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Influence of air-deck blasting on fragmentation in jointed rocks in an open-pit manganese mine
Engineering Geology 57 (2000) 13–29 www.elsevier.nl/locate/enggeo
Influence of air-deck blasting on fragmentation in jointed rocks in an open-pit manganese mine J.C. Jhanwar a, *, J.L. Jethwa a, A.H. Reddy b a Central Mining Research Institute Regional Centre, 54-B, Shankar Nagar, Nagpur, Maharastra 440 010, India b Department of Mining Engineering, Visvesvaraya Regional College of Engineering, Nagpur 440 010, India Received 14 January 1999; accepted for publication 13 September 1999
Abstract The influence of air-deck blasting on fragmentation in jointed rock masses of a manganese open-pit mine has been investigated. It is revealed that air-deck blasting improves the degree of fragmentation and produces more uniform fragmentation compared with conventional blasting. Mean fragment size (MFS) reduces from a range of 0.36–1.0 m to a range of 0.27–0.51 m. Fragmentation index (FI ) and blast-induced fragmentation increase from 1.38 to 4.34 and from 37–65% to 66–100% respectively. Empirical correlations have been developed to estimate MFS and FI from conventional and air-deck blasting on the basis of Bieniawski’s rock mass rating, powder factor and ratio of spacing to burden. Results indicate that air-deck blasting could be more effective in very low to low strength moderately jointed rock masses than in medium strength highly jointed rock masses. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Air-deck; Blasting; Fragmentation; Jointed rock; Open pit
1. Introduction Blasting with air-decked explosive charges has a long history. It has become an increasingly popular technique in production and presplit blasting in open-pit mines. In this technique, an air gap is introduced in an explosive column in the blast hole as a means of optimising the rock breakage for a given charge length. Field experiments and theoretical studies conducted by Melnikov and Marchenko (1971) using air-decked explosive charges revealed its benefits over conventional methods of blasting. Following studies of fracture network in thick Plexiglas * Corresponding author. Fax: +91-712-527628. E-mail address: [email protected] (J.C. Jhanwar)
blocks using air-decked explosive charges, Fourney et al. (1981) demonstrated that when a shock wave reaches the stemming it is reflected back to reinforce the stress field and that the time period over which it acted on the surrounding material increased by a factor of between two and five. Since then, air-deck blasting has been applied in a variety of applications, like presplitting, controlling ground vibration and fly rock, reducing fines and improving blast economics, by Marchenko (1982), Chiappetta and Memmele (1987), Mead et al. (1993), Moxon et al. (1993) and Bussey and Borg (1995). In spite of these studies, the mechanism governing rock fragmentation with air-deck blasting has still not been fully understood and its use does not always improve blasting results. The potential areas of application of this technique in production
0013-7952/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 1 3 -7 9 5 2 ( 9 9 ) 0 0 12 5 - 8
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Nomenclature P rate of decay of pressure (MPa/m) C,C empirical constants 1 2 M mass of explosive (kg) n number of moles of gas R universal gas constant T temperature of gas (°C ) S displacement of pressure front (m) R.L. reduced level (m) J volumetric joint count v S,S, spacing of joint set (m) 1 2 S,S 3 4 F/W footwall H/W hangwall RMR Bieniawski’s rock mass rating UCS uniaxial compressive strength (MPa) RQD rock quality designation MFS mean fragment size (m) M mass of fragments (%) p MSS mean sieve size (m) PF powder factor (kg/m3) S/B ratio of spacing and burden ADL air-deck length as a fraction of original charge length (ratio of air-deck length to original charge length) FI fragmentation index r correlation coefficient blasts of open-pit mines are yet to be established and its nature of impact on blast fragmentation in various rock masses remains to be ascertained. This paper discusses the results of experimental blasts conducted in jointed rocks of a manganese open-pit mine to evaluate the influence of air-deck blasting on fragmentation.
2. Theory of air-decking The technique of air-decking involves the use of one or more air gaps in the explosive column as a means of optimising fragmentation for a given charge length. The theory as proposed by Melnikov and Marchenko (1971) and Melnikov et al. (1979) postulates that shock waves, when reflected within the borehole, generate a secondary shock wave that extends the network of microfractures prior to gas pressurisation. The final borehole
pressure produced by an explosive is, however, reduced in this case, but the degree of fracture is increased as a result of repeated loading of the rock by a series of aftershocks. The three main pressure fronts, i.e. the shock front, pressure front due to formation of explosive products behind the detonation front and reflected waves from the bottom of the blasthole and/or from the base of the stemming travel within the air-deck for different distances and velocities, create these aftershocks. The duration of shock wave action on the surrounding rock mass is, therefore, prolonged. Consequently, the crack network in the rock mass in front of the borehole is increased in air-deck blasting. Since the magnitude of the shock front depends on the explosive formulation and decreases quickly with distance in air, the degree of fracture in the air-deck region finally depends on the length of the air-deck and the type of explosive.
J.C. Jhanwar et al. / Engineering Geology 57 (2000) 13–29
The rate of decay of a pressure wave as it passes through air is given by Kinney and Graham (1985) as P=C
(MnRT )1/3 1
S
C , 2
(1)
where M is the mass of explosive, n is number of moles of gas, R is the universal gas constant, T is temperature of the expanding gas, S is the displacement of the pressure front and C and C are 1 2 empirical constants. Moxon et al. (1993) extended this theory and opined that if the air-deck is placed in the middle of the explosive column, the pressure front produced due to explosive at either end of the airdeck collides at the centre of the air-deck. This interaction, they proposed, should develop a reinforced stress field and result in a more extensive radial crack pattern than if an air-deck was kept on the top of the charge. In the case of solid decking, the shock fronts from the opposite explosive columns also interact in the decking region, but they get attenuated due to high resistance offered by the solid material and, consequently, their intensities become too low to enable them to propagate further through the decking materials. It is for this reason that air-decking has an overriding advantage over solid decking.
3. Mining detail The mine known as Dongri-Buzurg open-pit manganese mine is situated in central India near the city of Nagpur ( Fig. 1). The overburden excavation and the ore production in this mine are 0.3×106 m3 and 0.18×106 t respectively. Shovels and hydraulic excavators are deployed in combination with 25 and 15 t dumpers for excavation of overburden and ore. Bench heights vary between 6 and 11 m. Drilling is done by means of tyre/crawler-mounted wagon drills with 100 mm hole diameter. The +37 mm ore fraction is treated as lumpy and the −37 mm ore fraction is fed into the high media separation plant. The manganiferous material is dumped separately.
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4. Geology Regionally the area forms an undulating terrain marked by an ENE–WSW trend in two ridges. The Ambagarh range in the south is supported by the folded Chorbaoli formation and forms a prominent physiographic feature. The Dongri-Buzurg mine is located on an eastêwest-trending hill range rising to 595 m reduced level (R.L.). To its east, is the Yederbuchi hill at 466 m R.L. The manganese ore horizon occurs in the lower part of the sequence of metasedimentary rocks of the Saucer group of pre-Cambrian age. The rock formations exposed belong to the Saucer series and comprise the Tirodi formation, Sitasaongi formation, manganese ore horizon and the Munsar formation with intrusive quartz veins and pegmatites, as shown in the geological cross-section of two bore holes (Fig. 2). The formations in the mine area occur in an inverted sequence representing an overturned limb of a regional anticline. The Munsar formation forms the core of the syncline as well as the footwall of the manganese ore horizon. The Sitasaongi formations followed by Tirodi gneisses form the hanging wall. The manganese ore horizon occurs as a continuous bed at the stratigraphic contact of the overlying Sitasaongi formation and the underlying Munsar formation on the reversed limb of the regional anticline. The manganese ore horizon comprises alternating bands of manganese ore, manganese quartzite, gondite and rhodonites. The manganese ore deposits are of sedimentary, syngentic-banded type, which at this mine have been oxidised and enriched due to leaching in the central part of the deposit. The ore body at the Dongri-Buzurg mine is lensoid in shape, consisting of aluminium minerals of braunnite, pyrolusite and sillimanite. The footwall consists of muscovite schists and the hangwall rocks are biotite-gneisses and quartzitic-gneisses, grading into schistose rocks. Rock exposures in the footwall and hangwall are shown in Figs. 3 and 4 respectively. The contact between the ore body and the footwall is formed of 2 to 3 m wide decomposed or phyllitic clay. The strata, in general, strike due east–west, with a southerly dip
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Fig. 1. Location and plan of Dongri-Buzurg open-pit manganese mine (all dimensions are in metres).
varying from 55 to 60°. The ore body width varies from 6 to 35 m, with a strike length of 500 m and thickens towards the east.
sponding to each joint set. In the footwall side rock masses J varies from 5.8 to 9, which indicates v medium-sized in situ blocks. Similarly, in the hangwall side rock masses J varies from 11 to 21, v which indicates small-sized in situ blocks.
