Problem description

In some cases, when using the interpolate pore pressure option for soil volumes that have an (assumed) vertical side inside the model (e.g., a dry excavation with a retaining wall and a high water level outside the excavation, Figure 1), the interpolate pore pressure option may not give a correct result for the steady state pore pressures in the above mentioned versions.

Figure 1. Example of possible problematic geometry
with interpolate pore pressures outside a dry excavation

This is caused by the vertical side not being 100% vertical due to numerical tolerances (see Figure 2). This can result in a side that has a dry, deactivated cluster directly vertically above it. In the interpolate pore pressures option, the pore pressures will be interpolated vertically, so here directly at the side, the pore pressures may become equal to the pore pressure at the inside of the excavation. In this example this would be equal to the dry cluster: a zero pore pressure.

Figure 2. Slightly tilted wall including a slightly tilted soil
volume side outside the excavation (exaggerated)

When can this happen?

This was an issue in PLAXIS 3D 2013 and PLAXIS 3D AE.00 (and fixed since PLAXIS 3D AE.01)

Typical cases in which this can happen is when:

• you modelled an excavation boundary with a water pressure difference between the excavation side and the outside
• and when the excavation side is slightly tilted. This can happen for instance when the surface (wall) is created by extruding a line from the top level to the bottom level of the wall.

How to see this error

One of the easiest ways to check this, is to check if the pore pressure distribution (or groundwater head distribution) along this side. In the preview or results of this phase, hide all structural elements to show only the soil elements. Now check the shaded plot of water pressures and groundwater head values on this excavation side The head values should give a smooth transition over the volume with the interpolated pore pressures. If this is not the case, most likely the mentioned problem occurs in your model.

Solution

This issue is now solved in PLAXIS 3D AE.01
For older models to resolve this issue, a new mesh needs to be generated to force to regenerate and rewrite all water definitions for the calculation kernel.

When this issue occurs in previous versions, please change the settings for these clusters to a User-defined pore pressure distribution reflecting the volume’s top and bottom boundary pore pressure value.