Need for deep foundations

Текст C

DEEP FOUNDATIONS

 

In most cases deep foundations are utilized only where it is not possible to have shallow foundations. The decision to use deep foundations, the selection of the type of system, and the design of the elements of the system should all be done by building designers with considerable knowledge and experience in foundation design, including knowledge of the types of systems, their capabilities and drawbacks, and some of the problems connected with the use of such systems for building foundations.

The most common reasons for the use of deep foundations are the following:

1. Lack of Adequate Soil Conditions for Bearing Footings. There are a number of soil conditions that may make the usual bearing foundation elements near the bottom of the building not possible. The deep foundation thus is used essentially in order to reach a desirable bearing level at some distance from the bottom of the building.

2. Heavy Loads on the Foundations. In some cases the soil at upper levels may be sufficient for the use of bearing elements for relatively light loads, but the size of footings required, the need for limited settlement, or other factors occurring with extremely heavy loads, may require a deep foundation. High-rise buildings, long-span structures, and construction of massive elements of concrete or masonry are cases in which loads may become considerable and may be above the simple bearing capabilities of ordinary soils.

3. Instability of Ground-Level Soil. Use of deep foundations may be necessary where soil at the level of the bottom of the building is subject to some forms of change in the soil structure or the general state of the soil mass. This situation sometimes occurs at waterfront and hillside locations. In these cases the use of deep foundations is necessary in order to anchor the building to a more reliable, stable ground mass.

4. Support of Structures Sensitive to Settlement. In some situations, settlement of foundations is highly critical. Examples are buildings with stiff rigid frame structures and buildings housing equipment that requires precise and continuous alignment. Bearing foundations will settle on almost any soil other than solid bedrock. Deep foundations tend to have little settlement, especially when the elements are bearing on rock or on a highly consolidated soil stratum.

Deep foundations are usually used only in situations in which shallow footings are not possible. The primary reason is cost. Where ordinary footings can be used and their size is not excessive, the cost of deep foundations will rarely be competitive. If footings are possible but their size is excessive, it may be necessary to perform a cost analysis of alternatives. Such an analysis will usually require relatively complete designs of the alternate systems and can be difficult when the alternate foundation systems require significant differences in the building structure.

Foundation design and construction practices are often influenced by the location of a building. This may be partly because of the local climate and soil conditions. Where there is some history of construction in the area, however, it may also be due to local experience with particular foundation systems or construction techniques.

Most deep foundation elements, piles or piers, are constructed by special foundation contractors.

 

Notes:

alignment n. выравнивание

consolidate v. укреплять

history n. история

sensitive adj. восприимчивый

waterfront n. береговая линия