Wind Turbine Foundation Design - Rotational Stiffness

[ronanon]

Hello,

I have a problem that I'm hoping someone could help me with.

I need to check the soil conditions on a site for sufficient rotational stiffness. I have been given values for the youngs modulus, shear modulus and poissons ratio. From these values I can get the stiffness modulus.

My question is whether the stiffness values changes in there is ground water present? For different soils I have the parameters with no groundwater present, what if groundwater is present, do these parameters change, i.e. poisson's ratio, shear modulus, young's modulus.

Is there a certain factor they change by?

Thanks

 

[Mccoy]

ronanon,
the stiffness value independent of groundwater is the shear modulus, G

You talk of a generic 'stiffness' but that's used often as a catch-all term.

The elastic modulus E can take up different values depending on the drained or undrained conditions.

For rotational stiffness (rocking), G is the favourite parameter anyway, the simplified models I know are all based on that (rigorous elastic solution by poulos and Davis 1974, Gazetas 1991 and the cone model by wolf-deeks, 2004).

If as you say you have values of shear modulus G I'd be satisfied with that.

I'd just check whence G comes from and if it has been subject to a proper degradation scheme.

I'd also check the poisson modulus (required as an input in the formulas), that's the parameter which is related to groundwater.

Do you have any soil profiles?
Can you give any details about the stiffnes parameters you were given and how they've been measured?

Rocking stiffness is influenced by a limited depth below foundations, so G should be chosen within that influence thickness

 

[ronanon]

Thank you very much for your reply.

To answer a few of your questions;

I have attached the soil information received, however the actual method of testing was not described. I now understand that the main criteria are dynamic shear modulus, G and poissons ratio.

Following on from your response, G will not change with gorundwater but poissons ratio will. As the table does not provide me with a saturated and unsaturated values can I still design the foundation?

Thanks for the tip on the slection of G, for this example given, the foundation is founded on reidual clay (G = 96 MPa).

Can you estimate the variation in poissons ratio between staurated and unsaturated if you have one of the values?

Thanks again for your help

 

[Mccoy]

Ronanon,
it occurs to me only now that you are dealing with a wind turbine and you might use pile foundations instead of a rigid block.

Poisson does not seem a big problem to me. In coherent soils, like the top residual clay or the bottom paleosoils and clayey silt if saturated 0.45 would be a more proper value, the values in the table would indicate a partly saturated soil, whereas I would use a 0.2 for weathered rock mass, not so different from the 0.3 proposed for the highly weathered. You might even carry out a sensitivity analysis with poisson ranging from 0.2 to 0.45 in the top layer and take the most conservative result.

If you have a shallow foundation G would be the input parameter, unsensitive to water content (the G' notation here would not appear to make much sense).

If you have piled foundations, E is usually taken to get to the stiffness, the E' notation would indicate it is a drained modulus (presumably with no groundwater in the profile) and calculation methods will vary in a wide range, as it has been already discussed at times.

Gdyn is most times under conservative, possibly very much so, I don't know the SOP for degradation schemes in wind turbines, I would expect some degree of mobilized shear strenght though and related strain especially in the top residual clay. that would mean a lower G than 96 MPa.

The issue is interesting, we can get no further though if we don't know details such as type and geometry of foundation, layers thickness, and which calculation method you would use.

 

[ronanon]

Thank you very muhc for your help. It has been very useful. I will progress with the design and make further posts if necessary.

Thanks again.