Pile Foundations 2
description
Transcript of Pile Foundations 2
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Foundation Engineering
Kaniraj Shenbaga Universiti Malaysia Sarawak
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Categorize different types of deep
foundations in sand and clay and
calculate their vertical load carrying
capacity.
Course Learning Objective 1
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Pile Foundations
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Deep Foundations
D/B > 1, for deep
foundations
When 1 L/D 15, the
foundations are called
moderately deep
For deep foundations,
L/D 15. e.g. pile
foundation
GL
B
D
D
L
Shallow foundation
Pile foundation D = diameter or breadth
L = length of embedment
Soil
GL
Pile
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Design Criteria for Foundations
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The three design criteria applicable to all types
of foundations including shallow and deep
foundations are:
1. Location and depth criterion
2.Stability criterion or bearing capacity criterion
3.Settlement criterion
Design Criteria
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A foundation must be properly
located in the available area and
must be founded at the correct
depth.
Location and Depth Criterion
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A foundation is a structure that transfers the load of the
superstructure to the soil.
A foundation can fail in two ways. It can fail:
By structural failure, i.e. when the stress due to the load
exceeds the load bearing capacity of the structural material of
the foundation.
By rupture of the soil, i.e. when the stress on the soil due to the
load transferred by the foundation exceeds the load bearing
capacity of the soil.
Stability or Bearing Capacity Criterion
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The foundation must be safe from both
structural failure (i.e. failure by exceeding
the load bearing capacity of the structural
material) and failure by rupture of the soil
(i.e. failure by exceeding the load bearing
capacity of the soil).
Stability or Bearing Capacity Criterion
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Soil is compressible. The foundation load
causes compression or settlement of the
soil.
The foundation should not settle
excessively either damaging the building
or impairing its utility.
Settlement Criterion
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Load Capacity of Single Piles
Single Piles in Compression
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Ultimate load (Qu) is the maximum load that a pile
can carry. The pile has reached the failure state at
the ultimate load.
Allowable or safe load (Qall ) is the load that a pile
can carry safely. It is obtained by applying a factor
of safety to the ultimate load.
Ultimate and Allowable Loads
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To avoid failure by excessive compressive
stresses on the pile material, the allowable
pile load in compression is determined as:
=
fc = allowable compressive stress on pile
Ap = area of cross-section of pile
Structural Failure
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Single Piles in Compression
Pile Driving Formulae
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A large number of pile driving formulae available
in literature. They are also called as dynamic
formulae.
The pile driving formulae are:
Empirical formulae developed for driven piles only.
Generally applicable for driven piles in sands only.
Not very reliable; they can give unsatisfactory
results.
Pile Driving Formulae
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The static ultimate load is estimated from
the dynamic resistance of the pile to
driving.
Total energy delivered by the hammer is
used partly to penetrate the pile into the
ground and the rest of the energy is wasted
in several ways.
Principle of Dynamic Formulae
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Energy delivered by hammer blow =
(Pile resistance x penetration of pile tip) + Energy losses
Therefore, estimate the pile resistance (or ultimate load) from known or estimated values of other quantities.
Principle of Dynamic Formulae
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Engineering News Record (ENR) Formula
= +
Qu = ultimate load of pile
in compression
Note: C and s should be in the
same units and Eh should be in
consistent units.
eh = hammer efficiency
Eh = rated energy of hammer
Eh = = Whh (for drop hammers)
Wh = Weight of hammer
h = height of fall of hammer
s = set (penetration of pile point
per hammer blow)
C = 25 mm for drop hammer
C = 2.5 mm for single acting steam
hammer
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Danish Formula
= +
=
.
Note: C1 and s should be in
the same units and Eh
should be in consistent
units.
eh = hammer efficiency
Eh = rated energy of hammer
s = set (penetration of pile
point per hammer blow)
L = length of pile
Ep = modulus of elasticity of pile
Ap = area of cross-section of pile
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Drop hammer hammer falls down by gravity
Air/Steam hammer ram is forced by steam
Single acting hammer ram is forced only up
Double acting or differential hammer ram is forced both up
and down
Diesel hammer
Hydraulic hammer under water hammers
Note: Manufacturers of hammers usually specify the rated
energy of the hammers.
Pile Driving Hammers
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http://www.vulcanhammer.com/onshore/specsi.php
Hammer number
Eh
1 kJ = 1 kNm
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http://www.apeholland.com/page/en/diesel-hammers/
Eh
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The factor of safety (FS) to be used on
ultimate load varies from one formula
to another.
FS varies from 4 to 6 in ENR formula
FS varies from 3 to 6 in Danish formula
Factor of Safety in Dynamic Formulae
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Using Danish formula determine the safe load on a 450
mm diameter and 18 m long driven concrete pile. The pile
is driven into sand using a Raymond differential hammer
(model 150 C) which has a rated energy of 66.1 kN-m and a
ram weight of 66.7 kN. The hammer efficiency is 0.75.
Four blows are required for the last 25 mm penetration of
the pile. Youngs modulus of pile material = 21x106 kN/m2,
and FS = 4.
Exercise 1
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Refer to Exercise 1. Determine the safe load
on the pile using ENR formula. Use:
C = 2.5 (Wp/Wh) mm (Wp = weight of pile
including pile helmet)
Unit weight of concrete = 23.6 kN/m3,
Weight of pile helmet = 5.1 kN
FS = 5.
Exercise 2