8th Lumb Lecture

Eurocode 7
Good practice in geotechnical design
Brian Simpson
Arup

8th Lumb Lecture

Eurocode 7 Good practice in geotechnical design

Limit state design
Holistic design structures and ground
Practical approach to characteristic values
of soil parameters
ULS and SLS design requirements
Water pressures
Ground anchors
Retaining structures numerical analysis
The future

8th Lumb Lecture

Eurocode 7 Good practice in geotechnical design

Limit state design
Holistic design structures and ground
Practical approach to characteristic values
of soil parameters
ULS and SLS design requirements
Water pressures
Ground anchors
Retaining structures numerical analysis
The future

Limit state design

states beyond which the
structure no longer satisfies
the relevant design criteria
partial factor design ?
probabilistic design ?
concentration on what might
go wrong

EN1990 3.3 (3)

States prior to structural collapse, which,
for simplicity, are considered in place of
the collapse itself, may be treated as
ultimate limit states.

EN 1990

3.3 Ultimate limit states

Serious failures involving risk of injury or major cost.

Must be rendered very unlikely. An unrealistic possibility.
5

EN 1990

3.3 Ultimate limit states

Serious failures involving risk of injury or major cost.

Must be rendered very unlikely. An unrealistic possibility.
6

EN1997-1 2.4.7 Ultimate limit states STR, GEO

EN1990

3.4 Serviceability limit states

Inconveniences, disappointments and more manageable costs.

Should be rare, but it might be uneconomic to eliminate them
completely.
8

EN1990

3.4 Serviceability limit states

Inconveniences, disappointments and more manageable costs.

Should be rare, but it might be uneconomic to eliminate them
completely.
9

Limit state design

An understanding of limit state design can be obtained
by contrasting it with working state design.
Working state design: Analyse the expected, working
state, then apply margins of safety.
Limit state design: Analyse the unexpected states at
which the structure has reached an unacceptable limit.
Make sure the limit states are unrealistic (or at least
unlikely).

10

Soil failure without geometrical instability (large displacements)??

BP190a.28

11

Fundamental limit state requirement

Ed Rd
E{

Fd ; Xd ; ad} = Ed Rd = R{ Fd ; Xd ; ad}

E{F Frep; Xk/M; ad} = Ed Rd = R{F Frep; Xk/M; ad}

or E{F Frep; Xk/M; ad} = Ed Rd = Rk/R = RnR (LRFD)
or

E Ek = Ed Rd = Rk/R

so in total

E E{F Frep; Xk/M; ad} = Ed Rd = R{F Frep; Xk/M; ad}/R

E = action effects

d = design (= factored)

F = actions (loads)

k = characteristic (= unfactored)

R = resistance (=capacity)

rep = representative

X = material properties
a = dimensions/geometry
12

Partial factors recommended in EN1997-1 Annex A (+UKNA)

Values of partial factors recommended in EN1997-1 Annex A (+ UKNA)
Design approach 1
Combination 1--------------------Combination 2 ------------------------Combination 2 - piles & anchors
M1 or ..M2
A1
M1
R1
A2
M2
R1
A2
R4
Permanent
unfav
1,35
Actions
fav
Variable
unfav
1,5
1,3
1,3
1,25
1,25
tan '
Soil
Effective cohesion
1,25
1,25
Undrained strength
1,4
1,4
Unconfined strength
1,4
1,4
Weight density
Bearing
Spread
Sliding
footings
Base
1,3
Driven
Shaft (compression)
1,3
piles
1,3
Total/combined
(compression)
Shaft in tension
1,25
1,6
Base
1,25
1,6
Bored
Shaft (compression)
1,0
1,3
piles
Total/combined
1,15
1,5
(compression)
Shaft in tension
1,25
1,6
Base
1,1
1,45
CFA
Shaft (compression)
1,0
1,3
piles
1,1
1,4
Total/combined
(compression)
Shaft in tension
1,25
1,6
1,1
1,1
Anchors Temporary
Permanent
1,1
1,1
Retaining Bearing capacity
Sliding resistance
walls
Earth resistance
Earth resistance
Slopes

indicates partial factor = 1.0

13

Design approach 2
DA2 - Comb 1
A1
M1
1,35

R2

1,5

Design approach 3
DA2 - Slopes
A1
M=R2
1,35
1,5

DA3
A1
1,35
1,5

A2

R3

1,3

StructuralGeotech
actions actions

1,4
1,1
1,1
1,1
1,1
1,15
1,1
1,1
1,1
1,15
1,1
1,1
1,1
1,15
1,1
1,1
1,4
1,1
1,4

M2

1,25
1,25
1,4
1,4

1,1

1,1

1,1

1,1
C:\BX\BX-C\EC7\[Factors.xls]

25-Nov-06 17:26

Partial factors recommended in EN1997-1 Annex A

Values of partial factors recommended in EN1997-1 Annex A
Design approach 1
Combination 1--------------------Combination 2 ------------------------Combination 2 - piles & anchors
M1 or ..M2
A1
M1
R1
A2
M2
R1
A2
R4
Permanent
unfav
1,35
Actions
fav
Variable
unfav
1,5
1,3
1,3
1,25
1,25
tan '
Soil
Effective cohesion
1,25
1,25
Undrained strength
1,4
1,4
Unconfined strength
1,4
1,4
Weight density
Bearing
Spread
Sliding
footings
Base
1,3
Driven
Shaft (compression)
1,3
piles
1,3
Total/combined
(compression)
Shaft in tension
1,25
1,6
Base
1,25
1,6
Bored
Shaft (compression)
1,0
1,3
piles
Total/combined
1,15
1,5
(compression)
Shaft in tension
1,25
1,6
Base
1,1
1,45
CFA
Shaft (compression)
1,0
1,3
piles
1,1
1,4
Total/combined
(compression)
Shaft in tension
1,25
1,6
1,1
1,1
Anchors Temporary
Permanent
1,1
1,1
Retaining Bearing capacity
Sliding resistance
walls
Earth resistance
Earth resistance
Slopes

indicates partial factor = 1.0

14

Design approach 2
DA2 - Comb 1
A1
M1
1,35

R2

1,5

Design approach 3
DA2 - Slopes
A1
M=R2
1,35
1,5

DA3
A1
1,35
1,5

A2

R3

1,3

StructuralGeotech
actions actions

1,4
1,1
1,1
1,1
1,1
1,15
1,1
1,1
1,1
1,15
1,1
1,1
1,1
1,15
1,1
1,1
1,4
1,1
1,4

M2

1,25
1,25
1,4
1,4

1,1

1,1

1,1

1,1
C:\BX\BX-C\EC7\[Factors.xls]

