The greatest difficulty with which an architect has to contend in the groundworks of a building is that of the ground-water in low-lying lands. Springs on hillsides are easily dealt with, but the water which percolates through mud and gravel only a few feet below the surface of the ground, and rises and fails perhaps with the rise and fall of a neighbouring river or ditch, furnishes a more difficult problem. Nor is a site like this confined to plains; it may be found on the banks of most rivers, even in deep and narrow valleys.

A permanently high level of ground-water is dangerous to health, but fluctuating ground-water is much worse. It is one advantage of subsoil-drainage that it tends to prevent extreme rise of the water, and so lessen the degree of fluctuation.

The level of ground-water, it may be added, is always raised by capillarity. The amount of rise has been estimated at about 1 foot in sands, and 4 or 5 feet in clay and compact marl. The rise will be lessened in many soils by properly opening and draining them.

In many cases, it is a mere farce to talk of draining the subsoil to a depth of 6, 8, 10, or 12 feet; not until the ocean has been drained, can the level of the ground-water in many parts of these islands be permanently lowered. Where the sea has to be kept out by dikes and sluice-gates, it is of little use talking about subsoil-drain age. So difficult is it to render dwellings on such low-lying sites habitable, that by the London Building Act, 1894, the London County Council received power to prevent the erection of dwelling-houses upon them. In many elevated places, however, there are damp, even boggy, patches of ground, and these can easily be drained, because there is an outfall for the drain into the valleys below.

Not all ground requires underdrains: many rocky, sandy, and gravelly sites are sufficiently dry already. But every site must be judged by itself, as the nature of the ground varies greatly even in a short distance. It is better, however, to drain too much than too little. The drainage of clay soils renders them drier, and, by reducing the evaporation, warmer. Sandy and gravelly soils are naturally drier and warmer than clay; on account of their porosity water rapidly sinks through them, and they contain a considerable volume of air. ln these the fluctuation of ground-water and consequent exhalation of more or less impure ground-air are more to be feared than dampness.

Subsoil-drains are sometimes - especially where stone is plentiful - merely trenches cut to the necessary depth, and filled to the height of 2 or 3 feet with ' pieces of broken stone. The ground-water, taking the line of least resistance, finds its way along these "rubble drains" (for so they are called) to the appointed outlet. Sometimes a small square drain is formed at the bottom of the trench with stones, or with tile bottom and brick sides and top.

It is always better, however, to provide pipes for subsoil-drainage, as these permit the water to flow off more rapidly and are less liable to choke than the simple stone drains. They may be either round or D-shaped, and should not be less than 3 inches in diameter. The ordinary unsocketed agricultural drainpipes are frequently used, but there is some difficulty in keeping the ends of the pipes together, both horizontally and vertically. To obviate this, half collars 3 or 4 inches long are sometimes placed under the joints, or pipes with a socket on the lower half only are used. Frequently ordinary soeketed drain-pipes are used, but with the joints left without cement or clay. The two last methods are the best Sometimes, as in Brooke's patent, a D-shaped subsoil-drain is laid Hat-side down, and upon it the sewage-drain is supported by means of concave pipe-rests made to fit the curves of the two drains.

Whatever kind of pipe is used, the trench above should be filled with broken stone or screened gravel to the height of 2 or 3 feet.

In very wet and loose sandy soils, drains may carry away, little by little, considerable volumes of sand, and so endanger the ground and structures al>ove. At Worksop, the earthenware sewers have caused so much mischief in this way that large portions of them have had to be replaced with iron. In extreme cases of this kind, subsoil-drains will be best omitted, and the money thus saved expended on a good layer of asphalt with concrete bed over the whole site.

The depth of subsoil-drains should be as great as possible - the deeper the Utter, - but considerations of outfall and expense will frequently prevent the depth being more than 3 or 4 feet below the basement floor. Where the drains are shallow, they should be closer together than is necessary when they are deep.

The distance apart of subsoil-drains must depend upon their depth, the quantity of water, and the nature of the ground. The stiffer the ground, the closer must they be. In stiff clay they should be laid every 3 or 4 yards, in loamy clay every 5 or 6 yards, while in sand and gravel they may be 10 or 15 yards apart. Indeed, in the latter case, it is frequently sufficient to carry a drain around the outside of a building, as shown in fig 19. In the other cases, however, it will usually be necessary to lay branch drains across the site in addition to the important drain encircling it.

The outlet for ground-water must he arranged according to circumstances. In many towns now. especially where the sewage is treated chemically, there is a great objection to the ground-water being conveyed to the sewers, and spe-cial "sewers" for surface-water and ground-water are provided, emptying into the nearest stream at various convenient points. In the country the subsoil-drains may be carried to the nearest stream, or, if there is sufficient fall, brought to the surface of the ground at some distance from the house and used for irrigation. Where, however, they must be connected with the sewage-conduits, they must be disconnected, as well from the house-drain as from the public sewer. This may be done by means of a disconnecting trap and air-shaft. A convenient place for the trap is the side of an inspection-chamber, as shown in Fig. 20. Instead of the pipe-shaft, a small chamber is sometimes built of bricks with a disconnecting trap between it and the sewer-disconnecting chamber