PRECISION BLASTING TECHNIQUES FOR AVALANCHE CONTROL

Kevin M Powell *Delta K Explosive Engineering Systems Ltd., Dunton Green, Kent, UK

ABSTRACT: Experimental firings sponsored by the Center For Snow Science at Alta, Utah havedemonstrated the potential of a unique prototype shaped charge device designed to stimulate snow packand ice. These studies, conducted against stable snow pack, demonstrated a fourfold increase in cratervolume yield and introduced a novel application of "shock tube" technology to facilitate position control,detonation and dud recovery of manually deployed charges. The extraordinary penetration capability ofthe shaped charge mechanism has been exploited in many non-military applications to meet a widerange of rapid piercing and/or cutting requirements. The broader exploitation of the potential of theshaped charge mechanism has nevertheless remained confined to defence based applications. In thestudies reported in this paper, the inimitable ability of the shaped charge mechanism to project shockenergy, or a liner material, into a highly focussed energetic stream has been applied uniquely to thestimulation of snow pack. Recent research and development work, conducted within the UK, hasresulted in the integration of shaped charge technology into a "common" Avalauncher and hand chargedevice. The potential of the "common" charge configuration and spooled shock tube fire and controlsystem to improve the safety and cost effectiveness of explosives used in avalanche control operationswas successfully demonstrated atAlta in March 2000. Future programmes of study will include focussedshock/blast mechanisms for suspended wire traverse techniques, application of the shaped chargemechanism to helibombing, and a feasibility study into the design and development of non-fragmentingshaped charge ammunition for military artillery gun systems.

KEYWORDS: Avalanche, Explosives, Blasting, Shaped Charges, Artillery.

1. INTRODUCTION

This paper presents the results of a series ofexperimental studies conducted over the last twoyears to investigate the potential of the shapedcharge mechanism to enhance the shock andblast· yield of the basic explosive charge usedextensively in avalanche control operations.

The shaped charge effect, in the form of anunlined cavity in an explosive charge, wasdiscovered in the late 1800s through miningactivities. Although there are various examples ofthe introduction of a metallic liner into the hollowcaVity throughout the early 1900s, the "lined"shaped charge effect and its extraordinarypenetration potential was not reported formallyuntil the 1930s. Military warhead designers soonrecognised the potential of the shaped charge inthe anti-armour role. The shaped charge hastherefore remained the principal warhead-----------------------------------------------

* Correspondence author address: Kevin MPowell, Delta K EES Ltd., 170 London Road,Dunton Green, Kent, United Kingdom, TN132TA;tel & fax:01732 779018; e-mail:Delta_K@Compuserve.com

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mechanism embodied within anti-tank mines andmissiles to date.

A shaped charge device can produce controllableand highly directional sources of energy that canbe tailored to produce a wide range of outputcharacteristics. These include: purelyfocussed/directed shock transmission with orwithout significant penetration effects; shallowplate perforation and/or punching operations;continuous linear effects for more complexcutting/severing applications; ejection of a solidflight stable projectile for accurate long rangeshock transmission, perforating or disruption andfinally deep narrow penetration. Potentialapplications of the shaped charge mechanismtherefore far exceed that of its penetrationcapability, for which it is more commonlyrecognised.

2. THE SHAPED CHARGE MECHANISM

The basic components of a typical cylindricalshaped charge device are shown in Figure 1a.The variant shown consists of a thin conical liner(usually a metal), a body containing a smallquantity of explosive, a means of initiation and a

(1)

V=600m/s

g(v- u? = Pt tl

where Pi is the density of the jet and PI is thedensity of the target. Now the penetration p isgiven by utf where tf is the time required toconsume the jet, which is simply l/(v - u). The totalpenetration is therefore:

V=8,OOOm/s

@ 30Ilsec @ 33 JlSec @ 40Ilsecafter initiation after initiation after initiation

Figure 1c: Final collapse of the shaped chargeliner and subsequent extension of the jetfollowing complete detonation of the explosive.

At an impact velocity of 7,800m/s the local loadsarising from the pressure created at the point ofimpact between the jet and target material greatlyexceed the mechanical properties of all knownmaterials. The response of materials under suchloading conditions is to behave hydrodynamicallyLe. as a fluid - Cook (1958). Consequently, if oneconsiders the simplified case of a jet of uniformlength I, with uniform velocity v, penetrating a

. target at uniform velocity u, given continuity ofpressure across the junction between the jet andtarget, Bernoulli's theorem gives:

Figure 1c shows a sequence of the .development at 30, 33 and 40j.ls after initiati~the explosive having fully detonated after 7Figure 1c also shows that, for the type of Sha:charge shown in Figure 1a, a velocity gradient Ofabout 6000m/s would exist between the tip of thejet and its tail. This gradient causes the jet tostretch whilst in forward motion until it eventuallybreaks up into a number of individual elementseach being capable of a specific amount of

. penetration into a given target medium. Theslower "Slug" separates from the tail of the jetafter approximately 60j.ls.

