Text 2. Types of foundations

Foundation construction is an integral part of home building. It is what the whole house rests on. A firm foundation is a must, so don't use sand. The simplest and most common form of foundation comprises a strip of concrete under all the load-bearing walls. The depth and width of the concrete strip is determined by the nature of the ground and the load of the building.

Most soils consist of a mixture of solid particles, water and air. In addition, the soil near the surface of the ground will also contain organic material and this top soil must never be used as a base for a foundation. It varies greatly in volume due to changes in water content and is very unstable due to the amount of organic material it contains.

Increasing the pressure on a soil by applying the load of a building squeezes some of the water out of the soil causing it to consolidate and allowing slight settlement of the structure above. In non-cohesive soils such as sands, water movements are rapid and a building will normally complete its settlement during its construction. Cohesive soils such as clay lose their water much more gradually and buildings may slowly settle for many years before equilibrium is reached. The softer clays contain large amounts of water and thus permit extensive settlement.

Concrete has only been common in foundations for just over 100 years. In its simplest form it is a mixture of ordinary Portland cement, stone and water, mixed in varying proportions. Because of concrete's high strength, bricklayers can usually start building on the foundations a few days after it has been poured.

There are three basic types of foundation used in housing and these are shown in the diagram below. Of the three, the strip foundation is by far the most common.

1. 2. 3.

1. Strip Foundation - strip of concrete under all loadbearing walls. The strip width and depth depends on building load and nature of ground.

2. Pile Foundation - long, slender concrete members used to transfer loads through weak or unstable soil to ground of higher loadbearing capacity

3. Raft Foundation - concrete raft which spreads loads over whole ground floor area.

In practice both traditional strip and trench fill foundations become uneconomic if the required depth is more than 1000mm. There is a large volume of excavated material to dispose of and in wet weather the open trenches need constant cleaning out and may well require temporary support to prevent the banks collapsing. A sensible alternative is to use piled foundations.

Piling can take place in freezing or wet weather which can often stop traditional foundation methods. In addition there is less excavated material to dispose of. There are a variety of piling systems suitable for housing.

In granular soils such as compact sands and gravels end bearing piles can be used. Their use in housing is very rare as these soils are generally ideal for strip foundations. However, on blocks of flats they may sometimes offer cost savings over strip foundations. End bearing piles can be formed in a number of ways. Two of the most common are driven pre-formed (pre-cast) piles and driven, cast insitu piles.

Pier foundations are sometimes used as an alternative to piling. If the ground is full of old basements or other obstructions, or if the ground is too steep for piling rigs, pier foundations are sometimes specified. They usually comprise a series of thick concrete piers, formed insitu, which support a reinforced concrete ground beam. Large concrete rings, usually used for drainage inspection chambers, can provide a permanent formwork for the wet concrete. A typical house might be supported by six or eight piers. When constructing pier foundations it is usually necessary to excavate the whole area below the building to the required depth of the piers. When the piers have been poured the ground is replaced.

Дата добавления: 2014-12-23