Problem description

For the above-mentioned versions, when doing an undrained analysis taking into account unsaturated soil (when switching off the Ignore suction in the phase settings), the excess pore pressures might be too low. When loading or unloading the soil, excess pore pressures will develop in the unsaturated zone, though these excess pore pressures will be reduced compared to saturated soil. Unfortunately, in older Plaxis versions we used to use a too low bulk modulus of air for the unsaturated zone (1 kPa instead of 100 kPa). This lead to too low excess pore pressures in the unsaturated soil.

In case of compressive loading of the soil, this could mean that the shear resistance of the soil is overestimated, and that the model is less safe than one could conclude from the results of the analysis.

Especially for materials with a large unsaturated zone, like clays, this effect on the excess pore pressures could be significant.

Solution

In PLAXIS 2D 2019, PLAXIS 2D CONNECT Edition V20 and PLAXIS 3D CONNECT Edition V20, the development of the excess pore pressures in the unsaturated zone is now taking the compressibility of the water and the Soil-Water Characteristic Curve (SWCC) into account for a fluid bulk modulus when the Ignore suction-option is switched off for that phase.

For more details, please refer to the section called "Undrained behaviour for unsaturated soil" in the PLAXIS 2D 2019 Reference Manual (6.2.4).