Limitations of Commercial Explosives and Blasting
Caps and their Effect on In Situ Blast Design
Keith Britton
Consultant
San Leandro, California
ABSTRACT INTRODUCTION
The limitations of presently available
explosives products form a constraint for the in
situ blast designer. This is generally
unrecognized, but is a matter of some seriousness.
The ultimate effects on blast design are almost
always significant, often profound, and not
infrequently found to be the limiting factor
regarding practicability. The designer may be
denied the ability to optimize because of the need
to make substantial provision to compensate for
failure or substandard performance, technically
innapropriate product often being traded for
enhanced reliability. More seriously, constraints
render theoretically desirable geometries or timings
impractical. This may gravely compromise a process,
e.g. by limiting the resource applicability, or, if
more tolerant designs prove ineffective, result in
economic or technical failure.
Initiating systems are discussed in depth,
illustrated by experience from the Geokinetics
Lofreco Process, and using data from retort shots
and delay blasting cap characterization by GEO and
Sandia National Laboratory. Explosives related
difficulties are discussed in terms of their general
nature, with anecdotal illustration from field
experience. Finally, research and product needs are
summarized and ranked in terms of importance to a
1990s in situ industry.
In situ blast design is not easy, despite the facile
impression often given that we need merely to
distribute explosives according to the dictates of
the computer model currently in vogue and our field
results will dutifully reflect model predictions.
Unfortunately present models are primitive. None
yet models factors which produce first order effects
readily observable in the field. Further, practical
designs are typically far more complex than model
cases, to take into account local anomalies, and any
practicable design is further complicated by
constraints, notably regarding limitations of
commercial delay blasting caps and explosives.
These constraints, the subject of this paper, are
classified under 'explosivesengineering'
and are
generally seriously underappreciated. By analogy,
we don't build bridges with 10 kilometer spans
because real bridges are built with real steel, not
some theoretical flawless abstraction, and in
practice we need to account for strength losses in
fastening etc., following which we derate by a
healthy factor to guard against the occasional bad
weld ... It is commonly accepted that the gap
between the theoretical and the practical may be
wide, but the gap between the practical and the
practicable is often ignored. Unfortunately, for in
situ blasting, this latter may prove wide.
0271-0315/85/0018-0109 $00.20 109 1985 Colorado School of Mines
The extent of the problem was well illustrated by
work performed for the last Lofreco blast design.
During the design process, the writer logged, by
category, each option considered. The universe
from which the eventual design was selected thus
numbered the multiple of these numbers, for a total
of some 5 million. Naturally, not all cases were
explored as such, the work being circumscribed to
classes and thus more reflecting the sum rather than
the multiple of the number of options. Of those
options logged, more than two thirds were affected
to some degree by 'explosives engineering'
considerations. Shortlist candidates were studied
intensively in this regard, e.g. by Monte Carlo
analysis using assumed delay data. Of the total
design effort, around half could thus be assigned to
'explosives engineering'.
DELAY DETONATORS
Delay detonator characteristics prove important to
most blasts and well illustrate the degree and
subtlety of constraints upon design. A typical
delay blasting cap consists of a pyrotechnic delay
element which is indirectly ignited by shock and/or
heat from a detonation aessage or electrical
bridgewire. The delay element burns for a designed
time interval, then ignites a primary explosive,
which communicates detonation via a base charge to
the main charge. Variability in the delay element
leads to variability in overall firing time. Less
obviously, performance is affected by factors such
as aging, temperature and stimulus.
The delay element is typically manufactured by
drawing down a thick walled tube filled with a
suitable pyrotechnic mixture, producing a stock tube
which is then sheared to length. When the element
becomes too long or short, a different composition
is used, so coarse control on firing time is by
composition, fine by length. It follows, and is
borne out in practice, that the best relative
performance is from the longest delay of a
particular composition, and the most uniform fror.i
product selected to be same tube, same machine(s)
and same shift. This matters. Within the product
from a single manufacturer, such simple selection
may make the difference between adequate caps and
unusable or even hazardous ones.
STIMULUS
For consideration of timing and relibility, a
notional delay cap may be considered as primarily
two elements, an ignitor and a delay train. Since
the former acts rapidly, it might not be expected to
contribute much variability, but electrical
bridgewire initiation is sensitive to the level of
firing current. It is not possible to implement
practical firing circuits with balanced currents at
all bridgewires due to progressive loss through
ground loop leakage, see Fig. 1. Partial failure
can occur if a sensitive cap fires, breaking the
circuit, before all are heated to the point of
ignition, but this can be considered a pathological
case since competent design avoids this region.
