EXCAVATION OF SUBAQUEOUS ROCKS 41 



to the bucket-backs, and those for sand, etc., at an angle of 55 

 degrees. 



Rock picks were tried between the buckets of one of the stationary 

 dredges, but as they did not assist in removing rock they were not 

 employed in the hopper dredges. When in rock the dredges advance 

 by 6-ft. lengths, and dredge athwart for breadths up to 200 ft. 

 The rock is loaded into hoppers and deposited at sea, or (when 

 required for harbor purposes) into separate barges having grilled 

 hoppers, enabling the rock to be screened and discharged. 



In estimating the quantity of rock carried in the hoppers of the 

 dredges deduction has been made for water contained along with 

 the rock. 



As compared with blasting, the rockbreaker disintegrates and 

 breaks the rock into smaller pieces, so that the quantity lifted by 

 the dredges in a given time is about 15 per cent more than that 

 of blasted rock. 



The average quantity of blasted rock removed by one dredge 

 per day of 24 hours, and allowing for all stoppages, is 158 cu.yds., 

 and of that broken by the rockbreaker 182 cu.yds. The average 

 number of days per annum worked by the dredges is 227. The cost 

 per cu.yd. for dredging blasted rock is 2s. 6d. The cost per cu.yd. 

 of dredging rock broken by rockcutter is 2s. 2d. 



Allowing 4 per cent for interest, and 2J per cent for depreciation 

 on 19,000, the cost of a dredge, the additional cost per cu.yd. would 

 be 8.2d. for blasted rock and 7. Id. for rock broken by rockbreaker. 



Comparative Costs. The comparative cost of rock removal 

 under the two systems is as follows : 



PER CU.YD. 

 s. d. 



Drilling and blasting rock 3 



Dredging same, 2s. 6d.+8.2d 3 t 2.2 



6 2.2 



Breaking rock by rockbreaker 1 2.5 



Dredging same, 2s. 2d. + l.ld 2 9.1 



3 11.6 



Difference in cost per cu.yd. in favor of the removal of rock by 

 means of rockbreaker is 2s. 2.6d. 



