test, and the effect of joint density on rockburst proneness of the elastic‐brittle‐plastic rock mass was discussed. The results show that the relationship between the joint density and the dissipated energy index of the jointed rock mass is a logarithmic function. With the same total input energy, the higher the joint density, the more the damage dissipation energy. Even in the case of high joint density, the rock mass still has limited resistance to external failure. Under the same joint density, the strength of parallel jointed rock mass is better than that of the cross‐jointed rock mass, and the parallel jointed rock mass can accumulate more elastic strain energy and has higher rockburst proneness. The joint density is closely related to the ability of the rock mass to store high strain energy. The higher the joint density is, the weaker the ability to accumulate the elastic strain energy of rock mass is and the lower the rockburst proneness is. It is helpful to predict rockburst proneness by investigating and studying the properties of geological discontinuities. The research results have some theoretical and engineering guiding significance for the prediction of rockburst proneness of the jointed rock mass.

Geological mapping and stand-up time estimation based on core drill evaluation using RMR89 on Tunnel 3 Sigli Banda Aceh Toll Road

Tunnel 3 of the Sigli Banda Aceh Toll Road has been designed based on several site investigations. The site investigations have not included surface geological investigations. Subsurface geological investigations have been carried out but with less detailed drill core evaluation. This will cause less accuracy on the assessed tunnel stand-up time. This research aims to evaluate the engineering geological conditions at the tunnel construction site by conducting geological mapping with 1:5,000 scale and re-evaluating drill core using RMR89 method more thoroughly so that it can produce more accurate assessment of the tunnel stand-up time. The geological mapping result shows that the research area has three rock units, namely lapili, tuffaceous sandstones, and interbedded sandstone-siltstones, with moderate to fresh weathering degree. The drill core evaluation shows that research area has poor to good rock mass classes (34 – 67). The drill core evaluation results also indicate possible geological structures that may influence subsurface rock mass strength. The tunnel stand-up time obtained from the lowest RMR89 value for each drill core shows a value of 5 - 90 hours, implying immediate application of support system after tunnel excavation.

Structural Mechanical Characteristics and Instability Law of Roof Key Block Breaking in Gob-Side Roadway

The influence of mining on the upper section of working face leads to the fracture of the lateral key block of the roof. From the goaf to the coal body, a group of “left-middle-right” key blocks are formed. According to the three different spatial position structure relations formed by roadway and broken key block in practical engineering, the mechanical causes of broken structure of key block in roof of roadway along goaf are analyzed. FLAC3D is used to simulate and analyze the deformation characteristics and stress state of key block structure model before and after roadway excavation, and the mechanical characteristics and instability mechanism of key block sliding and breaking under three spatial structure modes are obtained. With the help of the mathematical model of material mechanics, the structural mechanical behavior of key block model of roof before and after roadway excavation and the temporal and spatial evolution law of unloading and breaking are studied. The results show that the complex influence factors of mining disturbance and low strength of the weak rock mass will weaken the internal balance of “masonry beam” structure. When the roadway is located below the fracture line of the key block, the middle key block will rotate and lose stability with the side hinge joint of the goaf as the axis; when the roadway is located in the fracture line of the key block, it is easy for the middle key block to slide and lose stability; when the roadway is located outside the fracture line of the key block, the middle key block is in the state of complete collapse, the mechanical transmission mechanism of the surrounding rock in the vertical direction is weakened, and the surrounding rock is the most stable.

Quantifying the competing influences of lithology and throw rate on bedrock river incision

AbstractRiver incision in upland areas is controlled by prevailing climatic and tectonic regimes, which are increasingly well described, and the nature of the bedrock lithology, which is still poorly constrained. Here, we calculated downstream variations in stream power and bedrock strength for six rivers crossing a normal fault in western Turkey, to derive new constraints on bedrock erodibility as function of rock type. These rivers were selected because they exhibit knick zones representing a transient response to an increase in throw rate, driven by fault linkage. Field measures of rock mass strength showed that the metamorphic units (gneisses and schists) in the catchments are ∼2 times harder than the sedimentary lithologies. Stream power increases downstream in all rivers, reaching a maxima upstream of the fault within the metamorphic bedrock but declining markedly where softer sedimentary rocks are encountered. We demonstrate a positive correlation between throw rate and stream power in the metamorphic rocks, characteristic of rivers obeying a detachment-limited model of erosion. We estimated bedrock erodibility in the metamorphic rocks as kb = 2.2–6.3 × 10−14 m s2 kg−1; in contrast, bedrock erodibility values were 5–30 times larger in the sedimentary units, with kb = 1.2–15 × 10−13 m s2 kg−1. However, in the sedimentary units, stream power does not scale predictably with fault throw rate, and we evaluated the extent to which the friable nature of the outcropping clastic bedrock alters the long-term erosional dynamics of the rivers. This study places new constraints on bedrock erodibilities upstream of active faults and demonstrates that the strength and characteristics of underlying bedrock exert a fundamental influence on river behavior.

Carbon foot print evaluation in tunneling construction using conventional methods

The construction sector is one of the most relevant related to the emissions of the greenhouse gases (GHG). Within this sector, the weight of tunneling construction is significant considering the resources employed (for example, tunnel construction represents about the 25% of the total concrete used in civil works in Spain). Nevertheless, it seems that there is still a gap to make the calculations of these emissions for each specific tunnel during its design and later during its construction, taking into consideration the specific parameters that influence them.The present work provides the fundamentals for applying in a simplified way the Life Cycle Assessment method (LCA). A simplified calculation model is propose to estimate the CO2 emissions of each step of the construction phase in the case of tunnels advanced through medium to low strength rockmass by conventional methods (drilling and blasting, roadheader or hydraulic breaker hammer). The CO2 emissions can be calculated by estimating the amount of diesel, electrical energy and materials consumed, and then using the conversion factors to CO2, from well recognized organizations or more domestic sources (as some Spanish institutions).The simplified calculation model has been tested with data of a real tunnel and then it is used to analyze CO2 emissions of different tunnels to determine the influence of different factors. The main conclusion is that, for this kind of tunnels, the most relevant contribution to CO2 emissions is that associated to the fabrication of concrete used in the support and lining of the tunnel which represents, in average, about 80% of the total.

Strength anisotropy of jointed rock slope considering mining damage: a numerical analysis method

A combination of Hoek–Brown strength criterion and geological strength index (GSI) rock mass classification is a common method to determine rock mass strength. However, the conventional method involving the determination of disturbance coefficient (D) cannot reflect the anisotropic characteristics of strength of jointed rock mass when considering mining damage. A numerical analysis method is proposed to determine the value of D using jointed rock damage tensor and mining damage evolution simulation. The anisotropy of rock mass strength at different mining depths for a high jointed rock slope in Dagushan open-pit mine was analyzed by mining damage simulation. Additionally, the Rosenblueth moment point estimation principle was combined with Monte-Carlo method to obtain the random strength of a jointed rock mass considering the effects of mining probability damage. The results show that the influence of the unloading due to excavation to stress state of rock mass is relatively obvious within 100 m away from the slope surface, however, the area of drastic change of damage is limited to a range of ∼50 m. Moreover, the reliability of local slope, analyzed using limit equilibrium method (Janbu), shows that consideration of spatiotemporal anisotropy of rock mass with mining damage is closer to reality.