There are Maintenance rooms all around the game in various locations.
After taking the Zombrex and doing the first case (getting the zombrex) run back to the safe house and wait for the next case to pop up.
Sullivan will give you a key to the Maintenance rooms.
Simply run inside 10 of them and you'll get the achievement. ( check your map for Wrench icons )
Can also be done from the beginning of sandbox mode as they are all unlocked from the beginning.