Shallow and deep foundations signify the relative depth of the soil on which buildings are founded. When the depth of a foundation is less than the width of the footing and is less than ten feet deep, it is a shallow foundation. Shallow foundations are used when surface soils are strong enough to support the imposed loads. If the depth of a foundation is more than the width of the building foundation, it is a deep foundation. Deep foundations are often used to transfer building loads deeper into the ground.
Conditions where deep foundations are used
· Soil near the surface that has relatively weak bearing capacities (700 pounds per square foot or less)
· Soils near the surface that contain expansive clays (shrink/swell soils)
· Surface soils that are vulnerable to being removed by erosion or scour
Classification of deep foundations
Deep foundations are classified into three categories:
· Pile foundations
· Well foundations
· Caisson foundations
Types of foundations and basic mechanisms involved in the classification of deep foundations are reviewed in our FE Civil exam review course for those preparing to become an engineer in training.
A pile foundation is defined as a series of columns constructed or inserted into the ground to transmit loads to a lower level of subsoil. A pile is a long cylinder made up of a strong material, such as concrete. Piles are pushed into the ground to act as a steady support for structures built on top of them. Piles transfer the loads from structures to hard strata, rocks, or soil with high bearing capacity. The piles support the structure by remaining solidly placed in the soil. As pile foundations are set in the soil, they are more tolerant to erosion and scour.
Installation of pile foundations
Piles are first cast at ground level and then hammered or driven into the ground using a pile driver. A pile driver is a machine that holds the pile vertical and hammers it into the ground. Blows are repeated by lifting a heavy weight and dropping it on top of the pile. Piles should be hammered into the ground until the refusal point is reached, which is the point where a pile cannot be driven into the soil any farther. The method of installing a pile is a major consideration in the structural integrity of pile foundations. The driven-pile method is an ideal option because it least disturbs the supporting soil around the pile and results in the highest bearing capacity for each pile. Since every pile has a zone of influence on the soil around it, piles must be spaced far enough apart from each other so that the loads are distributed evenly.
Categories of piles
· Depending on their function, piles are classified as bearing piles, friction piles, friction-cum-bearing piles, batter piles, guide piles, and sheet piles.
· Based on the composition of materials, piles are classified as timber piles, concrete piles, sand piles, or steel piles.
1)Bearing piles are driven into the ground until a hard stratum is reached. Bearing piles rest on hard strata and act as pillars to support the structure. Bearing piles allow vertical loads and transfer the building load to the hard stratum underneath.
2)Friction piles are used when the soil is soft and there are no hard strata available. These piles are long, and the surfaces are roughened to increase surface area and increase frictional resistance. They bear on frictional resistance between their outer surface and the soil in contact. Friction piles do not rest on hard strata.
3)Batter piles are driven inclined to resist inclined loads.
4)Guide piles are used in the formation of cofferdams to provide stable foundations for under-water construction.
Basic principles of pile foundations and their classifications are recommended topics to review prior to taking the FE Civil exam.
Types of piles based on shape and composition