huangyhg

definitions of piles, caissons...
hello. i am a mechanical engineer, and right now, we are beginning to work on the foundations of a large structure. the structural engineers often refer to different terms, all of which i used to think were different ways of saying "foundation". can you please explain to me (or direct me to a website with good illustrations) the difference between 'concrete piles' (both cast, and auger, and any others), 'caissons', and 'foundations'? i really appreciate it!
thanks,
kayla
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hello kayla,
drilled pier: a cast-in-place concrete system where a hole is drilled into the ground, the bit removed, a cage of steel dropped in and concrete tremied into the shaft to form a solid concrete foundation. these are sometimes designed with a belled end that is created with a special bit (a can-type apparatus with a swing out plate to form the bell).
auger-cast pile: these are created by drilling into the ground with a long-stem auger. this is similar in concept to your regular wood drill bit except that it is very long (+30 feet) and has a hollow center where grout can be pressure injected into the bottom of the shaft as the drill bit (hollow-stem auger) is slowly removed. great when water tables or loose material is present. steel can be inserted into the top portions of the pile but are not usually extended to the bottom as in the drilled pier.
caisson: same as a drilled pier
pile: usually the term is referred to a solid piece of material (steel pipe, h shape, precast section, etc) that is driven into the ground with a hammer.
foundation: a general term used to describe all of the above as well as shallow spread footings.
the best option would be for you to get a copy of a foundation textbook where the different types are usually described. try peck, hanson, thornburn, "foundation engineeering", published by wiley.
you might also note that the new ibc 2000 has redefined the difference between a "pile" and a "pier" based on the lengh/diameter ratio and not on any other specific criteria.
i think a caisson is similar to a drilled shaft (what was described above as a drilled pier), but i was under the impression that a caisson has a permanent steel. i'm not sure though.
as far as bridges, we refer to a pier as a support that is made of a cap beam supported on columns. the columns are supported by footings. the footings may be spread footings resting on soil / rock or may be cast on concrete piles.
a pile bent on the other hand would be a cap beam that is supported directly by piles. they tend to be less attractive since there are usually a larger number of them sticking up out of the ground to support the bridge. so we usually only use pile bents where the public would not see them easily.
i just thought i'd add one other thing about the term "pile". it is pretty general, there are lots of types of piles. some are used as structural supports, including precast prestressed concrete piles, steel pipe piles and h piles, etc. then there are sheet piles made of steel, concrete, aluminum, etc. and there are hybrids such as sheet pile walls made with pipe piles. they can be driven with a hammer, vibrated, water jetted, hydrolically pushed into the ground, etc.

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