Consolidation Rate

[wwen]

Hello All,

Can anyone point me to a reference on consolidation of a clay layer that is above the water table that will still be loaded above Pc. I am wondering if there is a rational/justification for using a Cv that is larger than the one from consol tests.

Thanks

 

[EireChch]

Your Cv is calculated from your consolidation tests data, which involves saturating your soul sample.

I wouldn’t go factoring up the Cv value and applying it to the clay above the watertable as it will give you more settlement in that layer than you probably have. If it’s non Saturated then you could just estimate settlement of that later using a modulus based method (Schmertmann etc). Then use your Cv data a one dimensional consolidation theory for the clay below the watertable.

Also, the watertable may not indicate a definitive line of what soils is saturated and non saturated. Fattdad is often highlights the above point, i believe...

 

[wwen]

Oh I am trying to see if there is any justification for increasing the rate of settlement and wondering if there is some kind of soil merchanic rationale for it (ie insitu soils squeezing airvoids). I know there is the NAVFAC correlation for Cv, I'm not too familiar with the Schmertmann method but isn't that intended to be applied based on CPT data?

 

[EireChch]

Schmertmann is just based on (Young's Modulus) Es, it can be derived from CPT or SPT. However, thinking about it more on the way home from work i have realised that Schmertmanns method is semi empirical and intended for granular soils. Brain fart on my behalf.

You could use any conventional elastic theory method. Janbu (1998) has a method which is little different as it uses the Janbu modulus number "m". This can be calculated based on your consol data.

 

[LRJ]

c_v doesn't control settlement: it just controls the rate of settlement, i.e. it is a time parameter. c_v relates to the time it takes water to flow out of the material. With a larger c_v you will finish primary consolidation more quickly, but your overall settlement for this stage shouldn't change. Depending on the model, you might initiate more total settlement if creep starts earlier - but that would be a fault with the model since creep goes on even throughout primary consolidation, so the total amount of creep should be (pretty much) the same, regardless of c_v.

I think it is reasonable to expect that soil with significant air voids would settle more rapidly than soil with voids full of fluid: there is no fluid pressure to keep the void space open. However, I'm not sure on how this might be quantified. There is a book on unsaturated soil mechanics by Lu and Likos - there might be some information in this book in the sections relating to hydraulic conductivity, k (the correct term for what geotechnical engineers call 'permeability'; k_v (or k_h) is related to c_v (or c_h) by a standard equation). I've been meaning to read this book for a while, but haven't got round to it. If you do find something out, I'd be keen to hear what the conclusion to this is.

 

[wwen]

Thanks guys - I will see if I can dig up that Lu and Likos reference and will update if I find anything.

 

[LRJ]

Something else to consider: soil at a particular degree of saturation may in fact be so much stronger/stiffer that it doesn't settle as quick. This is due to the suction within the clay.

Hope you get an answer with that book.

 

[EireChch]

Is this just for your own interest or research, or is it an actual problem.

If it is an actual problem, to avoid having to go into unsaturated soil mechanics, if the thickness of your "unsaturated" soil layer isnt too excessive (say i dunno, 3m?), could you just assume the entire layer is saturated? Then if you calculate your consolidation settlement (and immediate settlement) is less than your allowable settlement (say 25mm), then is that your problem solved?

 

[Okiryu]

I found this:

https://core.ac.uk/download/pdf/11603128.pdf

 

[Okiryu]

Check also if your soils are saturated. If not or if there are not chances to become saturated, I would calculate elastic settlement and assume that it will occur fast. Basically as the approach that EireChchis suggesting.

 

[fattdad]

A soil sample obtained from above the water table is placed into the odometer. It's then inundated. IT'S NOT SATURATED!

Cv obtained from such an odometer test would reflect the dissipation of pore pressure.

You can scale Cv when using PVDs, as Ch is usually greater than Cv.

f-d

ípapß gordo ainÆt no madre flaca!