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Acta Astronautica. Vol. 6, pp. 329-340. Pergamon Press 1979. Printed in Great Britain 
Rock breakage by expiosivest 
T. N. HAGAN 
Senior Mining Engineer (Applied Blasting Research), Explosives Division, ICI Australia Limited, 
Melbourne, Australia 
(Received 20 December 1977) 
Abstract--Explosive charges should provide a peak compressive strain at the blasthole wall that fails 
to cause crushing. Where higher peaks are generated, some strain wave energy is wasted in 
pulverising an annular section of rock immediately around the blasthole. The outgoing tangential 
tensile wave produces a highly-symmetrical radial crack pattern all the way around the blasthole. 
The number of radial cracks decreases at increasing distances from the blasthole. Any pre-existing 
cracks arrest the propagation of radial cracks prematurely. Symmetrical crack growth continues until 
the reflected radial tensile wave (which originates at an effective free face) interacts with the crack 
tips; this reflected wave then tends to open those radial cracks with which the wave front subtends a 
small acute angle. The explosion gases then stream into forward-looking cracks (and especially those 
favoured by the reflective wave), wedging them open and extending them towards the free face(s). 
Once the burden rock has been at least partially detached from the mass, the very high rate of decay 
of strain causes release-of-load fractures along cylindrical (or conical) shells around the blasthole. 
The reflected tensile wave is usually too weak to cause spa/ling at an external free face; it can, 
however, open up and extend pre-existing cracks and fissures. Internal spa/ling at open joints within 
the rock can be significant. Shear failure occurs where the pushing effect of the gases causes relative 
movement of adjacent sections of rock along blast-induced cracks and/or fissures. After radial 
cracking has been completed, the various adjacent wedge-shaped elements of the burden rock 
undergo bending, and fracturing by flexural rupture occurs in planes normal to the blasthole axis. 
Breakage by in-flight collisions takes place for certain bl~ist geometries and/or initiation sequences. 
Introduction 
OVERALL costs in the mining, quarry ing and construct ion industr ies are af fected 
apprec iab ly by the degree of f ragmentat ion (especia l ly the percentage of over- 
size and fines), d i sp lacement and looseness of the muckpi le . For this reason, 
there are strong inducements to design pr imary blasts which prov ide opt imum 
muckpi les . Unfor tunate ly , opt imum blast designs have been, and still are, 
p revented by our incomplete unders tand ing of: 
1. The mechan isms of f ragmentat ion of rock by blast ing; and 
2. The inf luence of exp los ive propert ies , rock propert ies , b last geometry , 
init iat ion sequence and de lay t iming on these breakage processes . 
P roduct ion of the opt imum network of f ractures (this being synonymous with 
opt imum f ragmentat ion) will not be poss ib le , o f course, unti l the f racture 
mechan isms in fu l l -scale blasts (not necessar i ly in laboratory exper iments) are 
ful ly understood. 
tPresented at the Sixth International Coloquium on Gasdynamics of Explosions and Reactive 
Systems Stockholm, Sweden, 22-26 August 1977. 
329 
330 T.N. Hagan 
Modes of rock failure 
Laboratory studiest have indicated several ways in which the (in situ) rock 
body may be expected to fracture. This information has been supplemented by 
data from: 
1. Mini-bench blasts (Dick et al., 1973); 
2. Single-hole blasts in large (15 tonne) unfissured blocks of rock (Bergman et 
al. 1973); and 
3. High-speed photographic studies of (full-scale) openpit blasts (Chung et al., 
1975). 
In commercial blasting operations, rock failure can occur through the 
mechanisms described below. In general, blast designs should be such as to 
maximize the extent and intensity of each of these mechanisms (and, hence, the 
overall degree of fragmentation); the first of these mechanisms (viz. crushing), 
however, should be avoided wherever possible. 
