Footings and Foundations in House Building
The foundations of a house will, for the lifetime of the building, probably never be seen again once the house is built, and being out of site means out of mind, but nevertheless, the foundations are a vital part of the construction.
The foundations are what transfer the weight of the building safely and evenly out into the ground and support the weight of the walls and roof, and give a sound base on which to build.
Types of foundations vary with the size, shape and mass of the proposed construction, the make-up of the ground, and the influence of adjacent buildings and trees.
In house building the usual approach will be to look at strip foundations. The lines of the building’s supporting walls, are the lines which will dug as trenches. The minimum depth is around one metre. If suitable subsoil has to be found at a deeper level, digging can go on down to a depth of two to two point five metres.
After this depth, it becomes both impractical and unsafe to work in. At the bottom of the trench, a layer of concrete around 250mm is laid, and blockwork built up to finished ground level, where brickwork takes over.
Trench-fill foundations are dug in the same lines as strip, but narrower, as the trench is simply filled with concrete up to within two brick courses of finished ground level. It can be narrower because unlike the strip foundation there is no need to work in it.
It is quicker and easier, but because of the quantities of concrete, is slightly more expensive than the block-worked strip.
If the ground is found to be difficult and trench work not suitable, a structural engineer can advise a more complicated, thus “engineered” foundation. The Raft, or mat foundation is used where the strata is considered unstable, meaning that strip foundations would have to uphold more than half of the ground area below the building.
It is basically a slab of concrete which covers the whole ground floor area of the building, spreading the weight of the build across the whole slab. Ground works remain fundamentally the same for different foundation types, requiring a backhoe loader or digger (Hanlon provide a range of equipment for the purpose) to excavate the necessary earth providing suitable trenches for concrete pouring.
Where soil is too weak to support the construction, and the firm level is too far down for trench work, pile foundations can be used.
These are a series of concrete columns, or piles, which are driven down to the load bearing strata, and connected at the top to each other by horizontal beams of concrete to build off.