Sawmliana, C (2005) An Investigation into the Blast Design Parameters for Ringhole Blasting and Induced Blasting in Blasting Gallery Method of Underground Coal Mines. PhD thesis, Indian School of Mines .

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Abstract

The ever increasing demand for coal in the country and the availability of substantial amount of thick seam coal reserves prompted the coal industries in India to search for suitable methods of thick seam extraction. Out of several methods that had been tried, Blasting Gallery (BG) method was one of the most economically viable methods for underground exploitation of thick seams. BG methods is, originally a French technique for underground exploitation of thick coal seams. In BG method, the entire seam thickness can be excavated in one lift by drilling a set of holes in a ring pattern with percentage of coal recovery ranging from 65 to 85%. BG method was initially introduced for extraction of already developed pillars at East Kartras Colliery (BCCL) and Chora-10 pit Colliery(ECL). It was subsequently extended for exploitation of vigin thick coal seam at GDK-10 Incline of SCCL. With the successful introduction in SCCL and numerous advantages offered by the method over the other methods for thick seam extraction, more mines are expected to adopt BG method in near future. However, there are certain problems associated with BG method that require extensive research for its successful implementation in the country. Due to strata control problem, BG panel at East Katras Colliery (BCCL) had already been closed after the occurrence of airblast and overriding of the pillars. Induced caving by blasting (or in short induced blasting) has become an integral part of BG method for controlling the roof strata. Blast fragmentation problem has also been reported during ring hole blasting in coal. The maximum explosive charge per hole has been restricted by the Directorate General of Mines Safety (DGMS) to 3.0kg for ringhole blasting in coal and 1.0 kga for induced blasting in stone roof. Moreover, no standard blast design patterns were available for practical use for both ringhole an induced blasting in BG panel. The generation of induced ground vibration and post blast gases were also high due to usage of more explosive charge in BG method. In line with the above mentioned problems, a research was carried out to investigate different blast design parameters for arriving at safe and optimum blasting patterns for both ringhole blasting in coal and induced blasting in stone roof in BG Method. The study was broadly divided into two parts, viz. ringhole blasting in coal and induced blasting in stone roof. Initially, different important blast design parameters were identified from the literature. Mathematical models were developed for determining different blast design parameters for both ringhole blasting and induced blasting that would be applicable for practical use in the field. These blast design parameters were then optimized base don the field investigation and analyses. Field investigations were carried out in three different mines of M/s. Singareni Colliery Company Limited (SCCL) viz. VK-7 Incline, GDK-8 Incline and GDK-10 Incline. Investigations were carried out with the existing blast design patterns practiced in the mines as well as with modified patterns for both ringhole blast in coal and induced blast in stone roof to achieve optimized blast design patterns. The existing blasting patterns, as practiced in the mines, were critically observed and studied. Based on the observations, blasting patterns were modified and experimental blast were conducted with the modified patterns. For experimental purposes, charge per exceeded 1.0kg in the case of induced blasting in stone roof. A new induced blasting pattern called as ‘Staggered Ring Breakage Method’ or SRBM Pattern was developed and experimented in all the three mines. SRBM pattern of induced blasting was found to yield better blast results, aiding more area of roof fall than conventional single row induced blasting as commonly practiced in the mines. Gr5ound vibration monitoring was conducted ato ascertain the level of vibration in the roof pillar and floor of the galleries. Monitoring of strata load and roof convergence was done to establish the righ time for conducting induced blasting in BG panel and also to study the impact of blasting on strata behaviour. Strata load and roof convergence were measured before and after the blast. Pre-blast and post-blast gases were also monitored to find out the percentage of CH4, CO, CO2 and O2 for each ringhole and induced blasting. For understanding the effect of decoupled charge and use of plastic pipes on blast fragmentation as observed in GDK-10 Incline, a few experimental blasts were conducted in concrete blocks of known strengths. The results, thus obtained were correlated with the actual field conditions. The results from the model blasts in concrete blocks revealed that thee was a significant reduction of explosion pressure from 0.51 GPa in 42 mm diameter to 0.23 GPa in 62 mm hole diameter. Such reduction of explosion pressure has affected the fragmentation adversely. Based on the findings, modified drilling and charging patterns have been suggested. The analyses of the investigations revealed that the main causes of poor fragmentation leading to the formation of oversized boulders during ringhole blasting in coal were improper drilling and charging of holes, insufficient hole depth at the corners with respect to the seam thickness and size of the gallery, misfire due to improper priming, presence of stone-band in the coal seam, hole jamming due to strata pressure and insufficient explosive charge per hole. Ring burden of 1.5 m and hole density of 1.3-1.4 m/m2 were found to be optimum in case of ringhole blasting for seam thickness ranging from 8-11m. Empirical equations have been developed to determine the optimum number oif holes, total hole length and linear charge concentration for 1/5th portion of the hole in the bottom etc. charge factors to be used for ringhole blasting have been suggested after considering the compressive strength of coal and nature of coal seam. An artifical Neural Network (ANN) has been used to train different blast design parameters and other important factors and to study their effect on the area of roof fall produced by induced blasting. The ANN results showed that among the blast design parameters for induced blasting, front burden, front spacing, charge factor and total explosive quantity fired in a round were having the most significant influence on caving of roof rock. Among the geomechanical properties and condition of the roof, the area of exposed roof in the level, density of roof-rock and number of free faces present in the blasting area were found to have significant influence on induced caving by blasting. The deviation of the predicted area of fall by the ANN model and the actual area of fall was under 6 percent. Based on the ANN results sand the field observations, optimum values of different blast design parameters have been suggested. Based on the field observations, peak particle velocity of 100 mm/s was considered as the threshold level of ground vibration in BG panels. Different vibration predictor equations have been established including energy-based predictor equation. Square root scaling (USBM) equation and modified equation of cube root scaling by incorporating stiffness factor provided best fit equations for roof vibration. For pillar vibration and floor vibration, cube root scaling was used to established predictor equations. Zone of disturbances produced by ringhole and induced blasting were determined taking 100 mm/s as threshold level of ground vibration for roof and pillar stability. Based on the zone of disturbances, the advance support requirement was 50 m , 70m and 110 m in VK-7 Incline, GDK-8 Incline and GDK-10 Incline mines respectively. These zones were more than 40m, which was the advanced support requirement imposed by the Regulatory Authorities (DGMS) at present. Daily increase in roof convergence of 3 mm or more and the corresponding strata load of 2 tonne or more have been recommended as the right time for conducting induced blasting to prevent airblast in BG panel. In general, no changes in strata behavior were observed before and just after the ring hole blasting were monitoring points were more that 10m from the goaf edge. Changes in strata behavior were observed only when areas of uncaved roof were more and monitoring points wre less than 10 m away from the goaf edge. The analysis of the post-blast gas samples collected from the blasting sites showed that the percentage of CO, CO2 and O2 attained their safe levels after 20 minutes when the total explosive charge fired was up tot 140 kg in the case of ringhole blasting in coal. In case of induced blasting, they attained the permissible level after 15 minutes of the blast, However, taking in to consideration of the possible strata movement, it has been recommended to re-enter the induced blasting site only after 30 minutes. Based on the field investigations and analyses of the results, as design methodology had been suggested for ringhole blasting in coal and induced blasting in stone roof

Item Type:	Thesis (PhD)
Subjects:	Blasting
Divisions:	UNSPECIFIED
Depositing User:	Dr. Satyendra Kumar Singh
Date Deposited:	08 Feb 2012 04:29
Last Modified:	08 Feb 2012 06:31
URI:	http://cimfr.csircentral.net/id/eprint/882

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