Crushing 
A zone of intensely crushed rock is often formed immediately around the 
charge. For fully-coupled charges of "average strength" in "primitive" rock, the 
thickness of the so-called "crushed zone" was estimated (Langefors and Kihls- 
trom, 1963) to be ~, 2 cm for a charge diameter (d) of 4 cm. This ratio of crushed 
zone thickness is consistent with the views of other investigators (Olsen and 
Fogelson, 1969). In a relatively porous sandstone, however, the distance over 
which the strain wave exhibited a supersonic velocity indicated a crushed zone 
thickness of (7.5) d (Duvall and Atchison, 1957). That supersonic velocities can 
extend over such large distances is supported by Drukovanyi et al. (1971). 
In the crushed zone, the peak of the cylindrically-expanding strain wave 
exceeds the dynamic compressive breaking strain of the rock; fracture then 
occurs during a period of volume compression, through the collapse of the 
intercrystalline or intergrain structure. At points beyond the crushed zone 
boundary, the peak compressive strain falls (due to divergence and energy 
absorption effects) to a value below the (compressive) elastic limit of the rock. 
The excessive fragmentation in the crushed zone is associated with a very high 
rate of energy dissipation. The thickness of the crushed zone increases with both 
the explosion pressure of the charge and the charge: blasthole diameter ratio; it 
varies also with the properties of both the rock and material between the charge 
and blasthole wall. 
Effects of charge properties on crushing. The peak blasthole pressure (Pb) 
exerted by the explosion gases can be calculated from the equation 
Pb ---~ XP D2 ( l ) 
where x is a function of the explosive's density (p) and D is the explosive's 
tSee Rinehart (1958), Hino (1959), Kolsky (1953) and Cook (1958). 
Rock breakage by explosives 331 
velocity of detonation (Hagan, 1977). As one would expect, the intensity and 
extent of crushing increase as Pb increases beyond a certain critical value. In 
view of the increasing use of fully-coupled charges (i.e. charges which are 
poured, loaded pneumatically or pumped to fill the entire cross-sectional area of 
the blasthole) and the need to avoid the fines which originate in the crushed 
zone, it is becoming more and more necessary to know the P~ values exerted by 
the charge in use. 
Ammonium nitrate/fuel oil (ANFO)-type compositions are, ceteris paribus, of 
lower brisance than water gel explosives and blasting agents. It follows, then, 
that in rocks which are crushed with relative ease, the need to maximize the use 
of ANFO is greater than the relative explosive cost data would suggest. The 
higher compressive peak strains generated (at the blasthole wall) by water gel 
compositions become beneficial (and economically attractive) in situations where 
crushing fails to occur. But the reduction of the crushed zone volume does not 
depend solely upon the composition of the charge; the density and diameter of 
the charge are also influential factors. 
The effect of decreases in p on Pb is greater than that indicated by eqn (1). 
This is because the lowering of p causes a significant decrease in D and, hence, a 
considerable reduction in D 2. Where the density of ANFO is lowered through 
the addition of polystyrene beads, Pb can be reduced to a small fraction of its 
initial value. For this reason, such low-density mixtures may well prove to be 
useful in dry-blasthole conditions where the strata is crushed by ANFO. In 
development headings at an Australian underground mine, a (blow-loaded) 
mixture of 50 ANFO/50 polystyrene (volume basis) was substituted for regular 
ANFO in the blastholes in the heart of theround (Greeff, 1977). The fact that the 
overall degree of fragmentation and displacement of the muckpile remained 
unchanged suggests that an appreciable fraction of the ANFO's strain wave 
energy was being wasted in crushing and pulverising the rock (a relatively weak 
yet massive pyroclastic) in the immediate vicinity of the blastholes. Such 
observations prompt the question: in what percentage of the rock volume 
excavated annually in the combined blasting operations of the world is explosive 
energy needlessly wasted in creating unwanted (often unmarketable) powdered 
rock (or mineral)? 
The peak blasthole pressure of blasting agents is also affected appreciably by 
variations in charge diameter (d), especially where the latter falls below about 
125 ram. This reduction in Pb is caused by the increasingly-rapid decline in D as d 
falls in the 125 mm to 25 mm range. Small diameter blastholes, therefore, have the 
advantage, in crushable strata at least, of generating energy in a form which is more 
capable of creating effective fragmentation. 
