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.
Easy to install and instructions easy to follow. However, it would be essential to have the hole former if you have stony ground.
The ground anchor sockets are not tough enough for very heavy clay soil with stones and roots in it. Most of them went in OK but a couple bent in at the top under the force of the hammer blows, trapping the driving cap in the top of the spike. The welds between the flat plate and round part of the driving cap then failed and the round part dropped inside the ground socket. I had to remove the ground socket and grind off the bent-over top lip and start again. Unfortunately you can't just move over a couple of inches if you hit hard ground, they anchor has to be in the right place for the legs of the pergola. In the end I had to use an auger to drill a hole, put the ground anchor in and backfill.