Part D The way ahead
D3 RESEARCH AND DEVELOPMENT NEEDS
Careful study of EC7-1 raises many questions, and research needs could be found in many of its clauses. Most of these needs, however, are not special to EC7 but merely reflect the levels of knowledge and uncertainty which characterise geotechnical design. A selection of research needs which the use of EC7 will particularly emphasise is provided here.
D3.1 Application of partial factors
It was noted in D2.2 that an acceptable scheme of applying partial factors is still not agreed. Although this debate might temporarily be curtailed by the production of the Euronorm, it will doubtless recur until broad agreement is found. Engineers with practical knowledge of geotechnical design, together with a broad understanding of the purposes of factors of safety and the options for their implementation should be involved in this debate.
The continuing study of both successful geotechnical designs and failures could form the basis of useful research in this area. In particular, a clear understanding is needed of the calculated factors of safety of structures as constructed, noting that the constructed structure often incorporates elements of safety which are not included in the minimum design calculated to a code.
In this study, both ULS and SLS failures should be considered, together with the relationship of these to factors of safety.
D3.2 Serviceability and deformations
The limit state approach requires more explicit consideration of serviceability limit states than has been the case previously. In part, this implies that the geotechnical profession must improve its ability to calculate deformations.
EC7 therefore provides strong encouragement for continuing research into the deformation properties of soils, and the numerical methods needed to use these.
It should not be inferred, however, that EC7 encourages heavy numerical analysis where it is not needed. Geotechnical understanding of deformations is based mainly on case histories, leading either to simple empirical rules or to more complex back-analysis. The collection, categorisation and simple interpretation of case histories remains of paramount importance.
The relationship between deformation and mobilised strength requires further understanding. This is expressed in the ‘mobilisation factor’ of BS 8002.
D3.3 Statistics and probability methods
Across Europe, there is considerable interest in the application of statistical methods to geotechnical analysis. The training of British engineers is probably inferior to that of their European counterparts in this respect.
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. The development of a research programme in this area is therefore to be encouraged.
Topics to be considered could include:
a statistical variations of common material properties, including study of their standard deviations and the significance of extreme values;
b the use of statistical methods in deriving characteristic values for material parameters;
c the relationship of factors of safety to probability of failure, and the use of reliability indices, β.
D3.4 Economy of design
The original purpose of the Eurocodes was to facilitate trade and fair
competition in Europe. Studies will be needed to check whether this is being achieved, both before the Eurocodes become influential and as they become more dominant. Emphasis on the economy of design achieved in the various nations will be of particular importance.
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EUROCODE 7: A COMMENTARYTable D3.1 Progress with European NADs and comments on EC7-1 (ENV 1997)
Country Position with ENV, NAD Specific issues/problems
Austria NAD published in 1996 Problems with errors in German translation. The 8 Austrian standards do not use γfactors but are being converted to EC7 design philosophy. Use Case B for structures, Case C for geometry. Same γon c′and tanφ′
Belgium Completion expected by end 1997. NAD may For characteristic value will propose in NAD ‘representative mean value’ with include specific design method from CPT results table of pessimistic default values. Prefer low γFand high γm. Support
Cases A, B, C. Want γmodel. As for Switzerland, those who use EC7 indicate
‘interesting and easy’, while those who do not indicate ‘difficult’
Czech Republic Available in Czech and on sale. Legal problems, with Have used LSD for many years; taking characteristic values from existing no Govt ministry taking overall responsibility. Likely standards, leading to reduced γon loads. Does EC1 Table 9.2 apply only to to adopt lower factor on pile resistance (ENV Tables buildings? Term ‘Geotechnical design’ not understood
7.1, 7.2 and 7.3) and reduced γGin Table 2.1 (eg from 1.5 to 1.4, 1.35 to 1.2 and 1.3 to 1.2)
Denmark NAD published. Updating Danish code to EC7 Some problems with Case B for retaining walls – leave out Case B?
(by end 1997?) – the ‘Danish Eurocode’!
