METHODS AND COSTS OF RIVER DREDGING 207 



As the dredging is completed the channel is swept, so that the 

 depths dredged are reliable and available. A twin-screw river steamer 

 and a testing scow have been used as a sweeping plant, but now a 

 new ice-breaking and sweeping tug has been designed for this 

 purpose. In the Detroit River Improvement a drag, consisting 

 of a bevel-shaped iron-shod beam loaded heavily with scrap iron 

 and suspended from one end of a dump scow is used to sweep the 

 bottom where earth has been removed. When dragging this con- 

 trivance over the dredged area, the cutting beam is usually set 

 about one foot below the required depth. This tool is effective in 

 soft material only. 



One of the six elevator dredges was built in 1894, one in 1897, 

 two in 1900, one in 1901 and one in 1902. These dredges are from 

 148 to 168 ft. long; have beams of 30 to 34 ft., and an average 

 draft of from 8 to 11 ft., and a working depth from 42.5 to 45 ft. 

 The buckets are of a capacity up to 1 cu.yd. and can excavate from 

 1000 to 3500 cu.ycls. of fairly hard material per day. Two of them, 

 have steel hulls, the rest wooden ones. These dredges have all 

 been built by the Canadian Government at their Sorel yards. 



In consideration of the character of the shale rock, of the strong 

 current, and of the fact that the work must be carried on without 

 interrupting navigation the elevator dredge is well adapted for 

 this work. A powerful dipper dredge may tear up a certain quantity 

 of soft rock more quickly, without stopping to make a clean, even 

 bottom. Chisel cutters and blasting plants may break up harder 

 material, but taking everything into consideration this type of 

 plant is particularly well designed for good, clean work, without 

 interrupting navigation. Only the best machinery can stand up 

 under the constant, steady work in the hard material encountered. 

 This is evidenced by the fact that the buckets suffer much from 

 each season's work. The dredge "La Fontaine," built in 1901, 

 after three seasons' work had a complete new set of solid steel 

 rock buckets put into her. The dredge " Baldwin " had her buckets 

 rebuilt after two seasons' work. At the end of the season of 1907 

 the total cost of renewals and repair on buckets alone was $71,826.12. 

 This makes an average of about $20,000 per dredge for new sets of 

 buckets. The " Laurier, " built in 1897, had a new set of j-cu.yd. 

 hardpan buckets put in her in 1906, at a cost of $19,336. 



The hydraulic dredge used is the well-known "J. Israel Tarte, " 

 designed by Mr. A. W. Robinson. This dredge was designed for 



