dtorage in excavetd rock caverns.pdf

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477 STORAGE INEXCAVATED ROCK CAVERNS

Rockstore 77Proceedingsof the irst nternational ymposium

Stockholm5-8 September 1977

Edited byMAGNUS BERGMAN

IN THREE VOLUMES

VOLUME

.sI:) 'i\S

PERGAMON PRESSOXFORD NEW YORK *TORONTO * SYDNEY * PARIS * FRANKFURT

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SMOOTH BLASTING FOR RELIABLEUNDERGROUND OPENINGSBert-Olof Svanholm*, Per-Anders Persson* and Bernt Larsson**Swedish Detonic ResearchFoundation,Stockholm, Sweden NitroNobelAB, Gyttorp, Sweden

We have gained experience in Sweden and abroad during the last decades on how to excavate economicallyunderground facilities that will function reliably for a long time. We have learnt by hard experience notto blast carelessly tunnels and storage rooms. We have come to realize that the remaining rock has to hetreated as a structural material in the underground building that is in effect created when an undergroundfacility is excavated. The blasting method has to be chosen with due respect to the predicted needs forreinforcement and support, the structural stability of the opening and the importance of undisturbedfunctioning of the facility. Careful smooth blasting orevents the early movement and loss of structuralstrength of the remaining rock, and greatly reduces the cost of the final support necessary. This papersurveys the modern tools for smooth blasting developed and extensively used in Sweden and abroad.Grace I l'expirience que nous avons acquise en Sulde et I lVetranger au courant de ces derniires dizainesd'annies nous sommes a mimes de rialiser des installations sousterraines assurant et une bonne Sconomie etune fonction satisfaisante pendant longtemps. L'experience nous a appris de nous mdfier de sautages nonprhm6ditls qui ne tiennent pas compte de ce que la montagne entourant la caviti ainsi crife est 3 considirercomme le materiel de construction pour lea installations. I1 faut donc choisir la mithode de sautage quipuisse satisfaire aux besoins de renforcement et de support et a la stabilite structure lle de la cavitiassurant ainsi un fonctionnement sans derangement de l'installation. La mithode de sautage de riglageempiche le diplacement initial, conserve lea forces structurelles de la montagne de l'entourage et riduitla nicessiti et lea frais d'un renforcement final. Ce papier donne une presentation des outils modernespour le sautage de riglage employis en Suede et ; litrranger.Wir haben in Schweden und im Auslande in den letzten Jahrzehnten grosse Erfahrung erreicht, wie Bergrkumefir Untergrundanlagen okonomisch und auf die Dauer zuverlgssig zu schaffen sind. Wir haben aus teurer Er-fabrung gelernt nachllssiges Sprengen, dass keine RUcksicht darauf nimmt dass der den Bergraum umgebendeBerg als Baumaterial fUr die Untergrundanlage zu behandeln ist, zu vermeiden. Die Sprengmethode soll somitmit RUcksicht auf den Bedarf an Verstarkung und StUtze, an StabilitAt des Bergraums und an der Stbrungs-freien Funktion der Anlage gewlhlt werden. Schonendes Sprengen verhindert die Initialbewegung des ge-bliebenen Berges und dessen Verlust an strukturaler StIrke. Es reduziert such erheblich die Kosten fUrweitere VersterkungsbedUrfnisse. Die vorliegende Arbeit gibt eine PrIsentation der modernen Werkzeuge furschonendes Sprengen die in Schweden und im Auslande in grossem Umfang zur Verwendung kommen.

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INTRODUCTIONUnderground construction in rock is a Swedishspeciality with its roots in the early miningindustry in the 15th century. The mining traditionhas been continued into our day, and LKAB's ironore mine in Kiruna in the north of Sweden is theworld's largest underground mine. In recent years,many large underground construction jobs have beencompleted in the competent Swedish rock for avariety of purposes, primarily for hydroelectricpower plants, military and civil defence in-stallations, and for storage (Figure 1 .The development of modern methods for such largescale excavation and construction in rock hasbeen, and is, a continuing joint effort involvingimportant sections of modern Swedish industry. Tobuild in rock has become a normal part of ourway of house building and construction for amodern industrial society.In 1975, 21 Mm3 of rock was excavated undergroundin Sweden. Out of this, no less than 5. Mm wastunnelling and about 2 Mm3 was underground

