Pillar Burst Research Articles

Instability Mechanism of Pillar Burst in Asymmetric Mining Based on Cusp Catastrophe Model

Pillars are often reserved asymmetrically in the mining process. The roof deflection curve under non-equal span conditions of adjacent stopes is derived by considering the roof-pillar system as a rock beam-pillar model. The pillar instability condition under asymmetric mining is determined based on instability theory and cusp catastrophe theory. Pillar burst represents the equilibrium stability of the roof-pillar system. The pillar failure may be in a violent manner or a gentle manner, depending on the post-peak stiffness ratio of the roof-pillar system. By calculating the factor of safety (FOS) and roof-pillar stiffness ratio K, the pillar stability with different stope spans can be evaluated. The theoretical results are validated by comparison with a case study and numerical simulation. When the stope spans are not equal, the pillar is affected by small-eccentric compression. Four pillar failure patterns under eccentric compression are proposed and explained. The main factors affecting pillar burst appear to include the geometric parameters and mechanical properties of the roof-pillar system. It is difficult to change the mechanical properties, but the stiffness ratio K can be increased by improving the geometric parameters, so as to minimize the burst tendency. Once K < 1 and the critical compression failure load is reached, the pillar on the larger stope span side fails first, and then, the whole pillar loses its stability. Considering the external work during the pillar unstable failure, the rockburst energy index is optimized.

Mechanism and Evolution Control of Wide Coal Pillar Bursts in Multithick Key Strata

Coal mine pillar burst frequently occurs in Western China, which seriously restricts safe production. This paper takes the 35 m coal pillar of the 3102 working face of MKQ coal mine as the engineering background. The mechanism and evolution control of pillar bursts in multithick key strata are studied using field investigation, theoretical analysis, and numerical simulation. The mechanism of dynamic and static stress‐induced pillar bursts was revealed combining the “O‐X” broken features for key strata and numerical simulation of pillar stress evolution. A prevention scheme is put forward for strata presplit blasting and adjusting coal pillar width to minimize the dynamic and static stresses. The results demonstrate the following. (1) In the multithick strata, the first and second near‐field subkey strata have perpendicular “O‐X” broken features, whereas the third far‐field subkey has parallel “O‐X” broken features. The working face has three kinds of periodic weighting phenomena: long, medium, and short. (2) The simulated vertical stress curve of 35 m coal pillar goes through three states: two‐peak, asymmetric trapezoidal and symmetrical trapezoidal shape with the different advancing position of working face. The stress concentration is extensively promoting a high‐risk area for rock burst. (3) The coal pillar burst was induced by the superposition of energy released by the key strata breaking and the elastic energy accumulated in the wide coal pillar. (4) The monitoring data showed that the long, medium, and short periodic weighting steps of multithick key strata are 141.6 m, 43.2–49.6 m, and 17.6–27.2 m, respectively. The microseismic events energy, frequency, and stress of hydraulic support increment are the highest during the long periodic weighting, and the spatial distribution of microseismic events coincides with the stress concentration area. The theoretical analysis is confirmed with the field practice.

Mechanism of Coal Burst and Prevention Practice in Deep Asymmetric Isolated Coal Pillar: A Case Study from YaoQiao Coal Mine