Effect of rock properties on crushing. The range of dynamic compressive 
breaking strains of rocks is probably such that there are blasting operations where 
sizeable crushed zones are normally formed, and others in which even high- 
velocity, high-strength charges fail to develop the compressive strain necessary for 
crushing. Crystalline rocks of very low porosity (e.g. granite) are expected to offer 
the greatest resistance to compressive failure, whereas weak and/or highly-porous 
rocks (e.g. sandstones, some iron ores) and minerals (e.g. coal) crush relatively 
332 T.N. Hagan 
easily. The frequent presence of semi-blastholes and "burn marks" on newly- 
created faces in the harder rocks is visible evidence that the crushed zone thickness 
is zero or very small in such conditions. Because the acoustic velocities of rocks 
decrease appreciably with increases in porosity, one would expect a reasonably 
good correlation between acoustic velocity and crushed zone thickness. 
Effects of decoupling and decking on crushing. The amount of (wasteful) 
crushing can be reduced, or eliminated altogether, by decoupling. This is best 
illustrated by considering a constant diameter charge in a blasthole of varying 
diameter. If crushing occurs where the charge is fully coupled, the blasthole 
diameter can be increased until the crushed zone is eliminated without any 
appreciable reduction in peak compressive strain at points beyond the initial 
crushed zone boundary. 
Where deck charges are used, crushing can occur only within a certain 
(restricted) distance of the surface of each charge. Where the materiai between 
consecutive charge decks is dry and of a granular consistency, the rate of decay 
of blasthole pressure within each charge deck is greater than that which exists 
for a full column charge. This change in the pressure-time profile is most 
pronounced, of course, where the material between consecutive charges is air, 
and it is the claims (Melnikov, 1962) of reduced crushing with air-decked charges 
that has aroused most attention. 
Elimination of crushing through improved charge design. The design of 
improved composit ions and/or geometries will depend significantly upon the 
consideration of dynamic compressive breaking strains o f rocks. The reasoning 
behind this statement is best illustrated by considering the relative effectiveness of 
two fully-coupled charges in a rock which crushes at 40kbar. Explosive A 
generates a compressive stress of 78 kbar at the blasthole wall whereas explosive B 
generates only 39 kbar. The variations of peak stress with propagation distance are 
shown in Fig. 1. The elbow of curve A (point b) indicates the crushed zone 
boundary. The creation of very large (rock) surface areas during pulverisation of 
the crushed zone causes a very rapid decrease in peak stress (limb ab, curve A); the 
rate of decay is much lower beyond the crushed zone boundary due to the rock's 
elastic behaviour (limb be, curve A). Because explosive B does not create new 
surfaces by crushing, it gives a slower initial rate of stress decay Icurve B). At 
distances beyond the crushed zone boundary, where strain wave characteristics 
have an appreciable influence on the overall degree of fragmentation, the peak 
strain for explosive A will be little greater than for explosive B. In effect, the 
crushed zone acts as a peak strain governor. If a rock fails at a compressive stress of 
x kbar, there is virtually no incentive to use a charge capable of generating peak 
stresses more than x kbar in the rock. Increases in the coupling and/or explosion 
pressure of charges, therefore, do not necessarily lead to larger fractured zones or 
overall improvements in the degree of fragmentation. 
Fracturing by relative radial motion 
Consider a cylindrical shell of rock immediately around the blasthole wail. 
When the strain wave front passes, this shell is subjected to intense radial 
Rock breakage by explosives 333 
ExptosLve A causes crushing ~ 60 ExptosLve B does not 
~. 50 
>~ ~. ~ dynamic elastic 
Distance from blasthote watt t, 
Fig. 1. Variation of peak compressive stress with distance from blasthole. 
compression, and tangential tensile strains develop. If these strains exceed the 
dynamic tensile breaking strain of the rock, fissures initiate. A zone of dense 
radial fractures (which may well be superimposed on the annular crushed zone) 
is thus formed immediately around the blasthole. This intense radial fracture 
zone terminates quite abruptly at that radial distance where the wave's tangential 
tensile strain attenuates to a value which is incapable of generating new cracks. 
