Pergamon Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. Vol. 31, No. I, pp. 79-84, 1994

Elsevier Science Ltd. Printed in Great Britain 0148-9062/94 $6.00 + 0.00

Technical Note

Controlled Blasting in Jointed Rocks R. BADALt

1. INTRODUCTION

An enormous amount of energy is liberated in a fraction of a second during detonation of an explosive, but the understanding of how the energy stored in the explosive is liberated and imparted into the rock mass for perform- ing the fragmentation task in many practical civil and mining engineering application is still unresolved due to the variable behaviour of the rock masses. With this knowledge, rapid excavation could be achieved more safely and economically. The stability of the rock mass for various projects could be designed and evaluated with more confidence and efficiency if rock mass break- age by explosives could be understood completely.

The term "controlled blasting" is somewhat synony- mous with stability of the rock mass at the final limit of excavations. For performing the task, numerous parame- ters are involved in achieving a controlled fracture plane on the peripheral surface of an excavation. In this group of variables, many are controllable parameters and a few are uncontrollable parameters. Much effort has been made to understand the functioning of the various con- trollable parameters to derive an explosive-induced frac- ture plane on the perimeter surface of excavations [1-5].

Systematic effort was made by a group of scientists at the University of Maryland for development of a con- trolled fracture plane by using the notched borehole tech- nique to direct the fracture plane, which allows up to a 38% larger hole spacing than in normal practice [5]. With a similar concept, Mohanty [6] recommended a satellite hole on either side of central charged hole, providing an equivalent notched hole effect. A smaller burden and closer hole spacing with small diameter holes was sug- gested to improve the perimeter blasting results [7]. A mini- mum burden was recommended for improved results [8].

The understanding of the controlled fracture plane in various rock mass conditions remains unsolved; how- ever, rock blasting engineers have always performed the task in the face of variable rock mass characteristics. The properties of a rock mass which contains various joints have a greater influence on blasting results than the blast geometry and explosive properties [9-11]. Sometimes, the joint planes add to the performance of controlled

tRock Mechanics Division, CSMRS, Olaf Palme Marg, Haus Khas, New Delhi 110016, India.

blasting techniques for the desired output; whereas, for some other situations, performance is reduced signifi- cantly. All depends on the nature of joints, such as their location, their properties and orientation. Worsey [12] demonstrated from his findings that the orientation of joints parallel to the perimeter boundary produced smooth and stable face conditions, while joints across the perimeter boundary produced an irregular breakage in between the holes. Bhandari and Badal [13, 14] and Badal [15] have also presented some results useful to this study relating to the post blast results and fracture development in jointed rocks. In order to explain the explosive-induced fracture development demonstrated by Badal [15] for all joint orientations used in this study, reassembly of broken fragments on the limestone model scale study indicated that each orientation is governed by its own fracture pattern. Even then, several improve- ments are possible for guidance on various joint orien- tations occurring in natural rock masses.

2. BLASTING MECHANISM

The energy liberated by an explosive on detonation is in a fraction of a second, but it is not very clear how this energy is imparted to the rock mass for performing the fragmentation task. Several useful contributions are available to explain the blasting mechanism, either based on model studies, field observations, or trial and error approaches. The studies have contributed towards the understanding and are suitable for explanation of the mechanism for a specific test environment or conditions, but may not explain fragmentation for others. However, in this study, the available information on blasting mechanisms can be broadly classified into two ideas of relative importance:

(a) the role of the stress waves imparted to the rock by the rapid release of energy during the detona- tion of an explosive in the blasthole; and

(b) the role of gasses released by detonation which create a quasistatic field around the blasthole extending radial cracks and moving the frag- mented rock mass.

Barker and Fourney [16] and Fourney et al. [17] demonstrated the role of transient stress waves in the development of fracture networks in models, and

79

S0 BADAL: TECHNI('AL NOI[

explained that the fracture initiated at the point where the leading front of the transient stress waves came into contact with the reflected transient stress waves. Badal and Bhandari [18] recognized that the transient stress waves are mainly responsible for fracture and fragmen- tation in a jointed rock mass and explained the develop- ment of fracture networks resulting in fragmentation.