Coal pillar bursts continue to be a severe dynamic hazard. Understanding its mechanism is of paramount importance and crucial in preventing and controlling its occurrence. The extreme roadway deformations from the asymmetric isolated coal pillars in the central mining district of YaoQiao Coal Mine have responded with frequent intense tremors, with risky isolated coal pillar bursts. The theoretical analysis, numerical simulation, and field measurements were done to research the impact of spatial overburden structure and stress distribution characteristics on the isolated coal pillar area, aiming to reveal the mechanism of coal pillar burst leading to the practice of prevention and control in the asymmetric isolated coal pillar area. The study shows that the overburden structure of the asymmetric is an asymmetric “T” structure in the strike‐profile, and the stress in the coal pillar is mostly asymmetric “saddle‐shaped” distribution, with the peak stress in the east side of the coal pillar, and the coal pillar is a “high stress serrated isolated coal pillar.” Numerical simulation results showed that the support pressure in the isolated coal pillar area on the strike profile was asymmetrically “saddle‐shaped” distribution. The peak vertical stress in the coal pillar area continued to rise and gradually shifted to the mining district′s deep part. As a result, the response of the roadway sides to the dynamic load disturbance was more pronounced. They developed a coal burst prevention and control program of deep‐hole blasting in the roof of asymmetrical isolated coal pillar roof and unloading pressure from coal seam borehole. Monitored data confirmed that the stress concentration was influential in the roadway’s surrounding rock in the asymmetric isolated coal pillar area, circumventing coal pillar burst accidents. The research outcomes reference the prevention and control of coal bursts at isolated working faces of coal pillars under similar conditions.

Instability and pillar burst mechanism in roof-pillar system with rock beam embedded in elastic foundation

Pillar burst depends not only on the pillar itself but also on other components of the roof-pillar system. Instability theory is applied to optimize the critical condition of pillar instability, thus minimizing the likelihood of a pillar burst event. The instability mechanism and evolution process of a pillar burst for a rock beam embedded in an elastic foundation (EF) are investigated. The unmined orebody (UMO) is considered to be an EF; the pillar’s strain-softening behavior is described by a Weibull distribution. Pillar burst events occur in the post-peak strain-softening stage of the pillar and are mainly dependent on the pillar strength and stiffness ratio rK of the UMO-roof-pillar system. The EF coefficient kf reflecting the UMO property is an important factor affecting rK. The maximum deviation of rK reaches up to 198% when the UMO is regarded as a rigid foundation (RF), which emphasizes the importance of kf in terms of overall system instability. A typical case of partial pillar recovery is simulated to demonstrate that EF hypothesis accurately reflects the UMO response in such a system. The instability conditions obtained by simulation analysis are consistent with the theoretical results. The instability of the UMO-roof-pillar system is the result of many factors, among which the thickness hb and span l of the rock beam are extremely sensitive. The lateral pressure coefficient does not significantly affect pillar instability and can be neglected to some extent. The pillar burst evolution process can be divided into a stability stage, subcritical instability stage, instability stage, and post-instability stage.

Effect of Stress Anisotropy on the Efficiency of Large-Scale Destress Blasting

Large-scale panel destressing is a rockburst control technique that is used to create a stress shadow in the ore pillar to be mined. The technique aims to reduce the pillar burst proneness by deviating the major induced principal stresses away from the concerned zone of interest. The destress panels, situated in the pillar hanging wall, are choke-blasted with high explosive energy density, and the blast-induced damage in the panel is accompanied by stress dissipation and stiffness reduction due to fragmentation in the panel. These two effects are traditionally modeled holistically with stiffness and stress reduction factors α and β, respectively, applied to the destressed zone. This paper focuses on the interpretation of Phase 3 destress blasting results at Copper Cliff Mine (CCM) where a stress increase (rather than decrease) was detected in the ore pillar crown, while a stress decrease was recorded in the ore pillar sill (as expected). It is hypothesized that high mining-induced stress anisotropy in the pillar crown caused blast-induced fractures to propagate in the orientation of the major principal stress, a condition that would hinder the destressing effect in that orientation. To verify the hypothesis, a series of panel anisotropic rock fragmentation and stress dissipation factors are iteratively tested in a 3-dimensional back analysis of the Phase 3 destress blast. The analysis takes into consideration the stope extraction schedule per the mine plan to better replicate the mining-induced stress condition in the panel and the ore pillar. The results show good agreement with stress measurements taken in situ using borehole stress cells installed in the ore pillar prior to destressing. The paper discusses the implications of preferential fracture propagation orientation and how it might affect the efficiency of destress blasting.

