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QA Session

Q & A Session

We had an interesting and lively Q&A session during the Numerical Simulation in Accordance with Geotechnical Design Codes webinar and we wanted to share with you the questions that we didn't have the time to address during the event.

Q: Was the FEA model used to check internal stability in the reinforced soil wall

A: No, only external stability. Tensar used their own design software for internal stability using conventional analysis methods. Checking internal stability by numerical simulation is possible but rather complicated due to all the interactions within it and the partial factors imposed on those interactions.

Q: Do Euro7-Numerical SImulation code.. do you have for Tunnel?

A: Tunnelling is outside the scope of Structural Eurocodes.

Q: I do have the following question :In Updated EC7 willthe advanced numerical modelling be linkedto advanced ground investigations and characterization thus we can have reliable models?

A: The revised Eurocode will have more rules and incentives for better ground investigation, in particular related to the Geotechnical Category of a structure.

Q: When we have a local failure above or around critical slip surface we may exclude that specific region causing local failure. In this case how would this exclusion affect the global safety and how can we be sure about its impact on the global safety?

A: Yes, you need to be careful here. Local failures may be considered unimportant for safety, but they are a serviceability problem of course and should be prevented if you think they are genuine. If you think that they will not occur due to some feature excluded from the model (e.g. a surface treatment or a detail in the geometry), then consider how that detail could be included to eliminate the improbable local failure but not affect the prediction of unsafe failures. If you prevent local failure by any means, then you run the risk of increasing the safety margin artificially.

Q: Have you ever implemented Design Approach 2 of the Eurocode and more specifically how can we implement the partial factors on ground pressure (=1.2) and permanent actions (=1.35)?

A: DA2 can be implemented for piles relatively straightforwardly. Also, for structures with external loads (e.g. spread foundation). For internal geotechnical loads (e.g. retaining walls) it is very difficult to apply factors to these loads, so you apply factors to the effect of these loads on output such as bending moment. Applying resistance factors is very difficult on many structure types except piles. Check the references at the end of the webinar for more information on this.

Q: In Eurocode 7, there 3 Design approaches, can you recommend which best approach for a specific problem? e.g. excavations, piled foundations, embankemnts problem.

A: Most countries already agree that DA3 is best for slopes/overall stability and DA2 for piles. The debate gets heated over spread foundations and retaining walls where there is most interaction between ground and structure. The dual factoring DA1 is best with numerical simulation for these types.

Q: We are outside of Eurozone, where we can get/ buy EN 7?

A: Official, controlled copies can be obtained from the National Standards Bodies (e.g. British Standards Institution) who are members of CEN, including in different languages. Unofficial versions may be available on the internet.

Q: How do we validate the Factor (for example the 1.25 etc) ?Do we need to do sensitive analysis?

A: Sensitivity analysis is always a good idea. EC7 wants to discourage designers from blindly applying the partial factors and expecting everything to be adequately safe. If ULS is very sensitive to a particular parameter that you are uncertain about, maybe a higher factor is appropriate or rather this should be taken into account in the selection of a more cautious characteristic value.

Q: Does eurocode 7 offer any method to model soil as nonlinear springs such as P-y , T-z and Q-z ?

A: No methods are mentioned specifically in EC7 Part 1. It is possible that Part 3 which covers specific structure types, or EC2 for concrete structures, may mention non-linear springs but I have not seen the revised versions yet.

Q: My another question, the displacement of soil are is dependent on stiffness matrix we define in elasticity and plasticity part (mostly only hardening).. So even if we apply partial factors, the stiffness behaviour should not be changed, right?

A: Yes, with advanced constitutive models including hardening models, reducing the strength may cause a reduction in stiffness for non-elastic stress states and this is something to bear in mind. Partial factors should not be applied directly to stiffness parameters. It is better to perform sensitivity analysis to understand effect of stiffness rather than apply partial factors.

Q: Just to add to Andrew’s references – CIRIA will bepublishing a report called Advanced Numerical Modleling in Goetechnical engineering in October which refers to NAFEMS reports.
This report we give more guidance from clients perspectives and talks about competencies.

A: Good point. I was on the Steering Committee but forgot to mention it! NAFEMS and CIRIA will organise a joint event for this, so look out for notifications.

Q: I am curios that why you modelled geogrid in solid instead of using geogrid material in that software? In solid, how can you define the axial rigidity? (EA) Thank you very much

A: The stiffness parameters (EA) for material models (e.g. linear, non-linear and visco-elastic) are determined from wide-width tensile tests across several ribs and on specimens of several apertures long, so EA is already averaged across the geogrid so it can be represented as a continuous membrane. Similarly, soil-geogrid interaction is measured in shear box and pull-out tests where the results are averaged so that soil-geogrid friction and soil particle to geogrid aperture interlock is all taken into account.