Blocking up holes that goblins use to ambush travellers when crossing the Misty Mountains.
'But their main gate used to come out on a different pass, one more easy to travel by, so that they often caught people benighted near their gates. Evidently people had given up going that way, and the goblins must have opened their new entrance at the top of the pass the dwarves had taken, quite recently, because it had been found quite safe up to now.
"I must see if I can't find a more or less decent giant to block it up again," said Gandalf, "or soon there will be no getting over the mountains at all."'
The Hobbit, Ch.6, "Out of the Frying-Pan into the Fire'