IN PLAXIS 2D AE and PLAXIS 2D 2015.00, pore pressures calculated for interfaces may be incorrect in the case that the option interpolate pore pressures is used for the adjacent soil clusters, which may result in incorrect stresses in the interface elements.
The resulting interface pore pressures can be checked in Output. To view these results for interfaces, you can double click on the interface directly or use the Select structures icon from the side toolbar to select it first and then press [Enter]. From the menu, select the pore pressure result type.
An easy way to check these interface pore pressures would be to compare it with the adjacent soil: draw a cross section just next to the interface inside the soil and view the pore pressures plot: this should point out the correct pore pressure values that should also be applied in the interface.
This is now fixed since PLAXIS 2D 2015.01
For older versions (PLAXIS 2D AE, PLAXIS 2D 2015.00) this is a possible work around: