stopping the tunneling under kennel

[rowdiebrindlepit]

I recently was given a female pit and when i was given her i went out and got an out door dog kennel for her and my other pup and when it is just one of them in there they are fine but when i put them together to play they like to try to dig under the fence and escape. i am thinking about putting up a shock line on the bottom so that when they touch it they will get a joly and hopefully stay away from the fence. but i dont really wanna have to shock my dogs into learning that it is unacceptable for them to do that. so i would like some suggestions on how to stop the tunneling.