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.