Download CIRIA C641 EC7 – Implications for UK Practice PDF

TitleCIRIA C641 EC7 – Implications for UK Practice
TagsGeotechnical Engineering Civil Engineering Solid Mechanics Nature
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Page 1

CIRIA C641 London, 2008

EC7 – implications for
UK practice

Eurocode 7 Geotechnical design

Richard Driscoll BRE

Peter Scott Buro Happold

John Powell BRE

Classic House, 174�180 Old Street, London EC1V 9BP
TEL: +44 (0)20 7549 3300 FAX: +44 (0)20 7253 0523

EMAIL: [email protected] WEBSITE: www.ciria.org

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Page 2

Summary

The introduction of the Eurocodes represents for most civil and structural engineers a
significant challenge in adapting to a very extensive set of new design and construction
requirements. This is particularly so for geotechnical engineers in that Eurocode 7 and
its associated new standards present some profound departures from traditional
practice. The aim of this publication is to provide geotechnical engineers with an
understanding of how the new documents will affect their day-to-day activities. Much
information on the detail of the new Eurocode system already exists, so this book
focuses on changes to common practice and their implications.

The book takes the reader through a logical sequence of activities, from site and
ground investigation to geotechnical element design, to construction practices
introduced by the new European Execution Standards. It then concludes with an
indication of the likely timing of full implementation and a prediction of the effect that
the changes will have on geotechnical practice in the UK.

The book seeks to give a clear overview of the main changes that will arise, adding in
appendices such detail of the Eurocode system that is necessary to understand these
changes. It illustrates the changes with a set of design examples covering mainstream
design challenges such as piles, retaining walls, embankments and slopes, and hydraulic
failure.

The book is authored by three specialists who have worked closely with the
development and introduction of Eurocode 7 and its application in the design office,
and the content has been carefully criticised by a panel of leading UK geotechnical
practitioners.

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Page 70

Clauses

2.4.7.3.3(1)

2.4.7.3.4.2

(1) & 9.2

Table A4

Table A.NA.4

Table A.NA.4

9.5.1 (6)

9.5.1 (7)

9.5.1 (6)

Fig C.1.1

Fig C.2.1

[1]

Table A.NA.3

NA.A1.2(B)

(Permanent

Unfavourable)

[2]

1
st
trial, d = 14.0m

Stability

Design Approach 1

Combination 1: A1 “+” M1 “+” R1

Design Soil Parameters (M1)

Weight density

= 1.0

Design d1 for gravel = 20/1 = 20kN/m³

Design d1 for clay = 18/1 = 18kN/m³

Angle of shearing resistance

’ = 1.0

Design tan ’d1 for gravel = (tan36 )/1 = tan36

Design tan ’d1 for clay = (tan25 )/1 = tan25

Effective cohesion

c’ = 1.0

Design c’d1 for gravel = 0kPa

Design c’d1 for clay = 5/1 = 5kPa

Soil/Wall interface,

Based on 9.5.1 (6), d = K. cv;d and cv;d = 33 (for gravel) and

cv;d = 25 (for clay).

Based on 9.5.1 (7), for concrete cast against soil, K = 1.0

Design d1 for gravel = 33

Design d1 for clay = 25

Coefficient of horizontal active earth pressure

For gravel, Ka = 0.22 ( d1/ ’d1 0.9)

For clay, Ka = 0.34 ( d1/ ’d1 1)

Coefficient of horizontal passive earth pressure

For gravel, Kp = 8.9 ( d1/ ’d1 0.9)

For clay, Kp = 4.0 ( d1/ ’d1 1)

Design Loads (A1)

Water Pressure at the back of the wall (A1)

Uw(z) = HT = (H1 + H2)/2

= (176.58 + 127.53)/2 =152kPa (at the toe of

the wall)

Lateral Design Water Load at the back of the wall (A1)

A lateral water load at the back of the wall is an unfavourable action as it

causes the wall to have a greater tendency to overturn.