construction other than tunnelling. The totallength of tunnels excavated in 1975 was nearly 50miles, or 6 miles per million inhabitants(BrInnfors, 1977). The methods of careful blastingand particularly smooth blasting were used formore than 802 of the tunnelling and for practical-ly all of the other underground construction. Allcontour blasting was done using the Nitro NobelGURITY and NABI1eO special charge system.It is the purpose of this paper to present theengineering background and practical experiencewith these methods.The terms to describe different methods of re-ducing the damage effects of blasting have notbecome universally standardized, and one word isoften used by different authors to mean differentthings. To avoid misunderstandings, figure 2 givessome definitions of the terms used in this paper..

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574 B-0 Svanholm, P-A Persson and B. Larsson

Figure 1. An oil storage chamber under construction. The gallery and part of the horizontalhole bench is excavated. Drilling of the vertical hole bench has not yet started.

ADVERSEEFFECT

CAREFULBLASTING~~~~~~~~~~~~~~~~~~~~~~~~IsAIR WATER GROUND FLYROCK CONTOUR

BLAST SHOCK VIBRATION DAMAGEI /\NCAUTIOUS SMOOTH PRE-

BLASTING BLASTING SPLITTINGCONTOUR BLASTING

METHODTO AVOIDDAMAGE

Figure 2. Methods to prevent blasting damage. The term contour blasting is usedhere not only to describe blasting that produces a geometrically clean-cut contourbut also blasting that is controlled by the requirements of strength and stabilityof the remaining rock adjacent to the contour. It includes several methods otherthan smooth blasting and presplitting, but these have found limited practical useunderground and are not dealt with in this paper.

THE IMPORTANCE OF DRILLING minimizes overbreak and brings down the consump-tion of concrete for lining.

The quality, strength, and stability of an under-ground cavern is determined to a large extent bythe quality of drilling. The boreholes are drilledto get neither more nor less than the requiredweight of explosive inside the rock in such a waythat the explosive energy is distributed withinthe rock in a precisely predetermined manner. Astable and strong cavern cannot be produced witha large random scatter in borehole positioningand hole direction.

But the vibration and damage in the remaining rocaused by a given charge also depends on the easwith which it can break loose its burden (thefixation or constriction factor). Thus, hole devations and positioning errors that increase theburden or the degree of fixation (constriction)always make the charge produce more damage to thremaining rock than it would otherwise have done(Langefors Kihlstr~m, 1963). And this is truefor all holes in the blast, not only for thecontour holes.articularly important in this respect are the

contour holes. These should be drilled with assmall a look-out angle as possible and be keptas parallell with the axis of the tunnel as ispossible. This produces a high strength even arch,

Modern drill rigs are now increasingly being equped with devices for automatic look-out angle seting and with instruments for aiming the borehol

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Smooth blasting 575'in the correct direction. Equally important iscareful setting-out of the positions of the bore-holes. For checking the resulting tunnel profile,photo-sectioning that can now be made by a laserand a rotating prism or mirror is a useful method.BLASTING WITHOUT DAMAGEBlasting for underground construction purposes isa cutting tool, not a bombing operation. In theearly days of cavern-building for underground oilstorage it was thought that as long as the re-quired volume of rock was removed, it would notmatter greatly if some parts of the roof caved in.We now know better. The functional requirementsand the presence in the cavern of pumps, tubes,and perhaps heating systems make it imperativethat the cave contour is well defined and stable.The cavern also must be a safe place to work induring the time of construction. This makes itnecessary to plan and carry out the entire blasr-ing operation - gallery, first and second bench -carefully and in a controlled manner with respectto the strength and stability of the final walland roof. This does not mean necessarily moreexpense. The use of the minimum required chargeweights and accurate bore hole positioning oftenleads to considerable savings in the explosivesconsumption, less drilling, and better fragment-ation (Gustafsson, 1973).Ground vibrations and rock damage.

through a vibrational motion, the peak particlevelocity of which is v. In the sine-wave approxi-mation, and for an elastic material, the strain cand stress a are related to v in a simple way,E ' ONY ' v/C; C v (3.1)

where Y is the elastic modulus of the rock mass.Thus, for each kind of rock mass and each type ofwave, rock damage occurs at approximately a givencritical level of particle velocity.For predicting v, we make use of a relation be-tween v, the charge weight Q nd the distance Rto a single charge.

v - 700 Q0 7 /R1.5 (3.2)In equation (3.2), v is in mm/sec. Q in kg, andR in m. For an extended charge, we get a firstapproximation of the resulting v by integrating(3.2) with respect to the position along thecharge, neglecting the difference in arrivaltime of the elemental waves from different partsof the charge. The result is in reasonable agree-ment with observed velocities over a large rangeof distances and hole diameters (Persson, Holmbergand Persson, 1977).