(This was, nevertheless, the cause of the failure of
an early large scale U.S. government in situ
experiment.) Firing current is limited on the high
side by onset of arcing, with erratic firing or
failure, so, as shown in Fig. 2, significant
variability is intrinsic to electrical ignition.
Further, comparative complexity increases
opportunity for unreliability.
J_ CURRENT LEAKAGE~
.5 AMP
3A CIRCUIT CURRENT
DETONATOR BRIDGEWIRE
r*T4.5 A | 4'A |3.5A7
Figure 1. Firing Circuit Current Leakage
The two other ignition systems are detonating cord
and shocktube. Both provide a most substantial and
uniform signal. The former involves severe stress,
which may cause problems, notably short failure by
immediate base charge detonation. This used to
occur, as evidenced by high speed film, with ICI
delays. It has not been observed by this author in
Du Pont or Ensign Bickford delays, though grossly
short firing has been noted for tne latter.
no
10
JinEEUffUjEl
ALL-FlRE REGION
rm~M
NO- FIRE REGION
1O00I 001 .01 0.1
TIME IN SECONDS
Does this have practical significance, provided one
purchases appropriate nominal delays? Indeed it
does, and occasionally in the most gross sense.
Investigation reveals that manufacturers cheerfully
vend product whicn is so bad that even the mean
firing times may be out of sequence, Winzer (1973)
and Fig. 5. Inadvertent use of such in a
conventional surface blast affects fragmentation and
may result in misfires and unexpected flyrock as the
muck relieves in an unplanned direction. For in
situ blasts the consequences of out of sequence
firing may also be profound.
Figure 2. Response of Blasting Caps to Firing
Current (source - Atlas Powder Company)
Shocktube, involving only the much lower pressures
of mixed phase or gas detonations, ought to be
superior and appears so in practice. Both of these,
however, ignite from a stimulus which propagates
slowly enough to result in effects which cannot be
ignored and which may greatly complicate design.
Notably, care must be taken to ensure that rock
strains from early shots do not prematurely cut
lines to later firing charges, and consideration
must be given to the systematic variation in firing
times from stimulus propagation.
Considering the notional multi-row layout of Fig. 7,
the out of sequence row will fire with twice the
normal burden -
and without the possibility of
upwards relief which is available to surface rounds.
It follows that material which was intended to
fracture and fragment with rotation to produce
porous rubble, remains sensibly ordered and massive.
Worse, since the explosion gases must follow the
pressure gradient to relieve through the unfired
row, likely with block slippages, misfires may occur
from cut off holes, dead pressed explosive or
damaged detonators. This would ada a zone of
unfractured boulders to a retort bed which must be
further compromised by the progressive effect on
subsequent firing blastholes of burden tightness
from the motion inhibited mass.
DELAY VARIANCE
Caps vary in accuracy, the discrepancy between mean
and nominal firing time, in precision, the scatter
about the mean, and also exhibit incidences of gross
error, short or long, and total failure to fire.
They are typically characterized in terms of mean
and standard deviation, derived by calculation.
This is helpful, but valid only where data is
normally distributed. It is more informative and
safer to plot the data on probability paper, which
has the property of linearizing data which has
normal (Gaussian) distribution. The quality of the
work then becomes readily apparent, from the point
scatter, as does curvilinear or other systematic
aberration. (Work quality proves critical else
experimental error obscures delay variance.) Typical
testing data are presented in Figs. 3-6.
The preceding is obviously idealized and extreme.
Unfortunately, the principle proves only too
applicable in actual rounds using caps with
correctly sequenced means. It has been shown by
coring, burn and heave analysis, that hard zones,
once established, become extraordinarily persistent,
and may impose a perceptible forward shadow on the
burn in addition to the progressive breakage effect.
This persistence is seen in Fig. 8, which is part of
a plot of surface displacement from a full scale
retort, computer enhanced by slope normalization and
vertical exaggeration. The shallow depression
arrowed, which is too subtle a feature to be
perceptible without such treatment, is evident for
more than 40 meters, from its origin in a badly
blasted zone to the limit of the retort.
Ill
2
tn 51
FIRING TIME (ms)
Figure 3. Typical Cap Characterization Data
(Ensign Bickford Nonel Period 15)
U 91 IN Ml I2D 131 li IM IH
FIRING TIME (ms)
Figure 5. Out of Sequence Caps -
Nominally Period 5
or 125 ms (Note: mean Q 103ms, caps
fire with period 4 except outlier 015Ous
which fires witn period 6.
w 31
570 HI S10
FIRING TIME (ms)
Figure '. Reject Batch- Unusable Scatter -
A.^ing
(Hnsign Bickford Nonel Period lb)
u
2
II\n^
^^^31 d\
\.
