Investigations and evidence collection can take one into unusual settings including inside confined spaces such as trash compactors, storm sewers, underground vaults, and into deep excavations or trenches. Many locations may appear relatively benign but can hold hidden hazards with potentially fatal consequences for any who enter unprepared. These hazards can include, depending on conditions; lack of oxygen, toxic or flammable gases, risk of entrapment/engulfment, physical or mechanical hazards, and crushing or entombment from soil collapse.

The composition of air in trenches and confined spaces can be changed by both natural and man-made activities. Use of industrial or piped chemicals, welding activities, or old hazardous materials spills might affect the composition of air as well as the natural products of biological decomposition. The composition is usually affected in two ways, either there is a toxic contaminant or the oxygen is displaced by the chemical. If dangerous atmospheric concentrations are detected, they can often be controlled through the use of local ventilation.

Access to any confined spaces must be kept open and clear for egress, rescue, and communication. In general, entrants into confined spaces should wear full body harnesses and a means for non-entry rescue (e.g. a retrieval system) unless it can be shown to create a greater hazard for the entrant.

Trenches and excavations can present similar hazards as confined spaces with the added danger of wall collapse. A cubic foot of soil weighs in the neighborhood of 100 lbs. A trench wall collapse might contain three to five cubic yards of soil, weighing from 8,000 to 14,000 lbs. At five feet in depth, OSHA mandates control measures to protect anyone who enters the excavation. These measures might include sloping the excavation walls or the use of systems such as trench shields or trench boxes to protect the entrants.

From: Watch Out Below by The Safety Guys