The quantitative determination of the geological strength index (GSI) and disturbance factor (D) in the Hoek–Brown failure criterion is of great value because it can increase the reliability of the ...rock engineering design. In this work, a quantitative method for estimating GSI and D values is developed by considering the acoustic wave velocity in the rock mass, Cp. These improved GSI and D values are then used to propose a new method for determining the mechanical parameters of the rock mass based on the original Hoek–Brown failure criterion (which we refer to as ‘the improved Hoek–Brown failure criterion’). The analysis results show that the new method to estimate GSI and D can accurately reflect the factors affecting the rock-mass quality and degree of disturbance before and after blasting or excavation. The new method has high reliability and is also applicable to rock masses that are quite broken (i.e. have low Cp values). More importantly, the mechanical parameters calculated using the improved Hoek–Brown criterion for the rock masses involved in our case studies are in good agreement with the results obtained through long-term monitoring of the deformation. The veracity of the improved Hoek–Brown failure criterion was thus verified in a practical engineering context. Therefore, the proposed method can be used to rapidly determine the mechanical parameters of the rock mass when there is an insufficient amount of test data available.
•New quantitative methods to estimate GSI and D are established based on the Cp value.•Hoek–Brown strength criterion is improved based on the improved GSI and D values.•Mechanical parameters can be estimated using improved Hoek–Brown strength criterion.
Earthquakes are one of the most significant external factors that induce the failure of anti-dip bedding rock slopes (ABRSs). The pseudo-static method is currently (and is likely to remain in the ...near future) the most effective and popular method used to evaluate the stability and design of such slopes under the action of earthquakes. In this work, based on the limit equilibrium method and genetic algorithm, a new method considering the inertial force of earthquakes is proposed to assess the stability of ABRSs. Results previously obtained from centrifuge tests and reported in the literature were used to investigate the veracity of the proposed method. A parametric study was then performed to investigate the effects of earthquakes and the mechanical properties of the rock layers on the failure mechanism of an ABRS. The results show that ABRSs are more likely to undergo a complex combination of shearing and flexural toppling failure under the combined action of gravity and earthquake. The direction of the earthquake-induced inertial force, tensile strength of the rock layers, and shear strength of the joints significantly affect the stability of ABRSs. On the other hand, these factors have little influence on the shape of the failure surface. The failure surface is stepped (with steps of different heights) rather than a plane. The failure surface and corresponding safety factor of an ABRS can be readily found using the proposed method, and the failure mode of each rock layer can be obtained. The method proposed in this work provides a convenient theoretical tool for the design of ABRSs in regions prone to seismic activity.
Soft-hard interbedded structures are commonly found in anti-dip rock slopes and endow the slopes with special properties. In this work, a simple two-beam model was first used to discuss the possible ...failure modes of rock layers with different lithology. The exact failure sequence of rock layers depends on their tensile strengths, elastic moduli, and thicknesses. Then, based on the deformation compatibility of rock layers, a novel theoretical method called the DCM for assessing the stability of soft-hard interbedded anti-dip rock slopes was proposed. A centrifuge test and a numerical model were used to verify the feasibility of the DCM. The calculated critical g-level of 47 g for the centrifuge model and a safety factor of 0.96 for the numerical model agree well with the results of the centrifuge test (45 g) and numerical simulation (0.98). Moreover, the traditional limit equilibrium method (LEM) was also adopted to explore these two models. The calculated results of 68 g and 1.79 using the traditional LEM were found to be much larger than the validated values, with deviations of more than 50%. This finding implies that the traditional LEM is not suitable for assessing the stability of soft-hard interbedded anti-dip rock slopes. In addition, the failure evolution of toppling failure in soft-hard interbedded anti-dip rock slopes was investigated using numerical modelling. The results show that the soft and hard rock layers do not fail at the same time; the rock layers of one type of lithology are destabilized first, then the other rock layers begin to fail until the entire slope undergoes flexural toppling.
•An analytical method is proposed for assessing the stability of soft-hard interbedded anti-dip rock slopes.•The failure sequences of rock layers in a compatible deformation zone can be judged.•The proposed method has high calculation accuracy and can be applied in practice.
Flexural toppling failure is a widespread issue in anti-dip bedding rock slopes. Rock bolts are usually employed to reinforce bedding rock slopes as they are low-cost and efficient. In this technical ...note, a mechanical model based on limit equilibrium theory is proposed to predict the stability of anti-dip bedding rock slopes locally reinforced by rock bolts. By considering a practical case, the proposed model is subsequently verified by comparing its results with numerical results using a discrete element method. Finally, we investigate the influence the positions of rock bolts have on the reinforcement effect. The results show that, for a given bolt angle, the reinforcement effect is controlled by the positions of the rock bolts. In particular, the installation position must start from the superimposed toppling zone (the zone within which the rock layers resemble superimposed cantilever beams). On the other hand, there is no reinforcement effect if the installation position starts from the cantilevered toppling zone (the zone within which the rock layers resemble individual cantilever beams). The method proposed provides theoretical guidance for the design of the reinforcements applied to anti-dip bedding rock slopes. The results produced should also help engineers to gain a better understanding of the reinforcement mechanisms underlying the action of the rock bolts used in anti-dip bedding rock slopes.
