Foundations for House Building
Looking at a brand new house, with shiny fresh paintwork, and clean, solid brick and mortar walls, with their knife-edge angles, it’s not easy to visualise a muddy series of holes, pits, or trenches in its place.
The foundations of a house are, of, course, its strength and stability, and even though they are not visible, once the bricks and mortar are being laid, they are a vital part of the building’s structure.
Before any part of building takes place, a soil survey will give indications as what to expect when the ground is opened up. This will give an idea of what sort of foundations are likely to be needed.
The soil survey can give an indication of the characteristics of the plot, taking samples of both topsoil and subsoil to give an analysis of the make-up of the ground.
Given reasonably stable ground, simple foundations, which are the most common methods, can be dug. The trench is required to be a minimum of one metre in depth, and a width of at least 600mm.
A covering of the base of the trench with around 250 mm of concrete will give a base to build blockwork up to the ground level, where it changes to brick or whatever the external skin is going to be. This is known as strip foundations.
Trench-fill foundations require a similar trench dug, but it is then simply filled with concrete almost to ground level. This is quicker, being less labour intensive, but dearer materials wise.
This form of foundation is used when soil is loose, or there is a high water table. Trenches can be dug deeper to find stable ground, but after 2 to 2.5 metres the trench becomes unpractical and unsafe to work, and the foundations will have to be engineered.
Raft foundations are used where the ground is basically stable, but the trench-work would be so extensive that it would cover over 50% of the ground area beneath the building, or the sub strata may be liable to movement. This could be in areas where mining once occurred, for example.
The raft is a slab of concrete the area of the whole house. Poured onto a bed of compressed hard core, the raft will take the weight of the building over the whole ground floor area of the building.
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Should stable ground have to found at a depth of over 2.5 metres, piles can be driven down to it, and the resulting tube filled with concrete to form a column. The piles support a ring-beam or ground-beam that spans from pile to pile, on which the house is built.