After the crash site, you will follow a system of tunnels and ridges filled with enemies (most with heavy machine guns, some with fusion rockets and rail drivers). When you meet the Red Faction miner in the caves, he will say that he will watch your back. At this time you will follow a tall tunnel to a bridge -- you do not want to be on the bridge. Between the miner and the bridge is a turn off that leads to a catwalk going over the bridge. Kill the two gaurds and follow the catwalk until it stops at a hole in the wall. Jump from the catwalk into the hole and follow it until you get the grenades. Use them to kill the turrets and the fusion rocket carrying guard on the other side of the large rock by the grenades. This saves time and health you would have spent trying to fight the guard and the turrets.