BUILDING CONSTRUCTION,
ARCH 203:
LECTURE IV
COURSE LECTURERS:
ARC. I. G. ALIYU, MNIA
ARC. ZAINAB SADA
31st January, 2023.
FOUNDATION SYSTEM
FOUNDATION SYSTEM
As shown, a building may be
considered to comprise of:
(a) superstructure:
(b) substructure:
(c) foundation system:
FOUNDATION SYSTEM
(a) superstructure: generally
refers to the part of the
building that is above the
ground level which serves the
purpose of its intended use.
FOUNDATION SYSTEM
(b) substructure: refers to the lower
portion of the building, also referred
to as basements, if provided.
It is usually located below ground
level therefore transmits the loads of
the superstructure to the Sub-soil.
FOUNDATION SYSTEM- DEFINATION
Note:
Where a basement does not exist, a building has
only a superstructure and foundations.
FOUNDATION SYSTEM
(c) foundation system: refers to
the structure that lies below the
substructure.
A foundation is therefore that
part of the structure which is in
direct contact with the ground to
which the loads are transmitted.
FOUNDATION SYSTEM
The soil which is located
immediately below the base of
the foundation is called the
sub-soil or foundation soil.
While the lowermost portion
of the foundation which is in
direct contact with the sub-soil
is called the footing.
FOUNDATION SYSTEM- FOUNDATION FOOTING.
The Foundation footing is a formation
which is in direct contact with the
ground.
Footings in foundations are
necessary because the soil on which
the load-bearing walls and
columns rest, is generally weaker
than the materials used to construct
the walls and columns.
FOUNDATION SYSTEM- FOUNDATION FOOTING.
Therefore, the walls and
columns generally need to be
widened at their bases to
ensure that the bearing pressure
on the soil is less than its
bearing capacity.
FOUNDATION SYSTEM- BASIC FUNCTIONS
The basic function of foundations is to transmit the dead
loads, super-imposed loads (live loads) and wind loads
from a building to the Sub-soil on which the building
rests, in such a way that :
Settlements are within permissible limits, without
causing cracks in the super-structure, and,
The soil does not fail.
FOUNDATION SYSTEM- ESSENTIAL REQUIREMENTS
OF A GOOD FOUNDATION
foundations should be constructed to satisfy the
following requirements;
1. The foundations shall be constructed to withstand
the dead and imposed loads and to transmit these to
the sub-soil in such a way that the pressure on it will
not cause settlement which would impair the stability
of the building or adjoining structures.
FOUNDATION SYSTEM- ESSENTIAL REQUIREMENTS
OF A GOOD FOUNDATION……CONT’D.
2. Foundation base should be rigid so that differential
settlements are minimized, specially for the case
when super-imposed loads are not evenly
distributed.
3. Foundations should be taken sufficiently deep to
guard the building against damage or distress caused
by swelling or shrinkage of the sub-soil.
FOUNDATION SYSTEM- ESSENTIAL REQUIREMENTS
OF A GOOD FOUNDATION ……CONT’D.
4. Foundations should be so
located so that its performance
may not be affected due to any
unexpected future influence.
FOUNDATION SYSTEM: SHALLOW AND DEEP
FOUNDATIONS
•Foundations are broadly classified as shallow or deep.
•The choice of foundation type is governed by:
a)
the characteristics of the soil near the surface and
b)
the pressure exerted on it by the loads on the
building.
FOUNDATION SYSTEM: SHALLOW FOUNDATIONS
•Shallow foundations extend a relatively short distance below
the ground and bear directly on the upper soil stratum.
•According to Terzaghi, a
foundation is shallow if its depth
is equal to or less than its
width.
•Because of their lower cost,
they are preferred over deep
foundations.
FOUNDATION SYSTEM: SHALLOW FOUNDATIONS
•However, because the soil’s bearing
capacity is low at smaller depths
from the surface, shallow
foundations are generally limited to
low- to mid-rise buildings.
