Home Solutions Workers need to enter a below-ground, concrete-lined vault to repair a leaking valve that controls the flow of wastewater from several buildings at your plant. The opening for the pit is outdoors and secured with a 36-inch diameter manhole cover. The pit is 12 feet deep, and the walls are 6 feet x 6 feet wide. The valve is at the bottom of the vault. You opened the cover, tested the atmosphere, and found oxygen levels to be at 16%. No toxic substances were detected. After ventilating the space for 15 minutes, the oxygen reading reached 18% and remained at that level.