5. Geo-technical investigations 5.1. Structural mapping
5.2. Rock mass rating (RMR)
The rock mass in both the footwall and hangwall sides of the mine consists of four joint sets, including one set of schistocity (Table 1). The volumetric joint count J of various rock v masses has been determined using the relation
Bieniawski’s RMR, as proposed by Bieniawski (1973), was determined for different rock masses to assess the rock mass quality at different locations (Table 2). The RMR at different locations in this mine varies from 24 to 65, which indicates poor to good rock mass conditions. The uniaxial compressive strength ( UCS ) of footwall rocks varies from 4 to 40 MPa, which indicates very low to low strength rocks, and it varies between 58
1 1 1 (2) J = + + , v S S S 1 2 3 where S , S and S are the joint spacings corre1 2 3
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Fig. 2. Geological cross-section of bore holes (all dimensions are in metres).
and 65 MPa in hangwall rocks, which indicates medium strength rocks.
6. Blasting experimentation The experiments were conducted in overburden rocks only. Blast performance, particularly the fragmentation in the overburden rocks, at this mine was observed to be controlled principally by structural features. Conventional blasting practice produced uneven fragmentation, large backbreak,
and higher ground vibration coupled with poor blast economics. A series of blast trials, which included conventional trials (with solid deck) and air-deck trials, were conducted in jointed rocks of footwall and hangwall side overburden benches to study the effects of air-deck on blast performance and blast economics. Each blast was monitored for various parameters, like fragmentation, throw, backbreak, ground vibration ,etc. Since this paper deals only with fragmentation the other parameters are not discussed here. In order to evaluate the air-deck effects only,
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Fig. 3. Exposure of footwall side rocks at Dongri-Buzurg mine.
Fig. 4. Exposure of hangwall side rocks at Dongri-Buzurg mine.
other blast design parameters, like bench height, hole diameter, burden and spacing, in conventional and air-deck blast trials were kept similar. 6.1. Conventional blasting A total of six conventional blasts using solid decks of drill cuttings were conducted at various locations in the overburden benches. Explosive loading in a blast hole of a typical conventional
blast is shown in Fig. 5. The salient features of these blast trials are shown in Table 3. Observation of the conventional blasting practice revealed the following problems: 1. generation of oversize boulders requiring secondary blasting; 2. uneven fragmentation; 3. excessive backbreak; 4. inefficient shovel loading (mucking) operation due to poor fragmentation.
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J.C. Jhanwar et al. / Engineering Geology 57 (2000) 13–29 Table 1 Discontinuity details at Dongri-Buzurg mine Discontinuity
Dip (deg)
Dip direction (deg)
Remark
Footwall Schistocity plane Joint set 1 Joint set 2 Joint set 3
50 40 35 50
170 270 345 220
Strike (E–W ), wavy in nature, very smooth schistocity surface Smooth and undulating joint surfaces; spacing 1 to 3 m Inclined joint, strike roughly E–W, spacing 0.4 m, smooth and planar surfaces Rough and planar surfaces; spacing: 1.75–2.0 m
Hangwall Schistocity Joint set 1 Joint set 2 Joint set 3
50 75 43 55
175 060 325 135
Very smooth surface Planar, smooth surface; spacing: 10–50 cm Planar, rough surface; spacing: 15 cm Rough, irregular; spacing: 3 m
Table 2 Bieniawski’s RMR at Dongri-Buzurg mine Locationa
F/W, 48 m L, Ch. 1036 m F/W, 44 m L, Ch. 1066 m F/W, 40 m L, Ch. 1005 m F/W, 44 m L, Ch. 1036 m F/W, 40 m L, Ch. 1005 m F/W, 48 m L, Ch. 853 m F/W, 48 m L, Ch. 914 m F/W, 48 m L, Ch. 884 m H/W, 3 m L, Ch. 914 m H/W, 3 m L, Ch. 884 m H/W, 10 m L, Ch. 884 m
Type of rock mass
Value (rating)
RMR
UCS RQD (MPa) (%)
Joint spacing
Joint condition
Ground water
Joint orientation
Micaceous schist
4 (1)
40 (8)
0.6–2 (15)
Dry (15)
Fair (−25)
24
Massive jointed, mica grains Micaceous quartzite schist Micaceous muscovite schist, foliated Weathered micaceous schist Muscovite schist
40 (4)
60 (13)
0.6–2 (15)
Slicken sided (10) Rough (20)
Dry (15)
Fair (−25)
42
4 (1)
45 (8)
0.6–2 (15)
Rough (15)
Dry (15)
Fair (−25)
29
20 (2)
40 (8)
0.6–2 (15)
Rough (15)
Dry (15)
Fair (−25)
30
40 (4)
40 (8)
>2.0 (20)
Weathered (18)
Dry (15)
Fair (-25)
40
20 (2)
40 (8)
0.2–0.6 (10)
Damp (10)
Fair (−25)
25
Muscovite schist
20 (2)
40 (8)
>2 (20)
Damp (10)
Fair (−25)
35
Muscovite schist
20 (2)
40 (8)
>2 (20)
Separation <1 mm (20) Separation <1 mm (20) Rough (12)
Damp (10)
Fair (−25)
27
Quartzite muscovite
70 (7)
65 (13)
0.6–2 (15)
Damp (10)
65
Quartzite muscovite gneisses Micaceous schists
70 (7)
65 (13)
0.06–0.2 (8)
40 (4)
65 (13)
0.06–0.2 (8)
Very favourable (0) Very favourable (0) Very favourable (0)
Separation <1 mm (20) Separation <1 mm (20) Rough (12)
Damp (10) Damp (10)
58 47
a Ch. is chainage and L is level.
6.2. Air-deck blasting In this technique, air-decks were maintained almost in the middle of the explosive column by lowering a dumbbell-shaped wooden spacer. An air-decked explosive charge in a blast hole of a
typical air-deck blast is shown in Fig. 6. Both slurry and ammonium nitrate–fuel oil (ANFO) explosives were used for trials to understand the influence of explosive also. A total of 11 such blasts were conducted at different locations in footwall and hangwall side
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Table 3 Details of conventional blasts at Dongri-Buzurg mine Blast number
Locationa
Bench height (m)
Hole depth (m)
Number of holes
Spacing, Burden (m)
Charge/ hole (kg)
PF (kg/m3)
1 2 3 4 5 6
F/W, 40 m L, Ch. 1188 m F/W, 48 m L, Ch. 1036 m F/W, 44 m L, Ch. 1097 m F/W, 44 m L, Ch. 1158 m F/W, 48 m L, Ch. 1097 m H/W, 10 m L, Ch. 884 m
11 7.5 6 6 7.5 7.5
10.5 7.0 6 6.5 7.5 8.0
24 13 14 9 9 11
3, 2.5 2.75, 2.5 2.5, 2.5 2.5, 2 3, 2.5 3, 2.5
30.58 19.46 13.9 11.12 22.24 22.24
0.37 0.38 0.37 0.37 0.40 0.40
a Ch. is chainage and L is level.
benches with combinations of spacing, burden, powder factor (PF ) and bench height by Chakraborty et al. (1995) and Jhanwar et al. (1996). The salient features of these blasts are given in Table 4.
representative fragment size. The average (mean) fragment size of a whole muck pile was estimated using
7. Assessment of fragmentation
The average fragment size was then plotted with the cumulative volume for conventional and airdeck blasts. These plots provide the post-blast fragment size distribution for a given muck pile. In the assessment of fragmentation, the optimum fragment size was taken to be at 0.2 m in view of the excavator bucket capacity of 1.7 m3 according to Rzhevsky (1995). Fragments above this optimum size were considered as oversize, whereas below this they were considered as undersize.
Post-blast fragmentation assessments were made on the basis of actual size measurements of around 200 fragments from each blast. These fragments were randomly selected from the muck piles at different stages of loading operations. The average length, width and thickness of each fragment were measured, and the volume was calculated by multiplication of the above three dimensions in respect of each fragment. The second largest dimension of a fragment was chosen as its
Fig. 5. Explosive loading pattern in a typical conventional blast.
MFS=
M MSS p . 100
(3)
Fig. 6. Explosive loading pattern in a typical air-deck blast.
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J.C. Jhanwar et al. / Engineering Geology 57 (2000) 13–29 Table 4 Details of air-deck blasts at Dongri-Buzurg mine (Chakraborty et al., 1995; Jhanwar et al., 1996) Blast number
Locationa
Bench height (m)
Hole depth (m)
No. of holes
Spacing× Burden (m×m)
Charge/ hole (kg)
PF (kg/m3)
Air-deck length (m)
Air-deck length/original charge length (ADL)
1 2 3 4 5 6 7 8 9 10 11
F/W, 44 m L, Ch. 1097 m F/W, 44 m L, Ch. 1036 m F/W, 44 m L, Ch. 1005 m F/W, 39 m L, Ch. 1005 m F/W, 48 m L, Ch. 1036 m F/W, 39 m L, Ch. 1188 m F/W, 48 m L, Ch. 853 m F/W, 48 m L, Ch. 914m F/W, 48 m L, Ch. 884 m H/W, 3 m L, Ch. 914 m H/W, 10 m L, Ch. 884 m
6.0 7.0 8.70 10.20 6.0 11.0 9.5 9.5 10.0 6.0 8.0
6.0 7.0 7.0 10.70 5.5 10.75 10.5 4.0 10.0 6.5 8.5
9 6 11 11 7 6 11 26 9 11 26
1.8×2.0 2.5×2.0 2.3×2.0 2.5×2.0 2.75×2.0 3.0×2.0 3.5×2.0 2.5×2.0 2.5×2.0 3.0×2.5 3.5×2.5
11.12 25.56 34.56 39.56 18.65 34.6 26.4 8.34 22 12.5 22.24
0.45 0.73 0.89 0.70 0.62 0.56 0.39 0.33 0.44 0.28 0.31
0.9 0.9 1.35 1.80 0.60 2.4 1.80 0.90 1.80 0.90 0.90
0.33 0.20 0.25 0.29 0.22 0.36 0.30 0.40 0.31 0.30 0.20
a Ch. is chainage and L is level.