FOUNDATION SYSTEM: SHALLOW FOUNDATIONS
There are basically two types of shallow foundations:
• Foundation systems consisting of footings
• Monolithic concrete foundation under the building’s
entire footprint.
FOUNDATION SYSTEM: SHALLOW FOUNDATIONS
However, from the point of view of design, shallow
foundations may be of the following types:
1. Spread footings.
2. Combined footings
3. Strap footings.
4. Mat foundation.
FOUNDATION SYSTEM:
FOUNDATIONS TYPE BASED ON SOIL TYPE
Building foundation systems based on soil type can be
classified into two categories:
(a) foundation systems on stable soils.
(b) foundation systems on expansive soils.
Most foundation systems suitable for expansive soils may
also be used where near-surface soils have a low bearing
capacity or are otherwise unsuitable.
FOUNDATION SYSTEM: FOUNDATION MATERIALS
The materials commonly used for foundations are;
concrete (reinforced with either reinforcing steel or
posttensioning tendons), plain (unreinforced) concrete and
reinforced masonry (concrete masonry units).
Other materials used are, unreinforced masonry, and
preservative-treated lumber and plywood.
FOUNDATION SYSTEM: SHALLOW FOUNDATIONS,
FOUNDATION SYSTEM WITH FOOTINGS
The widened base of a column or wall is called the footing .
Widening of the base is essential because the strength of
the soil supporting the column or wall is generally lower than
the strength of the material used for the column or wall.
Thus, the primary purpose of a footing is to distribute the
superimposed load on a large area of the soil so that the
pressure on the soil is less than or equal to the soil’s
strength (bearing capacity).
FOUNDATION SYSTEM: SHALLOW FOUNDATIONS,
FOUNDATION SYSTEM WITH FOOTINGS
Commonly used foundation systems with footings are:
• Footings under perimeter foundation walls and interior
piers (short columns) with an elevated ground floor.
• Footings under columns and load-bearing walls with a
concrete slab-on-ground
• Isolated column footings with or without a basement.
FOUNDATION SYSTEM: SHALLOW FOUNDATIONS,
TYPES OF FOOTINGS
footings are of the
following types:
• Continuous wall footings,
also called strip footings,
are commonly used where
the superimposed loads
are linear, generally from
a load-bearing wall,
Foundation system consisting of footings
under perimeter walls and under interior
piers—a system commonly used for lightframe buildings.
The ground floor of such buildings is
elevated, with a crawl space below.
FOUNDATION SYSTEM: SHALLOW FOUNDATIONS,
TYPES OF FOOTINGS -ISOLATED
• Isolated (independent)
footings are used where the
superimposed load is a point
load—e.g., from a column.
The bearing capacity required
under a column footing is
generally higher than that
under a wall footing because
of the concentrated load on
the column.
FOUNDATION SYSTEM: SHALLOW FOUNDATIONS,
TYPES OF FOOTINGS
Sometimes, particularly
with steel columns, a
stepped footing (with a
pedestal) is used to save
concrete,
Foundation system for this multistory
building consists of footings under the
columns.
The footings may consist of isolated
FOUNDATION SYSTEM: SHALLOW FOUNDATIONS,
TYPES OF FOOTINGS
Foundation system for this multistory
building consists of footings under the
columns.
The footings may consist of isolated
column footings, a mat foundation, or a
raft foundation
FOUNDATION SYSTEM: SHALLOW FOUNDATIONS,
TYPES OF FOOTINGS
Foundation system
consisting of footings
under columns and loadbearing walls. The
ground floor is a
concrete slab-on-ground.
FOUNDATION SYSTEM: SHALLOW FOUNDATIONS,
TYPES OF FOOTINGS
• A combined footing is a
combination of two isolated
column footings.
It is used where two or more
adjacent columns are closely
spaced and heavily loaded.
• Combining them into one footing reduces the excavation cost
and distributes the load over a larger area.
