Calculation of large deformations of soils

For some geotechnical problems it is necessary to take into account the effect that the deformations or displacements of the soil are large.

This may be the case where a structure penetrates into the soil (e.g. skirted foundation, anchor, spud-can, CPT, T-bar, etc.)  or for riser/pipe-soil interaction problem. In these cases the distribution of the displacement (and strain) pattern changes significantly and the configuration has to be updated during the process.

When analysing these types of problems one generally distinguish between:

• Large displacements problems, where the geometry (first of all) is updated in order to account for the effects of large displacements, e.g. large settlements of soft soil where the soil move according to a fixed ground water table.
• Large deformation problems, where the deformation is so large that the strain can not be defined as gradients of displacements within the initial configuration, e.g. large compaction of peat where the final soil volume is significantly smaller than the initial volume. In this case one need special expressions for strains and stresses which thus also affect the constitutive relationship.

In addition to the effects above, these type of problems may also involve a good description of the interaction between the soil and the structure, since the contact area may change or the contact behaviour change as the structure penetrates deeper into the soil. In these cases the contact can not be modelled by interface elements (as for instance used in Plaxis). Instead it has to be solved by contact surfaces that gradually change positions and shapes. This is available in ABAQUS.

The final problem is that the deformations may be so large that some finite elements becomes too distorted, causing highly inaccurate solutions or even divergence of the solution procedure. In this case it may be necessary to update the mesh or re-mesh. This can be obtained by using the Arbitrary Lagrangian-Eulerian (ALE) method in ABAQUS. Another possibility is to use the Material Point Method (MPM). NGI is sponsoring an on-going project where the MPM formulation is implemented into the Plaxis code.