Foundations

The structural design of a building depends greatly on the nature of the soil and underlying geologic conditions and modification by man of either of these factors

Ground Conditions

If a building is to be constructed in an area that has a history of earthquake activity, the earth must be investigated to a considerable depth. Faults in the crust of the earth beneath the soil must obviously be avoided. Some soils may liquefy when subjected to the shock waves of a quake and become like quicksand. In such cases, either construction must be avoided altogether or the foundation must be made deep enough to reach solid material below the potentially unstable soil. Certain clay soils have been found to expand 23 cm or more if subjected to long cycles of drying or wetting, thus producing powerful forces that can shear foundations and lift lightweight buildings. Some soils with high organic content may, over time, compress under the building load to a fraction of their original volume, causing the structure to settle. Other soils tend to slide under loads.

Soils that have been modified in some way often perform differently, especially when other soil has been added to or mixed with existing soil, or when the soil has been made wetter or drier than normal, or when cement or chemicals such as lime have been added. Sometimes the soil under a proposed building varies so greatly over the entire site that a building simply cannot be constructed safely or economically Soil and geologic analyses are necessary, therefore, to determine whether a proposed building can be supported adequately and what would be the most effective and economical method of support.

If there is sound bedrock a short distance below the surface of the construction site, the area over which the building loads are distributed can be quite small because of the strength of the rock. As progressively weaker rock and soils are encountered, however, the area over which the loads are distributed must be increased.

Types of Foundations

The most common types of foundation systems are classified as shallow and deep. Shallow foundation systems are several feet below the bottom of the building; examples are spread footings and mats. Deep foundations extend several dozen feet below the building; examples are piles and caissons. The foundation chosen for any particular building depends on the strength of the rock or soil, magnitude of structural loads, and depth of groundwater level.

The most economical foundation is the reinforced-concrete spread footing, which is used for buildings in areas where the subsurface conditions present no unusual difficulties. The foundation consists of concrete slabs located under each structural column and a continuous slab under load-bearing walls.

Mat foundations are typically used when the building loads are so extensive and the soil so weak that individual footings would cover more than half the building area. A mat is a flat concrete slab, heavily reinforced with steel, which carries the downward loads of the individual columns or walls. The mat load per unit area that is transmitted to the underlying soil is small in magnitude and is distributed over the entire area. For large mats supporting heavy buildings, the loads are distributed more evenly by using supplementary foundations and cross walls, which stiffen the mat.

Piles are used primarily in areas where near-surface soil conditions are poor. They are made of timber, concrete, or steel and are located in clusters. The piles are driven down to strong soil or rock at a predetermined depth, and each cluster is then covered by a cap of reinforced concrete. A pile may support its load either at the lower end or by skin friction along its entire length. The number of piles in each cluster is determined by the structural load and the average load-carrying capacity of each pile in the cluster. A timber pile is simply the trunk of a tree stripped of its branches and is thus limited in height. A concrete pile, on the other hand, may be of any reasonable length and may extend below groundwater level as well. For extremely heavy or tall buildings, steel piles, known as H-piles because of their shape, are used. H-piles are driven through to bedrock, often as far as 30 m below the surface. H-piles can be driven to great depths more easily than piles made of wood or concrete; although they are more expensive, the cost is usually justified for large buildings, which represent a substantial financial investment. Caisson foundations are used when soil of adequate bearing strength is found below surface layers of weak materials such as fill or peat. A caisson foundation consists of concrete columns constructed in cylindrical shafts excavated under the proposed structural column locations. The caisson foundations carry the building loads at their lower ends, which are often bell-shaped.

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