M
71 \ a ^\^
N a\
M
NJ
i i i
91 SI 93 94 IS K 97 I
FIRING TIME (ms)
Figure 6. Two Populations -
Two Produce Streams or
Careless Ihermal Test (Plotted suuj,roap
were first caps tested, cold from
magazine? )
112
<x>
Figure 7. Out of Sequence Rows
CONTROLLING THE ACCURACY PROBLEM
Figure 8. Surface Expression of Bad Breakage
(Profiles @ 7m intervals)
Hazards obviously increase where both accuracy and
precision are involved. Nominal firing times may be
monotonic, but this is disturbed by errors in
accuracy, resulting in crowding where errors in
adjacent intervals sum to shorten the interval.
This was investigated by Winzer (1978), who showed
that the probability of successful firing was only
19% for a 24 charge shot using the best of his
samples. Clearly, this is intolerable, especially
considering in situ shots which are an order of
magnitude larger.
The obvious first step to a solution, recommended to
Geokinetics in 1976, was the elimination of the
crowding problem. Rather than using a variety of
downhole delays, monotonocity was acheived by
separately sequencing groups of charges with uniform
downhole delays. A computer was built to perform
this for electrical delays, since sequential timers
were not then available and to permit sophisticated
control, but much tighter dispersions were available
from non-electric delays. Accordingly, standard
practice became the use of uniform shocktube
downhole delays, with uniform surface delays,
stacked as needed, for sequencing. These options
are not equivalent, since the firing stimulus is a
propagating wave for the latter. Both, however,
have the disadvantage that integrity must be
maintained until the firing stimulus has acted i.e.
lines must not be prematurely cut.
This constraint is unfortunate since it conditions
the length of the downhole delay, longer delays
typically exhibiting poorer precision. Ground
motions can extend surprisingly, for instance where
buckling of frozen soil is involved, and for
underground shots so can air blast and missile
damage. For the Lofreco Walking W approach, which
must also wait for adequate overburden motion, it
becomes critical and process limiting. It forms a
first order control on void percentage for the
finished retort bed, and is process limiting since
even the best product available is unusable beyond
600ms delay.
The Lofreco Walking W design comprizes an initiating
round, which fragments and forces an overburden into
upward motion, supplying void, and an adjacent
lateral extension, which redistributes the void with
further fragmentation. As seen in Figs, 9,10,
downhole delays are required for the initiating
round. These must be of sufficient duration to
extend beyond the firing times of the initiating
round precision charges and also the time required
for the timing wave to propagate into the lateral
extension to a point beyond the effective limit of
ground motion from the former, thus constraining
the designer to avoid any initiating round design
requiring extended precise timing. This curtails
choice, prevents bed optimization and acts to limit
the efficiency and effectiveness of overburden
lifting, but the situation then gets much worse.
The initiating round downhole delay must be
subtracted from that for the lateral extension to
give the delay between initiation of overburden
motion and use of the void space produced. Maximum
elevation, and hence void, occurs after 900-1200ms,
so less than half of that produced is useful. The
excess motion instead contributes to the serious
economic and environmental problems that Lofreco
faces from retort leakage. Further, this is process
and resource limiting, being one of the main reasons
why it has not yet been successfully demonstrated
with more than some 20 meters of uniform overburden.
113
lateral extension
initating ,/\
round y
Figure 9. Plan View - Lofreco Walking W
Zone of Edge Breakage Cutoffs
e Frozen Soil Bucklin;
Fired Initiating Round Below Rising Overburden Block
Figure 10. Side Elevation - Lofreco Walking W
serious with a separation of less than 3 standard
deviations, but are prima facie acceptable at 5.
On the face of it, this does'nt seem too bad, a
minimum of 25ms or so between rows, which hardly
seems restrictive. But this only considers the cap.
There must also be allowance for variance in the
initiating train leading to it, and factors, notably
temperature, which may affect it. (Further, the 7ms
quoted is not generally available, Winzer (1978)
found standard deviations exceeding 60ms for such
delays.) Sparse evidence suggests that delays in
actual rounds do suffer further variance. Sandia
National Laboratory instrumentation of Geokinetics
Retort 24 shot provided sufficient data to permit
such analysis. For that round single delays gave a
mean of 432ms and a standard deviation of 9.6ms.
