We might have similar gophers, Olga. Ours are called northern pocket gophers, and they're quite small, usually about 15-18cm long. I've never seen one out of its hole (alive), but I often see the ground shaking as the gopher tries to pull something down by the roots.
Sounds about right Jan, that's about the size of our critters. I have on occasion seen their heads poking out of the opening in the ground, almost taunting me. I swear I could see a smile on their face!!! I have also seen my leeks and carrots slowly disappearing in the ground, well, not so slowly, and I could be forgiven to think I was stuck in a bugs bunny cartoon! I have cried in rage and frustration many a time.
I don't quite go with the idea that they are solitary animals. Even if they are, their lifespan is 6 to 8 months, maybe 10. The female will give birth to up to 8 babies, 5 or 6 times a year. Solitary maybe, but it's a darn big family if you ask me. Especially if you consider that the gestation period is 3 weeks, the young ones are autonomous in 4 weeks and sexually mature at 2 months old. Like I said, a darn lot of solitary critters in my garden!!