New burn cut blast design in drives enhances drilling blasting efficiency with economy - a case...

NEW BURN CUT BLAST DESIGN IN DRIVES ENHANCES DRILLING &

BLASTING EFFICIENCY WITH ECONOMY - A CASE STUDY

CHANGING FROM ‘DECKED-BURN’ PATTERN TO ‘NON-DECKED’ BURN PATTERN FOR

DEVELOPMENT BLASTING OF DRIVAGE, DECLINES AND RAMPS IN AN

UNDERGROUND METAL MINES

Author: PARTHA DAS SHARMA, (B.Tech-Hons.) in Mining Engineering

(E.mail - [email protected])

Weblog: http://miningandblasting.wordpress.com/

Abstract: A new Burn-Cut blast pattern has been designed for drives, declines and ramps in underground metal mines, to replace a design (of Decked-Burn with more number of holes), which was giving number of blast failures, such as ‘Under_Blast’ - difficult to handle. The new Burn-cut design contains less number of blast-holes and Reamer than earlier Decked-Burn-cut. Decked system has been removed to make the charging operation easier. This enables to increase explosives energy in a hole and to reduce stemming length in order to eliminate above blast failures. Moreover, requirement of Detonators is reduced, as Decked system has been abolished. Total explosives quantity has been reduced marginally. Thus, drilling efficiency and cost effectiveness has been achieved. Entire process has been done by changing the original pattern / system in three phases.

A. Introduction – Excavations of drifts and drives are common features in any metal mining. The specific constrain in driving drivage is, unlike opencast bench blasting, absence of initial free faces. Therefore solid blasting is carried out, for which, Blast Design is most important factor, in order to, create free-faces for successive rows and column of holes.

The important factors on which generally progress of drifts and drives depends are:

1. Geology of strata and Rock mass condition 2. Appropriate blast design including drilling pattern, quantity and type of explosive, initiation

sequence 3. Types of drilling equipment used and length of drilling rod used 4. Dimension of Drives 5. Properties and VOD of Explosives used The cost and time benefit of the excavation are mostly decided by the ‘Rate of Advance’. Therefore, it is utmost important to have proper blast design with quality explosives used, in order to achieve maximum rate of advance (Pull) per blast. ‘Burn’ cut (Parallel Holes with Reamers) blast design is suitable for any dimension of Drift excavation and with proper Explosives, initiation sequences etc., it can give considerable amount of Rate of Advance.

The said underground metal mines working for excavation of hard Dolomite based rock; only development work is being carried out. Excavation for Development work such as Declines, Ore Drives

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NEW BURN CUT BLAST DESIGN IN DRIVES ENHANCES DRILLING & BLASTING EFFICIENCY WITH ECONOMY - A CASE STUDY

2 Author: Partha Das Sharma, B.Tech(Hons.) in Mining Engg. (E.mail: [email protected]) Weblog: http://miningandblasting.wordpress.com/

at various levels, Ramps connecting Ore Drives etc., is being done. In underground mines development headings provide; mine access for men & materials, ore & waste transportation, and ventilation paths.

Mechanism of creating additional free-faces for Drift blasting - In Drift blasting, explosives are required to perform in a difficult condition, as single free face (in the form of drift face) is available in contrast to bench blasting where at least two free faces exist. Hence, more drilling and explosives are required per unit volume of rock to be fragmented in the case of Drift blasting. A second free face, called ‘cut’, is created initially during the blasting process and the efficiency of the blast performance largely depends on the proper development of the cut. The Drift blasting mechanics can be conceptualized in two stages. Initially, a few holes called cut holes are blasted to develop a free face or void or cut along the tunnel axis. This represents a solid blasting condition where no initial free face is available. Once the cut is created, the remaining holes are blasted towards the cut. This stage of blasting is similar to bench blasting but with larger confinement. The results of Drift blasting depend primarily on the efficiency of the cut-hole blasting. The specific constrain in driving drives is, unlike opencast bench blasting, absence of initial free faces. Therefore solid blasting is carried out, for which, Blast Design is most important factor in order to create free-faces for successive rows and column of holes. The factors influencing the development of the cut and the overall blast results are dependent on a host of factors involving rock mass type, blast pattern, Explosives used and the Drift configurations.