This was marginally inadequate, so they were
doubled, to give a calculated mean of 426ms and 8ms
standard deviation. Analysis showed, neglecting one
long rogue, that the mean obtained but the standard
deviation deteriorated to 14ms. It is not, known
how much this may reflect error in observation, but
similar results were observed for R25, Figs. 11,12.
INTER-ROW EFFECTS
Why then, if this downhole delay is so critical, is
a longer one not used? The answer is that, while
accuracy problems are controled as above and Ensign
Bickford can supply extraordinarily precise delays
(6-7ms standard deviation) to this duration, nothing
longer is usable, due to a composition change. And
even this is no better than marginal. Consider a
rectilinear round shot row-wise, with 15 holes (same
delay period) per row. There are 14/15 chances that
the longest delay in this row is not positioned
directly in front of the shortest of the next row.
Multiplying the probabilities for successive rows of
a 10 row shot produces a probability of.5,
i.e. an
even bet. Snedecor's Rough Check (ly56) then
suggests a typical scatter range per row of 3.5
standard deviations. (Figures for 10 hole rows are
3 SD range and 2:1 probability in favor, for 25: 4
SD range and 2:1 against.) As might be expected,
actual designs require extensive study by Monte
Carlo methods rather than this cursory treatment,
with evaluation of consequences not just event
probability,however results from Lofreco studies do
surest that, as a rule of thumb, hazards become
Doubling the caps greatly reduces the misfire
probability, lowers the mean and reduces the scatter
somewhat (and has since been essayed by Los Alamos,
Schmitt (1985) for the same reasons) but this should
not be done without thoughtful consideration. It
also greatly increases the probability of gross
short firing. Winzer, investigating detonating cord
type delays, noted two gross short firings in some
400 caps. Geokinetics found a similar incidence for
the shocktube kind, implying about one per round
without doubling. For in situ rounds, this may have
far more serious consequences than a single late or
misfired hole. Lofreco experience was that solitary
misfires are not of great consequence unless in
critical positions, while zonal problems, as might
occur from a gross premature, usually are.
Assuming best available caps and similar field
deterioration (which may not be valid for electric
caps), we then require sometning like 40ms or more
between rows. Regarding the ideal inter-row delay,
we simply do not know. There has been little
research in this area and it tends to be conditioned
by factors other than fragmentation, e.g. allowance
for muck notion and, for Lofreco, driving overburden
114
FIRING TIME (ms)
Figure 11. Mean and Dispersion - Single Caps Test
Conditions, Double Caps in Actual Shot
O H
FIRING TIME (ms)
Figure 12. Cap Dispersion in Retort Shot -
Timing
MS Short of Planned (Note: Sinuousity
may indicate non-Gaussian data)
flexure. We do, however, know that it may of great
importance to retort beds. Bulk muck motion opens
primaryseparation planes
-
as transient planar
voids. If they attain larger dimensions than those
of the adjacent fragments, then rotations may occur,
with permanent propping and construction of a high
permeability feature in trie retort bed. This may be
used purposively, e.g. to bleed edge tonguing back
into the retort proper, or to speed renormalisation
of a front after bypassing bad breakage from a bad
hole. What we can't do, because of delay
imprecision, is stop this from happening by using
delays which are too short for significant facture
opening- 40ms represents nearly a meter of motion
for the specific charge used in Lofreco shots.
INTRA-ROW EFFECTS
It may be thought that row-wise shooting is an
unrealistic case anyway. As it happens, it is a
common component of commercial blast designs, though
often disguised as intra-row delaying. The Lofreco
Walking W, for instance, is a deeply dentate row and
best analysed as such. The complexities of such
geometries lie beyond the scope of this paper, but
the simple case of Fig. 13a suffices to illustrate
the general nature of delay variation problems.
Intra-row delay effects on fragmentation have been
studied at model scale, by Bergman (1974) and
Fourney (1979), and their results are consistent
with studies at full scale reported by Langefors
(1963). Summarized, and as illustrated in Figs.
14,15: From the Swedish field experience (which may
be biased towards massive hard rock) fragmentation
is optimized by a delay of 3-5ms per meter of
burden. For zero delay, a square pattern is
somewhat superior to a rectangular one with spacing
exceeding burden, but the latter beats on the former
by nearly a factor of 2 at optimum, for a total
improvement of about 3 compared to simultaneous
firing. For the square pattern, the curve of
fragmentation against scaled time is a sloped
sigmoid, but for a 2:1 spacing:burden ratio, the
sigmoid is very steep, implying great sensitivity at
timings of around lms/m. For layered formations,
the optimum is shorter and more pronounced, and the
penalty for seriously exceeding it may approach that
for simultaneous firing. Fragmentation has also
been observed (by boulder count) to pass through a
115
Figure 13. Various Shot Geometries, Firing
Sequences and their Effects on
Fragmentation and Permeability
oooooNOTIONAL SINGLE ROW
ALL HOLES SAME DELAY
Figure 13a.
