US2609750A - Process of blasting - Google Patents

Description

p 1952 D. M. M FARLAND PROCESS OF BLASTING Filed May 16, 1946 Fig. 5-

David M. McFarland,

IN VEN TOR. fizz 57% Fig.2

reduced vibration conditions.

Patented Sept. 9, 1952 PROCESS OF BLASTING David M. McFarland. West Ghester, Pa., assignor to Atlas Powder Company, Wilmington, Del., a

corporation of Delaware Application May 16, 194 Serial No. 670,193

' "'14 iGlaims.

' *frhe present invention relates to blasting conditions of backbreak,

- A-iurth'er object of the invention is the provision of a method of blastingproducti-ve of "Still another object of the present invention "is--the provision of a -method of blasting pro- "ductive of lessened throw of 'brokenmaterial.

'Qther'objec'ts oiithe invention will be apparent fromthefollowi ng description. 7 4 v I *Blasti-ng operations of any considerable size, -*snchas,"for example, quarrying-{or the fproductioniof stone suitable for crushing,- are usually cohductedby detonating explosive charges in a enmity-peter; holes drilled in -a row behind and generallyparallelto a iace. The explosive charges in the individual bore holes are conventionally piactically simultaneously-initiated by such means -aselectric blasting caps-or detonati-ng fuses. The-blast pushes out the burden in front "oi-the holes" creating a i new race along the l-ineo'f bore holes.

It is desirable that the blast break the blasted material into sizes whiehperm'it ready removal "by power shovels, drag-lines, or-other loading 'equipmen-t. Usually,'howeven-customary blasting operations, such as have been just described, res'ulti'na throwing out of much of the burden "in large"solid blockswhich cannot beadequa'tely handled but which :must be individually further drilled, charged "with explosives, and blasted to break them up into manageable sizes (secondary blasting). i

dinotherdisadvantage which commonly results from-the customary methodof blasting described above istheioccurrence of a large amount of backbreak. Backbreak is the formation of an uneven -face and of large crevices which extend into the newly created face. "Backbreak is an tioning, drilling, and loading of a new line of bore holes. Third, crevices permit wastage of explosives in subsequent blasts due to premature escape of gases through the crevices without effective action on the burden;

Customaryblasting technique is productive of another serious ,difiiculty in the'vi-brations produced. Many quarries and other blasting sites are located near inhabited or manufacturing areas, and the vibration waves set up by blasting are blamed for much damage to structures and equipment. To prevent complaints, many quarries must seriously limit the size and number of bore holes in each blast despite the fact that for best blasting efficiency a large number of bore holes in a blast is desirable.

A further dilhculty often encountered in blasting operations is excessive throwing of the blasted material. Undue throw often covers tracks used for haulage or damages equipment, and in spreading the blasted material over too large an area, makes its subsequent handling slow and expensive.

According to the present invention, a method is provided by which the above-mentioned disadvantages are ;-in a large measure avoided.

It has been found that if, in ablastof a row of at least threebore holes aligned'to break through a face and -f orm a new face therebehind, the explosive charges in adjacent bore holes are detonated at slightly difiering times the action of the blast is usually radically altered with the production of much improved breakage, greatly reduced backbreak, a most considerable diminution of "vibrational efiects,-and much reduced throw. v

Firing time separations should not be sumcientlygreattha-t the unity of the blast and the well known; desirable actionof multiple hole blasts on the burden are destroyed, and separations which are productive of optimum results vary somewhat with the nature of the material being blasted. In general, separations as high as about 150 milliseconds and as low as about '2 milliseconds produce improved'results, but it is usually preferred that the amount of separation lie'between about 204*millisec0nds and about 75' milliseconds.