•A theoretical model reflecting the effect of local rock bolts is proposed.•Reinforcement effect of rock bolts relies on the reinforcement position.•Influence of the height of rock layer on the reinforcement effect is discussed.
Flexural toppling failure (FTF) is one of the main types of toppling failures and frequently occurs in anti-inclined rock slopes. A new UDEC Trigon approach for simulating FTF of a model slope was ...presented in this paper. FTF of anti-inclined rock slopes characterized by tensile failure of rock columns was successfully captured with the new method. Subsequently, special effort was made to investigate the effects of joint cohesion and joint friction angle on flexural toppling movements' mechanisms. Furthermore, a limit equilibrium method, which can reflect the effects of the joint cohesion, was proposed to quantitatively evaluate the stability of anti-inclined rock slopes against FTF. The results demonstrate that the deformation process of FTF can be divided into three stages: elastic deformation due to cohesion, development of FTF after interlayer slip, and formation of the total failure surface. The inter-column normal forces will suddenly decrease when the failure surface begins to initiate inside the slope, which can be regarded as an instability indicator of anti-inclined rock slopes against FTF. Joint cohesion and joint friction angle were found to have significant effects on the stability of anti-inclined rock slopes, but make an insignificant impact on the shape, location, and formation process of the failure surface. The simulated results indicate that joint cohesion should be considered when using the limit equilibrium method to evaluate the stability of anti-inclined rock slopes against FTF. Inaccurate result will be predicted if the joint cohesion is neglected in the limit equilibrium method.
•Flexural toppling of rock slopes was accurately simulated using UDEC Trigon model.•Inter-column force during flexural toppling was studied.•Effects of joint cohesion and friction angle on flexural toppling were discussed.•A theoretical method including joint cohesion for flexural toppling was developed.
•Two theoretical models for block-flexure toppling failure were proposed.•Detailed damage information of the joints during block-flexure was studied.•Effects of cross joint spacing on block-flexure ...were discussed.
Block-flexure toppling failure is frequently encountered in rock slopes. In this paper, we first proposed two theoretical models to calculate the factor of safety against block-flexure toppling, namely, step by step analysis model (LEM-SSAM) and overall analysis model (LEM-OAM). Then, we developed two special MATLAB codes for a quick stability estimation based on the proposed methods, respectively. Next, the universal distinct element code (UDEC) was used to study the failure mechanisms of block-flexure toppling. In order to realistically simulate the excavation of slope and obtain the detailed damage information of joints, we developed two FISH functions in UDEC. Finally, we conducted a comparative analysis of the limit equilibrium method and the numerical simulations through twelve theoretical models. The results show that many aspects of block-flexure toppling failure have been captured using UDEC combined with the developed FISH functions. The LEM-OAM and UDEC agree well while a larger factor of safety is obtained using the LEM-SSAM. It is found that the dip angles and friction angles of the joints dipping steeply into face have a great influence on block-flexure toppling failure while the influence of the cross joint spacing is less significant.
Obtaining a reasonable assessment of the stability of high-level pillars stabilized using cemented backfill is particularly important when designing deeply-buried metal mines. This research ...determined the average vertical stress in high-level pillars in deeply-buried metal mines and also their strength when subjected to the confining pressure generated by cemented backfill. An expression was thus proposed for the stability of the cemented-backfill-stabilized high-level pillars in deeply-buried metal mines. The results show that the improved formula for estimating the strength of the high-level pillar gives a better reflection of the impact of on-site size and shape effects on the strength of the high-level pillars. The strength of a high-level pillar subject to on-site size effects was obtained by considering the compressive strength of the rock-mass of the pillar under confining pressure provided by the cemented backfill body in the deeply-buried mine. The shape effect was taken into account by considering that the ratio of the effective width to the height of the high-level pillar modifies its strength. At the same time, the concept of load transfer distance was introduced to determine the average vertical stress in the high-level pillar. This method effectively improves the tributary area method which overestimates the average vertical stress in the high-level pillar. When mining the ore body inside a pillar's zone of influence, the area of the orebody excavated should not exceed 0.80 (otherwise it will cause a significant increase in the average vertical axial stress in the high-level pillars). The new method proposed in this study allows the stability of high-level pillars stabilized using cemented backfill to be assessed when an insufficient amount of large-scale in situ testing has been conducted in a deeply-buried metal mine. It also provides a fast, economical, and reliable method for assessing the stability of mined-out areas in deeply-buried metal mines.
•An improved formula for estimating the strength of high-level pillars is derived.•A method for determining the average vertical stress in the high-level pillars is proposed.•A method for assessing the stability of the high-level pillars in deeply-buried metal mines is proposed.
