WHAT IS FOUNDATION ? 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. 1. 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 (9 in) 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. 2. Types of Foundations Designed to provide buildings with support and stability, foundations are the first structural components installed in most construction projects. Spread-footing foundations (A) are a common, economical choice for projects built on stable ground. Friction piles (B) distribute weight along their entire length, unlike caisson piers (C), which transmit the building’s load to the stable bedrock that only the ends of the piers contact. Mat foundations (D) are reinforced concrete slabs used when building loads are relatively large, and ground conditions are unstable; these foundations carry the downward load of a building as a unit “floating” on the soil. 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 (Figure 1). 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 (100 ft) 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. 3. Groundwater Level Foundation construction is complicated by groundwater flowing above the bottom of the proposed foundation level. In such cases the sides of the excavation may be undermined and cave in. Lowering the groundwater level by pumping the water out of the excavation usually requires the installation of braced sheathing to shore up, or retain, the sides of the excavation to prevent any cave-ins. When the amount of water within the excavation is excessive, ordinary pumping methods, which bring to the surface loose soil mixed with the water, can undermine the foundations of buildings on adjoining property. To prevent damage caused by soil movement, well point dewatering is often used. Well points are small pipes with a perforated screen at one end. They are driven or jetted into the ground so that the screen, which prevents soil from flowing in with the water, is below groundwater level. These pipes are linked to a common manifold (pipe) that is connected to a water pump. In this way the groundwater is removed from below the excavation without damaging nearby property. Dewatering may also make it unnecessary to sheathe the sides of the excavation, providing the soil will not slide into the excavation because of its composition or because of vibrations from nearby heavy traffic or machinery. .
Posted on: Wed, 30 Jul 2014 18:41:49 +0000
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