As will hereinafter be more fully described, the firing'timeseparaticns employed in accordance withthe present invention may be obtained by a number of diiferent means, as for example, by the use of electric blasting caps havingljdifferent firing times, electric blasting caps connected'to difierent circuits which are energized .the. explosive charges in adjacent holes. usually-preferred, however, that-all holes in a first firing bore hole.

after the actuation, in each hole of the first group, of the detonation initiator therein. For example, the bore holes of a first group may advantageously be initiated by means ofinstantaneous electric blasting caps in a single circuit,

while the bore holes of a second group, each of which is in alternating position with a bore hole of the first group, are then initiated at a time or times slightly separated from and after the time of initiation of the bore holes of the first group by means of short delay electric blasting caps connected in the same circuit as the instantaneous electric blasting cap utilized to initiate-the bore holes of the firstgroup. In this manner all of the explosives in theblast may be detonated within a very short time period, as from about 2 to about 150 milliseconds and preferably from about to about "75 milliseconds,

and firing time separations of the same order between the initiation of the explosive charge in each bore hole and the initiation of the charge or charges in the hole or holes adjacent to'it may be positively obtained.

Since it is chiefly important that firing of adjacent holes be separated by times within the effective range, desirable blasting effects may also be obtained without the alternating scheme just describ ed. Thus a blast according to the invention is conducted when a second'hole is fired after a first hole and a third hole is fired after a second hole, provided that the difference in firing-time between adjacent holes is within the effective range. The difierence in firing time between the first and third holes may be outside the effective range. e

Moreovenas it is the time difierential between adjacent holes that is'important, it can be seen .that it is not necessary to fire any hole at substantially zero time after the actuation, in each ,hole', of the detonation initiator therein. Thus, where the bore holes are grouped and initiated in alternating relation, as described above, there -may be a time interval or delay between'the initiation of the blasting caps relied upon to initiate the bore holes of the first group and their actual initiation of the bore holes, provided that the time interval or delay between the initiation of theblasting caps relied upon to initiate the bore holes of the second group is longer and provides the proper differential between the initiation of It is blast which are fired in accordance with the process of the present invention be fired within as short a time as is practicably consonant with the obtention of efiective time separations between adjacent holes.

ing difiering firing times it is desirable that the ing to the present invention may be obtained in any convenient manner, it is usually preferred to obtain them by means of delay electric blasting caps having particularly short delay periods not greater than a few tenths ofa second and 4 preferably not greater than about 3001 millisecf onds.

Delay electric blasting caps, as is well known in the art, are blasting caps which fire a de- 1 layed period after the application of current to I them. Most delay" electric blasting caps obtain their delay period by means of a fuse train 10- cated between the ignition material and the primary detonating composition in the cap, and the delay time is determined by the burning time of the fuse train.

While particular short delay electric blasting caps form no part of the present inventionit may be explained that a preferred short'delay :electric blasting cap may be provided by interposiiig V a fuse element, or delay train, made up of a metal tube into which is pressed a'charge of black powder, between an initiating means such as an electric match and a primary detonating explosive in the cap. When the delay train is made'of a metal tube 0.140 inch in diameter and T's inch long into which 30 to;96 mesh granular black powder is pressed under about ,450 pounds to a height of about 0.068, inch, a delay period averagingabout 40 milliseconds with a standard deviation (the square root of the sum of the firing time deviations of a number of caps from the average delay period of the caps divided by the number of caps i. e. the .rootfmean square deviation) therefrom of about 16 milliseconds is obtained. Caps of this type will hereinafter be designated as A type 'short delay electric blasting caps. When somewhat more black powder is employed so that the height of; the black .powder charge, is about 0.133 inch and when an electric match is spaced about inch above .the metal delay tube, a short delay electric blast- "called B type short delay electricblasting caps.

The shortdelay electric blastingcaps just described are carefully sealed and are not vented as is customary in conventional delay electric blasting caps of longer delay periods employing, black powder delay trains, for thebuilding-up of pressure in the caps is generally important towthe development of high burning rates; 7, .1, f v Delay electric blasting caps'applicableiorzuse in the present invention are more specifically: described and claimed in co-pending application, Serial No. 682,995, filed July 12, 1946, byJohn E. Jefiery for Delay Electric Blasting Caps;

It is to be particularly noted that shortdelay electric blasting caps as described above normally vary somewhat-in delay period. The variation in timing is in each case: within the effective range of time separation required for the present inven,- tion. It can be shownbyconventional statistical methods that when'caps having a standard deviation of 16 milliseconds, such as the A or B-caps noted ab ove, are distributed at "random among a series of bore holes, %,of the-adjacentholes will be expected, to fire with time separationswithin the limits of about 3 1nilliseconds andabput 37 milliseconds ee simpps Engineers Manual of Statistical Methods, 1941., John Wylie & Sons, Inc., New York, especially chapter XII and appendixC). Hence the invention may be practice'd by initiating a row of holes by means of a number of similarly prepared shortdelay electric blasting caps when the individual variations in firing times among the caps are adequate to provide thedesirable effects. i I