G = 1.35

Ud1;a = 1.35 x {(19.62 x 2 x 0.5) + (19.62 x 16) +

[(152 – 19.62) x 16 x 0.5]}

= 1.35 x [(19.62) + (313.92) + (1059.04)]
Ud1;a = 1880

kN/m run

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Page 71

Annex C

Eqn C.1

Table A.NA.3

NA.A1.2(B)

(Permanent

Unfavourable)

[3]

Table A.NA.3

NA.A1.2(B)

(Permanent

Unfavourable)

[4]

Annex C

Eqn C.2

Active lateral earth pressure at the back of the wall (A1)

a(z) = Ka ( .z + q) – 2c Ka (in this case q = 0)

Lateral Characteristic Earth Pressures (A1)

’a1 (z) = Ka [( d x z) – (Uw(z))] – 2c Ka
’a1 (0) = 0kPa

’a1 (2) = 0.22 x (20 x 2) – 2 x 0 x 0.22 = 8.8kPa

’a1 (4) = 0.22 x [(20 x 4) – 19.62] – 2 x 0 x 0.22

= 13.3kPa

’a1 (4
+) = 0.34 x [(20 x 4) – 19.62] – 2 x 5 x 0.34

= 14.7kPa

’a1 (6+14) = 0.34 x {(20 x 4) + [18 x (20 – 4)] – 152)] – 2 x 5

x 0.34

= 67.6kPa

Lateral Design Earth Load (A1)

A lateral earth pressure is an unfavourable action as it causes the wall to

have a greater tendency to overturn.

G = 1.35

H’d1;a = 1.35 x {(8.8 x 2 x 0.5) + (8.8 x 2) + [(13.3 – 8.8) x 2 x

0.5] + (14.7 x 16) + [(67.6 – 14.7) x 16 x 0.5)}

= 1.35 x (8.8 + 17.6 + 4.5 + 235.2 + 423.2)

Water Pressure at the front of the wall (A1)

Uw(z) = HT = 152kPa (at the toe of the wall)

Lateral Design Water Load at the front of the wall (A1)

In examples 4 and 6 safety margins are applied to water levels, so

that through all stages of the calculations water pressures remain

unfactored. The alternative method shown in this example entails two

concepts being adopted:

1 Water pressures are factored (Clause 2.6.4.1(8))

2 Factors applied to water pressures on both sides of the retaining

wall are the same (2.4.2 (Note)). This is treating water as a “single

source” – a concept not fully explained in the code.

G = 1.35

Ud1;p = 1.35 x [HT x (14 – 1) x 0.5]

= 1.35 x 152 x 13 x 0.5kN/m run

Passive lateral earth pressure at the front of the wall (R1)

p(z) = Kp ( .z + q) + 2c Kp (in this case q = 0)

Lateral Characteristic Earth Pressures (R1)

H’d1;a =

930.56kN/m

run

Ud1;p =

1333.8kN/m

run

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Page 139

92 Although EC7-1 does not require this, it may be more sensible to check that:

Vtemp ≤ Gtemp + Rtemp

Vperm ≤ Gperm + Rperm

93 As has been said already, BSs make recommendations using the word “should” while
“Principle” clauses in the “execution” BS ENs use the word “shall”.

94 Indeed, this is a CDM requirement anyway.

95 The number of pages in a standard is only an indication of level of detail.

96 1 Scope.
2 Normative references.
3 Terms, definitions and symbols.
4 Specific needs.
5 Site investigation.
6 Materials and products.
7 Design considerations.
8 Execution.
9 Testing, supervision and monitoring.
10 Records.
11 Special requirements.

Annexes.

97 The following text is adapted from the National strategy for implementation of the
structural Eurocodes: Design guidance, a report prepared for The Office of the Deputy
Prime Minister by The Institution of Structural Engineers, April 2004.

98 See ODPM Simple guide (Driscoll et al, 2005).

99 Note that the UK National Annex was published in 2007.

100 For testing standards, withdrawal of BS documents has to be carried out within six
months of publication of the equivalent BS EN.

101 It will be interesting to see if clients will be willing to pay for better quality.

102 It is considered likely that statistical methods, well applied, could add to the
geotechnical profession’s understanding of uncertainty and safety in design. At worst,
it is important that geotechnical engineers ensure that their work is not damaged by
spurious, but plausible, uses of statistics which they are unable to challenge through
lack of knowledge.

