A foundation consisting partly of rock and partly of some softer stratum is most dangerous and untrustworthy, as the latter will yield more than the former, causing unequal settlement and fracture of the superstructure.

If the softer parts are of small extent they may be arched over, using the adjacent portions of rock as abutments.

If this be not possible they should be consolidated by driving in piles close together (see page 226), or excavating to a good depth and filling in with concrete.

If neither of these expedients can be adopted the building over the soft parts should first be carried up and allowed to settle to its bearings, and then the remainder built upon the hard rock, the latter being kept distinct from the former.

Gravel, when sound, makes one of the best possible foundations, as it is incompressible, and not affected by atmospheric influences.

If loose and coarse it may be greatly improved by grouting it with thin mortar, or sometimes a thin layer of concrete is spread on the bottom of the trench.

If very unsound it may be necessary to proceed as in the case of loose sand (see below).

Chalk varies immensely in its nature and characteristics; sometimes it is found as hard as rock, in other cases as soft as butter.

The hardest description of chalk may be treated as rock if its permanent dryness can be ensured.

The softness of chalk is caused by wet; before using it as a foundation all water should be removed and prevented from recurring.

This may be done by draining the trench, punning the sides with clay to prevent the ingress of water, or by putting in concrete at weak points.

Springs should, however, rather be diverted than dammed out, as otherwise they will very likely burst through in rainy weather.

Clay is a good soil to build upon when it is sound, tolerably dry, and protected from the action of the atmosphere by making the foundations deep or covering the bottom of the trenches with concrete.

Clay is very liable, especially in hot weather, to crack and form deep fissures, by which water is led below the surface, which will injure the footings unless they are placed deep enough to be out of the reach of the fissures and well drained.

When these precautions are neglected the clay undergoes continual changes in bulk from atmospheric influence, and becomes a very dangerous material to build upon.