7.1. Conventional blasting The fragmentation from six conventional blasts is depicted in Fig. 7. Oversize fragments in all blasts except blast 6 constitute more than 90% of
the fragments volume. In blast 6, which was taken in the hangwall side, the fragmentation was relatively uniform and 50% of the fragments volume was constituted by fragments within 0.4 m size, which was not very far from the optimum size of
Fig. 7. Fragmentation from conventional blasts.
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Fig. 8. Muck pile image of a conventional blast.
0.2 m. The fragments in the range of 1–1.1 m size accounted for only 5% of the volume, whereas the fragments larger than 1 m accounted for approximately 65%, 70%, 65%, 46% and 38% in blasts 1, 2, 3, 4 and 5 respectively. These blasts were taken in the footwall side rock masses. The difference in blast-induced fragmentation from footwall and hangwall rocks could be explained by the difference in in situ block sizes in these rock masses. The average in situ block sizes in footwall and hangwall rock masses were 1.0 m and 0.5 m respectively. The muck pile photograph of a typical conventional blast in footwall benches is shown in Fig. 8. It clearly indicates the presence of large boulders with uneven fragmentation. 7.2. Air-deck blasting The fragmentation from air-deck blasts 1 through 6 and 7 through 11 are shown in Figs. 9 and 10 respectively. It could be observed in Fig. 9 that oversize fragments are at a maximum in blast 6 and a minimum in blasts 3 and 4. The percentage of undersize fragments is the least in blast 6. In all other blasts, the undersize fragments vary between 15 and 20% of the total muck volume
and the oversize fragments vary between 70 and 80%. Blasts 3, 5 and 6 provide relatively uniform fragmentation, where approximately 50–70% of the fragments are close to the optimum size. In blasts 1 and 4, where the oversize fragments are as big as 1.25 m and 1.5 m respectively, in situ blocks constitute about 17% and 30% of the total muck volume. As shown in Fig. 10, blast 7 produced nonuniform fragmentation with some fragments as large as 1.8 m. Blasts 8, 10 and 11 provided relatively uniform fragmentation. In blast 11, over 70% of the fragmentation was close to the optimum size and thus could be considered as the best blast. The percentages of oversize in blasts 7 and 9 were about 70%. A muck pile photograph taken from a typical air-deck blast is shown in Fig. 11. It shows that the fragmentation is better than conventional blasting and is more uniform.
8. Impact of air-decking on fragmentation The influence of air-decking on fragmentation was evaluated in terms of mean fragment size (MFS), fragmentation index (FI ), and blast-
J.C. Jhanwar et al. / Engineering Geology 57 (2000) 13–29
Fig. 9. Fragmentation from air-deck blasts 1 through 6.
Fig. 10. Fragmentation from air-deck blasts 7 through 11.
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Fig. 12. Influence of RMR, PF and S/B on MFS in conventional blasting (MFS and PF are in metres and kilograms per cubic metre respectively).
Fig. 11. Muck pile image of an air-deck blast.
induced fragmentation. These three parameters were evaluated for conventional and air-deck blasts (Jhanwar, 1998). 8.1. MFS The MFSs of conventional and air-deck blasts were determined using Eq. (3). MFSs as obtained are shown in Table 5. It is interesting to note that the difference in the MFS between conventional and air-deck blasts is prominent in footwall rocks compared with the difference in hangwall rocks. This indicates that the air-deck technique is more effective in very low to low strength moderately jointed rocks than in medium strength highly jointed rocks. Regression analysis has indicated that the influence of Bieniawski’s RMR, PF and the ratio of spacing to burden significantly influences the MFS. Correlations between MFS and these parame-
Fig. 13. Influence of RMR, PF and S/B on MFS in air-deck blasting (MFS and PF are in metres and kilograms per cubic metre respectively).
ters were derived by regression analysis for conventional and air-deck blasting as shown here in Eq. (4) and Fig. 12, and Eq. (5) and Fig. 13 respectively. MFS=−0.05(RMR×PF×S/B)+1.54; (r=0.90)
(4)
MFS=−0.01(RMR×PF×S/B)+0.62; (r=0.87),
(5)
where MFS is in metres, PF is in kilograms per
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J.C. Jhanwar et al. / Engineering Geology 57 (2000) 13–29 Table 5 MFS from conventional and air-deck blasts Locationa
F/W, 44 m L, Ch. 1036 m– 1097 m F/W, 39 m L, Ch. 1036 m– 1188 m F/W, 48 m L, Ch. 914 m– 1097 m H/W, 10 m L, Ch. 884 m H/W, 3 m L, Ch. 914 m
Bieniawski’s RMR
Conventional blasts
Air-deck blasts
Blast no.
MFS (m)
Blast no.
42
3 4
0.95 0.80
1 2 3
0.51 0.38 0.27
40
1
0.78
4 6
0.48 0.46
25–35
2 5
1.0 0.68
5 7 8 9
0.23 0.49 0.32 0.42
50 65
6
0.36
10 11
0.32 0.27
MFS (m)
a Ch. is chainage and L is level.
cubic metre, S/B is the ratio of spacing and burden and r is the correlation coefficient. The MFS, as predicted from Eqs. (4) and (5) for different values of RMR, PF and S/B, in the cases of conventional and air-deck blasting is shown in Fig. 14. It can be seen in Fig. 14 that air-deck blasting produces a more uniform fragmentation compared with conventional blasting. Considering an optimum size of 0.2 m, air-deck blasts produced fragmentation that was not very far from the optimum
Fig. 14. Predicted MFS for conventional and air-deck blasting (MFS is measured in metres).
size, whereas the conventional blasts produced relatively uneven fragmentation in the form of oversized boulders and fines. In the case of air-deck blasting, MFS and PF have been correlated with Bieniawski’s RMR, airdeck length as a fraction of original charge length and the ratio of spacing and burden [Eq. (6) and Fig. 15]. It can be seen that MFS and PF together follow an asymptotic relationship with RMR, air-
Fig. 15. Influence of RMR, ADL and S/B on MFS and PF in air-deck blasting (MFS and PF are in metres and kilograms per cubic metre respectively).
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deck length and ratio of spacing to burden. MFS×PF=2.04(RMR×ADL×S/B)−1.04; (r=0.94),
(6)
where have already been defined and ADL is the air-deck length as a fraction of the original charge length. Air-deck blast numbers 4 and 8 and numbers 4 and 6 in Table 4 were excluded in the derivation of Eqs. (5) and (6) respectively to ensure good correlation. 8.2. The FI The FI, which defines blast-induced reduction in in situ block size, is estimated as the ratio of average in situ fragment size to average muck pile fragment size. The average in situ fragment size was estimated from the volumetric joint count. In the footwall rocks, average in situ fragment size was 1.0 m and in the hangwall rocks it was 0.5 m. The FI so calculated in respect of conventional and air-deck blasts is shown in Fig. 16. In the case of hangwall rocks, this is 1.38 in conventional blasting (blast 6 of conventional blasting, Fig. 16) and 1.56 and 1.58 in air-deck blasting (blasts 10 and 11, Fig. 16). Similarly, in the case of footwall rocks it varies between 1 and 1.47 in conventional blasting and 1.97 and 4.34 in air-deck blasting. From this, it is clear that the effect of airdecking in effecting further fragmentation is more pronounced in footwall rocks than in hangwall
Fig. 16. Blast-wise FI.
rocks. This indicates, therefore, that air-deck blasting could be more effective in very low to low strength moderately jointed rocks with medium size in situ blocks than in medium strength highly jointed rocks. In medium strength highly jointed rocks with smaller-sized in situ blocks, air-deck blasting does not help much in creating additional fragmentation. This could be due to the fact that in highly jointed rocks with small-sized in situ blocks the explosive energy is mainly utilised in loosening rather than in creating additional fragmentation, in contrast to moderately jointed rocks with medium-sized in situ blocks where the explosive energy is principally used to create new fractures. In moderately jointed rocks, therefore, the air-deck technique considerably improves the fragmentation. The critical FI for a given rock mass is principally governed by the bucket size of the mucking/loading equipment and the purpose of blasting. With larger bucket size a relatively low FI could serve the purpose, whereas if the bucket size is smaller a higher FI is required. This is because the optimum fragment size is governed by bucket size only. In this particular case, the bucket size was 1.7 m3 and, therefore, the optimum fragment size was 0.2 m. For this optimum fragment size, the FI was supposed to be 5.0 in footwall rock blasting and 2.5 in hangwall rock blasting. The in situ fragment sizes in footwall and hangwall were 1.0 m and 0.5 m respectively. The FI signifies blast-induced reduction in in situ fragments; it is, therefore, controlled by the rock mass, explosive charge and the blast design parameters. The rock mass has been identified by Bieniawski’s RMR, explosive charge by PF and blast design parameter by the ratio of spacing to burden. It is expected to improve with improvement in rock mass quality, increase in the PF and also with an increase in the ratio of spacing to burden. As the rock mass becomes more and more massive, the explosive energy utilisation in fragmentation becomes more and hence so does the FI. The increase in FI with an increase in spacing to burden ratio holds true up to a certain extent and beyond that limiting spacing to burden ratio, the FI may again decrease. The optimum ratio of spacing to burden for different rocks is different and varies between 1.2 and 1.5.
