Lecture 1 introduction & types of foundation

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

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

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

Dr. Abdulmannan Orabi IUST 3


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.



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.



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.


� 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


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


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:



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



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



Figure (1)

m

Z

45�

New footing

Existing footing

The difference in levels between adjacent foundations should not cause undesirable overlapping between stresses.


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





New footing

Existing footing

Ground surface

Excavation line

Soil bulges from loss

of lateral support

Figure (2)

S




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.


The two main types of foundation are :

�Shallow foundation (spread, combined,

and mat foundations) and

�Deep foundation: Pile foundation

Pier foundation

Types of Foundations


Sallow Foundation

�Shallow foundation

Foundation soil


Shallow Foundations


Types of Footing

Combined Footing

Combined

Footing

Isolated footing Strip footing / or

continuous

Strap footing or

cantilever


Isolated Footing

Df

H

Rectangular or Square footing with constant thickness

N


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


Strip Footing

q (kN/m)

2 5 9. 80

56 .1 4

Bearing Wall


Combined Footing

3 82 .9 1

2 4 2. 66

N1

N2


Combined Footing

Rectangular NA < NB

Property line


Combined Footing

Interior column

Exterior column

N1

N2


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


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


Deep Foundations


Retaining Walls

Road

Train

retaining

wall


Sheet Piles

� Sheet piles

ship

warehouse

sheet pile


Sheet Piles

� Sheet piles


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


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.



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.



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

36

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

��


Stability

�Sliding : To prevent the footing from sliding the resisting of the footing on the soil must exceed the applied horizontal load, thus

��

�

��

N

�� + �� − �� > �


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


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.


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


�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

�


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

��


Bearing capacity, kPa

Sett

lem

ent,

mm

Ultimate Bearing

Capacity

Safe Bearing

Capacity

Allowable Bearing Capacity

Ultimate & Allowable Bearing capacity


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.


Failure of medium dense sand ( After photo Dr. Abdulmannan Orabi , 1990)

Bearing Capacity


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


General Shear Failure

�This type of failure is seen in dense and stiff soil


Local Shear Failure

�This type of failure is seen in relatively loose and soft soil.


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.


Terzaghi’s Bearing Capacity

Dr. Abdulmannan Orabi IUST

General Equation

�Hansen’s equation

Df

V

η

+β


Lecture 1 introduction & types of foundation

Engineering

Transcript of Lecture 1 introduction & types of foundation