EXCAVATION OF SUBAQUEOUS ROCKS 39 



Section 2 Section 4 



Full rate 39% 35% 



Half rate 53 45 



Not paid 8 20 



It would seem from this that some other plan or method might 

 have greatly reduced the amount of work done for half pay or for 

 which no compensation was made, but it is difficult to decide upon 

 this without experimenting. 



COMPARISON OF THE TWO METHODS. 



A comparative test of the two methods of breaking subaqueous 

 rocks was made at Blyth, England, and reported upon by Mr. John 

 Watt Sandeman in Engineering, June 28, 1907. 



In the improvement proposed at Blyth it is necessary to remove 

 500,000 cu.yds. of sandstone rock and shale, and the work is now 

 being done by means of 2 Lobnitz rockbreakers and 2 700-ton 

 hopper dredges (of the clamshell type). Previous to 1906 about 

 150,000 cu.yds. of rock were broken up by means of drilling and 

 blasting, so that a comparison of the two methods can be made. 



The rock at Blyth is the sandstone of the Coal Measures, vary- 

 ing in character from friable stone containing fire clay, shale and 

 coal, to sandstone equal in hardness to basalt. 



Rock Drilling and Blasting. The drilling and blasting of the 

 rock was carried out by means of a barge having 6 drills, which 

 were lifted by steam power and guided by hand. 



The distance between the shot holes was 5 ft. in one direction 

 and 6 ft. 2 in. in the other. The blasting material, bellite, was 

 lowered in canisters through the drilling tubes, and fired by fuses 

 and detonators, the holes being. tamped with small gravel. 



The average quantity of rock drilled and blasted per week by 

 means of one barge was 488 cu.yds., and the average cost of drilling 

 and blasting was about 3s. per cu.yd. 



The quantity of rock blasted was ascertained by the number 

 of holes and their depth, which was 1 ft. more than that to which 

 the rock could be dredged, and a check was made by sounding 

 over an area of rock before blasting and after dredging. 



Rockbreaking. Each rockbreaker employed at Blyth consists of a 

 steel barge carrying shear-legs, from which is suspended a steel ram of 

 15 tons weight, 40 ft. to 50 ft. in length, and 17 in. to 19 in. in 

 diameter, having a renewable conical point, tempered so as to 



