Below Grade
Trenches, manholes, and low-lying areas may become hazardous from leaking gases heavier than air such as propane or from gasses such as carbon monoxide seeping through the soil and into the confined space.

Case study – Read the Case Study
A construction crew finished installing a 12-foot-deep manhole without incident. After the crew left the area, 265 pounds of nitroglycerin-based explosive in 20 boreholes, each 18 feet deep, were detonated 40-60 feet from the manhole. A worker who entered the manhole 45 minutes after the explosion collapsed within minutes, and two coworkers descended into the manhole to rescue him. One rescuer retrieved the unconscious worker before collapsing on
the surface, and the other rescuer died in the manhole.
An investigation determined that carbon monoxide released from the
explosion had migrated through the soil into the manhole. Carbon
monoxide concentrations at the bottom of the manhole two days after the incident were 1, 905 ppm (parts per million). This concentration was well above 1,200 ppm, the concentration classified as Immediately Dangerous to Life or Health (IDLH). Tests following ventilation of the manhole showed that high levels of carbon monoxide reappeared as a result of continued migration from the surrounding soil. Subsequent monitoring of the manhole showed a decline in carbon monoxide levels over the next 8 days.

Accumulation of contaminants in areas not classified as confined
spaces. (Please note)
A variety of spaces can become hazardous because of the products being used or the work being done in them.
These areas can be deadly even if they are not classified as confined spaces and even if the confined space provisions of the Regulations do not apply.