Scholarly article on topic 'Study of the Powder Factor in Surface Bench Blasting'

Study of the Powder Factor in Surface Bench Blasting Academic research paper on "Earth and related environmental sciences"

CC BY-NC-ND
0
0
Share paper
Keywords
{Sclerometer / "Rebound number" / "Powder Factor" / "Rock characterization" / "bench blasting."}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Fredj Mohamed, A. Hafsaoui, K. Talhi, K. Menacer

Abstract Study rock mass characterisation helps in selection and optimum usage of explosive in bench blasting. There are various methods to characterize the rock mass but use of Schmidt hammer in rock characterization before blasting may be a good option. The Schmidt hammer, due to its simplicity and capability of instant data production, has so far been a powerful tool utilized by many researchers to predict compressive strength of rocks. In this light, the present study was conducted in opencast phosphate mines to see the effect of Schmidt hammer rebound number or transformed compressive strength of rocks on the powder factor. The correlation was found sufficiently reliable to enable the determination of an optimum powder factor for surface bench blast in different rock types, maintaining the required blasting results regarding fragmentation.

Academic research paper on topic "Study of the Powder Factor in Surface Bench Blasting"

Available online at www.sciencedirect.com

ScienceDirect

Procedía

CrossMark

ELSEVIER

Earth and Planetary Science

Procedía Earth and Planetary Science 15 (2015) 892 - 899

World Multidisciplinary Earth Sciences Symposium, WMESS 2015

Study of the Powder Factor in Surface Bench Blasting

Fredj Mohameda*, A. Hafsaouia, K. Talhia, K. Menacera

University ofAnnaba, Mining Department Laboratory: Mineral Resources and planning, BP 12, Algeria Faculty of Earth Science.

Abstract

Study rock mass characterisation helps in selection and optimum usage of explosive in bench blasting. There are various methods to characterize the rock mass but use of Schmidt hammer in rock characterization before blasting may be a good option. The Schmidt hammer, due to its simplicity and capability of instant data production, has so far been a powerful tool utilized by many researchers to predict compressive strength of rocks. In this light, the present study was conducted in opencast phosphate mines to see the effect of Schmidt hammer rebound number or transformed compressive strength of rocks on the powder factor. The correlation was found sufficiently reliable to enable the determination of an optimum powder factor for surface bench blast in different rock types, maintaining the required blasting results regarding fragmentation.

©2015TheAuthors.PublishedbyElsevierB.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibilty of the Organizing Commitee of WMESS 2015.

Keywords: Sclerometer; Rebound number; Powder Factor; Rock characterization; bench blasting.

1. Introduction

Rock mass comprises several different rock types and is affected by different degrees of fracturing under varying stress condition. The strength of rock mass decreases with the increase in frequency of joints, bedding planes, fractures, pores and fissures and the deformability of rocks depend on their orientation (A.K. Sirveiya and N.R. Thote, 2013) Therefore, properties of rock mass are governed by the parameters of rock joints and rock material, as well as boundary conditions. Presence of discontinuities can affect the blasting result up to higher degree and play a very important role in achieving required blasting results with the charged explosive. The aim of rock blasting is to achieve the optimum fragmentation without generation any other blast induced nuisances. With the proper explosive quantity and powder factor and thus the generated energy, nuisances may be controlled. The optimum powder factor for the minimal overall mining cost for a specific blast condition may be defined as the powder factor required for

* Corresponding author. Tel.: +0-000-000-0000 ; fax: +0-000-000-0000 . E-mail address: author@institute.xxx

1878-5220 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibilty of the Organizing Commitee of WMESS 2015. doi:10.1016/j.proeps.2015.08.142

the optimum fragmentation, throw, ground vibration, etc. Presently, the powder factor is established through the trial blasts. However, the powder factor may be approximated using rock, blast design and explosive parameters. The powder factor is closely related with the efficient blasting (Jimeno C. L et al., 1995). With higher energy explosives, such as those containing large amounts of aluminum powder, a higher density charge can break more rock per unit weight than with lower energy explosives. Most of the commonly used explosive products have a similar energy values and similar rock breaking capabilities. Soft, low-density rock requires less explosive than hard, dense rock. Large whole patterns require less explosive per volume of rock Poor explosive distribution in larger diameter blast holes frequently results in coarser fragmentation. Massive rock with few existing planes of weakness requires a higher powder factor than a rock unit with numerous, closely spaced joints or fractures. The more free faces a blast has to break to, the lower the powder factor requirement. To determine the powder factor several approaches have been made by different researchers. These approaches consider those rock mass properties, which are the most significant parameters in a rock—explosive interaction. A review of the same has been aimed in this paper and establishes a relationship between powder factor and uniaxial compressive strength (UCS) of the rock which was obtained through rebound number.

