Shallow foundations (sometimes called 'spread footings') include pads ('isolated footings'), strip footings and rafts.
Deep foundations include piles, pile walls, diaphragm walls and caissons.

Shallow foundations are those built near to the finished ground surface; generally where the depth of the foundation is less than the width of the footing and less than 3 meters deep. These are not strict rules, but merely guidelines. Shallow foundations (sometimes called 'spread footings') include pads ('isolated footings'), strip footings and rafts.

Shallows foundations are used when surface soils are sufficiently strong and stiff to support the imposed loads; they are generally unsuitable in weak or highly compressible soils, such as poorly-compacted fill, peat, alluvial deposits, etc. This is significant when thinking about building on made lands in Boston, such as the Back Bay, which is filled with gravel, sand and/or mud to a depth of at least 5-10 meters.

• Pad foundations are used to support an individual concentrated load such as that due to a column. They may be circular, square or reactangular. They usually consist of a block or slab. Pad foundations are usually shallow, but deep pad foundations can also be used.
• Strip foundations are used to support a line of loads, either due to a wall, or if a line of columns need supporting where column positions are so close that individual pad foundations would be inappropriate.
• Raft foundations are used to spread the load from a structure over a large area, normally the entire area of the structure. They are used when column loads or other structural loads are close together and individual pad foundations would interact. A raft foundation normally consists of a concrete slab which extends over the entire loaded area. It may be stiffened by ribs or beams incorporated into the foundation. Raft foundations have the advantage of resisting settling because the weight is spread out over a large area. They are often needed on soft or loose soils as they can spread the loads over a larger area. The Federal Reserve Bank building on Atlantic Avenue is on a Raft Foundation.

Deep foundations are those well below the surface, usually at depths >3 m below finished ground level. Deep foundations can be used to transfer the weight of the structure to deeper, stronger layers if unsuitable soils are present near the surface.

• Piles are relatively long, slender posts that transmits the weight of a structure through weaker, top soil layers to stronger, deeper soil layers or solid rock. They are used when for reasons of money, construction, or soil conditions a shallow foundation is not practical.
• Piers are foundations for carrying a heavy structural load which is constructed on site in a deep excavation.
• Caissons are a form of deep foundation which are constructed above ground level, then sunk to the required level by excavating or dredging material from within the caisson.
• Compensated foundations are essentially deeply excavated basements, such as using a raft-type foundation built very deeply in the grou

Piles are often used because the surface soils are too weak to support the structure. It is important to understand that piles get support from both end bearing and skin friction. The amount of support that a given pile gets from either of these depends on the soil conditions. Piles can be used to support various different types of structural loads.

End bearing piles are those which terminate in hard, relatively impenetrable material such as rock or very dense sand and gravel. They derive most of their support from the resistance of the layers at the bottom of the pile. The Prudential Center and John Hancock buildings are examples of buildings built on end-bearing piles that are drilled into the bedrock.

Friction piles obtain a greater part of their support by skin friction. This tends to occur when piles do not reach an impenetrable layer but are driven for some distance into a penetrable soil. The longer the pile, the greater the friction force created by the pile pushing down. Boston City Hall and Massachusetts General Hospital are examples of a buildings built on Friction Piles

Piles that pass through layers of moderately- to poorly-compacted fill will be affected by negative skin friction, which produces a downward drag along the pile shaft and therefore an additional load on the pile. This occurs as the fill consolidates under its own weight.

For pictures of pilings being installed, visit this site

An end-bearing pile resting on an incompressible layer, such as solid rock.

A friction pile, which gets its strength by the fact that soil generally gets stronger as it gets deeper

This diagram illustrates skin friction, which is an additional force on a pile as a result of loose fill that settles along the pile.

The diagrams below show a simple example of end-bearing and friction-bearing pilings used to support a structure.

What do you think will happen if the friction between the pile and the ground is not enough to support the weight of the structure? Roll the cursor over the image below to find out.

In many cases, simply lengthening the pile will provide enough friction to support the structure. If the two forces balance, then the pile will not sink. Why do you think a longer pile creates more friction?

Piles may be pounded into place by the use of heavy weights or other machinery. These are known as displacement piles. These are usually made of steel or wood.

Another method of installing piles is by excavating the ground and placing the pile into the hole. These are known as non-displacement piles. Often in this case concrete is used - a hole is excavated and concrete is poured in to create the pile.

Problems with Pile degredation

In Boston, one of the major problems with older pile foundations is pile rot. Most of the original Back Bay buildings were constructed on wood friction piles. As long as the piles remain under the "water table" (the level of water in the ground), then the piles will remain intact. However, any part of a wood pile that is exposed above the water table is susceptible to rot. There have been many examples of buildings being damaged as a result of rotting piles.