B. Development work at the Mines: Followings are the major excavation for development work being

done at the said underground mines:

* Declines with Dimension – 5m x 3m, * Ore Drives for all the levels (Dimension -4.5m x 3m). * For connecting the Ore Drives, ‘Ramps’ are driven (with Dimension – 4.5m x 3m) at an interval of 100 to 150m. C. Drilling Equipment: As far as Drilling Equipment for Drifting is concerned, single-boomed Electro-hydraulic jumbo drills are used for Drifting. Parallel holes and Reamer holes can be drilled very easily and rate of penetration is also quite faster. D. Blast Design and Drilling & Blasting for excavation of various development activities at the said UG mines: For face blasting 40mm dia Cartridged Emulsion Explosives (each cartridge length 300mm, contain 390gm explosives) with VOD of about 4000m/s, are used for 45mm dia holes. Drilling length kept mostly 3.4m (or sometime 4.0m). Reamers of Dia 89mm are used. Long Delay Electric Detonators are used for initiation. Face blasting for Declines, Ramps, Ore-Drives and cross-cuts ‘Decked-Burn’ technique was used, using Long Delay Electric Detonators (as given in Original pattern, Fig. 1). The salient features of the ‘Decked-Burn’ system were: * The collar portion of the hole was blasted prior to the bottom. * Mid-column decking between the two charges in a hole was kept about 0.5m.





Fig. – 1

Original pattern - With above pattern (shown in Fig-1), for 4.5m x 3m face size number of drilling hole (with 3.4m) and Explosives requirements are given below:

Faces (with Dimension)

Number of Hole drilled

Number Holes charged

Explosives used in a round (Kg)

Detonator used (No.)

Stemming Length kept (m)

4.5m X 3m face (with 3.4m Drilling Length)

* 46 with 45mm Dia * 4 with Reamer 89mm dia (46+4)

42 120 50 (In burn holes two detonators were used)

1.3 to 1.6m

Fig - 2





E. Discussion on Decked-Burn Pattern (as shown in Fig – 1):

* As number of Holes were more, smaller quantity of explosives used per hole (6 to 8cartridges per

hole), resulting large stemming length (1.2 to 1.6m); causing a peculiar kind of failure – ‘UNDER-BLAST’

(as shown in Fig – 3), in which only inside is blasted and fragmented, whereas outside (Collar) rock

appears solid and intact.

* Difficult to handle the blasted rock with LHD loaders when there is ‘under_blast’.

* Difficult to deal with post blast sockets generated, sockets are hollow at the end (Fig – 3).

* To deal with above kind of failure re-drilling and re-blasting was common; affecting cycle time, cost

etc.

Fig-3

Fig – 4





F. Searching for solutions:

* It was thought to modify the Decked-Burn Pattern by reducing number of Holes, in order to put more

quantity of Explosives (7 to 10 cartridges) in individual holes to reduce stemming length (0.6 to 0.8m), to

overcome above type of ‘Under-Blast’ failures.

* Also simplification of Explosive Charging procedure was thought of by doing away with the ‘Decked

Charging’ procedure.

* For this, New Design of ‘Rectangular Burn’ (with four Reamers) has been put forth, wherein ‘Deck

Charging’ is eliminated.

* Later, ‘Four-Reamers’ have been reduced to ‘Three-Reamer’ Burn.

G. Solving the problem - Three phased initiative for Re-designing the Drilling Pattern:

* In the first phase, one vertical line was reduced in order to reduce number of Holes in the Pattern and

the same ‘Decked-Burn’ was kept intact.

* By implementation of one reduced line, 5 Holes were reduced in the pattern, accommodating more

number of cartridges in a hole in order to reduce stemming length within 1m.

* In the second phase, full ‘Burn’ was re-designed (Non-Decked) and incorporated in the above reduced-

hole pattern.

* In this second phase another 4 number of Holes were reduced and also ‘Decked-Charging’ was

removed – providing simplification of Explosives charging process and further reducing stemming length

to about 0.6 to 0.8m.

* In the third phase one reamer was reduced. Instead of four, three reamers were used, without

affecting quality of blasts and pull.

Fig - 5





Fig – 6

H. ‘DECKED-BURN’: Shortcomings (as experienced)

* For UG Drives Charging of Explosives is difficult and time-consuming. * Interchanging Delays between inside and outside column of explosives takes place and because of that blast failure occurs.

Idea of re-designing and to replace with ‘Non-Decked’ Burn came up in order to simplify the procedure of charging of explosives and to prevent blast failure due to inadvertent interchange of Delay timings between inside and outside explosives column.

