United States
Hong KongTrench protective systems by soil type
A protective system is required at 5 feet unless the excavation is entirely in stable rock. Beyond 20 feet the system must be designed by a registered professional engineer. Maximum slopes depend on soil type.
Subpart P, and the most numbers-dense material in the standards. Excavation kills people, and the rules reflect it.
The depth triggers
| Depth | Requirement |
|---|---|
| 5 feet | A protective system, unless entirely in stable rock |
| 4 feet | A means of egress within 25 feet of lateral travel |
| 4 feet | Atmospheric testing where a hazardous atmosphere could exist |
| 20 feet | A system designed by a registered professional engineer |
Note that four feet appears twice for different reasons, and five is the protective system trigger. Candidates blur them.
The three protective options
Sloping, shoring or shielding. Sloping cuts the walls back, shoring supports them, shielding protects workers without supporting the soil.
A trench box is shielding, not shoring. That distinction is asked.
Soil types and slopes
| Type | Unconfined compressive strength | Maximum slope |
|---|---|---|
| A | 1.5 tons per square foot or greater | Three quarters to one |
| B | Greater than 0.5, less than 1.5 | One to one |
| C | 0.5 or less | One and a half to one |
Weakest soil, widest cut. Candidates sometimes reverse it on the reasoning that weak soil needs a steeper cut, which is backwards - weak soil needs more room to stand up.
Spoil piles
At least 2 feet back from the edge, at 1926.651(j)(2). Close enough to be convenient is close enough to fall back in.
Atmospheric limits
Oxygen between 19.5 and 23.5 percent. Below is deficient, above is enriched and a fire risk.
Common questions
When is a trench protective system required?
At 5 feet deep, unless the excavation is entirely in stable rock.
What are the maximum slopes by soil type?
Three quarters to one for Type A, one to one for Type B, and one and a half to one for Type C.
When must an engineer design the system?
Beyond 20 feet deep.
How far back must spoil piles be?
At least 2 feet from the edge.
What are the acceptable oxygen levels?
Between 19.5 and 23.5 percent.