If left alone 12 hours a day, they will certainly find a way to let you know how unhappy they are. And, like all terriers, they are voracious chewers. However, I lavished attention on the dog when I returned, and I kept the long days to a minimum. A Cairn is not an appropriate dog for an outdoor life. He really needs to live in the house with his people.
Cairns are also rather well known for their digging ability.
A separate area of the yard, without prized shrubs or flowers would be suitable for your Cairn. Some people advocate filling the holes with water or other things to stop the digging. In my opinion, a better policy would be to find an out of the way area for your dog to dig, and not to worry about it. Cairn Terriers can be quite verbal. A Cairn left on a line in the backyard may well become a problem barker, due to loneliness.