INLAND DRAINAGE 113 



BLASTING DITCHES IN SWAMPS 



A cheaper and much more satisfactory way of ditching in 

 swamp areas is by use of dynamite. 



There are two distinct methods of blasting ditches — the pro- 

 pagated or transmitted and the electric. The propagated 

 method can be used only in wet soils, while the electric method 

 can be used in fairly dry, as well as wet soils. The explosives 

 and blasting supplies needed and the methods of loading vary 

 somewhat in the two methods. 



Propagated firing requires only the explosive itself and a cap 

 and some fuse for each couple of hundred feet of ditch, while 

 electric firing requires wiring, an electric blasting cap for each 

 charge and a blasting machine or other source of current. This 

 advantage of the former method is sometimes offset, however, 

 by the fact that it requires nearly twice as many holes as electric 

 firing. Theoretically, both methods require about the same 

 amount of explosive; in practice, however, more will be used in 

 transmitted firing, unless the work is done very skilfully. The 

 economy of this method comes largely from a saving of electric 

 blasting caps and time and the need for little equipment. 



MAKING TRIAL SHOTS 



In blasting ditches by the propagated method, the first thing 

 to do is to try a few trial shots to determine the best depth and 

 spacing for the holes. For ditches up to 3 to 3^ feet deep, 

 the depth of the holes will generally be from 24 to 30 inches and 

 the spacing between holes from 18 to 24 inches, although it may 

 be necessary to change these figures slightly. If water covers 

 the cartridges in the holes, no tamping is necessary; otherwise 

 the cartridges should be well tamped with earth. Eight to ten 

 holes should thus be loaded for the test shots; one hole should be 

 charged with an extra primer cartridge, and it is also well to put 

 one additional in each hole adjoining the primer. 



If the test shot makes too large a ditch, the spacing may be 

 increased a little, but should seldom be greater than 24 inches 

 and then only in warm soil. If the ditch is too deep, it will be an 

 indication that the depth of holes should be decreased. 



Small ditches, say 2 feet deep and 3 feet wide, may be excavated 

 by using half cartridge charges, but in such case the spacing be- 

 tween holes should seldom be more than 18 to 20 inches. Larger 



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