103 It should not be inferred however that BS EN 1997-1 encourages reliance on
numerical analysis where it is not needed. Our understanding of deformations is based
mainly upon case histories, leading either to simple empirical rules or to more
complex back-analysis. The collection, categorisation and simple interpretation of case
histories will remain of paramount importance.

104 The following text has been adapted from IStructE, 2004.

105 Alternatively, he could require an inspection and probing at each footing location, so
avoiding this most conservative assumption.

106 Annex A of BS EN 1997-1 contains 17 tables of sets of factor values to cover the three
design approaches, the different limit states, EQU, GEO, STR, UPL and HYD and
different geotechnical elements.

107 Some explanation of DAs 2 and 3 is given in the informative Annex B of BS EN 1997-
1, while additional discussion and comparisons of results using the 3 DAs may be
found in Frank et al (2004).

108 For example, when, to calculate the action resulting from water pressure, a factor
greater than 1 is applied to the known depth of a free water surface, leading to a
design value of the free water surface that would be above ground level, a physically
impossible situation. In such cases, partial factors γE are applied to the effects of the
action using expression 2.6b.

109 This is akin to the ENV requirement that both Cases B and C were in principle
required to be checked.

110 For readers familiar with ENV 1997-1, Combination 1 is the old Case B.

111 These are the action factor values given by BS EN 1990.

112 In Annex A of BS EN 1997-1, R1 takes values of 1. These are not shown in Table
A3.1, except for piles and anchorages for which the R values are > 1.0, except for
driven piles.

113 For readers familiar with ENV 1997-1, Combination 2 is the old Case C.

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Page 140

114 In fact, R1 takes the value 1 but, for simplicity, this is not shown in Table A3.1

115 In most instances, Case A would not be applicable and the need to carry out two
checks, for Cases B and C, would be eliminated if the result was obvious in advance.
However, in some specific instances, it would not be immediately obvious which of
Cases B and C took precedence and two checks would be required. For a discussion
on the Cases A, B and C in the ENV 1997-1, see Simpson & Driscoll (1998).

116 Department for the Environment, Transport and the Regions.

117 The published and draft BS ENs that were examined for conflict are shown below
with the BS codes with which they were compared.

118 EN 1538 tolerances relate to diaphragm walls constructed with guide walls, whereas
BS 8002 remarks that it is not usual practice to form guide walls for bored pile walls.

119 This change could impact on data used in correlations. BS EN 1536 states that other
rates may be agreed.

120 See BS EN 1997-1 Clause 7.6.2.2(4).

121 Note that this number applies for the ENV version of EC7-1. In BS EN 1997-1 it
reduces to 1.37 (1.05 × 1.3), for 3 pile tests, using the recommended partial factor
values for DA-1 Combination 2 in Table A.7 and the correlation factors in Table A.9.
The value reduces to 1.3 (1 × 1.3) for five or more pile tests.

122 A tacit assumption in directly comparing combinations of partial factors with an
equivalent overall (or lumped) factor of safety is that the characteristic strength
adopted for use with the former is the same as the strength adopted when applying
the latter.

123 Note that this number applies for the ENV version of EC7-1. In BS EN 1997-1 the
recommended factor values would give 1.56 (1.2 × 1.3) for mean shaft resistance and
1.92 (1.2 × 1.6) for mean base resistance, assuming three pile tests. Even greater
reductions result for five and more pile tests (see Table A.9).

CIRIA C641126

BS EN or draft EN (“pr…”) Subject Equivalent BS

BS EN 1536 Bored piles BS 8004, BS 8008

BS EN 1537 Ground anchors BS 8081, BS 8002

BS EN 1538 Diaphragm walls BS 8002, BS 8004

BS EN 12063 Sheet pile walls BS 8004, BS 8002

BS EN 12699:2001 Displacement piles BS 8004

BS EN 12715 Grouting BS 8004

BS EN 12716:2001 Jet grouting BS 8004

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