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