A foundation is that part of a building which is in direct contact with the ground and its primary aim is to transmit and spread the loads (dead and live) from the building over a sufficient area of soil to avoid undue settlement because of the failure of the underlying soil. A good foundation should satisfy the following requirements: a) It shall safely sustain and transmit to the ground the loads (dead, live and wind) in such a way as not to cause any settlement or other movement which would impair stability or cause damage to the whole or any part of the buildings or both.
b) It shall be taken down to such a depth or be so constructed as safeguard the building from swelling, shrinking of freezing of the subsoil.
c) The materials used must be capable of adequately resisting any attack by sulphates of any other deleterious matter and chemicals present in the subsoil.
FACTORS AFFECTING THE CHOICE OF FOUNDTION
The following are the factors affecting the choice of foundation: ^ Soil type
^ Site condition (topography/slope, water content/flood plain) ^ Type of construction
^ Magnitude of the load and intensity
^ Economic and other constructionak considerations
^ Type of building
^ The possible effect of weather element e.g. rain.
TYPES OF FOUNDATION
The major types of foundation are:
^ STRIP FOUNDATION: This is the commonest type of foundation used or adopted in building one or two storey building and at most four storeys on firm non-shrinkable sub-soils such as gravel or laterite. It consists of continuous strip of concrete either in ratio 1:3:6 or 1:2:4.
can be normal, Deep or Wide.
~NORMAL STRIP FOUNDATION: is when the width of the foundation footing is not more than three times the wall thickness
~DEEP STRIP FOUNDATION: is when the depth of the foundation is up to 900mm. It is used especially in shrinkable clayey soil to counteract the variations of soil conditions at different seasons.
~WIDE STRIP FOUNDATION: is used to spread the load over a larger area of soil; it is used in soil with low bearing capasity such as clayey, marshy ground, silt and build-up soils. The concrete is usually reinforced in both the transverse and longitudinal direction.
^ PAD FOUNDATION
These are isolated foundations constructed of reinforced concrete to support columns and mostly used in framed building construction. They can be used to carry point loads or can be designed so that the loads of the walls and the buildings are transferred through ground beams that rest on pad foundations. The pad foundation transfers the loads to a lower level where soil of sufficient load bearing strata exists. Pad foundation can be inform of isolated column foundation, continuous column foundation or combined column foundation.
^ RAFT FOUNDATION
A raft foundation is one which covers the entire area of the buildings may extend beyond it. It consits of concrete that is suitably reinforced and up to 300mm thick. It is best suited for use on soft natural ground, soft clay, alluvial deposits. Compressible fill or on ground that is liable to subsidence as in mining areas. Their advantage over strip foundation is in the ability to act as a single unit thus eliminating differential settlement.
Major forms of raft foundation
SOLID RAFT FOUNDATION: This consists primarily of reainforced concrete slab up to 300mm thick which is often thickend under load bearing walls.
BEAM AND SLAB RAFT FOUNDATION: This is like inverted reinforced concrete floor with upstand beams between the bases of columns. The beams collected the beams and slab spreading the loads over the whole area. However, if the surface of the slab is to serve as a floor, then the raft with downstand beams is used.
CELLULAR RAFT FOUNDATION: This is mostly used in basement
construction and can be constructed either as bouyancy, basement or box raft foundation.
This is frequently used where the subsoil is poor or uncertain bearing capacity of where there is likely to be appreciable ground movement as with firm or where it is necessary to transmit the building load through a weak and unstable soil conditions to a lower stratum of sufficient bearing capacity. The choice of the type to be used depends on the soil and advice of foundation experts while the distribution of the piles is influencfd by the design of the building, the loads to be carried and the load bearing capacity or the strenght of the piles.
Pile may be constructed of reinforced concrete, timber, steel or combination of any two.