Foundations For Framed Buildings

School of the Built
Environment
Heriot Watt University
Construction Management
and Surveying
Construction Technology 3
Foundations for
framed buildings
Foundations for framed buildings
(i) Aims and Learning Outcomes from this Unit
This unit aims to provide an understanding of the range of foundation technologies that
can be applied to framed structures.
At the end of the unit you should have an appreciation of the various alternative solutions
that are available and the economic and construction advantages or disadvantages that are
associated with them and an understanding of when they can and cannot be used.
Although a rudimentary example of load calculations is included in these notes, this is done
only to provide you with an appreciation of the magnitude of forces that have to be
supported by foundation systems for framed structures. This example calculation is
GREATLY SIMPLIFIED and does not fully reflect the engineering considerations or analysis
that would go into real world foundation design. Therefore, again it is stressed that at the
end of this unit, you will NOT be able to design foundation solutions from an engineering
perspective - that task must ALWAYS be carried out by structural engineers. However, you
should be able to discuss, describe and detail the various different foundation technologies
that are available in a variety of circumstances and you should be able to evaluate them in
terms of their appropriateness for consideration in a given building scenario. You should be
able to discuss possible options with engineers who can then perform the engineering
calculations necessary to deliver a safe solution.
(ii) Suggested Reading
Text Books:-
 Chapter 4- Structure & Fabric Pt 2: Foster and Greeno: 7th Edition: Prentice
Hall
 Chapter 3 - Barry’s Advanced Construction of Buildings: Emmitt and Gorse:

Blackwell Publishing
 Construction Technology Vol 2: Industrial and Commercial Buildings : Riley

and Cotgrave: Palgrave
Internet Sites:-
There are fewer internet sites that provide quality information for foundations in
comparison with what is available for structures, but you should find the following
sites useful:-
 www.info4education.com
 www.skanska.co.uk - will be a useful and helpful resource: You will need to

navigate to the ‘Market sectors’ page and thereafter to the link for ‘Civil
Engineering & Specialists, and then finally to the link for ‘Cementation
Foundations Skanska’. This will take you to the relevant page. There is no
direct URL for this page, so you must access the page the slow way!
(ii) Knowledge you should already have
Before proceeding with this unit you should be familiar with basic foundation
principles and should have an understanding of:-
(i) Strip Foundations
(ii) Pad Foundations
(iii) Plain Slab Raft Foundations.
If you are not familiar with these, you should refer to Chapter 4 of Construction
Technology 2 by Riley and Cotgrave (see above for full reference).
2.1 Introduction
The design of foundations for large scale commercial buildings broadly follows the same principals
and functional considerations, as small scale commercial (or domestic) construction.
However, the technical solutions that are employed are designed to meet the problems posed by
much larger and heavier buildings.
2.2 General Principles
Foundations for commercial buildings must be designed to perform their function adequately:
The foundation is required to safely transfer the loads carried by a building to the ground and to
ensure that these loads are appropriately distributed through the ground where the building
rests.
The foundation is required to provide resistance to uplift forces that may exert themselves upon
large, particularly tall buildings. Wind loading can produce overturning moments that make tall
structures tend towards rotation. The foundations are required to contribute towards that
buildings ability to resist such forces.
The foundations must also resist the forces set up by settlement, differential movement, subsidence
and other possible ground movements (e.g. earthquakes).
Designing an appropriate foundation system requires consideration of a number of factors. These

factors will significantly affect the selection of an appropriate foundation system or indeed, as is
often the case, an appropriate combination of different foundation systems. The design of
substructures and superstructures is closely linked and the design of one cannot be undertaken
efficiently in isolation from the design of the other. The concept design will inform the design of
foundations; this in turn will influence the solutions appropriate to the construction of the
superstructure.
It is important to consider which foundation system will be most appropriate for a proposed building
on a particular site. The basic considerations are as follows:
The existing ground conditions

The loads transferred from the building
The distribution of the loads from the building to the bearing strata
The settlement that will occur under load conditions
How the substructure and the superstructure will behave under differential movement
conditions
The wind loads and other forces acting upon the building.
The foundation system must also be considered from an economic, performance and construction
point of view.
2.2.1 Ground conditions
The primary factor in designing a foundation is an understanding of the ground conditions. Data is
required concerning:
The density of the soil below foundation level,

The cohesion (stickiness) of the soil,
The effective pressure of the overburden soil at the level of the foundation (the weight of soil
that lies above the level at which the foundation will be placed),
The angle of shearing resistance (this relates to the ease with which one soil particle will 'slide'
over another soil particle),
The depth of the proposed foundation,
The width of the proposed foundation footing,
The shape of the proposed foundation.
The acquisition, interpretation and analysis of this necessary data remains primarily an engineering
responsibility. However it is important to understand the practices of site investigation and the
science of soil mechanics.
Site exploration
The purpose of site exploration, or a sub-soil survey is to provide the building designer with a
knowledge of the nature, character and uniformity of the soil on which the building is to be placed
and to obtain samples of the soil for laboratory analysis to obtain some of the data which has been
identified above.
The site exploration will normally include:
An inspection of the site; the natural surface of the ground, and details concerning the
natural vegetation present.
An inspection of secondary information such as can be obtained from local authorities,
geological surveys, aerial photographs and ordinance survey maps.
Detailed knowledge of the nature and variability of the sub-soil is necessary in order to evaluate its
likely behaviour when placed under load conditions. A start to gaining such information is to dig
trial pits and to take borehole samples. Trial pits can be excavated to give information down to a
depth of approximately 2 to 3 metres. Generally, where information from greater depths is required
it will be necessary to acquire samples by drilling boreholes.
The data that is necessary to design foundations systems cannot be obtained from only a couple of
boreholes or from a couple of trial pits, rather these should be dug or drilled around the whole area
of the site in order that the variability of the soil can be ascertained. This will allow the designer to
provide for any weak strata that only occur in one particular area of a site and will allow precautions
to be taken in relation to any weak or unusual columns of soil that may be identified as resent
within the area that the proposed building will be in contact with. It is worth noting that, where
practical, information obtained from trial pits is preferable to that obtained by means of boreholes
simply because the trial pit allows the soil to be examined in situ whereas the borehole removes the
soil from its original situation.
Once acquired, samples require to be analysed and tested in the laboratory to determine the data
described above. Obtaining this data, interpreting it and understanding how it may be applied in
relation to the behaviour of soils under load forms the basis of soil mechanics.
Soil mechanics
Originally, it was assumed that no settlement of buildings of any kind would take place provided
that the load imposed upon a soil did not exceed a permissible bearing value that was allocated to
the various major soil types by experience and by rule of thumb. Foundation design was therefore
based on altering the width of a foundation.
Designers could therefore ensure that the permissible bearing values were not exceeded simply by
manipulating the dimensions of footings. This method, of course, was not particularly well
engineered and proved false once it was realised that all loads imposed on soils will produce some
degree of settlement. It was realised that the amount of settlement depended on many factors in
addition to the actual pressure imposed at the base of the foundation.
The purpose of soil tests is to provide information needed to determine how much load can be
safely placed on the soil, namely, the density of the soil below foundation level, the undrained
cohesion of the soil, the effective pressure of the overburden soil at the level of the foundation and
the angle of shearing resistance.
The foundation engineering report
Eurocode 7 requires that a foundation engineering report be prepared and that this should be
submitted to the building client or owner as part of the foundation design process. In other words,
whether employed by the design team or by a client organisation the contents of this report are of
importance to all involved; therefore it is useful that all involved in technological aspects of building
design have an appreciation of foundation engineering reports.
The foundation engineering report provides a considered analysis of all of the data that has been
obtained from trial pits, boreholes, site observations, historical secondary information and
laboratory tests.
It usually includes the following sections:
Introduction
General description of site
General geology of area
Description of soil conditions found in boreholes and trial pits
Laboratory test results
Discussion of the results of the investigation in relation to foundation design and
construction
Conclusions/recommendations.
The report will conclude with a recommendation of a suitable foundation system based on all of the
factual data.
2.2.2 Load distribution through soils
Before an appropriate foundation system can be selected it is necessary to understand something

about the distribution of pressure through soil and to appreciate the mechanisms that can lead to
the failure of a soil under load.
Pressure is not distributed beneath the surface of a footing in a uniform manner. The way that
pressure is distributed depends on:
The stiffness or rigidity of the foundation structure

The nature and characteristics of the soil
The depth of the foundation beneath ground level.
General patterns of pressure distribution can be generated. If points of equal pressure beneath a
proposed footing are connected it is possible to construct a ‘bulb of pressure’ diagram. A bulb of
pressure diagram gives a general indication of the way that the load carried by the foundation will
be distributed through the soil. The designer can determine the likely stress at various points in the
soil beneath the foundation. This can be compared with the strength data that have been
determined following laboratory testing of site samples.
Thus, the designer can understand how the soil will behave at various depths under load and can
understand whether or not a load imposed at a particular level in the soil is likely to produce failure
in some lower and weaker level.
Figure 2.1: Illustration of Pressure bulbs and Combined Pressure bulbs (taken from Barry's
Construction Technology).
If there are a number of foundation footings close to one another, individual bulb of pressure
diagrams can be combined so as to produce one overall combined pressure bulb. Since there are
frequently strata of different characteristics beneath a given building it is important that some
knowledge of the intensity of pressure at the different levels is obtained and understood.
An understanding of the distribution of contact pressures between foundation and soil will guide
the engineer and the designer in their choice of foundation system and superstructure. For example,
the foundations of a building on a cohesionless soil such as sand could be designed so that the more
heavily loaded columns are towards the edge of the foundation where contact pressure is least.
Light loads could be placed towards the centre where the contact pressure is greatest. Such an
arrangement would tend towards a uniform loading of the soil and would therefore allow the
building to settle in a uniform fashion.
Figure 2.2: Contact pressure distribution for uniformly loaded foundations (taken from Barry's
Construction Technology)
Figure 2.3: Position of foundations on different soil types (taken from Barry's Construction
Technology)
The failure mechanisms of soil
When a load is applied to a soil by a foundation, a degree of settlement will almost always take
place. For the foundation to be safe, this settlement must be relatively small and should be mainly
elastic. That is to say that if the load were removed the soil would recover. With increased loading
settlement will increase until the soil deforms and this will continue until the ultimate bearing
capacity of the soil is reached. If load is applied beyond the ultimate bearing capacity the soil will
enter its failure zone and the foundation will sink and tilt.
Three modes of failure have been identified and these depend on the characteristics of the soil.
These are:-
General shear failure

