This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The general method of doing this consists in making the soil more dense. This may be done by driving a large number of piles into the soil, especially if the piles will be always under the water line in that ground. Driving the piles compresses the soil; and if the piles are always under water, they, will be free from decay. If the soil is sufficiently firm so that the pile can be withdrawn and the hole will retain its form even temporarily, it is better to draw the pile and then immediately fill the hole with sand, which is rammed into the hole as compactly as possible. This gives us a type of piling known as sand piles.
A soft, clayey subsoil may frequently be improved by covering it with gravel, which is rammed and pressed into the clay. Such a device is not very effective, but it may sometimes be sufficiently effective for its purpose.
A subsoil is often very soft because it is saturated with water which cannot readily escape. Frequently a system of deep drainage which will reduce the natural level of the ground-water considerably below the desired depth of the bottom of the foundation, will transform the subsoil into a dry, firm soil which is amply strong for its purpose. Even when the subsoil is very soft, it will sustain a heavy load, provided that it can be confined. While excavating for the foundations of the tower of Trinity Church in New York City, a large pocket of quicksand was discovered directly under the proposed tower. Owing to the volume of the quicksand, it was found to be impracticable to drain it all out; but it was also discovered that the quicksand was confined within a pocket of firm soil. A thick layer of concrete was then laid across the top, which effectively sealed up the pocket of quicksand, and the result has been perfectly satisfactory.