EXCAVATION OF SUBAQUEOUS ROCKS BY A LARGE BLAST 47 



The contractors decided to undermine the coffer-dam by drilling 

 long lift holes, using the large Ingersoll-Sergeant, H 9 submarine 

 drills mounted on timbers laid in the bottom of the pit excavated 

 inside the coffer-dam. In this way 203 lift holes, having a dip of 

 1 in 10 from the horizontal, were drilled 50 to 79 ft. deep and 5 ft. 



FIG. 12. Plan of the Ledge of Rock at Henderson's Point. 



apart, all around inside of the pit. The holes started 5 ft. 

 below the bottom of the grade to which dredging must be 

 carried, so as to make sure of leaving no hog backs after blasting. 

 Fig. 13. 



The diameter of the holes at the collar was 6 in. and the deepest 



FIG. 13. Method of Excavation followed at Henderson's Point. 



holes were 2 in. in diameter at the bottom. A cross bit was 

 used for starting a hole, but plain chisel bits were found very 

 effective for the greater depths. The rock was exceedingly seamy, 

 many hard quartz veins occurring at frequent intervals in some 

 holes. 



The average progress of drilling was 30 ft. per 10-hour shift, 



