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Types of Foundation

Major Building PartsMajor Building Parts

SuperstructureSuperstructure

SubstructureSubstructure

FoundationFoundation

FootingFooting

Definition

Footings are structural members used to support columns and walls

and to transmit and distribute their loads to the soil in such a way

that the load bearing capacity of the soil is not exceeded, excessive

settlement, differential settlement,or rotation are prevented and

adequate safety against overturning or sliding is maintained.

Depth and location of foundation depends on 1. Zone of significant volume changes in soil.

2. Adjacent structures and property lines.

3. Ground water

4. Underground defects

Depth and location of foundationDepth and location of foundation

Zone of significant volume changes in soil :

Clays having high plasticity shrink and swell considerably

up on drying and wetting respectively.Volume change is

greatest near ground. Decreases with increasing depth.

Volume changes usually insignificant below a depth from

1.5-3.0 m and does not occur below.

Depth and location of foundationDepth and location of foundation

Adjacent structures and property lines.

Depth and location of foundationDepth and location of foundation

Structures may be damaged by the construction of new foundations, as a result of vibrations, undermining by excavation or lowering of the water table. After new foundations have been constructed, the (new) loads they place on the soil may cause settlement of previously existing structures as a result of new stress pattern in the surrounding soil.

In general, deeper the foundations and closer to the old structure, greater will be the potential for damage to old structures.

450 Existing Footing

New Footing

Limit for bottom of deeper Footing

Part extending

property line Property line

Primary Factors Affecting Foundation Choice

Subsurface soilSubsurface soil

Ground water conditionsGround water conditions

Structural requirementsStructural requirements

Secondary Factors Affecting Foundation Choice

Construction access, methods & site conditionsConstruction access, methods & site conditions

Environmental factorsEnvironmental factors

Building Codes & RegulationsBuilding Codes & Regulations

Impact on surrounding structuresImpact on surrounding structures

Construction scheduleConstruction schedule

Construction risksConstruction risks

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Types of Foundation

Types of Foundation Shallows Foundations Deep FoundationsShallows Foundations

Shallow foundations are generally where the founding depth (Df) is less than the width of the footing and less than 3m. Shallow 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.

FootingFooting

Definition

Footings are structural members used to support columns and walls and to transmit and distribute their loads to the soil in such a way that the load bearing capacity of the soil is not exceeded, excessive settlement, differential settlement,or rotation are prevented and adequate safety against overturning or sliding is maintained.

Types of FootingTypes of Footing

Wall footings are used to support structural walls that carry loads for other floors or to support nonstructural walls.

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Types of Footings

Pad or column footings (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

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Combined FootingUsually support two columns, or three columns not in a row.

Combined footings are used when tow columns are so close that single footings cannot be used or when one column is located at or near a property

Types of Footings

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 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.

Types of Footings

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Types of footings

 Continuous footings

 Support a row of three or more columns. They have limited

width and continue under all columns.  

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Types of footings

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 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.

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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.

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Mat (Raft) footings (Thickened slabs)

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Types of Foundation

Deep Foundations

Deep foundations are those with 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.

They are used when there are weak (“bad”) soils near the surface or when loads are very high, such as very large skyscrapers. They derive their support from deeper soils or bedrock

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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.

Common Types of Deep Foundations

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Types of Foundation

Pile caps are thick slabs used to tie a group of piles together to support and transmit column loads to the piles.

Typical pile foundation

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PiersAre foundations for carrying a heavy structural load which is

constructed in-situ in a deep excavation.

CaissonsAre 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.

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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.

Distribution of Soil PressureDistribution of Soil Pressure

When the column load P is applied on the centroid of the footing, a uniform pressure is assumed to develop on the soil surface below the footing area.

However the actual distribution of the soil is not uniform, but depends on may factors especially the composition of the soil and degree of flexibility of the footing.

Distribution of Soil Pressure

Soil pressure distribution in cohesionless soil. Soil pressure distribution in cohesive soil.

Design ConsiderationsDesign Considerations

Footings must be designed to carry the column loads and transmit them to the soil safely while satisfying code limitations.

Bearing capacity of columns at their base

Dowel requirements

Development length of bars

Differential settlement

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4.

Questions

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1.Different types of foundations (Advantages and disadvantages)2.Factor affecting the choice of foundation type3.What are the factors governing the depth of foundation4.Differntiate between deep and shallow foundations