Part 10 (1/2)
In protecting the spring and in building up around it in order to put it underground, concrete is the most suitable material, although a large sewer pipe or a heavy cask or barrel will answer the purpose. It is usually sufficient to dig out the spring to a depth of four or five feet, and with a pump it is possible to keep the water down, so that the concrete walls may be laid. In building these walls, it is important to notice from which side the spring water comes, and on that side holes should be left in the wall. These openings may properly be connected with agricultural tile drains laid out from the spring in different directions, serving both to drain the ground and to add volume to the spring. It is often possible instead of pumping out water during construction to drain a spring temporarily, in places where the ground slopes rapidly, by carrying out a drainpipe from the lowest level; this drain is to be later stopped up.
The size of this spring reservoir depends on the average rate of flow of the spring and on the quant.i.ty of water used. If there is always an overflow from the spring, that is, if it always at all times of the year furnishes more water than is required by the house at that time of day when the greatest demand is made, then a two-foot sewer pipe is just as good as a concrete chamber ten feet square. But if at times the spring is low, so that the flow during the night must be saved to compensate for the excess consumption during the day, or if the rate at which the water is drawn at certain hours is greater than the average rate at which the spring flows, then storage must be allowed for in preparing the spring to act as a reservoir.
We have already estimated that a family of ten persons might use five hundred gallons of water a day, and the most exacting conditions would never require the spring to hold more than one day's supply. This would mean a chamber four feet deep and in area four by five feet. If the average supply of the spring is less than the average consumption of the family, then the spring must become a storage basin for the purpose of carrying water enough over the dry season, and the capacity of the basin must be computed from the number of days' storage required. It may not be out of place to suggest again the possibility of increasing the yield of the spring by laying draintile in a ditch running along the permeable stratum. These pipes may run fifty or one hundred feet each way from the main spring, so long as they continue to find ground water.
The walls of such a spring reservoir as here suggested for depths of six to eight feet need not be more than nine inches thick, whether built of brick or concrete. For greater depths the thickness should be increased to twelve inches.
[Ill.u.s.tration: FIG. 38.--A protected spring-chamber.]
The roof of the spring-chamber may be of plank, but this is temporary and undesirable. It is far better, for all spans up to ten feet, to make the roof a flat slab of concrete six inches thick, imbedding in the concrete in the bottom of the ma.s.s some one-half-inch iron rods, s.p.a.ced about a foot apart each way and extending well into the side walls. The size of these rods should increase with the size of the chamber, making them three-quarter-inch rods up to a nine-foot span, and one-inch rods up to a twelve-foot span. There should be some way of getting into the spring, preferably by an opening in one corner so arranged as to carry the side walls of the opening or manhole up above the ground, where it may be protected with an iron cover locked fast (see Fig. 38, after Imbeaux). Besides the outlet pipe from the spring, which will naturally pa.s.s through the side walls about halfway between top and bottom in order to get the best water, there should be a drainpipe from the lowest part of the inclosure, the valve of which can be reached through a valve box coming to the surface. In the figure the drainpipe is shown by the dotted line, and the twofold chamber is for the purpose of allowing an examination of the spring to be made at any time.
The concrete used in this work should be of good quality, one part of cement to five parts of gravel or to four parts of stone and two parts of sand. A concrete bottom, although sometimes used, is not necessary.
The position of the drain, of the house pipe, and of the several collection pipes must not be overlooked when the wall is being built, since it is much easier to leave a hole than to dig through the concrete afterwards.
_Stream supplies._
If the volume of a stream is more than enough for the maximum consumption, nothing is needed but to carry the intake pipe from the sh.o.r.e out under water and protect the end with a strainer. In this case, however, the stream may freeze down to the level of the strainer and even around the strainer, so that the supply of water in winter would be cut off. To avoid this possibility the intake pipe ought to be in a pool of water so deep that it never freezes, and this means sometimes creating a pool for this very purpose. If storage is to be provided, a reservoir must be built, and this intake pipe would naturally be placed at least two feet below the surface of the water.
_Dams._
If the stream is not deep, or if there is not a pool of satisfactory depth, or if the minimum flow of the stream is not adequate for the maximum needs of the consumers, a dam across the stream becomes a necessity. There are two or three types of dams suitable for a reservoir on a small stream, and they may be described briefly.
A dirt dam is not generally desirable, since in most cases the dam must also be used as a waste weir; that is, the freshets must run over the dam. This means that unless the crest of the dam is protected with timber or masonry the dam will be washed out; as happened, indeed, in the terrible flood at Johnstown, Pennsylvania, several years ago. If it is possible to carry the overflow water of the stream away in some other channel than over the dam, then a dirt dam is not objectionable, although always a dirt dam is best with a masonry core. A very good dam can be made by driving three-inch tongue-and-grooved planking tight together across a gulley and then filling in on each side so that the slope on each face is at least two feet horizontal for every foot in height. This last requirement means that if the dam is ten feet high, the width of the dam at the base shall be at least forty-five feet, the other five feet being required to give the proper thickness to the dam at the top.
