Supplied as a set of 4 the arch ground sockets come with a driving cap which makes installation easier. Once installed the arch uprights can then slide into the ground sockets for greater stability.
Its ok for getting an arch installed but you need to be careful lining up and checking for level.I was a little concerned that the tops which took the pounding with the special cap began to buckle and endangered the actual entry for the arch to sit in.
Great item made setting up the pergola so easy - no need to worry about concrete, just bash in these bad boys and you are set. Only word of warning is to make sure they go in straight down - anything slightly off perpendicular still works but is trickier.