Detonationfront(V=7,800m/s)

Detonationtransfersystem

Conical shapedcharge liner(Copper)

Electricdetonator~

Explosivefilling

Emergingjet tip __----I

Figure 1a: Principal components of a conicalshaped charge

Figure 1b shows the progression of the linercollapse process resulting from a simple axialinitiation ofthe explosive filling. As the detonationfront passes over the liner at a velocity of about7,800m/s, the applied pressure, in excess of250kilobars, causes the liner material to flow as afluid towards the charge axis. As the linermaterial is steadily fed into the axis, uponcollision, a stagnation zone is formed about whichapproximately 1/3 of the liner material turns toform a high velocity "jer flowing along the axis,away from the point of initiation and the remaindergathers in the opposite direction to form a solid"slug". The slug is in effect the waste product ofthe liner collapse process. It travels at less than1/10th of the jet tip velocity, at about 800m/s,typically that of a rifle bullet.

Detonationgases

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system to transfer and control the detonationgeometry.

Figure 1b: Early stage of the liner collapseprocess 5 microseconds after initiation.

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Detonatingcord

90s Safetyr- Fuze

Char e

Slot cut insnow 1.2mdeep

Figure 3: Vertical section through a typicalborehole produced by a copper lined shapedcharge in stable snow pack.

Figure 2: Charge set-up for experimental firingsin stable snow pack.

snow pack targets, research studies wereconfined to stable snow pack located at the footof a basin area in Alta. The charges were firedhorizontally at a depth of 1.2m below the surfaceas shown in Figure 2.

Early work studied and characterised theresponse of snow pack to jets produced byclassical high precision metallic shaped chargeliners. A typical result of a borehole produced bya copper lined variant is shown in Figure 3. Notethe relatively deep penetration of some 5m inlength, vertically above which is a significant levelof heave generated by the passage of the jet.

(2)p=l

In order to obtain detailed, non-volatile, data from

3. EXPERIMENTAL ICE AND SNOW PACKTARGETS

In general, the majority of shaped chargeapplications tend to exploit the extraordinarypenetration potential of long narrow jets.However, the high kinetic energy packaged in theshaped charge jet also offers the potential tostimulate a range of secondary reactions which donot necessarily have to be accompanied bysignificant levels of penetration. This potential togenerate a highly energetic environment, purelyfrom the kinetics of particle interaction, is the rootof applications presented in this paper. It is,f~rthermore, a simple step to consider bringing awide range of potentially reactive materialstogether, which in normal circumstances wouldremain relatively inert.

However, the response of the above two targetmaterials to penetration by a shaped charge jetwill be very different. Since the tensile strengthand ductility of concrete is significantly lower thanthat of the aluminium alloy, unless the concretetarget is relatively massive or heavily confined,there will be a tendency for the high internalpressures and associated compressiveshockwave to cause a catastrophic and explosivedisassociation of the target. This is particularlytrue of an ice target. Consequently, with anappropriate layout of shaped charges it is possibleto spal or scab a free surface. The mechanismand effect is therefore similar to that of benchblasting used by the quarrying industry - butwithout the need to drill and pack explosives!

For practical purposes the length of thepenetrating jet is normally determined from flashradiographic diagnostic techniques.

Equations (1) and (2) represent an idealisedmodel of the penetration process where the "rootdensity" relationship of equation (2) dominates thepenetration process. The peculiar consequencesof this relationship are that for a given length of jetand jet material, the penetration into two materialsof similar density but otherwise dissimilarmechanical properties (typically a structuralaluminium alloy and concrete) will be similar.

To fully characterise the response of the fullspectrum of snow pack states, ranging from freshsnow to near solid ice, typical of cornice build-ups,it was necessary to conduct tests against icetargets.

Unconfined ice was expected to have a violentresponse to shaped charge attack and, althoughinducing such an "explosive" response was oneof the main objectives of these studies, onceagain, a non-volatile target was required to enablepenetration data to be recovered. Manydifficulties were experienced finding massive icetargets suitable for this work, which resulted in thedesign of a fabricated configuration. Thisconsisted of a stack of water ice blocks,surrounded by a stack of steel confinementcylinders as shown in Figure 4.

