lecture 1 introduction & types of foundation
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Lecture
1
INTERNATIONAL UNIVERSITY FOR SCIENCE & TECHNOLOGY
�م وا����������� ا������ ا��و��� ا����� �
Dr. Abdulmannan Orabi
Civil Engineering and Environmental Department
303421: Foundation Engineering
Introduction & Types of Foundation
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2
References
ACI 318M-14 Building Code Requirements for Structural Concrete ( ACI 318M -14) and Commentary, American Concrete Institute, ISBN 978-0-87031-283-0.
Bowles , J.,E.,(1996) “Foundation Analysis and Design” -5th ed. McGraw-Hill, ISBN 0-07-912247-7.
Das, B., M. (2012), “ Principles of Foundation Engineering ” Eighth Edition, CENGAGE Learning, ISBN-13: 978-1-305-08155-0.
Syrian Arab Code for Construction 2012
Dr. Abdulmannan Orabi IUST
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A structure essentially consists of two parts, namely the super structure which is above the plinth level and the substructure which is below the plinth level.
Definition of Foundations
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Definition of Foundations
The substructure or foundation
is the part of a structures that is usually placed below the surface of the ground to transmit the load from the superstructure to the underlying soil or rock.
Generally about 30% of the total construction cost is spent on the foundation.
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Definition of Foundations
Footings are those parts of the foundation which
resting directly on the soil, support specific portion of building and distributed building loads directly to the soil.
The most efficient footing and foundation system is that transmit building loads mostly to the soil without exceeding the bearing capacity of the soil.
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Definition of Foundations
All soil compress noticeably when loaded and cause the supported structure to settle.If soil of sufficient bearing capacity lies immediately below the structure then the load can be spread by footings as shown below.
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� Loads must be carried by a foundation system would include the dead load of the building and live loads of it occupants and contents.
� Foundation system must resist lateral loads from both ground pressure and wind, and provide anchorage for the building superstructure against uplift and racking force.
Requirements for Foundations
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Requirements for Foundations
� The most critical factor in determine the foundation system of building is the type and bearing capacity of the soil to which the building loads are distributed. ( Adequate safety)
� Small settlements ( Total and differential settlement)� Construction problems ( stability of excavation
bottom heave ground water problems)� Economical requirement
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Footing depth and spacing
1. The forts line 2. Zones of high volume change due to moisture
fluctuations.3. Top soil or organic material4. Peat and muck 5. Unconsolidated material such as abandoned
garbage dumps and similar filled in areas.
Footings should be carried below:
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Footing depth and spacing
Foundations should be placed at an acceptable level with respect to adjacent foundations.Foundations should be placed sufficiently away from the edge of a sloping ground
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Footing depth and spacing
When footings are to be placed adjacent to an existing structure, as indicated in figure (1) the line from the base of the new footing to the bottom edge of the existing footing should be or less with the horizontal plane.From this requirement it follows that the distance mof figure (1) should be greater than the difference in elevation of the two footings, Z.
45�
Recommendations for footing adjacent to existing structures
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Footing depth and spacing
Figure (1)
m
Z
45�
New footing
Existing footing
The difference in levels between adjacent foundations should not cause undesirable overlapping between stresses.
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Excavation may cause settlement to old foundation due to lateral bulging in the excavation and/or shear failure due to reduction in overburden stress in the surrounding of old foundation
Footing depth and spacing
Recommendations for footing adjacent to existing structures
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Footing depth and spacing
New footing
Existing footing
Ground surface
Excavation line
Soil bulges from loss
of lateral support
Figure (2)
S
Recommendations for footing adjacent to existing structures
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Footing depth and spacing
Figure (2) indicates that if the new footing is lower than the existing footing there is a possibility that the soil may flow laterally from beneath the existing footing. This may increase the amount of excavation. Somewhat but, more importantly may result in settlement cracks in the existing building.
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The two main types of foundation are :
�Shallow foundation (spread, combined,
and mat foundations) and
�Deep foundation: Pile foundation
Pier foundation
Types of Foundations
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Sallow Foundation
�Shallow foundation
Foundation soil
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Shallow Foundations
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Types of Footing
Combined Footing
Combined
Footing
Isolated footing Strip footing / or
continuous
Strap footing or
cantilever
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Isolated Footing
Df
H
Rectangular or Square footing with constant thickness
N
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Isolated Footing
2 5 9. 70
1 87 .0 2
6 8 .1 2
1 2 2 .9 9
Df
H
N
B or L
H
N
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Strip Footing
q (kN/m)
2 5 9. 80
56 .1 4
Bearing Wall
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Combined Footing
3 82 .9 1
2 4 2. 66
N1
N2
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Combined Footing
Rectangular NA < NB
Property line
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Combined Footing
Interior column
Exterior column
N1
N2
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Raft ( Mat ) Foundation
Strip footing in both directions
Strip footing in both direction may be sufficient to spread the load and reduce soil pressure to acceptable levels. If not a raft foundation may be give suitable bearing pressure.