1.1 Powder Factor

The quantity of explosive required to fragment 1 m3 or 1 tonne of rock is known as powder factor (Jimeno C. L et al., 1995). It can serve a variety of purposes, such as an indicator of hardness of the rock, or the cost of the explosives needed, or even as a guide to planning a shot. Several possible combinations can express the powder factor. Ashby (1981) developed an empirical relationship to describe the powder factor required for adequate blast based on the fracture frequency representing the density of fracturing and effective friction angle representing the strength of structured rock mass (Ashby John P, 1981). According to Ashby the powder factor of rock with ANFO may be determined either from the graph (Fig. 1) drawn for the purpose or from the following equation:

powder Factor = 0.56 *p * + i) / fracture/meter . Kg/cu.m (1)

where,

^ = Basic friction angle, p = in-situ density ofrock formation, i = roughness inclination angle,

= friction angle, fracture/meter represents the fracture frequency.

POWDER FACTOR

Fig.l. Empirical relation between powder factor, fracture, frequency and joint shear strength (Ashby John P, 1981).

Table 1 : Classification of the uniaxial compressive strength of rocks (Dyno Nobel,2010 and Schmidt, E. 1951).

Rock Type_UCS(MPa)_P.F.(kg/m3)

Very Low Strength 1 - 5 0.15 -0.25

Low Strength 5-25 0.25-0.35

Medium Strength 25 -30 0.4-0.5

High Strength 50- 100 0.7-0.8

Very high strength 100 - 250

Extremely high strength >250

1.2 Schmidt Hammer

The Schmidt hammer rebound hardness test is a simple and non-destructive test originally developed in 1948 for a quick measurement of UCS (Cargill, J.S. and Shakoor A, 1990) and later was extended to estimate the hardness and strength of rock (Torabi, S. R, 2005 et Poole, RW, and Farmer, I W, 1980). The mechanism of operation is simple: a hammer released by a spring, indirectly impacts against the rock surface through a plunger and the rebound distance of the hammer is then read directly from the numerical scale or electronic display ranging from 10 to 100, Figure 2. In other words, the rebound distance of the hammer mass that strikes the rock through the plunger and under the force of a spring, indicates the rebound hardness. Obviously, the harder the surface, the higher the rebounds distance. Its rebound is dependent on the hardness of the rock and is measured by the test equipment suggested by ASTM C805-08. By reference to the conversion chart, the rebound value can be used to determine the compressive strength.

This test is quick, cheap and non-destructive. In rock engineering, it is widely used for its simplicity, portability and the capability of instant data production. Today varieties of Schmidt hammers are available for use, such as the models of L-type and N-type. ASTM D5873 describes the procedure for testing of rock. Presently, Schmidt hammer can be used to predict the uniaxial compressive strength of rocks, the performances of tunnel boring machines (TBM), advance speed of drilling machines as well as the evaluation of discontinuities in rock formations. The following three of widely accepted test procedures with different Schmidt hammer rebound techniques were selected and applied on rock samples

Test Procedure: 1 - Poole and Farmer (Hucka VA, 1965) suggested that the peak value from at least five continuous impacts at a point should be selected.

Test Procedure: 2 - Hucka (ISRM, 1981) recommended that the peak value from at least ten continuous impact at a point should be selected.

Test procedure: 3 - ISRM (Dyno Nobel, 2010) suggested that twenty rebound values from single impacts separated by at least a plunger diameter should be recorded and the upper ten values averaged. Each testing method was repeated at least three times on any rock type and the average value was recorded as the rebound number.

