Chapter III: How to Go to Work to Lay Out a System of Drains (3)

Having staked these guiding points of the drains, it is advisable to remove all of the 50-foot stakes, as these are of no further use, and would only cause confusion. It will now be easy to set the remaining stakes,--placing one at every 50 feet of the laterals, and at the intersections of all the lines.

A system for marking the stakes is indicated on the map, (in the _C_ series of drains,) which, to avoid the confusion which would result from too much detail on such a small scale, has been carried only to the extent necessary for illustration. The stakes of the line _C_ are marked _C1_, _C2_, _C3_, etc. The stakes of the sub-main _C7_, are marked _C7a_, _C7b_, _C7c_, etc. The stakes of the lateral which enters this drain at _C7a_, are marked _C7a/1_, _C7a/2_, _C7a/3,_ etc. etc. This system, which connects the lettering of each lateral with its own sub-main and main, is perfectly simple, and avoids the possibility of confusion. The position of the stakes should all be lettered on the map, at the original drawing, and the same designating marks put on the stakes in the field, as soon as set.

_Grade Stakes_, (pegs about 8 or 10 inches long,) should be placed close at the sides of the marked stakes, and driven nearly their full length into the ground. The tops of these stakes furnish fixed points of elevation from which to take the measurements, and to make the computations necessary to fix the depth of the drain at each stake. If the measurements were taken from the surface of the ground, a slight change of position in placing the instrument, would often make a difference of some inches in the depth of the drain.

*Taking the Levels.*--For accurate work, it is necessary to ascertain the comparative levels of the tops of all of the grade stakes; or the distance of each one of them below an imaginary horizontal plane. This plane, (in which we use only such lines as are directly above the drains,) may be called the "Datum Line." Its elevation should be such that it will be above the highest part of the land, and, for convenience, it is fixed at the elevation of the levelling instrument when it is so placed as to look over the highest part of the field.

_Levelling Instruments_ are of various kinds. The best for the work in hand, is the common railroad level, which is shown in Fig. 6. This is supported on three legs, which bring it to about the level of the eye. Its essential parts are a telescope, which has two cross-hairs intersecting each other in the line of sight, and which may be turned on its pivot toward any point of the horizon; a bubble glass placed exactly parallel to the line of sight, and firmly secured in its position so as to turn with the telescope; and an apparatus for raising or depressing any side of the instrument by means of set-screws. The instrument is firmly screwed to the tripod, and placed at a point convenient for looking over a considerable part of the highest land. By the use of the set-screws, the plane in which the instrument revolves is brought to a level, so that in whatever direction the instrument is pointed, the bubble will be in the center of the glass. The line of sight, whichever way it is turned, is now in our imaginary plane. A convenient position for the instrument in the field under consideration, would be at the point, east of the center, marked _K_, which is about 3 feet below the level of the highest part of the ground. The telescope should stand about 5 feet above the surface of the ground directly under it.

_The Levelling-Rod_, (See Fig. 7,) is usually 12 feet long, is divided into feet and hundredths of a foot, and has a movable target which may be placed at any part of its entire length. This is carried by an attendant, who holds it perpendicularly on the top of the grade-stake, while the operator, looking through the telescope, directs him to move the target up and down until its center is exactly in the line of sight. The attendant then reads the elevation, and the operator records it as the distance below the _datum-line_ of the top of the grade-stake. For convenience, the letterings of the stakes should be systematically entered in a small field book, before the work commences, and this should be accompanied by such a sketch of the plan as will serve as a guide to the location of the lines on the ground.

The following is the form of the field book for the main drain _C_, with the levels recorded:

LETTERING OF THE STAKE. DEPTH FROM DATUM LINE. Silt Basin 18.20 C 1 15.44 C 2 14.36 C 3 12.85 C 4 12.18 C 5 11.79 C 6 11.69 C 7 11.55 C 8 11.37 C 9 11.06 C 10 8.94 C 11 8.52 C 12 7.86 C 13 7.70 C 14 7.39 C 15 7.06 C 16 6.73

The levelling should be continued in this manner, until the grades of all the points are recorded in the field book.

Fig. 21 - PROFILE OF DRAIN C.

Horizontal Scale, 66 ft. to the inch.
Vertical Scale, 15 ft. to the inch.

1 to 17. Numbers of Stakes.
(82) etc. Distances between Stakes.
18.20 etc. Depths from _datum-line_ to surface.
2.50 etc. Depths of ditch.
20.70 etc. Depths from _datum-line_ to drain.

If, from too great depression of the lower parts of the field, or too great distances for observation, it becomes necessary to take up a new position with the instrument, the new level should be connected, by measurement, with the old one, and the new observations should be computed to the original plane.

It is not necessary that these levels should be noted on the map,--they are needed only for computing the depth of cutting, and if entered on the map, might be mistaken for the figures indicating the depth, which it is more important to have recorded in their proper positions, for convenience of reference during the work.

*The Depth and Grade of the Drains.*--Having now staked out the lines upon the land, and ascertained and recorded the elevations at the different stakes, it becomes necessary to determine at what depth the tile shall be placed at each point, so as to give the proper fall to each line, and to bring all of the lines of the system into accord. As the simplest means of illustrating the principle on which this work should be done, it will be convenient to go through with the process with reference to the main drain _C_, of the plan under consideration. A profile of this line is shown in Fig. 21, where the line is broken at stake No. 7, and continued in the lower section of the diagram. The topmost line, from "Silt Basin" to "17," is the horizontal datum-line. The numbers above the vertical lines indicate the stakes; the figures in brackets between these, the number of feet between the stakes; and the heavy figures at the left of the vertical lines, the recorded measurements of depth from the datum-line to the surface of the ground, which is indicated by the irregular line next below the datum-line. The vertical measurements are, of course, very much exaggerated, to make the profile more marked, but they are in the proper relation to each other.

