I am running a 2D model with CPE8P elements. These are 8 noded elements, plane strain, having pore pressure degree of freedom. Ideally there shouldn't be any out of plane (OOP) strain (E33) in these elements. But I am observing OOP strain. Can anyone provide an explanation as to why this is happening.

Additional Information (not absolutely needed to answer the question):

A horizontal well is modelled. A vertical plane for modelling is considered.

The model has the first step as the geostatic step, wherein initial stresses get established in the rock mass.

In the second step, a borehole is made by removing a semicircular portion. And in its place, a casing is introduced.