CEN (European Committee for Standardization)

Fleming WGK

Orr TLL

P. J. Vardanega

Poulos HG

Schneider HR

Standards Australia

T. L. L. Orr

English

Crossref

Proceedings of the Institution of Civil Engineers - Geotechnical Engineering

Discussion: How Eurocode 7 has affected geotechnical design: a review

Orr, T. L. L.

Vardanega, P. J.

Explore Bristol Research

                          Orr, T. L. L., & Vardanega, P. J. (2013). Discussion: How Eurocode 7 hasaffected geotechnical design: a review. Proceedings of the Institution of CivilEngineers: Geotechnical Engineering, 166(6), 610-612.10.1680/geng.13.00011Link to published version (if available):10.1680/geng.13.00011Link to publication record in Explore Bristol ResearchPDF-documentUniversity of Bristol - Explore Bristol ResearchGeneral rightsThis document is made available in accordance with publisher policies. Please cite only the publishedversion using the reference above. Full terms of use are available:http://www.bristol.ac.uk/pure/about/ebr-terms.htmlTake down policyExplore Bristol Research is a digital archive and the intention is that deposited content should not beremoved. However, if you believe that this version of the work breaches copyright law please contactopen-access@bristol.ac.uk and include the following information in your message:• Your contact details• Bibliographic details for the item, including a URL• An outline of the nature of the complaintOn receipt of your message the Open Access Team will immediately investigate your claim, make aninitial judgement of the validity of the claim and, where appropriate, withdraw the item in questionfrom public view.Proceedings of the Institution of Civil EngineersGeotechnical Engineering 166 December 2013 Issue GE6Pages 610–612 http://dx.doi.org/10.1680/geng.13.00011Paper 1300011Received 22/01/2013 Accepted 20/02/2013Published online 24/06/2013ICE Publishing: All rights reservedGeotechnical EngineeringVolume 166 Issue GE6DiscussionOrr and VardanegaDiscussion: How Eurocode 7has affected geotechnicaldesign: a reviewT. L. L. Orr BA, BAI, MSc, PhD, EURING, MICE, FIEIAssociate Professor, Department of Civil Structural and EnvironmentalEngineering, Trinity College Dublin, Dublin, IrelandP. J. Vardanega MEngSc, PhD, MIEAust, MASCEResearch Associate, Laing O’Rourke Centre for Construction Engineeringand Technology, Department of Engineering, University of Cambridge,Cambridge, UKContribution by P. J. VardanegaThe discusser read the author’s paper with great interest, andacknowledges his contributions to the Eurocode 7 developmentprocess and the associated academic commentary (e.g. Orr, 2000,2012a, 2012b). The paper is an excellent summary of the presentstate of Eurocode development. The Eurocodes cannot be allowedto ossify. In this spirit there are a few points from the paperworthy of discussion with regard to future code development.ULS against SLSThe author, in reference to Vardanega et al. (2012a) and otherpapers, summarisesModel factors have been introduced in the UK NA to EN 1997-1, sothat the overall safety level of pile designs to Eurocode 7 is similar toformer practice, and the occurrence of an SLS as well as a ULS issufficiently unlikely.Although this is an accurate description of what the nationalannex (NA) (BSI, 2007) may achieve in practice, it showsconfusion between the two limit states, ultimate limit state (ULS)and serviceability limit state (SLS).SLS design requires that settlements, and hence strains, beprevented from exceeding some value. Recent work describingthe link between mobilised strength and soil strains mathemati-cally is worth mentioning. Using a large laboratory database,Vardanega and Bolton (2011) have shown that a power-lawfunction can be used to describe strength mobilisation in claysand silts1M¼ mobcu¼ 12ªªM¼2 b5:where M is the mobilisation factor (equivalent to a factor ofsafety on shear strength, BS 8002 (BSI, 1994); mob is themobilised shear strength; cu is the undrained shear strength; ª isthe shear strain; ªM¼2 is the mobilisation strain (defined as theshear strain at a shear stress of 0.5cu); and b is a power exponent.Vardanega et al. (2012b) conducted a laboratory study, andreported that, for kaolin, b and ªM¼2 both correlate with over-consolidation ratio. Equation (5) is used in Vardanega et al.