Finland Translation and NAD published; use is ‘rare’. Available Too many Cases; sometimes conservative; problems of interpretation with in English (see Table D3.2 for details of NAD) Chapter 7. How to deal with variable water levels?
France ENV & NAD published; NAD is ‘short’; legal problem: Cannot calibrate to existing practice because of ambiguity of characteristic all public works contracts must refer to French value definition; therefore new definition of characteristic value required, standards only eg ‘nominal value’. Unify Cases B, C? AFNOR mirror committee meets every
3 months; a testing committee of 20 people meets 10 times per year!
Germany Commentary published; calculation examples to be Over 500 calibrations point to need for ‘safety classes’ with γon tanφvarying published. See Table D3.3 for details of NAD from 1.2 to 1.3. Want other specific changes including omission of 5% in
characteristic value definition
Greece Translation complete; draft NAD in circulation but No use of EC7 except for Athens Metro, with mixing-up of Cases B, C?
Govt approval not received. NAD does not change γ Greeks writing a guide book. EC1 limited to buildings values and includes 50 pages of calculation methods
Holland No translation yet. NAD ready but ‘legal problems’; Trial calculations indicate problems with sheet pile wall design includes calculation methods. Some use by
international contractors
Ireland NAD published but need EN to encourage use. In Concern about Cases B, C for retaining walls NAD, Paragraph 2.4.6(7): Total settlements limited
to 25 mm not 50 mm. Adopts [boxed values] used in ENV. Paragraph 6.6.1(2) modified to read: Normally, this depth may be taken as the greater of 1.5 times the width of the footing or the depth at which the effective vertical stress due to the foundation load amounts to 20% of the effective overburden stress
Italy Translation of EC7-1 completed; distribution during Some problems achieving Ministry Public Works recognition 1997? Not known who will write NAD. No interest
in Italy?
Norway Translation complete; draft NAD complete Problem with characteristic value definition; like ‘National Annexes’ concept during 1997?
Portugal Translation and NAD complete. Boxed γ’s retained EC7 not sufficient on rocks. National Regulations being developed Slovakia Translating EC7; completion and NAD expected Only small differences between national standards and EC7, usually
during 1997? concerning ‘execution’. γ’s similar. Term ‘Geotechnical design’ not understood Spain Translation published. NAD published – very short Emphasise importance of geological models as opposed to calculations.
and boxed γ’s retained Cannot use EC7 to design. Important ground, eg rocks, partially-saturated soil not sufficiently covered
Sweden Will complete translation in 1997. NAD will have only Still testing Eurocode and NAD: results showing ~15% increase in costs of one case, not three cases A, B, C. Will have safety shallow foundations
levels
Switzerland NAD in German (not published); rejects section on EC7 ‘OK’ for those who have tried, not ‘OK’ from those who haven’t Anchors
Table D3.2 Details of the German National Application Document Features
Makes extensive reference to the following DIN documents:
● DIN V 1054-100 Calculation of partial safety factors used in earthworks and foundation engineering
● DIN V 4017-100 Calculation of design capacity of shallow foundations using the concept of partial safety factors
● DIN V 4019-100 Settlement calculations using the concept of partial safety factors
● DIN V 4084-100 Calculation of slope and terrain rupture using the concept of partial safety factors
