When you say the tortoise alternates front/back when digging, I'm guessing you mean she is digging down in a forward direction. She is just trying to get away, and after she digs with the front legs she pushes away that dirt with the back legs. If a female were digging a nest, she would back up and put her back-end over the hole and start scooping out dirt with the back feet only.

Get yourself a good UVB light (or get the tortoise out into the sunshine a couple hours each day) and sprinkle calcium over her food, and the shell should firm up. Good lighting, good diet plenty of sunshine (or good UVB lighting), and lots of exercise...that's the prescription for a healthy tortoise.

Yvonne G.