4_1 shallow foundations
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G. R. Dodagoudar
Department of Civil Engineering
Indian Institute of Technology Madras
Chennai – 600 036
Foundation Engineering –
Principles and Practices
Shear Strength of Soils: A
Review
Shallow Foundations
Bearing capacity
G. R. Dodagoudar
DEPARTMENT OF CIVIL ENGINEERING
I I T MADRAS, CHENNAI - 600 036.
Foundation
A foundation is that part of a structure which transmits loads
directly to the underlying soil.
The function of a foundation is to transfer the structural loads
from a building safely into the ground.
The structural loads include the dead, super imposed
and wind loads. To perform the function, the foundation
must be properly designed and constructed.
Its stability depends on the behaviour under load of the soil
on which it rests and this is affected partly by the design of
the foundation and partly by the characteristics of the soil.
It is necessary in the design and construction of foundation to
pay attention to the nature and strength of the materials
to be used for the foundations as well as the likely behaviour
under load of the soils on which the foundation rests.
Importance
Foundation
Figure 1 Buildings — the Sears Tower in Chicago is one of the tallest
buildings in the world. It needs massive foundations to transmit the
structural loads into the ground. The design of these foundations depends
on the nature of the underlying soils.
Geotechnical engineers are responsible for assessing these soil conditions
and developing suitable foundation designs.
Footings
Mats/Rafts
Bearing Pressure
Example
CHOICE OF FOUNDATION TYPE AND PRELIMINARY
SELECTION
Background and History of Bearing
Capacity
In the era before Karl Terzaghi’s bearing capacity
equation was introduced in the United States,
engineers went to their local building codes to
determine what the allowable bearing pressure was
for design of their foundations.
If they were working outside of a major metropolitan
area, where no local experience or building codes
existed, they had to refer to general texts on
“foundation soils.”
Before introducing the bearing capacity equation in
his 1943 textbook, Theoretical Soil Mechanics
(Terzaghi, 1943), Terzaghi discusses a footing’s load
settlement curve and how to determine a footing’s
failure loading from the load test curve.
Then Terzaghi goes on to suggest that the bearing
capacity equation is a simplified method
of estimating the failure loading of a shallow footing.
He discusses a general shear failure and a local
shear failure of foundation soils, indicating that
excessive settlement of loose sands results in a
local failure before a general shear failure can be
mobilized.
Early geotechnical texts such as Soil Mechanics in
Engineering Practice by Terzaghi and Peck, first
printed in 1948, include charts that give allowable
bearing pressures for 1 inch of settlement for
footings of varying widths on sands of varying
density.
Early pioneers of geotechnical practice knew
that performance of foundations was directly
related to foundation settlement.
A foundation that was load tested and designed for a
high allowable bearing pressure may not experience
a general shear failure, but if the footing settled
excessively, it failed to support the structure properly.
Evolution of Failure
Surfaces
Set-up for plate load test
Set-up for plate load test
Problems
Solution
Solution
Solution
Use general BCT
The allowable bearing capacity of clays, silty
clays and plastic silts may be limited
either by the requirement of an adequate factor of
safety against shear failure
or by settlement considerations.
The allowable bearing capacity of a sand depends
primarily on the density index, the stress history, the
position of the water table relative to foundation
level and the size of the foundation.
Of secondary importance are particle shape and
grading. Both the magnitude of settlement and the
value of the shear strength parameter φ ′ are
strongly dependent on density index: the denser the
sand the less scope there is for particle
rearrangement.
However, the magnitude of settlement is also
influenced by the stress history of the deposit, i.e.,
whether the sand is normally consolidated or
overconsolidated and the previous stress path.
If two sands having the same grading were to exist
at the same density index but one were normally
consolidated and the other overconsolidated, the
settlement would be greater in the normally
consolidated sand for identical loading conditions.
Problems
Solution
Results from elastic theory indicate that the increase
in vertical stress in the soil below the centre of a strip
footing of width B is approximately 20% of the
foundation pressure at a depth of 3B.
In the case of a square footing the corresponding
depth is 1.5B.
For practical purposes these depths can normally be
accepted as the limits of the zones of influence of the
respective foundations and are called the significant
depths.
An alternative approach is to take the significant
depth as that at which the vertical stress is 20% of the
effective overburden pressure. It is essential that the
soil conditions are known within the significant depth
of any foundation.