The Interior Impervious Layer condition acts as an interior sound hard wall for the saturating fluid in the porous material. The condition adds a so-called slit to the pressure degree of freedom and makes it discontinuous at the boundary. Note that the condition does not add a slit to the displacement degree of freedom. The condition can be added from the Fluid Conditions submenu and it can be combined with any of the conditions found in the Porous Matrix Conditions submenu.