pile soil dynamic spring constant estimation

huangyhg

pile / soil dynamic spring constant estimation
we have a situation. equipments (compressor, generators) would be supported on "piled raft" foundation. the soil is really bad on the site. rafts were not good enough for controlling the settlement,hence the wise people on the project made the decision to go for piled raft. couple of facts:
1) piles don't bear on hard staratum, and they are only friction piles.
2) soil is stratified with improved top sand layer followed by various clay and sand layers at bottom.
3) piles are hollow steel piles (steel pipe piles) with sand filled inside.
we are trying to estimate the static and dynamic soil spring constants for soil as well as the pile. it being piled-raft, both soil and friction pile will come into action.
these spring constants will be input to the dynamic analysis model in staad.
appreciate your help on this
thanks
skc
ps : i realize this topic has been discussed over and over again. somehow i couldn't find the specific information i was looking for, hence posted the message.
this is the type situation where pile load tests really "shine". the slope of the load test results graph is the spring constant that you need (for the pile). of course the graph won't be straight lines, so you will have to use engineering judgment to interpret the results.
suggest forgetting about the load carrying ability of the raft itself - most likely that is negligible compared to what the (friction) piles can do.
imho, design of this type foundation is one of the most interesting kind - takes as much "art" as "science" to get a foundation that works properly.
i've done several of these and the key is that you must have a dynamic load test as well as a static.  a static load test is no good for vibration calculations in these situations.  the dynamic stiffness of a pile supported foundation (even a friction pile one) is much greater than the static stiffness.  it is this stiffness that you will use when doing your vibration calculations to make sure it doesn't come apart or cause discomfort when you crank up the equipment.
for some background and help on how to calculate/estimate this type of loading, see "design of structures and foundations for vibrating machines", arya, o'neil, & pincus.
slideruleera & swearingen,
thanks for your input. i appreciate you time and effort.