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FI=0.12(RMR×PF×S/B)−0.33;
(r=0.71), (8)
Fig. 17. Influence of RMR, PF and S/B on FI in conventional blasting (PF is measured in kilograms per cubic metre).
The FI so predicted for conventional and airdeck blasting from Eqs. (7) and (8) is depicted in Fig. 19. It can be seen that, for all ranges of Bieniawski’s RMR, PF and ratio of spacing to burden, the FI is significantly higher in the case of air-deck blasting than for conventional blasting. In the case of air-deck blasting the FI is also correlated with Bieniawski’s RMR, air-deck length as fraction of original charge length and the ratio of spacing to burden, as shown in Eq. (9) and Fig. 20. FI=−0.08(RMR×ADL×S/B)+3.40; (r=0.82), (9) It can be seen that the FI is linearly related to these parameters. To ensure better correlation, blasts 1, 4, 7, 8 and 9 and blasts 1, 3, 5 and 8 were excluded in the derivation of Eqs. (8) and (9) respectively. Therefore, these correlations need to be further refined. The above correlations [Eqs. (4)– (9)] were obtained for Bieniawski’s RMR=24–65, PF=0.27–0.89 kg/m3, S/B=0.9–1.75 and air-deck length as a fraction of original charge length in the ranges of 0.20–0.40.
Fig. 18. Influence of RMR, PF and S/B on FI in air-deck blasting (PF is measured in kilograms per cubic metre).
In this study it was also found that the FI increased with Bieniawski’s RMR, PF and the ratio of spacing to burden. To define the FI by all these parameters collectively, it was therefore correlated with these parameters taken together in the cases of conventional and air-deck blasting as shown in Eq. (7) and Fig. 17, and Eq. (8) and Fig. 18 respectively. FI=0.03(RMR×PF×S/B)+0.73;
(r=0.73) (7)
Fig. 19. Predicted FI for conventional and air-deck blasting.
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J.C. Jhanwar et al. / Engineering Geology 57 (2000) 13–29
Fig. 21. Blast-induced fragmentation from conventional and air-deck blasting. Fig. 20. Influence of RMR, ADL and S/B on FI in air-deck blasting.
8.3. Blast-induced fragmentation Blast-induced fragmentation is hereby defined as the percentage muck volume made by fragments smaller than the in situ fragment size. This was estimated from Fig. 7 in the case of conventional blasts and from Figs. 9 and 10 in the case of airdeck blasts. Blast-induced fragmentations in conventional blasts 1, 2, 3, 4, 5 and 6 are 37%, 30%, 35%, 53%, 60% and 65% respectively. Similarly, the blastinduced fragmentations in the case of air-deck blast events 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 and 11 are 82%, 95%, 100%, 66%, 100%, 100%, 79%, 92%, 93%, 66% and 85% respectively. The blast-induced fragmentation for conventional and air-deck blasts is shown in Fig. 21. In the case of air-deck blasting, the blastinduced fragmentation is almost similar in footwall and in hangwall rocks. A comparison of blastinduced fragmentation for conventional and airdeck blasts reveals that air-deck blasting maximises the fragmentation.
tional blasting. In conventional blasting with solid material decking, shock front intensities become extremely low due to high resistance of solid materials and hence this results in poor breakage. (2) Air-deck blasting maximises the fragmentation as reflected by reduction in MFS, increase in FI and in blast-induced fragmentation. Fragmentation is influenced by the rock mass as characterised by its Bieniawski’s RMR, explosive factor as defined by PF, air-deck length as a fraction of original charge length (for air-deck blasting) and blast design parameters as defined by the ratio of spacing to burden. (3) The effectiveness of the air-deck technique in improving fragmentation is more pronounced in very low to low strength moderately jointed rocks than in medium strength highly jointed rocks with small-sized in situ blocks. In highly jointed rocks, since the explosive energy is principally utilised in loosening the rock mass rather than in causing additional fragmentation, the benefit of using air-decking is not that prominent. (4) The empirical correlations for estimation of MFS and FI are derived on the basis of a few blast results only and for a limited domain of rock type, and blast design parameters; therefore, scope exists for their further refinement.
9. Conclusions Acknowledgements (1) In jointed rocks, air-deck blasting improves the degree of fragmentation and produces more uniform fragmentation compared with conven-
The authors express their sincere thanks to the Director, Central Mining Research Institute,
J.C. Jhanwar et al. / Engineering Geology 57 (2000) 13–29
Dhanbad ( India). Thanks are also due to the mine management for providing facilities during field experiments and to Mr A.K. Chakraborty, Scientist, for his help during field trials. The views expressed in this paper are, however, of the authors and not necessarily of the institute to which they belong.
References Bieniawski, Z.T., 1973. Engineering classification of jointed rock masses. Transactions of South African Institute of Civil Engineers 15 (12), 335–344. Bussey, J., Borg, D.G., 1995. Presplitting with the new air-deck technique in: Proceedings of the Twentyfirst International Conference on Explosives and Blasting Techniques. Society for Explosives and Blasting Techniques, Tennesse, USA, pp. 197–213. Chiappetta, R.F., Memmele, M.E., 1987. Analytical high-speed photography to evaluate air-decks stemming retention and gas confinement in presplitting reclamation and gross motion studies in: Proceedings of the Second International Symposium on Rock Fragmentation by Blasting. Society for Experimental Mechanics, Bethel, CT, USA, pp. 257–301. Chakraborty, A.K., Murthy, V.M.S.R., Jhanwar, J.C., Jethwa, J.L., 1995. A feasibility study on air-deck blasting in: Proceedings of the Eighth National Convention of Mining Engineers. Institution of Engineers (India), Udaipur, India, pp. 68–74. Fourney, W.L., Barker, D.B., Holloway, D.C., 1981. Model
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studies of explosive well stimulation techniques. International Journal of Rock Mechanics and Mining Sciences and Geomechanics Abstracts 18, 113–127. Jhanwar, J.C., 1998. Investigations into air-deck blasting and its influence on blast performance and economics in openpit mines. M.E. Thesis, Nagpur University of Nagpur, India. Jhanwar, J.C., Chakraborty, A.K., Murthy, V.M.S.R., Jethwa, J.L., Dhar, B.B., 1996. Application of air-deck blasting technique to improve fragmentation and blast economics in the foot wall side of Dongri-Buzurg opencast mine, Unpublished Project Report, Number: MT/Collab/N/95, Central Mining Research Institute, Dhanbad, India. Kinney, G.F., Graham, K.J., 1985. Explosive Shocks in Air. Springer, New York. Marchenko, L.N., 1982. Raising the efficiency of a blast in rock crushing Fiziko-Tekhnicheskie Problemy Razrabotki Poleznykh Iskopaemykh (Soviet Mining Science) 18 (5), 1–6. Mead, D.J., Moxon, N.T., Danell, R.E., Richardson, S.B., 1993. The use of air-decks in production blasting in: Proceedings of the Fourth International Syposium on Rock Fragmentation by Blasting. A.A. Ballkema, Rotterdam, pp. 437–443. Melnikov, N.V., Marchenko, L.N., 1971. Effective methods of application of explosive energy in mining and construction. Twelfth Symposium on Dynamic Rock Mechanics. AIME, New York. Melnikov, N.V., Marchenko, L.N., Seinov, N.P., Zharikov, I.F., 1979. A method of enhanced rock blasting by blasting Fiziko-Tekhnicheskie Problemy Razrabotki Poleznykh Iskopaemykh (Journal of Mining Science) 6, 32–42. Moxon, N.T., Mead, D., Richardson, M., 1993. Air decked blasting techniques: some collaborative experiments. Transactions Institution of Mining and Metallurgy (Section A: Mining Industry 102, A25–A30. Rzhevsky, V.V., 1995. Opencast Mining Unit Operations. Mir, Moscow.
