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Foundations
What Is Foundation And Its Types
Civil Engineering
Foundation is the lowermost structure in any building. It will transfer the load from superstructure to the soil (substructure).
There would have been no need of foundation if the soil is good in
shear. The column would have been sufficient.
But column punches (settles or goes inside soil) because of a small
perimeter.
So, the foundation is provided just to increase perimeter so that load
is distributed over a large area.
In other words, foundation is the lowest part of the building or
the civil structure that is in direct contact with the soil which transfers
loads from the structure to the soil safely.
Types of Foundation
1. Shallow Foundations
2. Deep Foundations
Shallow 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.
These are not strict rules, but merely guidelines: basically, if
surface loading or other surface conditions will affect the bearing capacity of
a foundation it is ‘shallow’.
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.
Shallow Foundation Types:
1. Pad or column footings
(Isolated or Combined)
A - 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
B - 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 nonstructural walls.
5. Mat (Raft) footings (Thickened Slabs)
Consist 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 are used to spread the load from a structure over a
large area, normally the entire area of the structure.
They are used when column loads or other structural loads are close
together and individual pad foundations would interact.
A raft foundation normally consists of a concrete slab which extends
over the entire loaded area. It may be stiffened by ribs or beams incorporated
into the foundation.
Raft foundations have the advantage of reducing differential
settlements as the concrete slab resists differential movements between loading
positions.
They are often needed on soft or loose soils with low bearing capacity
as they can spread the loads over a larger area.
Deep Foundations
Deep foundations are those founded 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 can be used to transfer the loading to a deeper, more
competent strata at depth if unsuitable soils are present near the surface.
Deep foundations are used when there are weak (“bad”) soils near the
surface or when loads are very high, such as very large skyscrapers.
Deep foundations derive their support from deeper soils or bedrock
Common Types of Deep Foundations are:
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.
They are used for economic, constructional or soil condition
considerations, when it is desirable to transmit loads to strata beyond the
practical reach of shallow foundations.
In addition to supporting structures, piles are also used to anchor
structures against uplift forces and to assist structures in resisting lateral
and overturning forces.
Thick slabs used to tie a group of piles together to support and
transmit column loads to the piles.
2. Piers
These are foundations for carrying a heavy structural load which is
constructed in situ 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.
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