Figure 13e.
Figure 13f.
-- -
Figure 13b.
NOTIONAL DELAYED ROW
Figure 13g.
Figure 13c.Figure 13h.
Figure 13d.
Figure 13i.
116
2 3
DELAY RATIO ( S / M OF SUROEN)
Figure 14. Effect of Time Delay on Fragmentation
(after Bergman et al. and Langefors)
HOMOGENEOUS
DELAY TIME
Figure 15. Effect of Time Delay on Fragmentation
(after Fourney et al. and Singh)
Considering a representative burden of 5 meters, an
effectively simultaneous shot thus requires a firing
precision of no worse than some 4ms between adjacent
charges, or around 2ms standard deviation. This is
feasible v/ith few delays, even selected product and
short period shocktube ones. Either the implementer
is constrained, possibly to heroic measures, or the
designer is denied a valuable tool which may be
needed to control excessive fragmentation. This is
a serious problem for the Rotem oil shales, Engleaian
(1985), and presumably for brittle shales generally.
It is also important for in situ generally, mostly
for its implications for local permeability control,
e.^. to provide a high permeability zone at an gas
inlet/outlet to reduce pumping losses from the
radial flow problem. It may also be important to
minimize flexure in moved masses, the preceding
explaining the field observation that extreme
precision is needed to acheive this.
In actual practice, of course, real delays scatter
so much that a notional row acts as an intra-row
delayed one, but is indeterminate. Continuing the
example and assuming a 10ms effective standard
deviation for the delays, most adjacent charges will
be sufficiently mutually delayed to act in the
region of efficient fragmentation, though for some
the interval will be too short. Necessarily, the
resulting retort bed must become heterogenous
regarding both fragment size and permeability.
Further, it is not only the firing interval that is
not determinate, firing order isn't either. Thus
instead of the theoretical even slabwise breakage of
Fig. 13b, primary breakage more as in 13c will
actually occur, or for a rectangular pattern,
primary breakage and permeability/fragmentation as
in 13d. There some charges break a triangular prism
of burden from a tight mass, some a parallelogram
section with some lateral muck motion component, and
some, alone or in cooperation, a trapezoid.
pronounced minimum, with changes in size dispersion,
prior to stabilizing to the semi-infinite delay
value. By increasing delay, the sequence was:
Coarse with some fines; even fine; coarse and fine
mix; even coarse; coarse and fine mix. Thus poor
fragmentation not only involves increased mean
fragment size, but also greater dispersion of sizes,
a serious disadvantage for retorting. For practical
rounds, the consequences of all this prove horrid.
The best that can be done is to use ragged rows,
either by alternating delays or by using a grossly
rectangular staggered pattern as in Fig. 13e,f. Use
of a square pattern improves gross uniformity, but
at the cost of reduced design flexibility, increased
mean fragment size and decreased fragment size
uniformity. Increasing the delay scatter can
essentially eliminate the short delay effect, but
may involve comparable problem from excessive delay,
117
courting the deadly danger of reversed row firing,
and exaggerating channel development at primary
failure planes. In short, since random factors are
involved, there is no way of predicting detailed bed
condition. This may be tolerable for rounds which
are large enough to treat statistically, but is
embarassing for smaller charge numbers and makes
nonsense of many modelling efforts.
The obvious move, towards rendering results at least
determinate, is to purposively control firing
sequence with intra-row delays. The idea is fine,
or would be but for delay scatter. In principle,
one may prepare retort beds as in Figs. 13g. (The
sequences must be as in Fig. 13h rather than 13i
which simply produces inclined rows.) This returns
one to the previous probleaa set, but with the added
complication that the inter-row delay becomes some
integer multiple of the intra-row delay. The delay
target then becomes the optimum 3-5ms/m interval, or
a 10ms wide time slot for our 5m burden. Even if we
had delays of 5ms standard deviation, (to bring some
2/3rds of our charges within this limit) there still
remains the problem of precisely positioning the
mean firing time, in this instance to 20ms.
Electric blasting caps are made to fixed intervals,
which will rarely permit such optimization.
