Engineering Geology: Footings and Foundation

Topics: Concrete, Deep foundation, Geotechnical engineering Pages: 11 (3398 words) Published: March 13, 2013
FOUNDATION is the lowest supporting layer of a structure.
* Purpose: Transfer the building load to the soil.
* The size of the foundation is determined by the weight of the building and the load bearing capabilities of the soil. * When the load bearing capabilities of the soil is low, a footing is usually used. Types of Foundation

1. Shallows Foundations
* Shallow foundations are those founded near to the finished ground surface; generally where the founding depth (Df) is less than the width of the footing and less than 3m. * Shallows foundations are used when surface soils are sufficiently strong and stiff to support the imposed loads; they are generally unsuitable in weak or highly compressible soils, such as poorly-compacted fill, peat, recent lacustrine and alluvial deposits, etc. 2. Deep Foundations

* Deep foundations are those founding too deeply below the finished ground surface for their base bearing capacity to be affected by surface conditions, this is usually at depths >3 m below finished ground level. * Deep foundations are used when there are weak (“bad”) soils near the surface or when loads are very high, such as very large skyscrapers.

Shallow Foundation Types
1.Pad or column footings (Isolated or Combined)
Isolated - used to support single columns.  This is one of the most economical types of footings and is used when columns are spaced at relatively long distances. Combined - usually supports two columns, or three columns not in a row.  Combined footings are used when two columns are so close that single footings cannot be used or when one column is located at or near a property. 2. Cantilever or strap footings-consist of two single footings connected with a beam or a strap and support two single columns.  This type replaces a combined footing and is more economical.  3. Continuous footings - support a row of three or more columns.  They have limited width and continue under all columns. 4. Wall Footings - are used to support structural walls that carry loads for other floors or to support non-structural walls.    5. Mat (Raft) footings (Thickened slabs)

* consists of one footing usually placed under the entire building area.  They are used, when soil bearing capacity is low, column loads are heavy single footings cannot be used, piles are not used and differential settlement must be reduced.    * Raft foundations have the advantage of reducing differential settlements as the concrete slab resists differential movements between loading positions. Deep Foundation Types:

1. Pile foundations - are relatively long, slender members that transmit foundation loads through soil strata of low bearing capacity to deeper soil or rock strata having a high bearing capacity. 2. Piers - are foundations for carrying a heavy structural load which is constructed in a deep excavation. 3. Caissons - are a form of deep foundation which are constructed above ground level, then sunk to the required level by excavating or dredging material from within the caisson. 4. Compensated foundations - are deep foundations in which the relief of stress due to excavation is approximately balanced by the applied stress due to the foundation. The net stress applied is therefore very small. A compensated foundation normally comprises a deep basement. Types of Loads:

1. Dead Load
2. Live Load
3. Wind Load
4. Horizontal Pressures Below Grade
5. Forces (Uplift)
6. Earthquake
Push Forces
* The primary pushing force is frost heave.
* Can be a serious problem
* Some building codes allow footings above the frost line, but they must be insulated according to building code. Foundation Systems
* Four main types of foundation systems.
* Crawl space
* Full basement
* Slab at grade
* Elevated slab
Foundation Requirements
Three primary foundation requirements:
* Safe against Structural Failure
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