Problem description

When an outer boundary of the active model is submerged below the phreatic level, so called external water pressures will be added on this surface. E.g. the hydrostatic water pressures acting on soil under water, or a water pressure acting on walls in a submerged excavation.

However, when there is an active model boundary with an interface acting on the outside of the soil assigned to it, and the water pressures are time dependent in a transient groundwater flow, these external water pressures will be missing on this model boundary.

You can see this in the results, when you visualize the external water pressures. At the model boundary with the interface, you will see these external water pressures missing:

 

Solution

There are no straightforward workarounds possible here, however for some situations, a possible workaround could be to remove the interface on the outside of the soil.  The only other way to work around this problem, is to not use a time dependent water table or head. In order to still get the water variation in time, the approach below could work in some cases where the only change for the phase is a change in water level/head:

• split the transient phase into phases with a smaller time interval;
• for each phase, keep the transient groundwater flow calculation and keep the water level (or head) constant at a level that is representative for this phase’s time interval
  Note that this will change the external water levels and external water pressures instantly for each new phase, but the water pressures in the model will still respond time dependent on this change. And due to this discretization the smaller the time steps per phase, the closer the development of the external water pressures will be to the harmonic signal.

This issue is solved since PLAXIS 2D2012.01