Consideration of Rock Engineering

in Eurocode 7 (EN 1997)

Current state of the amendment

Herbert Walter

Member of Project Team 2 of WG1 / TG2

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▪Development of the Eurocodes, organization, time line etc.

▪Current stage of the development of EN 1990 and EN 1997

▪Reliability system (target reliability, verification methods etc.)

▪Consideration of rock mechanics and rock engineering in current drafts of EN 1990:2020 and EN 1997:2020

▪Controversial issues

Remark: All code text presented is draft and subject to change!

Consideration of Rock Engineering in Eurocode 7 (EN 1997)

Overview

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▪ M/515 “Mandate for amending existing Eurocodes and extending the

scope of structural Eurocodes” by the European Commission

▪ Requirements according to the Mandate:▪ Rules for assessment and strengthening of existing structures

▪ Rules for robustness

▪ Reduction of the number of Nationally Determined Parameters

▪ Improvement of ease of use

▪ Incorporation of recent developments

▪ …

Ref.: http://ec.europa.eu/growth/tools-databases/mandates/index.cfm?fuseaction=search.detail&id=523# (30. 9. 2017)

Mandate M/515 Amending existing Eurocodes and extending the scope of structural Eurocodes

2nd Generation of EN Eurocodes – Mandate M/515

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▪ CEN (European Committee for Standardization) / Technical Committee TC 250▪ WG 7 (EN 1990): Basis of structural design Convenor: P. Formichi

Secretary: Aiva Kukule (Latvia)

▪ SC 7 (EN 1997): Geotechnical design Convenor: Andrew Bond

Secretary: Mark Lurvink (Netherlands)

▪ 4 overlapping phases, start 2015, end 2020?

(WG7 now SC10)

Mandate M/515 Amending existing Eurocodes and extending the scope of structural Eurocodes

Implementation of Mandate M/515

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TC 250 Structural Eurocodes - Organization

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Timeline of Eurocode development

Courtesy: Andrew Bond

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▪ Phase 1: EN 1990 WG7.T1 “Evolution of EN 1990 – General”

(2015-2018) …

EN 1997 SC7.T1 “Harmonization and ease-of-use” (Project Team 1, Task Group 1)

EN 1997 SC7.T2 “General rules” (Project Team 2, Task Group 2)

…

▪ Phase 2: EN 1997: SC7.T3 “Ground Investigation” (PT 3, WG2/TG1 “Reorganization”)

(2017-2020) EN 1997: SC7.T4 “Foundations, slopes and ground improvement” (PT 4, WG3/TG1 “slopes”, WG3/TG2 “spread foundations”, WG3/TG3 “pile foundations”, WG3/TG7 “Ground improvement”)

EN 1997: SC7.T5 “Retaining structures, anchors and reinforced ground” (PT 5, WG3/TG4 “Retaining structures”, WG3/TG5 “Anchors”, WG3/TG6 “Reinforced soil”)

…

▪ Phase 3: EN 1997: SC7.T6 “Rock mechanics, dynamic design” (PT 6, WG1/TG3 “Rock mechanics”,(2018-2020?) WG1/TG4 “Dynamic design”)

▪ Phase 4 (2018-2020?): e.g. Harmonization between EN 1992, EN 1993 and EN 1994

Phases of EN 1997 draft

Phases

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SC 7 – Organizational structure 2016 - onwards

Courtesy: Andrew Bond

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▪ 3 parts:

▪EN 1997-1 General Rules

▪EN 1997-2 Ground Investigation

▪EN 1997-3 Geotechnical Constructions

Several categories of Eurocode users -primary target audience:Practitioners – competent engineers

Decision by TC 250 for 2nd generation of Eurocode 7

New structure of EN 1997

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▪ Types of paragraphs:▪ <REQ> “shall” - mandatory requirements▪ <RCM> “should” - recommendations▪ <PER> “may” - permission▪ <POS> “can” - possibility, option▪ Note: e.g. explanations, reference to other standards

▪ Annexes:▪ Normative▪ Informative (may become normative in some countries)

