Читать книгу Rural Hygiene - Henry N. Ogden - Страница 21

Fig. 3.—A grading that turns water away from the house.

Too much emphasis cannot be laid on the necessity for grading the ground surface away from the house. In some cases it may be sufficient to dig a broad shallow trench protected from wash by sods (Fig. 3). In other cases it may be desirable to pave the ditch with cobble stones or to build a cement gutter. In constructing such a surface drain, proper allowance must be made for the accumulation of snow and the resulting amount of water in the spring, so that the distance in which the ground slopes away from the house ought to be, if possible, at least ten feet, so that there can be no standing water to penetrate the house walls. The slope necessary to carry surface water away need not be great. A fall of one foot in one hundred will be ample, even on grassy areas, and if the surface is that of a macadam road or the gutters of a drive, this grade may be cut in two. A slope of more than one foot in one hundred is permissible up to a maximum of seven or eight feet per hundred, more than this being æsthetically objectionable and tending to make the house appear too high. Whenever gutters are built in driveways or ditches to intercept water coming down the slopes, a suitable outlet must be provided to carry the water thus collected either into underground pipes, by which the water is led to some stream or gulley, or directly into some well-marked surface depression.

Ground water.

The soil always contains water at a greater or less depth, and the elevation of this "ground water," as it is called, varies throughout the year partly with the rainfall and partly with the elevation of the water level in the near-by streams.

It is not at all unusual for this ground water to rise and fall six feet or more within the year, high levels coming usually in the spring and fall, and low levels in the late summer and winter. It is easily possible, then, that a house cellar may seem dry at the time of construction in summer and may develop water to a foot or more in depth after occupancy. The presence of such an amount of water in a cellar, whether injurious to health or not, is objectionable, and a subsoil trench should be provided in order to limit the height to which ground water may rise.

If a system of drainpipes is led around a house extending outward to include the surrounding yard, then the ground water will always be maintained at the level of those pipes, provided the system has a free outlet. Indeed, the question of an outlet for a drainage system is a most important factor, and no system of underdrains can be effective unless a stream or gulley or depression of some kind is available into which the drains may discharge. It is for this reason, quite as much as for any other, that the location of a house on a perfectly level bottom land is objectionable, since the ground there may be normally full of water with no existing depression into which it may be drained.

In the next chapter the proper method of laying drains close to the cellar wall, for the purpose of taking away the dampness from those walls, is described, but another system of drains is desirable, covering more area and more thoroughly drying the ground, provided the ground water needs attention at all. These drains should be laid like all agricultural drainage; and while substitution of broken stone, bundles of twigs, wooden boxes, or flat stone may be made, the only proper material to be used is burnt clay in the form of tile. These tiles are made in a variety of patterns, but the most common in use to-day is one which is octagonal outside and circular inside. They are about one foot in length and may be had from two to six inches inside diameter. The ordinary size for laterals is four-inch diameter, while the mains into which these laterals discharge are generally of six-inch diameter. These tiles are laid in trenches about fifteen feet apart, although in porous soil, such as coarse sand or gravel, this distance may be increased to twenty feet. If the tiles are laid more than four feet below the surface, this distance may be increased, and if the tiles are five feet deep, the distance apart of the several lines may be fifty feet.

The grade of the line must be carefully taken care of, and while it is possible to lay a line of tile with a carpenter's level and a sixteen-foot straightedge, it is much safer to have an engineer's or architect's level and set grade stakes, as in regular sewer work. A fall of one fourth of an inch to the foot is a proper grade, although a greater slope is not objectionable. It is sometimes desirable in soft ground to lay down a board six inches wide in the bottom of a trench on which to rest the tile, but, unless the ground is very soft, this is not necessary. Care must be taken, however, if the board is not used, to have the bottom of the trench very carefully smoothed so that a perfectly even grade in the tile is maintained. There are three ways of laying out a line of trench as shown in the following sketches (Fig. 4). It is usually sufficient to run parallel lines of tile from fifteen to fifty feet apart over the area which it is desired to drain, and let the ends of these lines enter a cross line which shall carry off the water led into it. This cross line should be six inches in diameter as a general rule, unless there is more than a mile of small drains, in which case the size of the cross pipe ought to be increased to eight inches. This cross line then becomes the main outlet, and great care must be taken to see that it has a perfectly free delivery at all times of the year. In cities and sometimes in small villages it is possible to discharge this outlet pipe into a regular public sewer, provided the sewer is deep enough, and provided the municipal ordinances allow such a connection. Otherwise, the outfall must be carried to a natural depression.

Fig. 4.—Modes of laying out drains.

In level ground, the problem of finding a suitable outlet is a serious one, and in many cases impossible of solution, so that the householder, being unable to find an outlet, must put up with the ground water and be as patient as possible during its prevalence. It does not do to trust one's eye to find a practicable outlet, since even a trained eye is easily deceived. An engineer with a level can tell in a few moments where a proper point of discharge may be found, and it is absurd to begrudge the small amount which it will cost, in view of the large expense involved in digging a long trench to no purpose.

Some years ago the writer was able to note the conditions in a house where the cellar excavation went three feet into limestone rock. The strata were perfectly level and the cellar floor of natural rock was apparently all that could be desired, smooth and flat, without involving any expense for concrete. One wall came where a vertical seam in the rock existed, and since this natural rock face was smooth and vertical and just where the cellar wall should go, it seemed unnecessary to dig it out and lay up masonry in its place. So it was left and the house built. When the spring rains came, however, the cellar was turned into a pond, water dripping everywhere from the vertical rock face, and coming up through the cellar bottom like springs. It cost a great deal more then to make the changes and improvements necessary in order to secure a dry cellar than it would have done at the outset. This serves as an illustration of the need of taking every precaution at the beginning to insure a dry and well-drained soil around and below the cellar walls.