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TheStructuralEngineer40
Technical Guidance Note
Technical

November 2012

Note 19 Level 1



Design principles

The bearing capacity of a soil is dependent
upon its structure, moisture content and
the type of foundation that is placed upon
it. It is important therefore to be familiar
with the various types of soil that can be
encountered. From simply knowing the soil
type, it is possible to develop reasonable
design solutions for any given sub-structure.
There are essentially five different types
of soil and/or strata (some of which have
further sub-divisions) that have an impact
on the design of foundations. Table 1
summarises these soils.

Foundation types
There are five core types of foundations
that are used within sub-structures. Most
are built using concrete, both mass and
reinforced, but it is possible to use steel
sections as piles. Figure 1 shows these types
of foundations.

Methods of assessing soil properties
Geotechnical engineering has a reputation
for being imprecise due to the variable
nature of soil and its interaction with
substructures placed upon it. To counter this
BS EN 1997-1 – Eurocode 7: Geotechnical
Design – Part 1 General Rules describes the
four differing methods that can be applied to
the properties of soil. All of them are equally
valid, with the major difference being that
some produce more efficient solutions than
others due to greater degrees of accuracy
of modelling the soil conditions.

Geotechnical design by calculation
This method is reliant on the quality of data

Soil bearing capacity
Introduction
When designing foundations for a structure there is a need to determine
the bearing capacity of the soil. This applies to all forms of foundation,
from a simple pad footing to a pile cap. The bearing stress capacity of
the soil is the key variable that has a direct impact on the form and size of
foundations. This Technical Guidance Note explains the principles of how
bearing capacity of soils are determined and how it impacts on the design
of foundations.

• Design principles

• Applied practice

• Worked example

• Further reading

• Web resources

IcoN
LeGeND

Table 1: Common types of soil and their bearing capacity characteristics

Soil type Description Typical foundation

Rock

Most commonly has a high bearing
capacity; its weakness lies with any

fissures that exist within its make-up and
its weathering state

Reinforced pad foundation that
serves more to fix the sub-

structure to the rock strata rather
than spread its load

Gravel

These are non-cohesive course soils
that tend to be mixed with sand. They
have a high bearing capacity and low

compressibility. The presence of ground
water can reduce its bearing capacity by
half and the soil’s relative density also has

an impact on its bearing capacity

Pad foundations due to the high
bearing capacity. Piling is

rare in these types of soils as it
is often not needed

Sand

Similar to gravel in many respects, sandy
soils also have a high bearing capacity

and low compressibility. Where it is loosely
compacted however, there is a risk of

significant settlement as load is applied.
Like gravel, the presence of ground water
has a detrimental effect on both the soil’s

bearing capacity and relative density

Similar to gravel

Clay

Clays are soils that are made up of very
small particles and are described as

‘cohesive’. They typically have a lower
bearing capacity than non-cohesive soils
and compress when placed under load,
which can occur over a long period of

time, causing settlement. This is countered
when they are over-consolidated at which

point their properties are very similar
to that of sand. Water has a significant
impact on clay soils with its properties

sensitive to the level of moisture content

Pad foundations to light 1-2
storey structures and then piled
foundations for all other forms

of structure. In cases where
settlement is undesirable e.g.

extensions to existing structures,
piling may be necessary

Silt

Silt have a relatively high bearing capacity
when confined, but their underlying

structure breaks down
when exposed to water. Silts can retain

volumes of water that can freeze,
causing the soil to heave

It is rare for structures to be
directly founded upon silt due to
its unpredictable nature. When
encountered, a piling solution is

adopted that passes through the
silt into a more solid strata

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www.thestructuralengineer.org

41

retrieved from geotechnical investigations
carried out on the prospective site.
Assumptions are made based on this data
and in some instances simplifications will
need to be applied to the calculation model
that can lead to conservative results. For
more details on this method see Clause 2.4
of BS EN 1997-1.

Geotechnical design by prescriptive
measures
In instances where the soil conditions of
the site are well known, it is possible to
prepare a set of parameters against which
any sub-structure can be designed. Due to
the generalised nature of this method, it’s
common for it to produce conservatively
designed solutions. For more information
see Clause 2.5 of BS EN 1997-1.

Geotechnical design based on load tests
and experimental models
In addition to geotechnical investigations
that focus on the soil type and location
of the water table, it is possible to carry
out tests to determine the soil’s bearing
capacity. These tests provide unique results
for that particular site and thus are more
accurate than making assumptions based on
data collected from a standard investigation.
This approach typically results in economical
design solutions due to the accuracy of the
data. Load tests however need to be at the
correct scale to ensure the test mirrors the
proposed foundation, which can prove to be
expensive. See Clause 2.6 of BS EN 1997-1.

Geotechnical design based on observation
In instances where it is not possible to
predict how the soil will interact with a
proposed substructure, it is possible to
apply an observational based method
of design. This requires the design of
the substructure to be altered as new
data is revealed about the soil during the
construction of the foundations. Careful
monitoring is needed throughout the
construction process, as well as quick

responses to the data being delivered,
in order to prevent delays during the
substructure works. This method is unlikely
to provide a practical approach to the
majority of foundation designs and is not
recommended for designing substructures
for buildings. For a more comprehensive
description see Clause 2.7 of BS EN 1997-1.