Realizing that optimization is an unlikely goal with
available delays, one might be tempted to bite the
bullet and elect to use long enough delays to
unequivocally define the firing sequence. Though
not maximized, fragmentation is then comparatively
even from mucn exceeding optimum delay. The author
did just that for an early Lofreco retort, smugly
producing probably the best ordered retort bed ever,
and to his later chagrin, one of the poorer burning.
Analysis, particularly regarding off gas components,
strongly suggested that each successive burden prism
was systematically bypassed by a flame front which
proceeded largely via those beautifully defined
primary fracture planes. This danger is probably
less acute for very large retorts, because of scale
and their different burn characteristics, but it can
hardly be ignored. Particularly, it limits control
of the timing problem by use of tolerant geometries.
It night also be noted that the extended inter-row
delay caused undesirable side effects.
It may be concluded that the designerneeds delays
of 4ms or better standard deviation, with adjustable
means or choice by about 2ms increments. Where
ground motions permit, mean firing time control can
be by sequential timer or surface delays, and this
can be quite elegant. A varying burden, for
instance, can sometimes be matched by running a
timing wave downslope and then reversing it, such
that two delays are systematically separated by the
varying propagation delay from the detonating cord.
Propagation delay effects become quite important at
large scale, both statistically and absolutely. At
Lofreco commercial scale it takes 20ms for the wave
to traverse a row. The absolute figure matters to
photography, analysis, zigzag firing etc., and the
statistical implications to muck motion vector
trends, (e.g. Center initiation thus tends to pile
to the center and relieve the edge.)
THE SUB-MILLISECOND REGIME
Electrical delays of around 4ms precision have been
produced by Atlas, and in the New Technology series
by Du Pont, Schmitt (1985). Improvement to l-2ns
precision would permit some use of wave interaction.
Using alternate top and bottom initiation for
nominally simultaneous charges, for instance, would
then cause interaction somewhere at a scale of
around lOia. Order of magnitude better precision is
needed though, to permit a further quantum change in
fragmentation control, from stress wave and other
charge interaction. The time regime for this
derives from detonation or stress wave velocities.
The former roughly range over 4000-6500 m/s, so it
follows that for phenomena to be precise to about
lm, delays must be precise (but not necessarily
accurate) to 0.1ms or better. An unpublished study
by this author (1979) showed that this was
technically feasible with digital electronics, and
electronic delays are presently being studied by at
least Atlas and ICI.
The potential from such exotic blast designs appears
substantial, as has been noted by earlier workers
considering conventional blasting. Attention has
centered in three fields, to which the author has
added a fourth. Tension wave superposition is used
in military applications to produce planar failures,
118
and might so function in rock. Generalized stress
wave interaction seems more applicable and
promising, and is timing tolerant for large systems.
Fourney (1978) has shown the value of multiple
stress wave transitions to fragmentation processes,
but this may be confounded in practice. Notably
poor fragmentation results from simultaneous firing,
despite demonstrable wave interaction. The third
field involves superposition of actual detonations,
usually with associated stress wave superpositions.
The author tried this in 1976, to initiate and drive
a fracture from the center of a column charge
initiated at both ends. Striking local blasthole
enlargement was observed (by caliper and borescope),
corresponding to the expected zone of pressure
enhancement, and a fracture was indeed driven into
witness holes at that level. But while this has
merit, applications seem few.
New, and more generally useful to in situ blasting,
is a technique developed by the author, primarily to
control axial gas flows within blastholes. Such
flows present a problem in that the relict blasthole
may act as a large diameter vent, dumping gas which
should entrain into the muck to an adjacent pressure
sink. Adequate stemming to prevent this reduces
seriously the explosives capacity of the blasthole,
and simultaneously introduces a local zone of poor
fragmentation at the stemming position. This is
avoided by arranging that simultaneously applied
detonation heads compress and sinter a very short
stemming column, producing a high integrity plug
before the isostatic stressfield collapses. Applied
to the lowest portion of Lofreco blastholes, this
resulted in pronounced curvilinear fracturing in way
of the stemming, and open beddings propped with
comminuted material at the lowest levels of the
retort bed -
of obvious value for drainage and gas
flow, and indicative of previously absent motions.
For lack of appropriate delays, this was acheived by
the clumsy expedient of using detonating cord to
outrun the detonation in one column charge and so
precisely time the reverse firing, the so called
'Fast Leader Technique*, Britton (1984). As
implemented, the method suffers from difficulties
which limit its applications. Choice of explosive
is limited by need to provide speed differential,
its performance is likely degraded by the detonating
cord, there are dimensional limits and geometry is
inflexible .