I. Phase one - with above pattern (shown in Fig-5), for 4.5m x 3m face size number of drilling hole (with 3.4m) and Explosives requirements are given below:









37 118.5 45 (In burn holes two detonators were used)

0.8 to 1m





Fig – 7

J. Phase two - with above pattern (shown in Fig-7), for 4.5m x 3m face size number of drilling hole (with 3.4m) and Explosives requirements are given below:










0.6 to 0.8m





Fig – 8

K. Discussion on Trials and Performance of Blasts:

* Extensive trial-blast conducted at all the dimensions of drives, i.e., 4.5mx3m, 5mx3m and 5mx4m with

3.4m and 4.0m length of drilling.

* No failure as of ‘UNDER-BLAST’ observed.

* No re-blasting was also carried out.

* The performance of blasts (Average Pull obtained) is either same or better than the earlier pattern.

* It is observed that, with 4m length of drilling, at Ramp-Up pull is better than Ramp-Down.

* Side and Top corner sockets observed when there is deviation of Hole. Chances of Hole-deviation with 4m length are more than with 3.4m length.

L. Introduction of Third Phase: Reducing one Reamer hole (i.e., using only three reamers with 89mm Dia.): * Later, reduction of one Reamer hole was thought, keeping quantity of explosives and detonator timing

same.

* Number of trials was taken-up with this three reamer system (89mm Dia.) at both 4.5x3m and 5x3m

faces.

* All the trial blasts were successful; with similar performance as of four reamers.

* Drilling cost and time has been reduced further.





Fig -9

Phase three - with above pattern (shown in Fig-9), for 4.5m x 3m face size number of drilling hole (with 3.4m) and Explosives requirements are given below:










0.6 to 0.8m

M. Discussion on Advantages of New Pattern:

* With new Blast design, blast failure of ‘Under-blast’ kind and re-blasting is eliminated completely.

* As this new blast design does not use ‘Decked charge’ at the ‘Burn’, the number of Detonators

used are considerably less and also gives ease in charging (Charging time is reduced).

* As the number of drilled holes is considerably less, the Drilling (Percussion) Time is reduced.

* Thus, Cycle time of Drilling and Blasting is reduced considerably – Working Efficiency is enhanced.

* As drilling efficiency is enhanced (Number of Drilling is reduced) drilling cost also reduced.

* This is evident from the fact that, now two full faces are drilled and blasted in one shift, thereby

enhancing efficiency.





* Number of Detonators reduced considerably per blast and also quantity of Explosives (Thus, powder

factor, Detonator factor have been improved).

* Now this three reamer system has been continued in both 4.5m x 3m and 5m x 3m faces, which

further reduced cost and enhanced drilling and blasting efficiency.

N. Summary and conclusions:

Below tables shows how numbers of holes have been reduced; thus, enhancement of drilling and

blasting efficiency and working cost:

For face dimension 4.5m X 3m (with 3.4m Drilling Length):

Design Changes

Number of Hole drilled (45mm + 89mm dia)




Reduction in Drilling (no. of holes) from previous phase

Reduction in Explosives and Detonators quantity from previous phase

Expl Det

Original (46+4) 42 120 50 Decked- Burn --- ---

Phase - I (41+4) 37 118.5 45 Decked-Burn, reduced no. of holes by 5 (45mm)

1.5Kg 5 no.

Phase - II (37+4) 33 117.5 33 Non-Decked Burn, further reduction in no. of holes by 4 (45mm)

1.0 kg 12 no.

Phase - III (36+3) 33 117.5 33 Non-Decked Burn, further reduction in no. of holes by 2 (One 45mm and one 89mm dia. Reaming hole)

--- ---

Final reductions from Original pattern 11 no. of holes (10 of 45mm and 1 of 89mm dia)

2.5 kg 17 no.

Note: Average Time taken for drilling a hole of 3.4m length by Jumbo drill is 2.5min. Overall 20 to 22% of drilling time and cost have been saved. Similarly, Charging time has also reduced as ‘Decked-Burn’ system eliminated.

As number of drilling holes have been reduced considerably, the percussion time and drilling meterage

has been saved. Cost of drill Bits also saved considerably. Moreover, total 2.5kg of explosives and 17

numbers of Electric Delay Detonators have been saved – thus, reducing costs. Similar advantages

obtained for 5m x 3m faces with 3.4m drilling and also with 4m drilling at 4.5m x 3m faces.

Upon obtaining satisfactory performance result, enhancement of efficienciy and eliminating blast

failure such as Under_Blast etc.; the new blast pattern has been fully incorporated for all the

development section of the said underground mine.

Disclaimer: Views expressed in the article are solely of the author’s own and do not necessarily belong to any of the Company

or institution.



New burn cut blast design in drives enhances drilling blasting efficiency with economy - a case...

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