New fracture models for the progressive failure of rock slopes

An improvement to previously developed constitutive FISH User-Defined-Model subroutine by Venticinque (2013) is demonstrated here to simulate the initiation and progressive propagation of fractures through rock structures. This model is based on the amalgamating failure and fracture mechanics theory applied to the finite difference FLAC code. The prior validation of fracture propagation in isotropic rock has been modified to simulate fracture propagation in anisotropic rock. It is shown that the model is capable to accurately simulate fracture distributions in both isotropic and anisotropic rock mass. Furthermore, application of the model to study rock slope stability highlights several characteristics relevant to the progressive failure process of hard rock dry wall slopes. Moreover the model introduces new potential insight towards the effectiveness of rock and cable bolt supports. This work contributes towards improving safety in mines through an increased understanding of key fracture and progressive failure characteristics within geological structures

3D rock failure modelling of pillar behaviour during longwall retreat into normal and angled design gas drainage stubs

3D Rock failure modelling was used to perform a coal pillar stability analysis comparing normal and angled Gas Drainage Stub (GDS) design during the approach of a longwall. This paper aims to benchmark the response behaviour of rectangle and triangle shape fender pillars respectively formed during longwall retreat into normal and angled GDS designs. The results of this study are intended to provide a general hazard awareness of potential layout/support issues relevant to GDS design rather than providing a unique recommendation or overall mining solution. Using FLAC3D, computer modelling of the rock failure process simulated pillar loading and pillar failure of the fender pillar that is created between the GDS at the tailgate end of the longwall. A standard gas drainage stub, angled at 90° to the roadway and another angled at 70° stub was simulated to provide comparison between normal and angled GDS design scenario. The modelled strata section was based on typical moderate strength sandstone/siltstone roof and floor strata. A variety of geometrical parameters were varied in the models to assess sensitivity of pillar stability, in particular anticipated peak strengths were compared against a range of reducing pillar sizes, representing the reduction of the fender pillar size during longwall extraction. Pillar shape was determined to be a major influencing factor with respect to pillar strength. It was found that the triangular fender pillar created from the GDS driven at an angle to the tailgate ultimately results in reduced pillar strength and softer post failure modulus as compared to rectangular pillars of similar widths. This reduction in overall strength is mainly attributed to the geometry of the triangular pillars (i.e. a smaller load bearing area) and their reduced ability to generate internal confinement. Although the triangle pillar yields at a further distance away from the longwall face, the impacted zone is focused over a smaller area of the tailgate corner rather than over the entire total length of the stub

Dynamic analysis of fault slips and their influence on coal mine rib stability

Historical data indicate that in deep coal mines the presence of faults in close proximity to excavations affect the frequency of coal bursts. A number of researchers have attempted to correlate the fault geometries to the frequency and severity of coal bursts but dynamic numerical modelling has not been used to show how faults can affect coal ejection from the rib side. The dynamic numerical analysis presented here show how different orientations of fault slips may affect coal bursts. To prove the concept, 89 cases of slipping fault geometries were modelled using the FLAC3D software and their effect on rib stability investigated. The results indicate that there is a simple and logical correlation between the fault location, its slip velocity and the ejection of the yielded coal rib side. The seismic compressive wave generates rock/coal mass velocities that directly impact the rib side. If the coal rib is relatively disturbed and loose, these velocities can cause its ejection into the excavation. The slip direction typically impacts one side of the mine roadway only. A 1 m thick loose coal block attached to the 3 m high rib side in mine roadway was ejected at speeds ranging from 2.5 to 5 m/s depending on the fault location, its orientation and the maximum fault slip velocity modelled at 4 m/s

Numerical Model of Coal Burst Mechanism

Coal bursts present one of the most severe hazards for the underground coal mining industry. In Australia, coal burst events are becoming increasingly frequent as coal measures are mined progressively deeper. Until now, coal burst mechanisms haven’t been properly understood. A significant ongoing effort and a large number of research activities are searching for answers. This work is supported by the Australian Coal Association Research Program (ACARP) which aims to provide explanation of the probable mechanisms and key factors behind the coal burst phenomena. The available energy required to eject the coal rib into the mine opening has seeded the idea of momentum transfer from within the seam towards the rib side. This mechanism has a strong analogy to Newton’s Cradle device and hence conservation of momentum and energy principles that can account for momentum transfer between confined seam masses at a distance and ejected unconfined fractured mass at the free surface of the rib. Using dynamic analysis, preliminary numerical models successfully simulate fast ejection speeds of coal rib material and thus identify a probable common cause of coal bursts in mine roadways. Modelled coal mine roadway in 3 m thick seam at a depth of 550 m successfully simulated coal burst phenomena; laterally ejecting 3.92 tonnes of coal from the rib with velocities ranging up to 2.3 m/s. Recognising that ejection speeds are dependent on material properties, extent of trigger induced failure between coal/rock boundary and chosen geometry; a few modelled cases are presented here