The depth at the silt-basin is fixed at 2-1/2 feet (2.50.) The rise is rapid to stake 3, very slight from there to stake 7, very rapid from there to stake 10, a little less rapid from there to stake 11, and still less rapid from there to stake 17.

To establish the grade by the profile alone, the proper course would be to fix the depth at the stakes at which the inclination is to be changed, to draw straight lines between the points thus found, and then to measure the vertical distance from these lines to the line indicating the surface of the ground at the different stakes; thus, fixing the depth at stake 3, at 4 feet and 13 hundredths,(15) the line drawn from that point to the depth of 2.50, at the silt-basin, will be 3 feet and 62 hundredths (3.62) below stake 1, and 3 feet and 92 hundredths (3.92) below stake 2. At stake 7 it is necessary to go sufficiently deep to pass from 7 to 10, without coming too near the surface at 9, which is at the foot of a steep ascent. A line drawn straight from 4.59 feet below stake 10 to 4.17 feet at stake 17, would be unnecessarily deep at 11, 12, 13, and 14; and, consequently it is better to rise to 4.19 feet at 11. So far as this part of the drain is concerned, it would be well to continue the same rise to 12, but, in doing so, we would come too near the surface at 13, 14, and 15; or must considerably depress the line at 16, which would either make a bad break in the fall at that point, or carry the drain too deep at 17.

By the arrangement adopted, the grade is broken at 3, 7, 10, and 11. Between these points, it is a straight line, with the rate of fall indicated in the following table, which commences at the upper end of the drain and proceeds toward its outlet:

FROM STAKE, TO STAKE, DISTANCE. TOTAL FALL. RATE OF DEPTH. DEPTH. FALL. PER 100 FT. No. No. 246 ft. 2.46 ft. 1.09 ft. 17...4.17 11...4.19 ft. ft. No. No. 41 ft. 82 ft. 2.00 ft. 11...4.19 10...4.59 ft. ft. No. No. 91 ft. 2.49 ft. 2.83 ft. 10...4.59 7...4.47 ft. ft. No. No. 173 ft. 96 ft. 56 ft. 7...4.47 3...4.13 ft. ft. No. S. Basin 186 ft. 3.47 ft. 1.87 ft. 3...4.13 2.25 ft. ft.

It will be seen that the fall becomes more rapid as we ascend from stake 7, but below this point it is very much 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 silt-basin the fall is such that the drain will clear itself.

The drawing of regular profiles, for the more important 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.(16)

Fall. Depth Feet and from Decimals. Datum Line. No. of Distance Per 100 Between To To Depth of Remarks. Stake. Between Feet. Stakes. Drain. Surface. Drain. Stakes. Silt 20.70 18.20 2.50 ft Basin. ft. ft. C. 1. 82 ft. 2 ft. 1.64 ft. 19.06 " 15.44 " 3.48 ft C. 2. 39 ft. do. .78 ft. 18.28 " 14.36 " 3.83 ft C. 3. 65 ft. do. 1.30 16.98 " 12.85 " 4.13 ft ft. C. 4. 51 ft. .56 .28 ft. 16.70 " 12.18 " 4.52 ft C. 5. 43 ft. do. .24 ft. 16.46 " 11.79 " 4.67 ft C. 6. 47 ft. do. .26 ft. 16.20 " 11.69 " 4.51 ft C. 7. 32 ft. do. .18 ft. 16.02 " 11.55 " 4.47 ft Silt-Basin here. Made deep at Nos. 7 and 10 to pass a depression of the surface at No. 9. C. 8. 41 ft. 2.83 1.16 14.86 " 11.37 " 3.49 ft ft. C. 9. 12 ft. do. .34 ft. 14.52 " 11.06 " 3.46 ft C.10. 38 ft. do. .99 ft. 13.53 " 8.94 " 4.59 ft C.11. 41 ft. 2.00 .82 ft. 12.61 " 8.52 " 4.19 ft C.12. 41 ft. 1.09 .44 ft. 12.27 " 7.86 " 4.41 ft C.13. 41 ft. do. .44 ft. 11.83 " 7.70 " 4.13 ft C.14. 41 ft. do. .44 ft. 11.39 " 7.39 " 4.00 ft C.15. 41 ft. do. .44 ft. 10.95 " 7.06 " 3.89 ft C.16. 41 ft. do. .44 ft. 10.51 " 6.73 " 3.88 ft C.17. 41 ft. do. .44 ft. 10.07 " 5.90 " 4.17 ft

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
_upper_ 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 20.45 feet.
the Silt-Basin
Depth of the Drain at 16.98 feet.
the Stake No. 3
----
Difference 3.47 feet.
Distance between the 186.-- feet.
two

1.86)3.47(1.865 or 1.87

1 86
----
1 610
1 488
----
1 220
1 116
----
1 040
930
----
110

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:
1.87x82=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 _C10_, and from _I_ to the head of _C17_,--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 1/4 tile even with the top of the 2-1/2 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.