Punching shear failure
Local shear failure.
General shear failure occurs when a wedge of soil directly beneath the footing reaches plastic failure
and is consequently forced downwards by the load. This pushes the soil on each side outwards
causing it to shear along a curved slip plane so that heaving of the soil takes place on each side of
the building at the surface. The soil on each side of the building is unlikely to be equally strong; as a
result failure in practice is likely to occur on one side.
In the case of punching shear failure, no failure surface is evident and no surface bulging is evident.
The soil beneath the foundation is simply compressed and the foundation sinks straight into the
ground.
Local shear failure is an intermediate case between the general shear failure and the punching shear
failure.
Figure 2.4: Mechanisms of soil failure (taken from Structure and Fabric Part 2)
Therefore, the foundations must be of adequate size, shape and depth and arranged such that the
soil does not reach the point of plastic failure which would permit one of the above three failures to
occur.
The ultimate bearing capacity
It is generally accepted that the bearing capacity of a soil is determined by three principal factors;
these are:
The soil cohesion,

The surcharge pressure or overburden; and,
The self weight of the soil.
Therefore, put very simply, the ultimate bearing capacity will be determined by:
Factor for soil + Factor for + Weight of soil

cohesion surcharge pressure
The shape and size of the foundation itself will also influence the bearing capacity. At the start of
the foundation design process we are not sure of the size and shape of the foundation. Foundation
design is an iterative process.
So, a footing of a certain shape and dimensions has to be assumed at the outset to calculate the
initial bearing capacity of the soil. Once this is established for that assumed size and shape, the
designer can look at the superstructure design and ask whether or not this bearing capacity is
suitable. If it is the initial size and shape of the footing may be accepted and designed in detail. If
not, the designer may wish to determine whether the superstructure or the foundation should be
revised to suit. For example, the loads imposed could be reduced by placing columns closer
together. Alternatively, the size of the footing could be increased or the same size of footing could
be tried at a lower depth, all the time checking to make sure that the bearing capacity becomes
acceptable at the new loads, sizes or depths.
Additional factors that must be considered are:
The level of the water table

Appropriate factors of safety.
The Water Table
The location of the water table can have a particularly significant impact upon the design of
foundations since the bearing capacity can be affected by as much as 50% depending on whether or
not the water table is well beneath the base of the foundation or whether it is at ground level.
Factors of Safety
Because soil mechanics is a relatively approximate science in as much as that many of the variables
require to be estimated approximately, the factors of safety tend to be relatively large, in the order
of 3.0 to 3.5. These factors are important, but we must remember that the engineering design
responsibility belongs to the structural engineer. What the factor of safety (fos) means is that once
determined, the ultimate bearing capacity will be divided by the fos to get the Allowable Bearing
Pressure. It is this pressure that the foundation must be designed to so that the actual force applied
to the soil is in the region of 1/3 less than the pressure that would cause the soil to enter a failure
mode.
2.2.3 Settlement
Settlement should not be viewed as a possibility but rather a certainty. Its effects on the
superstructure of a building will depend on the magnitude of the settlement, its uniformity across
the area of the building footprint, the length of time over which it takes place, and finally upon the
nature of the superstructure itself.
There are three principal causes of ‘normal’ settlement:
Elastic compression - Lateral bulging of the soil as it takes the weight

Plastic flow - Related to failure of the soil
Consolidation - Expulsion of voids and water causes soil to compact.
Settlement due to consolidation in cohesive soils such as clays may continue for a number of years
after the completion of the building. The compressibility of clay is relatively large.
Because the reduction of volume in clay soils takes place by the expulsion of water, consolidation
takes place very slowly.
Figure 2.5: Settlement of buildings on different soil types (from Barry's Construction Technology)
In sand and gravels consolidation under load is relatively small. The rate of settlement normally
keeps pace with the construction of the building. Upon building completion no further settlement
may be expected apart from that which may be caused by unforeseen events.
Uniform settlement over the whole area of the building is normal and provided that it is not severe
will do the superstructure little harm. However, differential settlement or relative movement can set
up stresses in the building superstructure as it is subjected to distortion and twisting forces.
Because a degree of settlement, both uniform and differential, is usually inevitable, it is necessary to
design the foundation and the structure such that movement is accommodated. The impact that
settlement will have upon a building depends to a large extent upon the rigidity of the structure
which supports it.
In load bearing masonry structures the beams are simply supported. Unequal settlement, will allow
the beam to lower very slightly as the supporting wall settles. There may be a small degree of
damage to interior fittings, but the structure can generally cope with small amounts of differential
movement.
In a rigid frame structure even small movements from column to column can cause secondary
shearing forces and bending moments in columns, beams and floor slabs; this could compromise
the frame if the stresses begin to exceed the design forces that were originally estimated. For this
reason, differential settlement (or relative movement) represents a more serious problem for
framed structures (particularly rigid framed structures) than for masonry or non- rigid
superstructures.
Buildings with non-rigid structures can therefore be supported on non-rigid foundations whereas;
buildings with rigid frames tend to require more rigid foundations to resist unequal movements and
thereby to allow the building to settle as a whole.
Where large movements, such as may occur on subsidence sites are encountered, special forms of
construction must be devised. In designing to allow for resistance to differential settlement, the
following issues should be considered:
Attempts should be made to avoid differential settlement as far as possible through an appropriate
design of the building superstructure. This can be done by attempting to ensure that the loads
transferred down each column are pretty much equal. An equal amount of applied pressure under
independent bases cannot be achieved if the loads on some are almost entirely dead loads whilst
the loads on others consist of both dead loads and live loads. It is therefore advisable to plan the
structure of a building such that the ratio of live loads to dead loads is broadly the same for all
columns.
Designers must design foundations to deal with eccentricity from wind loading (particularly on tall
structures) which may cause loads on one side of a structure to be continuously greater than on the
other. This situation can lead to differential settlement, so the designer should attempt to arrange
the structure (as far as practically possible) to maximise dead loads so as to keep the resultant forces
(from overturning moments) within the middle third of the building.
Designers must ensure that each independent foundation is designed around an allowable bearing
pressure such that excessive independent movement can be avoided.
Since most damage arising from differential movement is due to the distortion of rectangular
panels between beams and columns, and since the angle of racking depends on the distance
between columns and the amount of movement, it is possible to formulate rules on the same
principals as those developed to limit the deflections in beam and slab design. The uncertainty and
approximate nature of settlement is such that the rules cannot be applied with the same confidence
as in beam and slab situations. Internal walls and lift shafts can further stiffen structures meaning
that the effects of differential movement become localised.
One final aspect of differential settlement must be considered in relation to tall sections of buildings
joined with less tall or lightweight buildings, for example a skyscraper building that is connected to a
'podium' building at its base. Assuming that the settlement of each section of the building is
perfectly uniform, each will settle by a different amount simply as a result of the differences in
weight. This does not pose any particular problems for each section over and above those which
have been described. However, it does pose a problem where the two sections of the building
connect. If the two sections of the building are structurally continuous then distortion will occur at
the interface between the sections. The structural sections must be discontinuous, that is that they
can move independently of one another. But the design must also allow the building to function
correctly. For example, flexible joints between services may be required and attempts must be
made to ensure that floor plates do not end up with significant vertical differences.
Figure 2.6: Relative settlement (from Barry's Construction Technology)
2.3 Choice of foundation system
The issues that must be addressed in considering the selection of an appropriate foundation system
for a large commercial building are basically as follows:
What area of foundation is required for each structural element to transfer the load from that
element to the ground without exceeding the allowable bearing pressure of the soil and at
what depth must this foundation be placed.
Which is the most appropriate technological solution from an economic point of view.
Which is the most appropriate technology from a construction point of view.
How will the superstructure and the substructure be designed to work together to resist
forces set up by differential settlement.
2.3.1 Determining an appropriate bearing area
For foundations of a 'spread' nature where the force imposed is dispersed over a wide area in
accordance with the basic rule that:
Force (or Stress) = Load/ Area....
....determining the appropriate or required bearing areas is a relatively straightforward matter, once
the characteristics of the soil have been established. Preliminary foundation designs can begin once
a rough idea of the nature of the building to be placed on them is known.
The applied load can obviously be manipulated by altering the bearing area. In this way the designer
can alter the size and shape of the foundation in order to get the design loads into the ground such
that the allowable bearing pressure is not exceeded.
There may, however, be instances when the bearing area cannot be manipulated successfully in
order to safely transfer the loads using a spread or near surface foundation system. Where the
allowable bearing pressure of the soil is too low to support the loads at or near the surface the
designer will have to identify a means of getting the loads down to a layer in the ground where the
bearing capacity is higher. The most common method to achieve this is to use piles. The manner in
which some piles transfer load to the soil is substantially different to the conventional 'spread'
method outlined (dependent on the type of pile used). Therefore, piles and other deep foundation
systems may be thought of as a special case and we will address this when looking at various
different kinds of piles.
We will now look at an example of how the necessary bearing area can be determined, simply in
order that you gain an appreciation of the magnitude of load that can be encountered in a typical
commercial building. As we work through this example, please bear in mind that it is very much
simplified and incomplete in engineering terms. A structural engineer would require a plethora of
additional data and calculations to perform this task when compared to this example.
Figure 2.7 shows a typical building frame arrangement plan:
A B C D E F
6.000 6.000 6.000 6.000 6.000
C1 C4 C4 C4 C4 C1
1
B1 B1 B1 B1 B1
B B B B B B
1 B3 1 B3 1 B3 1 B3 1 B3 1
SD1 SD1 SD1 SD1 SD1 6.000
B3 B3 B3 B3 B3
C2 C5 C5 C5 C5 C2
2
B1 B1 B1 B1 B1
B3 B3 B3 B3 B3
SD1 SD1 SD1 SD1 SD1
B B3 B B3 B B3 B B3 B B3 B 9.000
2 2 2 2 2 2
B3 B3 B3 B3 B3
C6 B3 C3 B3 C3 B3 C3 B3 C3 B3 C6
3
B1 B1 B1 B1 B1
Figure 2.7: General steel frame arrangement plan for a 3 storey office building.
The steel sections are as follows:-
B1 = 406 x 130 x 39 Universal Beam C1 = 254 x 254 x 71 Universal Column
C4 = 305 x 305 x 79 Universal Column C5 = 305 x 305 x 186 Universal Column
C6 = 254 x 254 x 85 Universal Column
SD1 = Structural Concrete Composite Floor using Corus ComFlor 80 Composite Floor
Decking- depth of slab = 150mm.
Load imposed by ComFloor Deck = 0.75kN/m2
Note that this building has 3 storeys above ground level + a concrete roof having the same construction
as the floors. The ground floor slab is ground supported and is to be disregarded in foundation
assessments.
The last numbers in the steel beam and column section details represents its weight expressed in
kg/m, so for example, B1 weighs 39kg/m. We must remember that engineers do not perform load
calculations in kg, so we will need to convert these weights to loads using the unit of kN. There are
100kg in 1 KN, so we simply need to divide these numbers by 100 to complete the conversion.
There is some other data that we need to perform our loading calculation so that we can obtain a
reasonable estimate of the force that must be safely transferred to the soil beneath this building.
We need to know the force that will be imposed by concrete floors and we need to know the floor to
floor heights of the columns.
Concrete has a mass of 2400kg/m3
The floor to floor height spanned by the columns is 3m.
Estimating the design force to be supported by the foundation
We will consider only column C5 in position B2:-
A B C D E F
6.000 6.000 6.000 6.000 6.000
C1 C4 C4 C4 C4 C1
1
B1 B1 B1 B1 B1
B B B B B B
1 B3 1 B3 1 B3 1 B3 1 B3 1
SD1 SD1 SD1 SD1 SD1
B3 B3 B3 B3 B3
C2 C5 C5 C5 C5 C2
2
B1 B1 B1 B1 B1
B3 B3 B3 B3 B3
SD1 SD1 SD1 SD1 SD1
B B3 B B3 B B3 B B3 B B3 B
2 2 2 2 2 2
B3 B3 B3 B3 B3
C6 B3 C3 B3 C3 B3 C3 B3 C3 B3 C6
3
B1 B1 B1 B1 B1
Figure 2.8 Column C5

