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Factor of Safety

Factor of Safety

Factor of Safety

Could someone clarify a comment given in a soils report about the factor of safety? A soils report gives cast-in-place pile a allowable bearing capacity is in undisturbed silt till of 360 kPa.  Later in the report it states that "normally a factor of safety of 3 should be allowed in calculations of safe loading value of end bearing piles and to ensure that settlements remain within the previously noted limits".

Do I take 3 x 360 = 1080 kPa and compared that against my factored loads (limit states design), or used unfactored load and compare it against 360 kPa?

Thank you in advance for your support!

RE: Factor of Safety

The word, "allowable" implies that the value given (360kPa) is what you can safely have at the end of the pile, i.e. it includes the factor of safety already.  The next comment in the report does contradict it though.  It would be a good idea to call the geotechnical engineer and ask which it is.  Normally, when we give recommendations in our geotech reports, we include the factor of safety in the pressure or value where applicable so that the client doesn't need to decide what the appropriate factor of safety is.  The geotech engineer would probably be more than happy to clear that up for you.

RE: Factor of Safety

I agree with MRM, you should talk to the geotechnical engineer who wrote the report.  When I see the word "capacity", I think of only strength considerations, not settlement.  If the report later discusses a net allowable bearing "pressure" then I would assume settlement has also been considered.  Sometimes the allowable bearing "capacity" is further reduced to keep settlement within tolerable limits.  In my opinion a good report should tell you what value you should use (considering both strength and settlement issues), and just note what factor of safety was considered in the recommended value.

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