Piled Raft Design - Subgrade Modulus - Pile Spring Stiffness.

[JohnnySm]

Dear All,

Designing a RC piled Raft. medium dense clay of approximately 100kn/m2 CU value. Just want some friendly advice.

2 options as i see it

1. model the raft as a slab supported on a continuous set of springs. The K value (modulus of subgrade reaction) is required? No plate bearing test has been done on site and with no geotechnical specialist involved either, is it acceptable to just assume a very soft sub strata with K value of 5000kn/m2/m? springs at 1m c/c's would therefore result in 5000Kn/m k value for my model.
Model the piles as springs? No deflection values have been provided by a piling specialist.. again is it acceptable to assume a value here? I wouldn't even know where to start?

2. Or alternatively is it simpler to design the slab as a flat slab? this would ignore the concrete soil interaction and assume all the load to pass through the piles? Or is that unreasonably conservative, i imagine the rebar tonnage would rocket?

Thank you in advance.

 

[WARose]

If you are having to use a raft foundation (in the truest sense of the phrase) and you are using piles......you definitely need the input of a geotechnical consultant. That's just about mandatory for anything involving piles. I can give you some typical spring constants here.....but that's a lot of guesses that won't sound too good in a deposition.

As far as the model goes: yes, I've seen some people in such a situation treat the mat like it's sitting on air. Off the top of my head, I'd say that is probably the worst case scenario for the mat/raft and the piles. But you are still going to need a geotechnical report to give you the proper info on those piles.