Prediction of rock load emphasizing excavation damage of in situ rockscaused by blasting in coal mines

Roof failure in coal mines is strongly related to the frequency of laminations and their movement when the load acts upon them. Detachment of roof bolts from mine roof due to improper estimation of extent of weak zone is one of the major problems in underground coal mines, thus affecting the safety and productivity of workings. The most popular and practiced method for roof support design in Indian coal mines is the Central Mining Research Institute-ISM geomechanical classification system. Irrespective of such an established system of support design, accidents due to roof fall still persist. Here we review various available classification systems for rock load estimation and identify their limitations. The study has been extended taking into consideration the case study of KTK-6 incline of Singareni Collieries Company Limited by proposing a modified rock mass classification system based on seismic wave velocity as a key descriptor. A modified rock mass rating (RMR) system (RMRdyn) with inclusion of seismic velocity as one of the parameters is proposed for the estimation of rock load. Enhancement in rock load by 20% has been found for RMRCMRI-ISM values less than 40 according to the new rock load relation. This resulted in under-supporting of the roof and thus might have caused failures. For cases with RMRCMRI-ISM values more than 60, the earlier equation overestimates rock load by about 25% resulting in over-supporting. Thus, estimation of rock load from the proposed new equation appears to be more rational as it takes into account the actual damage zone

Relevance of Shape of Fragments on Flyrock Travel Distance: An Insight fro Concrete Model Experiments Using ANN

Flyrock are fragments that travel beyond the acceptable distances in rock blasting in surface mining. Their occurrence poses a great threat to life and property that may or may not belong to the owner of a mine. Hence, it becomes imperative to predict flyrock travel distance (range) that in turn facilitates definition of the blasting danger zone (or secure area) and take necessary safety precautions. The rock properties, blast design, and explosive loading parameters determine the distance travelled by flyrock. The kinematic equations do not work in such predictions owing to air drag that is specific to weight, shape and size of the flyrock. The importance of shape of the flyrock fragment thus assumes importance. In order to assess the importance of the parameters that determine their effect on the flyrock range, experiments on concrete models were conducted. Complete data of these parameters (136 datasets) were analysed using artificial neural networking (ANN). ANN proved to be a good tool to assess the relative importance and sensitivity of parameters. From the analysis, it emerged that the initial velocity, launch angle, and length of the fragments are of prime importance. Since, spherocity is difficult to ascertain, length of the fragments can prove to be a substitute descriptor for flyrock modelling. This will simplify the prediction techniques although launch angle is difficult to predict. Some insights for further R&D in this direction are also included in the paper

Flyrock in bench blasting: a comprehensive review

Flyrock is unwanted throw of rock fragments during bench blasting in mines and civil constructions. Perfunctory attempts by researchers to predict the flyrock range using mathematical, empirical and ANN based models do not address the issue in totality. Thus, flyrock continues to haunt the blaster. The research on the subject is, thus, still in its infancy. This paper identifies the lacunae, through a comprehensive review of the existing models, and suggests measures for better prediction and understanding of the problem on a holistic plane. One of the main reasons for improper predictions is the lack of data on flyrock in comparison to blast vibrations owing to statutory restrictions, avoidance of reporting and consequent constraints on experimentation. While fragmentation and throw of rock accompanied by subsequent vibration and air overpressure are essential constituents of the blasting, flyrock is not. This probably is one of the main errors in predictive domains. In addition, rock mass properties play a major role in heaving of rock fragments during blasting. Barring density of the rock, other rock mass properties have practically been ignored in all the models. At the end of this paper, for future investigations, a methodology for prediction of flyrock is also given