There are four types of foundation: Deep Strip, Raft, Trenchfill and Piled. Wherever possible, establish ground conditions by means of a survey in order to pre-determine the foundation method best for your site.
Deep Strip Foundations
Deep Strip foundations are the least expensive and are used when ground conditions are good. A concrete strip, sometimes reinforced with steel mesh, supports the walls. The trench depth is variable but in most cases should be at least 1m with a width of 600mm. The concrete should have a minimum depth of 225mm.
Trenchfill foundations fill the excavation, almost to the top, with concrete. Although more expensive than a Deep Strip foundation this method gets you out of the ground quickly. The Trenchfill foundation is used when soil is loose or in areas with a high water table. In areas with heavy clay and in the presence of trees, Trenchfill foundations may be taken deeper, to a level where the moisture content of the subsoil is unaffected. It may also require the provision of mesh reinforcement and the trench sides might need to be lined on one or both sides with a compressible material an /or a slip membrane.
Raft foundations are used where the ground is inherently stable but where conditions deep below the surface, such as mining, might lead to ground movement. The reinforced raft is cast on top of consolidated hardcore and is shaped at the edge to provide a step upon which both leafs of the wall are constructed.
Piled foundations are used where good bearing ground can only be found at deeper levels or where physical restraints make other forms of foundation impossible. A Piled foundation is usually carried out by specialist contractors. The piles can be dug, bored or driven into the ground. They support a concrete ring beam or ground beam that spans from pile to pile, upon which the house is built. This ring beam can be prefabricated, but if it is cast on site, it may be necessary to have reinforcement cages ready-made up to an engineer's specification. In certain situations, it may also be necessary to have compressible materials on hand to line the underside or sides of the beam.
- Concrete burns the skin so always wear protective gloves and footwear.
- Ensure the base of any foundation trench is clean, level, dry and free of any loose material. Trench sides should be cut square with the base. The ideal situation is to excavate the trenches and pour the concrete the same day. Special mixes that flow around whole channel are available, which do away with barrowing.
- Foundations poured in winter conditions must be done following strict guidelines and should be covered over.
- It is best to consult a weather forecast for possible frost conditions because overnight frost can ruin concrete.
- If you decide to use a concrete pump to place concrete, give the suppliers plenty of warning of your intention.
- Locate your nearest hire depot for urgent requirements such a water pump in the event of flooding.
Overcrowding on Site
Bear in mind that during the build, a number of items may be needed at short notice such as scaffolding, diggers, dumpers or cranes, so ensure there is adequate space for them. It is also a good idea, if you have room, to position a skip on site for the gathering and regular removal of rubbish as the build progresses.