(2012c) along with a ‘Randolph-style’ calculation method (e.g.Fleming et al., 2009) to estimate settlements of bored piles instiff clays.For a ULS calculation, a characteristic value should be used todetermine the design resistance, with no further factors beingapplied to the characteristic value itself. If this is followed, then areduction of DAs in Eurocode 7 may be possible. Factoring thecomputed resistance allows for the inevitable uncertainty in thecalculation method itself – aside from uncertainty in the soilparameters (e.g. undrained strength). On the other hand, for SLScalculations, a mobilisation factor is needed to control settle-ments.Characteristic values and assignment of partial factorsThe author reports that 94% of UK engineers require more generalguidance on the selection of characteristic values. For piledfoundations in stiff clays, Vardanega et al. (2012c) advocate a 5thpercentile design line (cu with depth) for base resistance, and a50th percentile design line for shaft friction. Future developmentsin Eurocode 7 concerning the determination of characteristicvalues should require that the design calculation in question (e.g.pile base or shaft resistance) inform the method used to determinethe characteristic value (alongside issues of data availability,variability and the ‘limit state being considered’). Values of partialfactors should be informed on the basis of risk, not merely thecountry that the design project is being completed in.Poulos (2004, 2011) presented a new approach for assigninggeotechnical reduction factors (GRF) (similar to partial factors)that has been incorporated in the Australian Piling Code AS2159-2009 (Standards Australia, 2009). A risk analysis matrix is usedto determine the geotechnical reduction factors (similar to partialfactors in the Eurocode), and an example applying the method topiles in stiff clays is detailed in Vardanega et al. (2012b).This approach allows the engineer to adjust the GRF in relation tothe design circumstances. The code gives a range of values for theGRF, with the final value selected by the risk analysis exercise. Asimilar methodology could be adapted within the Eurocode 7framework to determine each partial factor on a job-by-job basis.610Designers who are prepared (or able) to do more extensive fieldinvestigations, detailed design calculations and laboratory studieswould inevitably be more confident in specifying lower partialfactors for a specific project if they thought it safe to do so.Author’s replyThe discusser has provided interesting comments on aspects ofEurocode 7 that are indeed worthy of discussion regarding itsfuture development and are, in fact, currently being addressed bythe SC7 Evolution Groups referred to in the paper.The discusser’s point – that model factors have been introducedin the UK NA so that the overall safety level of piles designedto Eurocode 7 is similar to former practice, and the occurrenceof both an SLS and a ULS is sufficiently unlikely – showsconfusion between these two limit states. There should be nosuch confusion when designing to Eurocode 7, because Euro-code 7 clearly states in Section 2.1(1)P that ‘it shall be verifiedthat no relevant limit state . . . is exceeded.’ SLS verification canbe either by a direct method, comparing calculated settlementsto the allowable values, or by an indirect method that ensuresthat a sufficiently low value of the shear strength is mobilised tokeep the settlements within the SLS limits. In pile design, owingto installation effects and uncertainties in the calculation modelsand ground properties, calculations in most cases provide onlyan approximate estimate of the pile settlements. Thereforeincreased factors, as in the UK NA, may be used in a bearingcalculation to ensure that a sufficiently low shear strength ismobilised, and prevent an SLS as well as a ULS. The discusser’sexample of a power-law function relating soil shear strain tomobilised strength is interesting, in that it offers a direct methodto estimate pile settlement. However, it should not be overlookedthat the SLS requirements in Eurocode 7 relate to the supportedstructure, and not to the settlement of an individual pile.Regarding selection of characteristic values, the discusser recom-mends the 50th percentile of cu with depth to determine thecharacteristic pile shaft friction. This is not consistent withEurocode 7’s definition of the characteristic value as a cautiousestimate of the mean value, which must be less than the 50thpercentile. How cautious this mean value should be depends onthe data