risk of cavities into bedrock layer

Dear,

I'm busy with a 3D model with secantpiles wall who come just 1 m above a weathered bedrock. This bedrock have a great risk of karst. I don't have idea to include the risk of cavities in the model.

The secantspiles are modelled as concrete volume. I can add a small gap at the bottom of piles but I would like to avoid the vertical friction at the end (last 2 m for example) of piles.

Is there possibity to integrate cavities on soil without soil collapsing result ? only to have an effect on the wall.

Thank for advices.

Jean-Philippe