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PILE FOUNDATIONS
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INTRODUCTION
• In situations where the soil at shallowdepth is poor. In order to transmit the loadsafely , the depth of foundation has to beincreased till a suitable soil stratum isreached.
• The manner in which soil resists stressesfrom shallow and deep foundation isdifferent.
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Types of iles"
• #aterial of construction,
• Cross $ section
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The %ritish &tandard Code of ractice for'oundations (%& )**+ places piles in three
cate-ories.
1.Large displacement piles
2.Small-displacement piles
3.Replacement piles
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• ile foundation is an e ample of deepfoundations.
• / pile is a relati0ely small diameter shaft,which is dri0en or bored into the soilstratum.
• ile -roups may be sub1ected to 0ertical,hori2ontal or a combination of these twoloads.
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• iles are useful in transferrin- loadthrou-h poor soil or water to a suitablebearin- stratum by means of end bearin-.
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• iles are installed in soft soils such that theload is transferred throu-h friction alon- the
len-th of piles, they are called friction piles.
• iles are also used to resist hori2ontal loadsas in the case of retainin- walls. These pilesare called as laterally loaded piles.
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• iles are sometimes used to resist upliftloads and thus are in tension.
• iles can also be used to pro0ideanchora-e a-ainst hori2ontal pull as incase of anchored bul3heads.
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• iles are fre4uently sub1ected to lateral forcesand moments,
• iles supportin- structures should ha0e theability to resist a lateral force of 5*6 of theapplied a ial load.
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• / 0ertical pile unrestrained at the head, thelateral loadin- on the pile head is initiallycarried by the soil close to the -roundsurface.
• /t a low loadin- the soil compresseselastically but the mo0ement is sufficient totransfer some pressure from the pile to thesoil at a -reater depth.
• /t a further sta-e of loadin- the soil yieldsplastically and transfers its load to -reater
depths.
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The de-ree of distribution of the soil reaction
depends on
(a the stiffness of the pile,
(b the stiffness of the soil, and
(c the fi ity of the ends of the pile.
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• A short rigid pile unrestrained at thetop and having a length to widthratio of less than 10 to 12 ( Figure6.18 a) tends to rotate and passiveresistance develops above the toe onthe opposite face to add to the
resistance of the soil near the groundsurface
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• The ri-id pile will fail by rotation when thepassi0e resistance of the soil at the head
and toe are e ceeded.
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The lower part remainin- 0ertical while theupper part deforms to a shape
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lon- pile restrainedat the head, hi-hbendin- stressesde0elop at the pointof restraint, fore ample, 1ust
beneath the pile cap,and the pile mayfracture at this point
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'actors which -o0ern the beha0iour of laterally loaded pilesare"
5. ile stiffness, which influences the deflection and
determines whether the failure mechanism is one ofthe rotation of a short ri-id element, or is due tofle ure followed by the failure in bendin- of a lon- pile.
7. The type of loadin-, whether sustained (as in the caseof earth pressure transmitted by a retainin- wall oralternatin- (say, from reciprocatin- machinery orpulsatin- (as from the traffic loadin- on a brid-e pier ,influences the de-ree of yieldin- of the soil.
8. 9 ternal influences such as scourin- around piles at
sea:bed le0el, or the seasonal shrin3a-e of clay soilsaway from the upper part of the pile shaft, affect theresistance of the soil at a shallow depth.
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