Joined: 28 September 2007
I am also not aware of any statistics. The earthing company that I deal with prefers welding as best practice, especially where we have multiple tapes to join onto the earth rods.
In these instances, the tapes are weleded together along with a fly lead and then the fly lead makes a single connection onto the rod. If the installation is done right you can get an earth testing clamp around the flylead to test the earth rod.
I guess it also depends upon your maintenance regime and the nature of your installation. If you are on an earth mat with buried rods that have no access I would have thought a weld would have been more secure than a bolted joints. All the earth mat joints underground are welded so the earth rods should be as well.
Where access is available to the earth rod then a bolted joint can be inspected, but if that isn't part of your maintenance regime then it might be better to utilise weld joints.