Another method by which the present invention may be employed makes use of ordinary instantaneous electric blasting caps connected to different circuits, the flow of current in which is initiated at spaced times by electrical or mechanical means which close the different circuits a short time apart. A multiple switch blasting machine is an example of a firing means which may be used when the invention is performed inthis manner. I

The multiple circuit method of obtaining the delay means, as just described, is not preferred for use in some formations where the firing of the holes in the first circuit may lead to the breaking of the wires of the second circuit before that circuit is energized, 'for'under such conditions misfires in the second circuit may wellre sult; Delay electric blasting capsarenot subject to this disadvantage since they may all be initiated at the same time when connected in single circuit. l

A further manner in which the present invention maybe practiced is by the use of detonating fuse. Normally when detonating-fuse is used, a trunk line of detonating fuse is connected to branch lines which run to individual bore holes. By varying among the different'holes the length of fuse extending from a bore hole to a trunk, thertime of initiationv of difierent holes may be easily varied. to provide the desirable effects of the present invention. However, since detonating fuse detonates at an exceedingly high velocity, the obtention of the desired variation in firing times requires the use of very considerable-lengths of fuse, and the practice of the invention by this last-mentioned means is not usually preferred. v

Another manner in which detonating fuse may be used to produce the effects of the present invention in a substantial way is by initiating a long row of bore holes by means of two trunk lines of detonating fuse. One trunk line may be connected to branch lines leading to one group of alternating holes, and the other trunk line may be connected to other branch lines leading to the other group of alternating holes. The trunk lines may be then initiated from opposite ends of the row. It may be seen that in this way the greater number of the adjacent holes are detonated at separated times. This same general method may be performed with a single blasting cap if the trunk line of detonating fuse detonated by it is joined to the other trunk line at the opposite end of the row. 7 V

The present invention is not limited in its application to blasting with a single row of bore holes, but may be applied where more than one row of holes parallel to a face are fired. When multiple rows are employed, it is preferred for the obtention of optimum effects that each row of holes be shot with a variation in timing amongst the holes as set forth above in describing single row blasts. Also when multiple row blasts are made it is usually preferable to time the firing such that holes in successive rows fire somewhat behind holes nearer the blasting face. The inventionfinds application in many different types of blasting operations. A number 6 of these will be described inconnection with: the

drawings inwhichx l I Figure l is a diagrammatic isometric view of a quarry face withbore holes drilled therein. 7

1 FigureZ is av diagrammatic cross sectional view ofa loaded borehole.

.Fig'ureB is a. diagrammatic isometric view ofa bank. provided with horizontal bore. holes. Figures is a diagrammatic. elevational view-of atun'nelfacaandi y 5 I Figure 5 is. a cross sectional View taken on line 5-.5 of the face shown in Figure 4 I g For quarry blasting shots it is usuallyfpre ferre-d that a line of holes ll' be drilled generally parallel to a vertical quarry face l3 as shown in Figure l. .-In a preferreduse of the invention bore holes such as-are shown in Figural are initiated. by :a series of shortsdelay electric blastingficap's of two different periods arranged in alternating relation. Thus onegroup of alternating bore holes may be initiated by means of A type short delay. electric blasting caps while-the intermediate group may be initiated by B type delay electric blasting caps, or short delay electric blasting caps maybe alternated with'instantaneous blastingcaps; In an alternating system of blasting it is usually preferred that the timesof initiation be arranged so that the hole nearest an open end of a row be fired before its adjacent holes.