3. EXPERIMENTAL STUDIES

The experimental procedure has been discussed by Badal [19] and Bhandari and Badal [14, 20]. The nomen- clature used for the joint orientations has been adopted uniformly in this text. Nine different orientations were used.

The nomenclature is based on the most dominating joint planes direction in relation to the blasting direction, as given below:

(a) horizontal joints (0) ; (b) parallel vertical joints (90°); (c) perpendicular vertical joints (90°); (d) perpendicular 60 ° down dip joints; (e) perpendicular 60 ~' up dip joints; (f) parallel 60 ° dip joints; (g) perpendicular 30 ° down dip joints; (h) perpendicular 30 ° up dip joints; and (i) parallel 30 ° dip joints.

The geomechanical properties of the limestone slabs used for this study are as given below:

Compressive strength Tensile strength Young's modulus P-wave velocity Density

97.10 M N / m 2 5.90 M N / m 2

6551.40 MN/m: 4091.00 m/sec 2670 kg/m 3

Small-scale experiments on blocks of limestone (600 X 300 x 300 mm) were carried out using long cylin- drical explosive charges on bench shaped models. These blocks were prepared using limestone slabs (20-30 mm thick) cut to the desired size in a bench shape and then held together with Plaster of Paris as a cement medium. These experiments were carried out with a vertical hole of 6.2 mm dia. A 6 g/m detonating cord was used as an explosive charge (Fig. 1). A large canvas collecting chamber was used which enabled the collection of broken fragments and a study of post-blast remaining rock conditions on the blasted blocks.

Fig. I . Test block o[" limestone with vcrtical paralte[ .loinls containing two vertical holes with detonating card in Ihc holc:'~, al>,t a clec/ric

detonator taped al lhe protruding c~nd.

evaluated and discussed under various terlns such as backbreak, overbreak, crust lYacture, remaining rock condition, cratering effect, toe conditions, etc.

4. I. Horizontal joints

Horizontal joints are the most common type of joints occurring in the rock mass (mostly bedding planes). Figure 2a and b shows the front and top views of a block after tests with horizontal joints. Pronounced disturb- ances in the remaining rocks were observed with a small S :B ratio; when the ratio was increased, a gradual reduction in disturbance was observed. Overbreak and loose rocks on the sides of the blocks were commonly observed with the larger S : B ratios. A cratering effect was apparent in all the tests including the larger S : B ratio. The toe was absent in all the ratios used but with the larger S : B ratio, a hump between the holes ap- peared.

4. RESULTS AND DISCUSSIONS

In order to demonstrate the performance of an explo- sive-induced fracture plane on the perimeter face for the stable remaining rock mass, a large number of obser- vations were obtained from the test blocks blasted with various orientations of the joint planes. The outcome of these results not only demonstrates the influence of joint orientation in the development of fracture planes on the perimeter face, but also shows that there is a pronounced disturbance inflicted on remaining rocks on faces. Exper- imental tests carried out and results obtained were

Fig. 2. (a) Front view of blasted block with horizontal joints illustrat- ing small hump inbetween the holes and face disturbances, with S:B ratio of 3. (b) Top view of blasted block with horizontal joints illustrating cratering effect on the top slab and disturbances on the

remaining face, with S : B ratio of 2

BADAL: TECHNICAL NOTE 81

4.2. Parallel vertical joints

The blasting direction with this orientation produced disturbances in the remaining rocks. Similar results were obtained by Worsey [12]. Figures 3a and b show the front and top views of the block after testing. When the S : B ratio was larger than 2, unbroken rocks were encountered on the face in between the blast holes as toe. Overbreak on the face was absent, while backbreak was observed on the slabs containing explosive and some- times in the adjacent rocks. The increase in the number of joint planes (frequency) in between the holes reduced stress wave propagat ion and then waves become weak- ened for further fracture growth in the direction in which they travelled, resulting in an irregular face cut with the larger S : B ratios.

