LRFD Driven Pile Design 1

[beebs]

thread256-229716

In reference to the above thread, if we use phi(static)=0.4 to determine the length of the pile and during construction use dynamic testing phi(dyn) =0.65 the production pile lengths will be one third shorter and will invite claims from the contractor because the change in quantities is greater than 25%. This is the standard practice for the state agencies we work for. Furthermore, if we use phi(dyn) = 0.65 and depending on the psi load factor we use to calculate the factored load the equivalent factor of safety is equal to 2.0 which is similar to the factor of safety we generally use in ASD pile design with dynamic testing. If this is not the case what is the relationship between phi(dyn) and phi(stat)?

 

[DRC1]

I think you are making the assumption that a static analysis and PDA analysis should give you the same length for a pile. However since the static method is less reliable, it will give you a longer pile length to achive the same load as a PDA test if the soil conditions are exactly as predicted. Since this is rarely the case, properly done static analysis by practioners experienced in the subject soils and PDA test may be much closer than you are predicting. Or not. Static analysis is a rough guess. I am not a fan of the methods ASSHTO uses for static analysis. Really the idea is to give you a good place to start with your pile test program. Note that bearing capacity is only one aspect of pile design. settlement, lateral loads and group effects need to be considered. In short that there is so much variation in soils and the static analysis is so different from PDA that you can not assume that both methods will get you to the same answer.
Final note under ASD, static load test were given an FS of 2 while dynamic testing generally was about 2.5 and staic was 3