● DIN V 4085-100 Calculation of earth pressures using the concept of partial safety factors
● DIN 4126-100 Design of diaphragm walls using the concept of partial safety factors Uses three combinations of actions and three safety classes, grouped into three load cases Replaces EC7, Table 2.1 with Table 3, taken from DIN V 1054-100
EC7, 3.2: introduces the use of a ‘geotechnical expert’ for Geotechnical Category 3 and (usually) GC 2 situations
Supplements EC7, 3.2.3: ‘Design investigations’ are introduced EC7, 4.2.2: requires a construction log for GC 2 and 3
EC7, 6.3: a national standard on the interaction of stiff structures and ground is envisaged EC7, 6.4: in Germany, ground is regarded as frost-free below 0.8 m
EC7, 7.4.1: ‘analytical calculation methods’ are excluded
EC7, 7.7.2.1: reference is made to Arbeitskreis ‘Baugruben’ (EB 62) (Excavation Working Group) for the interaction of grouped piles in tension
EC7, 8.3.2.2: additional safety margins apply
EC7, 8.5.1: the only differentiation between ULS design earth pressure and SLS design earth pressure is in the use of different γvalues
EC7, 8.5.4: DIN V 4085-100 gives more detailed specifications
Table D3.3 Partial safety factors for actions
ULS Action Symbol Load Case Load Case Load Case
1 2 3
1A Permanent, unfavourable γGsup 1.00 1.00 1.00
Permanent, favourable γGinf 0.90 0.90 0.95
Liquid Pressure γF 1.00 1.00 1.00
Variable, unfavourable γQsup 1.50 1.00 1.00
1B Permanent, unfavourable γGsup 1.35 1.20 1.00
Permanent, favourable γGinf 1.00 1.00 1.00
Liquid Pressure γF 1.35 1.20 1.00
Variable, unfavourable γQsup 1.50 1.30 1.00
Perm. Transverse Pressure γH 1.35 1.20 1.00
Perm. Shaft Resistance γM 1.35 1.20 1.00
Perm. Earth Pressure γEg 1.35 1.20 1.00
Variable Earth Pr, unfav γEq 1.50 1.30 1.00
Earth Pr. at rest, perm γE0g 1.20 1.10 1.00
Earth Pr. at rest, var, unfav γE0q 1.35 1.20 1.00
1C Permanent γG 1.00 1.00 1.00
Liquid Pressure γF 1.00 1.00 1.00
Variable, unfavourable γQsup 1.30 1.20 1.00
Perm. Transverse Press. γH 1.00 1.00 1.00
Perm. Shaft Resistance γM 1.00 1.00 1.00
Perm. Earth Pressure Variable Earth Pressure
2 Permanent – 1.00 1.00 1.00
Variable 1.00 1.00 1.00
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EUROCODE 7: A COMMENTARYTable D3.4 Partial safety factors for soil resistances
ULS Soil Resistance Symbol Load Case Load Case Load Case
1 2 3
1B Passive Earth Pressure γEp 1.40 1.30 1.20
Bearing Capacity γS 1.40 1.30 1.20
Sliding Capacity γSt 1.50 1.35 1.20
Piles, axial γP 1.40 1.20 1.10
Injection anchors γA 1.10 1.10 1.10
Soil nails γN 1.20 1.10 1.05
Flexible reinforcement γB 1.40 1.30 1.20
1C tanφ γφ 1.25 1.15 1.10
c′ γc 1.60 1.50 1.40
cu γcu 1.40 1.30 1.20
Piles, axial γP 1.60 1.40 1.20
Injection anchors γA 1.30 1.20 1.10
Soil nails γN 1.30 1.20 1.10
Flexible reinforcement γB 1.40 1.30 1.20
Table D3.5 Details of the Finnish National Application Document Features
NAD + SFS-ENV 1997-1:1994 presents alternatives to the Collection of Finnish Construction Regulations (B1, B3)
Adds specific recommendations for highway structures
Paragraph 2.1(5): use of Geotechnical Categories (GCs) may become a Principle, not an Application Rule
Paragraph 2.1(2)P: attention to specific Finnish geological and climatic conditions Paragraph 2.1(5): further definition of the GCs for Finnish use
Paragraph 2.4.2(10)P: for stability analysis, assume 50-year worst case ground water level; for settlement analysis, use average maximum and minimum values
Paragraph 2.4.2(14)P–Table 2.1: Case C γvalues changed as follows: cu– 1.55, qu– 1.6;
furthermore, γφand γccan be decreased by up to 10% for transient loading where risk of material damage is minor, and shall be increased by 10% where risk of injury or material damage is large.
