How to allow horizontal deformation from wave loading?

Hi, 

I am a civil engineering student and am trying to model a reclamation project with Plaxis-2D. I was trying to study the lateral deformation caused by wave loading on the artificial island, but since the boundaries are fixed, no lateral deformation is allowed. I tried to use line displacement and set x-displacement as "free" while keeping other boundary conditions as default settings. However, an error was shown when I calculate the result. I knew setting line displacement as "free" means that the model is floating in the air, and thought only setting x-displacement as "free" would be fine. Apparently that was not the case. Therefore, I would like to know, if I wanted to simulate the lateral deformation caused by the wave loading, what should I do to allow the calculation to do done? Thanks!

Regards,

Lokhei LEE