available and the nature of the design situation. A numberof authors have provided guidance on the selection of character-istic soil properties: for example, Schneider and Schneider (2012)have published a statistical method that takes account of thefailure surface extent, soil variability and scale of fluctuation.The discusser states that the partial factor values should berelated to the risk, and not merely to the country in which theproject is being constructed. However, the responsibility forsetting safety levels for structures in a particular country is anational issue: hence NAs have been prepared by each country,with partial factor values providing the safety level judgedappropriate for that country. A system of reliability differentia-tion related to the risk level is offered in the head Eurocode, EN1990 (CEN, 2002), in the form of reliability classes andconsequences classes to modify the partial factor values,depending on the design situation. This system has not beenadopted in the UK NA, but has been adopted in the NAs ofsome countries where the partial factor values vary dependingon the design situation (Orr, 2012a, 2012b). An alternativemethod for taking account of the risk level is offered in Section2.4.6.2(1)P, which permits the design values of soil parametersto be assessed directly, instead of being obtained by applyingspecified partial factors to selected characteristic values. Hence,by choosing this method, the discusser could avail of thereduced risk arising from more extensive field investigations,detailed design calculations and laboratory studies when design-ing piles to Eurocode 7.REFERENCESBSI (1994) BS 8002:1994: Code of practice for earth retainingstructures. British Standards Institution, London, UK.BSI (2007) NA to BS EN 1997-1:2004: UK National Annex toEurocode 7. Geotechnical design. General rules. BritishStandards Institution, London, UK.CEN (European Committee for Standardization) (2002) EN1990:2002: Eurocode – Basis of structural design. EuropeanCommittee for Standardization, Brussels, Belgium.Fleming WGK, Weltman AJ, Randolph MF and Elson WK (2009)Piling Engineering, 3rd edn. Wiley, New York, USA.Orr TLL (2000) Selection of characteristic values and partialfactors in geotechnical designs to Eurocode 7. Computers andGeotechnics 26(3–4): 263–279.Orr TLL (2012a) How Eurocode 7 has affected geotechnicaldesign: a review. Proceedings of the Institution of CivilEngineers – Geotechnical Engineering 165(6): 337–350.Orr TLL (2012b) Implementing Eurocode 7 to achieve reliablegeotechnical designs. In Modern Geotechnical Design Codesof Practice (Arnold P, Fenton GA, Hicks MA andSchweckendiek T (eds)). IOS Press, Amsterdam, theNetherlands, pp. 72–86.Poulos HG (2004) An approach for assessing geotechnicalreduction factors for pile design. In Proceedings of the 9thAustralia New Zealand Conference on Geomechanics,Auckland, New Zealand (New Zealand Geotechnical Societyand Australian Geomechanics Society (eds)). Centre forContinuing Education, University of Auckland, New Zealand,vol. 1, pp. 109–115.Poulos HG (2011) The de Mello foundation engineering legacy.Soils and Rocks 34(1): 3–34.Schneider HR and Schneider MA (2012) Dealing withuncertainties in EC7 with emphasis on determination ofcharacteristic soil properties. In Modern Geotechnical DesignCodes of Practice (Arnold P, Fenton GA, Hicks MA andSchweckendiek T (eds)). IOS Press, Rotterdam, theNetherlands, pp. 87–101.Standards Australia (2009) AS2159-2009: Piling – design andinstallation. Standards Australia, Sydney, New South Wales,Australia.611Geotechnical EngineeringVolume 166 Issue GE6DiscussionOrr and VardanegaVardanega PJ and Bolton MD (2011) Strength mobilization inclays and Silts. Canadian Geotechnical Journal 48(10):1485–1503; Corrigendum, 49(5): 631.Vardanega PJ, Kolody E, Pennington SH, Morrison PRJ andSimpson B (2012a) Bored pile design in stiff clay I: codes ofpractice. Proceedings of the Institution of Civil Engineers –Geotechnical Engineering 165(4): 213–232.Vardanega PJ, Lau BH, Lam SY et al. (2012b) Laboratorymeasurement of strength mobilisation in kaolin: link to paststress history. Ge´otechnique Letters 2(1): 9–15.Vardanega PJ, Williamson MG and Bolton MD (2012c) Boredpile design in stiff clay II: mechanisms and uncertainty.Proceedings of the Institution of Civil Engineers –Geotechnical Engineering 165(4): 233–246.612Geotechnical EngineeringVolume 166 Issue GE6DiscussionOrr and Vardanega