25-Nov-06 17:26

Partial factors for DA1 UK National Annex

Design approach 1
Combination 1--------------------Combination 2 ------------------------Combination 2 - piles & anchors
M1 or ..M2
R4
A1
M1
R1
A2
M2
R1
A2
Permanent
unfav
1,35
Actions
fav
Variable
unfav
1,5
1,3
1,3
1,25
1,25
tan '
Soil
Effective cohesion
1,25
1,25
Undrained strength
1,4
1,4
Unconfined strength
1,4
1,4
Weight density
Bearing
Spread
UKNA
Sliding
footings
Base
1,7/1.5
Driven
Shaft (compression)
1.5/1.3
piles
Total/combined
1.7/1.5
(compression)
Shaft in tension
2.0/1.7
Base
2.0/1.7
Bored
Shaft (compression)
1.6/1.4
piles
Total/combined
2.0/1.7
(compression)
Shaft in tension
2.0/1.7
Base
As
CFA
Shaft (compression)
for
piles
bored
Total/combined
(compression)
Shaft in tension
piles
1,1
Anchors Temporary
Permanent
1,1
Retaining Bearing capacity
Sliding resistance
walls
Earth resistance
Earth resistance
Slopes

indicates partial factor = 1.0

C:\BX\BX-C\EC7\[Factors.xls]

15

EC7
values
1,3
1,3
1,3
1.6
1,6
1,3
1.5
1.6
1.45
1.3
1.4
1.6
1,1
1,1

2.4.7 Ultimate Limit States

2.4.7.1 General
General
2.4.7.1
(1)P Where
Where relevant,
relevant, itit shall
shall be
be verified
verified that
that the
the following
following limit
limit states
states are
are not
not exceeded:
exceeded:
(1)P
loss
loss of
of equilibrium
equilibrium of
of the
the structure
structure or
or the
the ground,
ground, considered
considered as
as aa rigid
rigid body,
body, in
in which
which the
the

strengths of
of structural
structural materials
materials and
and the
the ground
ground are
are insignificant
insignificant in
in providing
providing resistance
resistance
strengths
(EQU);
(EQU);
internal
internal failure
failure or
or excessive
excessive deformation
deformation of
of the
the structure
structure or
or structural
structural elements,
elements, including
including

e.g. footings,
footings, piles
piles or
or basement
basement walls,
walls, in
in which
which the
the strength
strength of
of structural
structural materials
materials is
is
e.g.
significant in
in providing
providing resistance
resistance (STR);
(STR);
significant
failure
failure or
or excessive
excessive deformation
deformation of
of the
the ground,
ground, in
in which
which the
the strength
strength of
of soil
soil or
or rock
rock is
is

significant in
in providing
providing resistance
resistance (GEO);
(GEO);
significant
loss
loss of
of equilibrium
equilibrium of
of the
the structure
structure or
or the
the ground
ground due
due to
to uplift
uplift by
by water
water pressure
pressure (buoyancy)
(buoyancy)

or other
other vertical
vertical actions
actions (UPL);
(UPL);
or
hydraulic
hydraulic heave,
heave, internal
internal erosion
erosion and
and piping
piping in
in the
the ground
ground caused
caused by
by hydraulic
hydraulic gradients
gradients

(HYD).
(HYD).

17

1
7

DA1 Combinations 1 and 2 correspond to STR and GEO?

Design approach 1

Actions

Soil

Combination 1--------------------Combination 2 ------------------------Combination 2 - piles & anchors

M1 or ..M2
A1
M1
R1
A2
M2
R1
A2
R4
Permanent
unfav
1,35
fav
Variable
unfav
1,5
1,3
1,3
1,25
1,25
tan '
Effective cohesion
1,25
1,25
Undrained strength
1,4
1,4
Unconfined strength
1,4
1,4
Weight density

STR

GEO

STR and GEO both designed for the same partial factors
18

8th Lumb Lecture

Eurocode 7 Good practice in geotechnical design

Limit state design
Holistic design structures and ground
Practical approach to characteristic values
of soil parameters
ULS and SLS design requirements
Water pressures
Ground anchors
Retaining structures numerical analysis
The future

Genting Highlands

BP87.59

BP106.30

BP111.22

BP112.43

BP119.43

BP124-F3.9

BP130.33

BP184.54

20
20

BP145a.8

Genting Highlands

BP87.60

BP184.55

BP106.31

BP111.23

BP112.44

BP119.44

BP124-F3.10

BP130.34

BP145a.9

FOS > 1 for characteristic

soil strengths

BP87.61

BP119.45

BP106.32

BP111.24

BP112.45

BP124-F3.11

BP130.35

BP145a.10

- but not big enough

The slope and retaining wall are

all part of the same problem.

BP87.62

BP106.33

BP111.25

BP112.46

BP119.46

BP124-F3.12

BP145a.11
Structure and
soil must be
designed together - consistently.
BP130.36

Partial factors for DA1 UK and MS National Annex

Design approach 1
Combination 1--------------------Combination 2 ------------------------Combination 2 - piles & anchors
M1 or ..M2
R4
A1
M1
R1
A2
M2
R1
A2
Permanent
unfav
1,35
Actions
fav
Variable
unfav
1,5
1,3
1,3
1,25
1,25
tan '
Soil
Effective cohesion
1,25
1,25
Undrained strength
1,4
1,4
Unconfined strength
1,4
1,4
Weight density
Bearing
Spread
UKNA
Sliding
footings
Base
1,7/1.5
Driven
Shaft (compression)
1.5/1.3
piles
Total/combined
1.7/1.5
(compression)
Shaft in tension
2.0/1.7
Base
2.0/1.7
Bored
Shaft (compression)
1.6/1.4
piles
Total/combined
2.0/1.7
(compression)
Shaft in tension
2.0/1.7
Base
As
CFA
Shaft (compression)
for
piles
bored
Total/combined
(compression)
Shaft in tension
piles
1,1
Anchors Temporary
Permanent
1,1
Retaining Bearing capacity
Sliding resistance
walls
Earth resistance
Earth resistance
Slopes

Should DA1-1 and DA1-2 give

the same result?
Then whats the point in doing
two calculations?

indicates partial factor = 1.0

C:\BX\BX-C\EC7\[Factors.xls]

24

EC7
values
1,3
1,3
1,3
1.6
1,6
1,3
1.5
1.6
1.45
1.3
1.4
1.6
1,1
1,1

EN1990 choice of partial factor values

BP145a.14

Design consistently at standard

deviations from the mean

0.7 and 0.8 or 1.0 and 0.4 ?

BP145a.15

Provided the uncertainties of loads and

resistances are reasonably similar

use this approach

But if one type of uncertainty is really dominant

Ratio of achieved to required

1.2

SAFETY RATIO

Less economic
1.1
1
0.9
0.8

Less safe

E=-0.7, R=0.8

0.7
0.6
0

0.2

0.4

0.6

E/(R+E)
C:\BX\BX-C\EC7\Papers\Paris Aug06\[Paris-Aug06.xls]

0.8

Ratio of achieved to required

SAFETY RATIO

1.2

Typical
foundations

1.1
1
0.9
0.8

Slope

E=-0.7, R=0.8

0.7
stability

Tower
foundations

0.6
0

0.2

0.4

0.6

E/(R+E)
C:\BX\BX-C\EC7\Papers\Paris Aug06\[Paris-Aug06.xls]

0.8

0.7 and 0.8 or 1.0 and 0.4 ?