4.3. Perpendicular vertical joints

The blasting direction perpendicular to the joint plane and parallel to the perimeter wall is shown in Figs 4a and b, front and top views of the block after testing. Small overbreak was present with the smaller S : B ratio while no backbreak was observed, except small crest fractures with smaller S : B ratios. The face between the holes was clean, showing hole marking on the remaining rocks. This orientation provides highly stable face conditions on the remaining rock after the blast.

4.4. Perpendicular 60 ° down dip joints

Figures 5a and b show the front and top views of a block after the test with perpendicular down dip joints. A pronounced backbreak, overbreak including crust fractures was observed on the remaining rocks of the face. An acute toe problem was also encountered with larger S : B ratios. Figure 4a shows the bot tom slab appearing as a toe on the face. Slippage of rock along

Fig. 4. (a) Front view of blasted block with perpendicular vertical joints illustrating clear hole marking on the face and toe in between the blast hole, with an S : B ratio of 4. (b) Top view of blasted block with perpendicular vertical joints illustrating a clean face cut with an

S : B ratio of 4.

joint planes, occurred as overbreak, was commonly observed on the face. The orientation of the joints inflicted a higher disturbance on the remaining rocks of the perimeter face.

4.5. Perpendicular 60 ° up dip joints

At this orientation of joints, stable face conditions were observed after the blast, while a small overbreak on the face was apparent with small S : B ratios up to 2, as shown in the front and top of Figs 6a and b. Crest fracture and toe was absent up to the larger S : B ratio. Sometimes a small depressed floor level was observed on the face, which demonstrates a pulling action exercised by explosive energy below the floor level. A clean face cut was apparent with the hole markings on the remain- ing rocks of the perimeter face.

4.6. Parallel 60 ° dip joints

Figure 7a and b shows front and top view of a block after the test. On the face, joints dipping 60 ° parallel to the blasting direction produced unequal breakage. The breakage of rock was more in the dip direction (down dip) side, while against the dip (up dip) direction it was less. Most of the breakage was limited in the slabs containing explosives and the back break up to the S : B ratio of 2, the toe was absent. With a further increase in ratio, the toe started appearing and took the shape of unbroken slabs.

Fig. 3. (a) Front view of blasted block with parallel vertical joint illustrating toe between the holes, with an S : B ratio of 3. (b) Top view of blasted block of parallel vertical joints illustrating irregular face cut with back movement in the slabs containing blast holes, with an S : B

ratio of 2.

4. 7. Perpendicular 30 ° down dip joints

Figures 8a and b show front and top views of a block after the test. A pronounced back break and overbreak was observed on the perimeter face. The cratering effect was also apparent, which includes crest fractures on the

~2 B:\I)AL: TE()HNICAL NO'IE

Fig. 5. (a) Front view of blasted block with perpendicular 6t) down dip joints illustrating back break and toe problem encountered with an S : B ratio of 3. (b) Top view of blasted block with perpendicular 60 down dip joints illustrating back break on the f~ce with an S : B ratio

of

face. With the larger S : B ratio, sometimes a toe ap- peared. A high disturbance in the remaining rocks was observed.

4.8. Perpendicular 30 up clip ,joints

Figures 9a and b show front and top views of a block after the test. The overbreak was observed with a smaller

Fig. 6. (a) Front view of blasted block with perpendicular 60 up dip joints illustrating clean hole marking on the face and a small over break with an S : B ratio of 3. (b) Top view of blasted block with perpendicular 6 0 up dip joints illustrating stable face cut with an

S : B ratio of 2.

Fig. 7. (a) Front view of blasted block with paraliel 0I) dip joints illustrating toe remains inbetween the blast holes with a~ 5 : B ratio of 3. (b) Top view of blasted block with parallel 60 dip joints illustrating

back break on the face with an S : B ratio of 2.