During construction the ice blocks were packedout w:>h sand and after firing the steel cylinderswere progressively removed from the top of thetarget to allow the ice blocks to be sectioned toreveal the borehole profile.

Figure 4: Experimental ice target beingsectioned to reveal the borehole produced by acopper shaped charger liner.

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4. THE DESIGN OF SHAPED CHARGES FORAVALANCHE CONTROL APPLICATIONS

The most effective way of cOupling andtransferring energy into a target medium via asupersonic impact is to improve the impedancematch between the target and impacting mediumIn practice this comes down to attempting t~

mat~h de~sities. F~r ~imple shaped chargedesigns With metallic liners, magnesium andaluminium offer the most practicable solutionagainst ice and snow.

However, a jet produced by solid metallic linerstends to have a narrow cross sectional area andconsequently a small area of intimate contactwiththe target medium. This tends to produce highpenetration with low diametric effects and this issubstantiated by the results of early stUdiesresults from which have been shown in Figures:3and 4. The design objectives to defeat ice andsnow pack were to induce two principal effects:firstly, a significant increase in the effectivesurface area of the jet material exposed to theenvironment within the target dUring thepenetration process, and secondly, attack of alarger cross sectional area of the target medium.

To meet these requirements, two basic shapedcharge configurations were tested, one with aconical liner geometry and one with a morecomplex cylindrical liner geometry. These werefilled with C4 explosive and designated Type 1and Type 3 respectively. The performance ofthese charges was compared directly to a controlset of 4 lightly cased blast charges eachcontaining an explosive content of 1kg.

Charge Types 2 and 4 consisted of smaller pairsof charges with geometries identical to those ofTypes 1 & 3 respectively. In each case the totalexplosive content for the paired charges was 1kgof C4 and the charges were arranged such thatthe jets produced by each pair would collide onthe charge axis after detonation. The generalarrangement for this paired configuration isshown in Figure 5.

The explosive compositions used in the fourcontrol blast charges consisted of: Pentolite, inthe form of a standard Avalauncher round(without tail fin); Trigran, a prilled aluminised TNTbased Canadian military explosive; C4, an RDXbased military plastic explosive and ANFO, acommon commercial blasting composition.

These compositions were chosen to introduce anarray of detonation output chara~teristics rang~ng

between high shock pressure with short durationto low detonation pressure with long duration.Typically, the short duration, high shock

Initiationterminationblock

(Trigran and ANFO). The crater profilesproduced for these 4 control charges are shownon the left side of Figure 6. It is immediatelyapparent that Trigran produced the highest cratervolume of 5.2m3

, followed by C4 at 4.6m3, ANFO

at 3.2m3. and Pentolite at 2.7m3

.

Trigran is a powerful Canadian blast composition,specifically formulated for battlefield engineeringapplications. Trigran therefore represents theupper limits of what could be achieved withmodem military explosive compositions, should itbecome commercially available. Note that theopen mouth of the crater suggests early ventingand that it may have performed better if placed alittle deeper.

Charge 1 Charge 2Conlrol - 1Kg Penlolile Experimental Charge Type 1 - 1Kg C4

Jet collision on axisFigure 5: Paired charge configurationcomprising 2 scaled down charges (Type 1shown) co-axially arranged such that the jetscollide upon simultaneous charge detonation.

compositions are used· for fracturing i.e.fragmentation effects, and the compositions,producing longer duration lower shock pressures,are used for pushing and heaving i.e. trenching.

5. RESULTS FROM EXPERIMENTAL FIRINGS

The cross section of the crater profiles producedby the 4 control blast charges and 4 experimentalshaped charge configurations are showncombined in Figure 6.

It is important to recognise that these were farfrom an exhaustive series of tests in terms ofabsolute performance of each composition ordevice. Indeed only one shot of each type wasfired. The objective of the study was to determinethe cratering characteristics of a group of simple"bare" charges with differing detonation pressuresand power. These could then act as a broadbaseline against which the benefits of variousshaped charge configurations could bedemonstrated.

Forthe baseline "bare" charges, two compositionswith high detonation pressure (C4 and Pentolite)were chosen for broad comparison with twocompositions of a lower detonation pressure

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Figure 6: Comparison of crater profiles for blastcharge and experimental shaped chargesconfigurations.