BP145a.15

Provided the uncertainties of loads and

resistances are reasonably similar

use this approach

But if one type of uncertainty is really dominant

Ratio of achieved to required

1.2

Uneconomic
E=-0.4, R=1.0

E=-1.0, R=0.4

SAFETY RATIO

1.1
1
0.9
0.8

Unsafe

E=-0.7, R=0.8

0.7
0.6
0

0.2

0.4

0.6

E/(R+E)
C:\BX\BX-C\EC7\Papers\Paris Aug06\[Paris-Aug06.xls]

0.8

Ratio of achieved to required

Uneconomic

1.1

E=-0.4, R=1.0

E=-1.0, R=0.4

SAFETY RATIO

1.2

1
0.9

Unsafe

0.8

E=-0.7, R=0.8

0.7
0.6
0

0.2

0.4

0.6

E/(R+E)
C:\BX\BX-C\EC7\Papers\Paris Aug06\[Paris-Aug06.xls]

0.8

Combinations 1 and 2 in EC7 - DA1

BP119.47

BP124-F3.13

BP129.50

BP106.34

BP145a.22

Design approach 1
Actions

Soil

Combination 1--------------------Combination 2 ------------------------Combination 2 - piles & anchors

M1 or ..M2
A1
M1
R1
A2
M2
R1
A2
R4
Permanent
unfav
1,35
fav
Variable
unfav
1,5
1,3
1,3
1,25
1,25
tan '
Effective cohesion
1,25
1,25
Undrained strength
1,4
1,4
Unconfined strength
1,4
1,4
Weight density

Just like load combinations, extended to include

variables on the resistance side.
All designs must comply with both combinations in
all respects, both geotechnical and structural
Turkstras principle for load combinations extended

BP111.53

BP112.47

th
8

Lumb
Lecture

8th Lumb Lecture

Eurocode 7 Good practice in geotechnical design

Limit state design
Holistic design structures and ground
Practical approach to characteristic
values of soil parameters
ULS and SLS design requirements
Water pressures
Ground anchors
Retaining structures numerical analysis
The future

Fundamental limit state requirement

Ed Rd
E{

Fd ; Xd ; ad} = Ed Rd = R{ Fd ; Xd ; ad}

E{F Frep; Xk/M; ad} = Ed Rd = R{F Frep; Xk/M; ad}

or E{F Frep; Xk/M; ad} = Ed Rd = Rk/R = RnR (LRFD)
or

E Ek = Ed Rd = Rk/R

so in total

E E{F Frep; Xk/M; ad} = Ed Rd = R{F Frep; Xk/M; ad}/R

E = action effects

d = design (= factored)

F = actions (loads)

k = characteristic (= unfactored)

R = resistance (=capacity)

rep = representative

X = material properties
a = dimensions/geometry
35

Fundamental limit state requirement

Ed Rd
E{

Fd ; Xd ; ad} = Ed Rd = R{ Fd ; Xd ; ad}

E{F Frep; Xk/M; ad} = Ed Rd = R{F Frep; Xk/M; ad}

or E{F Frep; Xk/M; ad} = Ed Rd = Rk/R = RnR (LRFD)
or

E Ek = Ed Rd = Rk/R

so in total

E E{F Frep; Xk/M; ad} = Ed Rd = R{F Frep; Xk/M; ad}/R

Concrete and steel: 2 standard deviations from the mean test result.

36

Characteristic values in EC7

37

Characteristic values in EC7 definition (2.4.5.2)

38

Characteristic values in EC7

2.4.3(4) also mentions:

39

Characteristic values in EC7

41

Characteristic values in EC7 zone of ground

Cautious worse than most probable.

Small building on estuarine beds near slope

42

Characteristic values in EC7 zone of ground

43

Characteristic values in EC7 zone of ground

2.4.5.2 Characteristic values of geotechnical parameters

Thoughtful interpretation not simple averaging

7.6.2.2

44

Characteristic values in EC7 definition (2.4.5.2)

45

0.5 SD below the mean?

A suggestion:
When:
a limit state depends on the value of a parameter averaged over a large
amount of ground (ie a mean value), and
the ground property varies in a homogeneous, random manner, and
at least 10 test values are available
Then: A value 0.5SD below the mean of the test results provides a useful
indication of the characteristic value
(Contribution to Discussion Session 2.3, XIV ICSMFE, Hamburg. Balkema.,
Schneider H R (1997) Definition and determination of characteristic soil properties.
Discussion to ISSMFE Conference, Hamburg.)
46

0.5 SD below the mean?

- a useful consideration, not a rule
1.2

5% fractile of
mean values

0.8

More remote when

dependent on
specific small zone.

0.6

0.4

5% fractile of
test resuts
0.2

Results of
soil tests

0
-3

-2.5

C:\bx\EC7\[EC7.xls]
47

-2

-1.5

SD from mean

-1

-0.5

Mean

26-May-03 10:10

0.5

A USA proposal 25% fractile

0.5

0.45

5% fractile of
mean values

0.4

75% exceedence
for tests

PROBABILITY DENSITY

0.35

0.3

0.25

0.2

5% fractile
of test
results

0.15

0.1

Results of
soil tests

0.05

0
-3

-2

-1

TEST RESULTS (SD from mean)

C:\BX\BX-C\EC7\[EC7a.xls]

48

14-May-09 11:20

Characteristic values in EC7

NOT a fractile of the results of particular, specified laboratory tests on
specimens of material.
A cautious estimate of the value affecting the occurrence of the limit state
Take account of time effects, brittleness, soil fabric and structure, the effects
of construction processes and the extent of the body of ground involved in a
limit state
The designers expertise and understanding of the ground are all encapsulated
in the characteristic value
Consider both project-specific information and a wider body of geotechnical
knowledge and experience.
Characteristic = moderately conservative = representative (BS8002) = what
good designers have always done.
49

8th Lumb Lecture

Eurocode 7 Good practice in geotechnical design

Limit state design
Holistic design structures and ground
Practical approach to characteristic values
of soil parameters
ULS and SLS design requirements
Water pressures
Ground anchors
Retaining structures numerical analysis
The future

Square footing

Ultimate bearing capacity

Undrained: R/B = (+2) cusu + q

Gk = 700 kN
= 17 kN/m3

1m

B?

Drained:

R/B = c' Ncsc + q' Nqsq + 0.5 ' B Ns

cu = 50 kPa
c=0, = 25
= 20 kN/m3

Limiting settlements:
20 mm short term (undrained)
30 mm long term (drained)
Partial factors Cu = 1.4
To satisfy ULS requirements:
Undrained:
B = 1.73 m
Drained:
B = 2.02 m
SLS:

51

=1.25

Working bearing pressure = 237 kPa

Working bearing pressure = 174 kPa

6.6

52

Serviceability limit state design

Square footing

Ultimate bearing capacity

Undrained: R/B = (+2) cusu + q

Gk = 700 kN
= 17 kN/m3

1m

B?

