18101 



THR CIVIL RNCINI':EII AND AI5< HII IXT .S .lOURNAT- 



IG.: 



ON BLASTING LIMESTONE ROCK. 



Some ./Iccomit of Blasting the UHule Linmtoiie in the County of 

 Antrim, in Inland. By William Bald, F.R.S.E., M.R.I.A., &c- 

 Read before the Institution of Civil Engineers. 



It becomes necessary to make a few short observations wliieh m.iy 

 perhaps be interesting to the scientific engineer. Along (he north 

 coast of Ireland from the Bay of Belfast to Lough Foyle, the country 

 consists of white limestone ; coliminar basalt, and some conglomerate 

 sandstone; but the hill of Carey consists of mica slate; and is of the 

 same formation as the Mull of Cantire, a part of the coast of Scotland 

 Ivino- opposite. The geologist can here easily trace the connecting 

 link in the formation, which joins the two countries, although a channel 

 yi) fithoms deep separates them. Numerous whin dykes intersect 

 tlie strata along this part of the Irish shore, they run nearly parallel to 

 each other in some cases, and are very remarkable in their structure. 



The study of the peculiar qualities of the respective rocks and 

 strata, -and their position and inclination, will enable the engineer to 

 work them in a more scientific manner. And in the construction of 

 harbours, lighthouses, lines of navigation, drainage, roads, &c. &c., an 

 intimate acquaintance with the component parts of the rocks will en- 

 able him to select those best suited to resist the action of time, whether 

 thev be placed under the dominion of the deep, exposed to the ravages 

 of the pholas, or subject to perhaps the more wasting influence of the 

 atmosphere; by such studies his skill will be alike visible in the se- 

 lection of the best material for the repaving of even a common road, 

 as it will be for that of the most splendid edifice destined to survive 

 ages. 



In constructing the Antrim coast road it became necessary to cut 

 through extensive and high masses of white limestone ; one of the sea 

 cliffs in the Little Deer Park, near Ulenarm Town, extended to a length 

 of nearly one thousand yards, rising from twenty feet to about two 

 hundred in height, washed at its base by a deep sea, and entirely ex- 

 posed to the run of the ocean in the north cliannel. 



Above the white limestone is situate the coluumar basalt, but no 

 ))art of the road was cut through this last mentioned rock. The white 

 limestone in Antrim differs from the chalk in England, in being more 

 indurated, while in other resjjccts it is similar to it in the quantities 

 of flint it con*.ains. This rock is close and fine in its texture, but it is 

 deeply fissured in many directions ; the scull veins it exhibits are ex- 

 tremely curious.* The inclinations of the limestone strata on this 

 part of the coast does not in general exceed 15° dipping into the land. 

 Under the lime rock strata lies the brownish red coloured sandstone. 



In blasting down those lofty clifi's of white limestone, the borings 

 were always made into the toe of the rocks, ami were so arranged that 

 the line of least resistance should not be in the direction of the line of 

 boring. Hundreds of tons of rock frequently rested on a base of a few 

 superficial feet, which being blasted away, the cliff above tumbled 

 down. The patent safety fuse was used, and which was attended 

 with the most beneficial results, copper tubes for putting in the charges, 

 and also copper needles. 



During three years operations not a man was lost, although upwards 

 of one hundred thousands tons of limestone were blasted down upon 

 less than one mile of the road. 



The following are the results of a few experiments maile upon 

 loose detached blocks of white limestone at Glenarm, Little Deer 

 Park. 



Table ok thk Worklng 1'roci;euings. 



From the above experiments it took one ounce of gunpowder to 

 rend asunder H-1-2 cubic feet of the white limestone when in blocks. 

 And from experiments made on the solid loose whinstone blocks, it 

 took about one ounce of gunpowder to blast asunder ll'7o cubic feet. 



Three experiments assigned the specific gravity of the white lime- 

 stone at '2,747, 2,709, "2,71)3 ; and the whinstone or basalt at 3,200, 

 being about 13 cubic feet of white limestone to (he Ion, and 11-20 

 cubic feet of the whinstone to the ton. 



