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.
There is great variation in clay soils, ranging from a very soft mass which will squeeze out in all directions when a very small pressure is applied, to shale or slate which will support a very heavy load. As the bearing capacity of ordinary clay is largely dependent upon its dryness, it is therefore very important that a clay soil should be well drained, and that a foundation laid on such a soil should be at a sufficient depth to be unaffected by the weather. If the clay cannot be easily drained, means should be taken to prevent the penetration of water. When the strata are not horizontal, great care must be taken to prevent the flow of the soil under pressure. When gravel or coarse sand is mixed with the clay, the bearing capacity of the soil is greatly increased.
The bearing capacity of a soft clay is from 2,000 to 4,000 pounds per square foot; of a thick bed of medium dry clay, 4,000 to 8,000 pounds per square foot, and for a thick bed of dry clay, 8,000 to 10,000 pounds per square foot.