The width of this zone depends on the dynamic tensile breaking strain and strain 
wave velocity of the rock, the peak strain at the blasthole wall, the explosive's 
velocity of detonation and, to a large degree, on the rate of energy absorption in 
the rock. 
Bordering this (inner) zone of dense radial fractures is an outer zone with 
much wider-spaced radial cracks. The latter cracks, symmetrically distributed 
around the blasthole, are the extensions of some of the cracks of the inner zone. 
Although the tensile strain of the wave has dropped below the critical value for 
initiating new cracks, it is still sufficiently active to extend pre-existing cracks; 
less strain is required to extend a crack than to form one. Extensions can take 
place approximately as long as tension is applied normal to the crack tip, i.e. as 
long as the tensile phase of the tangential strain has not completely passed by 
the slower-propagating crack tip. As soon as a crack begins to extend, the 
tangential stresses are released in its "immediate vicinity. This release acts in the 
form of a wave radiating from the crack tip. Thus, within a certain angle of the 
crack, practically no new radial fractures can start. At points sufficiently far 
away from the crack, the peak tensile strain is reached before the relief wave 
arrives and, hence, an extending crack forms there. The outer zone is thus made 
up of radially-running cracks alternating with fracture-free stress-relieved seg- 
ments. 
334 T.N. Hagan 
An approximately linear relationship exists between crack extension and length 
of pre-existing cracks. When blasting rocks with joints and bedding planes, 
therefore, the major crack generation will be in their direction and certainly no 
uniform fracture can be expected. 
The absence of a free face in no way prevents this type of fracturing. Radial 
fractures are produced in the solid stratum behind a newly-formed face. 
Experiments have shown (Olson and Fogelson, 1969; Repin and Panachev, 1969) 
that the propagation velocity of an elastic wave in rock behind a newly-formed 
face varies with depth of penetration into the face. The fissure densitydecreases 
and propagation velocity increases at increasing distances from the face. Where 
the peak strain at the blasthole wall cannot cause crushing, crack lengths in both 
radial fracture zones increase linearly with peak strain. For peak strains exceeding 
that required for crushing, an increase in peak strain does not increase the crack 
lengths, but results only in additional crushing around the blasthole. 
A joint or fissure plane which parallels and intersects the blasthole along the 
length of the charge causes a very large stress concentration. This joint opens 
under the action of the strain wave and allows little tension and radial fracturing 
to be caused elsewhere around the blasthole. Where such joints are located at 
some distance from the blasthole, the radial cracks are interrupted by the joint. 
Only where joints are very close to the blasthole wall, very narrow and/or 
water-filled is the tangential tensile strain on the other side of the joint 
sufficiently intense to reinitiate the radial crack. 
Fracturing by release of load 
For blasthole charges, the initial strain wave usually carries < 5% of the blast 
energy into the rock. Before the strain wave reaches the free face, however, the 
total energy transferred to the burden by the initial compression of the rock is 
claimed to be as much as 60-70% of the blast energy (Cook et al., 1966). After the 
compression wave has passed, a state of static equilibrium exists, the pressure of 
blasthole gases being balanced by the strains at the boundary of the crushed (or 
elastic) zone. When the pressure in the blasthole subsequently falls (as gases 
escape through stemming and radial cracks) this strain energy is released. 
Concentric shells of rock undergo radial expansion following the state of intense 
compression, and tangential release-of-load fractures occur in the immediate 
vicinity of the blasthole. Such concentric fracture surfaces are subsequently 
created nearer and nearer to the rock face. These fractures follow cylindrical 
and/or conical surfaces. 
