Is the belt damaged from one particular area or whole circumference is damaged?
If it's the first then something must have hit the belt from underneath... with so much lose gravel etc there it's quite possible.
If the belt is damaged all around then there is misalignment of pulleys or one the bearings is to blame and the said pulley is swirling instead of rotating steadily. This would be a more serious matter than the first case scenario.
Start the car and observe all connected pulleys yo the said belt... there should be no swirling.. all attached pulleys should be moving in a steady fashion.. identify the one which is swaying even slightly ... thats the one to blame..