The connection between ground vibrations and blast-ing damage to nearby constructions has long beenknown and damage criteria have been established.The same basic principles are applicable for esti-mating and predicting damage not only to nearbyalready completed caverns and tunnels in rock bu talso to avoid damage to the remaining rock adjac-ent to the new contour exposed by the blast itself.

C C/y - V/c - C v1 2 Rm n

Figure 3. The interrelation between strain(E), stress (a), and vibration velocity (v).c is the wave propagation velocity and Y isthe elastic modulus of the rock mass.

Fundamental to rock damage is stress, and stressis produced by the bending or stretching causedby the vibration of the rock. Figure 3 shows thesimplest case of blasting a charge near a flatrock surface. The sudden expansion of the chargeproduces a stress wave that propagates over thesurface like rings on the water. Downwards, thestress wave affects a semi-spherical volume asindicated in the figure. Each portion of the wavehas a characteristic propagation velocity c thatis some fraction of the sonic velocity which is amaterial property of the rock mass. As the wavepasses, each particle in the rock mass runs

Figure 4. Estimated vibration velocity asa function of distance for different linearcharge densities. Linear charge densitiesare given in kg Dynamex B per meter bore-hole, equivalent to 1.14 kg A 'FO per meter.

Figure 4 shows a diagram of v as a function of R,the perpendicular distance to an extended charge,with the linear charge density (charge weight pe runit length of borehole), A, as a parameter. Fora rock mass which gives incipient fracture at avibration velocity within the range 700-1000mm/sec, we find the radius of the zone of incipi-ent fracture to be about 0.25-0.35 m around a 3 mlong, 45 mm diameter borehole charged with a 17 mmGURIT charge. This charge has a linear chargedensity of 0.24 kg/m which is equivalent to


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576 B-0 Svanholm, P-A Persson and B. Larsson

0.20 kg ANF0/m.The fundamentals of-smooth blastin Eand

In smooth blasting and presplitting the linearcharge density in the contour boreholes is madevery low compared to that in ordinary blastholes.This gives a low initial borehole pressure. Inthis way the main result of the contour blastwhere the holes are allowed to cooperate is acrack that runs from borehole to borehole, andthe damage to the remaining rock is limited with-in a narrow aone close to the contour.In smooth blasting the contour charges are initi-ated last in the round. In presplitting, the con-tour charges are initiated before the rest of thecharges, mostly in a separate round. The crackrunning from hole to hole then has to be accommo-dated by elastic deformation of the rock on bothsides, because no rock is broken loose. Therefore,presplitting needs a closer spacing of contourholes, about 30-752 of that for smooth blasting,and preaplitting thus becomes more expensive(Figure 5). because of this, and because it isoften difficult to fit in an extra blastingoperation in the shiftcycle, presplitting is usedvery seldom underground in Sweden. Smooth blast-ing is the main method used, particularly intunnelling.

22 mm NABIT-. I-1

0.2 I n 17 mm GURy-t mm GURIT

20 40 6 80d mm

Figure 6. Empirical minimum required linearcharge concentration for smooth blasting andpresplitting as a function of hole diameter(full curve, f - ad where s - 90 kg/m 3) andrecommended practical hole diameter rangesfor NABIT and GURIT charges.

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Eur 1.0ICL;a

The ratio between hole spacingsmooth blasting should be keptThe normal recommendation is

E and burden V forwell below unity.

0.5

ENy S 0.8.In this way the damage to the remaining rock isminimized.For best results in smooth blasting, the chargesin the contour should be initiated simultaneouslyso that they can cooperate completely. However,because the extension of the crack between theholes involves the relatively slow dynamic motionof the considerable mass of material within thePRESPLITTING burden, the latitude for time statter in the timesof detonation is larger than might be expected.Table 1 shows the factors that determine the

20 40 60 80 initial borehole pressure, i.e. the pressure ofthe reaction products before they have done anyd mm work on the rock. For ease of understanding thed n table has been made for one drillhole diameter.Figure 5. Recommended ranges of hole spacingas a function of hole diameter for smoothblasting and presplittingSmooth blasting: E - kSd, where k5 - 15-16.Presplitting: E a k d, where kp - 8-12.