An extra-deep multiple-point borehole extensometer, applicable up to 300 m depth of cover, is introduced for the deformation monitoring of the deep rock masses (i.e., the rock mass below the ground ...surface) in the eastern area of Jinshandian Iron-ore Mine, and the in situ monitoring results collected over eight years are analysed. Combined with the deformation monitoring results on the ground surface collected by using a Global Positioning System (GPS), Level Monitoring Technique, and a three-dimensional Laser Scanning System (Leica HDS 880), the ground movement mechanism of the deep rock masses induced by underground mining is investigated. Results show that the extra-deep multiple-point borehole extensometer can capture the deformation information pertaining to deep rock masses in metal mines with complex geological conditions such as Jinshandian Iron-ore Mine. The moment that the deformation of the monitoring point starts to increase significantly can be used to determine whether the major deformation has transferred to the deep rock mass at the monitoring points, or not: the monitoring results from such critical moments indicate that the major deformation of the overburden strata gradually develops from the lower part to the upper part, consistent with the results from a physical modelling experiment. In Jingshandian Iron-ore Mine, the second caving of the deep rock masses reaches the ground surface above the centre of the mined-out area, and then causes the ground collapse; under the combined effect of the underground mining in Jinshandian Iron-ore Mine and the remaining private mined-out areas, large-scale ground surface collapses and cracks can be observed in mining-affected areas. The deep rock masses surrounding the mined-out areas are in a state of global deformation, and the boundary line of the deep break zone is a fold line. Currently, the rock masses in the major deformation zone move downwards under gravity on the shear surfaces in Jingshandian Iron-ore Mine.
•Extra-deep borehole extensometers are used for deep deformation monitoring.•Eight years’ in situ monitoring data from deep holes are analysed.•Ground movement mechanisms in deep rock masses are investigated.
During metal mining, the deformation and failure of rock mass are very important factors that dominate rock movement and zoning. Numerical modelling provides a feasible way to study the deformation ...and failure of rock mass. In this study, a numerical method of UDEC was adopted to study strata movement mechanism influenced by NWW-trending joints at the footwall at Chengchao Iron Mine, and the Mohr-Coulomb model and the Coulomb-slip model with residual strength were adopted as the constitutive models of rock masses and joints, respectively. The whole strata movement process was simulated and the strata movement mechanism at footwall and zoning phenomenon was clarified. In the chimney caving development stage, gravity is the driving force and surface deformations are mainly subsidence deformation of the surface above the ore body. In the post-chimney deformation stage, horizontal tectonic stress is the driving force and surface deformation and subsidence are dominated by horizontal deformation of the hanging wall and footwall in the mining area. After the level −395.0 m is mined out, the deformation characteristics of deep surrounding rock were revealed by means of numerical modelling, the maximum failure depth of the cantilever beam and the spatial distribution of deep surrounding rock zones can be determined. At last, the influencing boundary of surface deformation expands rapidly in a linear way with the excavation step and the expanding velocity of the horizontal deformation is greater than that of the subsidence in the post-chimney deformation stage.
The engineering geological and hydrogeological conditions in the eastern mining area of the Chengchao Iron Mine are analyzed in detail. The results are combined with the results of site ...investigations and in situ monitoring data, allowing the characteristics of the ground-surface collapse events to be summarized. The mechanism by which ground-surface collapse is induced by underground orebody extraction or de-watering activity was then investigated. The results show that the ground-surface in the mining areas experienced three collapse processes: karst collapse, mixed collapse, and mining-induced collapse. Karst-collapse sinkholes were found during the construction stage of the mine and are generally formed in the marble part of the marble–granite contact zone. These sinkholes show a ‘bead-on-a-string’ distribution, and lines connecting their centers are consistent with the strike of the marble. Such sinkholes are mainly formed due to changes in the dynamic conditions of groundwater caused by de-watering activity. Furthermore, the development level of the collapse mainly depends on the location and scale of the karst caves within the marble. Mixed collapse occurs in the initial mining stage and mainly results from the combined effect of underground orebody excavation and de-watering activity. Mining-induced ground-surface collapse pits mostly develop in the diorite and hornstone and generally show a zonal distribution along the strike of the orebodies from east to west. Mining-induced collapse occurs in the form of chimney caving. First, a caved pipe is formed where the rock masses above the caved zone are highly fractured. Then, the rock masses around the caved pipe gradually cave, and finally a large caved-zone is formed above the mined-out area. Mixed collapse is of particular interest as it is a peculiar type of subsidence involving the superposition of collapse processes related to both mining and karst cavities, and it can provide some instructive insight into the collapsing behavior of the ground-surface.
•Characteristics of ground collapse and corresponding mechanism are investigated.•Ground surface experiences karst, mixed, and mining-induced collapse.•Mixed collapse is related to both orebody mining and overlying solution cavities.
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