How do you get into the dark trenches in dragon age origins

go to the deep roads go to caridans cross then make your way through the caverns until you come to the road and youll see the exit to ortan thaig then you go throughb the thaig till you see a lonely scavenger he will run away. then you will see a bridge you cross the bridge and run into atunnel soon you will stumble upon brankas campsite and youll fight a queen spider then their is a tunnel leading to the dead trenches not the dark trenches.