A CASE STUDY OF CONTROLLED BLASTING WITH e-DET AT BANKOLA OPENCAST MINE OF EASTERN COALFIELDS LIMITED
Bankola Open Cast Mine is playing on the Jambad Top (R-VIII T) and Jambad bottom (R-VIII B) seam. The thickness of the top coal seam varies from 3.2 to 4.6m. The heaviness of the bottom bed is between 6.2 to 7.0m. The overburden cover is between 17 to 24m. The inter-burden between the cover and the bottom seams varies from 1.5 to 3.0 m. The OB is being taken away by a shovel dumper combination. 100 millimeter and 160 mm DIA. Electric Drills are being used in 3.5 m to 6.0m high benches for blasting with Large Diameter explosives .
The Bankola UG colliery, which is around 160m from the open cast working, is developing the Bonbhal seam. The heaviness of the line is 2.7 to 3.05m. It is approximately 74 to 84m below surface. The mine management of Bankola Patch OCP of Eastern Coalfields Limited applied for DGMS permission for extraction of pillars of Jambad Top (R-VIII T) and Jambad Bottom (R-VIII B) seam by open cast method with deep-hole blasting and deployment of HEMM The permission was granted by Director of Mines Safety, Sitarampur granted with a condition that all the blasting operations should be carried out with the use of electronic detonators along with a scientific study of the effect of blast induced ground vibrations on the following:
- a) Stability of the tower and roof of underlying Bankola seam and Bonbhal seam of Bankola colliery.
- b) Surface structures (dwelling houses and school located at 100 to 150 m) at a nearby hamlet.
Accordingly, the mine management engaged CMPDI Ltd, Asansol who in turn recommended use of electronic detonators for controlled shooting. The mine sought the services of M/s Gulf Oil Corporation Limited for the supply and application of their electronic detonators (e-Det) for the above subject
Electronic detonator (e-DET)
e-Det is an indigenously developed electronic detonator by M/s GOCL. e-sets are practiced in connection with a programmable e-Xploder device. The e-Xploder transfers desired delay interval data to the firing circuit and sends coded signal to the detonators to initiate the timing sequence in the ignition circuit. The e-Det is having delay number 0 & 1-30 and a maximum delay window of 3000ms. The desired delay between 1-100 ms can be set on the e-Xploder, which is custom planned to dismiss me-Det.
The service end of the e-Det is inserted into the cap sensitive booster charge and lowered down the hole, and the other terminal is linked up to the surface connector. All the holes are linked in series with the surface connecter, which is ultimately tied to the e-Xploder for firing.
Blasting Trials and Analysis
During the tribulation period, first round of blasts was monitored for blast vibration. The details of the results are presented in Table-I. From ten blasts, fifteen readings of ground vibration levels were brought on the surface at different positions near the residential structures, club and Kali temple of the village and at school buildings near ECL quarters.
The vibration monitoring sta
tions were picked out in consultation with the mine officials and the distance from the blast site to the monitoring stations were measured by the mine surveyor. The vibrations were monitored with a varying explosive charge per delay and with varying distances. The minimum peak particle velocity observed on the surface about the Gram Pradhan’s house in the hamlet was 2.35 mm / sea at a distance of 170m from the fire site with maximum explosive charge per delay of 25 kilo, in blasting round consisting of total explosive charge 525 kg. The frequency level was recorded as 17 Hz.
The minimum peak particle velocity was observed on pillar on Bonbahal seam as 3.30 mm / Sec at a radial distance of 87.8 m from the blast site with maximum explosive charge per delay as 12.5 kg in a smashing round of 75 kg where frequency was not usable.
The minimum peak particle velocity observed on roof of Bonbahal seam was 6.35 mm / sea at a length of 114.56 m from the blast site with maximum explosive charge per delay as 19.5 kg in a smashing round of 389.2 kg where Zc frequency was 51 Hz.
The maximum peak particle velocity was observed along the surface (within the mine premises) as 16.0 mm / sea at a length of 90 m from the blast site with maximum explosive charge per delay of 25 kilo in a smashing round of 750 kg where the frequency level was 8Hz. The reasons for the high point of vibration were due to the excessive toe burden in the first attack. This was the first blow in the mine and prior to this the HEMM were excavating the soft strata of the mine.
The maximum peak particle velocity was observed on roof of the Bonbahal seam as 25.1 mm / Sec at a radial distance of 87.73 m from the blast site with maximum explosive charge per delay as 25 kg in a smashing round of 319.7 kg where Zc frequency was 85 Hz.
The maximum peak particle velocity was observed on roof of the Bonbahal seam as 25.1 mm/Sec at a radial distance of 87.73 m from the blast site with maximum explosive charge per delay as 25 kg in a smashing round of 319.7 kg where the frequency level was 85 Hz.
The following information (Table 1) was generated during the trial blast of Bankola OC patch:
BLASTWISE DETAILS OF PEAK PARTICLE VELOCITY AND AIR OVER PRESSURE IN BANKOLA MINE TRIAL BLASTS WITH e-DET
(KG) Total Charge
(KG) Blast Results
17.02.09 25 750 90 16 100
19.02.09 20 417 150
23.02.09 19.60 411.6 100
26.02.09 25.0 525 135
02.03.09 16 319.70 155 2.79 100
02.03.09 19.50 389.0 140 2.98 100
08.03.09 16 305.8 140 2.16 100
15.03.09 22 525 140 2.60 112
26.03.09 25 375 158 3.18 110
26.03.09 12.5 75 110 2.67 110
27.03.09 11 412.5 120
31.03.09 19.40 465.36 110
01.04.09 19.5 350 120 1.71 100
10.04.09 19.5 525 130 1.18 100
24.04.09 20.8 250 100 1.84 100
With regular blasting operations, the formation of benches in the mine became proper and the ground vibration levels in the successive blasts reduced considerably. No fly rock was kept. The atomization of the blasted muck was satisfactory and desirable to the excavator in use.
Further development with e-DET-
GOCL is now ready with the fully “Field Programmable’ version where individual detonators can be programmed at the field commensurate to need. The raw version is having a programmable delay window of 9000 milliseconds in increments of 1 millisecond delay. This will open up fantastic opportunities to the blasting engineer while carrying out blasts, whether in mining or civil infrastructure sector.
More significantly in the Indian scenario, use of e-DETs will help unlock millions of tonnes of coal currently locked up in the opencast mine benches presently un-mineable, due to proximity to habitations. DGMS is presently not allowing mining of benches within 100 meters to dwellings. With the usage of e-DET, there survives a potential to cut down this distance to 50 ms, thus helping mining of millions of ones of coal which otherwise would have been left un-mined in the quarries.
Authors are grateful to the officials of Bankola Area and CMPDIL for giving them an opportunity to acquire relevant data and prove the efficacy of e-Det for controlling blasting. The authors also thank GOCL Management for according them permission to present this case study report.