Numerical model of dynamic rock fracture process during coal burst

Coal bursts present one of the most severe hazards challenging the safe operations in underground coal work environments. In Australia, these events are becoming increasingly frequent as coal measures are mined progressively deeper. This study is supported by the Australian Coal Association Research Program (ACARP) which aims to better understand the phenomena of coal burst. In this paper the dynamic fracture process of coal bursts was successfully simulated in the coal roadway. This was achieved using dynamic analysis utilising DRFM2D routine by Venticinque and Nemcik (2017) in FLAC2D (Itasca, 2015) which complemented previous study observations by Venticinque and Nemcik (2018). This is significant because until now the evolving dynamic rock fracture process during coal burst remained unknown. Additionally, coal/rock burst events were shown from simulation as being largely driven by the propagation of shear fractures from within the rib. This was demonstrated to produce effect forcing the dynamic conversion and release of potential energy stored as compressive strain in the rib into kinetic movement of the entire rib section. This entire process was shown to occur very fast taking approximately 0.2 seconds for a coal burst to fully establish, with ejection of several meters of rib at a velocity of 1.6 m/s produced in the model of an underground coal roadway having 550 m depth of cover

Modelling of Dynamic Fracture Propagation in Coal Pillars using FLAC 2D

For many years the empirical prediction of pillar stability has been the dominant method for pillar design. Now, with the advanced numerical modelling of dynamic fracture propagation, it is possible to study the correlation between the empirical pillar load estimates and the actual pillar fracture mechanics. An upgraded version to previously developed constitutive FISH subroutine in FLAC 2D driving the User-Defined-Model which simulates compressive failure behaviour of coal pillars through the development of dynamic fracture propagation. Special insight on peak strength and post failure behaviour is presented through analysis of fracture development as a function of the pillar width to height ratio. Results derived as part of this study show that the numerical fracture propagation in coal pillar produces similar results to the classical empirical estimations of pillar peak loads. The classical progressive shear failure in pillar ribs was observed in the models depicting the probable rib failure mechanisms. Such model is well suited towards providing better understanding of fracture behaviour in rock or coal mass to improve safety when mining within or around complex geological structures

Modelling dynamic fracture propagation in rock

Fracture propagation in brittle rock is very fast and highly dynamic. Typically this process consists of fracture initiation, propagation and termination. Growth of micro-fractures is conceptually and numerically well established, however, current practices to model fracture propagation in rock employs slow evolving static regimes that do not represent the true nature of fracture propagation in the laboratory or the field. This paper presents a newly developed numerical approach using Micro-Brittle Dynamics theory to model the propagation of fractures through rock in real time. This work presented here is based on a newly developed Dynamic Rock Fracture Model (DRFM2D ) and validated against laboratory experiments

Advanced Numerical Modelling of Dynamic Fractures in Rock

The application of numerical analysis towards modelling real world problems requires fundamental understanding of the system behaviour as it occurs in nature. The choice of the numerical model schemes depends largely on the modelled problems that may be either: static equilibrium such as simulation of the stable mine excavation, continually yielding/caving strata such as goaf formation or dynamic events to simulate fast propagation of fractures in brittle rock or other dynamic behaviour. My original contribution to knowledge is the novel development of a simple explicit, dynamic numerical scheme for solving complex nonlinear dynamic fracture mechanisms in rock. This was delivered in the form of the DRFM2D model package comprising several executable codes and functions in FLAC2D. A major benefit of pursuing with such simplified numerical approach was a versatility of application that is achievable as a first-pass alternative towards otherwise expensive and hard to reproduce physical models

Dynamic events at longwall face, CSM MIne, Czech Republic

Presented here are the details of the seismic events that occurred at longwall 11 located at the CSM mine in the Ostrava coal region, Czech Republic. This longwall was excavated in a very complex area located within the shaft protective pillar and adjacent to the 50 m wide and steeply inclined fault zone at a depth of 850 m. In addition, 10 longwalls were extracted below each other over many years in several sloping seams located on the other side of the large sloping fault zone resulting in complex stress fields and large subsidence. The immediate roof above longwall 11 was a very strong sandstone and sandy siltstone with a uniaxial compression strength of 80 – 160 MPa. When the longwall started, continuous seismic monitoring of the longwall area indicated 470 small seismic events with energy smaller than \u3c102 J. The first high energy event of 3.3*105 J occurred when the longwall advanced 85m past the starting line. Some 30 minutes later a rockburst occurred registering energy of 2.2 *106 J, causing significant rockburst damage at the tailgate located near the large tectonic zone. The roadway steel arches were significantly deformed and the maximum floor heave reached up to 1.5 m. To investigate the complex strata behavior in that area, a large FLAC3D model 0.27 km3 in volume was constructed and 10 longwalls were extracted in several sloping seams adjacent to the large fault zone. The model under construction is now ready to study the complex strata behaviour and the associated stress fields together with the dynamic strata behaviour to match the modelled seismic events with those measured underground