Notes to accompany 1014 “Foundations”Slides 1-3:

Buildings are heavy. If they move they crack. They are supported by the ground so it is essential that the load on the ground is spread out widely enough to distribute the weight of the building over sufficient area so that the ground can support it. It will move a bit, but so long as that movement is imperceptible, it is acceptable.

Slides 4-7:

Never build on topsoil. Never build on peat. They are organic and will change volume as organic matter decays. You can build on just about any other type of subsoil, but the type of foundation will have to be chosen and designed to suit the subsoil you discover underground. In extreme cases, you may have to take the weight of the building right down through all of the subsoil to bedrock underneath. On some city centre skyscrapers, the foundations can be over 30m deep. Be wary of clay. It can contain corrosive chemicals and it can change volume as moisture content varies through the year. The forces exerted by swelling clay can easily crush a building.

Slide 8-9

Study the textbooks to learn the terminology. Make sure you know what piles, ground beams, footings, retaining walls, trench fill, shallow strip, deep strip etc. means. Sometimes different terms are used for the same thing by different builders. Sometimes very similar terms mean very different things. If you have any doubt on a building site what is is being talked about, ask. It is better to seem ignorant than to make a stupid, avoidable mistake.

Slides 10-20

Wide and narrow strip foundations are the most widely used for domestic construction. Make sure you understand what the differences are and which is most appropriate. They are only sensible if the foundations are less than 2m deep. If you have to go deeper, use piles. If you are at all uncertain about sub soil conditions it may well be better to opt for piles in any case. Some builders use them all the time just to be certain they will not encounter deep excavation problems.

Slide 21-25

Wide raft foundations can be used on subsoils which are weak but stable. They are uncommon in domestic construction but are used widely for single storey commercial buildings like shops and warehouses. The strength of the reinforced concrete floor is also useful in these situations, but of little benefit in a house.

Slide 26-31

The overriding point about basement construction is that you must involve a structural engineer in their design. The walls must resist ground pressure pushing them in. The basement floor must resist the buoyancy up force of wet ground. You are in essence building a sealed boat underground and it will try to float up if the water table rises. The walls must be water proof. This is most difficult to achieve at the angle between the basement walls and floors. Ideally any waterproof layer should be on the outside of the walls and floor, so that water pressure pushes it tighter onto the wall/floor. If it is applied on the inside, which is easier, water pressure will keep trying to push it off the wall.

Slides 32 - 38

Piled foundations are simply columns in the ground, which support a beam at ground level, on which the building is constructed. It is best to construct the ground beam on a crushable layer, usually foam plastic a some type, so that there is no risk of it being pushed upwards by swelling clay subsoils. The ground beam needs to be steel reinforced so that it can resist bending forces.

Slides 39 – 43

Piles can work by either transferring building loads to the sides of the pile (friction piles), which rely on friction between the pile and the ground to support the building; or they can transfer the load right through the subsoil to their very lowest ends (end bearing piles), which avoids putting any load into potentially unstable upper layers of the subsoil.

Piles can be replacement (or bored) piles, where a hole is drilled and then reinforced concrete inserted; or driven, where a solid prefabricated pile is literally hammered into the ground until it won’t go in any more. Timber is no longer used for driven piles, they are all either steel or precast reinforced concrete.

Slide 44

To understand foundations, you really need to know all about concrete…