Bearing capacity ch#05(geotech)

GEOTECHNICAL ENGINEERING - II
Engr. Nauman Ijaz

Bearing Capacity of the Soil

Chapter # 05
UNIVERSITY OF SOUTH ASIA

FOUNDATION
It is the bottom most structural element of
the sub structure which transmits the
structural load including its own weight on
to and / into the soil
underneath/surrounding with out casing
shear failure or bearing capacity failure
(sudden collapse) and excessive
settlement.

CONTACT PRESSURE
The pressure generated by the
structural loading and self weight of
the member on to or into the soil
immediately underneath is called
Contact pressure (σo).

σo = Q / A
The contact pressure is independent of
soil parameters; it depends only on the
load and the x-sectional area of the
element carrying the load.

Q = 1000KN

σo = Q / A
= 1000/(0.5 × 0.5)
= 4000 Kpa
A

A

0.5m
Fig # 01
0.5m
Sec A-A

Super-Structure and
Sub- Structure
The part of the structure which is above
the GSL and can be seen with naked eye
is known as Super-Structure.
That part of structure which is below the
GSL and can not be seen with naked eyes
is known as Sub-Structure.

Q = 1000KN

Super- Structure
GSL

Sub- Structure
Df

B×L (2×2.5 m²)

σo = Q / A
Fig # 02

= 1000/(2 × 2.5)
= 200 Kpa

Foundation Depth (Df)
It is the depth below the lowest
adjacent ground to the bottom of the
foundation.
Need or Purpose of a Foundation
Foundation is needed to transfer the
load to the underlying soil assuming
safety against bearing capacity failure
and excessive settlement.

This can be done by reducing the contact
pressure such that it is either equal to or less
than allowable bearing capacity (ABC) of soil.
i.e σo < qa.
In Fig- 1, the contact pressure under the concrete
column is 4000Kpa which is much less than 21MPa

(crushing strength of concrete) but much
greater than 200KPa (ABC) of soil and
needed to be reduced prior to transfer it to
the soil underneath the column.
The reduction can be achieved by;

Lateral spreading of load using a large
pad underneath the column (Fig # 02)
σo = 1000 /5 = 200Kpa = ABCof soil
The larger pad is known as Spread footing.

FLOATING FOUNDATION
Balance Partly or completely the load
added to the load removed due to
excavation is known as Floating
foundation.i.e Provide basements.

Types of Foundation
Foundation may be characterized as
being either “ Shallow” or “Deep”.

Shallow Foundation
Are those located just below the lowest
part of the super structure which they
support ( and get support from the soil
just beneath the footing) and a least
width generally greater than their
depth beneath the ground surface, i.e
Df / B < 1
Df = 3 m (generally)

Deep Foundations
Are those which extend considerably
deeper into the earth ( and get supported
from the side friction (skin friction) and / or
bottom (end bearing) and generally with a
foundation depth to width ratio (D/B)
exceeding five.

TYPES OF FOUNDATION
Shallow foundations may be classified
in several ways as below;

SPREAD FOOTING OR
INDIVIDUAL FOOTING
This type of foundation supports one column
only as shown below. This footing is also
known as Pad footing or isolated footing. It
can be square or rectangular in shape. This
type of footing is the easiest to design and
construct and most economical therefore.

For this type of footing, length to
breadth ratio (L/B) < 5.

PLAN

GSL

ELEVATION

CONTINUOUS FOOTING
If a footing is extended in one direction
to support a long structure such as
wall, it is called a continuous footing or
a wall footing or a strip footing as
shown below.
Loads are usually expressed in force
per unit length of the footing.
For this type of footing , Length to
Breadth ratio (L/B) > 5.

A strip footing is also provided for a row
of columns which are closely spaced that
their spread footings overlap or nearly
touch each other.
In such a case it is more economical to
provide a strip footing than a number of
spread footing in one line.

COMBINED FOOTING
A combined footing is a larger footing
supporting two or more columns in one row.
This results in a more even load distribution
in the underlying soil or rock, and
consequently there is less chances of
differential settlement to occur.
While these footings are usually rectangular
in shape, these can be trapezoidal ( to
accommodate unequal column loading or
close property lines)

STRAP FOOTING
Two or more footings joined by a beam (called Strap)
is called Strap Footing.
This type is also known as a cantilever footing or
pump-handle foundation.
This form accommodates wide column spacing's or
close property lines.
Strap is designed as a rigid beam to with stand
bending moments, shear stresses.
The strap simply acts as a connecting beam and does
not take any soil reaction.
To make this sure, soil below is dug and made loose.

MAT OR RAFT FOOTING
A large slab supporting a number of
columns not all of which are in a straight
line is known as Mat or Raft or Mass
foundation.
These are usually considered where the
base soil has a low bearing capacity and /
or column loads are so large that the sum
of areas of all individual or combined
footings exceeds one half the total building
area ( to economize on frame costs).

