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 4
BASIC UNDERSTANDING OF PILING DEEP FOUNDATIONS
DEEP FOUNDATIONS
PILE FOUNDATION
Piles are structural members that are made of steel, concrete or/and timber.
They are used to build pile foundation, which are deep and usually cost more than shallow foundation.
An alternative to other foundation techniques when suitable bearing capacity soil is not found near the lowest floor of the structure.
The unsuitability of the upper regions of the subsoil may be due to:
Low bearing capacity of subsoil
Heavy point loads of the structure exceeding the soil bearing capacity.
Presence of high compressible soils near the surface.
Sub soils such as clay which may be capable of moisture movement.
High water table.
DEEP FOUNDATIONS
CLASSIFICATION OF PILES
Piles may transmit their load to lower levels of sub soils by:
End bearing piles – piles act as columns carrying loads through weak sub soils to firm strata.
Friction piles – where there is no firm strata at reasonable depth, end bearing piles become very long and uneconomical therefore friction piles are driven through the weak sub soils to a specified depths. The friction or floating pile is mainly supported by the adhesion and friction action of the soil around the perimeter of the pile shaft.
DEEP FOUNDATIONS
TYPES OF PILES
Steel Piles – may be steel H section, box or tube (pipe). Suitable for all ranges of loading. Can be readily cut down or extended (by welding). Steel piles require protective coating to guard against corrosion.
Concrete piles – can be divided into 2 types:
precast piles cast in place piles
Suitable for all ranges of loading. Readily adaptable to various sizes and shape. Cast in place piles are of 2 categories:
Cased piles – the casing of a steel or concrete tube or shell is driven and left in the ground and the casing is filled with concrete.
Uncased piles – casing is driven into ground and filled with concrete. The casing is gradually withdrawn.
DEEP FOUNDATIONS
Steel Piles
Concrete piles - Precast Piles
Concrete piles - Cast In Place Piles
DEEP FOUNDATIONS
Timber Piles – Suitable for light loads, cannot withstand high driving stress. Subject to decay due to fluctuating water table. Composite Piles – the upper and lower portions of composite piles are made of different materials e.g. steel and concrete or timber and concrete.
Timber Piles
DEEP FOUNDATIONS
DISPLACEMENT PILES AND REPLACEMENT PILES
displacement piles – piles may be performed and driven thus displacing the soil through which they pass and classified as displacement piles.
Replacement piles – soil is bored out and subsequently replaced by a pile shaft.
DEEP FOUNDATIONS
COLUMNS, PILE CAP AND PILING
DEEP FOUNDATIONS
PRECAST CONCRETE PILES
DEEP FOUNDATIONS
Precast Concrete Piles – remainder of piles above ground level are cut off
DEEP FOUNDATIONS
DEEP FOUNDATIONS
Formwork for casting Pile Cap
On top of the group of piles,
a pile cap will be constructed.
DEEP FOUNDATIONS
The completed pile cap
SUITABILITY OF FOUNDATION TYPES TO VARIOUS SOILS SOIL TYPES AND SITE CONDITION Rock , solid chalk, sands and gravels or sands and gravels with only proportion of clay, dense silty sands
FOUNDATION TYPE
REMARKS
Shallow strip foundation Pad foundation
Keep above water wherever possible. Slope on sand liable in erosion.
Uniform, firm and stiff clays: 1) Where vegetation is insignificant 2) Where trees and shrubs are growing or to be planted close to the site 3) Where trees are felled to clear the site and construction is due to start soon
Strip or pad foundations Bored piles
Bored piles R.C bored piles
With these soils downhill creep may occur on slopes greater than 1:10. Unreinforced piles may be broken by slowly moving slopes.
Thin reinforced rafts Basement rafts
Soft clays, soft silty clays
Strip foundations Rafts
Settlement of strips or rafts must be expected. In soft soils of variables thickness it is preferable to pile to firmer stratum.
Filled ground (reclaimed land)
Pier foundations Driven piles
If fill is sound, carefully placed and compacted in thin layers, strip foundations are adequate.
Mining and other subsidence areas
Special raft foundations
END OF LECTURE THANK YOU