SUBMITTED BY:
Saad Ahmed (287388)
Raja Dilawar Riaz (297151)
Usama Majid (289280)
Ibtesam Ashjha (321672)
Ali Salman Satti (283307)
Abdul Rahman (285519)
CE-121 ENGINEERING GEOLOGY
Assignment # 2: Types of Foundations
Submitted to: A/P Abdul Jabbar
Submitted on:13/03/2020
INTRODUCTION
The strength of a building lies in its foundation. The main purpose of the
foundation is to hold the structure above it and keep it upright. On the contrary, a
poorly constructed foundation can be dangerous to the occupants and the
neighborhood. With high-rise buildings touching the sky these days, it has become all
the more important to have powerful foundations. Therefore, it is highly essential to
determine the quality of construction when looking for office complexes, outlets for
retail stores or flats for sale in Palakkad or elsewhere in the country.
What is the purpose of having a foundation? A foundation plays three major roles in
the construction of a structure. The basic part is to support the load of the entire
building. Good and a strong foundation keeps the building standing while the forces
of nature wreak havoc. Well-built foundations keep the occupants of the building
safe during calamities such as earthquake, floods, strong winds etc. The foundation
must be built such that, it keeps the ground moisture from seeping in and weakening
the structure.
According to the construction experts and engineers, the foundation must be
able to withstand the “dead” load and “live” loads. The dead load is the weight or
the load of the basic structure itself. This is called dead load as it remains constant.
On the other hand, the live load is the weight of the people and other objects that
they bring with them. The foundation must be firm and must be able to channel the
weight of the entire building to the ground. If the building is being constructed on
sloping regions or moist ground, the foundation has to be customized and durable.
TYPES OF FOUNDATION
Foundations are classified as shallow and deep foundations. Types of foundations
under shallow and deep foundations for building construction and their uses are
discussed. It is advisable to know suitability of each types of foundation before
their selection in any construction project.
Following are different types of foundations used in construction:
1. Shallow foundation
 Strip foundation
 Raft or mat foundation
 Combined footing
 Individual footing or isolated footing
2. Deep Foundation
 Pile foundation
 Drilled Shafts or caissons
SHALLOW FOUNDATION
A shallow foundation is a type of building foundation that transfers building loads to
the earth very near to the surface, rather than to a subsurface layer or a range of
depths as does a deep foundation. Shallow foundations include spread footing
foundations, mat-slab foundations, slab-on-grade foundations, pad foundations,
rubble trench foundations and earthbag foundations.
1) STRAP FOUNDATION
A cantilever footing is a component of a
building’s foundation. It is a type of
combined footing, consisting of two or
more column footings connected by a
concrete beam. This type of beam is
called a strap beam. It is used to help
distribute the weight of either heavily or
eccentrically loaded column footings to
adjacent footings. Strap footings are
similar to combined footing. Generally, when the beam cannot be extended beyond
the property line the exterior footing is connected by strap beam with interior
footing. To get a very rough idea of the size of the footing, the engineer will take the
total load on the column and divide it by the safe bearing capacity (SBC) of the soil.
A cantilever footing is often used in
conjunction with columns that are located
along a building’s property or lot line.
Typically, columns are centered on column
footings, but in conditions where columns
are located directly adjacent to the
property line, the column footings may be
offset so that they do not encroach onto the adjacent property.
This results in an eccentric load on a portion of the footing, causing it to tilt to one
side. The strap beam restrains the tendency of the footing to overturn by connecting
it to nearby footings.
They are used where the column load is
not directly over the centre of the
footing. This distance is called
eccentricity when multiply by the
column load produces an overturning
moment. This causes the bearing
pressure of the footing to increase on
one side and reduce on the other. Therefore, a ground beam is required to
counteract this overturning moment so the bearing pressure becomes uniform.
These are normally used when building very close to existing building where the
columns are located up to the boundary.
2) RAFT FOUNDATION
Raft foundations (sometimes known as Mat Foundations) are a large concrete slab
which can support a number of columns and walls.
The slab is spread out under the entire building or at least a large part of it which
lowers the contact pressure compared to the traditionally used strip or trench
footings.
Because of the speed and volume of houses required after the second world war, the
raft foundation was widely used. The raft foundation was cheaper, easier to install
and most importantly, did not require as much excavation as the usual strip
foundations.