Raft foundation
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Deep Foundations
� Pile Foundations
If a soil pressure has insufficient
bearing capacity then it is necessary to use deep foundation, such as pile, to transmit the load to deeper, firmer strata.
bed rock
weak soil
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Deep Foundations
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Deep Foundations
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Retaining Walls
Road
Train
retaining
wall
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Sheet Piles
� Sheet piles
ship
warehouse
sheet pile
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Sheet Piles
� Sheet piles
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Bearing Pressure Under Footings
Actual pressure is not uniform due to:
1) Footing flexibility
2) Depth of footing below ground surface
3) Type of soil
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Soil Pressure Distribution
For a concentrically loaded pad footing the bearing pressure distribution for a cohesive soil is the pressure are higher near the edge because the load produces a shear resistance a round the perimeter which adds to the upward pressure.
Footing
N
Soil pressure distribution in cohesive soil.
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Soil Pressure Distribution
For cohesionless soil the pressure are higher at the center because the individual grains of soil at the perimeter can shift very slightly outwards to where the soil stresses are less. N
Footing
Soil pressure distribution in cohesionless soil.
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Soil Pressure Distribution
For design purpose, provided the load is symmetrical with respect to the bearing area, it is usual to disregard these variations and assume an uniform bearing pressure.
N
Axially Loaded Footings: Assume uniform pressure
q, bearing pressure
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Stability
Foundation and retaining walls must be designed to resist both overturning and sliding.
�Overturning : To prevent overturning the restoring moment due to the vertical load must be greater than the overturning moment, thus
� × � > M N
M
a
�� �
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Stability
�Sliding : To prevent the footing from sliding the resisting of the footing on the soil must exceed the applied horizontal load, thus
��
�
����
N
�� + �� − �� > �
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Stability
�� + �� − �� > �
Where:
�� is the friction resistance under the base
�� = ����� + ����
� is the friction angle between concrete and soil
��is the passive resistance due to horizontal movement
��is the active resistance due to horizontal movement
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Bearing capacity of soil
Bearing capacity of soil :
� It is defined as the maximum load per unit area which the soil will resist safely without displacement
� The bearing capacity of the soil can be found by loading the soil, noting the settlement and by dividing the maximum load by the area on
which the load is applied.
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Main components of a structure including soil
� Foundation soil is that
portion of ground
which is subjected to
additional stresses when
foundation and
superstructure are
constructed on the ground.
Super Structure
Foundation
Foundation Soil
Ground Level
Bearing capacity of soil
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�The maximum load is obtained from the graph between the settlement and load.
�Ultimate Bearing Capacity ( ) it is the
maximum pressure that a foundation soil can withstand without undergoing shear failure
Ultimate Bearing capacity of soil
�
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Allowable Bearing Pressure ( ) : It is the maximum pressure the
foundation soil is subjected to considering both shear failure and settlement.
Ultimate Bearing capacity of soil
���
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Bearing capacity, kPa
Sett
lem
ent,
mm
Ultimate Bearing
Capacity
Safe Bearing
Capacity
Allowable Bearing Capacity
Ultimate & Allowable Bearing capacity
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Artificial methods to improve bearing capacity of soil
�By increasing the depth of foundation.
�By draining the sub-soil water.
�By compacting the soil.
�By confining the soil mass.
�By cement grouting.
�By injecting chemicals like silicates etc.
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Failure of medium dense sand ( After photo Dr. Abdulmannan Orabi , 1990)
Bearing Capacity
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Modes of shear failure
�Depending on the stiffness of foundation soil and depth of foundation, the following are the modes of shear failure experienced by the foundation soil.
1. General shear failure
2. Local shear failure
3. Punching shear failure
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General Shear Failure
�This type of failure is seen in dense and stiff soil
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Local Shear Failure
�This type of failure is seen in relatively loose and soft soil.
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Punching Shear Failure
�This type of failure is seen in loose and soft soil and at deeper elevations. The failure surface in soil will not extend to the ground surface.
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Terzaghi’s Bearing Capacity
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General Equation
�Hansen’s equation
Df
V
η
+β
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