Protection of Underground Mine Structures Due to Adjoining Open-pit Mine Blasting By Pradeep K Singh Chief Scientist & Professor, Academy of Scientific & Innovative Research CSIR Central Institute of Mining and Fuel Research, Dhanbad, 826 015 CSIR CIMFR
Introduction The safety and stability of underground mine openings, sidewalls/pillars, water dams, etc. In close proximity to operating open pit mines are often endangered from blast induced vibrations. These blasts generate seismic disturbances, which in turn may damage the support system and may potentially affect the stability of roof and sidewalls/pillars of underground galleries in underground mines. The open-pit mine is currently working at 350 m depth and is designed to reach 420 m deep. It has been planned to operate the underground mine maintaining 50 m parting from the planned ultimate level of the open-pit mine. The present depth of underground working lies between 370 and 410 m below the surface level.
Introduction The impact of 86 open-pit blasts has been documented in the underground openings and 258 blast vibration data has been recorded. The vibrations were recorded simultaneously in the roof, side walls and at floor levels. The vibrations generated due to detonation of explosives with shock tubes and electronic delay detonators have been recorded in underground openings for comparative assessment. Ground vibration recorded at roof, pillar and floor were analysed separately and threshold value of the vibration for the safety of underground workings has been determined based on the Rock Mass Rating (RMR) of the roof rock.
Salient features Mining Lease Area: 1200 Ha Mining Method: Open Cast up to 420 m depth & Underground down below Mine Working : O/C Drill & Blast, Loading& hauling with shovel-dumper combination. (34 &15 cum shovels with 221&95MT dumpers) Present Mine Pit Dimension: 1998 m X 1027 m Bench Height: 10m in Ore & Overburden benches Berm Width: 9.6/6.1 m in F/W and 6.4/5.4 in H/W Bench angle: 60 degree in F/W, 70 degree in HW Pit Access: Ramp one each in H/W & F/W, width 30m Ultimate Pit Wall Slopes: H/W: 42, F/W 35 Ultimate Pit Depth: 420 m from surface Ore Body Strike Dip - 1550m, N50 E 60-70 SE Avg. width 58m Host Rock Graphite, Mica, Sillimanite, Gneiss North Ventilat ion Shaft As Built development Main Decli ne Main Hoisti ng Shaft South Ventilat ion Shaft Shaft Access Underground resource Galena OB Open Pit As of now Open Pit Stage 5 shell CSIR CIMFR
Major milestones at a glance Commencement of Mining Operation & 0.9 MTPA Beneficiation Plant : 1991 1st Phase expansion of capacity enhancement to 1.35 MTPA : 1997 Commissioning of Stream 2 increasing capacity of plant to 3.75 MTPA : 2005 Commissioning of Stream 3 increasing capacity of plant to 5.00 MTPA : 2008 Capacity enhancement to 6.15 MTPA for mining and 6.5 MTPA for beneficiation : 2010 Addition of 220 MT class truck and 34 cum. Shovel Commissioning of Stream 4 Underground Mine Decline Development Commencement : 2010 Underground Mine Production Commencement through decline : 2012 Underground Mine Shaft Sinking Commencement : 2012 Concurrent Open-pit and Underground Mining ( producing 6 MTPA ) continues till : 2020 CSIR CIMFR
Overview of Open-pit mine
Stable benches at Open-pit mine CSIR CIMFR
Experimental site details Rampura Agucha is a stratiform, sedimenthosted lead zinc deposit, occurs in Pre-Cambrian Banded Gneissic complex and forms a part of Mangalwar complex of Bhilwara geological cycle (3.2 2.5 billion years) of Archean age. The rocks have been subjected to polyphase deformations and high-grade metamorphism. The Rampura Agucha mixed sulfide deposit is a massive lens shaped orebody with a NE-SW strike length of 1500 m and a width varying from a few metres in the NE direction widening to as much as 120 m in the central to SW section. Rampura Agucha mine is the largest and richest lead zinc deposit in, containing ore reserves and resources of 107.33 Mt with grade of 13.9 % zinc and 2 % lead.
Experimental site details MINING OPERATION DETAILS The Rampura Agucha open-pit mine works with shovel dumper combination deploying large sized heavy earth moving machines like 34 m 3 shovels and 220 t trucks navigation with truck dispatch system (TDS). Rampura Agucha open pit lead zinc mine, which is producing 5.7 Mt/a of ore and has started its underground part, which is slated to produce initially 2 2.4 Mt/a and 4.5 Mt/a in the near future. The diameters of the blastholes are 115 mm and 165 mm. The initiation system includes detonating cord (for presplit), shock tubes and electronic delay detonators.