Regardless of the method of soil analysis
adopted, all results must be interpreted by
a suitably qualified geotechnical engineer,
which can then be passed onto the designer
of the substructure.

Determining un-drained soil design
bearing capacity
BS EN 1997-1 states that the ultimate bearing
resistance of the soil must be greater than
the applied bearing pressure from the
substructure. In numerical terms this is
expressed thus:

V Rd d#

Where:
Vd is the design vertical load, that is acting
normal to the foundation’s base.
Rd is the design bearing resistance of the soil.

There are two equations for calculating base
bearing capacity of a given soil. They are
dependent on the condition of the soil, which
is referred to as ‘drained’ or ‘un-drained’.
For cohesive soils such as clay, un-drained
design approach applies when placed under
a short term load, as the force would be
resisted by pore pressure rather than the
grains that form the soil.

For un-drained soil Rd is defined thus:

A'
R

( 2)c b s i qd u;d c c c$ $ $= + +r

Where:
A’ is the effective base area of the foundation

cu;d is the design un-drained shear strength
bc is the base inclination factor, if it is
placed on sloping ground
sc is the shape factor of the foundation
ic is the load inclination factor
q is the overburden pressure at the base of
the foundation

The effective area is based on how the load
is applied to the foundation. If the load is
eccentric to the centre of the foundation,
then the area over which the load is applied
to the soil from the foundation, is reduced.
For the purposes of this note however, the
assumption of all loads acting normal to the
base with no eccentricity, will be made.

The design un-drained shear strength is
defined as:

c
c

u;d
cu

u:k= c
Where:
cu;k is the undrained shear strength of the
soil, which is a measured property
γuc is the partial factor for the undrained
shear strength

The overburden pressure is the vertical
effective weight of the soil that is located
above the strata level where the foundation
is to be installed.

This note does not cover bases on inclined
slopes for the sake of simplicity. Hence the
base inclination and load inclination factors
are not discussed.

Determining drained soil design
bearing capacity
In the case of drained soils, reliance can be
placed on the friction between the particles
within the soil. As such the equation for
determining bearing capacity includes the
factors that are influenced by the angle of
friction (φ)

For drained soil, Rd is defined thus:

• Figure 1 Typical types of foundation

A'
R

c' N b s i q' N b s i ' B' N b s id d c c c c q q q q 1 2$ $ $ $ $ $ $ $ $ $ $ $ $ $= + + c c c c c

(2)

(1)

(3)

(4)

Page 3

TheStructuralEngineer42
Technical Guidance Note
Technical

November 2012

Note 19 Level 1



Where:
c’d is the design effective cohesion
q’ is the overburden pressure at the base of
the foundation
γ’ is the effective weight density of the soil
at the strata level of the foundation
bc , bq and bγ are base inclination factors
sc , sq and sγ are shape factors – see Table 2
for derivation
Nc , Nq and Nγ are the bearing capacity factors
(Table 3). They are the soil cohesion,
vertical effective stress and buoyant
density factors respectively

Partial factors to soil properties
BS EN 1997-1 requires all material properties
of soils to have a partial factor applied to
them. This is due to the adoption of limit
state theory to the design of substructures.
There are two sets of factors that need
to be applied to the material based on
the applied load combination that is being
considered. In the UK the following load
combinations are used:

Table 2: Shape factors for drained soil
bearing capacity

Foundation
shape

Shape
factor

Equation in
degrees

Rectangle

sq 1 + (B’ / L’ ) sin φ’d

sγ 1 – 0.3 (B’/L’ )

sc (sq Nq – 1)/(Nq – 1)

Square or
circle

sq 1 + sin φ’d

sγ 0.7

sc (sq Nq – 1)/(Nq – 1)

*Note: B’ and L’ are effective width and length of the foundation
and φ’d is the design value for the angle of friction of the soil.

Table 3: Bearing capacity factors

Nq Nc Nγ

0 1 5.14 0

16 4 11 1

18 5 13 2

20 6 14 3

22 7 16 5

24 9 19 7

26 11 22 10

28 14 25 14

30 18 30 20

32 23 35 27

34 29 42 38

36 37 50 53

38 48 61 74

40 64 75 106

A pad foundation measuring 0.75m x 0.75m with a thickness of 500mm is to be placed on
a site with a sand/gravel soil. The water table is 3m below ground level and footings are
founded 1.5m below ground level. The load combinations onto the pad footing are
750 kN/m2 for Combination 1 and 385 kN/m2 for Combination 2. Determine whether the
soil can accommodate this applied bearing pressure.

Soil Properties: φ’ = 30º, γ’ = 17 kN/m3, c’=0

Worked example

Combination 1: Permanent load x 1.35 +
Variable load x 1.5 matched with set ‘M1’
properties. This is described as Set B in BS
EN 1990.

Combination 2: Permanent load x 1.00 +
Variable load x 1.3 matched with set ‘M2’
properties. This is described as Set C in BS
EN 1990.

The load set providing the worst condition is
deemed to be the design case.

Table 4 lists the values of the partial factors
for material properties mentioned in this note.

ic , iq and iγ are load inclination factors

For the sake of simplicity the inclination of
base and load are not considered here.

φ’d (in degrees)

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