Hi,
I am working on as similar problem; determine the dynamic properties for soil nails by applying Impulse-Response technique. In order to model the contact between the soil and the nail, I started with a very simple case. I modelled the contact between the soil and a steel rod by itself using Gap element. The results were identical to the ones obtained by experimental procedures.
In the model, I did not define any kind of friction coefficient since I am not I applying a pullout force; the force I am using is very small in amplitude.
I have a doubt that using Gap element in a lateral direction sounds strange, but the results I am getting are promising.
So, as for your problem, I would say use Gap element as a contact element in the lateral direction with defining coefficient of friction and use Gap element as well to model the gap opening at the foot ending due to pull out load.
I hope that would work
Yours truly,