The surface texture significantly affects the rebound (R) number obtained. Tests performed on a rough-textured finish will typically result in crushing of the surface paste, resulting in a lower number. Alternately, tests performed on the same concrete that has a hard, smooth texture will typically result in a higher R-number. Therefore, it is recommended that test areas with a rough surface be ground to a uniform smoothness. This can be achieved easily with a Carborundum stone or similar abrasive stone. The device itself should be serviced and verified annually or whenever there is a reason to doubt proper performance. Verification of proper performance of the device includes the use of a test anvil. The required dimensions and steel hardness is listed in ASTM C805. By impacting the proper test anvil with a properly functioning device will typically result in rebound numbers of 80 ± 2.1 the device is believed to not be functioning properly, it is recommended to send it back to the manufacturer or experienced facility for repairs and re-verification.

2. Objective

The objective of this study was to investigate the influence of rock mass strength on explosive requirement for surface bench blasting in different rock types, maintaining the required blasting results in terms of fragmentation

3. Field description and research methodology

To meet the stated objective, a field study was conducted onlO working rock faces (benches) of an opencast phosphate mine of SOMIPHOS, a subsidiary of phosphate Algeria. The study benches were 10m and 15m high which were subsequently excavated by 5m3 rope shovels in conjunction with 35 &50 tonne rear dump trucks. Rock strata are highly fractured. Rock uniaxial compressive strength varies from 24 MPa to 73 MPa. The general strike of formation and associated phosphate seam is NW-SE and the dip varied from 4 to 5 degree towards South West. Mine's stripping ratio was approximately equal to 3. The blast holes sizes was 160 mm and were blasted by the emulsion explosives.

Fig.2. Study mine showing shovel benches.

The tests included the application of an NR-type Schmidt hammer to assess the hardness of the rock in as many points as practicable in the phosphate production areas. At each point about 20 cm by 20 cm surface of the rock was prepared by cleaning the area and performing about 50 impact in grid pattern on each bench before blasting, Figure 5. Among the numbers obtained, the mean value was considered as the Schmidt number for that bench. This procedure of performing Schmidt test was a compromise to the ISRM (1981) suggested method (Brown, E.T, 1981) where ten higher numbers are selected from twenty tests in the selected area. It is argued that the ISRM (1981) suggested method suffers from some shortcomings due to very selective nature of the procedure (Goktan, R.M. and Ayday, C, 1993). The reasoning behind this is the fact that eliminating a great number of the low numbers inevitably results in erroneous outcomes as low numbers might be the reaction of inherently weak portion of the rock and not merely the effect of test deficiencies.

Fig.3. Blast whole section and firing sequence.

Fig. 4. Rock testing with Schmidt hammer at field scale.

Table 2: Rebound values and corresponding uniaxial compressive strength (UCS) obtained from conversion chart.

Blast No. Rebound Values Angle of impact (Deg.) Average Rebound Values UCS (MPa)

B-l Varies between 33-46 90 39 32

B-2 Varies between 34-52 90 43 40

B-3 Varies between 34-47 90 40 34

B-4 Varies between 33-53 90 43 40

B-5 Varies between 33-51 90 42 38

B-6 Varies between 36-58 0 46 52

B-7 Varies between 36-61 0 48 56

B-8 Varies between 36-68 0 51 62

B-9 Varies between 36-60 0 48 56

B-10 Varies between 38-61 0 49 58

4. Results and discussions

Table 3: Blast design parameters at overburden benches for blast B1 to B5.

