Foundation Settlement & Raft on Piles

[BatMan]

Dealing w/ high water table with moderate seismicity here..
Geotech recommended using a raft on pile to bedrock at 40-ft below grade (some at 80-ft)
1. When does it trigger for geotech to recommend using piles over a mat foundation? If a mat settles one or two inches over a large area relative to other corner, does it really affect anything if functionality of the structure is not compromised? Is there a code recommended value?
2. Method of getting an idea of designing the raft on piles: I've read a paper recommending first running an analysis of the raft as a slab-on-grade w/ soil spring or subgrade modulus. Then check for settlement. If the settlement is less than 3 inches, then no need for piles. otherwise use piles and re-iterate the analysis. What is basis of this approach?
I always appreciate any feedback here.

 

[oldestguy]

Your last question probably can be be answered after more info is provided, such as structural loadings and tolerance for settlement. That also ties in with plumbing beneath and its tolerance. I, for one, question spring constant approach without running laboratory consolidation tests and using that info for settlement estimating.

 

[ATSE]

Will you have piles between 40ft and 80ft? Does that mean the site depth to bedrock varies quite a bit across the mat-slab foundation length?
Seems like you would expect a bit of differential settlement if there were no piles or ground improvement
If you use "soil springs" for your soil-structure interaction, you need to consider long term consolidation, as oldestguy has noted. And keep in mind the equivalent soil spring will not be constant for the entire foundation footprint.

 

[Okiryu]

Perhaps, the piled raft was recommended because you may have liquefaction issues?