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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