202 level 2 principles of building construction, information and communication

City & Guilds Construction

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Foundations

Unit 202: Principles of building construction, information and communication



Aims and objectivesAim:

• 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 typesThere 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 continuedStrip 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 continuedStep 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 continuedBasic principle of pile foundations:



Piled foundations continuedInstallation of piled foundations:



Foundations – factors to considerBefore 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 structureThe 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 structureThe 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 siteGreenfield and brownfield sites:

What is the difference?



Greenfield sitesGreenfield 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 sitesBrownfield 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 siteThe 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 costsThe 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?

202 level 2 principles of building construction, information and communication

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