Assessment of rockburst hazard by quantifying the consequence with plastic strain work and released energy in numerical models

Quantifying the rockburst consequence is of critical importance to reduce the hazards with preventative measures in underground mines and deep tunnels. Contours of energy components within a pillar model are plotted at different rockmass damage stages, and plastic strain work and released energy are proposed as indicators of rockmass damage consequence. One pillar model under different loading stiffness is simulated to assess indicators of pillar burst and the resulting damages. The results show the rockmass damage under soft loading stiffness has larger magnitude of plastic strain work and released energy than that which is under stiff loading stiffness, indicating the rockburst consequence can be quantified with plastic strain work and released energy in numerical models. With the quantified rockburst consequence, preventative measures can be taken to avoid severe hazards to mine safety.

Predicting pillar burst by an explicit modelling of kinetic energy

Violent pillar failures known as pillar bursts are suspected to be a possible cause of large collapses in underground mines. A classical stability criterion for mine pillars, based on the relative stiffness of the host rock and the pillars during their post-peak unloading, was proposed by Starfield & Fairhurst (1968)6 and further demonstrated by Salamon (1970)7. An energy balance indicates that an excess kinetic energy is generated when this pillar stability criterion is violated. The present study focuses on demonstrating how an explicit numerical modelling method may be used to calculate and locate the damping of this kinetic energy during pillar failure, considering simple 2D geometries. Arguments in favour of the validation of the numerical results are provided by comparison to analytical calculations and to an empirical classification of rockbursts proposed by Ortlepp (1997)1. The good correlation between numerical, analytical and empirical approaches suggest that explicit numerical modelling of kinetic energy damping, following a procedure proposed in this paper, could be a useful tool for predicting zones submitted to a pillar burst hazard in underground mines and for consequently optimizing the mining method.

Pillar Burst Assessment Based on Large-scale Numerical Modeling

Stress concentration is a direct result of stress redistribution around an excavation, that may lead to rock bursting under specific geomechanical conditions. This paper presents a case-study of a rockburst that took place in the shaft station area of the Provence coal mine in Southern France. The mined coal seam has a 2.5 m thickness and a 10° dip angle. The rockburst occurred in 1993 at the shaft station level, where it is surrounded by several longwall panels that were excavated between 1984 and 1994. The area of the shaft station is at 1000 m depth. A very thin layer of stiff limestone occupied the middle of the exploited coal seam. A large-scale finite difference numerical model of the mine has been constructed by using FLAC3D. The model simulates the area of the shaft with its irregular pillars and the longwall panels excavated between 1984 and 1993. The excavations were performed in two steps. Firstly, the galleries of the shaft station area were excavated in order to determine their effect on the failed pillar. Then, the longwall panels were excavated year by year to detect the stress and strain energy increments induced on the pillars. The origin of the rockburst was analysed based on different rockburst criteria. The results show that the vertical stress increased in the shaft station pillars due to excavation of longwall panels. In addition, we found that the small pillars have higher burst tendency than the large ones. Finally, the Burst Potential Index (BPI) was found to be able to estimate the pillar burst tendency based on the energy storage rate (ESR), however, this criterion (BPI) is based on calculating the stress and the energy changes in the vertical direction only.