Rafts are most often used these days when the strata is unstable or (because of this)
a normal strip foundation would cover more than 50% of the ground area beneath
the building. There are also situations (usually in areas where mining has occurred)
where there may be areas of movement in the strata.
Raft Foundations are built is these following steps:
1.The soil removed down to correct depth
2.The foundation bed is then compacted by ramming
3.Lay reinforcement on spacers over the foundation bed
4.Pour the concrete over the reinforcement
WHEN ARE THEY USED?
Raft foundations are usually preferred under a number of circumstances:
1.It is used for large loads, which is why they are so common in commercial building
which tend to be much larger, and therefore heavier, than domestic homes
2.The soil has a low bearing capacity so the weight of the building needs to be spread
out over a large area to create a stable foundation
3.The ratio of individual footings to total floor space is high. Typically, if the footings
would cover over half of the construction area then raft foundation would be used
4.If the walls of the building are so close that it would cause the individual footings to
overlap, then raft foundations should be used
The foundation may stiffen by ribs or beams built in during construction which will
add extra strength and rigidity.
TYPES OF RAFT FOUNDATIONS:
There are five main types of raft foundation:
1.Solid slab rafts.
2.Slab beam rafts.
3.Cellular rafts.
4.Piled raft foundations.
5.Balancing (or floating) rafts.
Solid Slab Rafts:
Solid Slab Rafts are typically used for small
buildings where it’s possible to position
columns at regular distances.
Slab beam rafts:
This type of raft foundation is typically used in a
build that involves column loads that are not
evenly distributed across the footprint of the
building.
Cellular Rafts:
This type of raft foundation comprises two
concrete slabs, which lock together
via ground beams.
Piled rafts:
Piled raft foundations are less common in
residential building projects, and are more often
seen in larger, high-rise developments as well as in
structures that require a single foundation
element, such as silos, tanks and chimneys.
Balancing rafts:
Balancing rafts or floating foundations are
increasingly used in cases where the reduction of
soil settlements is of absolute importance, as well
as in projects where the soil bearing capacity is
extremely low.
3) COMBINED FOOTING
Whenever two or more columns in a straight line are carried on a single spread
footing, it is called a combined footing. Isolated footings for each column are
generally the economical.
Combined footings are provided only when it is absolutely necessary, as
 When two columns are close together,
causing overlap of adjacent isolated footings.
 Where soil bearing capacity is low, causing
overlap of adjacent isolated footings.
 Proximity of building line or existing building
or sewer, adjacent to a building column.
When the soil under the foundation does not
have ample strength to bear the load of the structure, then footings are provided.
The function of the footing is to act as the supporting agent of the foundation. It also
protects the foundation from settling down beyond the permissible limit.
Combined footing is basically a combination of various footings, which utilizes the
properties of different footing in a single footing based on the requirement of the
structure. It carries two or more columns along a straight line.
The reasons for providing combined footings are as below.
• It is necessary to provide the combined footing in case of overlapping, when two
columns are close enough to each other.
• They are provided where the load bearing capacity
of the soil is low, and it will go below the optimum
limit.
• It is required to provide combined footing in the
structure, when the resultant loading is close enough
to the column of the building.
Depending on the shape of the combined footing,
there are three types of combined footing, namely,
rectangular, trapezoidal and T-shaped combined
footing.
Consider the figure for different types of combined
footing.
Steps for Design of Combined Footing
 Locate the point of application of the column loads on the footing.
 Proportion the footing such that the resultant of loads passes through the
center of footing.
 Compute the area of footing such that the allowable soil pressure is not
exceeded.
 Calculate the shear forces and
bending moments at the salient
points and hence draw SFD and
BMD.
 Fix the depth of footing from the
maximum bending moment.
 Calculate the transverse bending
moment and design the
transverse section for depth and
reinforcement. Check for
anchorage and shear.
 Check the footing for longitudinal shear and hence design the longitudinal
steel
 Design the reinforcement for the longitudinal moment and place them in the
appropriate positions.
 Check the development length for longitudinal steel
 Curtail the longitudinal bars for economy
 Draw and detail the reinforcement
 Prepare the bar bending schedule
4) ISOLATED FOOTING
Isolated footings are commonly used for shallow
foundations in order to carry and spread
concentrated loads caused by columns or pillars.
