This soil is found in every condition, varying from slate or shale, which will support any load that can come upon it, to a soft, damp material, which will squeeze out in every direction when a moderately heavy pressure is brought upon it.
In the first place, the top of the footings must be carried below the frost line to prevent heaving, and for the same reason the outside face of the wall should be built with a slight batter and perfectly smooth. The frost line varies with different localities, attaining a depth of six feet in some of the very Northern States, although between three and four feet is the usual depth in the so-called Northern States. The effect of freezing and thawing on clay soils is very much greater than on other soils.
The surface of the ground around the building should be graded so that the rain water will run away from the building, and in most clays subsoil drains are necessary. When the clay occurs in inclined layers, great care must be exercised to prevent it from sliding, and when building on a side hill the utmost precautions must be taken to exclude water from the soil, for if the clay becomes wet the pressure of the walls may cause it to ooze from under the footings. The erection of very heavy buildings in such locations must be considered hazardous, even when every precaution is taken.
Should it be necessary to carry a portion of the foundations to a greater depth than the rest, the lower portion of the walls should be built as described in Section 7, and care must be taken to prevent the upper part of the bed from slipping. Wherever possible, the footings should be carried all around the building at the same level.
10. In Eastern Maine, where the soil is a heavy blue clay, and freezes to the depth of four feet, it is customary to build the foundation walls as shown in Fig. 3, the footings being laid dry, to act as a drain, and the bottom of the trench being slightly inclined to one corner, from whence a drain is carried to take away the water. The portion of the trench outside of the wall is also filled with broken stone or gravel to prevent the clay from freezing to the side of the wall. In the better class of work the outside of the wall is plastered smooth with cement. Sometimes a tile drain is laid just outside and a little below the footings.
11. If the clay contains coarse sand or gravel its supporting power is increased, and it is less liable to slide or ooze away.
In Colorado the top soil consists principally of clay, mixed with fine sand, and as long as it is kept dry will sustain a great load without settlement. As soon as the soil becomes wet, however, it turns into a soft mud, which is very compressible and treacherous. For this reason the footings of heavy buildings are carried through the clay to the sand below. A peculiarity of this soil is that, although it freezes, it has never been known to heave, so that two-story buildings are often built directly on top of the ground, and as long as water is kept away from the walls no injury results.