After you escape the cell block, you head downwards into the sewers beneath the Asylum. You are brought to a maze of tunnels and gates that make up long corridors. The only way to escape this area is by going down a manhole even further downwards. To do this you need to empty the water in the manhole tunnel by turning two water valves off. One is to the right of the area (where Walker drops down to stalk you) and is fairly straight forward to reach. It's behind a green door.
The other valve is opposite of the other one of the main chamber. You have to trek down a few tunnels to find it but watch out as Walker is patrolling the perimeter. Once you turn the valve the achievement unlocks. Make your way back to the main chamber and down the shaft to continue the story.
Not much help in a solution for this as it isn't hard at all and its impossible to miss, unless you quit playing the game entirely.