An early Occidental VMIS retort design illustrates
potential from options presently denied the designer
through both lack of sub-millisecond delays and
suitable explosives. It used a vertical slot as
both free face and void source for two double rows
of peripherally positioned column charges. This was
much cheaper to prepare than later designs, but the
burn proved intolerable, being dominated by an axial
tongue, with subsiduary edge tonguing. The design
failed to solve two fundamental problems. First,
slabbed burden tends to fragment unevenly. There is
greater energy density adjacent the charge and in
the spall zone near the free face and fragments from
the latter have both enhanced velocity and great
freedom to rotate. Second, all muck motion was
inward, which tends to leave a planar void behind
the last firing charges.
Granted adequate delays, low detonation pressure and
dead pressing resistant explosive, one might instead
have proceeded as in Fig. 16, by initially
detatching and moving inwardly massive burden prisms
containing further charges. The charges could then
be fired at a time when the voids were distributed
evenly and sufficiently narrow to control rotations.
Fragmentation is then even and extremely efficient,
because of the much greater specific free face area,
wave interactions and avoidance of stress wave loss,
and, since final muck motion is outward, no edge
void develops. Permeability is then also even.
Parent Rock
Detatched Burden Prisms
Blasthole
Figure 16. An Hypothetical VMIS Shot Design
(plan view)
119
TEMPERATURE
One side benefit from a move to electronic delays
would be reduced temperature sensitivity. Thermal
coefficients seem not to be measured by makers of
delay caps, which is unfortunate, as the data has
considerable practical significance. From tests at
the Kamp Kerogen site on 400-600ms delays made by
Ensign Bickford, 0.8ms/C was adopted as a
representative figure. Rock temperature for this
site is around llc, presumptively ambient for
emplaced charges. Testing, however, more commonly
reflects noon air temperatures, say 32C for summer,
-18C for winter. This results in a 15ms shift in
mean firing time, or up to double the standard
deviation, which can hardly be ignored in design.
For testing, unless care is taken to ensure samples
are at thermal equilibrium and temperatures are
accurately recorded, one may primarily measure
temperature rather than delay variance, see Fig. 6.
Delays of different length may differ in thermal
coefficient, whether or not composition differs,
since the thermal masses are not the same. Further,
and somewhat surprisingly, variance may be affected
as well as mean, as shown by analysis of Winzer's
data on Du Pont delays. Cap characterization thus
requires sampling at differing temperatures to
establish the coefficient and thoughtful examination
of the data prior to any normalization to increase
sample size. One might be teaipted to avoid this
work, and reduce costs, by using an environmental
chamber set at rock ambient temperature, but the
implied assumption turns out to be questionable.
The explosive is not often loaded at the temperature
of the rock and may require significant time to
equilibriate. As an example, Ireco ships its nearly
saturated Ammonium Nitrate solution as hot as 70 C,
and cooling is presumably buffered by the phase
change as it fudges or crystallizes. As in Figs.
17,18, Ireco quotes an exponential cooling curve,
but field measurements at Kamp Kerogen, using
thermocouples, more suggested linear cooling. Close
examination of the data reveals an anomoly, in that
one would expect rapid and nearly linear heating of
the thermocouple on loading. The data for one unit
does not show this, suggesting that temperature
gradients are significant within the explosive mass.
Caution would then suggest that perhaps as much as
36 hours sleep time is needed to render thermal
effects negligible. This is not always available,
requiring planning of the loading sequence to
control effects from differential cooling.
TIME (HRS)
Figure 17. Cooling Curve of Ireco 1100 Explosive iti
a 310nm Blasthole (source Ireco)
,,
RETORT 23 HOLE :
130
F15:
L. 110F16 ;
"-*
ieo -
U 90/ x^^-^H^ ;
OL //~~"
30 I/~
~^^^irr-
70 / "~~^^--^~~~~^a ~~^"^ *~
ct*0
-
LU eo .
a.40
u 30 -
r-
28
10
;
0 12 3 4 5 6 11 12 13 14 15 16 17 18 11
ELAPSED HOURS
Figure 18. Cooling Curve of Ireco 1100 Explosive in
a 250mm Blasthole at Kamp Kerogen
EXPLOSIVES
It is not only in its effect on delays that hot
explosive causes problems. According to Ireco data,
their 1100 series material shrinks some 10/6 in
cooling. This causes too great a geometrical change
to be ignored for research, and probably also for
commercial in situ blasting, though it matters
little for conventional work. (This caused an
irritating and expensive rewrite to the CAD/CAM
program in use at Kamp Kerogen, as the original
could not be easily patcned.) One meter shrinkage
120
(from a 10m column) adds to downline tensions and
hence increased misfire incidence. Premature
cooling, however, causes more problems than excess
of heat. Chilling of the chemicals increases
viscosity, interferes with metering, mixing and
delivery, (not infrequently with hose rupture or
loss down the hole) and may cause fudging. Density
control suffers, with performance variability to the
point of unreliability. The logistics needed to
avoid this become formidable on remote sites and
with demand exceeding 100 tonnes/day.
