These foundations can be cast as deep-fill (filling most of the trench) or shallow-fill (where the minimum thickness to transfer the load to the soil is provided).
There are other types of foundations that may be used if the ground conditions do not make trench fill practicable. It is advisable to contact a structural engineer or speak to building control for further advice.
Factors to be taken into account of when designing a foundation:
Type of soil
The type of soil that the foundation will sit on is important for two reasons:
- it should be able to bear the weight (load) of the foundation and the extension - different soils have different load bearing capabilities.
- the way it reacts to variations in moisture content (such as in prolonged rainy or dry seasons) can lead to the soil expanding or contracting. This is a particular issue with some clay soils. These changes mainly occur up to a certain depth (typically about 0.75m) therefore foundations should be made deeper so they are not affected by ground movement (although see "Trees" below).
It is important to ensure that the excavation for the new foundation does not undermine adjacent structures. In general it is good practice to excavate at least to the same depth as the bottom of the foundation to the adjacent building. If the excavation runs alongside an existing footing then care will be needed - for example, by excavating and concreting the foundation in shorter sections to avoid undermining a whole length of an adjacent structure (see also guidance on the The Party Wall etc. Act 1996).
Trees will draw moisture from the ground around them and beyond through their root system. As moisture is drawn from the ground it will have a tendency to shrink. How much the ground will shrink will depend on the following factors:
- Type of soil - Clay soils shrink more than other types of soil. Therefore excessive movement of the ground could cause damage to the foundation and the structure it supports.
- Size and type of tree - How large a tree or shrub will grow (its mature height), and the tree type will determine how much moisture it generally draws from the ground.
The presence of trees in clay soil areas can mean foundations need to be significantly deeper than might be first expected, although if the trees are far enough away, there may be no impact. Note: If existing trees are removed or significantly reduced in size, all or some of the moisture in the root system will be released over time into the soil and, if the soil is clay for example, could cause swelling of the soil and damage to nearby foundations and structure(s) supported.
Drains and sewers
As the weight (load) from the foundation of a building is transferred to the soil it spreads downwards outside the footprint of the foundation at a typical angle of 45 degrees. If a drain or sewer is within the area covered by that 45 degrees area there is a risk that it could be affected by the load from the foundation and possibly crack. Therefore,the foundation excavation should normally be at least to the same depth as the bottom (invert) of the deepest part of the drain, sewer or its trench.
Size and construction of new building
The foundation will need to support more weight (load) from a two storey building compared to a single storey. This has a significant factor in determining design, particularly in respect of its depth and width. This is directly related to the bearing capacity of the soil supporting it. The width of the foundation is also governed by the wall thickness.
Generally the topsoil is taken away and good undisturbed ground is found i.e. ground that has not been built on. In some cases there are areas which have previously been backfilled, such as above where drains have been laid or to level a site, which consist generally of soft, mixed soil with foreign objects. The foundation can not be poured until undisturbed ground has been found.
Some properties have been constructed on landfill sites which may require a more extensive form of foundation like piling as the depth of undisturbed ground could be many metres deep. An alternative may be a "raft" foundation. A structural engineer will be able to advise you further.
For health and safety reasons, care should be taken when working in trenches due to the risk of collapse causing potentially serious injury.