FOUNDATION SYSTEM: SHALLOW FOUNDATIONS,
TYPES OF FOOTINGS
FOUNDATION SYSTEM: SHALLOW FOUNDATIONS,
TYPES OF FOOTINGS
A combined footing is also used where an exterior column
must be placed within the property line or adjacent to an
existing building,
FOUNDATION SYSTEM: DEEP FOUNDATIONS
Deep foundations consist of either piles or piers and are
generally used for high-rise buildings.
They are used where soil of adequate strength is not
available close enough to the surface for the use of shallow
foundations.
However, where the soil near the surface is unsuitable (e.g.,
subject to swell and shrink phenomenon),
Deep foundations are utilized for low-rise buildings as well
so that the loads are transferred to a suitable stratum.
FOUNDATION SYSTEM: DEEP FOUNDATIONS
Deep foundations may be of the following types:
1. Deep strip, rectangular or square footings.
2. Pile foundation.
3. Pier foundation or drilled caisson foundation.
4. Well foundation or caissons.
FOUNDATION SYSTEM: DEEP FOUNDATIONS
•The choice of a foundation system for low to mid-rise
buildings is governed by the type of near-surface soil.
•If the near-surface soil is stable and has adequate
strength, shallow foundations are appropriate.
•On the other hand, if the near-surface soil is either weak
or unstable (i.e., expansive), deep foundations must be
utilized.
•In the case of lightly loaded low-rise buildings, shallow
foundations can be designed for use on expansive soils.
FOUNDATION SYSTEM: PILE FOUNDATION
Pile foundation is that type of deep foundation in
which the loads are taken to a low level by means of
vertical members which may be of timber, concrete
or steel.
FOUNDATION SYSTEM: PILE FOUNDATION
Pile foundation may be adopted;
(i) instead of a raft foundation where no firm bearing
strata exists at any reasonable depth and the loading is
uneven,
(ii) when a firm bearing strata does exist but at a depth
such as to make strip or spread footing uneconomical, and
(iii) when pumping of sub-soil water would be too costly or
timbering to excavations too difficult to permit the
construction of normal foundations.
FOUNDATION SYSTEM: PILE FOUNDATION
Piles used for building foundation
may be of four types:
(i) End bearing pile
(ii) Friction pile
(iii) Combined end bearing and
friction pile
(iv) Compaction piles
FOUNDATION SYSTEM: PILE FOUNDATION - END
BEARING PILES
End bearing piles are used to transfer load through water
or soft soil to a suitable bearing stratum.
Such piles are used to carry heavy loads safely to hard
strata.
Multi-storeyed buildings are invariably founded on end
bearing piles, so that the settlements are minimized.
FOUNDATION SYSTEM: PILE FOUNDATION-
FRICTION PILES
Friction piles are used to transfer loads to a depth of a
friction-load-carrying material by means of skin friction
along the length of the pile.
Such piles are generally used in granular soil where the
depth of hard stratum is very great.
FOUNDATION SYSTEM: PILE FOUNDATIONFRICTION PILES..,,,CONT’D
Such piles are more common, specially when the end
bearing piles pass through granular soils.
Fig shows a pile which
transfers the super-imposed
load both through side
friction as well as end
bearing.
FOUNDATION SYSTEM: PILE FOUNDATION-COMPACTION
PILES
Compaction piles are used to compact loose granular soils,
thus increasing their bearing capacity.
The compaction piles themselves do not carry a load.
Hence they may be of weaker material (such as timber,
bamboo sticks etc.)-sometimes of sand only.
The pile tube, driven to compact the soil, is gradually taken
out and sand is filled in its place thus forming a ‘sand pile’.
FOUNDATION SYSTEM: PIER FOUNDATION
A Pier foundation consists of a cylindrical column of large
diameter to support and transfer large super-imposed loads
to the firm strata below.
The difference between pile foundation and pier foundation
lies in the method of construction.
Though pile foundations transfer the load through friction
and/or bearing, pier foundations transfer the load only
through bearing.
FOUNDATION SYSTEM: PIER FOUNDATION
Generally, pier foundation is shallower in depth than the pile
foundation.
Pier foundation is preferred in a location where the top
strata consists of decomposed rock overlying a strata of
sound rock.
