Sleeper frame for artificial grass.

Waiting for more sand to fill the other side but been thinking about the stability of this.... The sleepers will be buried with gravel around the outside all the way to the fence. It has a retaining wall on the left and against on the other 2 sides. I have added wooden pegs outside but none inside apart from the retaining wall on the left. Do I need some inside ?

Also do I have to nail grass onto wooden frame which I have not installed/ or sleeper? Grass is 4cm pile and I'm thinking of pinning it down with u pins directly onto the sand inside of sleepers from about 2cm therefore leaving 2cm above the sleepers.