Foundation Engineering

Kaniraj Shenbaga Universiti Malaysia Sarawak

Categorize different types of deep

foundations in sand and clay and

calculate their vertical load carrying

capacity.

Course Learning Objective 1

Pile Foundations

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

Design Criteria for Foundations

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

A foundation must be properly

located in the available area and

must be founded at the correct

depth.

Location and Depth Criterion

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

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

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

Load Capacity of Single Piles

Single Piles in Compression

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

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

Single Piles in Compression

Pile Driving Formulae

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

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

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

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

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

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

http://www.vulcanhammer.com/onshore/specsi.php

Hammer number

Eh

1 kJ = 1 kNm

http://www.apeholland.com/page/en/diesel-hammers/

Eh

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

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

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