CHAPTER 1 SUBSTRUCTURE-LECTURE 3 Shallow Foundation

CHAPTER 1: SUB-STRUCTURE 

LECTURE 1 - SOIL INVESTIGATIONS

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LECTURE 2 - CHOICE OF FOUNDATION IN RELATION TO SOIL CONDITION

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LECTURE 3 - SHALLOW FOUNDATIONS: PAD, STRIP & RAFT

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LECTURE 4 - BASIC UNDERSTANDING OF PILING DEEP FOUNDATIONS

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LECTURE 5 - BASEMENT CONSTRUCTION & WATERPROOFING

LECTURE 3 SHALLOW FOUNDATIONS

 SHALLOW FOUNDATIONS 

A Foundation System Which Transmit Load Of Structure To Ground At The Nearest Point To The Ground Floor.

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Can Be Classifieds Into 3 Different Types : 1. Strip Foundation 2. Pad Foundation 3. Raft Foundation

SHALLOW FOUNDATIONS

 STRIP FOUNDATION

SHALLOW FOUNDATIONS

 STRIP FOUNDATION 

Consists of a continuous strip, usually of concrete, under load bearing walls.

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The continuous strip serves as a level base on which the wall is built and is of such width that it is necessary to spread the load of foundation to an area of subsoil.

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The width of concrete foundation depends on the bearing capacity of the subsoil and load of the foundation

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Thickness of the concrete should be not less than the projection of the strip on each sides of the wall.

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The practical depth minimum of the strip foundation is usually 450 mm to allow for the removal of top soil and variations in ground level.

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Tensile reinforcement is required in the lower face of the strip with distribution bars in the second layer running longitudinally.

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The reinforcement will also assist the strip in spanning any weak pockets of soil encountered in the excavations.

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Also suitable for sloping sites.

SHALLOW FOUNDATIONS

 FOUNDATIONS ON SLOPING SITE

SHALLOW FOUNDATIONS

 PAD FOUNDATIONS

SHALLOW FOUNDATIONS

 PAD FOUNDATIONS

SHALLOW FOUNDATIONS

 PAD FOUNDATIONS 

Used to support and transmit load from piers and columns.

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More economical than excavating deep trenches for strip foundation.

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Most economical is square shape but if columns are close to each other, it may be necessary to use a rectangular shape.

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Reaction of the foundation to the load and ground pressure is to cup therefore main steel bars are required in both directions.

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Spacing of piers or columns is determined by the most economical construction.

SHALLOW FOUNDATIONS

 PAD FOUNDATIONS

SHALLOW FOUNDATIONS

PAD FOUNDATIONS

SHALLOW FOUNDATIONS

 RAFT FOUNDATIONS 

Principle of this foundation is to spread the load over entire area of site

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Consist of a raft of reinforced concrete under the whole building.

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Used for buildings on compressible grounds such as soft clay, alluvial deposits and compressible fill materials where strip foundation would not provide a stable foundation.

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Useful where column loads are heavy.

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Basically divided into 3 (three) types:

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Solid slab raft

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Beam and slab raft

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Cellular raft

Capable of accommodating small settlements in soil.

RAFT FOUNDATIONS

 SOLID SLAB RAFT FOUNDATIONS 

Constructed of uniform thickness over the whole raft area.

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Slab is reinforced at top and bottom.

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Where ground has poor compressibility, a wide toe raft foundation is recommended.

RAFT FOUNDATIONS

 SOLID SLAB RAFT FOUNDATIONS

RAFT FOUNDATIONS

 BEAM AND SLAB RAFT FOUNDATIONS 

An alternative to solid slab.

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Beams are used to distribute the column loads over a raft area which results in the reduction of slab thickness.

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Beams can be up stand or down stand depending upon the bearing capacity of the subsoil near the surface.

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Down stand beams will give a saving on excavation.

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Up stand beam creates a usable void below the ground floor if a suspended slab is used.

RAFT FOUNDATIONS

 BEAM AND SLAB RAFT FOUNDATIONS

RAFT FOUNDATIONS

 CELLULAR SLAB RAFT FOUNDATIONS  used when reasonable bearing capacity subsoil can be found at depths where beam and slab techniques become un economic.  construction is similar to reinforced concrete basements, except that the internal walls are used to spread the loads over the raft and voids into cells.  Openings can be introduced in the cell wall allowing the voids to be utilised for housing of services, store rooms or general accommodation.

SHALLOW FOUNDATIONS

 BLINDINGS 

A layer of 50mm to 75 mm of weak concrete or coarse sand should be placed under all the reinforce concrete foundations.

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Function is to fill any weak pockets encountered during excavation and provide true level surface where reinforcement can be positioned.

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Either lay blinding before assembling formwork or place within formwork and allow to set before positioning reinforcement and placing concrete.

SHALLOW FOUNDATIONS

 SETTLEMENT OF FOUNDATIONS 

Vertical downward movement of a base of a structure

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Effect depends on the :

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magnitude

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uniformity

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length of time over which it takes place

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nature of structure

Settlement may be caused by: 

imposed weight of structure on soil

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changes in moisture content

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subsidence due to mining

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general earth movement

SETTLEMENT OF FOUNDATIONS

RELATIVE/DIFFERENTIAL / UNEVEN SETTLEMENT 

Could cause distortion or could lead to structural failure

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Occurs on clay soil

UNIFORM /EVEN SETTLEMENT 

Minimal settlement, not excessive

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Occurs on sandy soil

CONTACT PRESSURES 

Definition – pressure distribution from the foundation to the soil. Pressure intensities depend upon the rigidity of the footing, type of soil and condition of soil.

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Knowledge of distribution of contact pressures between the foundation and the soil would assists in determining the most suitable arrangement of foundation to avoid relative settlement

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Building foundation on a non-cohesive soil (sand) should be designed so that the heavily loaded columns be towards the edge of the foundation (contact pressure is least) and the lightly loaded columns towards the center to allow uniformity of settlement

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For foundation on a cohesive soil (clay),the heavily loaded columns should towards the center where contact pressure is least.

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Building foundation on a noncohesive soil (sand) should be designed so that the heavily loaded columns be towards the edge of the foundation (contact pressure is least) and the lightly loaded columns towards the center to allow uniformity of settlement.

END OF LECTURE THANK YOU