According to another method of operation, each hole can. be fired by means of short delay'electric blasting caps which have a normal variation in firing time amongstthem of an-amountsuf ficient to produce eifective'time separation Often in quarry blasting it is desirablethat more-than one row of. holes paralleltoaxface be employed. The setup for such a blast would look similar to that shown in Figure 1' except that at least one more row of holes would appear behind those already shown.v Multiple row blasting provides an opportunity for combining blasting oper-. ations accordingto the present method withmore conventional blasting operations, although it is usually preferred that all holes be fired in accordance with the present invention. Thus a front row of holes might be fired with instantaneous electric blasting caps or with alternating instantaneous electric blasting caps and A type short delay electric blasting caps, while a back row might be fired with "13 type delay electric blastmg caps.

In Figure 2 is shown an exemplary loading scheme for bore holes such as may be employed according to the present invention. In the bore hole, represented generally at 20, is shown a main explosive charge 22, a charge of gravel stemming 24, an explosive deck charge 26, a further gravel stemming charge 28, a second explosive deck charge 39, and a final stemming charge 32. The bore hole is initiated in the main charge by means of two similar electric blasting caps 34 and 36 to which current is led by wires 38 and Ml, each of which includes two individual leg wires 42 and M, and it and 48. The deck charges are initiated from the main charge by means of a line of detonating fuse 50 running from the main charge to the top deck charge.

The present invention is not limited to any particular loading'schemes. That just described has been found productive of particularly good results but other systems may be employed. For example, the individual holes instead of being directly initiated by electric blasting caps may be initiated by lines of detonating fuse which are a 7 in-turn initiated by means of electric blasting caps located either inside or. outside the bore holes.

In Figure 3 is shown .a setup for horizontal blasting such as is used in a stripping. operation where a seam of coal is to be uncovered. Horizontalholes are drilled into a bank I 3 and generally .parallel.to the. material to be uncovcred.- Usually it is preferred when the bank is high also to drill a row of vertical holes ll.

.Thefiring of a .row of horizontal holes in a stripping blast according to the invention finds particular advantage because it is often productive of'much less undesirable shattering of the material to be uncovered. The improved breaking and vibration conditions effected-by the present' invention are, of course, also important in stripping operations; When vertical holes are employed in addition to horizontal holes they alsomay be fired according to the present invention, or they may be so fired and thehorizontal holes fired conventionally.

It should be noted that in stripping operations where both horizontal and vertical holes are shot. it is often desirable that one of the series be fired somewhat ahead 'of the other. .Whether a horizontal series or a vertical series is fired first usually depends upon the handling equipment which is available for. removal of the blasted material.

- For example, a horizontal row of holes might be fired with alternating instantaneous electric blasting caps and A type short delay electric blasting caps while a vertical row of holes might be fired by means of B" type short delay blasting caps.

In some stripping blasts,.-it is desirable to provide multiple rows of either orboth of vertical and horizontal holes.

Figures 4 and 5 illustrate a setup for use where the process of the invention is employed in underground work. Figure 4 represents a front elevation of the face l3 of a tunnel with a row of holes I l drilled across it. As appears from Figure 5,. therow of holes. is: drilled. downwardly and at an angle. In this way the lower corner of the face is blastedout. 'Ihe'individual holes in the row are discharged at anumberof separated times within. the effectiverange'. .In angle blasting of this'typ'e fit is usually preferred that the angle made by the .holes withthe face be not less than Subsequent rows; or holes may-be drilled parallel to the, row just described; -.The invention will be further; illustratedby the. following examples which, describe specific blasting operations: Example!" A, blast was made in a quarry from which a massivehblue dolomitic limestone is mined and crushed for firing. The stone at thisparticular quarry has long been noted for being especially hard to break up into manageable sizes without excessive secondary jblasting. A line ofseven seveninch diameterholes was drilled according to the general scheme shown in Figure '1 of the drawing. Each hole was loaded principally with low;veloc ity 60% strength dynamite in the general manner shown in Figure 2 of the drawing except that five of the. holes were supplied with a third'deck charge. 1 Theexplosive for all but the top deck charge was in the form ofsix inch diameter 25 pound cartridges. 1% by-8 inchfcars tridges were used to make up-the top deck charge. The stemming was finely crushed stone. Each hole was initiated in thermain charge by two primer cartridges each made up of a short delay electric blasting cap embedded in a 1% by 8 inch dynamite cartridge. Oneprimer was inserted just prior to the second cartridge loaded into the hole, and the other primer wasinserted just prior to the last cartridge ofv the main charge; Primacord detonating fuse was used to'carry detonation from the'main charge to r. the deck charges. More specific data relative to-the charging of the various holes is'set forth in the table below. The blasting caps were all fired in a single series. V a

7 TABLE Distance Distance Estimated Weight of H01 8 No Hole Height from from weight Dynamite Depth of Face Hole to Preceding of a in Main Face 1 Hole Burden Charge ft. ft; ft. tons lbs.

