I've recently had a retainer wall built in the landscape using the dry-stack method (see photos). Unfortunately, there has already been two occasions within past month of gopher activity behind the wall which has caused the soil to spill out onto the patio. I would like to seal the crevices between the stones with concrete to keep the gophers from pushing the soil outward. I'm not sure if portland cement or posthole concrete would be preferable. Although there isn't a french drain behind the wall, I don't think that drainage will be a problem as there should be plenty of smaller weep cracks for water to drain.
Consider one of the landscape or post-hole expanding foams. (example)
I've never used it, but it looks handy. Nice looking retaining wall!