United States
Hong KongSoil classification explained
Type A is 1.5 tons per square foot or greater, Type B above 0.5 and below 1.5, Type C at 0.5 or less. Maximum slopes are three quarters to one, one to one, and one and a half to one respectively.
A small table that generates a disproportionate number of questions, because it can be asked from either end.
The grid
| 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 |
Stable rock is a fourth classification and needs no protective system at all, which is the exception written into the five-foot rule.
Why weaker soil gets a flatter slope
Because it will not stand up steeply. Type C is the weakest and needs the most room to lie back at a safe angle.
The instinct is that weak soil needs a steeper cut to hold, which is backwards. Weak soil needs a wider excavation, which costs more and is why contractors are tempted to classify optimistically.
What downgrades a classification
Previously disturbed soil is Type C regardless of what it would otherwise be. So is soil subject to vibration, or with water seeping through it, or sloping layers dipping into the excavation.
A backfilled trench being reopened is disturbed soil. That is a common scenario and the answer is Type C.
Who classifies it
A competent person, using at least one visual and one manual test. Not a guess and not the excavator operator's opinion.
Layered soils
Where layers differ, the excavation is classified by the weakest layer, unless a more stable layer lies below a less stable one and the analysis supports otherwise.
The safe default is the weakest.
Common questions
What are the OSHA soil types?
Type A at 1.5 tons per square foot or greater, Type B above 0.5 and below 1.5, and Type C at 0.5 or less, plus stable rock.
What are the maximum slopes?
Three quarters to one for Type A, one to one for Type B, one and a half to one for Type C.
Why does weaker soil get a flatter slope?
Because it will not stand up steeply and needs more room to lie back safely.
What makes soil Type C automatically?
Previous disturbance, vibration, water seepage, or layers dipping into the excavation.
Who classifies the soil?
A competent person, using at least one visual and one manual test.