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In order to reveal the load transfer mechanism during cascading pillar failure, compressive tests on treble-pillar specimens were conducted under soft and stiff loading conditions, where the stiffness of the test machine was adjusted with a disc spring group. Experimental results showed that the load transfer behavior of treble-pillar specimen could only be reproduced under soft loading condition when the rapid elastic rebound is achievable with disc spring group. The load transfer behavior of treble-pillar specimen is governed by energy storage characteristics of test machine and the mechanical properties of three rock specimens. In this respect, the failure behavior of treble-pillar specimen under soft loading condition was summarized into the following three failure modes: successive failure mode, compound failure mode and domino failure mode. Additionally, a theoretical model was proposed to further explain the physical mechanism of load transfer behavior, where the theoretical results of load transfer and elastic rebound of disc spring group were in good agreement with the experimental results. Finally, it was concluded that the elastic deformation of near-field surrounding rockmass (or the soft loading condition) was the necessary condition for load transfer of multiple pillars; and the rapid elastic rebound of near-field surrounding rockmass was the physical essence of load transfer behavior. This study may contribute to understanding the load transfer mechanism among pillars and to optimizing the design of room-and-pillar stopes during underground mining.HighlightsThe soft loading condition of test machine is realized by adjusting the stiffness of disc spring group.The experiments on treble-pillar specimens are conducted to reveal the load transfer mechanism during cascading pillar failure.Three failure modes of treble-pillar specimen are successive failure, compound failure and domino failure.

This study investigated the use of a limit equilibrium model to simulate the time-dependent scaling of hard rock pillars. In the manganese bord and pillar mines in South Africa, extensive scaling is observed for pillars characterised by a high joint density. It appears that the scaling occurs in a time-dependent fashion. Evidence for this is the ongoing deterioration of pillars in old areas, even after the pillars are reinforced with thin spray-on liners. Monitoring of selected pillars were conducted in an attempt to quantify the rate of time-dependent scaling. Contrary to expectations, almost no additional scaling was recorded for the pillars during a 3-month monitoring period. The scaling distance for pillars of different ages could be measured and it seems as if most of the scaling occurred soon after the pillars are formed. Only a limited amount of additional time-dependent scaling seems to occur after this. Numerical simulations of the time-dependent scaling were conducted using a displacement discontinuity code and a limit equilibrium constitutive model. The postulated exponential decay of the failed rock mass strength at the edges of the pillars resulted in simulated behaviour that is qualitatively similar to the underground observations. The results from this study are encouraging and the method can be used to investigate the long-term stability of bord and pillar excavations. Further work is required to improve on the calibration of the model and to better quantify the rate of scaling of the underground pillars.HighlightsTime-dependent scaling gradually reduces the strength of pillars. This paper presents a study of this behaviour in a hard rock bord and pillar mine.A numerical modelling approach to simulate time-dependent pillar failure, on a mine-wide scale, is described in the paper. It consists of a displacement discontinuity boundary element method with a time-dependent limit equilibrium model.The behaviour of the hard rock pillars in the manganese mines in South Africa is used to test the proposed model. It provides valuable data for researchers interested in case studies of time-dependent pillar strength.The proposed modelling methodology seems valuable to design layouts where long-term stability is a requirement. Although the focus in this paper is on hard rock mines, it can also be used for coal pillars.

With the increase of mining scale and depth, the cascading pillar failure (CPF) disaster has gradually become one of the core technical challenges for safe mining. This paper studies the CPF disaster of multi-pillar (P 1-2 , P 2-3 , P 3-4 and P 4-5 ) at 848 m level of Alhada Lead-Zinc Mine based on the RFPA 2D numerical simulation software and stiffness theory. The numerical simulation results indicate that the load transfer effect induces the domino instabilities of double pillar (P 2-3 and P 3-4 ) and double pillar (P 1-2 and P 4-5 ), which is characterized by the '2 + 2' compound failure mode. The physical essence of load transfer effect is revealed through numerical simulation, which is the elastic rebound of surrounding rockmass (such as roof-floor). In addition, the theoretical model of rebound overload mechanism of roof-multi-pillar-floor system is established based on the catastrophe theory, and the instability criterion, sudden jump and energy release of roof-multi-pillar-floor system are derived. Finally, the main influencing factors of load transfer effect such as pillar spacing and damage fracture zone are quantitatively analyzed, and the load transfer law shows that the load transfer effect decreases with the increasing pillar spacing and is hindered by the structural plane in the surrounding rockmass.