S : B ratio up to 2, but, as the S : B ratio increased, the quantity of overbreak was reduced. Backbreak was observed with a smaller S: B ratio; if this ratio increased, backbreak reduced significantly. The toe was absent in all ratios, even with the larger S: B ratios used. The perimeter face between the blast hole was clear, showing hole marking on the remaining rocks.

4.9. Parallel 3 0 dip joints

Figures 10a and b show front and top views of a block after the test. The breakage observed was less and not equal on the face. In the dip direction (down dip) of the joint, more breakage was observed compared with the dip (up dip) side end of the face. Moreover, break and backbreak were observed in the slabs containing explo- sives. With the large S : B ratios, the toe appeared to develop as an unbroken face inbetween the holes, and resulted in an irregular perimeter face after blasting.

The results obtained from these studies have not shown any systematic trend towards the correlation of stability in the remaining rocks. However, after analyz- ing the experimental blasting results, certain significant observations could be made to provide a trend of damage inflicted on rock mass. From horizontal to vertical joints, either of" the joint orientations that are parallel or perpendicular to the blasting direction have a gradual improvement in the stable remaining rock mass, except for orientation perpendicular down dip joints, either gentle or steep dipping, which resulted in a highly disturbed rock mass on the blasted face. [While joints perpendicular to up dipping from the face in both cases either gently or steeply dipping joints.] However, the joints gently dipping parallel to the blasting direction showed the damage in the remaining rocks at the up dip direction side of the face; the tace remained stable in the down dip direction of the face, but the steeply dipping parallel joints resulted in less damage on the remaining rocks. The influence of the geomet~-ical parameters such

BADAL: TECHNICAL NOTE 83

Fig. 8. (a) Front view of blasted block with perpendicular 3if' down dip joints illustrating back break and toe problems encountered with an S :B ratio of 3. (b) Top view of blasted block with perpendicular 30 ~ down dip joints illustrating back break and disturbances on the

face with an S :B ratio of 2.

as spacing and burden was apparent. Normally, with the larger S : B ratios, less disturbance occurred in the remaining rocks; however, the parallel joints with smaller S : B ratios provide a stable face but with the larger S : B ratio the unbroken rock remained inbetween the holes and the disturbed rock on the blasted faces.

5. SIGNIFICANCE AND CONCLUSIONS

The results obtained from these studies have demon- strated the trends in stability of the remaining rock masses in the blasted bench faces. The influence of geometrical parameters has also been demonstrated in

Fig. 9. (a) Front view of blasted block with perpendicular 30' up dip joints illustrating (a stable face with small over break with an S : B ratio of 2. (b) Top view of blasted block with perpendicular 3 0 up dip joints

illustrating a small over break with an S : B ratio of 3.

Fig. 10. (a) Front view of a blasted block with parallel 30 ° dip joints illustrating toe and back break with an S :B ratio of 4. (b) Top view of blasted block with parallel 30 ° dip joints illustrating back break and

disturbances on the face with an S:B ratio of 1.

these studies and it has been found that in general, the larger S : B ratios produced more stability than smaller S :B ratios. However, the joint oriented parallel to the blasting direction resulted in adverse conditions with the larger S : B ratios.

The knowledge gained from these experimental stud- ies finds application for the stability in rock masses in the peripheral zones of blasted excavations. Optimal geo- metrical parameters such as hole diameter, burden, spacing, coupling ratio, delay in detonation, explosive type etc. should be decided on the basis of joint orien- tation and rock mass characteristics. As a first approxi- mation, the blast parameters can be decided on the basis of these guidelines. After a blast, the various blast design parameters should be evaluated carefully according to the results achieved from the previous blast and a suitable modification can be employed in the course of a blasting operation. Thus, it is recommended that a blasting engineer should take a meticulous look at orientation of rock discontinuities, their location and properties at each stage of the blasting operation, from initial drilling parameters to post blasting results, to achieve the objective--that is a stable rock mass around blasted excavations.

