Soil Modeling

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a.moussa posted this 3 weeks ago

Dear All,

I am trying to model a pile embedded in a soil. The applied loads are static concentrated loads at the pile cap. Could anyone provide me with the general modeling procedure for creating the soil model in Ansys. Thank you in advance for your kind help and cooperation.  

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peteroznewman posted this 3 weeks ago

Hello Moussa,

I am not an expert in soil modeling, but I did find a few articles on the Internet (1) (2).

What results do you want from your model?

Are the static loads on the pile cap vertically downward?

Are you modeling a single pile or a group of piles connected to a cap?

Are the piles supported at the bottom on bedrock or only soil?

Are you going to have different layers of soil properties?

Is the cap above the soil or in the soil?

Are you on a Student license or a full license? The Student license has a limit of < 32,000 nodes+elements which will make it difficult to build a continuum model, but you can easily build a beam model with nonlinear springs to represent the soil, however, that was in paper (1) that was studying earthquake ground motion causing inertial side loads on the structure.

The procedure if you use beam models is to create a beam for the pile with the correct cross-section and material properties and attach springs. The procedure if you use a continuum model is you create a block and a pile, subtract the pile from the block, share topology and assign materials to the pile and soil.

If you go with a continuum model, ANSYS provides a list of Geomechanical material models, some of which are appropriate for soil modeling.

 

 

 

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Rana Nasser posted this 2 weeks ago

Hi,

I'm using ansys 15.0 and I can't find the "Geomechanical" material models in the engineering data toolbox, any help?

this is the engineering data toolbox in my version

peteroznewman posted this 2 weeks ago

It looks like there are a few variations of Drucker-Prager, which has been used to model soil in the literature.

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Rana Nasser posted this 2 weeks ago

 but I tried to use the linear drucker - Prager model it gives me the ability to add " yeild stress at zeto pressure and slope in degrees" only, the other models in the "brittle/granular" material models didn't give me what I need also. I need to add the bearing capacity, the unit weight of the soil, c, and angle of friction!, so what is the material model can I use?

 

Rana Nasser posted this 2 weeks ago

Their is an other question, I will use this material in a transient analysis simulation and I didn't fined the " brittle/granular" material models in the engineering data of the transient structural, so I added separated engineering data from the tool box to define the soil and linked it with the engineering data of the transient structural. Is there any negative effect of this?!

 

peteroznewman posted this 2 weeks ago

Engineering Data is configured as a Toolbox. When you create a new material name, it has no properties. To do Static Structural (without Inertial loads), you only need Elasticity, so you add Isotropic Elasticity and provide Young's Modulus and Poisson's Ratio.  Now you want to do Transient Structural, you have to add Density, which you could use for the unit weight of the soil. Now you want to do Elastic-Plastic behavior, you add Bilinear Plasticity and provide the Yield Strength and Tangent Modulus. If there are properties in a material that are not needed by the analysis, it doesn't hurt if they are defined, they are just not used.

ANSYS keeps track of which properties are compatible and deactivates an old one if a new, incompatible property is added. For example, if you add Orthotropic Elasticity to a material that already has Isotropic Elasticity, the Isotropic Elasticity will me marked with a red x. You could delete that property.

ANSYS keeps track of which properties are needed for an analysis, so if you have finished a Static Structural analysis with a material that has no density, and drop a Transient Structural on that Model item, you will get a ? in the Setup cell because of the missing material property. If you try to solve, it will give you a message saying the density property is missing.

 

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Rana Nasser posted this 2 weeks ago

Peter I can't find enough words to express how I'm thankful for your continuous help!!  

I want to make sure that I got you, you mean that there is no problem in adding the a separated engineering data tool box to add material properties which not supported in the transient structural engineering data tool box because ansys will deal with the material properties which are needed in the analysis and ignore the others and if there are two opposite properties it will alarm me and stop solving.

** actually I have used the density property to represent the soil unit weight comes from the test by dividing gama by 10 and adding the result in the density cell, but I thought that if there are material models prepared in ansys to represent soil material it will include explicit gama.      

peteroznewman posted this 2 weeks ago

Rana, you are welcome!

You understand correctly, you can't put incompatible material models together since Engineering Data will disable one before you even start the solver. Once you have built up a good soil model, you can save that material definition and reuse it in many models.

I don't know anything about soil models or what gama is and why you would divide by 10, but density is simply mass/volume, in the right units.

Here is what Drucker-Prager looks like in ANSYS 18.2...

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