OSHA Excavation Safety: Complete Guide to Soil Types and Maximum Allowable Slopes

​Understanding OSHA's Subpart P requirements for excavation slopes is crucial for workplace safety. Different soil types require different slope angles to prevent cave-ins and ensure worker protection. This comprehensive guide breaks down the maximum allowable slopes for each soil classification.

​Understanding Soil Classifications

​Stable Rock

• Definition: Natural solid mineral matter that can be excavated with vertical sides
• Characteristics: Remains intact and stable when exposed
• Examples: Granite, sandstone, marble
• Maximum Allowable Slope: Vertical (90 degrees)

​Type A Soils

• Definition: Cohesive soils with high unconfined compressive strength
• Characteristics:
  • Clay, silty clay, sandy clay, clay loam
  • Unconfined compressive strength ≥ 1.5 tons per square foot
  • No fissures, no subject to vibration
• Maximum Allowable Slope: 3/4:1 (53 degrees)
• Note: Soil cannot be Type A if:
  • Subject to vibration
  • Previously disturbed
  • Fissured
  • Part of a sloped, layered system

​Type B Soils

• Definition: Cohesive soils with medium unconfined compressive strength
• Characteristics:
  • Angular gravel
  • Silt, silt loam, sandy loam
  • Previously disturbed Type A soils
  • Soils subject to vibration
  • Dry unstable rock
  • Unconfined compressive strength 0.5-1.5 tons per square foot
• Maximum Allowable Slope: 1:1 (45 degrees)

​Type C Soils

• Definition: Cohesive soils with lowest unconfined compressive strength
• Characteristics:
  • Granular soils including gravel, sand, and loamy sand
  • Submerged soil or soils with freely seeping water
  • Submerged rock that is not stable
  • Unconfined compressive strength < 0.5 tons per square foot
• Maximum Allowable Slope: 1½:1 (34 degrees)

​Simple Slope Excavations

For excavations less than 20 feet deep:

• Stable Rock: Vertical (90°)
• Type A Soil: 3/4:1 (53°)
• Type B Soil: 1:1 (45°)
• Type C Soil: 1½:1 (34°)

More Information

For more information. Take our online OSHA Subpart P - Excavation Standards Training Course.

We can OSHA assist with the creation of your Safety Manual for Excavation & Trenching work.