The Ireco explosives typify truck mixed slurries and
well illustrate the effects which particular product
choices may have on blast design. Wet holes are a
common problem for in situ, and much of the reason
for accepting the expense of slurries, but their use
does not eliminate difficulty. A typical problem
occurs when explosive falls out as the delivery hose
is being raised from a completed load. Hitting the
surface of water displaced by the main charge, it
spreads to form a plug or cap, encapsulating the
water. This may support stemming for awhile, but
then rapidly subside, stressing downlines or,
presumably, tangling them with risk of misfire.
Their 1100 series is physically similar to a stiff
grease and dense, so does not suffer this problem
and will normally support stemming, but an attempt
to use inert decks in wet blastholes showed that,
while the material is waterproof in the sense of not
dissolving, it will disperse as globules in agitated
water, translocating and defeating stemming. One
must then either totally dewater, use an exotic stem
consist, abandon decking or use another explosive.
control over the retort bed condition by means of
explosives selection, even with the constraint of a
cylindrical blasthole. This is a most important
finding, since it becomes ever more clear that the
fragmentation step is the technical key to in situ
recovery. Put another way, if we fail to control
bed parameters in blasting, we do not have anything
like enough control to bail ourselves out during the
burn. Looking at the economics, fragmentationcosts
form only a small proportion of overall expense, so
a sharp increase, if it results in only a modest
improvement in recovery or reduction in retorting
cost, may well be worthwhile. We can probably
afford better materials. Our needs are certainly
delays precise to 4ms or better, possibly electronic
delays precise to 0.1ms, and explosives of unusually
low detonation velocity but with high energy and gas
volume .
REFERENCES
Bergman, O.R., Wu, F.C., and Edl, J.W., 1974, "Model
Rock Blasting Measures Effect of Delays and Hole
Patterns on Rock Fragmentation". Engineering and
Mining Journal.
Britton, K.R.C,, 1984, in "The Mechanics of Oil
Shale", ed. Smith, J.W. and Chong, K., Elsevier.
Englman, R., Jaeger, Z., and Slotky, D., 1985,
"Distribution of Fragments in a Series of Explosions
in the Rotem Oil Shale Fields -
Theory and
Experiment", Society of Explosives Engineers, 11th.
Annual Conference on Explosives and Blasting.
The 1100 series is offered in a range of specific
energies, but will only detonate at a rather high
velocity, which mismatches badly to the acoustic
impedance of high grade shales. Their 600 series
can be supplied in a similar specific energy range,
and can be formulated to much better match the
properties of high grade shales, but not lean ones.
Using both becomes a logistics problem since the
truck setups differ, and, as was found out on a
large shot at Kamp Kerogen, they are chemically
incompatible at their interface.
Large scale experiments using the range of
properties available in these Ireco product lines
proved that it is possible to exert significant
Fourney, W.L., and Barker, D.B., 1978, "Photoelastic
Investigation of Fragmentation Mechanisms, Parts
I&II". Report to NSF by Photomechanics Lab.,
University of Maryland.
Fourney, W.L., and Barker, D.B., 1979, "Effect of
Time Delay on Fragmentation in a Jointed Model".
Report to NSF by Photomechanics Lab., University of
Maryland .
Langfors, U., 1963, "The Modern Technique of Rock
Blasting", John Wiley, New York; Almqvist and
Wiksell, Stockholm.
121
Schmitt, G.C, and Dick, R.D., 1985, "Use of Corrtex
to Measure Explosive Performance and Stem Behavior
in Oil Shale Fragmentation Tests", Society of
Explosives Engineers, 11th. Annual Conference on
Explosives and Blasting.
Snedecor, G.W., 1956, "Statistical Methods", Iowa
State University Press, p.44.
Winzer, S.R., 1978, "The Firing Times of MS Delay
Blasting Caps and their Effect on Blasting
Performance", Report to NSF, Martin Marietta
Laboratories.
122