Trench cave-ins kill an average of 40 workers per year in the United States, making excavation one of the most dangerous construction operations. A single cubic yard of soil weighs approximately 3,000 pounds, enough to crush and suffocate a worker in seconds. OSHA's excavation standard (29 CFR 1926 Subpart P) requires protective systems for all trenches 5 feet deep or more, but many fatalities occur in shallower trenches or where protection was available but not used.
This guide covers the competent person requirements that form the foundation of trench safety, soil classification methods that determine the required protection, sloping and shoring options for each soil type, and the daily inspection requirements that catch changing conditions before they cause failures. Every contractor who digs trenches should understand these requirements as thoroughly as they understand their equipment.
Competent Person Requirements
OSHA defines a competent person as someone capable of identifying existing and predictable hazards in the surroundings or working conditions that are hazardous or dangerous to employees, and who has authorization to take prompt corrective measures to eliminate them. For excavation work, the competent person must be on-site whenever workers are in or near a trench.
The competent person's responsibilities include: classifying soil at the excavation site using at least one visual and one manual test method; designing or selecting the protective system (sloping, shoring, or shielding) appropriate for the soil type and trench depth; inspecting the excavation daily before work begins, after every rainstorm, and whenever conditions change; and having the authority to stop work and remove workers from the trench if hazardous conditions develop.
The competent person is not required to hold a specific certification or license. OSHA defines competency by knowledge and authority, not by credential. However, the person must demonstrably understand soil mechanics, OSHA excavation standards, and protective system design. Training programs from OSHA Training Institute Education Centers and manufacturer-specific shoring training are the standard preparation paths.
• Must classify soil BEFORE workers enter the trench
• Must inspect daily before work begins and after rainstorms
• Must have authority to stop work and evacuate immediately
• Must select or design the appropriate protective system
• Cannot delegate these duties to someone without equivalent training and authority
Trench Volume & OSHA Sloping Calculator
Calculate trench excavation volume and OSHA 1926.652 required slope angles by soil type.
Soil Classification: Type A, B, C, and Stable Rock
OSHA classifies soil into four categories based on unconfined compressive strength and other characteristics. The classification directly determines the required sloping angle, shoring design loads, and shielding requirements. The competent person must perform at least one visual test and one manual test (thumb penetration, ribbon test, or pocket penetrometer) to classify the soil.
Stable Rock: Solid mineral matter that can be excavated with vertical sides and remain intact. True stable rock is uncommon in trench work. Most rock has joints, fractures, or weathered zones that prevent it from qualifying.
Type A: Cohesive soils with unconfined compressive strength of 1.5 tsf (tons per square foot) or greater. Examples: clay, silty clay, hardpan. Type A cannot be used if the soil is fissured, subject to vibration, previously disturbed, or part of a layered system dipping toward the excavation.
Type B: Cohesive soils with compressive strength between 0.5 and 1.5 tsf, or granular soils including angular gravel, silt, and silty loam. Previously disturbed Type A soil and fissured Type A both downgrade to Type B.
Type C: Cohesive soils with compressive strength of 0.5 tsf or less, granular soils including sand and gravel, submerged or seeping soils, and any soil where water is freely seeping from the excavation face. Type C requires the flattest sloping angle or heaviest shoring.
Thumb penetration: Press thumb firmly into the excavation face.
• Thumb does not indent = Type A candidate
• Thumb penetrates up to 1/4" with moderate pressure = Type B
• Thumb penetrates easily = Type C
Visual clues for downgrading: Fissures, layering dipping into trench, water seepage, or vibration from nearby traffic automatically move soil down one or more classes.
Trench Volume & OSHA Sloping Calculator
Calculate trench excavation volume and OSHA 1926.652 required slope angles by soil type.
Protective Systems: Sloping, Shoring, and Shielding
Sloping cuts the trench walls back at an angle steep enough to prevent cave-in. Maximum allowable slopes by soil type: Stable Rock may be cut vertical (90°). Type A allows 3/4:1 (53° from horizontal). Type B allows 1:1 (45°). Type C requires 1½:1 (34°). These are the steepest allowable angles. The competent person may require flatter slopes based on site conditions such as surcharge loads, adjacent structures, or seepage.
Shoring uses structural members (timber, aluminum hydraulic, or pneumatic systems) to support the trench walls and prevent soil movement. Timber shoring is designed per OSHA's Appendix C tables based on soil type, trench depth, and horizontal spacing. Hydraulic shoring is faster to install, adjustable, and does not require workers to enter an unprotected trench during installation.
Shielding (trench boxes/shields) does not prevent soil movement. Instead, it protects workers inside the shield if a cave-in occurs. The shield must extend at least 18 inches above the surrounding grade when material could roll into the trench. Workers must stay inside the shield at all times; working between the shield and the trench face is prohibited.
Stable Rock: Vertical (90°)
Type A: 3/4H : 1V (53°)
Type B: 1H : 1V (45°)
Type C: 1½H : 1V (34°)
Example: A 10 ft deep trench in Type C soil must be sloped back 15 ft on each side (10 × 1.5 = 15 ft).
Trench Volume & OSHA Sloping Calculator
Calculate trench excavation volume and OSHA 1926.652 required slope angles by soil type.
Access, Egress, and Daily Inspections
OSHA requires a means of egress (ladder, ramp, or stairway) in trenches 4 feet deep or more, positioned so that no worker must travel more than 25 feet laterally to reach one. In a 100-foot-long trench, at least three access points are needed. Ladders must extend at least 3 feet above the trench edge and be secured to prevent displacement.
Spoil pile placement: Excavated material must be placed at least 2 feet back from the edge of the trench. Spoil piled at the edge adds surcharge loading to the trench walls and dramatically increases cave-in risk. In Type C soil, setting spoil back 4 to 6 feet provides much better margin.
Daily inspections: The competent person must inspect the trench before the start of each shift, after every rainstorm or hazard-increasing event, and as conditions change. Inspections must check for: cracking or spalling on the trench face, water accumulation or seepage, undermining of the trench bottom, surcharge loads added near the edge, and proper functioning of shoring or shielding systems.
Utilities: Before excavation begins, contact the local one-call system (811 in the US) to locate underground utilities. Hand-dig or use vacuum excavation within the tolerance zone (typically 18 to 24 inches on each side of marked utilities). Striking a pressurized gas line or energized power cable in a trench is immediately life-threatening.
• Egress (ladder/ramp) within 25 ft lateral travel in trenches ≥4 ft deep
• Spoil pile set back at least 2 ft from trench edge
• Protective system required for trenches ≥5 ft deep
• Call 811 before you dig. Utility strikes kill
• Never enter a trench without inspecting first
• Keep heavy equipment back from the trench edge
Trench Volume & OSHA Sloping Calculator
Calculate trench excavation volume and OSHA 1926.652 required slope angles by soil type.