There are 2 categories of load we must assess:-
Self Weight (the weight of the structural materials- sometimes called the ‘dead loads); and
Imposed loads (from people and furniture- sometimes called the ‘live loads’)
The self weight consists of:-
The weight of the ½ of the span of each of the 4 main beams that connect to the column,
PLUS,
The weight of ½ of the span of the secondary beams that connect to the main beams that
connect to the columns!
The weight of the proprietary metal deck
The weight of the Structural Concrete Floor slab for the area that is carried by the above
beams.
ALL OF THIS NEEDS TO BE CALCULATED AND THEN MULTIPLIED BY THE NUMBER OF FLOORS
PLUS THE ROOF.
AND then finally we need to add the self weight of the column to all of the above….
This calculation can be performed as follows:-
Weight of main beams = (9m x 39kg) + (4.5m x 74kg) = 684kg = 6.84kN
Weight of secondary beams = (18m x 37kg) = 666kg = 6.66kN
Weight of metal decking = (7.5m x 6m x 0.75kN) = 33.75kN = 33.75kN
Weight of structural floor = (7.5m x 6m x 0.150m x 2400kg) = 16200kg = 162kN
Sub- Total = 209.25kN
There are 3 floors above GL plus the roof which has the same construction
as the floor, so this load needs to be multiplied by 4……
Therefore, the load carried by the column is
= 837kN
Finally, we need to add the weight of the column itself.

We know the floor to floor height is 3 m, so the column is
12m in length, at a weight of 79kg/ m Load is = 9.48kN
TOTAL = 846.5kN
Allow for factor of Safety of 1.2 COLUMN DESIGN (DEAD) LOAD = 1012.2kN
Now for the live loads:-
We will assume an imposed live load (uniformly distributed load or UDL) of 4.5kN/m 2.
Remembering that it is only the floor slabs that carry the live loads (the roof supports no people or
furniture), our calculation is as follows:-
Area of floor supported = (7.5m x 6.0m) = 45m2
UDL = 4.5kN/m2
Imposed Live load = 45m2 x 4.5kN/m2 = 202.5kN
No. of floors for which this applies at Column C5 =3
Live load = 3 x 202.5kN = 607.5kN
Multiply by F.O.S of 1.2 = 1.2 x 607.5kN = 729kN
Design load for Imposed load = 729kN
Now to obtain the total design load for column C5, we add the dead loads and the live loads
together
TOTAL DESIGN LOAD CARRIED BY COLUMN = DEAD LOAD + LIVE LOAD
= 1012.2kN + 729kN= 1741.2kN
This is our estimate of the load that must be supported by the foundation and consequently the soil.
If we adopt a spread foundation, we can work out the area of the foundation that we will need to
distribute this load safely into the soil provided that we know the allowable bearing pressure for the
soil.
In this case, lets assume that the ultimate bearing capacity of the soil is 750kN/m2 .
Remember, the allowable bearing pressure is the Ultimate Bearing Capacity divided by a suitable
FOS. In this case lets assume that a FOS of 3.0 is suitable.
This means that the allowable bearing pressure is 250kN/m 2. So, to support the load of 1741.2kN,
we would need a foundation having an area of 1741.2kN/250 = 6.96m2.
Having arrived at this point, alternative foundation technologies may be evaluated in terms of their
suitability from economic and construction points of view.
Reminder: This calculation process is very much simplified and is illustrative only. Many more variables
and factors must be analysed and taken into account in the engineering of a foundation. This is only to
give you a feel for the kind of forces that can be involved in foundation technology.
Selecting an appropriate technology from an economic viewpoint
Because there will often be more than one foundation technology that will be able to transfer the
loads to the soil safely, initially it is important to establish the economic attributes of the various
alternative technological solutions that are available.
For example, it may be technologically possible to safely transfer the loads from the columns in a
heavily loaded framed structure to the ground via large pad foundations. However, to safely do this
the size of the footings may be such that they are practically overlapping. Whilst this may not
present any undue technical problems, there are a number of issues that should be considered from
an economic viewpoint. First, large pad footings will require being of a larger section and will require
more reinforcement to effectively resist shear forces. This adds expense. Second, large pad footings
require more formwork and more construction effort. Therefore, if the pads are virtually
overlapping then a comparison of the original solution with alternatives on purely economic
grounds should be made. The alternatives that may be evaluated in such a case might be:
Altering the positions of the columns to reduce the loads on each footing. This may mean
introducing more columns to the whole structure or just to the ground floor. By reducing the
distance between columns the distance between the pad footings can be increased which
may make the foundation system more economic. However, the building as a whole must be
considered- introducing more columns may become uneconomic overall.
Use a raft foundation instead of individual pad footings.
Use deep foundation systems to take the loads down to a greater depth in the soil if the
bearing capacity is known to be higher at increased depths. Again, the cost of taking
foundations deeper generally increases, so the designer must be sure that any increased
costs are offset against savings made from the original design.
It should be clear from the above example that the selection of the most appropriate system is a bit
of a ‘balancing act’. All of the engineering options can be juggled until an optimum is identified. In
certain circumstances there may be very few suitable options from an engineering viewpoint which
means that there is less ‘balancing’ to do. For example circumstances may dictate that a piling
system must be used to get the loads to a greater depth.
In such a case you are then evaluating which piling system is the most economic rather than which
overall foundation system. A degree of economic evaluation will generally be advisable at least in
the early stages of design.
For greatest economy, it is important to transfer all of the loads in a building directly from the point
of application to the point of support in the foundation. This assumes more importance when the
loads are large. For example, a raft foundation may be an economic method of transferring the
loads from a large number of lightly loaded columns to a weak soil. On the other hand if the
columns are lesser in number meaning that they each carry higher loads, it may be more economic
to use a deep piling system to transfer the loads directly through the weak soil to bedrock.
In most instances, technological solutions can be engineered to work at a cost. However, this does
not mean that they will be economic. Whilst there may be a number of suitable answers from an
engineering perspective, there will normally be one solution which is more appropriate to another
on economic grounds.
2.3.3 Selecting an appropriate solution from a construction point of view
This is very closely linked to the decisions that may be taken on economic grounds, however, the
focus here is not so much on how much things cost but rather on the efficiency of the construction
process. There are obviously cost benefits associated with smooth and efficient construction
practices. Therefore, ultimately, the selection of an appropriate system from a construction point of
view can reduce cost. In essence, the technologist must consider the actual processes that will be
involved in putting the foundation into the ground. To return to the earlier example, it may be more
efficient in terms of resource (e.g. excavation plant; site manager, etc.) usage to construct a raft
foundation than to excavate many or large individual pad footings. Alternatively, it may be cheaper
to install a driven pile foundation as opposed to engaging in a complicated excavation involving
earthwork support and ground dewatering etc. Once more, it is a ‘balancing act’. The technologist
must attempt to select the most appropriate solution from a technological (i.e. engineering,
economic and resource) point of view.
There are some interesting solutions that are applied in order to get complex foundation systems
into the ground whilst maximising resource usage and minimising the overall construction time of
building projects, most notably the ‘top-down’ method applied to basement construction.
Each of these influencing factors informs the choice of foundation system and allows it to be
evaluated in relation to its appropriateness for the building and the client and the user.
2.4 Types of foundation
The foundation systems that are appropriate to large commercial buildings can be broadly
categorised as follows:
Slab foundations
Raft foundations
Piled foundations
Piers
Mass concrete matts (often in combination with piles or piers in the case of large or tall
buildings, or buildings with unusual ground circumstances).
2.4.1 Slab foundations
Pad foundations
Figure 2.9: Reinforced concrete pad foundation with ground beams - captive bolts for steel
columns are clearly visible (top); Reinforcement cage for pad foundation (bottom).
This type of foundation is very common for supporting low to medium framed structures on soils of
reasonable bearing capacity. The footing is basically a square or rectangle of concrete which
supports the weight of a single column. Reinforcement is required in both directions to resist the
dual bending stresses that are set up by the single point load imposed by the column. Shear
reinforcement is not normally necessary.
Since the footings act in an independent manner and since the structures normally supported are
rigid or semi-rigid, differential movement is a design issue. Actions must be taken to limit
differential movement or to minimise its effects. For this reason, independent pad footings are
often tied together using ground beams. This allows the foundation to act in a semi-rigid fashion
and helps to resist the independent movement of each footing. Ground beams also provide lateral
restraint to the footings preventing sideways movements.
Designers should also aim to ensure that the loads imposed upon different footings are not
substantially different so as to minimise the likelihood of differential movement.
Continuous or combined column foundations
This kind of foundation is suitable for the same generic type as in the case of the pad footing but is
appropriate in different circumstances.
First, it is used when the bearing capacity of a soil is such that to safely transfer loads, independent
pad footings would have to be either excessively large (in terms of footprint area). Or alternatively
columns would require to be located closer together than may be desirable.
Secondly, the combined column is appropriate where there is some restriction that would limit the
spread of a pad footing at right angles to a line of columns. Such a restriction may be a site
boundary or another existing building.
This situation is now relatively common on inner city urban sites. Thirdly, the combined column
foundation is suitable if a pocket of weak soil must be bridged. Finally, the combined column
footing may be appropriate to resist excessive differential movement.
Figure 2.10: Continuous column foundation (from Barry’s Construction Technology)
A continuous or combined column foundation is in essence a strip foundation, however, its

