Text 1. Foundations in construction

Find out the differences between shallow and deep foundations, total settlement and differential settlement and some other essential details concerning foundations.

Every building needs permanent stability. In order to have stability, buildings should have foundations. A foundation is a structure that transfers loads to the ground. Foundations are generally broken into two categories: shallow foundations and deep foundations.

Shallow foundations are usually embedded a meter or so into soil. One common type is the spread footing. It consists of strips or pads of concrete (or other materials) which extend below the frost line and transfer the weight from walls and columns to the soil or bedrock. Another common type is the slab-on-grade foundation where the weight of the building is transferred to the soil through a concrete slab placed at the surface.

Deep foundations are used to transfer a load from a structure through an upper weak layer of soil to a stronger deeper layer of soil. There are different types of deep foundations including piles, drilled shafts, caissons, piers, and earth stabilized columns. Historically, piles were wood, later steel, reinforced concrete, and pre-tensioned concrete. Sometimes these foundations penetrate bedrock.

The primary design concerns are settlement and bearing capacity. When considering settlement, total settlement and differential settlement is normally considered. Differential settlement is when one part of a foundation settles more than another part. This can cause problems to the structure the foundation is supporting. It is necessary that a foundation should not be loaded beyond its bearing capacity or the foundation will "fail". For a typical modern three-bedroom detached house the total of the dead and imposed loads is about 120 tonnes and most types of ground can easily carry this load using simple foundations. The exact dead and imposed loads can be easily calculated.

Other design considerations include scour and frost heave. Scour takes place when flowing water removes supporting soil from around a foundation (like a pier supporting a bridge over a river). Frost heave occurs when water in the ground freezes to form ice lenses.

Changes in soil moisture can cause expansive clay to swell and shrink. This swelling can vary across the footing due to seasonal changes or the effects of vegetation removing moisture. The variation in swell can cause the soil to distort, cracking the structure over it. This is a particular problem for house footings in semi-arid climates where wet winters are followed by hot dry summers.

When structures are built in areas of permafrost, special consideration must be given to the thermal effect the structure will have on the permafrost. Generally, the structure is designed in a way that tries to prevent the permafrost from melting.

Дата добавления: 2014-12-23