Consolidation for compensated raft foundation

[hamza41]

Dear soil experts,
We have to calculate settlements for some foundations of different types within an industrial project and we are facing a very soft clay layer of 7.5m thick at 5m depth.
In some cases the applied loads are almost equal, or sometimes even less than the excavated soil load.
Is consolidation still a problem in this case ? (may be due to clay swell and void ratio change) In this case, there should be another formula (other than Sc = Cc/(1+e0) log⁡〖((P0+ΔP)/P0)〗) to calculate this kind of settlement.
Thank you for your help

 

[LRJ]

Clay swell could be an issue, so I'd recommend to check it. The full consolidation equation includes a component for the initial loading (S[sub]s[/sub]) as well as post-yield loading (S[sub]c[/sub], which you state above). Swelling will be calculated using that S[sub]s[/sub] part of the equation.

 

[Okiryu]

At a level corresponding to the bottom of the foundation, the applied stresses from your loads are equal to the effective stresses at that level. So, you will not have changes in effective stresses thru the soft clay and then settlements are not a concern. However, as LRJ mentioned, heave and subsequent settlement should be checked.

You can refer to the book "Foundation Engineering for Difficult Subsoil Conditions" by Leonardo Zeevaert. This book has a full chapter about compensated foundations. Dr. Zeevaert was an expert on this type of foundations which was used to deal with the very soft clays in Mexico City. For compensated foundations, this book also talk about potential issues for settlement outside the excavation.

 

[hamza41]

Thank you.

 

[dik]

Millennium Towers comes to mind...

Dik

 

[hamza41]

But in our case the loads coming from foundation are almost same or less than the excavated earth load.