Foundations 3

City & Guilds Construction
© 2013 City and Guilds of London Institute. All rights reserved. 1 of 20
PowerPointpresentation
Foundations
Unit 202: Principles of building construction,
information and communication

Aims and objectives
Aim:
• Introduce learners to selecting foundations.
Objectives:
• List the different types of foundations used on houses.
• List the foundations used on commercial buildings.
• Describe factors to be considered when selecting foundations.
• Explain how foundations are laid/installed.

Foundation types
There are three main types of foundations used within construction:
• strip and wide strip
• raft
• piled.

Strip foundations
• Strip foundations are arguably the cheapest form
of foundation available to designers and
constructors.
• Strip foundations involve the removal of a strip of
land to a set depth and width. Concrete is then
poured into the strip/trench and tamped level.
• Strip foundations can only be used on firm, good
bearing strata, and to a depth of no more than
1.5-2m to be economical.
Section through a
strip foundation

Strip foundations continued
Step 1. Excavate trench/strip.
Step 2. Pour concrete.
Step 3. Construct walls to DPC.
Step 4. Backfill as required.
Plan Section

Strip foundations continued
Strip foundation – with reinforcement

Raft foundations
• Raft foundations are often used where the ground has poor load-
bearing capacity.
• For example, if you were to poke your fingers into sand they would
sink to a certain depth. However if you were to hold your hand flat
across the same sand, the sand would offer more resistance
because the load is applied across a greater area. This is the
principal of a raft foundation.

Raft foundations continued
Step 1: Excavate soil.
Step 2: Stabilise soil (using hardcore).
Step 3: Pour concrete raft.
Step 4: Construct walls.

Piled foundations
• Piled foundations are frequently used on large,
heavy buildings, or where the ground has poor
load-bearing capabilities.
• However, piled foundations are more
expensive than both strip and raft foundations.

Piled foundations continued
Basic principle of pile foundations:

Piled foundations continued
Installation of piled foundations:

Foundations – factors to consider
Before foundation types are decided on, a number of factors regarding
the structure and the site it is to be constructed on will have to be
considered, including:
• the weight of the proposed structure
• the purpose of the proposed structure
• the soil of the proposed site (soil mechanics)
• the history of the proposed site
• the cost feasibility.

Weight of proposed structure
The weight and distribution of loads of the proposed structure will
have a direct impact on the loads undertaken by the substrata. For
example, a six-storey building will apply a greater pressure than a
two-storey building.

Purpose of proposed structure
The applied weights of the structure will also be affected by the
proposed use. For example, the foundations at Wembley Stadium
would be under more load than that of a typical office building. Many
buildings have mixed-use requirements, such as an office block
attached to a factory which may house heavy machinery.
Richard Cavalleri / Shutterstock.com

Soil of proposed site
Greenfield and brownfield sites:
What is the difference?

Greenfield sites
Greenfield sites are areas which have never been developed (built on).
They are highly sought after by developers as they are often easier and
cheaper to construct on than brownfield sites.
Planning constraints are often used to protect greenfield sites
(greenbelt laws, for example).

Brownfield sites
Brownfield sites involve land that has either been developed or is currently in
use. The redevelopment of previously developed land often incurs higher build
costs as a result of:
• demolition
• decontamination
• site clearance
• proximity of adjacent structures
• traffic problems.

History of proposed site
The recent history of any proposed site will
have to be considered.
Many areas around the country are affected
by previous mining operations, for example.
Previous structures on the sites may have
included basements, and could also have
been used for chemical processes, for
example a petrol station.
Think about the research that must have
gone into the history of the site when the
Shard was constructed.
olavs / Shutterstock.com

Feasibility costs
The cost of the foundations could consume a high proportion of the
total cost of the building.
This might rule out the feasibility of constructing in some areas. Often
the costs of the substructure can be a third of the total budget.

Any questions?

Student to work on
Please go and work on activities:
7,8,9,10
And worksheet:
3

Foundations 3

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Foundations 3