Residential Building Inspector B1Footings and FoundationsMedium
An inspector is reviewing the trench excavation for a continuous footing. The soil is classified as Type C-6 (silty clay, moist) and the trench depth is 6 feet. According to OSHA and general safety practices adopted by the IRC, what is the MAXIMUM allowable slope for the sides of this trench if shoring is not used?
- A2:1 (26.5 degrees)
- B1:1 (45 degrees)
- C1.5:1 (34 degrees)
- D0.75:1 (53 degrees)
Show answer & explanationAnswer & explanation
Correct answer: C. 1.5:1 (34 degrees)
For Type C soils, OSHA regulations (which IRC often references for safety) mandate a maximum allowable slope of 1.5:1 (horizontal to vertical), which equates to approximately 34 degrees from the horizontal. This is a critical safety measure to prevent trench collapse.
Why the other options are wrong
- A. This slope is overly conservative and not the maximum allowed for Type C soils.
- B. This slope is for Type B soils, which are more stable than Type C.
- D. This slope is for Type A soils, the most stable soil type.
Trench Slope for Type C Soil
For Type C soils, the maximum allowable slope for an open trench excavation (without shoring) is 1.5 horizontal to 1 vertical (34 degrees) to prevent collapse and ensure worker safety.
- OSHA classifies soils for excavation safety.
- Type C soils are the least stable and require the flattest slopes.
- Slopes are expressed as horizontal:vertical ratio.
Memory trick: Soil's type dictates the slope, lest hope be lost.