Place this trap in the middle of a tunnel. Just before a junction in sewers would work great. The characters almost reach the junction, but the way is blocked by a large iron grate, inset into the masonry. A successful Bend Bars will bend the grate far enough to allow passage. However, there is a cumulative 20% chance each time a character passes through the opening that the tunnel will collapse, dealing 3d6 damage to the passing character and effectively permanently blocking the passage.