Geostatic Step Questions

[Taylor W]

Hello all,

I have several questions about the geostatic step. I am trying to model a well with a radius of 8.75 inches using 2D axisymmetric elements surrounded by infinite elements. It also includes pore pressure and I am using a gravity load.

1. Is the weight of the water included in the gravity load, or do I need to apply an initial pore pressure to model it?

2. Is the pore pressure output showing the excess pressure beyond the water's hydrostatic pressure, or does it include the water's hydrostatic pressure?

3. I know the initial horizontal and vertical in situ stresses and am applying them by a geostatic stress predefined field. Do I need to apply them to the whole model, or should I exclude the infinite elements?

Thanks!

 

[FEA way]

There are 2 approaches:
- total pore pressure analysis - includes fluid weight; performed when loading caused by fluid weight is large; this type of solution is obtained when gravity load is included in the model; density defined in material properties applies only to the dry material while the contribution from pore fluid to gravity load is defined by providing the specific weight for permeability in material properties
- excess pore pressure analysis - disregards fluid weight; this solution is obtained when gravity load is not included in the model

Infinite elements in Abaqus don't have a pore pressure degree of freedom.

 

[Taylor W]

So if there's no initial pore pressure other than the hydrostatic pressure, should the initial pore pressure be 0?

 

[FEA way]

If the initial pore pressure is not defined by the user then the solver assumes that it's zero.

 

[Taylor W]

Yes, but if I want to prescribe a pore pressure that is only the hydrostatic pressure without any excess, would I set it to 0 or to the hydrostatic pressure? For example, the hydrostatic pressure in my example is 24.8 MPa at y = 0 and 25.1 MPa at y = -30. I know how to set a linear pore pressure, but I need to know which one is correct, 0 or the linear case?

 

[FEA way]

In such a case I would use 0 initial pore pressure but it's best to solve some simple examples to see how it works in practice.