Problem description

When using circular shapes in Plaxis 3D AE, PLAXIS 3D 2013 and earlier to generate e.g. circular piles consisting of volume elements or plate elements, the resulting elements (volume elements, plate elements and interface elements) are not fully curved (isoparametric), but they have straight sides (see figure below). As a result, at the connection between two adjacent straight elements a significant corner may occur.

It was found that in this way possible locking may occur in the interface elements, particularly if the circular shape (e.g. a pile) is subjected to lateral loading. As a result, the interface elements will not yield, and the (lateral) pile bearing might be over-estimated due to the following reasons:

• Any given (reduced) interface shear strength is not taken into effect because the large normal stiffness of the adjacent interface will prevent lateral shear stresses to develop.
• The earliest possibility to yield is in the stress points of the adjacent soil volume elements outside the pile, which increases the effective pile diameter.

Hence, care has to be taken with the interpretation of the lateral pile bearing capacity from the results of the current analysis.

Note: This issue is about ultimate limit state (ULS) loading conditions, the results under working load conditions (SLS) give reasonable results, even with the mentioned interface locking problem.

Solution

With the release of PLAXIS 3D 2016 we made huge changes to the internal definition of the geometry data: now the geometry is described with parametric functions, and this solves this issue with interface locking. Please see the related article on Fully Parametric geometry for more information.

For older versions (PLAXIS 3D AE, 3D 2013) in order to overcome this issue, a special version of the PLAXIS 3D calculation kernel is available upon request, in which the mesh is optimised to match a smooth circle.  A successful analysis with this special kernel is shown in the related article "PLAXIS 3D Benchmark for Bearing Capacity of Suction Anchors" by R. Witasse