nomenclature distinguishes it from the strip footing because it does not support a continuous load
from a wall etc., therefore, it cannot support loads in the same way as the conventional strip
footing. Rather, it supports loads like an inverted continuous beam spanning a series of columns.
Thus a considerable amount of reinforcement is required to resist bending and shear stresses that
are not generated to such an extent in the traditional strip footing.
The continuous column foundation is designed as a continuous beam subjected to point loads from
above whilst simultaneously subjected to a distributed upwards load from the soil below. In a
practical sense, the continuous column foundation can be thought of as an upside-down beam. The
stresses are, therefore, a reverse picture of those set up in a normal continuous beam; consequently
the reinforcement also follows a reverse pattern from that which is generally found in conventional
beams.
The continuous column foundation will be appropriate to the same building types as the normal pad
footing but in the circumstances described.
Combined column foundations
When the distance between the line of columns and some restriction, such as an existing building, is
not sufficient to obtain a symmetrical load distribution, a combined column foundation can be used.
In this case, the columns closest to the line of the restriction are linked to an inner line of columns on
a footing which is designed to distribute the pressure from both columns in an even manner.
The design problem in this kind of foundation is the achievement of a footing that transmits the
pressure to the ground in an even fashion; that is the centre of gravity of the footing must be in line
with the centre of gravity of the imposed column loads. Otherwise eccentricity will be a factor
subjecting the footing to a tilting tendency and producing and uneven distribution of pressure on
the soil immediately beneath the footing.
Figure 2.11: Combined column foundation (from Barry’s Construction Technology).
This means that the shape of the footing must be altered to balance the turning moment that
produces the tendency to tilt. This is typically achieved by resorting to trapezoidal plan forms.
Alternatively, the designer can work with the design of the structure to produce unequal loads down
columns in order to shift the centre of gravity of the imposed load.
Reinforcement is required in a similar fashion to that necessary in the case of the continuous column
foundation except in this instance the footing acts as an inverted simply-supported floor slab as
opposed to an inverted continuous beam.
Combined column footings are appropriate for medium rise framed structures that require to be
supported in the circumstances described above. However, you should note that the use of
combined column footings is not restricted to those conditions and it may be considered
appropriate in a situation where independent footings would be close to overlapping (as described
earlier).
Balanced foundations
These foundations may be used as alternatives to combine column foundations and would be
appropriate in similar circumstances. A cantilever approach may be adopted to balance the footing
or alternatively a simple balance base approach may be used.
Cantilevered approach
In this case the tendency for the structure to tilt and rotate around the internal column must be
counterbalanced by the dead load that is transferred down the same internal column.
Figure 2.12: Cantilever beam foundation and balanced base foundations (from Barry’s Construction
Technology)
Balance base approach
The load from the internal column must again balance the overturning moment produced by the
external column so that the slab is balanced overall and rotation or eccentricity is avoided.
2.4.2 Raft foundations
Raft foundations are most commonly used to support buildings that are to be placed on soils of low
bearing capacity or on soils that have varying levels of compressibility that may lead to difficulties in
relation to differential settlement.
Raft foundations can be used to smooth the construction process where the structure is a simple
grid of relatively closely spaced columns. In these cases an overall raft avoids disruption and
inconveniences that are associated with a relatively large number of excavations for large numbers
of independent pad foundations and ground beams.
It is sometimes suggested that if more than 50% of the footprint of the building is going to be
occupied by individual pad footings or by wide strip footings, it will be more economical to provide
an overall raft foundation instead.
This is not always true since a raft carrying unequal column loads required complex reinforcement to
avoid unacceptable cracking and deflection, however, in the case where column loads are pretty
much equal the raft will often present a more economic solution.
Once more, your attention is drawn to the interdependency which exists between the design of the
superstructure and the design of the substructure.
There are a number of different kinds of raft foundations:
Plain slab rafts

Stiffened edge rafts
Slab and beam rafts
Cellular rafts.
Plain slab rafts
Plain slab rafts can be used in soils where large settlements are not expected. As a result a large
degree of stiffness or rigidity is not generally necessary. The plain raft is of constant thickness
throughout and typically only requires reinforcement in the top and bottom to resist bending
stresses set up by column point loads and evenly distributed ground pressure; it is essentially an
inverted two way spanning floor slab.
This is a relatively straightforward foundation system and is it appropriate to many buildings that
are of a lightweight nature; for example, low rise (often single storey) offices, industrial units or
warehouses etc.
Structurally, the plain raft slab performs in a similar manner to the continuous column foundation,
however, in two directions as opposed to just one. It is easier and more efficient to simply run the
reinforcing bars across the full length and width of the raft as opposed to designing overlapping
reinforcement to alternate between areas of bending in the top and bottom halves of the slab.
Stiffened edge raft foundation
This foundation subscribes to the same basic principals as the plain slab raft except for the
perimeter of the raft being thickened slightly. This serves to stiffen the raft giving a higher degree of
structural integrity and efficiency and is also helps to prevent the ingress of moisture at ground floor
level. This type of foundation is, once more, extremely common and the types of structure
illustrated demonstrate the generic nature of buildings that may be placed on stiffened edge rafts.
Slab and beam raft foundation
The rafts that have been illustrated so far are suitable for carrying lightweight, low rise structures,
however, to support loads imposed by larger, more substantial structures, these rafts would require
to be substantially thicker. They would quickly become uneconomic in the case of larger buildings.
Therefore, to allow rafts to be used economically to support larger buildings, concrete must be used
in a more structurally efficient manner. Consequently, it is necessary to use a raft that comprises
beams and slabs where large loads have to be supported. Using beams and slabs allows a very stiff
foundation to be constructed. The stiffness of the raft is the central issue since it is this that is of
principal importance in preventing distortion to the structural frame. Therefore, the stiffer the raft,
the greater loads it can support, or put the other way the greater the loads to be supported the
stiffer the raft will have to be. There are two approaches to constructing slab and beam raft
foundations: the upstand raft or the downstand raft.
Figure 2.13: Beam and slab raft – upstand raft and downstand raft (from Barry’s Construction
Technology.
The downstand raft has an advantage in that is offers a level surface which can double as the ground
floor slab, however, it is necessary to excavate a series of trenches for the downstand beams. This
causes difficulties in loose or sandy soils which require the excavated trenches to be supported.
Nevertheless, there are no major disadvantages in stiff clay soils.
The upstand beam design means that the beams are formed in clean and dry conditions above the
base slab, however the obvious disadvantage is that a floor slab must now be constructed to act as
the ground floor. This either required formwork, fill or alternatively precast concrete units might be
considered appropriate.
2.4.3 Piled foundation systems
Piled foundations are significantly different from the foundation systems that we have looked at up
until now. Whilst the other systems we have examined generally sit relatively close to ground level
and transmit the loads to the soil via some shape of a flat bearing area, the pile transfers the loads
to deep soil strata and can transmit the loads to the ground in a different fashion depending upon
the type of pile that is used.
There are a number of reasons for adopting piled foundation systems. These are as follows:
To support buildings on sites where the soil has a low bearing capacity at the kinds of depths
that are reasonable for the foundation systems that have been previously described.
To support buildings which have very uneven loadings in different areas making the adoption
of a raft system inadvisable or undesirable.
To support buildings where soil of reasonable bearing capacity is present but at a depth
which makes the use of alternative systems uneconomic (usually depths over 3.00m to
4.50m).
To support buildings on shrinkable clay soils to resist the effects of seasonal movements on
the building.
The main purpose of the pile foundation, which is essentially a column placed deep into the ground,
is to get the loads down into the soil at deeper levels where it can safely bear the imposed loads
without compromising the buildings superstructure.
Piles can transmit loads to soils in one of two ways - friction or end-bearing. Piles that transfer loads
principally by friction are termed ‘friction piles’ whereas piles that transfer load at their bases are
termed ‘end-bearing piles’.
Another classification is frequently applied to piling systems. Displacement piles are piles which are
driven into the ground. As the piles are forced into the soil they displace it. This produces what is
termed ‘skin friction’ and allows the pile to transfer some of the load by friction against the
surrounding soil. Alternatively a replacement pile (or a no-displacement pile) is bored into the soil.
The soil around the pile is not disturbed, friction is not set up and concrete is introduced to replace
the excavated material; this type of pile can only transfer the loads at its end.
Driven displacement piles are often precise concrete. Alternatively they may be formed by placing
in-situ concrete into a hole that was formed by driving (in this case the piles are referred to as
‘driven tube piles’). Bored, non-displacement piles are formed by placing in-situ concrete into a hole
that was formed by drilling or auguring.
In order to ensure that a friction pile will effectively lower the bulb of pressure to the lower soil
strata, it is necessary to ensure that the piles are relatively long when compared to the overall
length and width of the building. The wider a foundation is, the deeper the bulb of pressure will
penetrate (refer to earlier notes). So, unless friction piles are driven to a depth which is significantly
deeper than the width of the building, the bulb of pressure will not be lowered effectively (refer to
illustrations). This is not a problem in the case of end bearing piles since the point of load application
begins at the base of the pile, thus the bulb of pressure will begin at the level of the base of the pile
and will be effectively lowered into the stronger soil strata.
Piles may be placed individually beneath columns or alternatively, where column loads are
sufficiently large, it may be necessary to group two or three piles together under a single column. In
this case, the term ‘pile group’ is used.
Driven displacement pile systems
A number of materials can be used to form driven piles; these are:
Timber
Steel
Concrete.
Timber is not particularly suitable for piled foundations to large buildings for a variety of reasons.
Steel sections (either ‘H’ sections of box sections) may be used when the piles are required to
penetrate to substantial depths making the use of concrete sections impractical, however, such
situations are untypical. In practice, the most common solution for driven piled foundations to
medium to large scale buildings will be to use concrete.
Precast concrete piles