A Novel Experimental Technique to Simulate Pillar Burst in Laboratory

Pillar burst is one type of rockburst that occurs in underground mines. Simulating the stress change and obtaining insight into the pillar burst phenomenon under laboratory conditions are essential for studying the rock behavior during pillar burst in situ. To study the failure mechanism, a novel experimental technique was proposed and a series of tests were conducted on some granite specimens using a true-triaxial strainburst test system. Acoustic emission (AE) sensors were used to monitor the rock fracturing process. The damage evolution process was investigated using techniques such as macro and micro fracture characteristics observation, AE energy evolution, and b value analysis and fractal dimension analysis of cracks on fragments. The obtained results indicate that stepped loading and unloading simulated the pillar burst phenomenon well. Four deformation stages are divided as initial stress state, unloading step I, unloading step II, and final burst. It is observed that AE energy has a sharp increase at the initial stress state, accumulates slowly at unloading steps I and II, and increases dramatically at peak stress. Meanwhile, the mean b values fluctuate around 3.50 for the first three deformation stages and then decrease to 2.86 at the final stage, indicating the generation of a large amount of macro fractures. Before the test, the fractal dimension values are discrete and mainly vary between 1.10 and 1.25, whereas after failure the values concentrate around 1.25–1.35.

Stochastic theory for jerky deformation in small crystal volumes with pre-existing dislocations

A statistical theory is proposed to describe the jerky deformation of micron-sized crystal pillars. The probability for a specimen surviving the applied load without generating a strain burst of a given order is analytically expressed in terms of the nucleation rate of that burst. The survival probability can be measured from an ensemble of macroscopically similar deformation experiments to obtain the nucleation rate. From experiments on aluminium pillars, the nucleation rate is found to increase with the pillar size and to decrease with the burst order, indicating that more sources are present in a larger specimen and that the available nucleating sources are progressively exhausted by the occurrence of bursts. The activation volume measured is roughly independent of the pillar size and burst order, indicating a constant mechanism for burst nucleation.

Assessment of horizontal pillar burst in deep hard rock mines

The paper examines three methods for assessing the stability of horizontal pillars in deep underground hard rock mines. Such pillars could suffer from strain-type rockburst when they become too thin in an environment characterized by high in situ stress and hard, brittle rock. The methods presented are: potential stress failure (PSF), energy release rate (ERR) and the Burst Potential Index (BPI). It is shown through a numerical model of a characteristic crown pillar problem that the BPI is the most suitable of the three. Contrary to the PSF method, the BPI method accounts for the rock mass strength and deformability.

Seismic Moment Tensor Resolution on a Local Scale: Simulated Rockburst and Mine-induced Seismic Events in the Kopanang Gold Mine, South Africa

Seismic records contain information about the effect of the source as well as the effect of wave propagation through the rock mass. The effect of wave propagation is usually not well known as only simplified models of geological structures are available. Therefore, the information about the source retrieved by inverting seismograms may include errors due to incomplete knowledge of the rock mass along the propagation path, which in turn cause a distortion in the calculated moment tensor (MT). The distortion of the MT on a local scale was observed by inverting records of a simulated rockburst conducted at the Kopanang gold mine in South Africa. A dominant isotropic component of the explosive characteristics was found from the inversion. The deviatoric components retrieved from the blast are spurious. A test of their stability indicated that they are not significant, assuming an uncertainty above 5% for velocities and 10% for attenuation within the homogeneous model available for the mine. Thus, the retrieval of the MT from records of local networks in mines using a homogeneous model of the rock mass seems to be feasible. However, the homogeneous model of the rock mass can only be applied to close stations, within a few kilometers of the source. The seismic records from distant stations were too complex to be modelled by a homogeneous rock mass. Records of six mine-induced seismic events recorded at the Kopanang gold mine were also inverted. A vertical linear dipole along the pressure (P) axis was found for three of the events, suggesting a pillar burst. The mechanism of two events contains an isotropic implosion together with a nearly vertical dip-slip, and seems to indicate a combination of a cavity collapse with a down dip-slip along a nearly vertical fault. One event corresponds to a dipole along the tensional (T) axis. However, it is vertical, thus its association with tensile faulting of the hangingwall is uncertain.

Dynamic jump modelling of a rock pillar burst (in Polish) : Drescher, A; Lietz, J Arch Gorn, V26, N2, 1981, P383–403

Dynamic jump modelling of a rock pillar burst (in Polish) : Drescher, A; Lietz, J Arch Gorn, V26, N2, 1981, P383–403