EXCAVATING MACHINERY USED IN LAND DRAINAGE. 17 



In planning drainage for a small area which can be drained by one 

 dredged ditch from 6 to about 12 miles in length, the engineer will 

 frequently make the above-mentioned mistake of designing his ditch 

 too wide at the outlet. On large projects, where the amount of 

 excavation is great, this condition will not occur so often, since there 

 is usually sufficient yardage to warrant the installation of two or 

 more plants. The engineer should remember that if the ditches are 

 so planned that one machine can do all the work, even though the 

 yardage is sufficient to justify two dredges, the cost of construction 

 will be reduced. However, the time required to do the work with 

 one machine may be so great that the district would rather pay the 

 additional cost involved in installing two plants. 



A contract may consist of a number of ditches all but one of which 

 are suited to a given size of machine. This ditch is too wide to be 

 cut by the dredge at one cutting, and the yardage is insufficient to 

 justify the installation of another dredge. The difficulty may be 

 overcome by making a double cut. This, however, requires the use 

 of either vertical or the convertible type of spuds. 



If the best prices are to be obtained, each H-yard dredge on a 

 project should have a minimum of 250,000 cubic yards and each 

 3-yard dredge not less than 500,000 cubic yards. 



THE FLOATING GRAB-BUCKET DREDGE. 



In construction the floating grab-bucket dredge differs from the 

 dipper type only in the appliances for handling the material. In- 

 stead of using a dipper and dipper handle, an orange-peel or a clam- 

 shell bucket is suspended from the end of the boom. The bucket of 

 the orange-peel type is the one more generally used for drainage work. 



A much longer boom can be used with the grab-bucket dredge 

 than with the dipper dredge. From 75 to 85 feet is about the maxi- 

 mum length of boom that can be successfully operated on a dipper 

 dredge, while booms as long as 240 feet have been used on grab- 

 bucket dredges. This feature is of especial importance in levee con- 

 struction, as it is desired to deposit the material as far from the 

 stream as possible. 



While the dipper dredge pulls itself ahead by means of the dipper, 

 with a grab-bucket dredge some type of " pull-ahead" line is neces- 

 sary. Generally three auxiliary drums are provided, which are used 

 for operating the two spuds and for overhauling the pull-ahead line, 

 which is securely fastened to the bucket. The bucket is dropped 

 into the material, the hoisting line is slackened, and the pull-ahead 

 line is drawn taut, thus pulling the dredge ahead. In other cases 

 the pull-ahead line may be anchored to a "dead man" buried some 

 distance ahead of the machine. 

 4908°— Bull. 300—15 3 



