Last week I addressed briefly the history of basements in the UK and the advantages of going into the ground https://www.t-spacearchitects.co.uk/news/you-dig-it/. This week I want to touch on the challenges - cost and buildability.
What about the challenges?
Cost typically plays a major role in deciding whether or not a basement is viable. A typical basement extension will cost approximately £3,000 - £4,000 per square meter, however a number of factors can influence the final value including ground conditions, water level, existing services and drainage, waterproofing method, adjacent properties, etc.
How is a basement built and how does it work?
The foundations of a typical house without a basement are located 1.0 - 1.2m below ground level, and they have the role of distributing the weight of the house safely onto the stiff ground layer underneath. Digging a large 3 meter deep basement directly underneath the property will no doubt disrupt this initial condition, therefore a solution is needed to maintain the integrity of the foundations and of the building.
For small houses the solution is typically found in underpinnings which are essentially thick concrete 'walls' built in stages underneath the existing foundations which transfer the weight of the house from the initial level of 1.0-1.2m below ground, to that of the proposed basement, say -3.0m. Having transferred the weight of the existing walls to a lower level, additional reinforced concrete walls are typically needed to form the perimeter of the basement and provide a retaining structure to work against lateral earth and water pressures.
The reinforced concrete walls and basement slab will form the shell of the new basement which is then finished internally with thermal insulation, waterproofing, plaster, etc. depending on the design.
Other important considerations are temporary works, which are needed to prop against the inward 'push' of the ground while the permanent concrete walls are being built; access requirements for machinery and removing the excavated spoil.
Each project will be tailor-made to take into account all of the above, including the factors that might influence or have a significant impact on the new construction.
Groundwater level plays an important role in choosing the basement waterproofing method as a high water table requires an enhanced and costly solution. There are three forms of construction that protect against the ingress of water, and for habitable areas, typically a combination of the three are used together depending on design.
Type A - Barrier protection - Typically comprises of membranes or waterproof renders and are installed typically on the structural element to provide a single barrier against the entry of water
Type B - Integral protection - Where the main structure also plays a role of waterproofing - an example being waterproof type concrete
Type C - Drained protection - The most effective waterproofing method that collects any moisture that find its way through the walls, collects it in the interior cavity and evacuates it typically with the help of a pump installed underneath the basement floor.
As I mentioned last week basement works might seem like a daunting task - aside from the many advantages it can provide, each new project certainly has its technical challenges whether at planning, design or construction stage - however we at T-Space are always happy to assist in your decision making, offer technical guidance along the way and give you an idea about the possibilities and feasibility of your new basement extension.
Szilard Biro is in-house Structural Engineer at T-Space.
Link to the project featured.