61 56 v 14 1 24 1, 568 325 62 57 13 17 1,049 350 62 57 13 17 l, 049 325 62 57 13 18 1,111 325 63 58 13 16 1, 005 3 325 63 I 58 13 17 1,068 350 63 '58 12 17 986 350 1 Distance from termination of preceding blast.

Depth of Weight of Depth of Weight of De th of Wei ht of Height of bottom of crgplosive bottom of explosive botFom of expfiisive Hole No. mam first in first f 'second insecond top. intop' charge deck deck deck deck deck deck charge charge charge I charge charge charge It. ft. lbs. lbs; t. be. 1 3O 12% 12% f G l 5 f. 33 r 37% 6. 6 3 32 50 none '6 5 4 33 50 37361 6 5 5 i 26., 50 none 1 6 5 6 33 50 37% 6 5 7 as a 7% 6 The short delay electric blasting caps used were of the A- type. his to be noted that the time separation between adjacent holes is lessened when two short delay electric blast- ;ingcaps are employed in each hole, asin this example. The expected separation under these conditions is the value obtained for the use of zone cap in a hole, as noted above, divided by the square root of two; Thus, when two caps are 'usedin each hole if the caps are from a group having a standard deviation of 1-6 it will beer;- pected that in at least 90% of the cases the sep- :aration in firing time between adjacent holes will be at least about 2 milliseconds.

When the blast was set off very little vibration was'noticeable and the noise was greatly mufiied as'compared with instantaneously fired blasts. The breakage obtained was excellent as evidenced bycomparison'with a generally similar blast made immediatelyprior and adjacent to it and initiated entirely with the use of instantaneous electric blasting caps; After the prior blast it was necessary that 198 holes be drilled in the blasted rock for secondaryblasting operations. After the blast of this example only 19 holes were drilled for secondary blasting.

Example Ii Another blast was made in the same quarry as that in which the blast described in Example I was made. The manner or loading this blast was generally similar to the blast of Example I However, in this blast each of the 1st, 3rd, 5th, and 7th holes was initiated by meansof instan= taneous electric blasting caps, while each of the 2nd, 4th, and 6th holes was initiated by means of A type short perioddelay electric blasting caps, all caps being fired in a single series. The results of this blast were better even than those of Example .I and this alternating method of blasting is usually preferreifor it insures an effective separation in firing time between all adjacent holes.

Example III Another blast was made in the same quarry in which those of Example I and II were made. The same general loading scheme was employed except that in this lastblast there were only six 110165;; The holes in this blast; however, were loaded with dynamite of two different low velocities, a dynamite of lower velocity (about 3,600 feet per second measured unconfined in PA by 8 inch cartridges) being employed in holes 1, 3, and 5 and a dynamite of a higher velocity (about 6,800 feet per second measured unconfined in 1%; by 8 inch cartridges) being employed in holes 2, 4, and 6. Holes 1, 3, and 5 were initiated by means of A type short delay electric blasting caps and holes 2, 4, and 6 were initiated by means of instantateous electric blasting caps.

This blast appeared to move out unusually slowly and produced a peculiarly muiiled sound. The breakage produced was very good.

Example .I V

The method of shooting cf'this example produces adequatejseparatioh of firing times between'adjacent holes and also avoids simultaneous firing even of alternate holes. This blast came out welland the breakage produced was exceptionally good. I

'E'ramplc V y In another limestone quarry a blast of thirty four holes each about 5 /2 inches in diameter was made. In this blast the hol'es "were divided into two parallel rows of seveneteen holes. The dynamite mp1cyed was packaged in five inch diameter 25 pound cartridges. In the front row beginning with the first hole, alternating holes were initiated by means 'oi type short period delay electric blasting caps 'and beginning with the second hole alternating'hole's in the vfront ro'wwe're initiated by m an of instantaneous electric blasting c'ap's. In the back row each hole was initiate by mea s of B type short period delay electric blasting caps. This blast was also highly successful and produced a degree of breakage notordinarily ex erienced at this location.