Figure 6 shows a curve of the minimum requiredlinear charge concentrations for smooth blastingand presplitting versus hole diameter, and thecorresponding recommended special charge systems.

Ideally, in homogeneous rock, smooth blastingcould be done equally easily with holes of anydiameter, by choosing the charge diameter and thehole spacing proportional to the hole diameter tokeep the initial borehole pressure and the re-sulting rock stress the same. If the boreholepressure were low enough, so that only cracksconnecting the holes were created, then the largediameter holes would give no more damage to theremaining rock than the small diameter holes.


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Smooth blasting 577

Table 1. Initial borehole pressures for different explosives and charge diameters.Explosive Dynamex a ANFO NAZIT CURITHole diameter, mm 45 45 45 45 45 45Charge diameter. m 3 45 40 32 45 22 17Explosive density, kg/dm 1.15 1.45 1.45 0.95 1.1 1.2Weight strength (ANFO a 1 1.14 1.14 1.14 1.0 1.09 0.83Linear charge concentration

real. kg/r 2.0 1.6 1.12 1.5 0.40 0.24kg ANFO/m 2.3 1.82 1.28 1.5 0.44 0.20Initial borehole pressurebar 55,000 32,000 13,000 21,000 3,300 900MPa 5,500 3,200 1,300 2,100 330 90

Table 2. Recommended smooth blasting parameters using the CURIT and WAbIT systems.Drill hole Charge concentration, Charge type vdiameter, sm kg ANFO/m and size Burden, m Hole spacing, m25-32 0.08 11 em CURIT 0.30-0.45 0.25-0.3525-48 0.20 17 mm CURIT 0.70-0.90 0.50-0.7051-64 0.44 22 mm NABIT 1.00-1.10 0.80-0.90

In real rock, there is always a given structureof already existing cracks, weak planes, flaws,and fissures. These are more'easily damaged thanthe homogeneous rock. In reality, therefore, thelarge hole diameter smooth blasting gives an in-ferior result and more damage to the remainingrock than the small hole diameter smooth blasting.Because of the rock structure, the large diametercharge creates damage within a large zone, where-as the small diameter charge has a very limiteddamage tone (Persson, 1973).SELECTING EXPLOSIVES AND DETONATORS.

diameter considerably larger than the charge(Johansson * Persson, 1970). Table 2 shows thefull-range of recommended drillhole diameters,burden, and spacing for the standard range ofspecial NABIT and CURIT charges. In the table,the charge concentration is given in equivalentweights of ANFO (Table 2 .Table 3 shows the characteristics of CURIT. Thisexplosive system has been used for more than 20years for a great variety of smooth blasting andpresplitting applications.It is very easy to ruin the results of a wellplanned smooth blasting by chargingthe rest of the holes in the round carelesslywith a high charge concentration. We can see fromfigure 4 that a 48 mm drill hole fully chargedwith ANFO with linear charge concentrationabout 1.8 kg/m gives a crack zone of more than1.5 m. The recommended burden for a 1? mm CURITcharge in a 48 em hole is 0.8-0.9 m (Table 2 .Obviously the row of holes next to the contourrow also has to be charged with a reduced chargeconcentration.

The choice of explosives for contour blasting isdetermined by the required strength, stability,and lifetime of the completed construction. Eventhough there is an increasing demand for carefulblasting of mining drifts because of laboursafety requirements, the lifetime of a miningdrift is limited. In a machine hall in an under-ground hydro-electric power plant the situationis different. There, the roof must be 1002 safeagainst falling rock for the very long lifetimeof the plant.In the former case it can be acceptable to use aromewhat stronger charge such as the 25 mm KABITin a borehole diameter down to 38 mm For theroof of the machine hall it is necessary to pre-scribe the use of 17 em GURIT and a hole diameterup to 48 mm. NABIT is a nitroglycerine sensitizedexplosive of a strength approximately equal tothat of ANFO; GURIT is also a nitroglycerineexplosive but with a deliberately low gas volumeand explosion energy.A great deal of effort has been put into the de-velopment of these special charges to ensure re-liable functioning of such a low energy linearcharge when it detonates in a borehole of a