Parameters B-l B-2 B-3 B-4 B-5

Burden (m) 4 4.5 4 4.5 4

Spacing (m) 4.5 5 4.5 5 4

Hole depth (m) 12.5 14 12.5 14.5 15

Sub-grade (m) 1.25 1.4 1.25 1.45 0.5

No. of holes 7 12 8 14 25

Bench height (m) 11.25 12.6 11.25 14.05 14.5

Length of bench (m) 18 30 18 35 20

Width of bench (m) 8 9 8 9 17.5

Types of explosive Anfomile+ Anfomile+ Anfomile+ Anfomile+ Anfomile+

Marmanitelll Marmanitelll Marmanitelll Marmanitelll Marmanitelll

Explosive Quantity (kg) 1085 2200 1280 3120 4675

Anfomile/Marmanitelll (Kg) 8.4 3.6 9.6 4.2 7.5

Rebound Number(N) 39 43 40 43 42

UCS(MPa) of rock mass (from rebound value) 32 40 34 40 38

P.F(kg/m3) 0.77 0.65 0.80 0.91 0.91

Excavated volume of rock (m3) 1418 3402 1620 3443 5150

Fragmentation assessment by visual inspection Good Good Good Good Good

Excavator (Shovel) cycle time (s) ^ 25 26 25 26

The study was conducted for the 10 blasts. To calculate the uniaxial compressive strength (UCS) of the rock, 50-impact values in grid pattern on each bench were recorded, Figure 5. In this mine all blasts were drilled on square drilling pattern and fired on row to row firing pattern with inter-row and inter whole delays. A representative drilling and firing pattern is given in figure 4. The blast holes were bottom initiated with shock tube initiation system. These data are given in Table 1 and 2.

Table 4: Blast design parameters at overburden benches for blast B6 to BIO.

Parameters B-6 B-7 B-8 B-9 B-10

Burden(m) 4 4.5 4 3.5 3.5

Spacing(m) 4.2 5 4.5 4 4

Hole depth(m) 12.5 14.5 14.5 14 15

Sub-grade (m) 1.25 1.45 1.45 1.4 0

No. of holes 27 16 9 14 30

Bench height (m) 11.25 13.05 13.05 12.6 15

Length of bench (m) 37.8 40 13.5 28 24

Width of bench(m) 12 9 12 7 17.5

Anfomile+ Anfomile+ Anfomile+ Anfomile+ Anfomile+

Types of explosive Marmanitelll Marmanitelll Marmanitelll Marmanitelll Marmanitelll

Explosive Quantity (kg) 4260 3420 1935 2660 5610

Anfomile/Marmanitelll (Kg) 37.6 4.8 2.7 19.6 9

Rebound Number (N) 46 48 51 48 49

UCS(MPa) of rock mass 52 56 62 56 58

P.F(kg/m3) 0.84 0.73 0.92 1.08 0.89

Excavated volume ofrock(m3) 5103 4698 2114 2470 5619

Fragmentation assessment by visual inspection good good good good good

Excavator (Shovel) cycle time 27 25 26 25 26

(sec) 27 25 26 25 26

4.1 Relationship between Rebound Number (N) of Rock and Powder Factor (PF)

Rebound number (N) versus (vs) powder factor (PF) relationship for analyzed blasts is deduced from tables 3 and 4. The results are plotted graphically and are shown in figure 5.

Rebound Number vs Uniaxial compressive strength (UCS)

1,08 -0,96 -0,84 -—. 0,720,60 -& 0,48- PH

--- r r r —I—I—I— « r III 1 1 1 1 __1__1__1. J 1 1 1 1 1 1 1 __1__1__1. J 1 1 1 III -LJ-JJ III __i__i__LIi iiil ill —L-LJ_J II __1__1__1__1 r r r —(-—(-—)— !-¡-=* -1—1-4-4 1 1 1 1 1 1 1 1 -U-l-4-4 1 1 1 1 II A

* ! i 1 1 ---1---1---■---■---1--- ■ ■ ---1---1---4--- L 1 1 1 1 ---1---

-I--1--- — 1— 1— ■— —1—1—I— l_ l_ l_ l_ III A T 1—LJ III 1 1 1 1 __1__1__1__1__1 I I I r I- I- I- I- I---1---1---1------1--- _ 1_ 1_ 1_ 1_ 1_ 1 1 1 1 1 1 1 1 —)—1—1--1—1 1 1 1 1 1 1 1 1 —I—1—1--1—1 1 1 1 1 1 1 1 1 —I—1—1--1—1 1 1 1 1 1 1 1 1 . _l__1__1__1 —1—1—1—1—1 III 1 1 1 1 —1—U4-4-J III 1 1 1 1 __L_l__1__l_J

0,36 - 1 1 1 1 1 ---4---11 1

0,24 - r- !-

0,12 - 1 1 __1__1__ 1 1 —i—i—:—i—i— 1 1 1 1 1—1—1— 1 1 1 1 1

3 9,0 40,5 42 ,0 4 3 I ,5 45,0 46,5 4 8 ebound Number (N) ,0 49,5 5 1 ,0 5 2,5

Fig.5. Rebound number (N) of rock vs Powder factor.

It is evident from the figure 5 that the powder factor is increases as the Schmidt hammer rebound number is increases. Increase in rebound means rock mass is compact which requires more chemical energy to break.