Accurately distinguishing the stability of the residual coal pillars formed by the room-and-pillar mining method is significant for the safe mining of adjacent coal seams. In this study, the correlation between the rapid decrease in vertical stress and the connectivity of the internal dissipative energy core during the instability of coal pillars is revealed. Then, a new method for distinguishing the stability of coal pillars based on the above correlation is proposed, overcoming the shortcomings of previous studies that only used the plastic zone range to determine the stability of coal pillars. Based on this discriminant index and simulation method, the mechanism of residual coal pillar failure as well as the dynamic instability and expansion characteristics of multi-pillars have been revealed. The engineering method of grouting and filling to enhance the bearing capacity of coal pillars is proposed, and an in-depth study is conducted on the improvement effect of different strength filling materials on the bearing capacity of coal pillars. And the reasonable filling body strength is determined to be greater than 3MPa. The new discrimination method has important guiding significance for the analysis of coal pillar stability and the formulation of safety protection technical measures on engineering scales.

In this paper, various methods have been used to control and evaluate engineering difficulties in mining accurately. Different unstable scenarios occurring at the surfaces of underground mine walls, have been identified by comparing 3D terrestrial laser scanning surveys and subsequent point cloud 3D analysis. These techniques, combined with a change detection analysis approach and the integration of rock mechanics’ modelling, represent an asset for the assessment and management of the risk in mining. The change detection analysis can be used as control of mining and industrial processes as well as to identify valid model scenarios for establishing possible failure causes. A pillar spalling failure has been identified in an Italian underground marble quarry and this topic represents the basis of the present paper. A Finite-Element Method was used to verify the occurrence of relatively high-stress concentrations in the pillar. The FEM modelling revealed that stresses in the proximity of the pillar may have sufficient magnitude to induce cracks growth and spalling failure.

Irregular coal pillars left in longwall working faces are prone to stress concentration, resulting in failure and instability of coal pillar. Revealing the failure mechanism of the coal pillars is essential for the accurate prevention of coal pillar-type rockburst. Based on the geological conditions and the residual coal pillars in the 14320 working face of the Dongtan coal mine, this study investigates the failure mechanism and stress evolution characteristics in the abnormal area with irregular coal pillars through microseismic (MS) monitoring, moment tensor inversion, and velocity tomography of MS. The results show that (1) the MS events at the edge of the coal pillars are significantly more greater than those in the core area, in which, failure types of tension and compression occur in the edge and core areas, respectively; (2) the spatial parameters (strike φ, dip angle δ, slip angle γ) of the failure plane in the irregular coal pillar area were determined. The boundary of the irregular coal pillars was dominated by reverse fault sliding, and the core area is dominated by normal fault sliding; and (3) the stress field distribution characteristics of the working face and irregular coal pillars were determined using P-wave velocity tomography. The research findings provide a reference for analyzing the mechanisms of coal pillar failure, instability and the induced rockbursts.HighlightsMoment tensor theory is used to analyze the fracture mechanism of coal pillar in mines.Moment tensor theory reveals rupture types and occurrence of the rupture face in different regions of coal pillar.Stress evolution law of coal pillar is obtained by microseismic velocity tomography.

Underground mining using the room and pillar method leaves pillars to support the overlying strata. When longwall mining is used to extract a lower coal seam beneath the room and pillar mine, the bending of the interburden strata can induce pillar failure and roof caving. In this study, two physical model experiments with different pillar width/height ratios were performed, and a non-contact displacement measurement technique was used to record and analyse the deformation and failure processes. The failure mechanism of pillars and the roof in a room and pillar mine induced by longwall mining in a lower coal seam was investigated. The results show that, during the longwall retreat, the immediate roof of the longwall panel collapses and the main roof bends resulting in load redistribution among a series of overlying pillars. Eventually pillar failure and unsteady strata movement are induced. Physical model tests indicate the entire deformation and failure process can be divided into two stages, i.e. steady deformation and failure propagation. In addition, it is observed that different width/height ratios of pillars lead to different failure patterns in the second stage. For model #1 with higher pillar width/height ratios, load redistribution is induced by the bending of the main roof of the longwall mine, but most panel pillars remain stable. The unsteady movement of the main roof of the longwall causes an upward failure propagation in the overburden strata. For model #2 with lower pillar width/height ratios, the failure propagates downward and it is triggered by the progressive collapse of the panel pillars during the load redistribution process.

