Overconsolidation vs Normally Consolidated Clay:

[geo6]

Recently we are designing a large earth dam that will be gradually raised over the years for a total height of >60 m. There is extensive weak clay within the footprint: some at the shallow depth and some at deeper depth.

The shallower clay was found to be over-consolidated (OCR around 9) while the deeper clay appeared to be behaving in a normally consolidated manner.

Due to dyke loading, both clays will eventually behave in a normally consolidated manner. (which means that loadings from the dyke will eventually exceed the pre-consolidation pressure in the shallow clay.)

Now here comes my question:

1. Is overconsolidated clay desirable in this case (additional loading) provided that the loading does not exceed the pre-consolidation pressure).I know for building projects this means slower and less settle ment but for a large tailings dam this could be neglible?

2. Does the question of which one is more desirable also depend on the drainage condition? Say for a poor drainage condition (single drainage, very thick layer etc.) , normally consolidated clay is NOT desirable since they will try to settle more under loading but the excess pore pressure has nowhere to go due to the poor drainage condition ???

3. On the 1st question, Overconsolidation is a good thing because of higher shear strength that comes with it? Undrained shear strength is used since the normally consolidated manner will govern the stability from the beginning.

But again doesn't NC clay gain more strength at higher stress level as it consolidates???? (The normalized stress ratio is used in design)



Sorry for the lengthy discussion but any clarification is extremely welcomed !!!


Thanks !!!

 

[edi1478]

I hope you solved the problem. If not,the problem is of not great importance in OCR ratio and NC ratio. The problem arise from distribution on these lences in profile and in plan.And what do you intend to put on these clay (rockfill or balast with clay core, concrete mask,...).If you put clay on clay, this is . Normally, we are not accepting OCR clay in dam design (a dam must admit settlements of an order 0,5-1%H without cracking). If the clay is under D/S shell, the problem is a bit complicated. The fondation drainage is in contradiction with this material. And you must be sure there is not water circulation under it. Under U/S shell, you must take in consideration for slope stability in condition of saturated material. The problem totally differ from a civil foundation.
So, if you really want an answer, you must provide for discution a dam section, a longitudinal profile and a plan, with distribution of this material on sections (and the rest of geological condition).
edi47
edimihaesteanu@yahoo.com

 

[fattdad]

it's all about the undrained shear strength. The o/c clay will have greater undrained shear strength in contrast with its overburden stress than the n/c clay. Irrespective of this fact, you have to control loading to safely mobilize undrained shear strength AND you have to allow for the relief of excess pore pressure to realize an increase in undrained shear strength before additional load is applied.

f-d

¡papá gordo ain’t no madre flaca!