Precast concrete piles for driving come in a variety of shapes and forms. They may be square,
rectangular, circular and they may be solid in section or they may be hollow.
Precast concrete piles are typically 15.00m to 18.00m long although when necessary precast piles
can be manufactures at lengths over 30.00m. Alternatively, very deep precast piles can be formed
by using steel connectors to join independent concrete pile units of up to 10.00m sections together.
In this manner the precast piles can be driven to virtually any desirable depth.
Reinforcement in precast piles must be arranged in such a way as to resist the loads that are to be
placed on them by the building columns in addition to resisting forces and stressed that are set up
during transportation and driving.
Figure 2.14: Precast concrete pile foundation (from Barry’s Construction Technology)
Driven piles are most appropriate for sites that are relatively unconstrained, that is green or brown
field sites that are not located in constrained urban sites with many existing buildings in close
proximity. This is simply because much noise pollution is generated by the driving process and the
vibration caused may damage finishings and fittings in adjacent buildings. Additionally, bulky plant
is often required for pile driving; access for this and sufficient working space must be made available
for it to operate in an efficient manner.
As a result, the nature and location of a given site together with the physical requirements of the
proposed building will determine whether or not driven piles are an appropriate foundation system.
Driven tube piles (sometimes referred to as shell piles)
In this case a steel or concrete tube is driven into the ground. In-situ concrete is then placed into the
tube to form the piled foundation. The tube itself may be left in place, or it may be removed as the
concrete is placed. Since the tube itself still requires to be driven into the ground the same
constraints apply as were evident in the case of the precast driven pile, namely space for driving
apparatus and limitations with regard to noise and vibration pollution.
In the case of a steel tube, the driving can be achieved by a very similar rig and method to that used
to drive precast concrete piles. However, where concrete sections are used to form the shell or tube,
the driving must be achieved using a mandrel (refer to notes concerning the installation of
foundation systems). This allows the tube to be driven into the ground as a result of pressure that is
applied to the toe of the tube as opposed to the head of the tube. There are various methods that
can be employed in order to ensure that the concrete is properly placed and compacted within the
tube to form the completed pile.
Figure 2.15: Driven cast in-place pile (from Barry’s Construction Technology)
Bored piles (rotary augured percussion piles and continuous flight augured (CFA) piles)
Although a slight simplification, we can think of these piles as acting only in an end-bearing manner.
They are appropriate for sites that are constrained by existing buildings or by other considerations
that may limit the noise and vibration pollution that is typically associated with driven and driven
tube piles.
In essence bored piles are formed by drilling the hole into the ground and removing the soil.
Reinforcement and concrete can then be placed and the pile foundation is formed relatively easily
with comparatively little noise and vibration.
There are a number of methods appropriate for drilling and placing the piles; these are discussed in
later when we deal with the installation of foundation systems. Because these piles can be thought
of as acting only in an end bearing fashion they can be considered to spread imposed loads at their
base in the same manner as the near surface spread foundations discussed earlier. This means that
there is a benefit associated with enlarging the toe of end bearing piles since the larger the bearing
area the lesser are the stresses imposed on the soil. This allows the piles to be sunk to lesser depths
than may otherwise be the case. The process of enlarging the base of end bearing piles is known as
‘under-reaming’ and under-reamed piles are illustrated below.
Barrette Foundations
A barrette is a form of pile foundation, but in this instance the plan shape of the foundation is
rectangular instead of circular. Barrettes are normally excavated using rotary grabs or hydrofraise
equipment (discussed in Unit 3). The advantage of the barrette is that its rectangular shape
increases the bearing area when compared to the circular shape of conventional bored piles. The
bearing area can be further increased by arranging two or 3 barrettes so that they intersect to form
'T' , '+' , 'H' or 'C' shapes. This can be a more effective way of increasing the bearing area of a piled
foundation when compared to under-reaming of conventional bored piles and it is possible for a
single barrette foundation to replace a number of separate conventional bored piles in a typical pile
group. Barrettes can also be useful where the foundation will be required to resist large lateral
loads, such as those caused by wind action on the structure they support. In such instances the long
length of the rectangle can be orientated in the direction of the lateral force.
Figure 2.16 A cruciform barrette foundation under construction (source: GeoEng2000;

Thasnanipan et al, 2000)
Cylinder piles
These are piles that have a very large diameter, typically between 600mm and 3.00m (diameters
over 3.00m can be achieved if necessary). They are appropriate when the loads to be carried are
substantial.
You should note that the loads that can be imposed by tall buildings of, say, 60-80 storeys can be
immense. For example, the single point load at the base of one steel column in the Cesar Tower in
Manhattan (completed in 1989) was calculated at 3500 tons or 35000kN (the weight of 5000 average
sized cars).
Cylinder piles are not normally reinforced. They are designed for use principally in cohesive soils and
offer many advantages over conventional piles where the loads to be supported are large.
This is normally related to the fact that the need for pile groups under heavily loaded columns, or
the need for a large number of piles generally, can be removed by placing a few very large cylinder
piles which can carry the same amount of applied loading in a satisfactory manner.
The construction of a few large piles is more straightforward than the construction of many small
piles or pile groups since fewer holes have to be sunk and concrete can be placed more efficiently.
Reinforcement is generally unnecessary except where the cylinder pile passes through weak soil
strata close to the foot of the building. Additionally fewer pile caps and ground beams together with
the associated works are necessary once more speeding the construction process.
In the case of large, particularly tall structures, the cylinder pile represents an economic alternative
to placing a relatively large number of very long piles that would otherwise be required to carry the
loads.
Deep shaft foundations
Deep shaft foundations are very similar to piled foundations; in particular, they are virtually identical
to cylinder piles, the major difference being the methods of construction.
Deep shaft foundation may have diameters from 2.50m to 8.00m and may be sunk as far as is
practically possible depending upon the construction method that is employed (shaft foundations
have been sunk to depths of approximately 40.00m by hand excavation methods).
Shaft foundations may be sunk in a variety of ways. As above, they may be hand excavated from
scratch (in which case some form of temporary support either by sheet steel or by timber is required
together with meticulous attention to health and safety practices and legislation). Alternatively they
may be sunk using Caissons (which will be discussed shortly) or they may be formed using
contiguous piles (which are discussed in relation to methods of permanent earthwork support in
foundations. In either case a lining formed by the caisson or by the contiguous piles is typical.
Alternatively, the lining may be formed within the excavation using special formwork sections.
Whatever method is used to form the lining it is typically backfilled with mass concrete.
Shaft foundations can carry enormous loads and are often more efficient than installing a large
number of individual piles or pile groups. They have been used in connection with other ancillary
foundation systems on buildings such as Embankment Place (London) and the Development Bank
of Singapore.
Figure 2.17: Deep shaft foundations (from Tomlinson – Foundation Engineering)
2.4.4 Pier Foundations
Brick or concrete piers may be constructed as an alternative to short piled foundations when soil of
good bearing capacity is present at depths of approximately 4.50m below ground level. However,
such piers are generally not used to support great loads and you may already have come across
masonry pier foundations in relation to domestic of small scale buildings. Of particular interest in
the context of larger scale building in relation to pier foundations is a form of pier foundation known
as cylinder of monolith foundation.
Caissons and monolith foundations
These foundation systems are appropriate when the loads to be transferred to the soil are large and
the soil close to the base of the building is either waterlogged or of weak bearing capacity such that
the loads have to be transferred to deeper strata (as in the case of pile and shaft foundations).
Cylinder foundations should not be confused with piled foundations, particularly with cylinder piles
as there are many apparent similarities.
A cylinder foundation is formed using a caisson, which is in essence a large diameter tube made of
steel or concrete (masonry can also be used but is out-dated and untypical in this regard). The
bottom edge of a concrete caisson is formed into a cutting edge. The caisson is allowed to sink into
the soil under its own weight or with the aid of kentledge.
After having been sunk into the ground to the correct depth, the caisson is left permanently in
position. It can be completely filled with concrete or alternatively it may be sealed at the base with a
concrete ‘plug’ and at the top with a concrete floor slab; the interior of such a caisson can be left
empty or can be filled with sand or water. Otherwise it may be left open at the top and bottom; such
caissons are referred to as ‘open caissons’ and can be used where ground conditions permit.
Caissons may be sunk to depths of 45.00m or more. Although there are not theoretical limits
concerning how far a caisson may be sunk, there are practical limitations since substantial mass is
required to overcome the friction that acts against the skin of the caisson as it is sunk into the soil
(the caisson can transfer load by friction in a similar manner to the driven pile foundation).
Lubricants can be used to overcome friction, however, in some instances the use of a large cylinder
pile may be considered as an alternative when the depths involved are very great.
A pneumatic caisson many be used when it is necessary to exclude water or silt from the interior of
the caisson. In this case the caisson must act as an airtight chamber; the air pressure keeping the
water and silt from penetrating the caisson. Pneumatic caissons may be sunk in any kind of soil,
however, it is rare for them to be sunk beyond depths of 30.00m since the air pressure which is
necessary to maintain the integrity of the caisson beyond such depths is approaching the greatest
pressure in which humans can work.
Caissons are typically associated with foundation systems for bridges and other civil engineering
structures that involve forming foundations on river beds and such like. Further details can be
obtained from the texts cited in your reading list.
Monoliths
A monolith is basically a series of caissons that are connected and constructed as one large single
unit. In other words, a monolith is a large caisson with a number of unconnected excavation pits.
Monoliths are appropriate when the area of the foundation requires to be large and when the
applied loading is high. They have the advantage of considerable mass which helps in the process of
sinking them into the ground.
Figure 2.20: Caisson foundations
Figure 2.21: Caisson foundations (from Tomlinson – Foundation Engineering)
2.4.5 Concrete Mats
In the case of large tall buildings, the foundations above ie. Piles, Cylinder Piles, Shaft foundations
or Caissons, are often used in conjunction with mass reinforced concrete mats. Depending on the
size of the building, its loads and soil conditions, RC mats could be 1.5 to 3m thick. The purpose of
the mat is to transfer the large loads from the tall building evenly into the foundation system that
lies beneath the mat. An alternative reason for using a mat of this type would be when the building
must span a large obstacle that lies beneath the building within the footprint of the building. Such
an obstacle could be one or more railway tunnels which could be part of a subway, metro or
underground system.
2.5 The construction of foundation systems
Foundation systems for some buildings are relatively straightforward. However, when we examine
large complex buildings, particularly tall buildings with deep basements, these are much more
complicated.
Selecting a solution that is appropriate to a given building from a technological point of view is
important. It is often the case that there will be more than one engineering solution. It may be
equally suitable to use deep bored piles in a relatively large number of pile groups or you may be
able to found the same building on just a few deep shaft foundations. One of the major
considerations is that of how it will be constructed. The cost of installing a foundation system in
terms of time, resources, plant and equipment is typically far more significant than the raw material
costs.
There is obviously a clear divide between the construction and installation of foundation that are
relatively close to ground level and those which may be considered to be deep foundations. As a
result the systems we have examined can be basically divided as follows:
Foundations which are relatively close to ground surface
Isolated column foundations (pads)