Similarly, γcuand γqucan be decreased by up to 15% and shall be increased by 20%
Paragraph 2.4.3(5)P and (6): the definition of characteristic value is changed somewhat and the reference to the use of statistical methods is removed
Paragraph 2.4.6(7): additional limiting values of rotation are quoted for structures of different materials
Paragraph 3.2.3(9)P: calls for closer spacing of exploration points than does the ENV Calls up Finnish site investigation, soil and rock classification and testing documentation Clause 3.3: introduces Weight Sounding tests
Paragraph 3.4.1(1)P: the presentation system in publication SGY 201 is used Paragraph 3.4.1(2): ‘radon’ and ‘frost susceptibility’ added
Section 5: several constraints and amendments applied to the use and properties of fill materials Clause 6.2: several additions concerning frost heave and its avoidance
Paragraph 6.6.1(2): additional text on compressible and organic soils and ground water level change
Paragraph 7.3.2.2(2): Addition: negative skin friction and transient actions need not be considered simultaneously
Paragraph 7.4.2(4)P: Add:’Handling and transport of piles’
Paragraph 7.4.2(5): Pile design classified as GC 2 or 3
Paragraph 7.6.3.2(6)P, Table 7.1: replaced by an extensive set of factors according to number of loaded piles, whether tests are static or dynamic
Paragraph 7.6.3.3(4)P: factor value changed to 1.6 Paragraph 7.6.3.3(9)P: introduces a plugging coefficient
Paragraph 7.6.3.4(1)P: specifies minimum values for the product ξx γt
Paragraph 7.7.2.3(3): Addition: Characteristic tensile resistance of tensile piles ... assessed from characteristic compression resistance of ... shaft ÷ 2 (long-term) or ÷ 1.6 (short-term) Paragraph 7.9(5): delete comment on established practice
Paragraph 7.10(5)P: Storage period for pile records and other documents at discretion of builder/client; as-built drawings stored for service life
Section 8: extensive reference to RIL 181 and RIL 194
Paragraph 8.3.2.1(2): Additional requirement: ... a level surface load of at least q = 10 kN/m2 should be assumed in allowance of over-fill ... etc
Paragraph 8.3.2.2(1)P: specific guidance on determination of design water level, with γvalues related to duration of period of water level observation
Paragraph 8.8.5(6)P: changes γvalue from 1.25 to 1.3 for temporary anchorages Paragraph 9.2(1)P: warns of frost melting
Paragraph 9.3(3)P: ... most unfavourable value occurring during the service time, or the value recurring once during 50 years, is selected as the characteristic value ...
Annexes: ... little experience ... of ... some ... in Finnish conditions ... should calibrate ... to the Finnish practice ... (without) sufficient information and earlier experience ...
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EUROCODE 7: A COMMENTARYTable D3.6 Details of the Irish National Application Document Features
Paragraph 2.4.2(15): For ease of application, a table of öd values is provided for Cases A, B and C Paragraph 2.4.6(7): total settlements for normal strip and pad footings are limited to 25 mm not the 50 mm in EC7
Paragraphs 3.1(3), 3.2.3(6)P and 3.3.2(2)P: Reference Standards: Relevant documents are BS 1377 and BS 5930
Paragraphs 5.3.2(1)P and 5.3.4(2): Proctor density to be derived using Method 3.4 of BS 1377-4
Paragraph 6.4(2)P: width of foundation should not be less than that specified in Part E of Technical Guidance Document A of the Building Regulations
Paragraph 6.6.1(2), fourth sentence, amended to read: Normally, this depth may be taken as the greater of 1.5 times the width of the footing or the depth at which the effective vertical stress due to the foundation load amounts to 20% of the effective overburden stress
Paragraph 7.6.3.3(4): a clarifying statement is made, similar to the U.K. NAD Paragraphs 8.3.2.1(1) and 8.3.2.1(2) are both to be satisfied
Paragraph 8.6.6(4) (and 2.4.2(15)): a model factor of unity to be used for Cases A, B and C