In modern building construction are usually made of concrete, and must be carried down to a layer of earth or rock strong enough to bear the weight of the building and its contents and to withstand the force of the wind blowing on surefaces of the building. The nature of the ground has to be studied and the weight it can safely carry has to be culculated most carefully, for if too great a load is applied the ground will sink, or “settle”, causing the building to crack. In the 1930’s, for example, it was discovered that the supporting power of the ground beneath St. Paul’s Cathedral, in London, was becoming seriously reduced by the pumping of water from the neighboring sites, and a special Act of Parliament was passed to restrict any more buildings or excavations in that area. In some countries the special stresses caused by earthquakes must also be allowed for. Rock, coarse sand, and gravels normally provide good support, but finer soils, such as clay, may present considerable problems. The clay subsoils of London, for example, is one of the factors that limits the height and size of building there; whereas in New York, where the ground consists of solid rock, towering skyscrapers can be built.
The main type of concrete foundations are “independent”, “strip”, “raft”, and ” pile”. For bridge building and other heavy engineering structures “cassion” foundations are used. Independent foundations are used to support columns or piers which are unevenly spaced and unequally loaded. A pad of concrete, plain or reinforced, is placed under each column or pier, the base of the pad extending far enough in all directions to spread the load evenly over the ground. Strip foundations are used when a row of columns is so closely spaced that independent foundations would nearly meet. When the columns are both near together and regularly spaced, and carry nearly equal loads, raft (sometimes called “slab” ) foundations, extending the whole length and breadth of a building, are used instead of strip foundations. The raft may be a thick slab of mass concrete, or a thin slab of reinforced concrete, laid between lines of concrete beams joining the feet of the columns. A raft foundations is often suitable in very soft ground, for it enables the building to “float” as a whole, where separate, independent foundations might sink by unequal amounts. Piles are used to take the weight of a building when it is imposible to dig down to a solid layer of earth, either because the ground is water-logged or because there is a very great depth of poor soil. Piles are solid lengths of timber, steel, or reinforced concrete, generally from 20 to 80 feet long driven into the ground by a heavy steam hammer. Sometimes hollow steel tubes are used, being driven into the ground in the same way as solid piles and afterwards filled with concrete. Before the concrete hardens, the tubes are withdrawn and can be used again.