Continuous column foundations
Combined column foundations
Cantilevered foundations
Balanced base foundations
Solid rafts
Beam and slab rafts
Cellular rafts.
Deep foundations
Friction piles
End bearing piles
Mass concrete cylinders
Caissons
Basements.
2.5.1 Installing foundations systems close to ground level
The principal issues that need to be addressed in the case of foundation systems that are relatively
close to ground level are:
Excavation
Temporary support to excavation
Formwork and placement of reinforcement
Ground water control.
Excavation
The excavation required for wide strip foundations, pad footings, continuous column foundations
and so on are relatively simple and will normally be easily within the scope of a normal JCB
excavator or equivalent, it would be unusual to require plant more excessive than this. In the case of
very large raft foundation systems, or even to remove the top soil to get the site to reduced level for
foundation construction, it will normally be more efficient to use larger plant.
How large depends upon the amount of material to be cleared, however, since excavation plant is
always necessary whatever foundation system is selected the plant requirements for these kinds of
foundations is not really of major importance. This is simple because there is little to distinguish the
systems from the point of view of making a technological decision on the grounds of the plant
requirements.
Temporary support to excavations and trenches
This depends to a large extent on the soil conditions and the depth to which the foundation is to be
before and upon the type of foundation itself. The construction regulations, together with the
Health and Safety at Work Act require that any excavation trench that is over 1.2m deep must be
adequately shored to prevent collapse.
There are many methods available to support foundations. Traditional methods use timber,
however, this is now considered uneconomical when compared to a number of proprietary systems
that are available.
Excavation for shallow pad foundations and raft foundations will not normally require support,
depending of course on the soil type since non-cohesive soils often require
support, in such soils, formwork is sometimes necessary since the sidewalls of the excavation will
not necessarily be self supporting even at shallow depths, sides of excavations therefore need to be
shored.
Formwork
Where soils are not self supporting, formwork will be necessary in order to shape the foundations as
required. Once more timber formwork was traditional and has been used for many years but it is not
often more economical and more effective, particularly on large sites, to use proprietary systems.
There are not substantial differences between the near surface foundation solutions from a
technological point of view in connection with formwork requirements. It is correct to say, in a non-
cohesive soil, a series of pad footings connected by ground beams will require more formwork than
would an overall raft which could be used to support the same building, however, the pad footings
may be cheaper and faster to construct despite such as additional need for formwork provided that
they are not excessively large in relation to the individual areas necessary for each footing.
Nevertheless it is still beneficial to examine all of the options from a technological standpoint.
Ground water control
This can be one of the most difficult problems in excavation work. Water inflow will occur wherever
an excavation is taken below the water table. Where this is substantial erosion can take place and a
footing may become unstable due to upward pressure from the water.
Figure 2.22: Water causes most difficulties for substructure
Considerations in connection with ground water control are based much more upon the soil type
than upon the foundation system at this level. In other words, if there is a high water table, the issue
will require to be addresses regardless of whether you are considering using a pad foundation, a
strip foundation, a continue column foundation or a raft foundation, etc. This means that from a
technological point of view there is little to differentiate the available solutions from one another
based on the construction in relation to ground water control.
Nevertheless, it is still of some importance for you, the technologist, to understand some of the
approaches that can be adopted where water inflow to an excavation is likely. All of these, obviously
are aimed at removing the water from an excavation or towards preventing it from arriving there in
the first instance.
Pumping from open sumps; or

Ground water lowering
Pumping from open sumps
This is a cheap solution that is appropriate in the most soil conditions. It is also a simple solution
involving only the excavation of a small drainage ditch around the perimeter of the main excavation
with a fall set to a corner where a sump pump is located. The main excavation is therefore kept free
from water ingress. The downside with this method is that the water table remains high with the
result that water still flows towards the excavation with a risk of causing erosion to the walls of the
excavation and subsequently the risk of collapse.
Ground water lowering
This can be achieved by the use of well points around an excavation. These are drilled into the
ground to a depth of approximately 1 metre and are connected by riser pipes to a header pip. The
water is then pumped out. The principal advantage of this approach is that the water is drawn away
from the main area of the excavation allowing the side walls of the excavation to be steeper than
would be the case above without the same risk of failure. As mentioned earlier, dewatering or the
removal of ground water is an issue that the technologists require to address, however, the
foundation systems themselves are not differentiated in terms of these considerations.
Summary in relation to the installation of near surface foundation systems
The construction of foundation systems relatively close to ground level is relatively straightforward.
Excavations require to be supported and formwork is often necessary particularly in non-cohesive
soils. However, this is once more, relatively, simple. Because these foundation systems are installed
relatively close to ground level, the installation considerations related to them all are very much the
same.
For example, all require a degree of excavation, all require a degree of earthwork support if depths
are in excess if 1.2m, all require ground water control (if the water table is known to be relatively
high) and all will usually require a degree of formwork if the soil itself is not capable of supporting
itself.
The possible exception is the raft foundations which will require greater excavation than the other
systems but less formwork, all things being equal. Nevertheless, in broad terms, there is little to
choose between the systems in terms of their implications from the points of view of construction.
Rather in general, each will be selected as an appropriate settlement rather than being selected on
purely technological grounds because one will be faster to construct than another.
2.5.2 Installing deep foundation systems
Driven piles
As discussed previously, driven piles are basically hammered into the ground until the required
depth is reached. There are not great problems presented by this approach except on sites with high
water tables where buoyancy can cause difficulties. This is simply due to the fact that the vibration
from hammering one pile into the ground can cause other piles that were previously driven to
‘bounce’ back out. Practical considerations relate to the pollution from the hammering process both
in relation to noise and vibration, however, these have been mentioned previously. The apparatus
required to drive piles into the ground is fairly uncomplicated and most piling rigs follow a standard
format. In certain circumstances, precise piles can be installed hydraulically using a continuous
applied pressure as opposed to hammering the pile into the ground. Such an approach removed
some of the difficulties in connection with vibration and noise pollution, however, the plant
necessary and the installation process is specialised; this must be borne in mind.
Driven tube or shell piles

