Undrained groundflow analysis

In case you want to perform an undrained groundflow analysis, but do not want to have both the material velocity and groundflow equations at the same time in system matrix, you can use group_materi_undrained_capacity. Then the following equation will be used to determine the total groundwater pressure changes in an element:

$$C \; \dot{p}_{total} = \frac{\partial v_i}{\partial x_i}$$

which actually is the groundflow storage equation without permeability. The above equation can be solved on an element-by-element level, so that the groundflow hydraulic head and the storage equation do not need to be added to the complete system matrix. The capacity $C$ should be specified in group_materi_undrained_capacity. Results for the pressure in a element will be written to element_intpnt_materi_undrained_pressure. Application of this undrained analysis can be switched off and on with control_materi_undrained_apply.

This option is convenient to prevent the need for large, and ill-conditioned, system matrices in coupled soil - groundwater analysis. Typically the computational strategy may be like this:

...
(include capacity for undrained analysis in relevant groups)
group_materi_undrained_capacity ...
...
(set the hydraulic heads, and fix them for the remainder of the calculation)
control_reset_dof ...-pres
bounda_dof ...-tpres
...
(solve material displacements in the remainder of the calculation)
control_timesteps ...
control_materi_undrained_apply ... -yes
...

The advantage of the above computational strategy is that never a system matrix with both material velocities and groundflow pressures needs to be solved. When solving the remainder of the calculation Tochnog uses the fixed total pressure from the hydraulic heads plus the excessive undrained pressure of the remainder of the calculation as the full total pressure (when determining total stresses from effective stresses plus full total pressure). Alternatively to setting the hydraulic head at the start with the control_reset_dof, you can also solve the gravity state for hydraulic heads and material displacements (at the expense of a system matrix with both material velocities and groundflow hydraulic pressures in this gravity calculation; but in the gravity calculation only and the remainder of the calculation).