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CV503 Foundation Engineering
MS Civil Engineering
(Fall-2018)
Pile Foundations
Principles of Foundation Engineering by Braja M. Das (7th SI Edition)
Dr. Mubashir Aziz
Associate Professor, Department of Civil Engineering
National University of Computer and Emerging Sciences
Lahore, Pakistan. mubashir.aziz@nu.edu.pk
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PILE FOUNDATIONS
Pile Foundations:
• Deep foundations (Df ≥ 5Bf)
• Structural members made of steel, concrete or timber
• Cost more than shallow foundations
Design of Deep Foundations
• The bearing capacity (resistance to shear) – usually governs the design
• Settlement (performance limits)
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Conditions Requiring the Use of Pile Foundations
1. When upper soil layers are highly 2. When subjected to
compressible and too weak horizontal forces
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3. When expansive and collapsible 4. When foundations are 5. When foundations are subjected to
soils are present at the site subjected to uplift forces loss support due to scouring / erosion
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6. When soils around foundations 7. When fender systems are 8. When future excavations are
are subjected to liquefaction required to protect bridge planned which would require
during seismic events piers from vessel impact underpinning of shallow foundations
Types of Piles & Their Structural Characteristics
• Different type of piles are used depending on;
The type of load to be transferred
The subsoil conditions
The location of GWT
• Piles types based on material
Steel piles
Concrete piles (PCC, RCC, PSC)
Wooden (timber) piles
Composite piles
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STEEL PILES
• Pipe piles or rolled steel H-section piles
• Wide-flange and I-section steel beams can also be used
• In many cases, pipe piles are filled with concrete after driving
Allowable structural capacity of steel piles is;
Qall = As fs
Where; As = x-area of steel, fs = allowable stress of steel ≈ 0.33 - 0.50 fy
Steel piles are spliced by welding or riveting
H-pile Pipe pile H-pile (rivets / bolts)
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In hard driving conditions, steel piles can be fitted with driving
points or shoes
Steel piles are subjected to corrosion
• Swamps, peats, and other organic soils
• Soils with pH > 7.0 are not so corrosive
Provide:
• Additional thickness of steel
• Epoxy coatings
• Concrete encasement
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Some general facts about steel piles
• Usual length: 15 m to 60 m
• Usual load: 300 kN to 1200 kN
Advantages:
• Easy to handle w.r.t. cutoff and extension to desired length
• Can stand high driving stresses
• Can penetrate hard layers (dense gravel / soft rock)
• High load-carrying capacity
Disadvantages:
• Relatively costly
• High noise during pile driving
• Subject to corrosion
• May be damaged/deflected during driving through hard layers
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CONCRETE PILES
i. Precast piles
ii. Cast in-situ piles
PRECAST PILES
a. Ordinary Reinforcement (RCC)
b. Precast Prestressed (PSC)
Reinforcement is provided to resist the bending
moment developed during pick up and transportation,
the vertical load, and the bending moment caused by
lateral loads
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GENERAL FACTS: Precast Concrete Piles
• Usual length: 10 m to 15 m,
• Usual load: 300 kN to 3000 kN
Advantages:
• Can be subjected to hard driving
• Corrosion resistant
• Can be easily combined with a concrete superstructure
Disadvantages:
• Difficult to achieve proper cutoff
• Difficult to transport
GENERAL FACTS: Precast Prestressed Concrete Piles
• Usual length: 10 m to 45 m (Max. length: 60 m)
• Maximum load: 7500 kN to 8500 kN
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Cast In-situ Concrete Piles
• Built by making a hole in the ground and filling it with concrete.
• Can be CASED or UNCASED (Casing – steel pipe)
• Both types may have a pedestal at the bottom (under-reaming)
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Under-reamed piles (cast in-situ & precast piles)
Piles with enlarged bases. It can only be formed in stable soils. In such conditions they allow very
high load bearing capacities.
The diameter of the under-ream bulbs (Du) is normally 2.5 times the diameter of the pile stem.