Influence of air-deck blasting on fragmentation in jointed rocks in an open-pit manganese mine J.C. Jhanwar a, *, J.L. Jethwa a, A.H. Reddy b a Central Mining Research Institute Regional Centre, 54-B, Shankar Nagar, Nagpur, Maharastra 440 010, India b Department of Mining Engineering, Visvesvaraya Regional College of Engineering, Nagpur 440 010, India Received 14 January 1999; accepted for publication 13 September 1999
Abstract The influence of air-deck blasting on fragmentation in jointed rock masses of a manganese open-pit mine has been investigated. It is revealed that air-deck blasting improves the degree of fragmentation and produces more uniform fragmentation compared with conventional blasting. Mean fragment size (MFS) reduces from a range of 0.36–1.0 m to a range of 0.27–0.51 m. Fragmentation index (FI ) and blast-induced fragmentation increase from 1.38 to 4.34 and from 37–65% to 66–100% respectively. Empirical correlations have been developed to estimate MFS and FI from conventional and air-deck blasting on the basis of Bieniawski’s rock mass rating, powder factor and ratio of spacing to burden. Results indicate that air-deck blasting could be more effective in very low to low strength moderately jointed rock masses than in medium strength highly jointed rock masses. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Air-deck; Blasting; Fragmentation; Jointed rock; Open pit
1. Introduction Blasting with air-decked explosive charges has a long history. It has become an increasingly popular technique in production and presplit blasting in open-pit mines. In this technique, an air gap is introduced in an explosive column in the blast hole as a means of optimising the rock breakage for a given charge length. Field experiments and theoretical studies conducted by Melnikov and Marchenko (1971) using air-decked explosive charges revealed its benefits over conventional methods of blasting. Following studies of fracture network in thick Plexiglas * Corresponding author. Fax: +91-712-527628. E-mail address: [email protected] (J.C. Jhanwar)
blocks using air-decked explosive charges, Fourney et al. (1981) demonstrated that when a shock wave reaches the stemming it is reflected back to reinforce the stress field and that the time period over which it acted on the surrounding material increased by a factor of between two and five. Since then, air-deck blasting has been applied in a variety of applications, like presplitting, controlling ground vibration and fly rock, reducing fines and improving blast economics, by Marchenko (1982), Chiappetta and Memmele (1987), Mead et al. (1993), Moxon et al. (1993) and Bussey and Borg (1995). In spite of these studies, the mechanism governing rock fragmentation with air-deck blasting has still not been fully understood and its use does not always improve blasting results. The potential areas of application of this technique in production
0013-7952/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 1 3 -7 9 5 2 ( 9 9 ) 0 0 12 5 - 8
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Nomenclature P rate of decay of pressure (MPa/m) C,C empirical constants 1 2 M mass of explosive (kg) n number of moles of gas R universal gas constant T temperature of gas (°C ) S displacement of pressure front (m) R.L. reduced level (m) J volumetric joint count v S,S, spacing of joint set (m) 1 2 S,S 3 4 F/W footwall H/W hangwall RMR Bieniawski’s rock mass rating UCS uniaxial compressive strength (MPa) RQD rock quality designation MFS mean fragment size (m) M mass of fragments (%) p MSS mean sieve size (m) PF powder factor (kg/m3) S/B ratio of spacing and burden ADL air-deck length as a fraction of original charge length (ratio of air-deck length to original charge length) FI fragmentation index r correlation coefficient blasts of open-pit mines are yet to be established and its nature of impact on blast fragmentation in various rock masses remains to be ascertained. This paper discusses the results of experimental blasts conducted in jointed rocks of a manganese open-pit mine to evaluate the influence of air-deck blasting on fragmentation.
2. Theory of air-decking The technique of air-decking involves the use of one or more air gaps in the explosive column as a means of optimising fragmentation for a given charge length. The theory as proposed by Melnikov and Marchenko (1971) and Melnikov et al. (1979) postulates that shock waves, when reflected within the borehole, generate a secondary shock wave that extends the network of microfractures prior to gas pressurisation. The final borehole
pressure produced by an explosive is, however, reduced in this case, but the degree of fracture is increased as a result of repeated loading of the rock by a series of aftershocks. The three main pressure fronts, i.e. the shock front, pressure front due to formation of explosive products behind the detonation front and reflected waves from the bottom of the blasthole and/or from the base of the stemming travel within the air-deck for different distances and velocities, create these aftershocks. The duration of shock wave action on the surrounding rock mass is, therefore, prolonged. Consequently, the crack network in the rock mass in front of the borehole is increased in air-deck blasting. Since the magnitude of the shock front depends on the explosive formulation and decreases quickly with distance in air, the degree of fracture in the air-deck region finally depends on the length of the air-deck and the type of explosive.
J.C. Jhanwar et al. / Engineering Geology 57 (2000) 13–29
The rate of decay of a pressure wave as it passes through air is given by Kinney and Graham (1985) as P=C
(MnRT )1/3 1
S
C , 2
(1)
where M is the mass of explosive, n is number of moles of gas, R is the universal gas constant, T is temperature of the expanding gas, S is the displacement of the pressure front and C and C are 1 2 empirical constants. Moxon et al. (1993) extended this theory and opined that if the air-deck is placed in the middle of the explosive column, the pressure front produced due to explosive at either end of the airdeck collides at the centre of the air-deck. This interaction, they proposed, should develop a reinforced stress field and result in a more extensive radial crack pattern than if an air-deck was kept on the top of the charge. In the case of solid decking, the shock fronts from the opposite explosive columns also interact in the decking region, but they get attenuated due to high resistance offered by the solid material and, consequently, their intensities become too low to enable them to propagate further through the decking materials. It is for this reason that air-decking has an overriding advantage over solid decking.
3. Mining detail The mine known as Dongri-Buzurg open-pit manganese mine is situated in central India near the city of Nagpur ( Fig. 1). The overburden excavation and the ore production in this mine are 0.3×106 m3 and 0.18×106 t respectively. Shovels and hydraulic excavators are deployed in combination with 25 and 15 t dumpers for excavation of overburden and ore. Bench heights vary between 6 and 11 m. Drilling is done by means of tyre/crawler-mounted wagon drills with 100 mm hole diameter. The +37 mm ore fraction is treated as lumpy and the −37 mm ore fraction is fed into the high media separation plant. The manganiferous material is dumped separately.
15
4. Geology Regionally the area forms an undulating terrain marked by an ENE–WSW trend in two ridges. The Ambagarh range in the south is supported by the folded Chorbaoli formation and forms a prominent physiographic feature. The Dongri-Buzurg mine is located on an eastêwest-trending hill range rising to 595 m reduced level (R.L.). To its east, is the Yederbuchi hill at 466 m R.L. The manganese ore horizon occurs in the lower part of the sequence of metasedimentary rocks of the Saucer group of pre-Cambrian age. The rock formations exposed belong to the Saucer series and comprise the Tirodi formation, Sitasaongi formation, manganese ore horizon and the Munsar formation with intrusive quartz veins and pegmatites, as shown in the geological cross-section of two bore holes (Fig. 2). The formations in the mine area occur in an inverted sequence representing an overturned limb of a regional anticline. The Munsar formation forms the core of the syncline as well as the footwall of the manganese ore horizon. The Sitasaongi formations followed by Tirodi gneisses form the hanging wall. The manganese ore horizon occurs as a continuous bed at the stratigraphic contact of the overlying Sitasaongi formation and the underlying Munsar formation on the reversed limb of the regional anticline. The manganese ore horizon comprises alternating bands of manganese ore, manganese quartzite, gondite and rhodonites. The manganese ore deposits are of sedimentary, syngentic-banded type, which at this mine have been oxidised and enriched due to leaching in the central part of the deposit. The ore body at the Dongri-Buzurg mine is lensoid in shape, consisting of aluminium minerals of braunnite, pyrolusite and sillimanite. The footwall consists of muscovite schists and the hangwall rocks are biotite-gneisses and quartzitic-gneisses, grading into schistose rocks. Rock exposures in the footwall and hangwall are shown in Figs. 3 and 4 respectively. The contact between the ore body and the footwall is formed of 2 to 3 m wide decomposed or phyllitic clay. The strata, in general, strike due east–west, with a southerly dip
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J.C. Jhanwar et al. / Engineering Geology 57 (2000) 13–29
Fig. 1. Location and plan of Dongri-Buzurg open-pit manganese mine (all dimensions are in metres).
varying from 55 to 60°. The ore body width varies from 6 to 35 m, with a strike length of 500 m and thickens towards the east.
sponding to each joint set. In the footwall side rock masses J varies from 5.8 to 9, which indicates v medium-sized in situ blocks. Similarly, in the hangwall side rock masses J varies from 11 to 21, v which indicates small-sized in situ blocks.
