"Draining on a sound clay," says the writer of a prize essay, "free from stones, may be executed at a cheaper rate per rod, in length, than on almost any other kind of soil, as, from the firmness of the clay, the work may be done with narrow spades, and but a small quantity of soil requires to be removed. The draining of wet sands or grounds, or clays in which veins of sand abound, is more expensive than on sound clays, because a broader spade has to be used, and consequently a larger amount of soil removed; and draining stony or rocky soils is still more expensive, because the pick has to be used. This adds considerably to the expense."
Great stress is laid, by all experienced persons, upon using narrow spades, and opening ditches as narrow as possible.
It is somewhat more convenient for unskillful laborers to work in a wide ditch than in a narrow one, and although the laborers frequently protest that they cannot work so rapidly in narrow ditches, yet it is found that, in contract work, by the rod, they usually open the ditches very narrow.
Indeed, it will be found that, generally, the cost of excavation bears a pretty constant proportion to the number of cubic feet of earth thrown out.
It will surprise those unaccustomed to these estimates, to observe how rapidly the quantity excavated, increases with the increased width of the ditch.
To enable the reader accurately to compute the measurement of drains of any dimensions likely to be adopted, a table and explanations, found in the Report of the Board of Health, already quoted, are given below. The dimensions, or contents of any drain, are found by multiplying together the length, depth, and mean width of the drain.
"Thus, if a drain is 300 yards long, and the cutting 3 feet deep, 20 inches wide at the top, and 4 inches wide at the bottom, the mean width would be 12 inches (or the half of the sum of 20 and 4), and if we multiply 300, the length, by 1, the depth in yards, and by 1/3, the mean width in yards, and the product would be 100 cubic yards. The following table will serve to facilitate such calculations.
"Along the top of the table is placed the mean widths in inches, and on the left-hand side the depths of the drains, extending from 30 inches to 5 feet. The numbers in the body of the table express cubic yards, and decimals of a yard. In making use of the table, it is necessary first to find the mean width of the drain, from the widths at the top and bottom. Thus, if a drain 3 feet deep were 16 inches wide at the top, and 4 inches at the bottom, the mean width would be half of 16 added to 4, or 10; then, by looking in the table for the column under 10 (width), and opposite 36 (inches of depth), we find the number of cubic yards in each rod of such a drain to be 1.53, or somewhat more than one and a half. If we compare this with another drain 20 inches wide at the top, 4 inches at the bottom, and 4½ feet deep, we have the mean width 12, and looking at the table under 12 and opposite 54, we find 2.75 cubic yards, or two and three-quarters to the rod. In this case, the quantity of earth to be removed is nearly twice as much as in the other, and hence, as far as regards the digging, the cost of the labor will be nearly double. But in the case of deep drains, the cost increases slightly for another reason, namely, the increased labor of lifting the earth to the surface from a greater depth."
Under the title of the "Depth of Drains," other reasons are suggested why shallow drains are more easily wrought than deeper drains. The widths given in English treatises, and found perfectly practicable there, with proper drainage-tools, will seem to us exceedingly narrow. Mr. Parkes gives the width of the top of a four-foot drain 18 inches, of a three-and-a-half foot drain 16 inches, and of a three-foot drain 12 inches. He gives the width of drains for tiles, three inches at bottom, and those for stones, eight inches. Of the cost of excavating a given number of cubic yards of earth from drains, it is difficult to give reliable estimates. In the writer's own field, where a pick was used to loosen the lower two feet of earth, the labor of opening and filling drains 4 feet deep, and of the mean width of 14 inches, all by hand labor, has been, in a mile of drains, being our first experiments, about one day's labor to three rods in length. The excavated earth of such a drain, measures not quite three cubic yards. (Exactly, 2.85.)