(2012a) Bored pile design in stiff clay I: codes of practice.

(2012a) How Eurocode 7 has affected geotechnical design: a review.

(2012b) Implementing Eurocode 7 to achieve reliable geotechnical designs.

(2012b) Laboratory measurement of strength mobilisation in kaolin: link to past stress history.

(2012c) Bored pile design in stiff clay II: mechanisms and uncertainty.

An approach for assessing geotechnical reduction factors for pile design.

AS2159-2009: Piling – design and installation. Standards

BS 8002:1994: Code of practice for earth retaining structures. British Standards Institution,

Committee for Standardization

Dealing with uncertainties in EC7 with emphasis on determination of characteristic soil properties.

NA to BS EN 1997-1:2004: UK National Annex to Eurocode 7. Geotechnical design. General rules. British Standards Institution,

Piling Engineering, 3rd edn.

Selection of characteristic values and partial factors in geotechnical designs to Eurocode 7.

The de Mello foundation engineering legacy.

Orr, T. L. L., & Vardanega, P. J. (2013). Discussion: How Eurocode 7has affected geotechnical design: a review. Proceedings of the ICE -Geotechnical Engineering, 166(6), 610-612.https://doi.org/10.1680/geng.13.00011Publisher's PDF, also known as Version of recordLink to published version (if available):10.1680/geng.13.00011Link to publication record on the Bristol Research PortalPDF-documentUniversity of Bristol – Bristol Research PortalGeneral rightsThis document is made available in accordance with publisher policies. Please cite only thepublished version using the reference above. Full terms of use are available:http://www.bristol.ac.uk/red/research-policy/pure/user-guides/brp-terms/Proceedings of the Institution of Civil EngineersGeotechnical Engineering 166 December 2013 Issue GE6Pages 610–612 http://dx.doi.org/10.1680/geng.13.00011Paper 1300011Received 22/01/2013 Accepted 20/02/2013Published online 24/06/2013ICE Publishing: All rights reservedGeotechnical EngineeringVolume 166 Issue GE6DiscussionOrr and VardanegaDiscussion: How Eurocode 7has affected geotechnicaldesign: a reviewT. L. L. Orr BA, BAI, MSc, PhD, EURING, MICE, FIEIAssociate Professor, Department of Civil Structural and EnvironmentalEngineering, Trinity College Dublin, Dublin, IrelandP. J. Vardanega MEngSc, PhD, MIEAust, MASCEResearch Associate, Laing O’Rourke Centre for Construction Engineeringand Technology, Department of Engineering, University of Cambridge,Cambridge, UKContribution by P. J. VardanegaThe discusser read the author’s paper with great interest, andacknowledges his contributions to the Eurocode 7 developmentprocess and the associated academic commentary (e.g. Orr, 2000,2012a, 2012b). The paper is an excellent summary of the presentstate of Eurocode development. The Eurocodes cannot be allowedto ossify. In this spirit there are a few points from the paperworthy of discussion with regard to future code development.ULS against SLSThe author, in reference to Vardanega et al. (2012a) and otherpapers, summarisesModel factors have been introduced in the UK NA to EN 1997-1, sothat the overall safety level of pile designs to Eurocode 7 is similar toformer practice, and the occurrence of an SLS as well as a ULS issufficiently unlikely.Although this is an accurate description of what the nationalannex (NA) (BSI, 2007) may achieve in practice, it showsconfusion between the two limit states, ultimate limit state (ULS)and serviceability limit state (SLS).SLS design requires that settlements, and hence strains, beprevented from exceeding some value. Recent work describingthe link between mobilised strength and soil strains mathemati-cally is worth mentioning. Using a large laboratory database,Vardanega and Bolton (2011) have shown that a power-lawfunction can be used to describe strength mobilisation in claysand silts1M¼ mobcu¼ 12ªªM¼2 b5:where M is the mobilisation factor (equivalent to a factor ofsafety on shear strength, BS 8002 (BSI, 1994); mob is themobilised shear strength; cu is the undrained shear strength; ª isthe shear strain; ªM¼2 is the mobilisation strain (defined as theshear strain at a shear stress of 0.5cu); and b is a power exponent.Vardanega et al. (2012b) conducted a laboratory study, andreported that, for kaolin, b and ªM¼2 both correlate with over-consolidation ratio. Equation (5) is used in Vardanega et al.