Excavations, Concrete and Masonry
How cave-ins, hazardous atmospheres and water are controlled in trenches and excavations, and the OSHA rules for formwork, reinforcing steel, concrete placing and masonry construction.
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8.1 Excavations
Cave-ins are the most dangerous trench and excavation hazard. Other hazards can also kill: asphyxiation in a confined space with too little oxygen, toxic fumes, flammable gases, falls, and water that collects and causes drowning.
| Term | Meaning |
|---|---|
| Excavation | A man-made cut, cavity, trench or depression formed by removing earth. |
| Trench | A narrow excavation below the surface in which the depth is greater than the width, and the width is not more than 15 feet. |
| Shoring | A structure, such as a hydraulic, mechanical or timber system, that supports the sides of an excavation to prevent cave-ins. |
| Shield | A structure strong enough to resist a cave-in and protect workers inside it (often called a trench box). |
| Sloping | Cutting the sides of the excavation back at a specified angle. |
| Hazardous atmosphere | One that may cause death, illness or injury because it is explosive, flammable, poisonous, corrosive, oxidising, irritating, oxygen deficient, toxic or otherwise harmful. |
| Confined space | A space with limited openings for entry and exit, poor natural ventilation, that may contain or produce hazardous substances, and is not meant for continuous occupancy. |
Scope and exemptions
The standard applies to all man-made open excavations in the earth's surface, and trenches are excavations by definition. House foundation and basement excavations are exempt from the protective system requirements if they are less than 7 and a half feet deep, or are benched at least 2 feet horizontally for every 5 feet or less of vertical height, and the horizontal width at the bottom is as wide as practicable and not less than 2 feet, and there is no water, surface tension crack or other condition that reduces stability.
Protective systems
Every excavation 5 feet deep or more needs a protective system, unless the excavation is made entirely in stable rock. Trenches less than 5 feet deep need one too if a competent person decides that there is a hazard of cave-in. The three kinds of system are:
- Sloping or benching the sides to a safe angle.
- Shoring, with supports against the sides.
- Shielding, with a trench box.
The right choice depends on the soil, which a competent person classifies: stable rock, Type A, Type B or Type C (the least stable). For example, the maximum allowable slope for a simple slope in Type A soil is three quarters of a foot horizontal to one foot vertical, in Type B one to one, and in Type C one and a half to one. Use the tables in the standard, and have the competent person confirm the soil type.
A study aid, not published or endorsed by IOSH or OSHA. Laws and limits differ between countries.