EXCAVATION OF SUBAQUEOUS ROCKS BY A LARGE BLAST 43 



these shafts two sets of parallel galleries were run at right angles 

 to each other, undermining the whole nine acres of reef and leaving 

 it standing on pillars about 15 ft. square and about 25 ft. center to 

 center. The roof in the cross galleries was then blasted down, 

 leaving it as thin as the character of the rock and the location under 

 the river bed would permit. The average thickness of the roof 

 was 18.8 ft., while the least thickness was 10 ft. As the soundings, 

 although taken with great care, could hardly indicate the difference 

 between rock conglomerates and boulders, the cutting of the 

 roof of the small galleries was carried on with great caution. Only 

 one drill hole was fired at a time, thus requiring a large expenditure 

 for both drilling and explosives; 2.3 pounds of explosives and 11.97 





FIG. 10. Plan of Flood Rock at Hell Gate, X. Y. 



ft. of drilling were required per cu.j'd. The height of the galleries 

 varied from 4 to 33 ft., while they were 10 ft. wide. Fig. 10 shows 

 the honeycombing of the Flood Rock. The rock was fissured and 

 an inrush of w T ater was only prevented by walling the seams with 

 Portland cement. The seams gave continuous trouble until the 

 work was completed. The amount of work required to undermine 

 the reef consisted of 21,669 ft. of tunnels driven, 80,232 cu.yds. of 

 rock excavated, and about 480,000 Ibs. of high explosives consumed. 

 Only one man was killed during the operations. 



It was the intention of Gen. Newton to excavate a cavity suf- 

 ficiently large to receive the debris from the roof and leave a depth 

 of 26 ft. at mean low water, after the final blast. But on account 

 of the extra expense of timbering and the fact that it required 1 .45 

 cu.yds. of space to contain one cu.yd. of roof after it was broken. 



