Consolidation: Minimum Pore Pressures and Degree of Consolidation analysis may give spurious displacements of boundary nodes

When performing a consolidation analysis three possibilities exist:

• Staged Construction (SC);
• Minimum Pore Pressure (MPP);
• and Degree of Consolidation (DoC).

For an SC calculation, a time interval must be specified for the duration of the consolidation process whereas for the other two options the duration of the consolidation process is, in fact, a result of the calculation by specifying the end of consolidation. This end of consolidation is specified either as the maximum allowed excess pore pressure anywhere in the model or the degree of consolidation that must be at least reached anywhere in the model.

However, this is not the only difference between the two calculation types. More important is that an SC calculation can deal with changes in the geometry (construction, excavation, changes of water level and/or loads) during the consolidation process. These changes lead to a so-called unbalance between the externally applied loads and the internal stresses in the model and this unbalance will be solved. On the other hand, the MPP and DoC analysis assume a fixed geometry: they are pure consolidation analyses that do not take into account the possibility of an unbalance.
This may have consequences when performing a consolidation analysis directly after an undrained analysis (Plastic or Dynamic). Such an undrained analysis will generate excess pore pressures even on the nodes on the contour of the model, for instance, the soil surface. A consolidation analysis, however, would consider the soil surface an open boundary where the excess pore pressures are by definition zero. Hence, at the start of the consolidation analysis, the excess pore pressures on the soil surface and any other boundary that is not explicitly set as a closed flow boundary are set to zero. By doing so, the total stresses at those boundary nodes are changed and so an unbalance is created. If the consolidation analysis is an SC calculation, this unbalance will be solved as part of the consolidation analysis whereas for an MPP and DoC analysis this unbalance will remain throughout the calculation. For the MPP and DoC analyses, this may lead to spurious displacements of boundary nodes, especially if the soil is cohesionless and does not allow for tensile stresses.

It is therefore in general not recommended to use an MPP or DoC type of consolidation analysis directly after an undrained Plastic or Dynamic analysis but rather use an SC type of consolidation analysis first. This SC type of consolidation phase can then be followed by an MPP or DoC consolidation phase. The SC consolidation analysis will set the excess pore pressures on the boundaries to zero and solve the unbalance so that for the MPP or DoC analysis there will be no unbalance left to influence the results.