Usually the spiral grows with the snail. But if injured they have the capability to regenerate as long as the center of the spiral where its vital parts are situated is unharmed.
If you walk on a snail you can try to save it: take it home and put it on a small box on top of a lettuce leaf.
Prepare the folowing mixture: powder an eggshell with a pestle and mix in equal parts with wheat flour, place on the lettuce leaf next to the snail. It takes about a week to get better.