For a single blasthole, the velocity of rock fragments from a quarry face 
undergoes significant variations (Petkof et al., 1961). These velocity changes 
occur in steps rather than in a continuous manner. Similar step velocity in- 
creases were observed (Hino, 1959) in laboratory studies. A prima [acie 
explanation of this phenomenon is that of in-flight impacts between suc- 
cessively-ejected layers of rock. Such impacts cannot be explained by spalling 
theory, however, since this predicts that successive layers of rock are ejected at 
Rock breakage by explosives 335 
diminishing velocities. If, as is suggested, release-of-load fracturing starts near 
the blasthole and progresses towards the face, the whole burden is detached near 
the blasthole and it moves forward at an initially low velocity. As each additional 
layer is detached from the moving burden, the velocity of the face jumps to a 
higher value. This mechanism of step velocity increases, therefore, would appear 
to be based upon release of load rather than in-flight collisions. 
Where large multi-row blasts with an "in-line" initiation sequence (Hagan, 
1975a, 1975b) are fired, the compressive strain waves and gas expansion effects 
from all blastholes in a given row act in unison on the rock mass behind the 
design excavation limit. There is also some superposition of the effects produced 
by the individual rows. Such blasts can cause vertical overbreak cracks parallel 
to and up to 60 m behind the newly-created face (McIntyre and Hagan, 1976). 
These overbreak cracks are caused by release of load. In weak and/or highly 
incompetent strata, at least, the behaviour of the rock may be likened to that of a 
multi-layered mass of rubber which is impacted, at normal incidence, by a falling 
heavy steel plate. After simultaneous contact over the upper surface of the 
rubber, the plate continues to compress the mass until its momentum is ex- 
hausted. The highly-compressed layers then accelerate the plate in the opposite 
direction and, in ejecting it vertically upwards, separate from each other. Such 
separation between adjacent layers explains the so-called tension fractures so 
often observed (and to be avoided wherever possible) in open pit and strip mine 
operations where poor blasting practice encourages pit wall instability. 
Reflection breakage or "spalling" 
When the compressive strain wave strikes a free face, two reflected waves, 
one tensile and the other shear, are generated. If the reflected tensile wave is 
sufficiently strong, "spalling" occurs progressively from any effective free face 
back towards the blasthole. For burdens normally used in rock blasting, 
however, "spalling" of the face does not usually occur. This statement is based 
on the results of high-speed photographic studies of bench blasts. 
Where a wide air-filled joint (or bedding plane) lies close and parallel to the 
blasthole, the surface of the fissure causes internal "spalling" through reflection 
of the compressive strain wave. This produces increased fragmentation between 
the blasthole and the fissure, but (because of the greater attenuation and 
dispersion of the outward-propagating strain wave) tends to cause poor breakage 
beyond the fissure. In highly-fissured strata, therefore, internal "spalling", 
together with crushing and/or dense radial fracturing encourages superfrag- 
mentation of the rock close to the blasthole. The excessive degree of breakage in 
this zone is often achieved at the expense of satisfactory fragmentation at points 
well within the burden rock. 
Very narrow fissures may be too fine (especially where they are filled with 
water) to cause significant reflection of the radial compressive wave. Because of 
their inability to transmit tensile stress, however, they separate under the 
influence of the reflected tensile wave. 
Even where they are too weak to initiate new fractures, reflected tensile 
336 T.N. Hagan 
Ref(ected 
Face tens,re wave 
, .Xeo- • - - j - 
- ~ . " 180-80 
~IIl I, ~ R?a{ cracks 
Fig. 2. Radial fractures and the (weak) reflected tensile wave front. 
waves can interact with the stress field at the tips of suitably-oriented radial 
cracks and cause considerable extensions. As is shown in Fig. 2, the reflected 
tensile wave will be most effective in extending those radial cracks which run at 
large acute angles to these tensile forces. The easiest path for the gases to the 
free face, therefore, should be by the cracks at angles 0 ° and (180-0 °) to the 
face. It is difficult to assign a precise value to 0, since there appears to be no 
accurate method of determining the relative crack-extending abilities of: 
1. Stronger (tension) waves at a relatively small angle of incidence; and 
2. Weaker waves at normal (or approaching normal) incidence. 
Any study of this problem, of course, requires an appreciation of the 
following facts. 