▪ No repetition of content (not even of content of EN 1990 in EN 1997)

▪ No textbook-material

▪ Sources and references should be available for every paragraph

Decision by TC 250 for 2nd generation of Eurocode 7

Formal requirements

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▪Draft of EN 1997-1

▪Reduction in number of National Choices

▪Enhanced ease of use

▪Reliability discrimination

(geotechnical complexity, validation methods, …)

▪Ground water pressures

▪Numerical models

▪Alignment with EN 1990

Project Team 2 (PT2) – EN 1997 General Rules

Tasks distributed among Task Groups (TGs) and

Project Teams (PTs) - e. g. PT 2 of SC7 (for EN 1997)

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▪EN 1990:▪ Official draft of April 2017

▪ Reorganization of the formulas

▪ Reduced partial factor on some permanent actions and on water

▪ Focus on semi-probabilistic approach in the partial safety format

▪ Smaller number of limit states (STR + GEO + EQU combined)

▪ New development since then: ▪ Table with „Design Cases“

▪ Ongoing development (difficult to catch up with in PT2)

EN 1990 – Basis of structural design

Current stage of development of EN 1990

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April 2017 - draft

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September 2017 - draft

Action or effect Partial factor F for Design Cases DC1 to DC4

Type

Group

Sym-bol

Resul-ting

effect

DC1 1 DC2 2 DC3 4 DC4 5

(a) 3 (b)

All Static equilibrium

and uplift Geotechnical

design

Equation (6.4) (6.4) (6.4) (6.6) & (6.8)

Perma-nent action (Gk)

All (excl. water)

G un-

favour-able

1,35KF 1,35 KF 1,0 1,0

Not used

Water pres-sures

G,w 1,2 KF 1,2 KF 1,0 1,0

(All) G,fav favour-able

1,0 1,0 1,0 1,0

Varia-ble action (Qk)

All (excl. water)

Q

un-favour-able

1,5 KF 1,5 KF 1,5 KF 1,3 1,1

Water pres-sures

Q,w 1,2 KF 1,2 KF 1,2 KF 1,0 1,0

(All) Q,fav favour

-able 0

Action-effects (E)

E un-favour-able Not used

1,35KF

E,fav favour-able

1,0

1 DC1 is used both for structural and geotechnical design.

2 DC2 is used for the combined verification of strength and static equilibrium, when the structure is sensitive to variations in permanent action arising from a single-source.

Values of F are taken from columns (a) or (b), whichever gives the less favourable outcome.

3 The value of is 0.85 unless the National Annex gives a different value.

4 DC3 is typically used for the design of slopes and embankments, spread foundations, and gravity retaining structures. See EN 1997 for details.

5 DC4 is typically used for the design of transversally loaded piles and embedded retaining walls and (in some countries) gravity retaining structures. See EN 1997 for details.

[DRAFTING NOTE: values in yellow for water pressures to be further discussed]

Action or effect Partial factor F for Design Cases DC1 to DC4

Type

Group

Sym-bol

Resul-ting

effect

DC1 1 DC2 2 DC3 4 DC4 5

(a) 3 (b)

All Static equilibrium

and uplift Geotechnical

design

Equation (6.4) (6.4) (6.4) (6.6) & (6.8)

Perma-nent action (Gk)

All (excl. water)

G un-

favour-able

1,35KF 1,35 KF 1,0 1,0

Not used

Water pres-sures

G,w 1,2 KF 1,2 KF 1,0 1,0

(All) G,fav favour-able

1,0 1,0 1,0 1,0

Varia-ble action (Qk)

All (excl. water)

Q

un-favour-able

1,5 KF 1,5 KF 1,5 KF 1,3 1,1

Water pres-sures

Q,w 1,2 KF 1,2 KF 1,2 KF 1,0 1,0

(All) Q,fav favour

-able 0

Action-effects (E)

E un-favour-able Not used

1,35KF

E,fav favour-able

1,0

1 DC1 is used both for structural and geotechnical design.

2 DC2 is used for the combined verification of strength and static equilibrium, when the structure is sensitive to variations in permanent action arising from a single-source.

