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Shallow foundations of a house versus the deep foundations of a Skyscraper.

A foundation (also called a groundsill) is a structure that transfers loads to the earth. Foundations are generally broken into two categories: shallow foundations and deep foundations.

Contents

Shallow foundation

Shallow foundation is, usually, embedded a meter or so into soil. One common type is the spread footing which consists of strips or pads of concrete (or other materials) which extend below the frost line and transfer the weight from walls and columns to the soil or bedrock. Another common type is the slab-on-grade foundation where the weight of the building is transferred to the soil through a concrete slab placed at the surface.

Deep foundation

A deep foundation is used to transfer a load from a structure through an upper weak layer of soil to a stronger deeper layer of soil. There are different types of deep foundations including helical piles, impact driven piles, drilled shafts, caissons, piers, and earth stabilized columns. The naming conventions for different types of foundations vary between different engineers. Historically, piles were wood, later steel, reinforced concrete, and pre-tensioned concrete.

Base-isolating foundation

Base isolator being tested at the UCSD Caltrans-SRMD facility

Base-isolating foundation, also known as seismic or base isolation system, is a collection of structural elements which should substantially decouple a superstructure from its substructure resting on a shaking ground thus protecting a building or non-building structure's integrity during a potentially devastating earthquake.

Base-isolating foundation is believed to be the most powerful tool of the contemporary earthquake engineering pertaining to the passive structural vibration control technologies [1].

Elevated building foundation

Bottom view of the Municipal Services Building [3] sitting on abutments of its elevated building foundation, City of Glendale, CA

Elevated building foundation (or EBF) is a kind of seismic vibration control technology which remains an integral part of a building superstructure [2]. It is conceived to shield the building's superstructure against potentially destructive components of the anticipated earthquakes including both lateral and vertical shaking.

This goal can be achieved by means of a proper choice of building materials, dimensions, and configuration of EBF for the particular construction site and local soil conditions.

As a result of multiple wave reflections and diffractions, as well as energy dissipations of the seismic waves in a process of their vertical propagation through horizontal strata of the EBF, any transmission of seismic wave energy into the building superstructure furnished with EBF will be decreased considerably which will decrease seismic loads and enhance seismic performance of the structure [3].

Design

Foundations are designed to have an adequate load capacity with limited settlement by a geotechnical engineer, and the foundation itself is designed structurally by a structural engineer.

The primary design concerns are settlement and bearing capacity. When considering settlement, total settlement and differential settlement is normally considered. Differential settlement is when one part of a foundation settles more than another part. This can cause problems to the structure the foundation is supporting. It is necessary that a foundation is not loaded beyond its bearing capacity or the foundation will "fail".

Other design considerations include scour and frost heave. Scour is when flowing water removes supporting soil from around a foundation (like a pier supporting a bridge over a river). Frost heave occurs when water in the ground freezes to form ice lenses.

Changes in soil moisture can cause expansive clay to swell and shrink. This swelling can vary across the footing due to seasonal changes or the effects of vegetation removing moisture. The variation in swell can cause the soil to distort, cracking the structure over it. This is a particular problem for house footings in semi-arid climates such as South Australia, Southwestern US, Turkey, Israel, Iran and South Africa where wet winters are followed by hot dry summers. Raft slabs with inherent stiffness have been developed in Australia with capabilities to resist this movement.

When structures are built in areas of permafrost, special consideration must be given to the thermal effect the structure will have on the permafrost. Generally, the structure is designed in a way that tries to prevent the permafrost from melting.

External links

References

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