Experimental site details Physico-Mechanical properties of rocks Rock Type Compressive Strength (MPa) AMP (Amphibolite) GBG (Garnet- Biotite- Gneiss) GBSG (Garnet-Biotite- Silimanite-Gneiss) Young s Modulus (GPa) Poisson s Ratio Tensile Strength (MPa) BRMR 74 18.6 0.13 8.7 62 59 14.8 0.10 7.6 60 44 12.3 0.17 6.8 50 ORE 68 23.2 0.07 7.9 62 PEG (Pegmatite) 76 13.2 0.05 6.5 72
Blasting details Blasting details Details of data Number of blasts 86 Number of peak particle velocity data recorded [Roof-32, Floor-98] 258 Sidewall-128, Range of total explosive weight detonated (kg) 79-95978 Range of explosive weight per delay detonated (kg) 30-825 Range of blast vibration monitoring distance (m) Range of recorded peak particle velocity (mm/s) Radial: 97-2171 Vertical: 60-758 Roof: 0.439-24.7 Sidewall: 0.46-35.1 Floor: 0.42-23.0 Range of dominant peak frequency (Hz) 13.9 124
Blasting details Experimented blast design for Hangwall at 220/210 m RL N ALG bench
Blasting details CHARGING PATTERN OF HOLES Blast Geometry 115kg Hole diameter No. of holes : 200 Hole depth Bench height Burden Spacing Explosive per hole Total charge : 165 mm : 11.5 m : 11 m : 4.0 m : 5.5 m : 115 kg : 23,000 kg Charge per delay within 8 ms : 230 kg (2 115) Effective charge per delay : 145 kg (2^1/3 115) Designed charge factor : 0.48 kg/m 3
Blasting details Experimented blast design for Ore at 110/100 m RL S ALG bench
Blasting details CHARGING PATTERN OF HOLES Blast Geometry 150kg Hole diameter No. of holes : 39 Hole depth Bench height Burden Spacing Explosive per hole Total charge Charge per delay within 8 ms Effective charge per delay : 165 mm : 11.5 m : 11 m : 4.0 m : 4.5 m : 150 kg : 5850 kg : 300 kg (2 150) : 189 kg (2^1/3 150) Designed charge factor : 0.76 kg/m 3
Blasting details
Blasting details
Blast vibration monitoring Triaxial geophones were used to monitor vibrations. The locations in the roof were at the junctions as well as in between the two junctions of galleries; in the side walls they were 1.2 1.6 m below the roof and at depths of 0.5 0.6 m inside pillars. Seismographs with eight as well as four channels provided with two/one tri-axial transducer(s) for monitoring vibration (in mm/s) were used. All the seismographs record vibration in three directions i.e. Longitudinal (L), Vertical (V) and Transverse (T).
Blast vibration monitoring Attachment of geophones of seismographs in the roof and sidewall
Underground vibration monitoring site CSIR CIMFR Overview of the development face
Result and analyses Plot of recorded dominant peak frequencies monitored on the ground surface and in the underground sidewalls/roof due to open pit blasting.
Result and analyses [A] [B] [A] Blast wave signature recorded on roof and sidewall at 200m [B] Fast Fourier Transform (FFT) analyses of [A]
Result and analyses Plot of recorded peak particle velocity on the surface and in sidewalls/pillars of underground openings due to open pit blasting
Result and analyses S. No. Location of blast Explosives weight per delay [kg] Radial distance [m] PPV in roof [mm/s] PPV in sidewall [mm/s] Ratio of recorded PPV in roof and sidewall 1. 170/160 ORE N ALG 363 817 1.01 0.637 1.6 2. 200/190 H/W N ALG 440 642 0.348 0.290 1.2 3. 180/170 FW N ALG 338 644 0.603 0.351 1.7 4. 200/190 ore S ALG 276 692 0.704 0.531 1.3 5. 170/60 HW S ALG 360 231 5.36 2.35 2.3 6. 180/70 HW S ALG 370 228 1.6 0.842 1.9 7. 120/10 FW S ALG 194 207 1.2 0.421 2.8 8. 170/60 HW S ALG 360 385 5.36 2.35 2.28 9. 180/70 HW S ALG 370 228 1.6 0.842 1.9 10. 140/30 HW S ALG TRIM 304 526 1.79 0.475 3.76
Result and analyses Damage criteria and Threshold value of vibration for the Safety of underground workings The PPV has traditionally been used as a criteria to establish the degree of blast damage. The degree of damage observed in the below ground openings is influenced by the RMR of roof rock. RMR of roof rock Threshold value of vibration in terms of peak particle velocity for the safety and stability of roof [mm/s] Threshold value of vibration in terms of peak particle velocity for safety and stability of pillars/sidewalls [mm/s] 20-30 50 20 30-40 50-70 20-30 40-50 70-100 30-40 50-60 100-120 40-50 60-80 120 50 CSIR CIMFR
Result and analyses The regression plot of recorded blast vibration data for roof and sidewall at 95 % line equation
Result and analyses Plots of mean peak particle velocity as a function of scaled distance for the data recorded in roof and sidewalls
Result and analyses The propagation equation of recorded vibration data due to Nonel and electronic initiation devices
Result and analyses Plot of recorded vibration data simultaneously at different scaled distances due to open-pit blasts at ground surface and in underground openings
Conclusion The comparative analyses of PPV recorded on surface and in UG openings due to open pit blasting revealed that the PPV recorded in the UG openings were much lower than those recorded on the surface at same scaled distances. It is recorded that along with the geometrical spreading, the underground voids plays important role in attenuation of ground vibration. The reduction in blast vibration was up to 42 % in comparison to surface PPV at a scaled distance of 25. High frequency ground vibration data were recorded in underground openings whereas on surface low frequency ground vibration data were monitored. The blast-induced ground vibration data generated from Nonel (shock tube) and electronic initiation system were analysed separately and found that electronic initiation system produced 8 to 10% lower level of PPV in comparison to Nonel (Shock tube) initiation system in the underground openings. The RMR values of the PPV monitoring locations ranges from 50 to 72. Therefore, the threshold values of PPV for the safety of UG roof, pillars /sidewalls and floor would be 100 mm/s, 40 mm/s and 20 mm/s respectively
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