1. The stronger but more-glancing waves exert their influence at an earlier 
time and are required to extend the intercepted crack tips over a shorter distance 
(to the free face). 
2. As soon as one radial crack is extended, its propagation is encouraged 
while that of other cracks is suppressed. 
Gas extension of strain wave-generated fractures and natural fissures 
After strain wave emission, the reduced gas pressures cause a quasi-static 
stress field around the blasthole. During or after the formation of radial cracks 
by the strain wave, the gases start to expand and penetrate into these cracks. 
The radial cracks then extend under the influence of the stress concentrations 
(caused by pressurised gases in these cracks) at the cracks tips. 
The number and length of wave-induced radial cracks have a marked effect 
on the magnitude of gas-induced stresses around the blasthole. Where thenumber of cracks is large, the full gas pressure can be assumed to act not over 
the original blasthole, but over an "equivalent blasthole" having a diameter equal 
to that of the inner radially-fractured zone. The gas-induced stress field, there- 
fore, becomes much more extensive and considerable crack extension can be 
expected. The stresses around a blasthole with pressurised cracks one blasthole 
Rock breakage by explosives 337 
diameter long, for example, are nine times higher than those around the original 
pressurised blasthole. The critical pressure for crack propagation decreases 
markedly in the presence of radial cracks (Kutter and Fairhurst, 1971). Because 
stress concentration at the crack tip increases with crack length, the longest 
crack is the least stable and requires the lowest critical pressure, particularly 
near the blasthole where a small increase in length causes a large decrease in 
critical pressure. Therefore, longer fractures always extend first and propagate at 
a higher velocity than shorter adjacent fractures. The further they get ahead, the 
greater is the velocity difference until the shorter ones stop altogether. Except 
for the case of a single radial crack, the critical pressure increases with the 
number of cracks. For cracks of equal length, the stress at the crack tip 
decreases with the distance between these tips. For effective gas extension of 
fractures, therefore, the distance between radial fractures should not be too 
small; the inner (dense) radial fracture zone should be minimised so that the 
outer radial fractures are of sufficient length for optimum crack growth. The 
crushed and highly-shattered zones probably hinder gas extension of cracks as 
well as causing a very high rate of energy dissipation. 
Intense cracking behind blastholes in the more competent rocks is rarely 
visible. For this reason penetration of backward-facing radial cracks by high- 
pressure gases is considered to be minor. 
The extensions of explosion-generated cracks, as important as they may be, 
are often masked by those of pre-existing fractures (i.e. natural fissures plus 
cracks by previous blasts). Indeed, pre-existing cracks such as joints and 
bedding planes frequently dominate both the nature and extent of the fracture 
pattern. Pre-existing cracks are widened and extended, often to considerable 
lengths, while the formation of new cracks in their immediate vicinity is 
suppressed. Because major crack development is along such joints and bedding 
planes, uniform fragmentation can be achieved only where the (natural) fissures are 
closely-spaced and quite evenly distributed throughout the rock mass. In rocks of 
this type (i.e., densely fissured strata), high-pressure explosion gases can often 
produce satisfactory breakage without the creation of fracture surfaces by strain 
wave effects. Where fissures are on wide centres and/or of uneven distribution, 
however, a high percentage of oversize material can result. 
Fracturing by flexural rupture 
High speed photographic studies of bench blasting have indicated that 
fracturing by flexural rupture due to longitudinal bending (i.e. bowing along lines 
up and down the face) can occur (Ash, 1973). This bending takes place after 
completion of radial cracking. Fracturing by flexural rupture occurs along planes 
which are normal to the blasthole axis and is caused by tangential tensile strains 
associated with the bending of the burden rock segments by the expanding 
explosion gases. The amount of flexural rupture fracturing will depend upon the 
stiffness of the segments of burden rock formed by radial cracking. As pointed 
out by Ash (1973), the stiffness of a rock segment varies directly with the rock's 
modulus of elasticity and the third power of the ratio of the segment's thickness: 
length ratio. This may well be one reason for the poorer overall fragmentation 
338 T.N. Hagan 
observed where larger diameter blastholes and their corresponding enlarged 
blasthole patterns are employed. The use of higher benches with the 230-380 mm 
diameter blastholes in widespread current use would provide improved frag- 
mentation (especially in tough massive rocks) not only through flexural rupture 
but also as a result of the enhanced charge distribution in the rock to be broken. 