Values of F are taken from columns (a) or (b), whichever gives the less favourable outcome.

3 The value of is 0.85 unless the National Annex gives a different value.

4 DC3 is typically used for the design of slopes and embankments, spread foundations, and gravity retaining structures. See EN 1997 for details.

5 DC4 is typically used for the design of transversally loaded piles and embedded retaining walls and (in some countries) gravity retaining structures. See EN 1997 for details.

[DRAFTING NOTE: values in yellow for water pressures to be further discussed]

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Reliability system – EN 1990, draft April 2017

Consequences Classes in EN 1990

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Reliability system – EN 1990, draft April 2017

Consequences Classes - examples and factors

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▪Permission to use risk-informed and reliability-based approaches in addition to the semi-probabilistic approach

▪Specification of levels of reliability

▪ Treating geometrical properties (joint orientations etc.) of rock masses differently from other geometrical data

▪Probably more …

EN 1990 – Details important for rock engineering

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▪ From ISO 2394 and Annex C of EN 1990: Stages:▪ Risk-informed

▪ Reliability based

▪ Semi-probabilistic approach via the partial factor format

▪Gross human error is a separate issue, cannot be covered by verification methods

▪Safety level is a national choice: Partial safety factors must be nationally determined parameters (NDPs)(Eurocodes give recommended values which may be accepted)

Reliability system – EN 1990

Safety aspects - general

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▪ 1.1(2) EN 1990 describes the basis for structural design and verification according to the limit state principle.

▪ 1.1(4) Design and verification in EN 1990 is based on the partial factor method. NOTE Annex C1 defines alternative reliability verification approaches

▪ 2.1(1) <REQ > A structure shall be designed and executed in such a way that it will, during its intended life, with appropriate degrees of reliability and in an economical way – sustain all actions and influences likely to occur during execution and use, and – meet the specified serviceability requirements for a structure or a structural element.

▪ 2.1.(2) <REQ> A structure shall be designed to have adequate : – structural resistance, – serviceability, – durability, consistently with the provisions related to robustness and sustainability.

Reliability system – EN 1990, draft April 2017

EN 1990 – Mainly partial factor method

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▪ 3.5(1) <REQ> Design for limit states shall be based on the use of structural and load models for relevant limit states.

▪ 3.5(6) <RCM> The requirements of 3.5(1) should be achieved by the partial factor method, described in section 6. NOTE : Annex C1 defines alternative reliability verification approaches that may be used when authorized by the client and the relevant authorities.

▪ Section 6 Verification by the partial factor method

▪ 6.1(7) <PER> Design values may be determined directly provided the resulting degree of reliability is no less than that required by this standard.NOTE 1: Guidance on the direct determination of design values is given in the other Eurocodes. NOTE 2: Further guidance on the verification of reliability is given in Annexes C and D.

Reliability system – EN 1990, draft April 2017

EN 1990 – Limit states, partial factor method

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▪ Annex C1, C1.1(3) Other methods than the partial factors method may be used for design situations that are accepted as not suitable for design using the partial factor format by the client and/or the relevant authorities (see C1.3 (4) and (6)).

▪ C1.3(4) The Eurocodes implement the semi-probabilistic approach via a partial factor design format. Except where stated otherwise in the Eurocodes, this approach should be applied for all design situations.

▪ C1.3(5) The use of the reliability-based approach described in this Annex may apply to design situations where the uncertainties concerning the representation of loads, load effects, material resistances, and system effects are outside the ranges that are covered by the partial factor design format.

▪ C1.3(6) Situations which are not covered by the partial factor design format may e.g. be associated with: - design situations where relevant loads or hazard scenarios are not covered by EN 1991; - the use of building materials or combination of different materials outside the usual application domain; - ground conditions (such as rock) which are strongly affected by discontinuities and other geometrical phenomena.