In such a condition, it becomes difficult to drive the bearing
piles through decomposed rock.
In the case of stiff clays, which offer large resistance to the
driving of a bearing pile, pier foundation can be conveniently
constructed.
FOUNDATION SYSTEM: PIER FOUNDATION
Pier foundations may be of the
following types:
(i) Masonry or concrete pier
(ii) Drilled caissons.
FOUNDATION SYSTEM: PIER FOUNDATION
When a good bearing stratum exists up to 5
m below ground level, brick, masonry or
concrete foundation piers in excavated pits
may be used .
The size and spacing of the piers depends
upon the;
depth of hard bed,
nature of overlying soil and
super-imposed loads.
FOUNDATION SYSTEM: PIER FOUNDATION
The terms drilled caissons, foundation pier or sub-pier are
interchangeably used to denote a cylindrical foundation.
A drilled caisson is largely a compressed member
subjected to an axial load at the top and reaction at the
bottom.
Drilled caissons are generally drilled with the mechanical
means.
FOUNDATION SYSTEM: PIER FOUNDATION
Drilled caissons may be of three types :
(i) concrete caisson with enlarged bottom [Fig. 2.9 (b)],
(ii) caisson of steel pipe with concrete filled in the pipe [Fíg.
2.9 (c)] and
(iii) caisson with concrete and steel core in steel pipe [Fig.
2.9 (d)].
FOUNDATION SYSTEM: WELL FOUNDATIONS
Well foundations or caissons are box-like structure-circular
or rectangular-which are sunk from the surface of either
land or water to the desired depth.
They are much larger in diameter than the pier foundations
or drilled caissons.
FOUNDATION SYSTEM: WELL FOUNDATIONS
Caisson foundations are used for major foundation works,
such as for:
(i) Bridge piers and abutments in rivers, lakes etc.,
(ii) Wharves, quay walls, docks.
(iii) Break waters and other structures for shore protections.
(iv) Large water front structures such as pump houses,
subjected to heavy vertical and horizontal loads.
FOUNDATION SYSTEM: WELL FOUNDATIONS
Well foundations or caissons are hollow from inside, which
may be filled with sand, and are plugged at the bottom.
The load is transferred though the perimeter wall, called
steining.
Note; Well foundations are not used for buildings.
FOUNDATION SYSTEM: FAILURES
• The foundations may fail due to the following reasons:
1. Unequal settlement of sub-soil. Unequal settlement of the
sub-soil may lead to cracks in the structural components and
rotation thereof.
Unequal settlement of sub-soil may be due to
(i) non-uniform nature of sub-soil throughout the foundation,
(ii) unequal load distribution of the soil strata, and
(iii) eccentric loading.
FOUNDATION SYSTEM: FAILURES
•The failures of foundation due to unequal settlement can be
checked by :
(i) resting the foundation on rigid strata, such as rock or
hard stratum,
(ii) proper design of the base of footing, so that it can resist
cracking,
(iii) limiting the pressure in the soil, and
(iv)avoiding eccentric loading.
FOUNDATION SYSTEM: FAILURES
2. Unequal settlement of masonry. As stated earlier,
foundation includes the portion of the structure which is
below ground level.
This portion of masonry, situated between the ground level
and concrete footing(base) has mortar joints which may
either shrink or compress, leading to unequal settlement of
masonry.
FOUNDATION SYSTEM: FAILURES
Due to this, the superstructure will also have cracks.
This could be checked by;
(i) using mortar of proper strength,
(ii) using thin mortar joints,
(iii) restricting the height of masonry to 1 m per day if lime
mortar is used and 1.5 m per day if cement mortar is used,
and (iv) properly watering the masonry.
FOUNDATION SYSTEM: CAUSES OF FAILURES
3. Sub-soil moisture movement.
This is one of the major causes of failures of footings on
cohesive soil, where the sub-soil water level fluctuates.
When water table drops down, shrinkage of sub-soil takes
place.
Due to this, there is lack of sub-soil support to the footings
which crack, resulting in the cracks in the building.
