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reduced, so much as to make it very likely that silt will be deposited, (see page 91), and the drain, thereby, obstructed. To provide against this, a silt-basin must be placed at this point which will collect the silt and prevent its entrance into the more nearly level tile below. The construction of this silt-basin is more particularly described in the next chapter. From stake 7 to the main siltbasin the fall is such that the drain will clear itself.
The drawing of regular profiles, for the more imporant drains, will be useful for the purpose of making the beginner familiar with the method of grading, and with the principles on which the grade and depth are computed; and sometimes, in passing over very irregular surfaces, this method will enable even a skilled drainer to hit upon the best adjustment in less time than by computation. Ordinarily, however, the form of computation given in the following table, which refers to the same drain, (C.) will be more expeditious, and its results are mathematically more correct.*
| Fall. Feet and Depth from
. 1. t. - - I --> - • *.
- - - 2 O .18 “ 2. -Basin here.
- now - “ 12.27 -
* The figures in this table, as well as in the next preceding one, are adopted for the published profile of drain C, Fig. 21, to avoid confusion. In ordinary cases, the points which are fixed as the basis of the computation are given in round numbers;–for instance, the depth at C3 would be assumed to be 4.10 or 4.20, instead of 4.13. The fractions given in the table, and in Fig. 21, arise from the fact that the decimals are not absolutely correct, being carried out only for two figures.
NoTE.—The method of making the foregoing computation is this: 1st. Enter the lettering of the stakes in the first column, commencing at the lower end of the drain. 2d. Enter the distances between each two stakes in the second column, placing the measurement on the line with the number of the wpper stake of the two. 3d. In the next to the last column enter, on the line with each stake, its depth below the datum-line, as recorded in the field book of levels, (See page 105.) 4th. On the first line of the last column, place the depth of the lower end of the drain, (this is established by the grade of the main or other outlet at which it discharges.) 5th. Add this depth to the first number of the line next preceding it, and enter the sum obtained on the first line of the fifth column, as the depth of the drain below the datum-line. 6th. Having reference to the grade of the surface, (as shown by the figures in the sixth column,) as well as to any necessity for placing the drain at certain depths at certain places, enter the desired depth, in pencil, in the last column, opposite the stakes marking those places. Then add together this depth and the corresponding surface measurement in the column next preceding, and enter the sum, in pencil, in the fifth column, as the depth from the datum-line to the desired position of the drain. (In the example in hand, these points are at Nos. 3, 7, 10, 11, and 17.) 7th. Subtract the second amount in the fifth column from the first amount for the total fall between the two points—in the example, “3" from “Silt-Basin.” Divide this total fall, (in feet and hundredths,) by one hundredth of the total number of feet between them. The result will be the rate of fall per 100 feet, and this should be entered, in the third column, opposite each of the intermediate distances between the points. Example: Depth of the Drain at the Silt-Basin.......... 20.45 feet. “ “ “ “ “ Stake No. 3........... 16.98 “
Difference.................................. 3.47. “
8th. Multiply the numbers of the second column by those of the third and divide the product by 100. The result will be the amount of fall between the stakes, (fourth column.)—Example: 1.87 x 82=153+100=1.53.
9th. Subtract the first number of the fourth column from the first number of the fifth column, (on the line above it,) and place the remainder on the next line of the fifth column.—Example: 20.70—1.64= 19.06.
Then, from this new amount, subtract the second number of the fourth column, for the next number of the fifth, and so on, until, in place of the entry in pencil, (Stake 3,) we place the exact result of the computation.
Proceed in like manner with the next interval,—3 to 7.
10th. Subtract the numbers in the sixth column from those in the fifth, and the remainders will be the depths to be entered in the last.
Under the head of “Remarks,” note any peculiarity of the drain which may require attention in the field.
The main lines A, D, and E, and the drain B, should next be graded on the plan set forth for C, and their laterals, all of which have considerable fall, and being all so steep as not to require silt-basins at any point, can, by a very simple application of the foregoing principles, be adjusted at the proper depths. In grading the stone and tile drain, (H, I) it is only necessary to adopt the depth of the last stakes of the laterals, with which it is connected, as it is immaterial in which direction the water flows. The ends of this drain,_from H to the head of the drain 0.10, and from I to the head of C 17,-should, of course, have a decided fall toward the drains.
The laterals which are placed at intervals of 20 feet, over the underground rock on the east side of the field, should be continued at a depth of about 3 feet for nearly their whole length, dropping in a distance of 8 or 10 feet at their lower ends to the top of the tile of the main. The intervals between the lower ends of C7c, C7d, and C7e, being considerably more than 20 feet, the drains may be gradually deepened, throughout their whole length from 3 feet at the upper ends to the depth of the top of the main at the lower ends.
The main drains F and G, being laid in flat land, their outlets being fixed at a depth of 3.50, (the floor of the main outlet,) and it being necessary to have them as deep as possible throughout their entire length, should be graded with great care on the least admissible fall. This, in ordinary agricultural drainage, may be fixed at .25, or 3 inches, per 100 feet. Their laterals should commence with the top of their + tile even with the top of the 24 collar of the main,_or.15 higher than the grade of the main, —and rise, at a uniform inclination of .25, to the upper end.
Having now computed the depth at which the tile is to lie, at each stake, and entered it on the map, we are ready to mark these depths on their respective stakes in the field, when the preliminary engineering of the work will be completed.
It has been deemed advisable in this chapter to consider the smallest details of the work of the draining engineer. Those who intend to drain in the best manner will find such details important. Those who propose to do their work less thoroughly, may still be guided by the principles on which they are based. Any person who will take the pains to mature the plans of his work as closely as has been here recommended, will as a consequence commence his operations in the field much more understandingly. The advantage of having everything decided beforehand,-so that the workmen need not be delayed for want of sufficient directions, and of making, on the map, such alterations as would have appeared necessary in the field, thus saving the cost of cutting ditches in the wrong places, will well repay the work of the evenings of a whole winter.
Knowing, now, precisely what is to be done; having the lines all staked out, and the stakes so marked as to be clearly designated; knowing the precise depth at which the drain is to be laid, at every point; having the requisite tiles on the ground, and thoroughly inspected, the operator is prepared to commence actual work.
He should determine how many men he will employ, and what tools they will require to work to advantage. It may be best that the work be done by two or three men, or it may be advisable to employ as many as can work without interfering with each other. In most cases, especially where there is much water to contend with, the . latter course will be the most economical, as the ditches will not be so liable to be injured by the softening of their bottoms, and the caving in of their sides.
The Tools Required are a subsoil plow, two garden lines, spades, shovels, and picks; narrow finishing spades, a finishing scoop, a tile pick, a scraper for filling the ditches, a heavy wooden maul for compacting the bottom filling, half a dozen boning-rods, a measuring rod, and a plumb rod. These should all be on hand at the outset, so that no delay in the work may result from the want of them.
Writers on drainage, almost without exception, recommend the use of elaborate sets of tools which are intended