Different ground conditions require differently engineered foundations and foundation design. Likewise frost and groundwater need special consideration, especially for Cellars and Basements. Avoid some early pitfalls and get your foundations right first time. .
The Right Foundations for Ground Conditions
Covered under Part A of the UK Building Regulations, foundations support the whole of the load transferred to them from the entire structure of the building, including dead loads, live loads, and imposed loads from, for example, snow and wind. This also means that the ground into, or upon which the foundations are laid, or set, must also provide a good foundation.
Is your supporting ground sand, clay, gravel, rock, river, or seabed? Might it be subject to movement from shrinkage, swelling, freezing or landslip? Is it the site of a former building, industrial use, or land fill? Is the ground on a steeply sloping site, and will there be need for a great deal of excavation and removal of soil to expensive land fill? (See The Risk of Unforeseen Ground Conditions by Brewer Consulting)
Engineered Foundation Design
Whatever the answer to these questions they will affect the type of foundation design appropriate for your building. If your extension will have a relatively lightweight structure, then maybe a simple strip foundation will be acceptable. But alternatively you might be better served with some form of piled or rafted structure, which of designed correctly could cost less.
Frost and Groundwater,
Generally speaking foundations have to be excavated to below the frost layer to avoid movement that can occur during the freezing and thawing process. Depending on the type of foundation to be specified, consideration of natural drainage, the (varying) level of the water table, and even underground running water courses also need to be considered. This is particularly important if you intend to have a full or partial cellar or basement.
Cellars and Basements
In countries with permafrost, or where the ground freezes to a great depth in winter, cellars and basements are logically good uses of the space created by such excavations. In the UK, where a depth of less than a metre may be sufficient to address this hazard, or where mass excavation can be carried out cheaply as part of a larger scheme, creating them can be very expensive. Not only must a basement be properly ‘tanked’ or waterproofed underneath and up all walls below ground level, it may also need special groundwater drainage consideration, possibly including the provision of a sump and pump to clear away rising water.(visit Basements for description and history)
Therefore, although conceptually attractive and attracting increasing attention as a means of getting more ‘living’ or otherwise usable space out of the site, unless land values in the area where your project is located are very expensive, or unless there are special conditions, like the need for a lot of underbuilding on a steeply sloping site, this may prove to be financially unviable. The Basement Information Centre provides promotional information on the provision of cellars and basements.
Modern building regulations also require the ground floor to be well insulated. This applies to the perimeter of the walls as well, meaning that where the ground floor is an integral part of the foundations, as is the case in many projects, the effect this might have on the integrity of the foundations must also be taken into consideration. Cellars and basements have to be insulated in the same way.
All of the foregoing will determine the way that the foundations are engineered and designed. You may also be required to submit evidence that the ground conditions have been properly considered in their design by your local building control authority.
(1). Don’t assume that standard, old style, trench and strip foundations will be adequate. While superficially these might seem least costly, there are an increasing number of cases where poor ground conditions end up requiring very deep excavations which cost a fortune to build up with either concrete or masonry. So make sure you obtain a suitable ground and soil survey.
(2). If you like the idea of having a basement and there are no special reasons for this, like building on a very sloping site, check out the likely costs of excavating for as well as building this before committing yourself to a design where the cellar or basement greatly influences the rest of your plans.