Varying Soil Parameters with respect to time for Pile foundation

You can do the reassigning of the material data set for a soil cluster with reduced strength, however, you need to be aware that if no out-of-balance is introduced then the model will not "feel" any change. This has to do with how Plaxis calculates equilibrium. If the change you do in the model, does not introduce any out-of-balance, e.g. different unit weight, activation/deactivation of any cluster or structure, then the equilibrium is already reached and therefore, there is nothing to solve. Thus, the reassigning of the material data set has no effect.

In general, a difference in weight between the changed materials can cause an out-of-balance, however, if the values are the same or very similar this might be skipped due to the tolerance allowed.

A Finite Element calculation is driven by out-of-balance forces in the model. These out-of-balance forces can be caused by a change in load/stresses Δσ or a change in displacements and strains Δε, and then uses the (simplified here) relationship:

Δσ = E * Δε.

That means that the calculation itself does not see an out of balance when you change the stiffness alone: there will be no change in stresses (Δσ = 0), hence no change in strains (Δε = 0), and vice versa.

If we would only change the stiffness E, but no change in σ or ε, then we will see:

0 = E * 0

Note that the Finite Element calculation uses increments to calculate the changes. So it is not the total that is used to calculate, but it looks at the differences, and builds up the displacements.

So one way to model such a reduction in strength would be to use an appropriate material model that degrades its strength with time and do a transient analysis such as a fully-coupled flow deformation analysis. Such a material model could be the soft soil creep model.

Another way is to include strength reduction is to either use different material strengths in different parallel phases such that they all start from the same initial phase but each phase will have its own reduced strength material properties assigned. Then do a transient analysis such as the fully coupled analysis. In essence which this type of method will do is basically you are running one phase with an initially high strength, then another phase that starts from the same initial phase and will have a slightly reduced strength, then another phase which starts again from the same initial phase and will have further reduction in its strength parameters and so on and so forth. So eventually you will be able to compare and combine the results to see the full spectrum of strength reduction analysis this way.

 

You can do the reassigning of the material data set for a soil cluster with reduced strength, however, you need to be aware that if no out-of-balance is introduced then the model will not "feel" any change. This has to do with how Plaxis calculates equilibrium. If the change you do in the model, does not introduce any out-of-balance, e.g. different unit weight, activation/deactivation of any cluster or structure, then the equilibrium is already reached and therefore, there is nothing to solve. Thus, the reassigning of the material data set has no effect.

In general, a difference in weight between the changed materials can cause an out-of-balance, however, if the values are the same or very similar this might be skipped due to the tolerance allowed.

A Finite Element calculation is driven by out-of-balance forces in the model. These out-of-balance forces can be caused by a change in load/stresses Δσ or a change in displacements and strains Δε, and then uses the (simplified here) relationship:

Δσ = E * Δε.

That means that the calculation itself does not see an out of balance when you change the stiffness alone: there will be no change in stresses (Δσ = 0), hence no change in strains (Δε = 0), and vice versa.

If we would only change the stiffness E, but no change in σ or ε, then we will see:

0 = E * 0

Note that the Finite Element calculation uses increments to calculate the changes. So it is not the total that is used to calculate, but it looks at the differences, and builds up the displacements.

So one way to model such a reduction in strength would be to use an appropriate material model that degrades its strength with time and do a transient analysis such as a fully-coupled flow deformation analysis. Such a material model could be the soft soil creep model.

Another way is to include strength reduction is to either use different material strengths in different parallel phases such that they all start from the same initial phase but each phase will have its own reduced strength material properties assigned. Then do a transient analysis such as the fully coupled analysis. In essence which this type of method will do is basically you are running one phase with an initially high strength, then another phase that starts from the same initial phase and will have a slightly reduced strength, then another phase which starts again from the same initial phase and will have further reduction in its strength parameters and so on and so forth. So eventually you will be able to compare and combine the results to see the full spectrum of strength reduction analysis this way.