[Ill.u.s.tration: FIG. 39.--Concrete core in a dam.]
In the second type of dam this central timber core is replaced with a thin wall of concrete as shown in Fig. 39, from six to twelve inches thick, sufficing to prevent small animals burrowing through the dam and at the same time to make the dam more nearly water-tight. Sometimes stone masonry is used, building a light wall to serve as the true dam, and then holding up this light wall with earth-filling on each side. If neither plank, stone, nor concrete can be used, the central core is made of the best earth available, a mixture of clay and sand preferably, and special pains are taken in the building to have this mixture well rammed and compacted.
The writer has recently heard of a dam on a small stream being made by the continual dumping of field stone from the farm into the brook at a certain definite place. This stone, of course, a.s.sumed a slope at each side and settled in place from year to year as the dam grew. The mud and silt of the stream filled up the holes between the stones, so that the dam was finally practically water-tight. This made a cheap construction and had the additional value of serving to use up stones from the fields. It was necessary, since the spring floods poured over the top of this dam, to protect the top stones, and a plank crest was put on, merely to keep the dam from being washed away.
The third type of dam is entirely of concrete or stone masonry, concrete to-day being preferable because more likely to be water-tight. The problem with a concrete dam is to get a foundation such that the impounded water will not leak out under the dam, imperiling the very existence of it. The ideal foundation, of course, is rock, and in a great many locations can be found in the small gulleys where the limestone and shale peculiar to this region will answer as well as more solid rock for dams not more than ten feet high; but with gravel banks on the sides or with soft sandy bottom, or where the clay soil becomes saturated with water at times, the gulley offers great difficulties for the construction of a dam. It will be wise, under such conditions, to carry a cut-off wall, not necessarily more than twelve inches thick, well into the bank, that is, about ten feet on each side, and under the dam this cut-off wall ought to go down until it reaches another stratum of sand or clay or rock. This cut-off wall, then, surrounding the main dam, shuts off the leakage, and the dam itself can be built without danger of undermining. In many large dams this cut-off wall is carried down more than a hundred feet, especially where the depth of water behind the dam is great. For small dams, a row of plank driven down behind a timber sill across and in the bed of the stream will often be sufficient.
[Ill.u.s.tration: FIG. 40.--Section of a flood dam.]
The cross section of the main dam, in cases where flood water in the spring runs over the dam, should be such that the bottom thickness is about one half the height, and Fig. 40 (after Wegman) shows a suitable cross-section of a dam ten feet high. Figure 41 (after Wegman) shows a cross-section intended to carry the water over the dam, especially in times of flood, without danger of erosion.
Sometimes, in a narrow gorge with rock sides, it is possible to save masonry by building the dam in the form of an arch upstream, the resistance to the force of the water being then furnished by the abutment action of the rock sides, instead of by the weight of the dam, as in ordinary construction. For a dam ten feet high, the necessary thickness of the curved dam would probably not be more than twelve inches, while the ordinary gravity dam would be three or four feet thick. The workmans.h.i.+p on the former, however, must be of a very superior order.
[Ill.u.s.tration: FIG. 41.--Section of a flood dam.]
It is never desirable to allow the water flowing over the dam to fall directly on the ground in front, since the falling water will rapidly carry away this soil and undermine the front of the dam. For this reason, the lower section of the dam is made curved, as shown in Fig.
41, giving the water a horizontal direction as it leaves the dam instead of a vertical. A plank floor is often added to carry even further from the dam any possible erosion (Fig. 40). Where it can be done, it is a good plan to provide a small body of still water below the dam, so that the force of the falling water may be distributed through the water on to the soil below.
There are other forms of dams often used. For example, brush dams, formerly common, are made by cutting off the tops of trees and dropping them in place and loading them with stones so as to make a ma.s.s of interwoven branches. These branches hold together particles of earth which are dumped in and form a dam.
Another dam that has been much used in rural communities is the old-fas.h.i.+oned crib dam, where logs are piled up crib fas.h.i.+on, held together at the corners by iron pins, a bottom spiked on, and the crib then filled with stone, a succession of these cribs across the stream forming the dam. Dirt is filled in on each side of this crib work, and, in some cases, cross timbers are set in, and both sides of the dam covered with tongue-and-grooved planking. But such dams are not permanent, and their construction involves an expense nearly equal to that of a permanent structure, and consequently they are not to be recommended.
_Waste weirs._
When the dam is made of earth with or without a core wall and when no opportunity exists for carrying the waste water around the dam, a waste weir of masonry through the dam must be provided, so that freshets may be carried off without destroying or was.h.i.+ng out the earth work.