CHAPTER 1: SUB-STRUCTURE
LECTURE 1 - SOIL INVESTIGATIONS
LECTURE 2 - CHOICE OF FOUNDATION IN RELATION TO SOIL CONDITION
LECTURE 3 - SHALLOW FOUNDATIONS: PAD, STRIP & RAFT
LECTURE 4 - BASIC UNDERSTANDING OF PILING DEEP FOUNDATIONS
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.
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.
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.
The width of concrete foundation depends on the bearing capacity of the subsoil and load of the foundation
Thickness of the concrete should be not less than the projection of the strip on each sides of the wall.
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.
Tensile reinforcement is required in the lower face of the strip with distribution bars in the second layer running longitudinally.
The reinforcement will also assist the strip in spanning any weak pockets of soil encountered in the excavations.
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.
More economical than excavating deep trenches for strip foundation.
Most economical is square shape but if columns are close to each other, it may be necessary to use a rectangular shape.
Reaction of the foundation to the load and ground pressure is to cup therefore main steel bars are required in both directions.
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
Consist of a raft of reinforced concrete under the whole building.
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.
Useful where column loads are heavy.
Basically divided into 3 (three) types:
Solid slab raft
Beam and slab raft
Cellular raft
Capable of accommodating small settlements in soil.
RAFT FOUNDATIONS
SOLID SLAB RAFT FOUNDATIONS
Constructed of uniform thickness over the whole raft area.
Slab is reinforced at top and bottom.
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.
Beams are used to distribute the column loads over a raft area which results in the reduction of slab thickness.
Beams can be up stand or down stand depending upon the bearing capacity of the subsoil near the surface.
Down stand beams will give a saving on excavation.
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.
Function is to fill any weak pockets encountered during excavation and provide true level surface where reinforcement can be positioned.
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
Effect depends on the :
magnitude
uniformity
length of time over which it takes place
nature of structure
Settlement may be caused by:
imposed weight of structure on soil
changes in moisture content
subsidence due to mining
general earth movement
SETTLEMENT OF FOUNDATIONS
RELATIVE/DIFFERENTIAL / UNEVEN SETTLEMENT
Could cause distortion or could lead to structural failure
Occurs on clay soil
UNIFORM /EVEN SETTLEMENT
Minimal settlement, not excessive
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.
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
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