5. Geo-technical investigations 5.1. Structural mapping
5.2. Rock mass rating (RMR)
The rock mass in both the footwall and hangwall sides of the mine consists of four joint sets, including one set of schistocity (Table 1). The volumetric joint count J of various rock v masses has been determined using the relation
Bieniawski’s RMR, as proposed by Bieniawski (1973), was determined for different rock masses to assess the rock mass quality at different locations (Table 2). The RMR at different locations in this mine varies from 24 to 65, which indicates poor to good rock mass conditions. The uniaxial compressive strength ( UCS ) of footwall rocks varies from 4 to 40 MPa, which indicates very low to low strength rocks, and it varies between 58
1 1 1 (2) J = + + , v S S S 1 2 3 where S , S and S are the joint spacings corre1 2 3
J.C. Jhanwar et al. / Engineering Geology 57 (2000) 13–29
17
Fig. 2. Geological cross-section of bore holes (all dimensions are in metres).
and 65 MPa in hangwall rocks, which indicates medium strength rocks.
6. Blasting experimentation The experiments were conducted in overburden rocks only. Blast performance, particularly the fragmentation in the overburden rocks, at this mine was observed to be controlled principally by structural features. Conventional blasting practice produced uneven fragmentation, large backbreak,
and higher ground vibration coupled with poor blast economics. A series of blast trials, which included conventional trials (with solid deck) and air-deck trials, were conducted in jointed rocks of footwall and hangwall side overburden benches to study the effects of air-deck on blast performance and blast economics. Each blast was monitored for various parameters, like fragmentation, throw, backbreak, ground vibration ,etc. Since this paper deals only with fragmentation the other parameters are not discussed here. In order to evaluate the air-deck effects only,
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J.C. Jhanwar et al. / Engineering Geology 57 (2000) 13–29
Fig. 3. Exposure of footwall side rocks at Dongri-Buzurg mine.
Fig. 4. Exposure of hangwall side rocks at Dongri-Buzurg mine.
other blast design parameters, like bench height, hole diameter, burden and spacing, in conventional and air-deck blast trials were kept similar. 6.1. Conventional blasting A total of six conventional blasts using solid decks of drill cuttings were conducted at various locations in the overburden benches. Explosive loading in a blast hole of a typical conventional
blast is shown in Fig. 5. The salient features of these blast trials are shown in Table 3. Observation of the conventional blasting practice revealed the following problems: 1. generation of oversize boulders requiring secondary blasting; 2. uneven fragmentation; 3. excessive backbreak; 4. inefficient shovel loading (mucking) operation due to poor fragmentation.
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J.C. Jhanwar et al. / Engineering Geology 57 (2000) 13–29 Table 1 Discontinuity details at Dongri-Buzurg mine Discontinuity
Dip (deg)
Dip direction (deg)
Remark
Footwall Schistocity plane Joint set 1 Joint set 2 Joint set 3
50 40 35 50
170 270 345 220
Strike (E–W ), wavy in nature, very smooth schistocity surface Smooth and undulating joint surfaces; spacing 1 to 3 m Inclined joint, strike roughly E–W, spacing 0.4 m, smooth and planar surfaces Rough and planar surfaces; spacing: 1.75–2.0 m
Hangwall Schistocity Joint set 1 Joint set 2 Joint set 3
50 75 43 55
175 060 325 135
Very smooth surface Planar, smooth surface; spacing: 10–50 cm Planar, rough surface; spacing: 15 cm Rough, irregular; spacing: 3 m
Table 2 Bieniawski’s RMR at Dongri-Buzurg mine Locationa
F/W, 48 m L, Ch. 1036 m F/W, 44 m L, Ch. 1066 m F/W, 40 m L, Ch. 1005 m F/W, 44 m L, Ch. 1036 m F/W, 40 m L, Ch. 1005 m F/W, 48 m L, Ch. 853 m F/W, 48 m L, Ch. 914 m F/W, 48 m L, Ch. 884 m H/W, 3 m L, Ch. 914 m H/W, 3 m L, Ch. 884 m H/W, 10 m L, Ch. 884 m
Type of rock mass
Value (rating)
RMR
UCS RQD (MPa) (%)
Joint spacing
Joint condition
Ground water
Joint orientation
Micaceous schist
4 (1)
40 (8)
0.6–2 (15)
Dry (15)
Fair (−25)
24
Massive jointed, mica grains Micaceous quartzite schist Micaceous muscovite schist, foliated Weathered micaceous schist Muscovite schist
40 (4)
60 (13)
0.6–2 (15)
Slicken sided (10) Rough (20)
Dry (15)
Fair (−25)
42
4 (1)
45 (8)
0.6–2 (15)
Rough (15)
Dry (15)
Fair (−25)
29
20 (2)
40 (8)
0.6–2 (15)
Rough (15)
Dry (15)
Fair (−25)
30
40 (4)
40 (8)
>2.0 (20)
Weathered (18)
Dry (15)
Fair (-25)
40
20 (2)
40 (8)
0.2–0.6 (10)
Damp (10)
Fair (−25)
25
Muscovite schist
20 (2)
40 (8)
>2 (20)
Damp (10)
Fair (−25)
35
Muscovite schist
20 (2)
40 (8)
>2 (20)
Separation <1 mm (20) Separation <1 mm (20) Rough (12)
Damp (10)
Fair (−25)
27
Quartzite muscovite
70 (7)
65 (13)
0.6–2 (15)
Damp (10)
65
Quartzite muscovite gneisses Micaceous schists
70 (7)
65 (13)
0.06–0.2 (8)
40 (4)
65 (13)
0.06–0.2 (8)
Very favourable (0) Very favourable (0) Very favourable (0)
Separation <1 mm (20) Separation <1 mm (20) Rough (12)
Damp (10) Damp (10)
58 47
a Ch. is chainage and L is level.
6.2. Air-deck blasting In this technique, air-decks were maintained almost in the middle of the explosive column by lowering a dumbbell-shaped wooden spacer. An air-decked explosive charge in a blast hole of a
typical air-deck blast is shown in Fig. 6. Both slurry and ammonium nitrate–fuel oil (ANFO) explosives were used for trials to understand the influence of explosive also. A total of 11 such blasts were conducted at different locations in footwall and hangwall side
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Table 3 Details of conventional blasts at Dongri-Buzurg mine Blast number
Locationa
Bench height (m)
Hole depth (m)
Number of holes
Spacing, Burden (m)
Charge/ hole (kg)
PF (kg/m3)
1 2 3 4 5 6
F/W, 40 m L, Ch. 1188 m F/W, 48 m L, Ch. 1036 m F/W, 44 m L, Ch. 1097 m F/W, 44 m L, Ch. 1158 m F/W, 48 m L, Ch. 1097 m H/W, 10 m L, Ch. 884 m
11 7.5 6 6 7.5 7.5
10.5 7.0 6 6.5 7.5 8.0
24 13 14 9 9 11
3, 2.5 2.75, 2.5 2.5, 2.5 2.5, 2 3, 2.5 3, 2.5
30.58 19.46 13.9 11.12 22.24 22.24
0.37 0.38 0.37 0.37 0.40 0.40
a Ch. is chainage and L is level.
benches with combinations of spacing, burden, powder factor (PF ) and bench height by Chakraborty et al. (1995) and Jhanwar et al. (1996). The salient features of these blasts are given in Table 4.
representative fragment size. The average (mean) fragment size of a whole muck pile was estimated using
7. Assessment of fragmentation
The average fragment size was then plotted with the cumulative volume for conventional and airdeck blasts. These plots provide the post-blast fragment size distribution for a given muck pile. In the assessment of fragmentation, the optimum fragment size was taken to be at 0.2 m in view of the excavator bucket capacity of 1.7 m3 according to Rzhevsky (1995). Fragments above this optimum size were considered as oversize, whereas below this they were considered as undersize.
Post-blast fragmentation assessments were made on the basis of actual size measurements of around 200 fragments from each blast. These fragments were randomly selected from the muck piles at different stages of loading operations. The average length, width and thickness of each fragment were measured, and the volume was calculated by multiplication of the above three dimensions in respect of each fragment. The second largest dimension of a fragment was chosen as its
Fig. 5. Explosive loading pattern in a typical conventional blast.
MFS=
M MSS p . 100
(3)
Fig. 6. Explosive loading pattern in a typical air-deck blast.