Drained:

R/B = c' Ncsc + q' Nqsq + 0.5 ' B Ns

cu = 50 kPa
c=0, = 25
= 20 kN/m3

Limiting settlements:
20 mm short term (undrained)
30 mm long term (drained)
Partial factors Cu = 1.4

53

=1.25

To satisfy ULS requirements:

Undrained:
B = 1.73 m
Drained:
B = 2.02 m

Working bearing pressure = 237 kPa

Working bearing pressure = 174 kPa

SLS:
SLS using cu/3
SLS using cu/2

Working bearing pressure = 120 kPa

Working bearing pressure = 171 kPa

B = 2.44 m
B = 2.04 m

/cu

Settlement prediction by Bolton et al

cu

max
cu/2 (M=2)
(M=2)

/M=2
Vardanega, P.J. and Bolton, M.D. (2011) Strength mobilization in clays and silts. Canadian Geotechnical Journal
48(10):1485-1503.
McMahon, B.T., Haigh, S.K., Bolton, M.D. (2014) Bearing capacity and settlement of circular shallow foundations
using a nonlinear constitutive relationship. Canadian Geotechnical Journal 51 (9): 995-1003.
54

Square footing
Gk = 700 kN
= 17 kN/m3

1m

B?

cu = 50 kPa
c=0, = 25
= 20

kN/m3

Limiting settlements:
20 mm short term (undrained)
30 mm long term (drained)

Settlement limits
55

Design for undrained ULS only

likely to fail at SLS.
Design for ULS drained
marginal at SLS
cu/3 small settlements
cu/2 non-linear

 = 17

1m

B?

cu = 50 kPa
c=0, = 25

mm
Settlement

Gk = 700 kN
kN/m3

Bolton

50

40

200

Drained

Bearing
pressure

150

Bolton

30

Limiting settlements:
E/Cu=200
Undrained
20 mm short term (undrained)
20
30 mm longE/Cu=300
term (drained)

100

50

10

F=2

= 20 kN/m3

F=3
0

0
1.60

1.80

2.00

2.20

Footing width

Settlement limits
57

2.40

2.60

Bearing pressure

Square footing

kPa

250

60

 Design for undrained ULS only

likely to fail at SLS.

Square footing
Gk = 700 kN
= 17 kN/m3

1m

B?

cu = 50 kPa
c=0, = 25
= 20 kN/m3

Limiting settlements:
20 mm short term (undrained)
30 mm long term (drained)

Settlement limits
58

Design for ULS drained

marginal at SLS
cu/3 small settlements
cu/2 non-linear
Necessary to check both ULS and
SLS: SLS may govern

Grand Egyptian Museum

Governed by SLS

59

Coventry University Engineering and Computing Building

Design for Collaborative Learning

Detailed Design

Coventry University Engineering and Computing Building

 Sand and clays

Governed by long
term bearing capacity
(ULS)
Careful consideration
of relevant load
combinations

62

63

Greengate Public
Realm
- footbridge near
Manchester

64

Section 7 Pile foundations

65

SLS also covered by ULS factors

66

8th Lumb Lecture

Eurocode 7 Good practice in geotechnical design

Limit state design
Holistic design structures and ground
Practical approach to characteristic values
of soil parameters
ULS and SLS design requirements
Water pressures
Ground anchors
Retaining structures numerical analysis
The future

Water has a way of seeping between any two theories!

2.4.7 Ultimate Limit States

2.4.7.1 General
General
2.4.7.1
(1)P Where
Where relevant,
relevant, itit shall
shall be
be verified
verified that
that the
the following
following limit
limit states
states are
are not
not exceeded:
exceeded:
(1)P
loss
loss of
of equilibrium
equilibrium of
of the
the structure
structure or
or the
the ground,
ground, considered
considered as
as aa rigid
rigid body,
body, in
in which
which the
the

strengths of
of structural
structural materials
materials and
and the
the ground
ground are
are insignificant
insignificant in
in providing
providing resistance
resistance
strengths
(EQU);
(EQU);
internal
internal failure
failure or
or excessive
excessive deformation
deformation of
of the
the structure
structure or
or structural
structural elements,
elements, including
including

e.g. footings,
footings, piles
piles or
or basement
basement walls,
walls, in
in which
which the
the strength
strength of
of structural
structural materials
materials is
is
e.g.
significant in
in providing
providing resistance
resistance (STR);
(STR);
significant
failure
failure or
or excessive
excessive deformation
deformation of
of the
the ground,
ground, in
in which
which the
the strength
strength of
of soil
soil or
or rock
rock is
is

significant in
in providing
providing resistance
resistance (GEO);
(GEO);
significant
loss
loss of
of equilibrium
equilibrium of
of the
the structure
structure or
or the
the ground
ground due
due to
to uplift
uplift by
by water
water pressure
pressure (buoyancy)
(buoyancy)

or other
other vertical
vertical actions
actions (UPL);
(UPL);
or
hydraulic
hydraulic heave,
heave, internal
internal erosion
erosion and
and piping
piping in
in the
the ground
ground caused
caused by
by hydraulic
hydraulic gradients
gradients

(HYD).
(HYD).

69

6
9

2.4.7 Ultimate Limit States

2.4.7.1 General
General
2.4.7.1
(1)P Where
Where relevant,
relevant, itit shall
shall be
be verified
verified that
that the
the following
following limit
limit states
states are
are not
not exceeded:
exceeded:
(1)P
loss
loss of
of equilibrium
equilibrium of
of the
the structure
structure or
or the
the ground,
ground, considered
considered as
as aa rigid
rigid body,
body, in
in which
which the
the

strengths of
of structural
structural materials
materials and
and the
the ground
ground are
are insignificant
insignificant in
in providing
providing resistance
resistance
strengths
(EQU);
(EQU);
internal
internal failure
failure or
or excessive
excessive deformation
deformation of
of the
the structure
structure or
or structural
structural elements,
elements, including
including

e.g. footings,
footings, piles
piles or
or basement
basement walls,
walls, in
in which
which the
the strength
strength of
of structural
structural materials
materials is
is
e.g.
W

significant in
in providing
providing resistance
resistance (STR);
(STR);
significant
F1

F2

failure
failure or
or excessive
excessive deformation
deformation of
of the
the ground,
ground, in
in which
which the
the strength
strength of
of soil
soil or
or rock
rock is
is

significant in
in providing
providing resistance
resistance (GEO);
(GEO);
significant
b

loss
loss of
of equilibrium
equilibrium of
of the
the structure
structure or
or the
the ground
ground due
due to
to uplift
uplift by
by water
water pressure
pressure (buoyancy)
(buoyancy)

or other
other vertical
vertical actions
actions (UPL);
(UPL);
or
hydraulic
hydraulic heave,
heave, internal
internal erosion
erosion and
and piping
piping in
in the
the ground
ground caused
caused by
by hydraulic
hydraulic gradients
gradients

(HYD).
(HYD).

70

7
0

2.4.7 Ultimate Limit States

2.4.7.1 General
General
2.4.7.1
(1)P Where
Where relevant,
relevant, itit shall
shall be
be verified
verified that
that the
the following
following limit
limit states
states are
are not
not exceeded:
exceeded:
(1)P
loss
loss of
of equilibrium
equilibrium of
of the
the structure
structure or
or the
the ground,
ground, considered
considered as
as aa rigid
rigid body,
body, in
in which
which the
the

strengths of
of structural
structural materials
materials and
and the
the ground
ground are
are insignificant
insignificant in
in providing
providing resistance
resistance
strengths
(EQU);
(EQU);
internal
internal failure
failure or
or excessive
excessive deformation
deformation of
of the
the structure
structure or
or structural
structural elements,
elements, including
including

e.g. footings,
footings, piles
piles or
or basement
basement walls,
walls, in
in which
which the
the strength
strength of
of structural
structural materials
materials is
is
e.g.
significant in
in providing
providing resistance
resistance (STR);
(STR);
significant

F = 1.0, 1.35, 1.5 etc

failure
failure or
or excessive
excessive deformation
deformation of
of the
the ground,
ground, in
in which
which the
the strength
strength of
of soil
soil or
or rock
rock is
is

significant in
in providing
providing resistance
resistance (GEO);
(GEO);
significant

F = 1.0, 1.35, 1.5 etc

loss
loss of
of equilibrium
equilibrium of
of the
the structure
structure or
or the
the ground
ground due
due to
to uplift
uplift by
by water
water pressure
pressure (buoyancy)
(buoyancy)

or other
other vertical
vertical actions
actions (UPL);
(UPL);
or

F,dst = 1.0/1.1, F,stb = 0.9

hydraulic
hydraulic heave,
heave, internal
internal erosion
erosion and
and piping
piping in
in the
the ground
ground caused
caused by
by hydraulic
hydraulic gradients
gradients

(HYD).
(HYD).

