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# Settlement on Clay

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## Settlement on Clay

(OP)
Hi all, I would like to know the limitations of various settlement equations, currently the soil samples are in the lab, but in the meantime, I would like to start off with rough prelim calcs. Basically it's a shallow footing on stiff clay (~3 meters).

I have decided to use the following equation for Initial + Primary Consolidation settlement. My concern lies with the swelling index
Cr, as from what I have researched, the Oedometer test performed to obtain this index is under the drained condition, so how well does this index resemble the dilative nature of stiff OC clay in undrained loading?

There is also this formula which is based on elastic theory, I know the initial settlement is elastic but does the formula also work on consolidation settlement? (My gut feeling tells me this is viable for soft clay hmm..)

I know there is no definitive answer when it comes to soil settlement, but it would be great if you could share some general insights on the topic, ideally on the equation side of things :)

### RE: Settlement on Clay

settlement theory/principles

total = elastic/initial + consolidation + creep

defining the material characteristics are necessary to work out which is important

for a saturated stiff clay (with no other information) it is consolidation

consolidation s = coefficient of compressibility x average stress increase through layer x layer thickness x factor

### RE: Settlement on Clay

do a consolidation test.
Obtain Pp
Anticipate the range of new loads.
Make sure the consolidation test goes over those loads.
Perform elastic theory to get delta sigma V for the applied loads.
Superimpose the change in stress with the soil layers - subdivide.
Run the numbers.

I also agree that, "Settlement" includes compression (i.e., modulus-based), consolidation (i.e., as influenced by pore pressures) and secondary, "Creep." There are different equations for each. That said, I don't typically consider elastic response when I have a consolidation test as the elastic response is built into the data.

f-d

ípapß gordo ainÆt no madre flaca!

### RE: Settlement on Clay

Keep in mind....elastic compression is initial in sands, final in materials such as clay and silts. You can't mobilize elastic compression until resistance occurs and during consolidation, there is little if any elastic resistance.

### RE: Settlement on Clay

If you have any further information about the "clay" like Plasticity Index and SPT-N values you can use Modulus of Volum Compressibility with:
consolidation s = coefficient of compressibility x average stress increase through layer x layer thickness
as maxim22 suggests.
(Note: Don't forget to calculate the effective depth of your load before you use Layer Thickness)

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