Acknowledgements--The author gratefully acknowledges Dr S. Bhandari for his valuable suggestions extended during the course of these studies, Dr V. M. Sharma, Director for all his encouragement, and Dr A. K. Dhawan and Mr R. B. Gangadhar for correcting the script.

Accepted .for publication 26 July 1993.

R E F E R E N C E S

I. Langefors U. and Khilstrom B. The Modern Techniques of Rock Blasting. Wiley, New York (1963).

2. Calder P. N. Pit Slope Manual, Chap. 7. Perimeter blasting. CAMET Report 77-14 (1977).

3. Rustan A. Linear Shaped Charge for contour blasting or stone cutting. 1st. Int. Syrup. Rock Fragmentation by Blasting, Lule~ (1983).

84 BADAL: TECHNICAL NOTE

4. Calder P. N. and Bauer A. Pre-split blast design lor open pit and undeground mines. 5th Int. Congr. Rock Mech. Melbourne, pp. 1425 1431 (1983).

5. Holloway D. C., Bjarnholt G. and Wilson W. H. A field study of Fracture Control Techniques for smooth wall blasting part 2. Int. Symp. Rock Fragmentation by Blasting, Keystone, pp. 646-556 (1987).

6. Mohanty B. Explosion generated Fracture in rock and rock like materials. Int. J. Engng Mech. 35, 889-898 (1990).

7. Coates D. F. Rock Mechanics Principles. Energy Mines and Resources, Canada. Monograph 874, pp. 2-28-35 (1981).

8. Mckenzie C. K. Blasting in hard rocks, techniques for diagnosis and modelling for damage and fragmentation. ISRM Congr., Montreal, pp. 1425-1431 (1987).

9. Ash R. L. The influence of geological discontinuities on rock blasting. Ph.D. Thesis, University of Minnesota (1973).

10. Hagan T. N. Rock breakage by explosive. NationalSymp. on Rock Fragmentation, Australian Geomechanics Society, Adelaide, pp. 1 16 (1973).

11. Bhandari S. Studies on rock fragmentation by blasting. Ph,D. Thesis, University of New South Wales (1975).

12. Worsey P. N. The effect of discontinuity orientation on the success of pre-split blasting. Proc. of Tenth Explosive and Blasting Tech- niques SEE, Lake Buena Vista, Florida, pp. 197 217 (1984).

13. Bhandari S. and Badal R. Role of burden and spacing on the geotechnical stability of surface mines. Int. Symp. on Geotechnical Stability in Surface Mines, Calgary (1986).

14. Bhandari S. and Badal R. Post blast studies in jointed rocks, b~i. J. Engng Fracture Mech. 35, 439-445 (1990).

15. Badal R. Influence of rock joints on fracture produced by explo- sive loading. Int. J. Rock Mech. Rock Engng (under revision) (1992).

16. Barker D. B. and Fourney W. L. Photoelastic investigations of fragmentation mechanics. Part 11. Flaw initiated network. Report, University of Maryland (1978).

17. Fourney W. L., Holloway D. C. and Barker D. B. Fragmentation in jointed rock material. Proc. Ist Int. Syrup. Rock Fragmentation by Blasting, Lule~, pp. 505-531 (1983).

18. Badal R. and Bhandari S. Fragmentation mechanism in rock joints. 1SRM Asia Regional Syrup. on Rock Slopes, New Delhi. pp. 377-385 (1992).

19. Badal R. A blast design parameters primarily depend on the orientation of rock joints: an experimental study. Int. Conf. on Engng Blasting Techniques, Beijing, China (1991).

20. Bhandari S. and Badal R. Relationship of joints orientation with hole spacing parameters in multi-hole blasting. Thirdlnt. Syrup. on Rock Fragmentation by Blasting, Brisbane, Australia, pp. 225---231 (1990).