Furthermore, mat foundations are useful in
reducing the differential settlements on
individual columns.
A particular advantage of mat for basement
at or below ground water table is to provide
a water barrier.

SELECTION OF FOUNDATION TYPE
The selection of the type of foundation for a
given structure-subsoil system is largely a
matter of judgment/elimination based on
both an analysis of scientific data and
experience.
It is not possible to establish rigorous
regulations and detailed recommendations
for the solution of all soil problems, as the
planning and designing of foundations for
structures is more of an art than a science.

1.

The type of foundation most appropriate for a
given structure depends on several factors
but commonly the principal factors are three
which are as follow:
The function of the structure and the
loads it must carry.
– Purpose of the structure i.e residential, office,
industrial, bridge etc
– Service life
– Loading number of stories, basement.
– Type i.e framed RCC, masonry, column
spacing etc.
– Construction method and schedule.

2.

Sub-surface Condition.
– Thickness and sequence of soil strata
with subsoil parameters.
– GWT position and function limits.
– Presence of any underground
anomalies.

3.

The cost of foundation in
comparison with the cost of the
super structure i.e funds available
for the construction and foundation.

COMPARISON OF SHALLOW
AND DEEP FOUNDATIONS
Sr/No

DESCRIPTION

SHALLOW FOUNDATION

DEEP FOUNDATION

1

Depth

Df / B < 1

Df / B > 4+

2

Load Distribution

Lateral Spread

Lateral and/or Vertical
spread.
•For end bearing lateral
spread.
•For frictional vertical
spread.
•Generally both.

3

Construction

•Open pit construction.
•Easy control and the best
QA/QC.
•Less skill labour is required.
•Min. Disturbance.
•During construction
dewatering is required for
shallow GWT.

•In hole or driven
•Difficult QA/QC.
•Very skilled labour is
required.
•Max.disturbance.
•Dewatering may or may
not be required.

Sr/No

DESCRIPTION

SHALLOW
FOUNDATION

DEEP
FOUNDATION

4

Cost

Less as compared
with deep
foundations.

Usually 3 times or
more costly than
shallow.

5

Structural Design
Consideration

Flexural bending

Axial Compression

6

Settlement

More than that of
deep foundation.

Usually 50% of the
shallow foundation
for similar loading.

7

Environmental Suitability

Does not suit to all
environments
specially for off
shores sites.

Suitable for all
environment
including off shore.

CRITERIA FOR FOUNDATION
DESIGN

1.

2.

When designing foundation; there are two
criteria which must be considered and
satisfied separately.
There must be accurate factor of safety
against a bearing capacity failure in the
soil i.e soil shouldn’t fail in shear.
The settlement and particularly the
differential settlement must be kept within
reasonable limits.

Causes of Deformation
Deformation of an element of soil is a function
of a change in effective stress (change in
volume) not change in total stress. Various
causes of deformation of a structure are listed
as follow;
1.
2.
3.
4.
5.

Application of structural loads.
Lowering of the ground water table.
Collapse of soil structure on wetting.
Heave of swelling soils.
Deterioration of the foundation ( Sulphate attack
on concrete, corrosion of steel piles, decay of
timber piles).

6. Vibration of sandy soil.
7. Seasonal moisture movement.
8. The effect of frost action.

DEFINITIONS OF
BEARING PRESSURE
Gross Bearing Pressure (q gross):
The intensity of vertical loading at the base
of foundation due to all loads above that
level.
2. Net Bearing Pressure: (q net):
The difference between q gross and the total
overburden pressure Po at foundation level
(i.e q net = q gross – Po). Usually q net is the
increase in pressure on the soil at
foundation level.
1.

3. Gross Effective Burden Pressure (q’gross):
The difference between the qgross and the
pore water pressure (u) at foundation level.
(i.e q’gross = qgross – u).
4. Net Effective Bearing Pressure (q’net):
The difference between q’gross and the
effective over burden pressure Po at
foundation level. (i.e q’net = q’gross – Po).
5. Ultimate Bearing Pressure (qf):
The value of bearing pressure at which the
ground fails in shear. It may be expressed as
gross or net or total effective pressure.

6. Maximum Safe Bearing Pressure (qs):
The value of bearing pressure at which the risk
of shear failure is acceptably low; may be
expressed as gross or net or effective pressure.
7. Allowable Bearing Pressure (qa):
Takes account the tolerance of the structure to
settlement and may be much less than qs.
8. Working Bearing Pressure (qw):
Bearing Pressure under working load. May be
expressed as gross or net or total or effective
pressure.

Bearing capacity ch#05(geotech)

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