Settlement calculation on undrained clay

[lcruzz01]

According to a geotechnical investigation, we´ve encountered a 7 meters lean clay stratum with moisture contents between 9% and 14%. Which method should I use to estimate the settlement of a 4 storey building? I believe using parameters obtained from consolidation tests might not reproduce the conditions encountered in the field. There is no groundwater in the upper 15 meters and the lean clay rests on a sandy clay stratum.

 

[lcruzz01]

Just correcting, the title should say "drained clay".

 

[oldestguy]

Do you have any data on the strength, such as unconfined compression or tri-axial compression tests. Is there any expected future moisture content change? How about Atterberg Limits?

 

[lcruzz01]

The report states undrained strengths between 806-2218 psf (I´m pretty sure they´re correlated from spt blowcounts). Regarding changes in moisture content in the future, one of my recommendations is to take measures to avoid water infiltration. Liquid limits between 30 and 45 and plastic limits in the range of 13 to 24.

 

[Mccoy]

I believe using parameters obtained from consolidation tests might not reproduce the conditions encountered in the field.

In my own experience, results of oedometer lab tests can be overconservative (in unsaturated silty-sandy clays). I ruminated much on the issue, without finding a final explanation. Maybe sample disturbance, maybe lab saturation without field saturation, maybe the lab curve is very different from the field curve.

Sometimes it's good to have a load-unload cycle but it doesn't Always improve things much.

Once when I really did not want an overconservative result I used the Marchetti flat dilatometer.

http://www.mccoy.it

 

[oldestguy]

If one does not have undisturbed samples for determining shear strength, he may lean on some correlation (if there is such a thing) with blow counts. Under the circumstances of what is shown, it would appear that setting a conservative design bearing capacity based on blow counts is about the best one can go, considering settlements will be slight. I'd not be running consolidation tests to begin with for such low moisture contents.