▪ More details can be found in C1.4 (uncertainty representation) and C1.5 Reliability-based design

Reliability system – EN 1990, draft April 2017

EN 1990 – Alternatives to the partial factor method

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▪ 2.2.2(1) Different levels of reliability may be adopted inter alia : o for structural resistance ; o for serviceability ; o for durability.

▪ 2.2.2(2) <RCM> The choice of the levels of reliability for a particular structure should take account of the relevant factors, including :– the possible cause and /or mode of attaining a limit state ;– the possible consequences of failure in terms of risk to life, injury, potential economic losses, see

2.2.3 ;– public aversion to failure ;– the expense and procedures necessary to reduce the risk of failure.

▪ NOTE after Table 2.1 Definition of consequences classes:Reliability classification can be represented by failure probability levels or target β indices of reliability levels (see Annex C) which take account of accepted or estimated statistical variability in effects of actions and resistances and model uncertainties.

▪ Drafting note there: “… Probability levels to be defined in Annex C“

Reliability system – EN 1990, draft April 2017

EN 1990 – Level of reliability

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▪ C1.6 Reliability requirements:▪ C1.6.1(1) Reliability requirements shall be prescribed by the relevant national authority.

▪ C1.6.2(2) In the partial factor method reliability requirements are implicitly satisfied through the use of partial factors specified in the National Annexes to the Eurocodes.

▪ C1.6.1.1(1) If the design situation can be directly related to a similar reference design situation that is covered by the partial safety factor design format, it should be demonstrated that the same reliability level than the reference design is obtained. NOTE: This relative comparison should be made based on similar probabilistic models.

▪ C1.6.1.1(2) When it is stated in the Eurocodes that a design and assessment situation is not covered by the partial safety factor design format of the Eurocodes, target reliability values should be defined. NOTE: Target reliability are given in the National Annex. In Table C1.2 tentative values are given, to assist national authorities to define values applicable in a Country.

Reliability system – EN 1990, draft April 2017

EN 1990 – Level of reliability

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▪ 4.3(3) <PER> Where their statistical distribution is sufficiently known, values of geometrical quantities that correspond to a prescribed fractile of the statistical distribution may be used.

▪ 6.3.6(1) <RCM> When the design of the structure is sensitive to deviations in a geometrical parameter, the design value of that parameter (ad) should be calculated from:

ad = anom ± Δ a

▪ Current wording in 6.3.6 for rock engineering (based on suggestions by L. Lamas and J. Harrison):

6.3.6(5) <PER> For geotechnical design, geometrical data may be treated as ground properties and described in a probabilistic way, as specified in EN 1997.

NOTE: For example, the spacing and orientation of discontinuities in rock are commonly accounted for in the selection of characteristic material properties of the rock mass.

Reliability system – EN 1990, draft April 2017 and later

EN 1990 - Geometrical data

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▪ First draft of whole EN1997-1 in April 2017

▪ Now dealing with comments (more than 1100)

▪ Second draft has to be ready by end of October 2017

▪ Comments and resulting changes (examples):▪ Reintroduction of Geotechnical Categories, but based explicitly on both,

geotechnical complexity and consequences of failure

▪ Amount of ground investigation, demands on validation of calculation models, monitoring etc. based on (new) Geotechnical Categories (no new classes which were proposed in April draft)

▪ Requirements on design check, execution inspection, qualification etc. based on (new) Geotechnical Categories, classes of EN 1990, Annex B may be used

Development of EN 1997

Current stage of development of EN 1997

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▪ Project Team PT6 will check all parts of EN 1997 in Phase 3 of the development of the Eurocodes what additional changes will be needed for integrating rock engineering

▪ Use general term “ground” instead of “soil” where appropriate, term “geotechnical units”

▪ Subsection 5.3 on “Rocks and rock masses” (based on existing text)

▪ Allow reliability-based methods in geotechnical analysis (4.4.2 (3))

▪ Subsection 4.4.5 on Observational Method has been rewritten

▪ New subsection on numerical methods

▪ Requirements on validation of calculations▪ E.g. sensitivity analysis

▪ Back analyses (4.3.3(2))

▪ Ground water levels: design values derived from required return period, mostly no partial safety factor