Table 3. Functional characteristics of CURITDetonation velocity m sec M 4000Gas volume t KTP m /kg 420Working factor kJ/kg 3700Weight strength

relative to ANFO 0.83Charge diameter mm 11 17Charge concentration kg CURIT/m 0.11 0.24Charge concentration kg ANFO/m 0.08 0.20


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578 3-0 Svanholm, P-A Perston and B. Laruson

DetonatorsThe time interval between detonation of adjacentcontour charges is important for the result ofsmooth blasting and presplitting. It has beenfound by practical experimentation that the short-er the time interval the better the result willbe. This implies that the best result should beobtained if the contour holes were initiatedsimultaneously. This can be done in presplittingand also in smooth blasting if the contour holesare initiated separately in a special round. Bothof these methods are expensive and difficult tofit into a modern high productivity cycle ofoperation. More usual is to combine, as is donein many parts of the world, in tunnelling, themillisecond delay series of detonators with thehalf-second series. The half-second detonatorswill then be used for the smooth blasting row.Because the time scatter in the half-second delayseries is large 100-200 milliseconds) this meansthat the result of the smooth blasting is not asgood as it could be .A better result is obtained by using the NitroNobel VA-SYSTEM4 of detonators. This system hasdelay intervals of a length between the milli-second and the half-second series (Table 4). Theconsiderably smaller scatter of the 100 milli-second VA-MS series gives a better smooth blastingresult and also an increased advance per roundthan the half-second series.

Table 4. Detonators, interval numbers, andinterval times.IntervalType of Interval time Scatter

detonator No. millisec millisecVA-MS 0-20 25 5-10VA-MS 24-801) 100 20-50VA-HS 1-12 500 100-200NONEL 3-20 25 5-10NONEL 24-802) 100-150 20-501) This series has the numbers 24, 28, 32.80.

) This series has the numbers 24. 28, 32 .. 44,50, 56. 80 .

The advantage of electric firing is the controlledfiring sequence obtained by means of short perioddetonators and also the control of the instant ofinitiation. One disadvantage is, however, the riskof unintentional initiation through other sourcesof electricity. The VA-SYSTEM has a relativelyhigh built-in degree of safety, but there arestill numerous regulations to be satisfied and insome cases even the VA detonators are not allowed.Traditional non-electric firing systems do notalways provide the precision required for modernblasting technique.The new NONELM initiating system, however, is anon-electric initiating system that is an alter-native to high-energy electric detonators. It can

satisfy the same demands for precision timing andcontrolled blast initiation and it is used moreand more in underground applications. The basicfeature in this system is the invention of theNONEL tube, i.e. a small plastic tube transport-ing self-sustaining shock wave (Persson, 1967,1968, 1976).The Nitro Nobel NONEL detonators are available inany required length and the range of delays is30 periods, comprising 25, 100 and 150 u llisecintervals. The time scatter is kept within thesame narrow limits as for VA millisecond delaydetonators to suit tunnelling where very highquality smooth blasting is demanded (Table 4 .BLAST PLANNING. SUPERVISION AND CONTROLBecause the remaining rock is an integral andloadcarrying part of each underground construct-ion, it is necessary that the rock is left sintact as possible after the blasting operationsare completed. This necessitates careful planning,supervision, and control. Firstly, the rock sur-face to be exposed must be protected during theblast by a well-planned smooth blasting of thecontour and by designing the charge pattern ofthe adjacent row so that cracks spread no longerfrom this row than from the contour row.Secondly, even when the contour blasting has beensuccessful, the result can still be spoiled bylater rounds. In an oil storage chamber, forexample, an originally intact roof may be damagedby blasting the lower bench without due care tothe ground vibrations. A gallery roof in graniteshould not be exposed to a higher ground vibrationvelocity than 70-100 mm/sec to avoid such damage.Rock fall has occurred in the past in severalcases bringing a risk of injury to personnel anddamage to equipment. When the height of fall isperhaps 30 m, the consequences of such a fallcould be catastrophical.Many oil storage chambers are built today quiteclose to older chambers that are already full ofoil. In these cases blasting must be planned andexecuted with an eye to the possible effects inthe roofs of the older chambers due to the groundvibration. Another important factor is theaccompanying air blast that could damage theconcrete plugs.Rock falling from the ceiling of an oil-filledchamber can do considerable damage to the tubesystem and pumps. It is no easy task to take careof securing such a failed roof in a chamber fullof oil. By measuring and monitoring the groundvibration and air blast, the blasting can becontrolled and the risks brought down to a mini-mum