Exam le T7;

in the limestone uarry of Example V, a hole blast was made in each hole of which the charge consisted of a main charge and onedeck charge. Beginning with the first hole the main charge in each alternatin hole was initiated by means of an A type short delay electric blast ing cap and beginning with the second hole each main charge was initiated by means of an instantaneous electric blasting cap. 'In this blast, however, each deck charge wasfir'ed by means cf an A type short delay electric blasting cap, no detonating fuse being employed.

Example VII In a mining operation in which a bank of over burden was stripped from a coal seam a single row of fourteen five inch horizontal holes averaging 39 feet in length were drilled. Each hole was charged with a main charge of low velocity dynamite and a deck charge of dynamite of somewhat higher velocity. Beginning with the first hole each alternate main charge was initiated by means of instantaneous electric blasting caps and beginning with the second hole each alternate main charge was initiated by means of A type short delay electric blasting caps. The deck charges in alternating holes, beginning with the first hole, were initiated by means of A type short delay electric blasting caps. Beginning with the second hole the deck charges were initiated 'by means of B type short delay electric blasting caps. The breakage produced in this blast was exceedingly good.

" Example VIII In a granite quarry having a face about 20 feet in height, 145 vertical holes were arranged in three parallel rows. Each hole was about 2 inches in diameter, and about 223 feet deep. The holes were about five feet apart, and each had been sprung with one 1%; by 8 inch gelatin dynamite cartridge. They were loaded with a total of 1800 pounds of 1 by 12 inch gelatin dynamite cartridges. In the front row beginning with the first hole, each alternate hole was initiated by means of instantaneous electric blasting caps, and beginning with the second hole in the front .row each alternate hole was initiated by means of A type short delay electric blasting caps. Each hole in the second row was initiated by meansof A, type short delay. electric blasting caps. The third-row of holes was initiated by means of B type short delay electric blasting caps. This blast showed amarked improvement in breakage over'conventional prior blasts using all instantaneous caps. A particularly great improvement obtained as a result of the blast of this example was practical elimination; of backbreak which had been particularly seriousat this quarry. w

p EmampleIX' 7 In-alimestone mine the'face of a tunnel was drilled with fifteen 1 /2 inch diagonal holes in the general manner shown in Figures 4; and 5 of"-the 'drawing. Each hole. was approximately feet in depth and aboutdfeet of tamping was used after the charge in each hole. A total of about-59 pounds of 40%. gelatin dynamite was contained man the holes. vBeginning with the first hole, each alternate hole was fired by means of an.A typeshort delay electric blasting cap having an average delay .period of about I 40 milliseconds and each alternate hole beginning with the second hole was fired by means of an instantaneous electric blasting cap. The results of this blast showed much improvement over prior conventional type blasts. Normally about seventy blocks requiring secondary blasting were produced. Only twenty-five required secondary blasting'as a result of this blast.

Example X a In another limestone quarry a further type of blasting in accordance withthe invention was performed- A row of six inch holes were each loaded with 75 pounds of 50% strength ammonia gelatin dynamite and 100 pounds of 60% strength ammonia gelatin dynamite. Each hole was feet deep and the holes were spaced 12 feet apart and 12 feet from the face of the quarry. The first six successive holes were each initiated by means of instantaneous electric blasting caps. The next seven successive holes in the row were initiated by means of A type short delay elec- L tric blasting caps, and the last seven holes were initiated by meansof B type short delay electric blasting caps. This blast was productiveof very goodresults, and while the breakage was good all along the line of holes, it was noted to be particularly good in front of the holes in an electric blasting cap within the hole or the first detonation of fuse within the bore hole.

where-a line'of detonating fuse running into the bore hole is relied upon to'initiate the hole.