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J.C. Jhanwar et al. / Engineering Geology 57 (2000) 13–29 Table 4 Details of air-deck blasts at Dongri-Buzurg mine (Chakraborty et al., 1995; Jhanwar et al., 1996) Blast number
Locationa
Bench height (m)
Hole depth (m)
No. of holes
Spacing× Burden (m×m)
Charge/ hole (kg)
PF (kg/m3)
Air-deck length (m)
Air-deck length/original charge length (ADL)
1 2 3 4 5 6 7 8 9 10 11
F/W, 44 m L, Ch. 1097 m F/W, 44 m L, Ch. 1036 m F/W, 44 m L, Ch. 1005 m F/W, 39 m L, Ch. 1005 m F/W, 48 m L, Ch. 1036 m F/W, 39 m L, Ch. 1188 m F/W, 48 m L, Ch. 853 m F/W, 48 m L, Ch. 914m F/W, 48 m L, Ch. 884 m H/W, 3 m L, Ch. 914 m H/W, 10 m L, Ch. 884 m
6.0 7.0 8.70 10.20 6.0 11.0 9.5 9.5 10.0 6.0 8.0
6.0 7.0 7.0 10.70 5.5 10.75 10.5 4.0 10.0 6.5 8.5
9 6 11 11 7 6 11 26 9 11 26
1.8×2.0 2.5×2.0 2.3×2.0 2.5×2.0 2.75×2.0 3.0×2.0 3.5×2.0 2.5×2.0 2.5×2.0 3.0×2.5 3.5×2.5
11.12 25.56 34.56 39.56 18.65 34.6 26.4 8.34 22 12.5 22.24
0.45 0.73 0.89 0.70 0.62 0.56 0.39 0.33 0.44 0.28 0.31
0.9 0.9 1.35 1.80 0.60 2.4 1.80 0.90 1.80 0.90 0.90
0.33 0.20 0.25 0.29 0.22 0.36 0.30 0.40 0.31 0.30 0.20
a Ch. is chainage and L is level.
7.1. Conventional blasting The fragmentation from six conventional blasts is depicted in Fig. 7. Oversize fragments in all blasts except blast 6 constitute more than 90% of
the fragments volume. In blast 6, which was taken in the hangwall side, the fragmentation was relatively uniform and 50% of the fragments volume was constituted by fragments within 0.4 m size, which was not very far from the optimum size of
Fig. 7. Fragmentation from conventional blasts.
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J.C. Jhanwar et al. / Engineering Geology 57 (2000) 13–29
Fig. 8. Muck pile image of a conventional blast.
0.2 m. The fragments in the range of 1–1.1 m size accounted for only 5% of the volume, whereas the fragments larger than 1 m accounted for approximately 65%, 70%, 65%, 46% and 38% in blasts 1, 2, 3, 4 and 5 respectively. These blasts were taken in the footwall side rock masses. The difference in blast-induced fragmentation from footwall and hangwall rocks could be explained by the difference in in situ block sizes in these rock masses. The average in situ block sizes in footwall and hangwall rock masses were 1.0 m and 0.5 m respectively. The muck pile photograph of a typical conventional blast in footwall benches is shown in Fig. 8. It clearly indicates the presence of large boulders with uneven fragmentation. 7.2. Air-deck blasting The fragmentation from air-deck blasts 1 through 6 and 7 through 11 are shown in Figs. 9 and 10 respectively. It could be observed in Fig. 9 that oversize fragments are at a maximum in blast 6 and a minimum in blasts 3 and 4. The percentage of undersize fragments is the least in blast 6. In all other blasts, the undersize fragments vary between 15 and 20% of the total muck volume
and the oversize fragments vary between 70 and 80%. Blasts 3, 5 and 6 provide relatively uniform fragmentation, where approximately 50–70% of the fragments are close to the optimum size. In blasts 1 and 4, where the oversize fragments are as big as 1.25 m and 1.5 m respectively, in situ blocks constitute about 17% and 30% of the total muck volume. As shown in Fig. 10, blast 7 produced nonuniform fragmentation with some fragments as large as 1.8 m. Blasts 8, 10 and 11 provided relatively uniform fragmentation. In blast 11, over 70% of the fragmentation was close to the optimum size and thus could be considered as the best blast. The percentages of oversize in blasts 7 and 9 were about 70%. A muck pile photograph taken from a typical air-deck blast is shown in Fig. 11. It shows that the fragmentation is better than conventional blasting and is more uniform.
8. Impact of air-decking on fragmentation The influence of air-decking on fragmentation was evaluated in terms of mean fragment size (MFS), fragmentation index (FI ), and blast-
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Fig. 9. Fragmentation from air-deck blasts 1 through 6.
Fig. 10. Fragmentation from air-deck blasts 7 through 11.
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Fig. 12. Influence of RMR, PF and S/B on MFS in conventional blasting (MFS and PF are in metres and kilograms per cubic metre respectively).
Fig. 11. Muck pile image of an air-deck blast.
induced fragmentation. These three parameters were evaluated for conventional and air-deck blasts (Jhanwar, 1998). 8.1. MFS The MFSs of conventional and air-deck blasts were determined using Eq. (3). MFSs as obtained are shown in Table 5. It is interesting to note that the difference in the MFS between conventional and air-deck blasts is prominent in footwall rocks compared with the difference in hangwall rocks. This indicates that the air-deck technique is more effective in very low to low strength moderately jointed rocks than in medium strength highly jointed rocks. Regression analysis has indicated that the influence of Bieniawski’s RMR, PF and the ratio of spacing to burden significantly influences the MFS. Correlations between MFS and these parame-
Fig. 13. Influence of RMR, PF and S/B on MFS in air-deck blasting (MFS and PF are in metres and kilograms per cubic metre respectively).
ters were derived by regression analysis for conventional and air-deck blasting as shown here in Eq. (4) and Fig. 12, and Eq. (5) and Fig. 13 respectively. MFS=−0.05(RMR×PF×S/B)+1.54; (r=0.90)
(4)
MFS=−0.01(RMR×PF×S/B)+0.62; (r=0.87),
(5)
where MFS is in metres, PF is in kilograms per
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J.C. Jhanwar et al. / Engineering Geology 57 (2000) 13–29 Table 5 MFS from conventional and air-deck blasts Locationa
F/W, 44 m L, Ch. 1036 m– 1097 m F/W, 39 m L, Ch. 1036 m– 1188 m F/W, 48 m L, Ch. 914 m– 1097 m H/W, 10 m L, Ch. 884 m H/W, 3 m L, Ch. 914 m
Bieniawski’s RMR
Conventional blasts
Air-deck blasts
Blast no.
MFS (m)
Blast no.
42
3 4
0.95 0.80
1 2 3
0.51 0.38 0.27
40
1
0.78
4 6
0.48 0.46
25–35
2 5
1.0 0.68
5 7 8 9
0.23 0.49 0.32 0.42
50 65
6
0.36
10 11
0.32 0.27
MFS (m)
a Ch. is chainage and L is level.
cubic metre, S/B is the ratio of spacing and burden and r is the correlation coefficient. The MFS, as predicted from Eqs. (4) and (5) for different values of RMR, PF and S/B, in the cases of conventional and air-deck blasting is shown in Fig. 14. It can be seen in Fig. 14 that air-deck blasting produces a more uniform fragmentation compared with conventional blasting. Considering an optimum size of 0.2 m, air-deck blasts produced fragmentation that was not very far from the optimum
Fig. 14. Predicted MFS for conventional and air-deck blasting (MFS is measured in metres).
size, whereas the conventional blasts produced relatively uneven fragmentation in the form of oversized boulders and fines. In the case of air-deck blasting, MFS and PF have been correlated with Bieniawski’s RMR, airdeck length as a fraction of original charge length and the ratio of spacing and burden [Eq. (6) and Fig. 15]. It can be seen that MFS and PF together follow an asymptotic relationship with RMR, air-
Fig. 15. Influence of RMR, ADL and S/B on MFS and PF in air-deck blasting (MFS and PF are in metres and kilograms per cubic metre respectively).
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deck length and ratio of spacing to burden. MFS×PF=2.04(RMR×ADL×S/B)−1.04; (r=0.94),
(6)
where have already been defined and ADL is the air-deck length as a fraction of the original charge length. Air-deck blast numbers 4 and 8 and numbers 4 and 6 in Table 4 were excluded in the derivation of Eqs. (5) and (6) respectively to ensure good correlation. 8.2. The FI The FI, which defines blast-induced reduction in in situ block size, is estimated as the ratio of average in situ fragment size to average muck pile fragment size. The average in situ fragment size was estimated from the volumetric joint count. In the footwall rocks, average in situ fragment size was 1.0 m and in the hangwall rocks it was 0.5 m. The FI so calculated in respect of conventional and air-deck blasts is shown in Fig. 16. In the case of hangwall rocks, this is 1.38 in conventional blasting (blast 6 of conventional blasting, Fig. 16) and 1.56 and 1.58 in air-deck blasting (blasts 10 and 11, Fig. 16). Similarly, in the case of footwall rocks it varies between 1 and 1.47 in conventional blasting and 1.97 and 4.34 in air-deck blasting. From this, it is clear that the effect of airdecking in effecting further fragmentation is more pronounced in footwall rocks than in hangwall
Fig. 16. Blast-wise FI.
rocks. This indicates, therefore, that air-deck blasting could be more effective in very low to low strength moderately jointed rocks with medium size in situ blocks than in medium strength highly jointed rocks. In medium strength highly jointed rocks with smaller-sized in situ blocks, air-deck blasting does not help much in creating additional fragmentation. This could be due to the fact that in highly jointed rocks with small-sized in situ blocks the explosive energy is mainly utilised in loosening rather than in creating additional fragmentation, in contrast to moderately jointed rocks with medium-sized in situ blocks where the explosive energy is principally used to create new fractures. In moderately jointed rocks, therefore, the air-deck technique considerably improves the fragmentation. The critical FI for a given rock mass is principally governed by the bucket size of the mucking/loading equipment and the purpose of blasting. With larger bucket size a relatively low FI could serve the purpose, whereas if the bucket size is smaller a higher FI is required. This is because the optimum fragment size is governed by bucket size only. In this particular case, the bucket size was 1.7 m3 and, therefore, the optimum fragment size was 0.2 m. For this optimum fragment size, the FI was supposed to be 5.0 in footwall rock blasting and 2.5 in hangwall rock blasting. The in situ fragment sizes in footwall and hangwall were 1.0 m and 0.5 m respectively. The FI signifies blast-induced reduction in in situ fragments; it is, therefore, controlled by the rock mass, explosive charge and the blast design parameters. The rock mass has been identified by Bieniawski’s RMR, explosive charge by PF and blast design parameter by the ratio of spacing to burden. It is expected to improve with improvement in rock mass quality, increase in the PF and also with an increase in the ratio of spacing to burden. As the rock mass becomes more and more massive, the explosive energy utilisation in fragmentation becomes more and hence so does the FI. The increase in FI with an increase in spacing to burden ratio holds true up to a certain extent and beyond that limiting spacing to burden ratio, the FI may again decrease. The optimum ratio of spacing to burden for different rocks is different and varies between 1.2 and 1.5.