C4 was probably the most energetic of the fourcompositions tested. With a high detonationpressure, it would not necessarily produce thebest cratering effects since snow pack is aneffective shock attenuator. However, the parallelsides of the C4 crater indicate that the 1.2memplacement depth was optimum for 1kg.

ANFO is not a particularly powerful compositionand is particularly difficult to detonate effectively

in small quantities. The smaller crater volume istherefore consistent with expectation.

The behaviour of the Pentolite charge (in the formofa standard Avalauncher round) was anomalous.It is assumed that the composition was SO/50PETNITNT which should have produced arelatively high detonation pressure and powerfactor, just a little lower than that of C4.

The crater profiles for the four experimentalshaped charge configurations tested are shownon the right hand side of Figure 6. Charge Type 1shows a remarkable increase in the crater volume,rising from 4.6m3 for the C4 control shot to 11.9m3

for the Type 1 charge. This result confirms theappro,ach taken towards optimising the linerdesign and clearly demonstrates the contributionmade by the shaped charge to the bare C4explosive equivalent of 1kg.

The results of the Type 3 charge also indicate thata significant increase in crater volume from 4.6m3

for the C4 control shot to 9.3m3 was produced bythe cylindrical liner configuration. The mostsignificant additional effect noted for this chargewas the unusually high ground shockaccompanying the firing which was commentedon by a number of spectators.

The Type 2 and Type 4 shots both producedvolumes that were marginally higher than the C4control shot but did not produce a yield greaterthan their single counterparts (Types 1 & 3respectively). Similar work conducted by theauthor for other applications has demonstratedthat there are considerable benefits to be gainedfrom the effects of colliding jets, so backgroundresearch work will continue in this area.

In summary, the results from the 1999 firingsclearly demonstrated the potential of the shapedcharge mechanism and established thefoundations of a database to support futurestudies. The unexpected performance ofthe Type1 shaped charge has also provided a firmfoundation for early product development.

6. A SHAPED CHARGE DEVICE FORAVALAUNCHER AND HAND CHARGEOPERATIONS

During the summer of 1999 an intensive period ofexperimental studies, conducted in the UK,concentrated on the feasibility of integrating the

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shaped charge effect into an Avalaunchprojectile. However, the many idiosynCrasies:the existing Avalauncher gun system, particUlarlywith respect to the ammunition, necessitated acomprehensive review of the existing system inorder to enable the programme to proceed. Thisalso provided an insight into the maximumperformance limits that could be introduced Withinthe current cost effectiveness envelope of thesystem.

Following extensive dynamic characterisation ofthe Avalauncher gun and ammunitionappropriate interim modifications were made, anda series of inert filled prototype shaped chargeprojectiles was successfully tested, With theexisting basic fin design. A significant feature ofthe new Avalauncher charges was that they weredesigned as part of an integrated systemproviding a "common" charge configuration thatcould be used for both Avalauncher and handcharge operations.

As part of this integrated system, a noveltechnique for controlling, retaining and firing handcharges was also introduced. This consisted ofathin fibreglass tube containing a coil of non­electric initiating line (generically referred to asshock tube). The coil of shock tube can either befitted to the charge or attached to the operator sothat when the charge is either thrown, launchedor released, the spool pays out in an orderlyfashion.

Figure 7 shows an example of the use of a shocktube spool with a standard 090 booster charge.The high strength of the shock tube (40kg deadload) allows the charge to be either re-positionedor suspended, typically over either a cable-line orcornice. The operator then has absolute controlover"instant fire" or "veto", using a hand held fireunit and, if necessary, subsequent recovery ofthecharge.

Pyrotechnic Safety Fuze is used extensively inthe US and Canada for detonating hand charges.although the use of this particular methodremains under review. Electronic time delays areprobably the most convenient replacement forSafety Fuze but even the military demands forsimilar reliable miniature time fuzes have metwith a number of fundamental safety andreliability problems. In the meantime the spooledshock tube option offers a practicable interimsolution that ensures a high degree of safety.

r

Figure 7: Section through a 090 booster (handcharge) fitted with a spool canister carrying23m of shock tube.

comprehensive control ov~r positioning andorientation of the charge: WIth delay.detonators,the benefits of accurate ripple delay firing of a pre-repared charge sequence can also be exploited.

~igure 8 shows the above 090 charge in mid-flightafter being launched by hand. The shock tubetether can be seen paying out from the spoolcannister, in this case, attached to the charge.

clearly visible surface "heave" and drop, effectiveover many metres about the penetration axis ofthe jet.

Figure 8: Deployment of hand charge shoWingshock tube paying out from a spool housingattached to the charge.