71

F,dst = 1.35, F,stb = 0.9

7
1

Design water pressures in EC7

72

2.4.2 Actions

The single source principle

(9)P Actions in which ground- and free-water forces predominate shall be

identified for special consideration with regard to deformations, fissuring,
variable permeability and erosion.
NOTE Unfavourable (or destabilising) and favourable (or stabilising)
permanent actions may in some situations be considered as coming from a
single source. If they are considered so, a single partial factor may be applied to
the sum of these actions or to the sum of their effects.

73

3rd International Symposium on Geotechnical Safety and Risk,

Munich, June 2011

Geotechnical safety in relation to water pressures

B. Simpson
Arup Geotechnics, London, UK

N. Vogt
Technische Universitt Mnchen,
Zentrum Geotechnik, Munich, Germany

A. J. van Seters
Fugro GeoServices, The Netherlands
Simpson, B, Vogt, N & van Seters AJ (2011) Geotechnical safety in relation to water
pressures. Proc 3rd Int Symp on Geotechnical Safety and Risk, Munich, pp 501-517.

Very simple problems

75

Slightly more complex problems

76

Explicitly accommodate the worst water pressures

that could reasonably occur

1m rise in water level multiplies BM

by about 2.5 outside the range
allowed by factors on the water
pressure or water force.

77

Use of an offset in water level?

7
8

HYD Equation 2.9

G S
z

79

EC7 {2.4.7.5(1)P} states: When considering a limit state of failure due to heave
by seepage of water in the ground (HYD, see 10.3), it shall be verified, for every
relevant soil column, that the design value of the destabilising total pore water
pressure (udst;d ) at the bottom of the column, or the design value of the seepage
force (Sdst;d) in the column is less than or equal to the stabilising total vertical
stress (stb;d) at the bottom of the column, or the submerged weight (Gstb;d) of
the same column:
udst;d stb;d
(2.9a) total stress (at the bottom of the column)
Sdst;d Gstb;d (2.9b) effective weight (within the column)

HYD Equation 2.9

G S
z

Annex A of EC7 provides values for partial factors to be used for HYD, G;dst = 1.35
and G;stb = 0.9. But the code does not state what quantities are to be factored.
EC7 {2.4.7.5(1)P} states: When considering a limit state of failure due to heave
Maybe:
by seepage
water
theground
(HYD,
G;dstofudst;k
inG;stb
(2.9a) see 10.3), it shall be verified, for every
stb;k
1.35/0.9 =
1.5
relevant and
soil column, that the design value of the destabilising
total
pore water
S ) atthe
Gstb;kof the(2.9b)
G;stb
pressure(u
bottom
column, or the design value of the seepage
G;dst
dst;ddst;k
force (Sdst;d) in the column is less than or equal to the stabilising total vertical
In
this (format,
areofapplied
to different
inweight
2.9 a and
stress
stb;d) atthe
(Gb.stb;d) of
thefactors
bottom
the column,
or thequantities
submerged
the same column:
udst;d stb;d
(2.9a) total stress (at the bottom of the column)
Sdst;d Gstb;d (2.9b) effective weight (within the column)
81

Orr, TLL (2005) Model

Solutions for Eurocode 7
Workshop Examples.
Trinity College, Dublin.

HYD Equation 2.9

H=?

7m

1m
3m

Uniform permeability

82

HYD Equation 2.9

udst;d stb;d
Sdst;d Gstb;d

(2.9a) total stress (at the bottom of the column)

(2.9b) effective weight (within the column)

Apply G;dst = 1.35 to:

Apply G;stb = 0.9 to:

Pore water pressure udst;k

Total stress stb;k

2.78

Seepage force Sdst;k

Buoyant weight Gstb;k

6.84

Excess pore pressure udst;k - wz

Buoyant density
6.84
Orr, density
T.L.L. 2005.
Buoyant
6.84
Model Solutions for Eurocode 7
Total Workshop
density Examples.
6.1
Trinity College, Dublin.

G;dst u(u
-G;stb
Excess head
wz)stb;k
/w
dst;k
dst;k

(2.9a)

Excess pore pressure or excess head

G;dst Sdst;k G;stb Gstb;k
(2.9b)
83

HYD Equation 2.9

udst;d stb;d
Sdst;d Gstb;d

84

(2.9a) total stress (at the bottom of the column)

(2.9b) effective weight (within the column)

Apply G;dst = 1.35 to:

Apply G;stb = 0.9 to:

Pore water pressure udst;k

Total stress stb;k

2.78

Seepage force Sdst;k

Buoyant weight Gstb;k

6.84

Excess pore pressure udst;k - wz

Buoyant density

6.84

Excess head (udst;k - wz) /w

Buoyant density

6.84

Excess pore pressure or excess head

Total density

6.1

Safety Against Hydraulic Heave (HYD in EC7)

Conclusions
Not good to factor total water pressures
- Factoring differential or excess water pressure
may be OK. (ie excess over hydrostatic)

85

Terzhagis rectangular block

G' = buoyant weight

S = seepage force

b=t/2

due to excess water pressure

t
G'

Dimensions t x t/2
FT = G'/S

Das (1983) Fig 2.47

86

Factors of safety for HYD

Das (1983) Fig 2.47

8
7

Essential to assess correct water pressures (permeabilities)

then FT seems to be irrelevant
6m

Level (m)

h/t = 2
FT 1.5

4
2
0
-2
-4
-6
-8
-10

FT = 1.17

1.00E-06

1.00E-05
1.00E-04
Permeability m/s

Permeability m/s

89

-12
1.00E-03

Why b=t/2? A narrower block would be more critical.

Include friction on the side of the block?

G' = buoyant weight

S = seepage force

b=t/2

due to excess water pressure

t
G'

Dimensions t x t/2
FT = G'/S

Das (1983) Fig 2.47

90

Equipotentials for uniform permeability FT = 1.5

h/t = 2
FT 1.5

6m

h = 6m

t = 3m
b

Simpson, B & Katsigiannis, G (2015) Safety considerations for the HYD limit state.
Submitted for ECSMGE, Edinburgh.

