Df ≤ B to 3B
Df = depth from ground level to the base of footing
B = width of footing
Foundation or footing should be below depth of frost penetration,
undermining by scour, and preferably below zone of seasonal variations
of the GWT.
Can be very difficult and sometimes impractical to meet all these criteria.
Footing must not cause sheer failure of the ground.
Footing must not settle excessively.
CVG3106 Soil Mechanics II
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The soil bearing capacity is the pressure that a foundation unit can
impose onto the supporting earth mass without causing overstressing
(shear failure).
Deformations, due to foundation loading, usually cause settlement.
Lateral movements associated with settlement are also of concern.
Failure of foundations can be due to:
¾ Shear failure (based on the permissible load)
¾ Settlement (permissible settlement)
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Bearing capacity equations: shear strength
properties of the soil (take into consideration
only shear failure criterion but NOT THE
SETTLEMENT)
Penetration resistance data (e.g., the SPT
data) and relate it to the bearing capacity and
settlement characteristics of the soil.
(Commonly used for sandy soils and shallow
foundations. NOT available for all type of
soils and foundations).
9 Takes into account shear strength and
settlement: Good design approach!
CVG3106 Soil Mechanics II
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Relating soil type to presumptive bearing
capacity (Limitation: does not consider
the soil compressibility and the possible
influence of weaker soil layers under the
bearing layer). Not a good approach for
foundation design! Reliable only if you
have prior experience)
Field load tests (commonly used for pile
foundations).
¾ Test results related to both bearing
capacity and settlement (Expensive)
¾ For every 10 piles, 1 pile is fully
tested.
CVG3106 Soil Mechanics II
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Bearing capacity is the capability of the soil beneath a foundation to support a
superstructure load.
The maximum load-bearing capability of the soil, that is the maximum stress
the soil can carry without failure, is called ultimate bearing capacity, qu.
Determination of ultimate bearing capacity depends on the foundation shape
(square, rectangular, circular), size, embedment depth, subsurface conditions,
and the failure mode.
In foundation design, a global factor of safety for bearing capacity is commonly
used to account for the approximation of design methodologies and
uncertainty of the subsoil parameters and to provide sufficient safety margin.
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Therefore, the allowable bearing capacity is used to compare with the
maximum stress due to the superstructure.
qall
qu
FS
Where:
qu = ultimate bearing capacity
qall = allowable bearing capacity
FS = factor of safety.
In foundation design, the acceptable factor of safety is generally not less than
3.0.
FS
qu
t qall
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A complete shear failure surface develops
from the corner(s) of the foundation and
extends to the ground surface.
Occurs in dense coarse-grained soils
(I’ ≥ 36o, Dr ≥ 71%)
The ground surface adjacent to the footing
bulges upward.
Soil displacement is accompanied by tilting
of the foundation.
General shear failure occurs suddenly and is
the most catastrophic to the structure.
It is the most common failure mode.
CVG3106 Soil Mechanics II
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