What is claimed is: V 1. A process of cooperative multiple bore hole blasting, wherein the blast is primarily detonated by electric blasting caps, which comprises detonating a'row of at least three charged bore holes generally aligned so as on firing to break away a face and form a new face therebehind, the

charges in the adjacent of at least three successively adjacent bore holes in said row being detonated attimes separated by fromz to 150 milliseconds and'within about 300 milliseconds of the time of electricalactuationv of the first firing blasting cap.

.2. A process of cooperative multiple bore hole blasting wherein a row of at least three charged bore holes is generally aligned so as on firingto break away a face and form a new face therebehindwhich comprises detonating the charges in alternate bore holes substantially simultaneously, the chargesin intermediate bore holes beingidetonated at times separated from 2 to 150 millisece onds from the time of detonation of charges in adjacent bore holes and within approximately 300 milliseconds of the time of actuation; of-the detonation initiator in the first firing bore hole.

3. A process of cooperative multiple bore hole blasting which comprises detonating a' row ofat least three charged bore holes generally aligned so as on'detonating to break away a face and form a new face therebehind, the charges in the adjacent of at least three successively adjacent bore holes in said row being detonated; at

times separated by from 2 to 150 milliseconds and within approximately 300 milliseconds of the time of actuation of the detonation initiator in the first firing of said last mentioned bore holes, said time separation of from 2 to 150 milliseconds being suflicient to provide a high degree breakage on the burden but insuflicient to destroy the cooperative efiect of multiple hole blasting on the burden among said last mentioned holes.

4. A process according to claim 1 wherein said row is generally parallel to said face.

5. A process according to claim 2 wherein said charges in intermediate bore holes are detonated at times separated by fromapproximately 20 to approximately milliseconds from the time of detonation of adjacent bore holes.

6. A process of cooperative multiple borehole blasting wherein a row of at least three charged bore holes is generally aligned so as on firing to break away a face and form a new face therebehind which comprises detonating the charges in' alternate bore holes substantially instantaneously, the charges in intermediate holes being detonated from about 20 to about '75 milliseconds from the time of detonation of charges in adjacent bore holes.

7. A process according to claim 3 where said time separation is a period from approximately 20 to approximately '75 milliseconds.

8. A process according to claim 3 wherein the charges in said boreholes are detonated in two different sets, each hole of one set alternating in position with a hole of a second set and each hole of said first-mentioned set firing before each hole of said second-mentioned set.

9 A process according to claim 3 wherein the charges in each successivelyadjacent bore hole in said row are detonated sequentially.

10. A process according to claim 3 wherein said time separation is a period of from approximately 20 to approximately 75 milliseconds, and wherein the charges in said bore holes are detonated in two different sets, each hole of one set alternating in position with a hole of a second set and each hole of said first mentioned set firing before each hole of said second mentioned set.

11. A process according to claim 1 wherein said time separation is a period from approximately 20 to approximately 75 milliseconds and where-' 13 in said electric blasting caps are simultaneously electrically actuated.

12. A process according to claim 1 wherein said time separation is a period from approximately 20 to approximately 75 milliseconds and wherein the charges in said bore holes are detonated in two difierent sets, each hole of one set alternating in position with a hole of a second set and each hole of said first mentioned set firing before each hole of said second mentioned set.

13. A process according to claim 2 wherein the blast is primarily initiated by electric blasting caps which are electrically actuated simultaneously.

14. A process of cooperative multiple bore hole blasting which comprises detonating a row of at least three charged bore holes generally aligned so as on detonatingto break away a face and form a new face therebehind, the charge in the adjacent of at least three successively adjacent bore holes in said row being detonated at times separated by from approximately 20 to approximately 75 milliseconds, the charges in said lastmentioned bore holes being detonated in two different sets, each hole of one set alternating in position with a hole of a second set and each hole of said first-mentioned set firing before each hole of said second-mentioned set, said time separation of from approximately 20 to approximately milliseconds being sufiicient to provide a high degree breakage on the burden but insufficient to destroy the cooperative effect of multiple hole blasting on the burden among said lastmentioned holes.

DAVID M. MCFARLAND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1 821,883 Du Pont May 29, 1906 1,570,733 Eschbach Jan. 26, 1926 2,239,123 Stoneking Apr. 22, 1941 2,475,875 Burrows et al July 12, 1949 OTHER REFERENCES

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