In underground operations where large diameter blastholes are being introduced, 
the lengths of charge decks should be maximised (consistent with satisfactory 
ground vibration levels) if best possible fragmentation is to be achieved. Bending 
along the lines across the face could also contribute fo flexural rupture fractur- 
ing in radial planes. 
As with the bending of a notched beam, any pronounced protrusions and/or 
indentations in the face will act as stress concentrators, and initiation of flexural 
rupture fractures will be facilitated at such points. The extensions of these 
fractures will tend to follow any bedding planes and/or joints present. 
Shear fracturing along fissures and strain wave-generated cracks 
A shearing ripping type of failure occurs when the heaving, pushing effect of 
the blasthole gases is sufficiently great to cause relative movement of adjacent 
elements of the burden along suitably-oriented fractures, bedding planes, joints 
etc. If the rock is relatively weak, some additional fracturing may occur in 
addition to the opening up of pre-existing cracks. The resistance to shearing 
varies greatly with rock type and blasting geometry. In bench blasting where 
pronounced horizontal bedding occurs, for example, clean breakage at floor level 
is accomplished with relative ease; the frictional and inertial forces tending to 
prevent relative horizontal movement along the floor-level plane are overcome 
by the sustained push of the blasthole gases. 
The pushing action of the blasthole gases tends to produce vertical shear 
fractures. Because that segment of the burden directly between the blasthole and 
face has the least inertia and is probably the most highly fractured, it tends to be 
propelled horizontally outwards before adjacent segments. (In practice, actual 
segment boundaries are probably determined by the location of pronounced 
radial fracture planes, joints and/or bedding planes.) The resulting relative 
motion of adjacent segments produces a shearing ripping action along the rough 
interface between these segments. This shearing action causes additional frac- 
turing. The development of vertical shear fractures will depend very largely on 
rock type and is encouraged by providing an effective time delay between the 
detonation of adjacent blastholes. Vertical shear fracturing is minimal when all 
holes in an effective row are fired simultaneously. 
Fracturing by in-flight collisions 
The importance of in-flight breakage depends very largely upon the blasthole 
pattern and/or the initiation sequence. Where millisecond delay initiation is used, 
for example, in-flight breakage is more pronounced for a full-face tunnelling 
round than for a single row of holes in a bench blasting operation. Because 
tunnel charges on the later delays propel rock fragments in the general direction 
of the tunnel axis, impact velocities tend to be high, conditions favouring 
Rock breakage by explosives 339 
"head-on" collisions. This may help to explain the improved fragmentation with 
millisecond (cf. 0.5 sec) delays in tunnelling. For bench blasthole patterns, 
in-flight breakage is more pronounced for V-type than for in-line initiation 
sequences (Hagan, 1975a, 1975b). 
Conclusions 
Rock breakage by crushing should be eliminated and intense radial fracturing 
minimised by using explosive charge geometries and initiation methods which 
are selected on the basis of the particular rock's dynamic breaking strains. A 
considerable amount of explosives energy (and operating expenditure) canbe 
wasted where highly-coupled charges of dense, high-velocity explosives are 
employed in the weaker and/or relatively porous rock types. 
If blasting efficiency is to be increased through improvements in ef fect ive 
fragmentation, it will be essential to promote our knowledge of: 
1. The important rock breakage mechanisms; and 
2. The effects of explosive properties, rock properties, blast geometry, 
initiation sequence and delay timing on these processes. 
The massive and increasing consumption of explosives in the world's mining, 
quarrying and construction operations provides a very considerable incentive to 
achieve optimum blasting conditions through the development and application of 
this knowledge. 
Acknowledgement--The author extends his thanks to ICI Australia Limited for permission to publish 
this paper. 
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