In these soils, as also in very loose gravel, care must be taken to exclude water, which might otherwise penetrate and wash away the soil, causing hollows in the foundation and subsidence in the superstructure.
Foundations in these soils require great care, more especially if the site is made up of different kinds, one more compressible than the other; in such a case unequal settlement may be apprehended, and should be guarded against.
The pressure is thus distributed by the concrete bed over a larger area, and does not bear so heavily on each superficial foot of the soil.
When the ground is marshy or of such a nature that it would not bear the weight required, even when distributed over a large area of concrete, more complicated arrangements must be adopted, according to the nature of the case.
1. A soft stratum of moderate depth overlying hard ground.
In this case the foundation should be carried down to the solid ground ; or, if this would be too expensive, a number of piers may be sunk, and arches turned from one to the other, upon which the building may rest, or similar piers may be used to support a timber platform; or again, instead of the piers, holes may be driven through the soft upper stratum and filled in with concrete, sand (if the ground will resist its lateral pressure), stones, or other incompressible material; or lastly, piles may be driven into the hard substratum to act as supports for a platform.
2. Ground very soft to an indefinite depth.
Such ground may be treated in several different ways.
Sometimes a wide trench filled with good concrete, of such a thickness as to resist fracture, will answer by distributing the pressure over such a large area that the soil is enabled to bear it, or a trench may be filled with moist sand carefully punned; in this case the natural soil must be able to retain the sand laterally, as it will press upon the sides as well as the bottom of the trench.
If the trench cannot be kept free from water, holes about 6 feet deep and 6 inches in diameter may be bored and filled with slightly moistened sand. These are better than timber piles, as the sand transmits the pressure upon it tolerably well, and driving is avoided, which shakes the ground.
Another plan is to form a raft of timber or fascines, which floats upon the nearly liquid soil and distributes the weight of the building over a large area.
In such a case it is important that the centre of gravity of the building should be immediately over that of the platform, and the latter should be evenly weighted, or it may sink more on one side than on the other.
A fascine platform consists simply of two or three courses formed of fascines (long bundles of brushwood) laid close together, the alternate courses being in opposite directions, and the whole being kept in position by wooden pickets.
Such platforms should either be at a depth where they will be constantly wet, or be so drained as to be permanently dry, otherwise the material will soon perish.
Again, a soft soil may be consolidated by driving into it, over an area larger than the proposed building, short piles quite close together; these are prevented from sinking by the friction of their sides against the soil.
In all these cases the pressure has a tendency to cause the ground immediately around the foundations to rise; this must be counteracted by placing stones or concrete upon it so as to act as a counterbalancing weight.
Sometimes before laying the platform the site is surrounded by sheet piling to prevent the lateral escape of the soft soil when the weight of the building comes upon it.
3. A crust of good ground overlying a soft substratum.
If the crust be thick enough to bear the weight required it should be left alone, care being taken not to cut or injure it.
In alluvial soils there is frequently a layer of clay over a stratum of soft mud; in such a case piles would do harm, as they would disturb and injure the crust of clay; it would be better, therefore, merely to pun and consolidate the clay with a rammer.
The upper crust should be sounded by striking it with a log; experience will tell whether it gives out a clear ring or a hollow sound; in the latter case it is not to be trusted.
When the upper crust is not thick enough to bear the weight originally intended, the area of the foundations may sometimes be increased so as to reduce the pressure, on each superficial foot, to what the crust can bear.
When the substratum is sand it will be safe to build upon if its lateral escape is prevented. A peaty substratum should, if possible, be thoroughly drained.
When the hard crust rests upon a soft stratum which crops out on a cliff or the bank of a river, particular care must be taken or it will be found to ooze out and cause a subsidence of the crust.
When the soft substratum is very shallow, open drains may be cut so as to encourage it to ooze out and permit the weighted crust to take its bearings; but if it be of considerable depth such excavations must be carefully avoided.