Endurance, to clear more debris within a short time frame?
Diplomacy, Bluff, or Intimidate, to get bystanders to help?
Perception, to spot tools that could make the job easier?
Dungeoneering, representing experience at clearing away rocks?
What I'm more curious about is why it should be a skill challenge. Is there a time limit, with failures meaning that people trapped in the debris die before they can be rescued? Is the debris precariously balanced so that a failure means that rocks fall, everyone gets injured?