Isolated footing can consist of either reinforced
or non-reinforced material. For non-reinforced
material, however, the height of the footing has
to be bigger in order to provide the necessary
spread of load. It is one of the most popular and
simplest types of foundations used world-wide.
This type of footing is used when:
Columns are not closely spaced,
Loads on footings are less, and
The safe bearing capacity of soil is generally high.
The isolated footings essentially consist of a
bottom slab. The three basic types of bottom
slabs are:
 Pad footing
 Stepped footing
 Sloped footing
Some popular isolated footings are:
 Square footing
 Rectangular footing
 Circular footing
DEEP FOUNDATION
A deep foundation is a type of foundation that transfers building loads to the earth
farther down from the surface than a shallow foundation does to a subsurface layer
or a range of depths. A pile or piling is a vertical structural element of a deep
foundation, driven or drilled deep into the ground at the building site.
1) PILE FOUNDATION
Deep foundation is required to carry
loads from a structure through weak
compressible soils or fills on to
stronger and less compressible soils
or rocks at depth, or for functional
reasons. Deep foundations are
founded too deeply below the
finished ground surface for their base
bearing capacity to be affected by surface conditions, this is usually at depths >3 m
below finished ground level.
Pile foundation is a type of deep foundation which is used to transfer heavy loads
from the structure to a hard rock stratum
much deep below the ground level. Pile
foundations are used to transfer heavy
loads of structures through columns to
hard soil strata which is much below
ground level where shallow foundations
such as spread footings and mat footings
cannot be used.
The selection of pile foundation depends
on the soil investigation data received from soil exploration bore holes at different
depths. Selection of appropriate pile for the desired strength and requirement
plays an important role in cost reduction and efficiency.
The factors that affect the selection of pile foundations are;
Soil conditions
Loads from structures
Nature of loads
Number of piles to be used
Cost of construction
Pile foundations are also used to prevent
uplift of structure due to lateral loads such
as earthquake and wind forces. These are
generally used for soils where soil
conditions near the ground surface is not
suitable for heavy loads. The depth of hard
rock strata may be 5m to 50m (15 feet to
150 feet) deep from the ground surface.
Pile foundation resists the loads from
structure by skin friction and by end
bearing. Use of pile foundations also prevents the differential settlement of
foundations.
2) DRILLED SHAFTS OR
CAISSON FOUNDATION
Drilled shafts, also called as caissons, is a
type of deep foundation and has action
similar to pile foundations discussed above,
but are high capacity cast-in-situ
foundations. It resists loads from structure
through shaft resistance, toe resistance
and combination of both of these.
The construction of drilled shafts or
caissons are done using an Auger. An auger
is a drilling device, or drill bit, used for
making holes in wood or in the ground.
Auger is as shown in the figure. Drilled shafts can transfer column loads larger than
pile foundations. It is used where depth of hard strata below ground level is location
within 10m to 100m (25 feet to 300 feet). Drilled shafts or caisson foundation is not
suitable when deep deposits of soft clays and loose, water-bearing granular soils
exists. It is also not suitable for soils where caving formations are difficult to stabilize.
DIMENSIONS: With available drilling equipment, shaft diameters up to 20 ft (6 m)
and depths exceeding 250ft (76 m)
are possible. However, for most
normal applications, diameters in
the range of 3 to 10 ft (1 to 3 m) are
typical.
NEED: In addition to above facts,
normal construction practices for
drilled shafts effectively eliminate
the noise and strong ground
vibrations that develop during pile
driving operations. For these and
other secondary reasons, drilled shafts have become both the technical and
economic foundation of choice for many design applications.
PRICE: In USA If caissons are pulled for reuse, the cost per cubic foot is $8.00 to
$18.20. If caissons are not pulled, use $23.50 to $47.00 per cubic foot. Normal highrise construction requires caisson to remain in place
Advantages of Caissons:
 Economics
 Minimizes pile cap needs
 Slightly less noise and reduced vibrations
 Easily adaptable to varying site conditions
 High axial and lateral loading capacity
Disadvantages of Caissons:
 Extremely sensitive to construction procedures
 Not good for contaminated sites
 Lack of construction expertise
 Lack of Qualified Inspectors