In this case two approaches are available. In the case of a driven tube pile, the steel tube complete
with driving toe, is driven into the ground in the same fashion as is followed for the precise concrete
pile. Once the tube is in place, a reinforcement cage can be placed and concrete introduced. In the
case of shell piles, a slightly different approach is necessary. Here the tube is formed by sections of
precast concrete. These are relatively weak and therefore cannot be driven into the ground by
applying pressure blows to the top of the tube. Instead, the pressure has to be transferred to the toe
of the pile. This is achieved using a ‘mandrel’. The sections of precast concrete tube are located on
the mandrel and it is driven into the ground, the pressure being applied at the base of the pile. Once
the shell has been driven to the required depth, the mandrel is removed, a reinforcement cage is
placed and concrete can be introduced to complete the pile.
Figure 2.23: Piled foundation installation
Bored piles
Deep holes for piles can be formed wither by percussive boring or by rotary drilling. Percussive
boring requires only a very light apparatus and can be used where headroom is restricted to
approximately 1.80m. The method involves allowing a steel tube to be sunk into the ground by
using a coring tool to remove material from within the hole allowing it to sink either be self weight
or with the minimal assistance. Once the first section of tube has been sunk, another section may be
attached and coring can continue allowing the tube to sink further and so on until the required
depth is reached. A reinforcement cage and concrete can then be introduced to the pile and the
shell tube can be slowly withdrawn. The concrete can then be compacted using either a drop
hammer or by using compressed air.
Figure 2.24: Contiguous bored pile walls
Rotary drilling or rotary augured piling is appropriate for larger piles on larger sites where there is
good access for sizable drilling rigs. This method uses limited length augers to remove soil from the
pile hole. An example of the type of drilling rig suitable for this purpose is illustrated below. When
necessary, temporary support for the drilled pile hole can be provided by inserting steel tubes. This
is not normally required in cohesive soils, however, there may be strata of weaker, non-cohesive soil
or fill material near to ground level. In such instances a relatively short steel tube may be used to
prevent collapse of the bored hole close to ground level.
From a practical perspective, piles of up to 1.80m in diameter can be installed by this method with
comparatively little vibration or noise pollution.
Figure 2.25: Rotary drilling rig

Continuous flight auger piling (CFA Piles)
In this case a relatively long auger with a hollow central stem is used to drill the pile hole. Once the
required depth has been reached the auger is slowly removed and a highly workable concrete is
introduced to the pile hole via the hollow stem at the centre of the auger.
From a practical viewpoint, this method of constructing a piled foundation is virtually vibrationless
meaning that it can be used on sites that are bounded by existing structures with a relatively low risk
of any kind of damage being incurred by those structures.
Under reaming
The under-reaming of a bored pile foundation is simply the enlargement of the diameter of the toe
of the pile to increase the bearing area. This is relatively easy to achieve in a cohesive soil but is
problematical, difficult and usually uneconomic in cohesionless soils. Therefore, under-reaming is
only really applicable in the case of reasonable cohesive soil conditions.
Excavating an under ream is relatively simple and can use two types of equipment known as ‘belling
buckets’. The first of these has arms or cutters that are hinged at the top whereas the second type
has these arms hinged at the bottom.
The buckets are operated by the drill rods of the auger apparatus or by Kelly bars. The top hinged
bucket produces a conical shaped under ream whereas the bottom hinged bucket produces a bell
shaped under ream.
It is uneconomical to produce under reams for piles having lesser diameters than approximately
760mm. Under reams of up to 3.7 metres are typical, however, if more robust equipment is
employed it is possible to produce an under ream of approximately 7 metres.
Completing the installation of piled foundations
Piles, one installed either by driving or by auguring, are not immediately able to support the loads
imposed by buildings in the post-installation state. To ensure that the building structure will sit on
the piles correctly and to ensure that the foundation will act correctly as a system, the installed piles
have to be cut down to the required level and capped. Ground beams may also have to be
constructed to connect pile caps.
Cutting down and constructing pile caps
All piles except CFA piles need to be cut down to level for two reasons:
To ensure that the building rests on piles that are structurally sound. This is of particular
importance in the case of driven displacement piles since the process that has been
previously described often damages the head of the pile (referred to as driving damage).
Alternatively, in the case of rotary augured replacement piles, there may be areas of poor
compaction (of the concrete) close to the pile head which, once more, can effect structural
integrity.
To allow the reinforcement from the piles to be tied into the pile cap to which the structural
column loads will be directly applied.
Thus the piles are cut down to level using jack hammers or proprietary cutters. CFA piles can only be
finished at ground level and the reinforcement can be left exposed for the purpose of tying into pile
caps. A cut off can be specified for a CFA pile, however, this adds considerable expense as cutting
down is not a cheap process.
The structural loads of the building must be transferred to the pile head via a pile cap which may be
thought of as a pad foundation which bears directly upon a pile or upon a pile group rather than
upon soil. There are a number of reinforcing details for pile caps depending on the shape of the cap
and upon the number of piles in the group.
To improve the overall stability of the foundation system as a whole it is normal to connect the
independent pile caps using a system of ground beams. When this is done, the two elements i.e. the
pile cap and the ground beam are designed as a complete system rather than as two separate
elements that are being combined.
2.6 Underpinning
Underpinning is the term used to describe the process of replacing the foundations beneath an
existing building, usually at a depth greater than that at which they were originally placed.
It is necessary in three circumstances:
When the existing foundation has suffered excessive settlement.

When a new structure is to be placed alongside the existing building in such a way that the
foundation of the existing building will be compromised; for example, as would happen if a
basement were to be constructed next to an adjoining building.
When the load bearing capacity of the existing foundation requires to be increased.
Underpinning is a highly specialised and skilled process and each building is different, having
different underpinning requirements. Therefore, the design of underpinning solutions is very
specialised and one-off solutions may frequently be necessary. Many specialise contractors are
available purely for the purpose of executing underpinning works.
Prior to beginning underpinning operations
This is a fairly obvious requirement, but it is worthy of mention. Before any work can be done, the
building in question must be thoroughly surveyed to check for any structural weaknesses and for
any effects of settlement that may be exacerbated during the course of the underpinning work.
‘Tell-tales’ should also be fixed and detailed measurements undertaken. In very old buildings a
degree of structural strengthening may be necessary before any underpinning can take place. This
may include grouting up any cracks, or using tie rods or pre-stressing cables to tie structural
members together. Openings may require attention, for example, it may be necessary to brick these
up to provide additional strength during the underpinning operations. In addition shoring and
propping may be necessary to reroute structural loads away from particular members altogether.
3.6.1 Methods of underpinning
Underpinning by continuous strip foundations
This is the simplest form of underpinning available. It basically involves excavating a series of ‘legs’
beneath the existing foundation. A ‘leg’ will extend to the depth that is required for the new
foundation and will be between 1.0m and 1.5m wide where the walls of the supported structure are
brickwork of normal construction. The ‘legs’ excavated at any one time should be evenly spaced
along the length of the structure and at no time should a total of more that ¼ of the total length of
the structure be unsupported at any one time.
Figure 2.26 Underpinning by continuous strip foundations (from Structure and Fabric Pt 2)
Once the excavation of the ‘leg’ is complete, a new section of strip foundation can be placed and
brickwork constructed up to the level of the existing foundation. Alternatively, the old section of
foundation may be removed and the brickwork continued up such that it courses in with the new
structure.
The work typically should be done in ‘legs’ numbered in groups of 6, with each ‘leg’ of the same
number being undertaken at the same time (refer to figure 2.25).
The concrete for the new foundation should be placed as quickly as possible once the excavation is
complete.
Where brickwork is not to be used in the ‘leg’ the concrete may be taken right up to the level of the
existing foundation. To ensure good bearing and to allow for the shrinkage of the placed concrete,
where this method is used, the concrete should normally be stopped 50mm to 100mm short of the
base of the existing foundation. It should then be left to cure and shrink before the final resulting
gap is filled with fine dry concrete or mortar. This stage is known as ‘pinning up’.
A final stage in the process sometimes involves grouting between the old and new structures to
close any remaining voids. Dry packing is usually better, however, grouting may be advantageous if
irregular shapes are evident.
Needles may sometimes be used to support sections of wall above ‘legs’ whilst underpinning work is
undertaken.
Underpinning of columns and bases
This can be achieved in a similar fashion except the ‘legs’ previously described are no longer
appropriate. Instead the structural loads must be transferred temporarily away from the existing
foundation. This can be achieved by the use of needles (refer to illustrations).
Once the load has been removed from the existing foundation the excavation can proceed down
below the base until the required depth for the new foundation is achieved. Then, in a similar
fashion to that previously described, the excavation may be backfilled with concrete to
approximately 50mm from the base of the original footing. The final gap may be pinned up once the
concrete has cured by packing with dry concrete or, alternatively, by grouting to ensure a
structurally sound connection, thereby minimising any settlement.
Underpinning by piers
In some instances, continuous walls can be underpinned by using piers to reduce the amount of
excavation necessary. In such instances, beams which span between the piers have to be inserted to
the base of the walls. The ‘Pynford’ method allows this to be achieved by inserting precast concrete
stools that take the structural loads until the new beam is in place (see illustration). A gap of 50mm
to 75mm is left to allow for ‘pinning up’ once the beam is in position. The piers themselves can be
constructed from brickwork or alternatively piles may be used (see next section).
Underpinning by piles
Piles are used in underpinning works when the depths that the new footings require to be taken to
are such that is would be uneconomical to use the methods that have been previously described or
when the ground conditions are such that it would be difficult to undertake the excavation work
necessitated by the other methods.
Typically the structural loads are transferred from existing structural members to the new piles
foundations by permanent ground beams or needles.
Because the working conditions encountered when installing underpinned foundation systems are
cramped and restricted and since vibration must be avoided to prevent damage to the existing
structure, the piles for underpinning are typically bored. However, this said, in certain situations,
precast piles can be jacked (not driven) into the ground by the application of a continuous pressure.
Micropiles can also be used to underpin strip footings.
Underpinning by injection
This method depends upon soil types and ground conditions. They are really only applicable in the
case of sandy or granular non-cohesive soils although you should note that there is a compensation
grouting technique which can be used in clay soils (refer to texts cited in reading lists). The injection
method is particularly useful where the purpose is to strengthen the foundations of an existing
building such that a new basement may be constructed alongside.
The method is essentially very straight forward (assuming that the soil conditions are appropriate).
A cement grout is injected at ground level. This does not require the needs for needles or other
temporary support structures to transfer existing structural loads. The cement grout fills the voids in
the soil and in so doing provides a block of consolidated ground beneath the existing foundation.
The bearing capacity of the soil immediately beneath the base of the existing foundation is
therefore dramatically improved.
2.7 Foundation Systems Summary
The design and construction of foundations for large buildings is a complex process. It is important
that ground conditions are fully investigated and understood. The long term ability to transfer loads
from the building to the ground depends on the foundations. They must be able to withstand the
forces that will ordinarily be placed on the building and exceptional loads in terms of high winds and
earthquakes.
There are numerous solutions available to allow construction to take place on almost any site.
However an appropriate foundation will be one that is based on the following factors:
Ground conditions
Structural efficiency
Economic considerations
Construction process considerations
Environmental considerations.
Foundation systems are in constant development to meet the needs of evolving performance
requirements. The construction industry seeks to provide faster delivery of projects and techniques
for foundation system construction are critical. Redevelopment of dense urban areas has
necessitated the need for alternative solutions to traditional foundation requirements. Land
reclamation poses similar problems. Climate change is another factor impacting on foundation
design. Predicted wind loads must be planned into future construction.
The further that structural design is developed the closer to the limit foundation design is taken.
This encourages the development of innovative solutions to meet the ever-increasing demands of
the built environment.
Advanced Foundation Systems
Self Assessment Questions
Completion of the Unit

