Heave in Over Consolidated Clays

[EireChch]

Hi all,

I am looking for some references for calculating heave in over consolidated clay caused by stress relief due to removal of overburden (i.e. deep excavation).

I have been through Bowels and Tomlinson but they both seem to just discuss the factor of safety against bottom heave. I would like to estimate the amount of heave caused due to stress relief.

Any help appreciated.

Thanks.

 

[fattdad]

seems like you take Cr and run the numbers? It'll be a negative load and you'll obtain a negative settlement - i.e., heave.

is it right? It'll be a gauge and you'll be able to scale the likely problem.

f-d

ípapß gordo ainÆt no madre flaca!

 

[EireChch]

Thanks for the response f-d. Yes that is a method that we discussed in the office. Like you said, it would give you ball park figure.

Bowels in his text (Foundation Analysis and Design-Fifth Edition, page 542) states that "In general, where heave is involved, considerable experience and engineering judgement are necessary in estimating probable soil response, for currently there are no reliable theories for the problem".

FEM is a possible tool, but that isnt available to me.

 

[aeoliantexan]

I was a bystander on a recent project involving a 70-foot deep, 180-foot diameter excavation in highly overconsolidated, slickensided, fat clay. I-D consolidation predicted some 9 or 10 inches of heave, and a FLAC analysis about half as much. The condition was complicated by an artesian aquifer beneath the clay, requiring dewatering. the excavation was made, foundations constructed, a heavy structure built and backfilled over a period of several months. Total movement was less than 1 inch. Apparently the clay was highly impermeable despite the slickensides, and the movement never caught up with the construction.

Would I count on that the next time? Gosh no!

 

[fattdad]

I would never disagree with running an FEM model. Problem is you have to characterize the modulus values for the stress range(s) of interest.

If you have the recompression index (ratio), which is not too-much governed by permeability, you sort of have the constrained modulus. Actually there are interpretation methods to get constrained modulus from an odometer test. Either way, with the odometer test, you should be able to fashion a conceptual, "Model" to gauge the consequences of offloading the clay. Such rebound would result from the stress change and you'd be able to see how much of a concern it's likely to be?

Then decide if you need FEM, 'cause the data to drive the model will take additional efforts.

So, is the juice worth the squeeze? Don't know?

f-d

ípapß gordo ainÆt no madre flaca!