A quick guide to foundations and ground conditions when building an extension.
One of the first stages to an extension project is the groundworks, and the ground conditions that you are building on will have a huge impact on this. How prepared and informed you and your builder are will determine whether or not you can be caught out and incur unexpected costs.
The main considerations for footings (or foundations as they are also called) are:
- Soil composition
- Proximity of trees
- Presence of 'made up ground'
How prepared and informed you and your builder are will determine whether or not you can be caught out and incur unexpected costs.
The main goal and purpose of foundations, as obvious as it may seem, is to find a good and stable strata (or bearing) to build the new structure onto. This is usually identified by finding 'virgin ground' during the dig, which will usually appear as golden ballast or sand. Once you have established that the dig has reached this virgin ground, you can use this as your bearing for the new foundation. Ordinarily the minimum depth of foundation is 1 metre, although this must be agreed with the building inspector. There are however some scenarios when you will be required to go deeper.
If the soil you are digging in is made up of clay and not sand and gravel, then you can expect to have to go deeper with your foundations. The reason for this is that clay expands and contracts with moisture which could lead to movement in the footings during particularly hot weather.
A trench foundation will become more expensive the deeper you have to dig due to labour, disposal of soil and the amount of concrete that will be required to back fill
If you are unlucky enough to encounter clay and trees, then these two combined will mean you have a very deep dig on your hands as trees draw moisture from the ground with their roots. The depth of excavation will depend on the type of tree and their proximity to the closed part of the footing. A clay master board will also be required which comes in sheets and is inserted to the side of the trench before the concrete is poured.
The proximity of trees to the foundation will also have an effect on how you have to approach things. The concern with trees is the moisture that they draw from the ground with their roots, which can undermine the footings and cause them to drop.
Be careful not to get caught out by hedges, they count as trees in this context
The type and size of tree will be considered by building control but generally speaking the bigger the tree and the closer it is, the more of an impact it will have on your foundations. Be careful not to get caught out by hedges, they count as trees in this context! In fact a Hawthorne hedge or shrub line is one of the worst you can encounter and will see you digging down to more than 2 metres.
Made Up Ground
Made up ground, or man made ground, is an area that has been built up using material brought onto the site. You usually find this on older housing estate developments when there was a lot less regulation; the last plot on the site was used as a way to dispose of all the unwanted material left over.
In these situations a test hole is crucial
Made up ground can cause a serious headache as there is no way of knowing how far it goes down without digging a test hole. In these situations a test hole is crucial, otherwise you could dig and dig a trench with no guarantee of getting to virgin ground all the while spending money on labour and removal of soil.
So if you find yourself in a less than ideal situation what can you do? It doesn't have to be a show stopper.
If there are trees present and you don't mind getting rid of them, then cut them down. You probably won't have to dig out the root and building control mostly will be happy for them to be taken down to stump. You should always consult building control however and work to their guidance.
On made up ground it could pay to look at an alternative type of foundation. The traditional trench or strip footing is popular as it is relatively easy and straightforward, however it can be expensive if you have to go deep as the labour, soil removal and extra concrete costs all add up.
You should always consult building control however and work to their guidance
In extreme situations piling could be the best way forward, which involves driving steel cylinders or columns into the ground to provide a strong platform and transfer the load safely. It is an expensive method and will require a structural engineer in most cases to specify the requirements, however it is better than a three metre deep trench which brings its own risks and costs.
Another solution in a built up ground situation is to dig pad foundations in each corner of the line of foundation. This gives the strata required to transfer the load safely, but only in four areas. By installing a ground RSJ (rolled steel joist or 'steel') this can be transferred across the full foundation and you can build up from there. This method requires less concrete and less digging, however it does require a structural engineers' design, and steel beams which carry a cost.
There are many alternative methods for foundations and always a way round problems that present, as with most things the key is preparedness and being well informed from an early stage.