37. One of the dangers that may be met with during the excavation is the presence of a clay bank between limestone, sandstone, or sand beds. These beds being watertight, that is, not permitting rain water to penetrate their thickness, currents or sheets of water flow over them, which may cause great derangement to the foundations, especially if this clay bank or stratum abuts against the foundation walls. Rain which will easily penetrate the gravel or loam above, or percolate through the crevices of the rock, will be arrested by this clay bank; little by little it will form cavities and produce a concealed current. If the cellar wall extends below this current, the water will penetrate it, and fill the cellar.

In this case, it will be necessary to turn aside this sheet of water by means of a drain, as shown at (a) in Fig. 5. Here a b is the gravel, sand, or loam bed that water can penetrate; c d, the clay bed on top of which water collects after every shower, which would be stopped by the foundation of the cellar e, and would soon penetrate it; f is the transverse drain with openings upward to receive the water, thus leaving the wall e dry. Fig. 5 (b) shows a view of part of the intercepting drain; a, a are openings for the water to enter.

Protection Of Foundation Walls 5

Fig. 5.

38. If foundations are laid in open clay, another difficulty is often met and must be guarded against.

Clay banks may slip, when their slopes present such a section as shown in Fig. 6, where a shows the rock bed under the clay bank or stratum b, and c is the gravel or loam on top of the clay. The rain water accumulating above will pass at e under this bank, and if the rain continues long will form from c to e a thick, soapy bed so that the clay bank may slip by its own weight especially if that weight be increased by that of the building d.

Protection Of Foundation Walls 6

Fig. 6.

This danger may be averted by building the drain f at the upper end of the clay bank c e, to intercept the water running between the clay bank and the ledge of rock. It would be well also to extend the foundation wall at d down to the rock, thus presenting a solid face to the pressure of the clay bank, and preventing absolutely the upheaval of the cellar floor and bulging of the wall d.

39. Excavation is usually measured by the cubic yard of 27 cubic feet, and the price paid varies according to locality, nature of the soil, distance material excavated must be hauled, and disposition to be made of it.

Twenty-four cubic feet of sand or 17 cubic feet of earth or clay, when comparatively dry, will weigh a ton.

One cubic yard of earth before removal will occupy about 1 1/2 cubic yards when dug, contains 21 struck bushels, and is considered a single load.