I'm just back from the latest B/526 meeting, where we discussed comments received on the draft UK National Annex to Eurocode 7 Part 1. (For those of you who don't know, B/526 is the BSI committee responsible for producing and maintaining British Standards covering geotechnical engineering.) The hope was to publish the NA by the end of 2006.
The good news is that considerable progress was made sifting through the feedback received from this summer's public comment period. The bad news is we didn't have time to complete the job in one meeting.
As expected, the most time consuming issue was discussion of pile design. Eurocode 7 allows piles to be designed in two ways:
- from test results, with calculated resistances modified by correlation factors that take account of the number of tests performed
- from ground parameters, with resistances modified by a model factor that takes account of uncertainties in the calculation model
The "ground parameter method" appears to be the one favoured by UK piling engineers, but details of how this method should work are missing from Eurocode 7. The main issues are:
- should characteristic ground parameters Xk be selected as "cautious estimate" or "best estimate" values?
- should a model factor γRd (as suggested by the code) or a correlation factor ξ (as used in the correlation factor method) be used to modify calculated resistances?
- at what stage in the calculations should the model factor appear?
- what value(s) should the model factor take?
- should there be different values of γRd for different pile types (bored, driven, CFA)?
- should the partial factors for base resistance (γb) and shaft (γs) differ according to pile type (as they currently do in the code)?
- should the values of γRd be incorporated directly into γb and γs so only one set of factors is used in calculations?
- how do the results of this method compare with traditional designs?
The current plan for resolving these issues is for a set of example calculations to be prepared for discussion at the next meeting of the Federation of Piling Specialists and for B/526 to decide on the issues above based on the feedback received.
If anyone wants to contribute to this process, please contact me for further info.