Single-bulb Multi-bulb
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Cased Cast In-Situ Piles
General Facts:
• Usual length: 5 m to 15 m (Max. length: 30m to 40 m)
• Usual load: 200 kN to 500 kN (Approx. max. load: 800 kN)
Advantages:
• Relatively cheap
• Allow for inspection before pouring concrete
• Easy to extend
Disadvantages
• Difficult to splice after concreting
• Thin casings may be damaged during driving
Allowable Load:
Qall = As fs + Ac fc
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Uncased Cast In-Situ Piles
General Facts:
• Usual length: 5 m to 15 m (Max. length: 30m to 40 m)
• Usual load: 300 kN to 500 kN (Approx. max. load: 700 kN)
Advantages:
• Initially economical
• Can be finished at any elevation
Disadvantages
• Voids may be created if concrete is placed rapidly
• Difficult to splice after concreting
• In soft soils, the sides of the hole may cave in, squeezing the concrete
Allowable Load:
Qall = Ac fc
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TIMBER PILES
• Tree trunks whose branches and bark are carefully
trimmed off
• Timber should be straight, sound, and without any
defects
ASCE Classifications:
Class A piles – carry heavy load
Class B piles – carry medium loads
Class C piles – Used in temporary construction work
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• Can not withstand hard driving stresses
• Can stay undamaged indefinitely if surrounded by saturated soil
• Subject to attack by various organisms in a marine environment
• Subject to attack by insects when located above GWT
• Allowable load-carrying capacity:
Qall = Ap fw (Ap = avg. x-area of pile; fw = allowable stress on timber)
• More axial compressive strength if load is parallel to grains
• Maximum length: 10 m to 20 m
• Usual load: 300 kN to 500 kN
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COMPOSITE PILES
• Steel – Concrete
• Timber – Concrete
• Forming proper joints between two dissimilar materials is difficult
• Not widely used.
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Estimating Pile Length
Piles are divided into three major categories depending on their length
and the mechanisms of load transfer to the soil.
a. Point / End Bearing Piles
b. Friction Piles
c. Compaction Piles
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a) Point / End Bearing Piles
• When bedrock or rocklike
material is present at a site
within a reasonable depth.
• Length of pile can be fairly
well-established.
• Qu = Qp + Qs
[ If Qs is very small, Qu ≈ Qp ]
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b) Friction Piles
• No layer of rock or rocklike material is present
at a site within a reasonable depth.
• Point bearing piles become very long and
expensive.
• Length of pile depends on shear strength of soil,
the applied load, and pile size.
• Qu = Qp + Qs [ friction-cum-end bearing piles ]
[ If Qp is very small, Qu ≈ Qs ]
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c) Compaction Piles
• Piles driven in granular soils to increase degree of compaction.
• Length of pile depends on;
i. Dr of soil before compaction
ii. Dr of soil required after compaction
iii. Required depth of compaction
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Installation of Piles (Driven Piles)
Methods of Pile Driving
1. Hammers
i. Drop Hammer
ii. Single-acting air or steam hammer
iii. Double-acting and differential air or steam hammer
iv. Diesel Hammer
2. Vibratory Pile Drivers
3. Jetting or Partial Augering
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Raised by air/steam pressure Raised & dropped by
Dropped by gravity air/steam pressure Air-Fuel mixture on
Raised by the anvil is ignited by
a winch dropping ram. Pile is
pushed downward
and ram raises up.
Drop Hammer
Single-acting air or Double-acting and differential Diesel Hammer
steam hammer air or steam hammer 26
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Vibratory Pile Drivers
Consist essentially of two counter-rotating weights producing sinusoidal dynamic
vertical force to drive the pile downward.
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Pile Driving by Jetting
• When the pile needs to penetrate a thin layer of hard soil
(sand/gravel).
• Water is discharged at the pile tip by a pipe (50 to 75 mm dia.) to wash
and loosen the sand and gravel.
• Piles also may be advanced by partial augering used to predrill the
holes part of the way.
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Load Transfer Mechanism
Load on pile at ground
surface (Qz=0) is gradually
increased from Zero to Qu
At ultimate load,
Q(z=0) = Qu
Thus,
Q1 = Qs & Q2 = Qp
Load carried by the pile
shaft at any depth, z
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When the load Q at the ground surface is gradually
increased:
• Qs will be fully mobilized at relative displacement
between soil and pile of 5 mm (driven piles) to 10 mm
(bored piles), irrespective of pile size and length.
• Qp will be fully mobilized until the tip of the pile has
moved about 10 to 25% of the pile width (or diameter).
• Hence, Qs is developed at much smaller pile
displacement compared with Qp.
Pile foundations are deep
foundations and that the
soil fails mostly in
punching mode.
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