Problem description

In the indicated Plaxis versions some issues exists with the calculation of the maximum value used for the bulk modulus of water. This issue is only relevant for Undrained (A or B), Consolidation and/or Transient flow calculations and only when using stiffer (sand or rock like) materials. In general the effects will be small for these situations.

The following is happening: with an Undrained (A or B), consolidation or transient flow analysis the program calculates a bulk modulus for the water (K_w) based on the Input value of the effective soil stiffness (K’). The program by default uses a large ratio between K_w and K’ such that the Plaxis calculation obtains realistic behaviour, but not such that the Plaxis calculation has numerical problems.

However, to prevent that the program may use unrealistic large values for K_w (if already a high K’ is entered) the value for K_w is limited to the actual value of the water bulk modulus (= 2E6 kPa) called K_w,max.

The following issues now exist:

• 2D2011, 2D2012 and 3D2012 versions: the actual value used in the calculation should be K_w,max / n, in which n is the porosity. However in these Plaxis versions the K_w,max is used directly instead of K_w,max / n;
• Plaxis 3D2010 and 3D2011 versions: no upper limit (K_w,max or K_w,max / n) is used at all.

In the first case this issue might cause too low excess pore pressure to be generated, a too slow consolidation process or might result in incorrect discharges in a transient groundwater flow calculation. In the second case the issue might cause too high excess pore pressures to be generated, a too fast consolidation process or result in incorrect discharges in a transient flow calculation.

Solution

This issue is solved in PLAXIS 2D 2012.00, however note the related issue with unit conversion which still exists in this version.

This issue is also solved for PLAXIS 3D: since 3D2013.00