CHAPTER IV 



EXCAVATION BY HAMMERING 



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EXCAVATION OF SUBAQUEOUS ROCKS. GENERAL DISCUSSION. 



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ROCK is the most difficult and expensive material to be removed 

 from beneath the water. When, in carrying out the work of 

 harbor or river improvements rock is encountered, its excavation 

 involves two distinct operations: 



(a) The breaking of the rock into small fragments. 



(6) The raising of the debris to the surface and its disposal in 

 any convenient manner. This last operation is accomplished by 

 means of any ordinary dredge, while the breaking of the rock into 

 small fragments is done in two different ways, viz., by hammering 

 and by blasting. 



The Method of Hammering. The breaking of the rock into 

 small fragments, by hammering, is accomplished by means of devices 

 which strike powerful blows. The blows are delivered by gravity, 

 when an extra heavy weight falling from a certain height strikes 

 the rock, as in the Lobnitz machine; or by steam or compressed- 

 air engines, when the rock is struck by iron bars placed in contin- 

 uation of the piston rod of a vertical engine, as in the Scott & 

 Godsir cutters. No matter which method is used the rock is reduced 

 into fragments of such size that they can be easily picked up by any 

 ordinary dredge, and in some cases though material is pulverized 

 in such a way that the finely divided particles can be carried away 

 by the current. 



The Method of Blasting. Submerged rocks can also be excavated 

 by blasting. Different methods are used, depending upon the 

 magnitude, depth and location of the rock to be removed. When 

 the rock is entirely submerged and must be removed from an exten- 

 sive area, but in shallow water, the most common method is by 

 successive small blasts as in the excavation of rock in the open air. 

 The only difference being that more powerful drilling machines are 

 used and they are mounted on scows or stages of special construction. 



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