THE IRRIGATION AGE. 



177 



no method so satisfactory as that in which a steam 

 dredge is used. These dredges as constructed for such 

 purposes are of three different types. One, known as 

 the floating dredge, begins operations at the upper end 

 of the channel and works toward the outlet. There 

 must be sufficient water in the ditch to float the boat 

 which carries the engine and excavating machinery. 

 The excavated earth is deposited on each side of the 

 ditch at a distance of 6 to 12 feet from the edge of the 

 channel. This style of dredge is adapted to the excava- 

 tion of large channels, varying from 12 to 60 feet wide 

 and as deep as is usually required. It has been success- 

 fully used in the middle West for twenty years and has 

 done more toward the reclamation of level lands than 

 any other agency which can be named. The sides of 

 the ditch as usually excavated have a slope of about % 

 to 1, and when completed a section has approximately 

 the form of the letter U. Where the ground is quite 

 unstable the side slopes should be not less than 1 to 1, 

 and the excavated earth should be left not nearer than 

 8 feet from the edge of the excavation. 



Another type of steam dredge will make ditches as 

 rnrrow as 4 feet at the bottom and as wide as 12 to 15 

 feet at the top, with side slopes of 45, the depths 

 ranging from 4 to 9 feet. This machine is placed at 

 the cutlet of the proposed ditch and is pulled upgrade 

 by means of a drum which winds up a cable previously 

 anchored ahead of the machine. No water is required 

 in the ditch in order to operate it. It excavates to its 

 full depth and grade and completes the work from the 

 outlet toward the source. The machine is not used as 

 generally as the floating dredge, not because it is less 

 efficient, but, being adapted only to the excavation of 

 smaller ditches where the ground is firm on both sur- 

 face and bottom, it is limited in its field and hence not 

 =o much in favor with contractors. 



The third steam machine which may be described 

 has similar limitations. It is constructed to move upon 

 the surface of the ground in advance of the excavation, 

 instead of following up the bottom of the ditch, as does 

 the one last described. The plant carrying the ma- 

 chinery rests upon long runners which slide upon mov- 

 able cross blocks, and is pulled by a cable, one end of 

 which is attached to a winding drum at the engine and 

 the other to a log anchored some distance ahead of the 

 machine, technically called a "dead man." The 

 mechanism for excavation consists of two dippers which 

 arc filled by being pulled toward the machine against 

 the earth and are dumped alternately. While one dip- 

 per is filling the other is being swung to the opposite 

 side and emptied. This dredge, like the last men- 

 tioned, is adapted to ditches of the smaller class and to 

 land which is sufficiently firm to support the machine 

 upon the surface. 



Dredges of the three types just described have been 

 in successful operation for ditching purposes for twelve 

 to twenty-five years. The boats are built and the ma- 

 chinery mounted upon the ground where the work is to 

 be done. The machines cost not less than $5,000 each. 

 They are operated by contractors, who provide them- 

 selves with full equipment and do the work by the 

 cubic yard, under specifications and measurements made 

 by an engineer. The work required by these machines 

 is performed at a cost of 7 to 13 cents per cubic yard, 

 large contracts being taken at lower figures than small 

 ones. Any of the dredges can excavate the larger and 

 longer channels required for drainage at a much less 

 cost than can be done by any other method. 



SECTION AND BEHAVIOR OF DITCHES. 



It has been found by experience that ditches may 

 be constructed with sides more nearly vertical than was 

 formerly thought practicable. In stiff loams and clays 

 it is not desirable to cut the sides with slopes greater 

 than 1 to' 1. Ditches made with the floating dredge, 

 usually have slopes of about 1/2 to 1. In any case, those 

 which carry large volumes of water change their form 



FIG. 7. Change in section of ditch 

 by erosion where side slopes are 

 45 degrees. 



FIG. 8 Change by erosion in 

 section of ditch with steep 

 side slopes. 



by reason of erosion and weathering of the earth and 

 assume approximately the forms illustrated in Figs. 

 7 and 8, so that it is of greater importance to secure 

 ample bottom width, in order to allow for this change, 

 than to attempt to make the exact slope desired and 

 expect it to remain as left by the machine. 



The excavated earth, which of course lies in un- 

 sightly masses along the edges of the ditch when the 

 work is finished, will, after weathering through one win- 

 ter, assume a much less formidable shape and can in a 

 year or two be worked down with a plow and scraper 

 until the land can be cultivated nearly to the bank of 

 the ditch. It is always well, however, to keep a strip 

 on each side bordering the ditch in grass in order to 

 prevent the banks from crumbling and to keep the ad- 

 joining cultivated soil from being washed into the ditch 

 in times of sudden and violent freshets. 



The grades upon which such ditches may be con- 

 structed are 6 inches per mile and upward, but a grade 

 of 3 feet per mile is required for the effectual and per- 

 manent scouring of small ditches excavated through 

 loam or clay. Large and deep ditches, made straight 

 and so situated that they will not receive silt or debris 

 in large quantities will usually be self-cleaning. 



There are a few facts bearing upon the drainage of 

 level lands which have been fairly well demonstrated 

 and which, in the planning of works, should not be dis- 

 regarded. One of them is that deep ditches are neces- 

 sary to accomplish the desired end. By this is meant 

 those excavated from 6 to 10 feet deep. In many in- 

 stances the entire grade for lateral drainage must be 

 made by additional depth of the outlet. The velocity 

 and carrying capacity of the ditch increases with the 

 depth. For example, a ditch carrying water 6 feet deep 

 will have a mean velocity 40 per cent greater than when 

 the water is only 2 feet deep. Water 8 feet deep will 

 have twice the velocity of that 1 foot deep in a ditch of 

 the same width. This partially explains why shallow 

 ditches are such marked failures as drainage outlets. 

 Their carrying capacity is comparatively small and their 

 action affects only the surface of the soil. 



(To Be Continued.) 



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