Pea-Gravel has been used for backfilling due to being self-compacting. I believe this to be a myth and disagree to an extent. If your trench is two or three feet deep and you are topping the last foot or so with road base, then Pea-Gravel may work in this situation. However, we are talking landscape irrigation trenches averaging 3” to 12” wide and about 12” deep.
I recommend placing and compacting ¾” limestone aggregate as a backfill material for trenches under synthetic turf. Limestone rock is clean, slow absorbing, and creates a structure that will allow excess water to filter through, preventing shrinkage and settlement. If you live in an area where limestone aggregate is not available, please give me a shout, and I will assist you with a substitute for your backfilling: 888-846-3598.