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. INTRODUCTION TO MODELLING COAL BURST Coal bursts are very difficult to predict as they are inherently not frequent, isolated and occur without warning. To minimise the occurrence of coal burst it is necessary to first understand how the mechanism of coal burst arises. Frequent miss-use of conventional elastic-plastic, strain softening and hence otherwise static based models are attributed towards significantly limiting both theoretical derivation and computational ability in analysing fast dynamically occurring events such as coal bursts. This highlights the serious shortcoming of trying to model dynamic material response behaviour of strata around excavations; hence such models should not be used. The importance of built in dynamics is therefore recognised in dynamic analysis for incorporating inertial effects, enabling real time simulation of dynamic ground movement. Likewise, when using these models, correct approach is necessary to isolate and observe what mechanisms are taking place. A 2-Dimesional Dynamic Rock Fracture Model (DRFM2D) has been undergoing development at the University of Wollongong for more than 5 years. The theoretical basis of this model has been successfully implemented across numerous problems with outputs producing model fractures as observed in laboratory tests Venticinque and Nemcik (2017). Subsequently the DRFM2D subroutine in FLAC (Itasca, 2015) is capable of producing the associative dynamic effects from fractures in real time and in doing so is well suited towards capturing the conversion of kinetic motion response naturally produced in the rib during coal burst. 1 University of Wollongong: Email: gv667@uowmail.edu. au Tel: +61 414 799 325; jnemcik@uow.edu. au Tel: +61 2 4221 449 Coal Opertors’ Conference University of Wollongong, February 2020 276 MODELLING PROCEDURE Using FLAC2D (Itasca, 2015), an excavation of a standard underground coal mining roadway 5m width by 3m height was modelled within in a 3m thick coal seam bound by competent sandstone roof and floor strata at a depth of 550m, illustrated in Figure 1 and having rock mass strata properties listed in Table 1. Table 1 – Rock mass strata properties. Figure 1: Model of strata Initially, the model was brought to static equilibrium prior to dynamic solution being initiated as per described in Itasca (2015). In order to simulate the dynamic coal burst event, bond failure was artificially induced within a single interface element located at the roofline between the coal and sandstone strata 4m from the rib side. With reference to the vertical stress profile of the rib the artificially failed zone coincided with the maximum vertical and horizontal stress location at the coal/rock interface as well as the inflection point of the stress gradient profile before steeply Coal Opertors’ Conference University of Wollongong, February 2020 277 declining towards the rib side shown in Figure 2. From Venticinque and Nemcik (2018), this was identified as being the most appropriate region for coal burst to be initiated from. Figure 2: Stress profile along coal roof line adjacent to rib face prior to coal burst occurring. Following artificial initiation of coal burst from behind the rib, the DRFM2D subroutine was executed enabling the real time dynamic propagation of fractures to naturally evolve concurrent with the dynamic response effects evolving within the rib. During simulations, dynamic fracture propagation and velocity mass response of the rib was monitored with respect time.