The most spectacular effects obtained from thenew charges were the 4 fired against establishedcornice build-ups. In one area many Pentoliteboosters had been fired to try to dislodge thecornice without success. However, a singleshaped charge brought the full length of thecornice down. Since it was not clear if the cornicehad been weakened by previous work, the sametests were repeated at other sites with equallyspectacular results.

The twenty prototype charges fired at Altaresulted in a spectacular and convincing displayof the versatility of the shaped chargemechanism. The success of the technologydemonstrated to date has prompted a jointfunding initiative from a number of West CoastDOTs to provide a sustained period of funding forfurther work.

7. CURRENT PROGRAMME

090Charge

Coiled,....--- Shock

Spool

Following the inert dynamic tests conducted in theUK, twenty of the new charges were tested withlive explosive fillings in Alta in March 2000. Ten ofthe new charges were assigned to hand chargeoperations and the remainder were fired from anAvalauncher gas gun. Funding constraintsprecluded re-design of the fin component, sothese firings were conducted using the existing findesign. This combination was not suitedaerodynamically, consequently accuracy and flightstability remained poor. An optical instrumentationtechnique was employed to determine muzzlevelocity and this was correlated with anelectronically monitored chamber pressure. All ofthe new charges behaved and performed asexpected although particularly stable conditionsdid not result in any releases that day.

Of the ten hand charges, six were deployed invarious surface, sub-surface and air burstpositions and compared directly with standard 21bPentolite boosters. The effects of the shapedcharge were visually spectacular and producedsignificantly greater sonic report. "Bamboo shots"arranged to fire at 90 degrees to the fall line withabout 1m separation produced both a significantincrease in sonic reports, accompanied by a

Between one and two thousand pre-productionrounds will be prepared for evaluation over the2000/2001 season to be fired under controlledconditions at a number of sites. Approximatelyhalf will be assigned to Avalauncher operations.and these will embody the fin improvementsarising from the latest (October 2000) UK firingprogramme to include: a redesigned "one piece"all plastic baseplate and draw pin assembly; a

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more robust fin/initiator design more compatibleaerodynamically with the new forebody; animproved safety pin arrangement that links thebaseplate to a fin vane. These features willimprove performance within the currentAvalauncher gun system and will primarilyintroduce the robustness necessary to supportplanned enhancements to future Avalauncher gunsystems.

Principal design features of the new chargedesigned for dual Avalauncher and Hand Chargeoperations are shown in Figure 9.

up to 20 % of AI. powder) would significantlyenhance blast yield from pellets 3, 4, 5 & 6 butpellets 1 & 2 would be a high density HMX a~dJoRDXlwax composition, better suited to the shap~charge mechanism.

The barrier shapes the geometry of the detonationfront and influences the way in which the shalledcharge liner collapses. Different effects can beboth introduced and controlled by altering thebarrier geometry. The introduction of a separatepellet (Pellet 2) containing this feature allowsconsiderable latitude to accommodate variousbarrier geometries.

Figure 9: Section through the Mk I AvalancheControl Charge.

The main body consists of an injection mouldedpolypropylene body and nacelle which firmly cliptogether via the joint ferrule, without the use ofscrews or rivets. The Joint Ferrule also retains theLiner and HE pellets within the Body component.

The hollow Nacelle provides aerodynamicstreamlining and stand off between the mouth ofthe shaped charge liner and target material andallows alternative Nacelle configurations to beintroduced to control the detonation delay time insoft snow pack.

This configuration has been selected to prOVide arobust research vehicle to support future researchstudies and a build standard upon which earlydevelopment of a commercially available productcan be based with the flexibility necessary to meetworldwide requirements.

In addition to the planned pre-productionassessment in Alta March 2001, a broad range ofvariants of the Mk I configuration will also betested against stable snow pack. Whenappropriate levels of funding are secured, R&Dwork will also commence on shaped chargesystems for' helibombing and a review of thefeasibility of introducing polymeric based shapedcharge replacement round for military artillery gunsystems.

Cook, M. A, 1958. The Science of HighExplosives. - Reinhold 1958.

8. REFERENCES

DetonatoWell

WaveShaping

Liner Banier

The Liner is pressed from aluminium powderbound with wax. This facilitates cost effectiveproduction and allows alternative linercompositions to be introduced to cater for specialconditions. Different liner geometries can readilybe introduced into the "Pellet 1 zone".

The explosive charge consists of six pre-pressedpellets. These allow alternative explosivecompositions to be introduced to meet specificrequirements. Typically, aluminised (addition of

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