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FI=0.12(RMR×PF×S/B)−0.33;
(r=0.71), (8)
Fig. 17. Influence of RMR, PF and S/B on FI in conventional blasting (PF is measured in kilograms per cubic metre).
The FI so predicted for conventional and airdeck blasting from Eqs. (7) and (8) is depicted in Fig. 19. It can be seen that, for all ranges of Bieniawski’s RMR, PF and ratio of spacing to burden, the FI is significantly higher in the case of air-deck blasting than for conventional blasting. In the case of air-deck blasting the FI is also correlated with Bieniawski’s RMR, air-deck length as fraction of original charge length and the ratio of spacing to burden, as shown in Eq. (9) and Fig. 20. FI=−0.08(RMR×ADL×S/B)+3.40; (r=0.82), (9) It can be seen that the FI is linearly related to these parameters. To ensure better correlation, blasts 1, 4, 7, 8 and 9 and blasts 1, 3, 5 and 8 were excluded in the derivation of Eqs. (8) and (9) respectively. Therefore, these correlations need to be further refined. The above correlations [Eqs. (4)– (9)] were obtained for Bieniawski’s RMR=24–65, PF=0.27–0.89 kg/m3, S/B=0.9–1.75 and air-deck length as a fraction of original charge length in the ranges of 0.20–0.40.
Fig. 18. Influence of RMR, PF and S/B on FI in air-deck blasting (PF is measured in kilograms per cubic metre).
In this study it was also found that the FI increased with Bieniawski’s RMR, PF and the ratio of spacing to burden. To define the FI by all these parameters collectively, it was therefore correlated with these parameters taken together in the cases of conventional and air-deck blasting as shown in Eq. (7) and Fig. 17, and Eq. (8) and Fig. 18 respectively. FI=0.03(RMR×PF×S/B)+0.73;
(r=0.73) (7)
Fig. 19. Predicted FI for conventional and air-deck blasting.
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Fig. 21. Blast-induced fragmentation from conventional and air-deck blasting. Fig. 20. Influence of RMR, ADL and S/B on FI in air-deck blasting.
8.3. Blast-induced fragmentation Blast-induced fragmentation is hereby defined as the percentage muck volume made by fragments smaller than the in situ fragment size. This was estimated from Fig. 7 in the case of conventional blasts and from Figs. 9 and 10 in the case of airdeck blasts. Blast-induced fragmentations in conventional blasts 1, 2, 3, 4, 5 and 6 are 37%, 30%, 35%, 53%, 60% and 65% respectively. Similarly, the blastinduced fragmentations in the case of air-deck blast events 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 and 11 are 82%, 95%, 100%, 66%, 100%, 100%, 79%, 92%, 93%, 66% and 85% respectively. The blast-induced fragmentation for conventional and air-deck blasts is shown in Fig. 21. In the case of air-deck blasting, the blastinduced fragmentation is almost similar in footwall and in hangwall rocks. A comparison of blastinduced fragmentation for conventional and airdeck blasts reveals that air-deck blasting maximises the fragmentation.
tional blasting. In conventional blasting with solid material decking, shock front intensities become extremely low due to high resistance of solid materials and hence this results in poor breakage. (2) Air-deck blasting maximises the fragmentation as reflected by reduction in MFS, increase in FI and in blast-induced fragmentation. Fragmentation is influenced by the rock mass as characterised by its Bieniawski’s RMR, explosive factor as defined by PF, air-deck length as a fraction of original charge length (for air-deck blasting) and blast design parameters as defined by the ratio of spacing to burden. (3) The effectiveness of the air-deck technique in improving fragmentation is more pronounced in very low to low strength moderately jointed rocks than in medium strength highly jointed rocks with small-sized in situ blocks. In highly jointed rocks, since the explosive energy is principally utilised in loosening the rock mass rather than in causing additional fragmentation, the benefit of using air-decking is not that prominent. (4) The empirical correlations for estimation of MFS and FI are derived on the basis of a few blast results only and for a limited domain of rock type, and blast design parameters; therefore, scope exists for their further refinement.
9. Conclusions Acknowledgements (1) In jointed rocks, air-deck blasting improves the degree of fragmentation and produces more uniform fragmentation compared with conven-
The authors express their sincere thanks to the Director, Central Mining Research Institute,
J.C. Jhanwar et al. / Engineering Geology 57 (2000) 13–29
Dhanbad ( India). Thanks are also due to the mine management for providing facilities during field experiments and to Mr A.K. Chakraborty, Scientist, for his help during field trials. The views expressed in this paper are, however, of the authors and not necessarily of the institute to which they belong.
References Bieniawski, Z.T., 1973. Engineering classification of jointed rock masses. Transactions of South African Institute of Civil Engineers 15 (12), 335–344. Bussey, J., Borg, D.G., 1995. Presplitting with the new air-deck technique in: Proceedings of the Twentyfirst International Conference on Explosives and Blasting Techniques. Society for Explosives and Blasting Techniques, Tennesse, USA, pp. 197–213. Chiappetta, R.F., Memmele, M.E., 1987. Analytical high-speed photography to evaluate air-decks stemming retention and gas confinement in presplitting reclamation and gross motion studies in: Proceedings of the Second International Symposium on Rock Fragmentation by Blasting. Society for Experimental Mechanics, Bethel, CT, USA, pp. 257–301. Chakraborty, A.K., Murthy, V.M.S.R., Jhanwar, J.C., Jethwa, J.L., 1995. A feasibility study on air-deck blasting in: Proceedings of the Eighth National Convention of Mining Engineers. Institution of Engineers (India), Udaipur, India, pp. 68–74. Fourney, W.L., Barker, D.B., Holloway, D.C., 1981. Model
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studies of explosive well stimulation techniques. International Journal of Rock Mechanics and Mining Sciences and Geomechanics Abstracts 18, 113–127. Jhanwar, J.C., 1998. Investigations into air-deck blasting and its influence on blast performance and economics in openpit mines. M.E. Thesis, Nagpur University of Nagpur, India. Jhanwar, J.C., Chakraborty, A.K., Murthy, V.M.S.R., Jethwa, J.L., Dhar, B.B., 1996. Application of air-deck blasting technique to improve fragmentation and blast economics in the foot wall side of Dongri-Buzurg opencast mine, Unpublished Project Report, Number: MT/Collab/N/95, Central Mining Research Institute, Dhanbad, India. Kinney, G.F., Graham, K.J., 1985. Explosive Shocks in Air. Springer, New York. Marchenko, L.N., 1982. Raising the efficiency of a blast in rock crushing Fiziko-Tekhnicheskie Problemy Razrabotki Poleznykh Iskopaemykh (Soviet Mining Science) 18 (5), 1–6. Mead, D.J., Moxon, N.T., Danell, R.E., Richardson, S.B., 1993. The use of air-decks in production blasting in: Proceedings of the Fourth International Syposium on Rock Fragmentation by Blasting. A.A. Ballkema, Rotterdam, pp. 437–443. Melnikov, N.V., Marchenko, L.N., 1971. Effective methods of application of explosive energy in mining and construction. Twelfth Symposium on Dynamic Rock Mechanics. AIME, New York. Melnikov, N.V., Marchenko, L.N., Seinov, N.P., Zharikov, I.F., 1979. A method of enhanced rock blasting by blasting Fiziko-Tekhnicheskie Problemy Razrabotki Poleznykh Iskopaemykh (Journal of Mining Science) 6, 32–42. Moxon, N.T., Mead, D., Richardson, M., 1993. Air decked blasting techniques: some collaborative experiments. Transactions Institution of Mining and Metallurgy (Section A: Mining Industry 102, A25–A30. Rzhevsky, V.V., 1995. Opencast Mining Unit Operations. Mir, Moscow.