This is a point upon which we must decide for our selves. There is no country where drainage is practiced, where the thermometer sinks, as in almost every Winter it does in New England, to 20° below zero (Fahrenheit).
All writers seem to assume that tile-drains must be injured by frost. What the effect of frost upon them is supposed to be, does not seem very clear. If filled with water, and frozen, they must, of course, burst by the expansion of the water in freezing; but it would probably rarely happen, that drainage-water, running in cold weather, could come from other than deep sources, and it must then be considerably above the freezing point. Still; we know that aqueduct pipes do freeze at considerable depths, though supplied from deep springs. Neither these nor gas-pipes are, in our New England towns, safe below frost, unless laid four feet below the surface; and instances occur where they freeze at a much greater depth, usually, however, under the beaten paths of streets, or in exposed positions, where the snow is blown away. In such places, the earth sometimes freezes solid to the depth of even six feet. It will be suggested at once that our fields, and especially our wet lands, do not freeze so deep, and this is true; but it must be borne in mind, that the very reason why our wet lands do not freeze deeper, may be, that they are filled with the very spring-water which makes them cold in Summer, indeed, but is warmer than the air in Winter, and so keeps out the frost. Drained lands will freeze deeper than undrained lands, and the farmer must be vigilant upon this point, or he may have his work ruined in a single Winter.
We are aware, that upon this, as every other point, ascertained facts may seem strangely to conflict. In the town of Lancaster, among the mountains in the coldest part of New Hampshire, many of the houses and barns of the village are supplied with water brought in aqueducts from the hills. We observed that the logs which form the conduit are, in many places, exposed to view on the surface of the ground, sometimes partly covered with earth, but generally very little protected. There has not been a Winter, perhaps in a half century, when the thermometer has not at times been 10° below Zero, and often it is even lower than that. Upon particular inquiry, we ascertained that very little inconvenience is experienced there from the freezing of the pipes. The water is drawn from deep springs in the mountains, and fills the pipes of from one to two-inch bore, passing usually not more than one or two hundred rods before it is discharged, and its warmth is sufficient, with the help of its usual snow covering, to protect it from the frost.
We have upon our own premises an aqueduct, which supplies a cattle-yard, which has never been covered more than two feet deep, and has never frozen in the nine years of its use. We should not, therefore, apprehend much danger from the freezing of pipes, even at shallow depths, if they carry all the Winter a considerable stream of spring-water; but in pipes which take merely the surface water that passes into them by percolation, we should expect little or no aid from the water in preventing frost. The water filtering downward in Winter must be nearly at the freezing point; and the pipes may be filled with solid ice, by the freezing of a very small quantity as it enters them.
Neither hard-burnt bricks nor hard-burnt tiles will crumble by mere exposure to the Winter weather above ground, though soft bricks or tiles will scarcely endure a single hard frost. Too much stress cannot be laid upon the importance of using hard-burnt tiles only, as the failure of a single tile may work extensive mischief. Writers seem to assume, that the freezing of the ground about the drains will displace the tiles, and so destroy their continuity, and this may be so; though we find no evidence, perhaps, that at three or four feet, there is any disturbance of the soil by freezing. We dig into clay, or into our strong subsoils, and find the earth, at three feet deep, as solid and undisturbed as at twice that depth, and no indication that the frost has touched it, though it has felt the grip of his icy fingers every year since the Flood. With these suggestions for warning and for encouragement, the subject must be left to the sound judgment of the farmer or engineer upon each farm, to make the matter so safe, that the owner need not have an anxious thought, as he wakes in a howling Winter night, lest his drains should be freezing.
Finally, in view of the various considerations that have been, suggested, as well as of the almost uniform authority of the ablest writers and practical men, it is safe to conclude, that, in general, in this country, wherever sufficient outfall can be had, four feet above the top of the tiles should be the minimum depth of drains.
CHAPTER VIII
ARRANGEMENT OF DRAINS.
Necessity of System.—What Fall is Necessary.—American Examples.—Outlets.—Wells and Relief-Pipes.—Peep holes.—How to secure Outlets.—Gate to Exclude Back-Water.—Gratings and Screens to keep out Frogs, Snakes, Moles, &c.—Mains, Submains, and Minors, how placed.—Capacity of Pipes.—Mains of Two Tiles.—Junction of Drains.—Effect of Curves and Angles on Currents.—Branch Pipes.—Draining into Wells or Swallow Holes.—Letter from Mr. Denton.
As every act is, or should be, a part of a great plan of life, so every stake that is set, and every line laid in the field, should have relation not only to general principles, but also to some comprehensive plan of operations.
Assuming, then, that the principles advocated in this treatise are adopted as to the details, that the depth preferred is not less than four feet—that the direction preferred is up and down the slope—that the distance apart may range from fifteen to sixty feet, and more in some cases, according to the depth of drains and the nature of the soil—that no tiles smaller than one and a half inch bore will be used, and none less than two inches except for the first one hundred yards, there still remains the application of all these principles to the particular work in hand. With the hope of assisting the deliberations of the farmer on this point, some additional suggestions will be made under appropriate heads.