Effect of friction on the Terzaghi block

10
9

Factor of safety

h/t = 2
FT 1.5

7
6
5
4

+ friction

3
2

No friction

1
0

b=t/2
0

4
Column width b (m)

Effect of friction on the Terzaghi block

2

h/t = 2

1.8

No friction

Factor of safety

1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0

b=t/2
0

4
Column width b (m)

Effect of friction on the Terzaghi block

2

h/t = 2

1.8

No friction

Factor of safety

1.6
1.4

h/t = 3

1.2
1

0.8
0.6
0.4
0.2
0

b=t/2
0

4
6
Column width b (m)

Effect of friction on the Terzaghi block

2

h/t = 2

1.8

No friction

Factor of safety

1.6
1.4

h/t = 3
h/t = 3.33

1.2
1

friction

0.8
0.6
0.4
0.2
0

b=t/2
0

4
6
Column width b (m)

88

Effect of friction on
the Terzaghi block

10

1.8

h/t = 2
FT 1.5

7
6
5
4

+ friction

3
2

No friction

1.6

Factor of safety

Factor of safety

h/t = 2

No friction

1.4

h/t = 3
h/t = 3.33

1.2
1

friction

0.8
0.6
0.4
0.2

0
0

4
Column width b (m)

4
6
Column width b (m)

88

Conclusions of EG9
Not good to factor total water pressures
-

Factoring differential water pressure may be OK.

Design for F= is no use if the pore pressures (permeability

distribution) are not properly understood.
ULS design water pressure derived without factors (1% chance)
-

No factors on effects of water pressure eg seepage force S.

But could be factors on structural effects of water pressures eg BM

Take directly assessed ULS design water pressures (1% chance)

with factored strengths of materials. Consider all failure
mechanisms. Simple!
Special case: Terzaghi block only consider one mechanism so
add a factor of safety (1.5?).
97

8th Lumb Lecture

Eurocode 7 Good practice in geotechnical design

Limit state design
Holistic design structures and ground
Practical approach to characteristic values
of soil parameters
ULS and SLS design requirements
Water pressures
Ground anchors
Retaining structures numerical analysis
The future

Ground anchors
EC7 Section 8
and the new UKNA

99

EC7 Section 8 Anchorages (existing)

EC7 Evolution Groups

EG0
EG1
EG2
EG3
EG4
EG5
EG6
EG7
EG8
EG9
EG10
EG11
EG12
EG13
EG14

Management and oversight

Anchors
Maintenance and simplification
Model solutions
Numerical models
Reinforced soil
Seismic design
Pile design
Harmonization
Water pressures
Calculation models
Characterization
Tunnelling
Rock mechanics
Ground improvement

Three European documents

EN 1537
Execution of special geotechnical work Ground anchors
EN ISO 22477-5
Geotechnical investigation and testing Testing of geotechnical structures Part 5:

Testing of anchorages
Eurocode 7 EN 1997-1 Section 8 Anchors + UKNA

And the existing British code

BS 8081 Ground anchorages (being revised as NCCI)

EN 1997-1:2004/A1:2013
Eurocode 7 (2004) with amendment (2013)
Section 8 - Anchors
8.1 General
8.2 Limit states
8.3 Design situations and actions
8.4 Design and construction considerations
8.5 Limit state design of anchors
8.6 Tests on anchors
8.7 Lock-off load for pre-stressed anchors
8.8 Supervision, monitoring and maintenance
+UKNA
103

Motherhood and apple pie?

Anchor
force

The life story of

a ground anchor
Time

104

Anchor
force
Check
behaviour

ULS: EULS;d = max (FULS;d , Fserv;d)

Proof
load, Pp

Sufficient to prevent supported structure

exceeding ULS
FServ ;d

FULS;d

Serv
FServ ;k

Lock-off
load, P0

FServ ;k
Pre-load and testing
EN 22477-5
BS 8081

Working life

SLS

Sufficient to prevent
supported structure
exceeding SLS

FULS;d

Time
FF
anchor
force,
including
effect
of lock
off
and sufficient
to prevent
Characteristic
a cautious
estimate
what
is likely
to happen
serv the
themaximum
force(k):
required
to prevent
anyofultimate
limit
state
in load,
the supported
structure
ULS
a serviceability limit state in the supported structure
105

106

Small factor a;ULS

Take the worst

Investigation or suitability tests must be used to check EULS;d

Investigation tests not used much on small contracts. Suitability tests on working anchors.

Investigation or suitability tests may optionally check behaviour at Fserv;k (NA)

All grouted anchors must have acceptance tests
Acceptance tests may check EULS;d and/or Fserv;k (NA)
107

Small factor a;ULS

Take the worst

108

110

CEN value: ULS = 1.0

UK value: ULS = 1.35 Fserv;k/EULS;d
< 1.0, if EULS;d > 1.35Fserv;k

CEN value: a;ULS = 1.1 = UK value

So RULS;m = 1.1xRULS;d 1.1EULS;d

Fserv;k??

So RULS;m = 1.1x (RULS;d EULS;d)x1.35 Fserv;k/EULS;d 1.5FServ;k

111

Anchor
force
Check
behaviour

ULS: EULS;d = max (FULS;d , Fserv;d)

Proof
load, Pp

FULS;d

RULS;m
1.5

FServ ;d

Serv
FServ ;k

Lock-off
load, P0

FServ ;k
Pre-load and testing
EN 22477-5
BS 8081

Working life

SLS

Sufficient to prevent
supported structure
exceeding SLS

FULS;d

Time
FF
anchor
force,
including
effect
of lock
off
and sufficient
to prevent
Characteristic
a cautious
estimate
what
is likely
to happen
serv the
themaximum
force(k):
required
to prevent
anyofultimate
limit
state
in load,
the supported
structure
ULS
a serviceability limit state in the supported structure
112

RULS;m = 1.1x (RULS;d EULS;d)x1.35 Fserv;k/EULS;d = 1.5FServ;k

RSLS;m = Fserv;k

Advice on design of anchors to achieve these

performance requirements will be provided in
BS 8081 (2015).

113

Summary
Anchor validation based only on
testing no reliance on
calculations.
No requirement for big overall
FOS.
But contractor will need to be
confident that every anchor will
pass the acceptance test. Low
creep at fairly high loads.
So he might introduce extra
margins to be sure of this.
EC7 gives the test criteria, but
doesnt advise how to achieve
them. BS8081 will do this.
114

8th Lumb Lecture

Eurocode 7 Good practice in geotechnical design

Limit state design
Holistic design structures and ground
Practical approach to characteristic values of
soil parameters
ULS and SLS design requirements
Water pressures
Ground anchors
Retaining structures numerical analysis
The future

How retaining
walls fail ULS
(Eurocode 7)

BP140.11a

Which governs ULS or SLS? Always SLS?

3:41 pm

3:41 pm

9.8 Serviceability limit state design

119

9.8.2 Displacements

120

BP168-4.33

8th Lumb Lecture

Eurocode 7 Good practice in geotechnical design

Limit state design
Holistic design structures and ground
Practical approach to characteristic values of
soil parameters
ULS and SLS design requirements
Water pressures
Ground anchors
Retaining structures numerical analysis
The future

9.8.2 Displacements

BP168-4.33

Numerical analysis often used for SLS.

Nothing new in EC7.

122

8th Lumb Lecture

Eurocode 7 Good practice in geotechnical design

Use of numerical methods for ULS

Can numerical methods be used for all design approaches?