Pillars are often used to support the roof in underground mining. The multi-pillar and roof system is regarded as a multi-pillar and rock beam model. For a system composed of n pillars, the interaction force between the roof and pillars is obtained by a semi-analytical and semi-numerical method. Pillar failure may be progressive or sudden, depending on the equilibrium stability of the system in the post-peak stage. The initial conditions of multi-pillar failure and influence of one pillar progressive failure or unstable failure on adjacent pillars are analyzed based on the instability theory. If one pillar fails gradually, the stress transfer between pillars is also progressive. Once the first pillar fails suddenly, part of the stress is transferred to adjacent pillars, which may lead to further unstable failure of adjacent pillars and cascading failure of multiple pillars. The factors of pillar unstable failure mainly include geometric and mechanical parameters of the system. The mechanical parameters cannot be changed; however, entry (or stope in metal mine) geometrical parameters can be adjusted to reduce the possibility of unstable failure. The variations of pillar stability with entry widths are analyzed according to the factor of safety (FoS) and roof-to-pillar stiffness ratio rk. If the pillars are arranged in a properly concentrated manner, FoS increases, rk decreases and the tendency of pillar unstable failure increases. Conversely, if the pillars are scattered close to the barrier pillars, the possibility of pillar unstable failure is reduced but the overall strength of all pillars is also reduced. Therefore, for a multi-pillar and roof system, the entry widths should be properly adjusted from an overall system perspective to ensure that both FoS and rk values are sufficiently large to minimize the possibility of pillar unstable failure.

The crown pillar in the stope structure belongs to the category of thick plate, and its thickness determines the stability of the engineering structure. The key to determining the safe thickness of the crown pillar lies in solving its deflection function. Previous researchers often used the Galerkin method and other methods except the Ritz method to solve the deflection function of the crown pillar under simple boundary conditions. However, these methods are difficult to solve for the deflection function of the crown pillar under complex boundary conditions. Therefore, this paper builds upon the Reissner plate theory and introduces the Ritz method while considering the influence of strain components εz, γyz, and γzx on the flexural deformation of the crown pillar. Thus, the Reissner–Ritz method for solving the deflection function of the crown pillar in the stope is developed. Taking the failure of the crown pillar in stope 27 in the + 280 m section of the Daxin Manganese Mine as an example, firstly, the maximum tensile stress of the crown pillar is compared with its tensile strength to determine the safe thickness of the crown pillar in stope 27. The correctness of the chosen safe thickness for the crown pillar in stope 27 is verified using the Reissner–Ritz method. Then, FLAC3D is used to model and analyze the 8 m thick crown pillar in stope 27, and the accuracy of the Reissner–Ritz method in determining the safe thickness of the crown pillar is verified through finite element analysis. Finally, a comparison is made between the Reissner–Ritz method and the Galerkin method in solving the deflection function of the crown pillar under the uniformly distributed load, considering both simple and complex boundary conditions. The research results show that the Reissner–Ritz method has significant advantages over the Galerkin method in solving the deflection function of the crown pillar under the uniformly distributed load, even under complex boundary conditions. The findings are of great significance for solving the deflection function of thick plates under both simple and complex boundary conditions under the uniformly distributed load and determining the safe thickness of thick plates.HighlightsBased on the Reissner thick plate theory, the Ritz method is introduced, taking into account the influence of strain components εz, γyz, and γzx on the bending deformation of the crown pillar, forming the Reissner–Ritz method for solving the deflection function of the crown pillar in the stope.The range of safe thickness values for the crown pillar is determined based on the thickness-span ratio and safety factor, incorporating engineering experience to finalize the safe thickness of the crown pillar in the stope.The accuracy of the Reissner–Ritz method in determining the safe thickness of the four-sided fixed support crown pillar under the uniformly distributed load is verified through finite element analysis.Comparing and analyzing the calculation results of the Reissner Ritz method with the Galerkin method.

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.