Power point foundations

City & Guilds Construction

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PowerPoint presentationFoundations

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 continuedStep 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?

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