Before you even set foot on site, it is strongly advisable to employ the services of a specialist engineer or soil expert to conduct a soil analysis. The integrity of your build may ultimately depend on the stability and strength of the underlying soil or rock. However, this is not usually a straightforward matter. In many cases the underlying matter may be a mixture of elements and form superimposed layers, each of which may contain a mixture of soil types.
Thankfully, there are a range of foundation solutions to meet every type of subsoil, ranging from traditional trench-fill or strip footing through to an engineered raft or even a pile solution. Make sure you consult with a local building inspector who will be able to agree your trench-depth on-site.
Where there is soil instability or potential settlement, your structural engineer will design a raft solution. This essentially integrates the foundation and slab into one concrete core with suitable reinforcement holding both together. Heavy clay soil usually requires deeper foundations, which can incur additional costs.
If conditions on site mean that you have to employ a special foundation, don’t panic. In reality, the costs can be easily absorbed during the build or covered by your contingency fund. In very worst cases, extra costs are unlikely to exceed 10% of the original budget.
In most cases homes built with either timber frame or masonry will share precisely the same foundations and no distinction will be made between the two. In a few cases – where loads are critical – the lightweight timber frame or other prefabricated-type structures may be able to have lesser foundations. This is particularly applicable with a piled and ringbeam foundation, where it might sometimes be possible to reduce the number of piles.
Principle types of foundation:
1. Standard Strip
For most soil conditions this is the best and most cost-effective way of constructing your foundations. A trench 450-600mm wide is dug 1-1.2m deep beneath all external and loadbearing walls. In the bottom of this a layer of concrete, at least 225mm thick, is placed and upon this the foundation walls are built in blockwork with two skins to external walls and a single skin to internal loadbearing walls.
In instances where the water-table is high or the trench sides are unstable it’s often better to revert to this type of foundation. The trenches are essentially the same as Standard Strip, except that, instead of just putting concrete in the bottom, you fill them almost to the top –usually within 200mm of the surface.
3. Deep Trenchfill
If you are working with heavy clay and are in close proximity to trees, it is recommended to take the foundation below the level at which a tree’s thirst for water is active. This can mean digging to a depth of 3m and filling the trenches with significant amounts of concrete. At these levels special care needs to be taken and shoring might be required.
4. Piled and Ringbeam
This kind of foundation is used in situations where the top layers of ground have poor bearing capacity and good bearing can only be found at deeper levels, such as that found with deep clay strata or brownfield site. The piles are driven or bored into the ground to support a reinforced concrete ringbeam (groundbeam) spanning from cap to cap to support the house walls.
5. Reinforced Concrete Raft
This system is used where the ground has good bearing but is inherently unstable due to geological or mining conditions far below the surface. A large hole is dug and filled with consolidated layers of hardcore. Upon this a specially designed reinforced-concrete raft is cast, and it is this raft or slab that supports the whole house.