COSTThe innocent rock mass often gets the blame forinsufficient stability that is really due torough and careless blasting. Where no precautionshave been taken to avoid blasting d m ge noknowledge of the real stability of the undisturbedrock can be gained from looking at the remainingwall What one can see are the remains of whatcould have been a perfectly safe and stable rock


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Smooth blasting 579

wall, had the available techniques for contourblasting been applied. The damaged ceiling andwalls of an underground chamber in rock oftenneed n amount of support and reinforcement thatreflect the extent of blasting damage causedrather than the real available rock mass proper-ties.This can be seen particularly clearly in the caseof a low strength rock mass where a tunnel can begiven a sufficiently long stand-up time for safeexcavation work to continue, simply by carefulcontour blasting followed quickly by a lightshotcrete treatment to prevent the early motionthat is so damaging to the long term stability ofthe finished tunnel.When weighed against the very high cosr of adifferent driving method and more extensivesupport, reinforcements and safety measures, theactual cost of contour blasting is often negli-gible.The additional cost of contour blasting is pro-portional to the surface area of contour exposedby the blast, and it increases with increasingborehole diameter in the rest of the round. Seve-ral minor factors enter into this cost, such asthe seemingly higher price of the special chargescompared to the same weight of a standard explo-sive), the cost of extra detonators, and theextra work of loading a small quantity of explo-sive into a large number of holes. The main costfactor, however, is the extra drilling necessarybecause of the relatively small burdens that thereduced charges can remove.In the price level of 1975, this cost is of theorder of 10-20 Sw.Crs./m 2 contour area dependingon the borehole diameter in the main blast. Thehigher figure is characteristic of the largediameter boreholes that require considerable re-ductions of charge in one or more rows of holesadjacent to the contour row.When we consider that a normal oil storage cham-ber has 0.03-0.04 square meters of ceiling areaper cubic meter of volume, the additional costfor good contour blasting is a very small priceto pay for obtaining a strong, stable roof witha minimum of support and reinforcement, and re-liable functioning of the storage facility.

REFERENCES1. BRUNNFORS, S., 1977. Rock tunnelling in Sweden- from conception to a completed product.Proceedings, First US-Swedish Underground

Workshop: Mechanical Boring or Drill- andBlast Tunnelling? Stockholm, December 5-10,1976. Document D3:1977, Statens Rid forByggnadsforskning, Stockholi.

2. LANGETORS, U., and ICIHLSTRBIM B., 1963. TheModern Technique of Rock Blasting , AlmqvistA Wiksell, Stockholm, pp 41, 301.3. PERSSON, P.A., HOLMBERG, R., and PERSSON, G.,

1977. Careful Blasting of Slopes in Open PitMines. Swedish Detonic Research FoundationReport DS 1977:4 (in Swedish).4. GUSTAFSSON, R., 1973. Swedish BlastingTechnique , SPI, Gothenburg, Sweden.S. PERSSON, P.A., 1973. The Influence of Blastingon the Remaining Rock. Swedish Detonic ResearchFoundation, Report DS 1973%15 (in Swedish).6. JOHANSSON, C.H., and PERSSON, P.A., 1970.

Detonics of High Explosives , Academic Press,London & New York.7. PERSSON, P.A., 1967. Low Energy Fuse for Trans-mission or Initiation of Detonation. Swedish

Patent 333 321, July 20, 1967.8. PERSSON, P.A., 1968. Fuse for Transmission or

Initiation of Detonation. Swedish Patent356 500, July 15, 1968.

9. PERSSON, P.A., 1976. Nitro Nobel's NONELPSystem - a New Detonator System. Proceedings,Pyroteknikdagen 1975. Sektionen fdr Detonikoch FdrbrAnning, Sundbyberg, 1976 (in Swedish).

10. GUSTAFSSON, R., 1976. Smooth Blasting. Pro-ceedings, International Symposium Tunnelling76, London, March 1-5, 1976, Institute ofMining and Metallurgy, London (in print).

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