The summary below is given to you as a checklist for what you should have learned and broadly
what you should now be able to do as a result of completing Unit 2. This sheet also presents a
number of tutorial and revision questions and a sample exam question for you to test whether you
have learned as much as you should have. If you identify any areas of deficiency, you will need to be
careful to revise the topic and fill in any of the gaps that you may be struggling with.
A note of caution. The notes issued end with a discussion of foundation systems that would be
capable of supporting loads from high rise structures including skyscrapers and large bridges. Such
foundations include cylinder piles, deep shaft foundations and caissons. These foundations are not
appropriate to low rise, long span structures or most office/ commercial buildings in the range 2-10
storeys in height. Normally at most a relatively straightforward piled solution would be the most
complex type of foundation required for buildings of this scale. Basically, in an exam scenario, think
about the context of the building in question and don't propose deep shaft foundations for a single
storey supermarket just because you may know what they are- they simply wouldn't be
appropriate.
Learning Checklist
 Have you understood the performance requirements demanded YES / NO

of foundation systems and the basic performance characteristics of
soil in terms of its load bearing properties?
 Do you understand how loads are distributed through various soil YES / NO
strata and do you appreciate how your choice of foundation could
be influenced by this?
 Do you appreciate the difference between surface foundations YES / NO

and deep foundations and do you know the main criteria that are
relevant to the selection of one or the other.
 Can you describe in detail at least 3 different surface foundation YES / NO

systems that could be used for framed structures?
 Can you describe in detail four different raft foundation systems YES / NO
and analyse their relative advantages or disadvantages?
 Can you discuss at least 4 different approaches to providing YES / NO

piled foundation systems and do you understand the reasons for
pile groups, pile caps and ground beams.
If you have answered "No" to any of the above questions, you need to do more work on the material
that has been presented in Unit 2 again.
Revision/ Tutorial Questions
1. What are the performance requirements that have to be satisfied by foundation systems
and how does the soil type and structure influence this?
2. Describe in detail, four different foundation systems that would be suitable for a framed
structure- you may choose either deep systems or near surface systems.
3. Describe the construction of a pad foundation system indicating its key characteristics and
the performance requirements that need to be satisfied.
4. Do the same as asked in Q3. for:-
 Raft Foundations (of any type),

 Continuous Column Foundations,
 Driven Pile foundations
 Bored Pile foundations
 Driven shell piles.
 Cylinder Piles
 Caissons
5. Describe fully why a strip foundation could not be considered suitable for a framed
structure and explain the difference between a strip foundation and a continuous column
foundation.
6. When might you be forced to opt for a continuous column foundation?
7. Why might you be forced to consider the use of a 'balanced footing'.
8. Sketch a detail showing the connection of a typical steel column to a typical pad
foundation.
9. Sketch a detail showing the connection of a typical steel column to a typical pile cap.
10. Why do foundations fail and what steps are necessary to prevent failure.
Food for thought...
Foundations are the single most important element of any building or facility. Foundation failure is
rare, but when it happens the impact is normally catastrophic. There are two main problems; one is
soil failure which occurs when the soil beneath a foundation is loaded beyond the point of its
ultimate bearing capacity. In this case the end result would ordinarily be a total or partial collapse of
the supported structure. The second problem is much more common and in fact happens to one
degree or another in almost all buildings. This is the issue of settlement. Providing the overall
degree of settlement is small no lasting problems will result. An important issue in foundation
engineering is calculating the degree of settlement that should be expected and of more
importance limiting the degree of differential settlement that may occur, particularly in framed
structures. If one part of a structure settles more than another, cracking and distortion may occur
and the columns and beams in a framed structure may be subjected to additional stresses that they
were not engineered for. Foundation failures are rare, but occasionally occur in the case of grain
silo's- why do you think this is the case? Additionally, what would you perceive to be the problem in
the case of the leaning tower of Pisa?
Sample Exam Questions...
Advanced Foundations
A client is considering building a new office headquarters building in the centre of Edinburgh. They
have secured planning permission for an 8 storey building. The finance for the project has been
obtained and the project is now moving from sketch design to detailed design. You have been
asked to provide advice to the client concerning the foundation system for the proposed building.
(i) Fully describe the main performance requirements that the foundation system for this new 8
storey building must accomplish.
(8 marks)
(ii) Discuss any specific problems that may be faced by the foundation systems for this building.
(6 marks)
(iii) Assuming that the soil conditions are poor to a depth of 19.000m below ground level and
assuming that there is a railway tunnel beneath the footprint of the office building, describe in
detail an approach for providing the foundations for this building and describe any special care
that would need to be taken for this scenario.
(16 marks)
Advanced Foundations
The factory building (Building A) illustrated on Drawing 1 is to be built upon a soil having the
following composition:-
0.0m - 1.6m Loose fill material of low bearing capacity.

1.6m - 2.5m Sandy Clay of moderate bearing capacity.
2.5m - 8.0m Boulder Clay of high bearing capacity.
(i) Discuss the main issues that need to be considered in selecting an appropriate foundation
approach for this building.
(7 marks)
(ii) Evaluate three foundation systems that could be considered appropriate for this particular
building given the indicated soil conditions.
(8 marks)
Drawing 1
14.000m
C Retail
20.000m
Retail
Existing
Hotel
10.000m C
Building C
Retail
8 Storey
20.000m A B B
Office Tower
18.000m
Retail 1 Storey Building B
Factory Retail
Building A
A
18.000m
Retail
18.000m
Retail
Building B
Building C
Building A
A
B
Section A - Section C - C
A
Section B - B

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School of the Built

(i) Aims and Learning Outcomes from this Unit

 Chapter 3 - Barry’s Advanced Construction of Buildings: Emmitt and Gorse:

 Construction Technology Vol 2: Industrial and Commercial Buildings : Riley

 www.skanska.co.uk - will be a useful and helpful resource: You will need to

(ii) Knowledge you should already have

(i) Strip Foundations

(ii) Pad Foundations

(iii) Plain Slab Raft Foundations.

2.2 General Principles

Designing an appropriate foundation system requires consideration of a number of factors. These

The existing ground conditions

The density of the soil below foundation level,

The site exploration will normally include:

The foundation engineering report

It usually includes the following sections:

Before an appropriate foundation system can be selected it is necessary to understand something

The stiffness or rigidity of the foundation structure

General shear failure

The soil cohesion,

Factor for soil + Factor for + Weight of soil

Additional factors that must be considered are:

The level of the water table

The Water Table

There are three principal causes of ‘normal’ settlement:

Elastic compression - Lateral bulging of the soil as it takes the weight

2.3 Choice of foundation system

Which is the most appropriate technology from a construction point of view.

Force (or Stress) = Load/ Area....

Figure 2.7 shows a typical building frame arrangement plan:

B1 = 406 x 130 x 39 Universal Beam C1 = 254 x 254 x 71 Universal Column

B2 = 457 x 152 x 74 Universal Beam C2 = 305 x 305 x 88 Universal Column

B3 = 305 x 127 x 37 Universal Beam C3 = 305 x 305 x 149 Universal Column

C4 = 305 x 305 x 79 Universal Column C5 = 305 x 305 x 186 Universal Column

C6 = 254 x 254 x 85 Universal Column

Load imposed by ComFloor Deck = 0.75kN/m2

Estimating the design force to be supported by the foundation

We will consider only column C5 in position B2:-

Figure 2.8 Column C5

The self weight consists of:-

The weight of the proprietary metal deck

This calculation can be performed as follows:-

Weight of main beams = (9m x 39kg) + (4.5m x 74kg) = 684kg = 6.84kN

Weight of secondary beams = (18m x 37kg) = 666kg = 6.66kN

Weight of metal decking = (7.5m x 6m x 0.75kN) = 33.75kN = 33.75kN

Weight of structural floor = (7.5m x 6m x 0.150m x 2400kg) = 16200kg = 162kN

Sub- Total = 209.25kN

Finally, we need to add the weight of the column itself.

Area of floor supported = (7.5m x 6.0m) = 45m2

Imposed Live load = 45m2 x 4.5kN/m2 = 202.5kN

No. of floors for which this applies at Column C5 =3

Live load = 3 x 202.5kN = 607.5kN

Multiply by F.O.S of 1.2 = 1.2 x 607.5kN = 729kN

Design load for Imposed load = 729kN

TOTAL DESIGN LOAD CARRIED BY COLUMN = DEAD LOAD + LIVE LOAD

= 1012.2kN + 729kN= 1741.2kN

Use a raft foundation instead of individual pad footings.

2.4 Types of foundation

Figure 2.10: Continuous column foundation (from Barry’s Construction Technology)