16 BULLETIN 300, U. S. DEPAKTMENT OF AGRICULTUEE. 



ber and size of stumps to be encountered is of the highest importance. 

 Owing to the expense of knocking down, transporting, and setting 

 up a dredge, it is necessary to select or use one of the size that will 

 do the most work at one building. This requires an intimate knowl- 

 edge of the layout of the proposed work, and of the accessibility of 

 the different portions. 



It is the opinion of many contractors that the use of dredges with 

 narrow hulls, say less than 18 feet, is to be avoided except where 

 the ground is so hard that the bank spuds rest firmly and bear the 

 weight of the swinging load; in soft ground it may be cheaper to 

 use a wider hull, even though it be necessary to make the ditch wider 

 than is specified. 



The various makers of dredges have compiled and made available 

 to prospective purchasers tables, giving full descriptions of their ma- 

 chines. These tables give for each of the numerous dipper capacities 

 such data as the following: Length of boom and dipper handle; dis- 

 tances machines will dig below water line and dump above water line; 

 distance from center of hull to center of dump; dimensions of hull 

 and amount of lumber required to build; sizes of hoisting and swing- 

 ing engines; and daily digging capacity of machine. With the aid 

 of these data, and having in mind the ditch specifications and the 

 factors enumerated in the preceding paragraph, the proper size of 

 dredge for a particular ditch may be determined. 



Where it will be necessary to cope with stumps, this factor will 

 often be the ruling one in determining the capacity of machine 

 needed. 



When designing a ditch, the engineer should always have in mind 

 the type and size of machine to which the work is adapted. So far 

 as is consistent with other considerations a ditch system should be 

 so designed as to give the contractor the greatest amount of exca- 

 vation for a given size of dredge. This point can best be illustrated 

 by a practical example. A certain ditch was designed with a bottom 

 width varying from 16 to 46 feet, and with a cut of about 7 feet 

 throughout the entire length of 15 miles. The ditch as planned was 

 too wide at its lower end to be constructed by an ordinary-sized 

 dredge, unless equipped with the telescopic or the convertible power 

 spuds. By making the cut deeper at the lower end, the width of 

 the ditch could have been made considerably less and an ordinary 

 dredge could have dug the ditch throughout. The necessity of using 

 two dredges of different sizes on such a comparatively small job 

 of courso tended to increase the unit cost of the work. Conditions 

 may, it is true, be such as to mako a deeper ditch impracticable, as, 

 for instance, scour due to too great a velocity, the lack of a free 

 outlet, the presence of rock, and other conditions. 