How should strength factors be applied?
Does FEM give the wrong failure mechanism?
Use of advanced soil models for ULS
Undrained behaviour and consolidation
K0 and soil stiffness
Staged construction

Simpson,
B and
Junaideen,
SM (2013)
EC7
Evolution
Group
4, chaired
by
UseAndrew
of numericalLees
analysis
with Eurocode 7.
Dr
(Cyprus)

18th South East Asia Geotechnical Conference, Singapore.

123

8th Lumb Lecture

Eurocode 7 Good practice in geotechnical design

Use of numerical methods for ULS

124

Can numerical methods be used for all design approaches?

How should strength factors be applied?
Does FEM give the wrong failure mechanism?
Use of advanced soil models for ULS
Undrained behaviour and consolidation
K0 and soil stiffness
Staged construction

Partial factors recommended in EN1997-1 Annex A (+UKNA)

Values of partial factors recommended in EN1997-1 Annex A (+ UKNA)
Design approach 1
Combination 1--------------------Combination 2 ------------------------Combination 2 - piles & anchors
M1 or ..M2
A1
M1
R1
A2
M2
R1
A2
R4
Permanent
unfav
1,35
Actions
fav
Variable
unfav
1,5
1,3
1,3
1,25
1,25
tan '
Soil
Effective cohesion
1,25
1,25
Undrained strength
1,4
1,4
Unconfined strength
1,4
1,4
Weight density
Bearing
Spread
Sliding
footings
Base
1,3
Driven
Shaft (compression)
1,3
piles
1,3
Total/combined
(compression)
Shaft in tension
1,25
1,6
Base
1,25
1,6
Bored
Shaft (compression)
1,0
1,3
piles
Total/combined
1,15
1,5
(compression)
Shaft in tension
1,25
1,6
Base
1,1
1,45
CFA
Shaft (compression)
1,0
1,3
piles
1,1
1,4
Total/combined
(compression)
Shaft in tension
1,25
1,6
1,1
1,1
Anchors Temporary
Permanent
1,1
1,1
Retaining Bearing capacity
Sliding resistance
walls
Earth resistance
Earth resistance
Slopes

Easy to factor primary input

material strengths and actions
Difficult to factor geotechnical
indicates partial factorand
= 1.0 action effects
resistances

125

Design approach 2
DA2 - Comb 1
A1
M1
1,35

R2

1,5

Design approach 3
DA2 - Slopes
A1
M=R2
1,35
1,5

DA3
A1
1,35
1,5

A2

DA2
unsuitable for
numerical
analysis

R3

1,3

StructuralGeotech
actions actions

1,4
1,1
1,1
1,1
1,1
1,15
1,1
1,1
1,1
1,15
1,1
1,1
1,1
1,15
1,1
1,1
1,4
1,1
1,4

M2

1,25
1,25
1,4
1,4

1,1

1,1

1,1

1,1
C:\BX\BX-C\EC7\[Factors.xls]

25-Nov-06 17:26

8th Lumb Lecture

Eurocode 7 Good practice in geotechnical design

Use of numerical methods for ULS

126

Can numerical methods be used for all design approaches?

How should strength factors be applied?
Does FEM give the wrong failure mechanism?
Use of advanced soil models for ULS
Undrained behaviour and consolidation
K0 and soil stiffness
Staged construction

Fundamental limit state requirement

Ed Rd
E{

Fd ; Xd ; ad} = Ed Rd = R{ Fd ; Xd ; ad}

E{F Frep; Xk/M; ad} = Ed Rd = R{F Frep; Xk/M; ad}

or E{F Frep; Xk/M; ad} = Ed Rd = Rk/R = RnR (LRFD)

E Ek = Ed Rd = Rk/R

or
so in total

E E{F Frep; Xk/M; ad} = Ed Rd = R{F Frep; Xk/M; ad}/R

(a) Reduce strength, increase the loads, and check equilibrium
OR
(b) Find the remaining FOS?
OR
(b) c- reduction
127

 reduction?
Pre-factored strength, or c-
Max wall displacement 48mm

Large displacement
Max wall displacement 48mm

xbcap5-Dec11ab.sfd

= 1.25

= 1.45

= 1.25
= 1.45

MR;d

10m

BENDING
MOMENT
500
129

1000

1500 kNm/m

8th Lumb Lecture

Eurocode 7 Good practice in geotechnical design

Use of numerical methods for ULS

130

Can numerical methods be used for all design approaches?

How should strength factors be applied?
Does FEM give the wrong failure mechanism?
Use of advanced soil models for ULS
Undrained behaviour and consolidation
K0 and soil stiffness
Staged construction

Wrong failure mechanism?

Max wall displacement 48mm

Large displacement
Max wall displacement 48mm

xbcap5-Dec11ab.sfd

= 1.25

= 1.45

There is no right failure mechanism

Because failure isnt the right answer!

EC7 is interested in proving success, not failure.

Finding FOS useful for design refinement, but not for final verification.
Plastic models of structural elements useful in ULS analysis.
131

8th Lumb Lecture

Eurocode 7 Good practice in geotechnical design

Use of numerical methods for ULS

132

Can numerical methods be used for all design approaches?

How should strength factors be applied?
Does FEM give the wrong failure mechanism?
Use of advanced soil models for ULS
Undrained behaviour and consolidation
K0 and soil stiffness
Staged construction

Factoring advanced models

, c, cu not explicit parameters
eg Cam Clay, BRICK, Lade etc
Change to Mohr-Coulomb for the factored calculation?
If c= this is the code factor on drained strength, however derived.
Consider: is the models drained strength more or less reliable than
those used in conventional practice?
eg the model might take good account of combinations of principal
stresses, direction (anisotropy), stress level etc.
- Possibly modify factors in the light of this.

133

8th Lumb Lecture

Eurocode 7 Good practice in geotechnical design

Use of numerical methods for ULS

134

Can numerical methods be used for all design approaches?

How should strength factors be applied?
Does FEM give the wrong failure mechanism?
Use of advanced soil models for ULS
Undrained behaviour and consolidation
K0 and soil stiffness
Staged construction

Undrained strength in effective stress models

Reliable computation of undrained strength from effective stress
parameters is very difficult.
EC7 generally requires a higher factor on undrained strength (eg
1.4 on cu) than on effective stress parameters (eg 1.25 on c, tan).

135

cu/1.4 doubles bending moment when sensitive

136

Undrained strength in effective stress models

Reliable computation of undrained strength from effective stress
parameters is very difficult.
EC7 generally requires a higher factor on undrained strength (eg
1.4 on cu) than on effective stress parameters (eg 1.25 on c,
tan).
The drafters assumed that effective stress parameters would be
used only for drained states.
The higher factor (eg 1.4) was considered appropriate for
characteristic values of cu based on measurement, which is
generally more reliable than values computed from effective
stress parameters.
So it is unreasonable to adopt a lower value for the latter.
137

Time-dependent analysis
Beyond EC7!
Geotechnical category 3

138

8th Lumb Lecture

Eurocode 7 Good practice in geotechnical design

Use of numerical methods for ULS

139

Can numerical methods be used for all design approaches?

How should strength factors be applied?
Does FEM give the wrong failure mechanism?
Use of advanced soil models for ULS
Undrained behaviour and consolidation
K0 and soil stiffness
Staged construction

Ko
In reality, K0 is not a simple function of soil strength (').
So it is not sensible, and not a Eurocode requirement, to factor
K0 or vary it as a function of '. In situ stresses are taken as a
separate parameter an action.

