Foundation rafts can often provide an economic solution when there are difficult ground conditions on the site. For example if there is made ground on the site, or the ground has a low bearing capacity. They can also sometimes be used when there are shallow mine workings beneath the site (depending on the depth and thickness of the workings).
A foundation raft is designed to spread the weight of the building over as large an area as possible area to reduce the bearing pressures on the soil. The raft is designed so that any minor differential settlements do not cause any significant cracking in the building, which would otherwise be the case.
The raft consists of a reinforced concrete slab with integral stiffening beams beneath the loadbearing walls.
Rafts are not normally an economic solution to reduce the foundation depth in shrinkable clay soils, because to comply with current regulations this normally results in an unacceptably thick layer of stone sub base being required beneath the raft.
We need the following information to design a raft foundation:
architectural layouts or sketch drawings with dimensions of the extension:
details of the existing house foundations (if possible), particularly if the existing house has piled foundations
details of the ground conditions, your local Building Inspector will be able to give you advice on this
details of any nearby drains or trees
All designs are carried out in accordance with BS 8110 (The structural use of concrete) and BS 8004:1986 (The code of practice for foundations).
The calculations provided will be suitable for Building Regulation approval.
You should satisfy yourself that the excavation for the foundations will not undermine any adjacent foundations or neighbouring ground
Excavations may fall under the Party Wall Act Please seek specialist advice.
click for party wall act information