▪ Additional information about derivation of characteristic values from site data + previous experience

EN 1997 and rock engineering

EN 1997-1 – Further details important for rock

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GCC 1 Low All the following conditions apply

· uniform ground conditions and standard construction technique

· isolated shallow foundations are systematically applied in the zone

· well established design methods available for the local conditions and the planned construction technique

· irrelevant ground-structure-interaction

GCC 2 Medium Covers everything not contained in the characteristics of GCC 1 and GCC3

GCC 3 High Any of the following applies

· difficult soils

· difficult geomorphologies

· significant thickness of made ground

· sliding ground

· steep soil slopes

· significant geometric variability

· significant sensitivity to ground water conditions

· significant complexity of the ground-structure-interaction

· little experience with calculation models for the current situation

Reliability system – EN 1997, draft Sept. 2017

Selection of Geotechnical Complexity Class

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Consequence

Class

(CC)

Geotechnical Complexity Class (GCC)

Low

(GCC1)

Medium

(GCC2)

High

(GCC3)

High (CC3) GC2 GC3 GC3

Medium (CC2) GC2 GC2 GC3

Low (CC1) GC1 GC2 GC2

Reliability system – EN 1997, draft Sept. 2017

(New) Geotechnical Categories

Amount of further requirements is based on Geotechnical Categories

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▪Several variants of a design

▪Start with a design with best-estimate parameters

▪Cover all foreseeable deviations

▪Plan of monitoring, observation and testing which allow rapid detection of changes

▪Ranges of admissible results of monitoring, observation and testing for each design variant

▪Preparation for immediate switching to a different variant if indicated by the measurements and observations

EN 1997 and rock engineering

Observational Method

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EN 1997 and rock engineering – Observational Method, draft Oct. 2017

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▪ 4.3.3 (2) <PER> Values of ground properties may also be obtained by comparingmonitoring results from closely related situations with corresponding calculations.

NOTE 1 Examples for closely related situations are1) earlier construction stages or nearby cross sections of the same geotechnical structure in the same geotechnical unit

2) field trials of comparable structures in the same geotechnical unit

▪ 7.1.1 (7) Validation of the calculation model may be achieved by the following measures for each Geotechnical Category (GC)

▪ GC1, GC2 …▪ GC3: Calibration of the calculation model for the specific site

Sensitivity analyses for all relevant parameters.NOTE: Guidelines on validation measures for specific geotechnical structures can be found in EN 1997-3 or National Annex

▪ 7.1.5.1 (3) (Numerical methods) <RCM> Parametric studies on critical parameters, including geometry, should be performed to determine the reliability of outputs.

NOTE: A basic parametric study could involve varying each critical parameter in turn by ±1 standard deviation and observing its effect on calculation outputs.

NOTE: Alternatively, lower bound critical parameters may be used where they result in conservative outputs.

EN 1997 and rock engineering, draft April 2017 and later

Back analysis, sensitivity studies

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▪Are reliability based approaches mandatory for rock engineering or just an option?

▪How do parametric studies fit into the overall picture?

▪How do estimated values for mean and standard deviation fit into the scheme?

▪Are there limits to limit state design?

▪Definitions:▪ Prescriptive measures▪ Empirical rules (empiricism – e. g. rock mass classification)

Open issues

Open issues – maybe for this workshop

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▪Should rock engineering be excluded from the scope of application of EN 1997?

▪ Is the Observational Method risk informed and neither reliability-based nor within the partial factor approach?

▪Where is the boundary between soil and rock, between rock engineering and soil engineering or foundation engineering etc (German: Lockergestein – Festgestein)?

Open issues

“Philosophical” issues

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▪Do we need a subsection in “Materials” about the differentiation between rock properties and rock mass properties? Is this differentiation sufficiently covered in EN 1997-2 (Ground Investigation)?

▪ To what detail should reliability-based methods be covered in EN 1997?

▪ For what types of problems do we have tools and sufficient data for performing reliability-based analysis?

Open issues

Issues for PT 6

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Thank you for your attention!