During upward movement of moisture, the soil (specially if it
is expansive) swells resulting in high swelling pressure.
FOUNDATION SYSTEM: CAUSES OF FAILURES–
4. LATERAL PRESSURE ON THE WALL.
The walls transmitting the load to the foundation may be
subjected to lateral pressure or thrust from a pitched roof
or an arch or wind action.
Due to this, the foundation will be subjected to a moment (or
resultant eccentric load).
If the foundation has not been designed for such situations, it
may fail by either overturning or by generation of tensile
stresses on one side and high compressive stresses on the
other side of the footing.
FOUNDATION SYSTEM: CAUSES OF FAILURES
5. Lateral Movement of sub-soil This is applicable to very soft
soil which are liable to move out or squeeze out laterally
under vertical loads, specially at locations where the ground
is sloping.
Such a situation may also arise in granular soils where a big
pit is excavated in the near vicinity of the foundation.
Due to such movement, excessive settlements take place, or
the structure may even collapse.
If such a situation exists, sheet piles should be driven to
prevent the lateral movement or escape of the soil.
FOUNDATION SYSTEM: CAUSES OF FAILURES7. ATMOSPHERIC ACTION.
The behaviour of foundation may be adversely affected due
to atmospheric agents such as sun, wind, and rains.
If the depth of foundation is shallow, moisture movements due
to rains or drought may cause trouble.
If the building lies in a low lying area, foundation may even
be scoured.
If the water remains stagnant near the foundation, it will
remain constantly damp, resulting in the decrease in the
strength of footing or foundation wall.
FOUNDATION SYSTEM: CAUSES OF FAILURES-
6. WEATHERING OF SUB-SOIL DUE TO TREES AND SHRUBS.
Sometimes, small trees, shrubs or hedge is grown very near to
the wall.
The roots of these shrubs absorb moisture from the foundation
soil, resulting in reduction of their voids and even weathering.
Due to this the ground near the wall depresses down.
If the roots penetrates below the level of footing, settlements
may increase, resulting in foundation cracks.
FOUNDATION SYSTEM: CAUSES OF FAILURES
Hence it is always recommended to provide suitable
protection along the external walls by;
(i) filling back the foundation trenches with good soil and
compacting it,
(ii) providing gentle ground slope away from the wall and
(iii) providing a narrow, sloping strip of impervious material
(such as of lime or lean cement concrete) along the exterior
walls.
FOUNDATION SYSTEM: SETTLEMENT
The vertical downward movement of the base of a
structure is called settlement and its effect upon the structure
depends on;
its magnitude,
its uniformity,
the length of the time over which it takes place, and
the nature of the structure itself.
FOUNDATION SYSTEM: SETTLEMENT
Foundation settlement may be caused by some or a
combination of the following reasons:
1. Elastic compression of the foundation and the underlying
soil.
2. Inelastic (or plastic) compression of the underlying soils,
which is much larger than the elastic compression.
FOUNDATION SYSTEM: SETTLEMENT
3. Ground water lowering.
Repeated lowering and raising of water level in loose
granular soil tends to compact the soil and cause settlement
of the ground surface.
Lowering of water level in fine grained soils cause
consolidation settlement.
FOUNDATION SYSTEM: SETTLEMENT
4. Vibrations due to pile driving, blasting and oscillating
machineries may cause settlement in deposits of granular
soils.
5. Seasonal swelling and shrinkage of expansive clays.
6. Ground movement on earth slopes, such as surface erosion,
slow creep or landslide.
7. Other causes such as adjacent excavation, mining
subsidence, underground erosion, etc.
ASSIGNMENT
1. With the aid of good Sketches, VI. Friction pile
Briefly illustrate the various types of
VII. Combined end bearing and
settlements.
friction pile
2. To scale 1:20 Draw a typical 3-D VIII. Compaction piles.
details of any four (4) of the
following foundation types;
I.
Spread footings.
II. Combined footings
III. Strap footings.
IV. Mat foundation.
V.
End bearing pile