ZIMSEC O Level · Building Technology and Design · Form 2, Form 3
Foundations spread the load of the building onto firm ground so that it does not sink or crack.
Types
- Strip foundation: a continuous strip of concrete under walls – the most common for houses.
- Pad foundation: separate pads under columns.
- Raft foundation: a slab under the whole building, used on weak soils.
- Pile foundation: long columns driven or cast deep into the ground for heavy buildings or weak soils.
Constructing a strip foundation
- Excavate trenches to the required depth and width (on firm subsoil, below topsoil).
- Level the bottom and place level pegs.
- Pour and compact concrete to the top of the pegs.
- Cure the concrete.
- Build the foundation brickwork up to above ground level.
Factors affecting foundation design
Type of soil (clay, sand, rock), load of the building, water table and the slope of the site.
Damp-proof course (DPC)
A waterproof layer (plastic sheet) laid in walls at least 150 mm above ground level to stop rising damp.
Exam tip: Draw a labelled section through a strip foundation – ground level, concrete strip, brickwork, DPC and floor slab.
Check yourself
- What is the purpose of a foundation?
- Which foundation is used under a whole building on weak soil?
- How high above ground is the DPC usually placed?
Show answers
- To spread the building's load onto firm ground and prevent settlement.
- A raft foundation.
- At least 150 mm above ground level.
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