Constrained Modulus (M) or Coefficient of Compressibility (mv)

[SuperSandman]

Hi Guys

I have CPTu data which gives me the correlated M of a clay. I also have the oedometer results of the same clay, which gives me mv for various stress increments. I understand that I can determine the settlement of the clay by using M or mv in their correct equations.

I do not understand how the expression M = 1/mv applies to the data which I have, because the mv from the oedometer varies for different stress ranges, whereas the M from CPTu data is not dependent on a stress range and is more or less constant throughout the clay layer.

Could someone please shed some light as to when it is appropriate to use M and mv in determining settlement, taking into account the CPTu and Oedometer data for the same clay is available for interpretation.

 

[L0k]

Hi SuperSandman

In my opinion the M from CPTu can be used straight forward since it is already in the overburden strength, while the oedometer-mv is depending on your stress range in each layer

 

[SuperSandman]

Thank you L0k

I have read a few CPTu manuals and some state that the constrained modulus (M) obtained from correlation with cone resistance is only applicable to a stress increments of up to 100kPa. The stress increment for my project is less than 100kPa.

The settlement which I have obtained using the stress increment from the oedometer mv is almost 3 times as much as the settlement obtained using the CPTu M.

It would be interesting to calculate settlement with the data I have for stress increments exceeding 100kPa, using both the oedometer mv and the CPTu M.

 

[L0k]

I believe insitu data is more accurate in this case using the constrained Modulus M - CPTu

 

[DougHole]

when you say max stress increment of 100 kPa does that mean 1 or 2 below...

1. the increase in total stress at the surface is a max of 100 kPa (e.g. a a 5 m thick fill of 20 kN/m3 = 100 kPa i.e. a max fill height of 5 m in this example)

2. or is it the increase in effective stress at any depth. e.g. say at 10 m depth, the original effective stress was 200 kPa and now its gone up to 250 kPa (upon loading) its a 50 kPa stress increment i.e. less than 100 kPa.

for a fill with a large extent, 1 and 2 are the same thing but for a local footing load that attenuates with depth, these will be different things...




Doug Hole
Junior Geotechnical Engineer

 

[Mccoy]

That's the big question mark of in situ tests, stress level. Supposedly it should be equal to overburden stress.
Sometimes it's taken as the maximum-frequency interval of loading stress values underneath shallow foundations, range which usually goes from 100 to 200 kPa (1 to 2 tsf).

Msot interesting thing though, and the OP noticed this as well, is that pretty often edometric modulus values from lab tests are smaller than edometric modulus values from various in situ correlations.
This means that, designing by lab tests, we may tend to design in a very conservative way (settlements-wise).

And, since it is pretty rare that any of us has an objective check of the real settlements value after building, we actually have no way to check how reliable are our methods.

This amounts to real bad science.

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