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Technical Note on the use of BR470 in Soft Clay

In the design of working platforms the design document BR470 states in Appendix A2:

“Where cu<20kPa the ground will be so soft that special measures will be needed to construct a working platform, and a more sophisticated type of design calculation is appropriate.”

This immediately begs the question as to what type of design is appropriate and whether a relatively simple adjustment of the BR470 method can be developed to extend its range of validity. This technical note considers this question and indeed whether the stipulation of a minimum undrained shear strength of 20kPa is an appropriate value.

The main problem in considering this question is the dearth of experimental field testing of working platforms. In the absence of such testing all that can be done is to consider theoretical analyses. To use one theoretical analysis to evaluate another theoretical analysis leads to concerns over the validity of the theoretical methods. In general, though, more sophisticated theoretical analyses can be considered to give a more accurate representation of field behaviour than simple methods and thus it is considered valid to use a more sophisticated method to assess a simple one (the BR470 method).

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