140

Soil stiffness
CIRIA Report C580 recommends that stiffness should be
reduced (halved) for ULS analysis. No other publication has a
similar requirement.

The reason for this was that

larger strains may be mobilised
in ULS analyses it was not an
additional safety margin.

This reasoning may apply to Strategy 1, but not so clearly to

Strategy 2 since, in many cases, most of the displacement has
already taken place when the strength is reduced. If the soil is
close to failure, stiffness will not be important.
So reduction of stiffness for ULS analysis is not recommended.
141

8th Lumb Lecture

Eurocode 7 Good practice in geotechnical design

Use of numerical methods for ULS

142

Can numerical methods be used for all design approaches?

How should strength factors be applied?
Does FEM give the wrong failure mechanism?
Use of advanced soil models for ULS
Undrained behaviour and consolidation
K0 and soil stiffness
Staged construction

ULS for staged construction single propped example

Strategy 1
Compute using
factored strength
Factor material
strengths

Strategy 2
Compute using unfactored
parameters parameters
characteristic

Compute using
factored parameters

Initial state

Initial state?

143

Excavate to 5m
wall cantilevering

Excavate to 5m
wall cantilevering

Install prop at 4m
depth

Install prop at
4m depth

Excavate to 10m

Excavate to 10m

No further factors
on strut forces or
BMs

Apply factors on
strut forces or
BMs

Could be critical for wall

bending moment

Could be critical for wall

length, bending moment
and prop force
No further factors
on strut forces or
BMs

Florence Rail Station

- 25m deep, 50m wide,
550m long
- Mezzanine level prop
- High groundwater level

Simpson, B and Hocombe, T

(2010) Implications of modern
design codes for earth retaining
structures. Proc ER2010,
ASCE Earth Retention
Conference 3, Seattle, Aug
2010.

151

Eurocode case study: High speed rail station, Florence, Italy

152

454m long, 52m wide and 27 to 32m deep

1.2 to 1.6m thick diaphragm walls
Three levels of temporary strutting.

Eurocode case study: High speed rail station, Florence, Italy

153

SLS analyzed as if London Clay using the BRICK model.

Time dependent swelling and consolidation.
Eurocode 7, DA1, Combinations 1 and 2 analysed using FE and Oasys FREW.

Eurocode case study: High speed rail station, Florence, Italy

154

Eurocode 7 readily used with FE for this large project.

Geotechnical and structural design readily coordinated.

Partial factors for DA1 - UKNA

Design approach 1
Combination 1--------------------Combination 2 ------------------------Combination 2 - piles & anchors
M1 or ..M2
R4
A1
M1
R1
A2
M2
R1
A2
Permanent
unfav
1,35
Actions
fav
Variable
unfav
1,5
1,3
1,3
1,25
1,25
tan '
Soil
Effective cohesion
1,25
1,25
Undrained strength
1,4
1,4
Unconfined strength
1,4
1,4
Weight density
Bearing
Spread
UKNA
Sliding
footings
Base
1,7/1.5
Driven
Shaft
(compression)
1.5/1.3
piles
1.7/1.5
Total/combined
(compression)
Shaft in tension
2.0/1.7
Base
2.0/1.7
Bored
Shaft (compression)
1.6/1.4
piles
Total/combined
2.0/1.7
(compression)
Shaft in tension
2.0/1.7
Base
As
CFA
Shaft (compression)
for
piles
bored
Total/combined
(compression)
Shaft in tension
piles
Temporary
1,1
Anchors
Permanent
1,1
Retaining Bearing capacity
Sliding resistance
walls
Earth resistance
Earth resistance
Slopes

indicates partial factor = 1.0

C:\BX\BX-C\EC7\[Factors.xls]

155

EC7
values
1,3
1,3
1,3
1.6
1,6
1,3
1.5
1.6
1.45
1.3
1.4
1.6
1,1
1,1

MSNA
1.87/1.65
1,7/1.5
1.65/1.43
1.5/1.3
1.87/1.65
1.7/1.5

2.0/1.7
2.20/1.87
2.0/1.7
2.20/1.87
1.6/1.4
1.76/1.54
2.0/1.7
2.20/1.87
2.0/1.7
2.20/1.87
As
for
bored
piles
1,1
1,1

ULS for staged construction single propped example

Strategy 1
Compute using
factored strength
Factor material
strengths

Strategy 2
Compute using unfactored
parameters parameters
characteristic

Compute using
factored parameters

Initial state

Initial state?

156

Excavate to 5m
wall cantilevering

Excavate to 5m
wall cantilevering

Install prop at 4m
depth

Install prop at
4m depth

Excavate to 10m

Excavate to 10m

No further factors
on strut forces or
BMs

Apply factors on
strut forces or
BMs

Could be critical for wall

bending moment

Could be critical for wall

length, bending moment
and prop force
No further factors
on strut forces or
BMs

Florence Station comparison of bending moments

Prop and excavation levels
Stage 1
2

45

40

Level (m)

35
30
25

C2, Strategy 1

20
C2, Strategy 2

15
10
5
-6,000

-4,000

-2,000

2,000

4,000

6,000

8,000

Bending Moment (kNm/m)

C1

C1

C1
C1

C2, factors all stages (Strategy 1) C2, factors all stages

C2, factors all stages
C2, factors all stages
C2,factoring stages separately (Strategy
C2, factoring
stages separately
2)

157

Summary numerical analysis

FEM analysis of SLS is conventional nothing new.
FEM can also be used for ULS
Design Approach 1 is well suited to this.
Difficult to distinguish favourable and unfavourable actions from
the ground the star approach for these.
The code requirement is best checked by applying fixed factors to
strength method (a).
c- reduction might be useful for design refinement method (b).
Plastic modelling of the structure would be beneficial.
When advanced soil models are used, it may be best to switch to
Mohr-Coulomb for the ULS check.
158

Summary numerical analysis

Great care is needed in modelling undrained situations using
effective stress parameters requires a good advanced model.
The full value of Cu should be applied for undrained materials.
Factoring of K0 and stiffness is not recommended.
Strategy 2 applying factors to stages individually is
recommended.
-

Analyse DA1-1 first, then check critical stages for DA1-2.

Computing effort might be reduced if stages for which DA1-2 is critical
can be established for a given range of situations.

EC7 Evolution Group

159

8th Lumb Lecture

Eurocode 7 Good practice in geotechnical design

Limit state design
Holistic design structures and ground
Practical approach to characteristic values
of soil parameters
ULS and SLS design requirements
Water pressures
Ground anchors
Retaining structures numerical analysis
The future

The future
Evolution groups => extensive revisions of most sections
About to start re-drafting for 2020(?)
Reorganised into three parts: General, Testing, Specific elements
Harmonisation simplifying the Design Approaches
Consequence classes variations to partial factors (1.25 1.2?)
Additional sections
- Reinforced ground
- Ground improvement
- Rock mechanics
Numerical analysis section or sub-section

161

8th Lumb Lecture

Eurocode 7 Good practice in geotechnical design

Limit state design
Holistic design structures and ground
Practical approach to characteristic values
of soil parameters
ULS and SLS design requirements
Water pressures
Ground anchors
Retaining structures numerical analysis
The future

Thanks for
listening