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  • FOUNDATION TYPES*

  • Shallow foundation (non-piled foundation)*

  • a foundation where the transfer of load to the soil is at a level close to the lowest floor of a building eg. strip, pad and raft footings

    should always be made as a priority due to the economic reason unless it is impractical to practice then only can deep foundation be employed

    *

  • Deep foundation

    (piled foundation)

    *

  • a foundation where the transfer of load to the soil is at a level much below the lowest floor of a building

    eg. piles and various types of pier foundation*

  • use when the ground conditions are incapable of supporting structural loads (impractical to use shallow foundation at an economic depth)

    *

  • Shallow foundation

    also called spread foundations

    may be divided into:: strip footing: pad footing: raft footing*

  • STRIP FOOTING

    *

  • a continuous strip of concrete under a continuous wall carrying a uniformly distributed load

    provides a continuous longitudinal bearing (continuous ground bearing under the load bearing walls)

    placed centrally under the walls*

  • also a continuous strip of concrete under several columns

    designed to redistribute bearing-stress concentrations and associated settlements in the event of variable bearing conditions or localized ground loss beneath footings*

  • the required area (length x width) at which the strip footing rests is related to the imposed load and the bearing capacity of the soil sketch

    in cases of light-loading building, resting on a reasonably strong soils, strip footing not wider than the wall it carries may suffice

    *

  • however, footing spread is usually provided

    footing spread (150mm on each side) is usually provided :

    i.to facilitate bricklayer a working space for building the lower courses of walls

    *

  • to provide stability to the load-bearing walls before it is tied-in by floors and roof

    iii.to provide working space for the hand excavation operation in deeper trench excavation*

  • when the edges of a foundation project beyond the faces of the wall it supports, bending due to cantilever action will occur as a result of the resistance of the soil and will cause bending and shear to the foundation

    *

  • to overcome the above: the angle of spread of loads from the wall base to the outer edge of the foundation must be not more than 45

    *

  • with the above understanding, this will lead to a thicker concrete strip (under a given load) for a safe bearing capacity, and any additional load to the design on the same soil, will lead to an increase in the thickness of the strip uneconomic

    *

  • to overcome the above, use strip footing with the following criteria:: thinner: wider: reinforced*

  • Wide Strip Footing reinforced concrete

    *

  • use in a situation where the load bearing capacity of the soil is low (eg. marshy ground, soft clay silt, made ground)

    site which may be subjected to unequal settlement (mining areas), or where the subsoil is not uniform in character

    *

  • Inverted T Beam Strip Footing

    reinforced concrete

    *

  • use in a situation where there is a danger of the foundation failing as a beam in the longitudinal direction*

  • Deep Strip Footing plain / mass concrete

    *

  • introduced to reduce the expenses involved in constructing orthodox strip footing to depths of more than 900mm deep in shrinkable clay soils

    *

  • firm or stiff shrinkable clays are strong and when carrying light loads necessitate quite small foundations, possibly no wider than the wall carried

    *

  • in a deep, narrow trench

    :excavation by hand is not possible:brickwork cannot be built up from the bottom :excavation by mechanical means is possible*

  • if the trench is filled with concrete up to the ground surface level

    :the difficulties of bricklaying are overcome:less soil to be excavated:backfilling is eliminated

    *

  • the deeper foundation provides greater resistance to fracture from unequal settlement

    *

  • `it is quicker to construct and it is quicker to complete as compared to the conventional strip footing, because:avoid planking and strutting (timbering) in a self-supporting soil:economic for large projects with long runs of straight trenching*

  • Stepped Strip Footing

    *

  • all foundations should bear horizontally on the soil

    if strip footing to a building on a sloping site are at the same level through out, theni.those part of the building on the higher side will be a greater distance below ground level

    *

  • ii. require deeper trenches with a greater amount of walling in the soil

    there are two ways in which this excessive building into the soil may be reduced

    *

  • i.by cut-and-fill or cut so as to provide a horizontal plane for which to build

    ii.by stepping down the slope so as to make the foundations parallel to the slope or ground level stepped footing

    *

  • the steps should be kept..

    :relatively short in length:sufficient in number along the length:uniform and in small heights..or otherwise may cause differential settlement

    *

  • most economic procedure, as this will reduce the amount of:trench excavation:backfilling:surplus soil removal:earthwork support

    *

  • RAFT FOOTING

    *

  • a large reinforced insitu concrete slab foundation covering the whole building area, and is designed to transmit all the loads from the building and distributed the load over the whole area under the raft

    *

  • best suited in the following conditions:

    i.column loads are extremely heavy, thus requiring large bases

    *

  • ii.columns are closely spaced in both directions and if pad footings are used, it would overlap and may almost cover the whole building area

    *

  • iii.ground bearing capacity is low, such as in: :compressible soft natural ground :compressible filled ground :ground that is liable to subsidence as in mining areas*

  • the raft should be extended (@ 300mm) beyond the external walls to spread the load

    the ground at the edge of the raft should be protected from deterioration by the weather and possible slipping away of soil by means of:

    *

  • i.incorporating concrete paving around the building

    ii.deepening the edge beam

    *

  • Solid Slab Raft Footing

    *

  • constructed of slab with uniform thickness through out

    slab being reinforced to counter the effect of load from the columns and the ground pressure *

  • Beam and Slab Raft Footing

    *

  • Used where poor soils are encountered

    constructed of beams and slab, the beams are used to distribute the column loads over the area of the raft, which results in the reduction of the slab thickness

    *

  • the beams can be upstand or downstand depending upon the bearing capacity of the soil near the surface

    using downstand beams will give a saving on excavation costs, whereas upstand beams will create a usable void below the ground floor if a suspended slab is used

    *

  • Cellular Raft Footing

    *

  • best used where a reasonable bearing capacity subsoil can only be found at depths where beam and slab techniques become uneconomic

    *

  • similar with basement construction except the internal walls are used to spread the load over the raft, and with walls form and divide into cells, openings can be incorporated for the housing of services

    *

  • the divided voids formed into cells and by incorporating openings within the cell walls will allows the voids to be utilised for the housing of services or store rooms *

  • PAD FOOTING

    *

  • most economical shallow foundation types but are more susceptible to differential settlement

    usually support single concentrated loads, such as those imposed by columns

    *

  • isolated or independent slab of concrete foundation to support concrete columns or steel pillars, detached brick or masonry piers

    the pier or column bearing on the centre point of the slab

    *

  • the thickness is govern by the same consideration as for strip footing and is made not less than the projection of the slab beyond the face of the column or pier, or the edge of the base plate of a steel stanchion

    *

  • in whatever circumstances the thickness should be less than 150mm thick, and should the base become very wide and thicker, the reduction in thickness can be effected by introducing reinforcement to the slab

    *

  • variation to pad footings:

    i.combined footings used where the bearing areas of closely spaced columns overlapped

    ii.cantilever footings designed to accommodate eccentric loads *

    *