10-04-2017, 08:39 PM
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OVERVIEW OF FOUNDATIONS FOR TALL BUILDINGS
Presented By:
M.A. SHAKER
Md. FEROZ ALI
M.A. JAVEED PASHA
Md. ABUBAKAR SIDDIQUE
VISHAL KUMAR KALYANI
ABSTRACT
Design of foundations for tall structures is a challenging and very interesting task.
In particular, floating foundations for tall structures in soft ground, structures founded on
rock, deep (pile) foundations, leaning instability are highlighted.
Design of foundation in seismic areas especially if the soil is likely to get liquefied is a
daunting task.
A solid building starts with a strong foundation, that part of structure that lies beneath the
ground level and is not seen .
INTRODUCTION
A tall building weighs a lot.
The material that can support such heavy loads is solid rock or strong bedrock is not easy.
A deep foundation is a boon as additional space gets provided.
A structural engineer has a verity of choices for the materials to build on top.
Geotechnical engineer has to deal with the material that lies below the structure.
It is always a very challenging task to design the foundation for tall structures.
What is foundation??
Foundation serves some other functions such as:
Prevent settlement of a structure.
Allow building over water or water-logged ground.
Resist uplifting or overturning forces due to wind.
Resist lateral forces due to soil movement.
Support unstable or existing structures.
Types Of Foundation
The choice for the type of foundation to b adopted depends on both the structure and the ground.
SHALLOW FOUNDATION
DEEP FOUNDATION
Shallow Foundation
This type of foundation usually refers to those being rested on stratum with adequate bearing
capacity.
This foundation is laid not more then 3mts below the ground level.
Common examples include pad, strip or raft foundation.
The selection of right type of shallow foundation normally depends on the magnitude and disposition
of the structural loads and the bearing capacity of subsoil.
A combination of two or three types of shallow foundation in one single structure is not uncommon.
Pile Foundation
Pile foundations are the part of a structure used to carry and transfer the load of the structure
to the bearing ground.
The main components of the foundation are the pile cap and the piles.
The main types of materials used for piles are Wood, steel and concrete.
Piles are long and slender members which transfer the load to deeper soil or rock of high bearing
capacity avoiding shallow soil of low bearing capacity.
Classification Of Piles
Classification of pile with respect to load transmission and functional behavior
End bearing piles (point bearing piles)
Friction piles (cohesion piles)
Combination of friction and cohesion piles
End bearing piles
These piles transfer their load on to a firm stratum located at a considerable depth below the base
of the structure and they derive most of their carrying capacity from the penetration resistance of
the soil at the toe of the pile
Even in weak soil a pile will not fail by buckling and this effect need only be considered if part
of the pile is unsupported .
Friction Piles
These piles transmit most of their load to the soil through skin friction.
This process of driving such piles close to each other in groups greatly reduces the porosity and
compressibility of the soil within and around the groups.
Therefore piles of this category are some times called compaction piles.
Combination of friction piles and cohesion piles
An extension of the end bearing pile when the bearing stratum is not hard, such as a firm clay .
A farther variation of the end bearing pile is piles with enlarged bearing areas.
This is achieved by forcing a bulb of concrete into the soft stratum immediately above the firm
layer to give an enlarged base.
Bored piles which are provided with a bell have a high tensile strength and can be used as tension
piles see in the fig.
Classification of pile with respect to type of material
Timber
Concrete
Steel
Composite piles
Timber piles
Used from earliest record time and still used for permanent works in regions where timber is
plentiful.
The timber should be in a good condition and should not have been attacked by insects.
It is essential that the timber is driven in the right direction and should not be driven into firm
ground. As this can easily damage the pile.
Keeping the timber below the ground water level will protect the timber against decay and
putrefaction.
To protect and strengthen the tip of the pile, timber piles can be provided with toe cover.
Pressure creosoting is the usual method of protecting timber piles.
Concrete pile
They are pre-caste so that they can be easily connected together in order to reach to the required
length see in fig (a).
Usually of square triangle, circle or octagonal section, they are produced in short length in one
meter intervals between 3 and 13 meters see in fig (b).
Composite piles
part of a timber pile which is installed above ground water could be vulnerable to insect attack
and decay.
To avoid this, concrete or steel pile is used above the ground water level, whilst wood pile is
installed under the ground water level see in the fig.
Steel pile
Steel/ Iron piles are suitable for handling and driving in long lengths.
Their relatively small cross-sectional area combined with their high strength
makes penetration easier in firm soil.
They can be easily cut off or joined by welding.
Advantages of different pile material
The piles are easy to handle
Can be driven in long lengths
Can be installed in large diameters
Can carry heavy loads.
Piles can be cast before excavations.
Pile lengths are readily adjustable.
Do not corrode.
Disadvantages of different pile material
The piles will corrode above the ground water level. Have a limited bearing
capacity.
Can be damaged during driving.
Sometimes problems with noise and vibration.
Cannot be driven with very large diameter.
Time consuming.
Relatively difficult to cut.
.
Displacement, heave, and disturbance of the soil during driving
Raft Foundation
Raft foundation is a large combined thick slab designed to supports the whole or a large part of a
structure.
Raft foundation is usually used when subsoil is weak and have low bearing capacity, when the
columns are closely located.
A raft foundation is also known as mat foundation.
A raft foundation is use to distribute the building pressure over a large area so the soil can bear
the stress.
Piled raft foundation:
Piled raft is a foundation system consisting of piles & raft.
The piles can be designed in such cases to carry loads close to their ultimate capacity.
It is assumed that the total applied load has to be carried only by the piles with certain FOS,
against bearing capacity.
Raft versus piled raft
Piled raft foundation is more economic foundations when compared to pure pile
foundations when considering bearing capacity.
Piled raft foundation reduces the settlement comparison with a raft foundation.
CONCLUSION
The paper presents a brief overview of the geotechnical aspects of the foundation for the
tall structures which have come into vogue lately in India.
The choice of foundation type depends both on structure and the ground.
Deep foundation, piles or large diameter shafts, are commonly are adopted foundation type for tall
structures in soft to medium ground. Several alternate
pile types and pile group configurations are possible. Piled rafts have become from the most
preferred alternative as the capacity of ground to resist the loads is included making the design
viable, rational and economic.