* The grey limestone with wliicli tlic Light-house of Clare Island is built 

 is much traversed by scull veins, and water enlers theui during severe rain 

 storms. 



An auger of H inch diameter. 



Depth of boring. 



Quantity of powder. 



15 inches deep. 



2G ditto 



30 ditto 



36 ditto 



48 ditto 



5 feet 



6 feet 



G inches. 



8 ditto. 



ditto. 

 12 ditto. 

 17 ditto. 

 20 ditto. 

 27 ditto. 



The above table exhibits the diameter of the auger or jumper used, 

 the depth sunk, and the number of inches of gunpowder" put in. 



The force of the explosion of gunpowder is assumed to be as the 

 cube of the length of the line of least resistance, thus if one ounce of 

 gunpowder will open a distance of one foot of rock, the table vvoidd 

 run thus : — • 



Line of least resistance. 

 If 1 foot of rock rc(|nires 

 2 feet would rciiuire 

 3 

 4 

 5 

 6 

 7 

 8 

 9 

 10 



Charge of powder exclusive of priming. 



1 ounce. 



8 ditto. 



27 ditto. 



64 ditto. 



- 125 ditto. 



- 21G ditto. 



- 343 ditto. 



- 512 ditto. 



- 729 ditto. 



- 1000 ditto. 



I am aware there is nuich difficulty in knowing exactly where the 

 line of least resistance is, because the rock may be fissured, or some 

 bed or opening may be near to the line bored, and this is the case 

 where the rocks are stratified; but the hypogene rocks, such _ as 

 granite and syenite, lying in large solid compact masses unstratified 

 will be different, and these rules may be usefully applied. In blasting 

 asunder loose detached blocks, a much greater quantity of material 

 will be blown asunder by the same (piantity of gunpowder than of rock 

 lying in close connecteil beds. 



It'is always desirable to work the rock out by the dip of the in- 

 clination of the strata, or as the quarrymen call it, the going way of 

 the rock. 



In the while limestone quarries lying in the high ground north of 

 the town of Belfast, where the limestone is quarried lor building and 

 agricuhural purposes, and also for export; two men will quarry out 

 at an average from eight to ten tons per day, the augurs or jumpers 

 generally used are l^f inches, and two inches diameter; and the in- 

 duration of the white limestone may be estimated when two men will 

 bore one foot deep in half an hour ; they generally put in about three 

 inches of powder for 15 inches deep, and 6 inches for about 2 feet 

 deep ; the expense for quarrying is about from fivepence to sixpence 

 ])er ton. There are nearly 13 cubic foot of the white limestone to the 

 ton, which is at the rate "of nearly about one shilling per cubic yard. 

 This white limestone is much esteemed in Glasgow and all the towns 

 on the Clyde, where it sells for five shillings per ton— but the quarrying 

 works near Belfast are carried on in a very limited manner, or rather 

 on a very small scale. 



Numerous experiments made hy military engineers assign the force 

 of the explosion of gunpowder to be as the culae of the length of the 

 line of least resistance. Vauban and Belidor,-!" both of them excellent 

 mathematicians, and also possessing great practical skill, ingenuity 

 and experience, investigated this subject, doubtless more particularly 

 with a view to the operations of war, than to those of the works of 

 the civil engineer. The law of the explosive force of gunpowder re- 

 mains the same in all the various forms it may be applied to matter, 

 whether in blasting out of rock or earth, or the destruction of the ma- 

 sonry of fortifications by blowing them iqi, or laying in ruin bridges 

 built over large and deep rivers to arrest the progress of hostile 

 armies. 



The total cubical contents of the four blocks of limestone given 

 above, amounted to 17 1',) cubic feet, and the quantity of powder used 

 12(i ounces, being at the rate of 1-94 ounces for each cubic yard blasted 

 asunder. But if the rate per cubic yard be deduced from the quan- 



* One pound of gun]iowdcr occupies 30 cubic inches. 

 f Bchdor, one of t)io uiosi scientific of the French engineers, has given the 

 following rule for liiidiiig the charge of a surcharged mine or globe of com- 

 pression, " is to nmltiply the length of the line of least resistance in feet by 

 00, and the product will he the weight of the powder in pounds." 



Z 