(2012c) along with a ‘Randolph-style’ calculation method (e.g.Fleming et al., 2009) to estimate settlements of bored piles instiff clays.For a ULS calculation, a characteristic value should be used todetermine the design resistance, with no further factors beingapplied to the characteristic value itself. If this is followed, then areduction of DAs in Eurocode 7 may be possible. Factoring thecomputed resistance allows for the inevitable uncertainty in thecalculation method itself – aside from uncertainty in the soilparameters (e.g. undrained strength). On the other hand, for SLScalculations, a mobilisation factor is needed to control settle-ments.Characteristic values and assignment of partial factorsThe author reports that 94% of UK engineers require more generalguidance on the selection of characteristic values. For piledfoundations in stiff clays, Vardanega et al. (2012c) advocate a 5thpercentile design line (cu with depth) for base resistance, and a50th percentile design line for shaft friction. Future developmentsin Eurocode 7 concerning the determination of characteristicvalues should require that the design calculation in question (e.g.pile base or shaft resistance) inform the method used to determinethe characteristic value (alongside issues of data availability,variability and the ‘limit state being considered’). Values of partialfactors should be informed on the basis of risk, not merely thecountry that the design project is being completed in.Poulos (2004, 2011) presented a new approach for assigninggeotechnical reduction factors (GRF) (similar to partial factors)that has been incorporated in the Australian Piling Code AS2159-2009 (Standards Australia, 2009). A risk analysis matrix is usedto determine the geotechnical reduction factors (similar to partialfactors in the Eurocode), and an example applying the method topiles in stiff clays is detailed in Vardanega et al. (2012b).This approach allows the engineer to adjust the GRF in relation tothe design circumstances. The code gives a range of values for theGRF, with the final value selected by the risk analysis exercise. Asimilar methodology could be adapted within the Eurocode 7framework to determine each partial factor on a job-by-job basis.610Designers who are prepared (or able) to do more extensive fieldinvestigations, detailed design calculations and laboratory studieswould inevitably be more confident in specifying lower partialfactors for a specific project if they thought it safe to do so.Author’s replyThe discusser has provided interesting comments on aspects ofEurocode 7 that are indeed worthy of discussion regarding itsfuture development and are, in fact, currently being addressed bythe SC7 Evolution Groups referred to in the paper.The discusser’s point – that model factors have been introducedin the UK NA so that the overall safety level of piles designedto Eurocode 7 is similar to former practice, and the occurrenceof both an SLS and a ULS is sufficiently unlikely – showsconfusion between these two limit states. There should be nosuch confusion when designing to Eurocode 7, because Euro-code 7 clearly states in Section 2.1(1)P that ‘it shall be verifiedthat no relevant limit state . . . is exceeded.’ SLS verification canbe either by a direct method, comparing calculated settlementsto the allowable values, or by an indirect method that ensuresthat a sufficiently low value of the shear strength is mobilised tokeep the settlements within the SLS limits. In pile design, owingto installation effects and uncertainties in the calculation modelsand ground properties, calculations in most cases provide onlyan approximate estimate of the pile settlements. Thereforeincreased factors, as in the UK NA, may be used in a bearingcalculation to ensure that a sufficiently low shear strength ismobilised, and prevent an SLS as well as a ULS. The discusser’sexample of a power-law function relating soil shear strain tomobilised strength is interesting, in that it offers a direct methodto estimate pile settlement. However, it should not be overlookedthat the SLS requirements in Eurocode 7 relate to the supportedstructure, and not to the settlement of an individual pile.Regarding selection of characteristic values, the discusser recom-mends the 50th percentile of cu with depth to determine thecharacteristic pile shaft friction. This is not consistent withEurocode 7’s definition of the characteristic value as a cautiousestimate of the mean value, which must be less than the 50thpercentile. How cautious this mean value should be depends onthe data available and the nature of the design situation. A numberof authors have provided guidance on the selection of character-istic soil properties: for