digging under the fence

Question
I fenced in my back yard for my golden retreiver. she won't stop digging under the fence. i have also tried putting her in a crate while away. she just pawed at the plastic bottom, until it was ruined. then i tried putting her on a leash extending the yard. well, she chewed it off. then i tried putting her on a chain. well, she broke it. i have tried other types of ropes, leashes, etc. and she will get tangled up on something. I have even tried placing heavy objects, like wheels, gray bricks around the fence line. i'm scared that she will get trapped or stuck. therefore, she roams for now. she won't leave the front porch, but i do not want her to get hurt. what else do u suggest?  i was told to try a electric line on the bottom of the fence, but i do not want her to feel that pain, because she is so stubborn that she won't stop trying. then she may inflict injuries on herself that will hurt and cost more that the electric fence.

Answer
I have my fence lined with flagstone.  So far it has contained 6 of my puppies and likely more guests than that.  There was one that dug under the unlined fence separating the yard and garden.  

Typical Goldens are poor candidates to be left by them selves.  Look into doggy daycare.