Therefore, more explosives were charged in the holes to get the required blasting results in terms of improved fragmentation.

4.2 Relationship between Uniaxial Compressive Strength (UCS) of Rock and Powder Factor (PF)

Uniaxial compressive strength vs PF relationship for analyzed blasts is deduced from tables 3 and 4. The results

are plotted graphically and are shown in figure 6.

Uniaxial Compressive Strength (US) vs Powder Factor (PF)

~i il il ii ill il il il il i

0,0 0 -1-.-1-1-1-1-1-1-1-1-rJ

28 32 36 40 44 48 52 56 60 64

UCS (MPa)

Fig. 6. Uniaxial compressive strength (UCS) of rock vs Powder factor (PF).

It is evident from the figure 6 that the powder factor is increases as the uniaxial compressive strength of rock is increases. Increase in UCS means rock mass is strong which requires more explosive energy to break. Therefore, more explosives were charged in the holes to get the required blasting results in terms of improved fragmentation. It was also observed that the cycle times of the excavators were remain almost constant due to the uniformity in obtained fragment sizes in each blast.

Conclusions

Blasting is still the cheapest means of breaking rock. The suitable results of blasting can only be obtained when the rock is properly understood. It is well known that rock nature changes from bench to bench and mines to mines. Therefore, rock mass characterization for each bench is essential to optimize the blasting results. Rock mass characterisation can also help in selection and optimum usage of explosive to improve the overall economy of the project. The use of Schmidt hammer in rock characterizing before blasting may be a good option. In this study it was observed that to maintain the rock fragmentation for best utilization of excavators the quantity of explosive for breaking the same volume of rock or powder factor is increases as the rebound value or uniaxial compressive strength is increases.

References

1. A.K. Sirveiya, N.R. Thote (2013). Assessing the effect of rock mass properties on rock fragmentation", Measurement and Analysis of Blast Fragmentation - Sanchidrian & Singh (Eds) Taylor & Francis Group, London, ISBN 978-0-415-62140-3, pp: 139-144.

2. Jimeno C. L., Jimeno E, & Carcedo (1995). Drilling and Blasting of Rocks, A.A. Balkema Publishers.

3. Ashby John P (1981). Production blasting and the development of open pit slopes, Proceedings of the 3rd. Int.Conf. on Stability in Open Conference on Stability in Open Pit Mining, Vancover, B.C.

4. Dyno Nobel (2010); Blasting and Explosives Quick Reference Guide-2010, REF0110/0210/AZZAUS/2K

5. Schmidt, E. (1951). A no destructive concrete tester. Concrete, 59(8), pp: 34-5.

6. Cargill, J.S. and Shakoor A. (1990). Evaluation of empirical methods for measuring the uniaxial strength of rock. Int. J. Rock Mech. Min. Sci., 27, pp: 495-503.

7. Torabi, S. R. (2005). Reliability of the application of Schmidt hammer in determination of the UCS, Final project report, in Persian, Shahrood University of Technology, Shahrood, Iran.

8. Poole, RW, and Farmer, IW. (1980). Consistency and repeatability of Schmidt hammer rebound data during field testing (Technical Note) Int. Journal Rock Mech Mm Sei Geomech Abstr 17, pp: 167-171

9. Hucka VA, (1965). A rapid method determining the strength of rocks m-situ Int Journal Rock Mech Mm Sei Geomech Abstr 2, pp: 127-134

10. ISRM (1981). Rock characterization testing and monitoring ISRM suggested methods, suggested methods for determining hardness and abrasiveness ofrocks, Part 3,101-3.

11. Torabi, S. R., Ataei, M., and Javanshir, M. (2010). Application of Schmidt rebound number for estimating rock strength under specific geological conditions. Journal of Mining & Environment,, Vol.1, No.2, 2010,

12. pp:l-8.

13. Brown,E.T.(1981).ISRMsuggestedmethods-rock characterization, testing and monitoring, PergamonPress,Oxford.

14. Goktan, R.M. and Ayday, C. (1993). A suggested improvement to the schmidt rebound hardness ISRM suggested method with particular referenceto rockmachineability", Technical Note, Int. J. Rock Mech.Min.Sci.&Geomech.Abstr.,30(3),pp:321-322.