example, Schneider and Schneider (2012)have published a statistical method that takes account of thefailure surface extent, soil variability and scale of fluctuation.The discusser states that the partial factor values should berelated to the risk, and not merely to the country in which theproject is being constructed. However, the responsibility forsetting safety levels for structures in a particular country is anational issue: hence NAs have been prepared by each country,with partial factor values providing the safety level judgedappropriate for that country. A system of reliability differentia-tion related to the risk level is offered in the head Eurocode, EN1990 (CEN, 2002), in the form of reliability classes andconsequences classes to modify the partial factor values,depending on the design situation. This system has not beenadopted in the UK NA, but has been adopted in the NAs ofsome countries where the partial factor values vary dependingon the design situation (Orr, 2012a, 2012b). An alternativemethod for taking account of the risk level is offered in Section2.4.6.2(1)P, which permits the design values of soil parametersto be assessed directly, instead of being obtained by applyingspecified partial factors to selected characteristic values. Hence,by choosing this method, the discusser could avail of thereduced risk arising from more extensive field investigations,detailed design calculations and laboratory studies when design-ing piles to Eurocode 7.REFERENCESBSI (1994) BS 8002:1994: Code of practice for earth retainingstructures. British Standards Institution, London, UK.BSI (2007) NA to BS EN 1997-1:2004: UK National Annex toEurocode 7. Geotechnical design. General rules. BritishStandards Institution, London, UK.CEN (European Committee for Standardization) (2002) EN1990:2002: Eurocode – Basis of structural design. EuropeanCommittee for Standardization, Brussels, Belgium.Fleming WGK, Weltman AJ, Randolph MF and Elson WK (2009)Piling Engineering, 3rd edn. Wiley, New York, USA.Orr TLL (2000) Selection of characteristic values and partialfactors in geotechnical designs to Eurocode 7. Computers andGeotechnics 26(3–4): 263–279.Orr TLL (2012a) How Eurocode 7 has affected geotechnicaldesign: a review. Proceedings of the Institution of CivilEngineers – Geotechnical Engineering 165(6): 337–350.Orr TLL (2012b) Implementing Eurocode 7 to achieve reliablegeotechnical designs. In Modern Geotechnical Design Codesof Practice (Arnold P, Fenton GA, Hicks MA andSchweckendiek T (eds)). IOS Press, Amsterdam, theNetherlands, pp. 72–86.Poulos HG (2004) An approach for assessing geotechnicalreduction factors for pile design. In Proceedings of the 9thAustralia New Zealand Conference on Geomechanics,Auckland, New Zealand (New Zealand Geotechnical Societyand Australian Geomechanics Society (eds)). Centre forContinuing Education, University of Auckland, New Zealand,vol. 1, pp. 109–115.Poulos HG (2011) The de Mello foundation engineering legacy.Soils and Rocks 34(1): 3–34.Schneider HR and Schneider MA (2012) Dealing withuncertainties in EC7 with emphasis on determination ofcharacteristic soil properties. In Modern Geotechnical DesignCodes of Practice (Arnold P, Fenton GA, Hicks MA andSchweckendiek T (eds)). IOS Press, Rotterdam, theNetherlands, pp. 87–101.Standards Australia (2009) AS2159-2009: Piling – design andinstallation. Standards Australia, Sydney, New South Wales,Australia.611Geotechnical EngineeringVolume 166 Issue GE6DiscussionOrr and VardanegaVardanega PJ and Bolton MD (2011) Strength mobilization inclays and Silts. Canadian Geotechnical Journal 48(10):1485–1503; Corrigendum, 49(5): 631.Vardanega PJ, Kolody E, Pennington SH, Morrison PRJ andSimpson B (2012a) Bored pile design in stiff clay I: codes ofpractice. Proceedings of the Institution of Civil Engineers –Geotechnical Engineering 165(4): 213–232.Vardanega PJ, Lau BH, Lam SY et al. (2012b) Laboratorymeasurement of strength mobilisation in kaolin: link to paststress history. Géotechnique Letters 2(1): 9–15.Vardanega PJ, Williamson MG and Bolton MD (2012c) Boredpile design in stiff clay II: mechanisms and uncertainty.Proceedings of the Institution of Civil Engineers –Geotechnical Engineering 165(4): 233–246.612Geotechnical EngineeringVolume 166 Issue GE6DiscussionOrr and Vardanega

https://research-information.bris.ac.uk/files/34277486/geng166_0610.pdf

Discussion: How Eurocode 7 has affected geotechnical design: a review

Abstract

Similar works

Full text

Available Versions

Crossref

Explore Bristol Research

Explore Bristol Research