CHAPTER VI 

 EXCAVATION OF SUBAQUEOUS ROCKS BY A LARGE BLAST 



THE removal of subaqueous rocks by a single huge blast can bo 

 accomplished in two different ways- either by honeycombing the 

 whole reef and firing the charges simultaneously, or by a coffer- 

 dam and series of lift holes. 



Mining or Honeycombing the Reef. This method was employed 

 by Gen. Newton in blasting the Flood Rock at Hell Gate, in Now 

 York Harbor, and was described by 1st Lieut. George Mc.C. 

 Derby, Corps of Engineers, U. S. A., in the Sanitary Engineer, 

 December, 1885. The following description is slightly condensed 

 from his paper: 



All the vessels plying between New York and Long Island Sound 

 go through Hell Gate, a narrow and crooked waterway between 

 the rocks, where the East River makes its way between Black- 

 well's Island and Ward's Island. Since the year 1848 it was estimated 

 that one vessel in every fifty attempting the passage was more or 

 less damaged by being thrown on the rocks. The channel was so 

 dammed by these obstructions, as to show a difference of level 

 of 1.9 ft. at high water, causing currents of 8J knots an hour. The 

 necessity of removing these obstructions was felt for many years, 

 and Congress appropriated money at different times. Such sums, 

 however, were inadequate for the purpose. When Gen. John Xowtnn, 

 Corps of Engineers, U. S. A., was placed in charge of this improve- 

 ment, he vigorously presented the real solution of the problem. (Jon. 

 Newton proposed to remove all the dangerous reefs to a depth of 

 26 ft. at mean low water and submitted detailed estimates of the 

 cost of the work and the time required to complete it. This project , 

 on account of its magnitude, was first only partially approved, but 

 after the successful demolition at Hallett's Point, Astoria, L. I., it 

 was adopted in its entirety. 



The method adopted by Gen. Newton at Flood Rock was as 

 follows: Two shafts wore sunk on the ridge of the roof and from 



