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8/12/2019 Geo Lecture 8
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CIVE 2004
Lecture 8
Shallow Foundations
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Principles of Foundation Design
Evaluation of the ability of ground to support structuralloads.
Designing the proper structural element (foundation) to
transmit the loads into the ground.
The design should be economical, with due safety against
failure and without unacceptable movement either during
or after construction.
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Types of Foundations
Shallow foundations (also called spread footings) arelocated near the ground surface.
The founding depth, D, is less than the width, B, of the
footing and less than 3m.
They include pads, strip footings and rafts.
Pad foundations are used to support individual point loads
such as that from a column. They may be square,
rectangular or circular.
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Types of Foundations
Strip foundations are used to support line loads, eitherfrom a load-bearing wall, or from a line of columns which
are so close to each other that individual pad foundations
are not appropriate.
Raft foundations are used to spread the loads from a
structure over a large area, normally over the entire
footprint of a building. They are used when column and/or
other structural loads are close together. A raft foundationnormally consists of a concrete slab but it may be stiffened
by ribs or beams incorporated into the foundation.
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Types of Foundations
Deep foundations When the soil immediately below a given structure is
weak, the load of the structure may be transmitted to a
greater depth.
They include piles, pile walls, diaphragm walls and
caissons.
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Requirements of a satisfactory foundation
3 main design criteria:
Adequate depth: foundation structure must be located at
a depth which will not be subjected to any future
influence which could adversely affect its performance.
Adequate factor of safety against shear failure (ultimate
limit state).
Limiting settlement: foundation must not settle or
deflect excessively so as to damage the structure or
adversely affect its serviceability (serviceability limit
state).
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Modes of bearing capacity failure
Failure mode depends generally on the size and shape of
the foundation, the relative density/compressibility of the
soil, drained or undrained behaviour and the relative
stiffnesses of the soil and footing/structure. Shear failure
can take place in three different modes (Whitlow P 456& 457):
General shear occurs in dense or overconsolidated soils
of low compressiblity. At failure, a plastic yield surfacedevelops under the footing, extending outward and
upward to the ground surface. Catastrophic collapse
and/or rotation of the foundation occurs.
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General Shear Failure
qu or qf = ultimate bearing
capacity, the bearing pressure
at which the ground fails in
shear.
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Local Shear Failure
qu, is referred to as the
first failure pressure.
qu or qfis the ultimate
bearing capacity.
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Punching Shear Failure
Punching shear failure occurs
in soft or loose soils of high
compressibility.
Failure takes place by small
sudden vertical movements of
the foundation. There is no
visible collapse nor substantial
tilting.
qu or qfis the ultimate bearingcapacity.
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Definitions
Ultimate bearing capacity, qf, - bearing pressure at which
the soil fails in shear.
Safe bearing capacity, qs, - bearing pressure that the soil
will safely support following the application of a factor of
safety.
Allowable bearing capacity, qa, - bearing pressure that the
soil will safely support and satisfies a limiting settlement.
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Definitions
Gross bearing pressure, q, - total pressure applied by the
foundation at founding level.
Net bearing pressure, qn, - net increase in pressure at
foundation level. It is the pressure that causes shear failure
and settlement.
In terms of total stress, qn = q so where so is the
overburden pressure
In terms of effective stress, q'n = q s'o where s'o isthe effective overburden pressure
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Definitions
Factor of safety is a factor applied to the net ultimate
bearing capacity to obtain a safe net bearing pressure.
Total/undrained stress case:
Effective/drained stress case:
o
of
n
nf
q
q
q
qF
s
s
o
of
n
nf
q
q
q
qF
'
'
s
s
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Definitions
Design bearing capacity or gross safe bearing capacity =
so = s'o, for effective stress cases
onf
s
F
qq s
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Ultimate bearing capacity
Terzaghis general equation for a strip foundation of width
B:
Gross ultimate bearing capacity
Net ultimate bearing capacity
s BNNcNq qocf2
1
oqocf BNNcNq ss 2
1
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Terzaghi bearing capacity factors
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Other bearing capacity factors
Whitlow: Table 11.2
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Ultimate bearing capacity
Effect of foundation shape: shape factors (Whitlow
Table 11.3)
Effect of depth: depth factors
Effect of groundwater table (Whitlow worked example
11.3)
Effect of load inclination
Effect of eccentric loading
Effect of ground surface inclination