Consideration of Rock Engineering in Eurocode 7 (EN 1997)

Transcription

1 Consideration of Rock Engineering in Eurocode 7 (EN 1997) Current state of the amendment Herbert Walter Member of Project Team 2 of WG1 / TG2

2 Consideration of Rock Engineering in Eurocode 7 (EN 1997) Overview 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! 2

3 Mandate M/515 Amending existing Eurocodes and extending the scope of structural Eurocodes 2 nd Generation of EN Eurocodes Mandate M/515 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.: ( ) 3

4 Mandate M/515 Amending existing Eurocodes and extending the scope of structural Eurocodes Implementation of Mandate M/515 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) 4

5 TC 250 Structural Eurocodes - Organization 5

6 Timeline of Eurocode development Courtesy: Andrew Bond 6

7 Phases of EN 1997 draft Phases Phase 1: ( ) Phase 2: EN 1990 WG7.T1 Evolution of EN 1990 General 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) EN 1997: SC7.T3 Ground Investigation (PT 3, WG2/TG1 Reorganization ) ( ) 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, ( ?) WG1/TG4 Dynamic design ) Phase 4 ( ?): e.g. Harmonization between EN 1992, EN 1993 and EN

8 SC 7 Organizational structure onwards Courtesy: Andrew Bond 8

9 Decision by TC 250 for 2nd generation of Eurocode 7 New structure of EN parts: EN General Rules EN Ground Investigation EN Geotechnical Constructions Several categories of Eurocode users - primary target audience: Practitioners competent engineers 9

10 Decision by TC 250 for 2nd generation of Eurocode 7 Formal requirements 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 10

11 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) Draft of EN Reduction in number of National Choices Enhanced ease of use Reliability discrimination (geotechnical complexity, validation methods, ) Ground water pressures Numerical models Alignment with EN

12 EN 1990 Basis of structural design Current stage of development of EN 1990 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) 12

13 April draft 13

14 Action or effect Partial factor F for Design Cases DC1 to DC4 Type Group DC1 1 DC2 2 DC3 4 DC4 5 Resulting (a) 3 (b) Sym -bol Static equilibrium Geotechnical effect All and uplift design Equation (6.4) (6.4) (6.4) (6.6) & (6.8) All (excl. G 1,35K F 1,35 K F 1,0 1,0 water) Perma -nent action (G k ) Water pressures Variable action (Q k ) unfavour -able G,w 1,2 K F 1,2 K F 1,0 1,0 (All) G,fav favour -able All Q (excl. water) (All) Action-effects (E) Q,w Q,fav E E,fav unfavour -able 1,0 1,0 1,0 1,0 Not used 1,5 K F 1,5 K F 1,5 K F 1,3 1,1 1,2 K F 1,2 K F 1,2 K F 1,0 1,0 Water pressures favour -able 0 unfavour -able Not used favour -able 1 DC1 is used both for structural and geotechnical design. 1,35K F 2 DC2 is used for the combined verification of strength and static equilibrium, when the 1,0 Perma -nent action (G k ) unfavour water) Water -able pressures G,w 1,2 K F 1,2 K F 1,0 1,0 (All) G,fav favour -able 1,0 1,0 1,0 1,0 All Q (excl. 1,5 K F 1,5 K F 1,5 K F 1,3 1,1 water) Variable action (Q k ) (All) Action-effects (E) Q,w Q,fav E E,fav unfavour -able Not used September draft 1,2 K F 1,2 K F 1,2 K F 1,0 1,0 Water pressures favour -able 0 unfavour -able Not used favour -able 1 DC1 is used both for structural and geotechnical design. 1,35K F 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] 1,0 14

15 Reliability system EN 1990, draft April 2017 Consequences Classes in EN

16 Reliability system EN 1990, draft April 2017 Consequences Classes - examples and factors 16

17 EN 1990 Details important for rock engineering 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 17

18 Reliability system EN 1990 Safety aspects - general 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) 18

19 Reliability system EN 1990, draft April 2017 EN 1990 Mainly partial factor method 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. 19

20 Reliability system EN 1990, draft April 2017 EN 1990 Limit states, partial factor method 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. 20

21 Reliability system EN 1990, draft April 2017 EN 1990 Alternatives to the partial factor method 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 21

22 Reliability system EN 1990, draft April 2017 EN 1990 Level of reliability 2.2.2(1) Different levels of reliability may be adopted inter alia : o for structural resistance ; o for serviceability ; o for durability (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 ; 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 22

23 Reliability system EN 1990, draft April 2017 EN 1990 Level of reliability 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. C (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. C (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. 23

24 Reliability system EN 1990, draft April 2017 and later EN Geometrical data 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 (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: a d = a nom ± Δ a Current wording in 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 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. 24

25 Development of EN 1997 Current stage of development of EN 1997 First draft of whole EN 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 25

26 EN 1997 and rock engineering EN Further details important for rock 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 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 26

27 Reliability system EN 1997, draft Sept Selection of Geotechnical Complexity Class 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 GCC 3 High Covers everything not contained in the characteristics of GCC 1 and GCC3 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 27

28 Reliability system EN 1997, draft Sept (New) Geotechnical Categories Consequence Geotechnical Complexity Class (GCC) Class Low Medium High (CC) (GCC1) (GCC2) (GCC3) High (CC3) GC2 GC3 GC3 Medium (CC2) GC2 GC2 GC3 Low (CC1) GC1 GC2 GC2 Amount of further requirements is based on Geotechnical Categories 28

29 EN 1997 and rock engineering Observational Method 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 29

30 EN 1997 and rock engineering Observational Method, draft Oct

31 EN 1997 and rock engineering, draft April 2017 and later Back analysis, sensitivity studies (2) <PER> Values of ground properties may also be obtained by comparing monitoring results from closely related situations with corresponding calculations. NOTE 1 Examples for closely related situations are 1) 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) 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 or National Annex (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. 31

32 Open issues Open issues maybe for this workshop 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) 32

33 Open issues Philosophical issues Should rock engineering be excluded from the scope of application of EN 1997? Is the Observational Method risk informed and neither reliabilitybased 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)? 33

34 Open issues Issues for PT 6 Do we need a subsection in Materials about the differentiation between rock properties and rock mass properties? Is this differentiation sufficiently covered in EN (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? 34

35 Thank you for your attention! 35