Excavation, Trenching, and Shoring

Governing Standard

OSHA 29 CFR 1926 Subpart P — Excavations

Applies to all open excavations made in the earth's surface, including trenches. A trench is defined as a narrow excavation made below the surface of the ground — depth greater than its width, and no wider than 15 feet.

Soil Classification

Soil type determines which protective system is required. The competent person must classify soil before work begins using visual and manual tests (e.g., thumb penetration test, ribbon test, dry strength test).

Type A— Most Stable

Cohesive soils with unconfined compressive strength ≥ 1.5 tsf. Examples: clay, silty clay. Cannot be Type A if fissured, subject to vibration, or previously disturbed.

Type B— Moderate

Cohesive soils with unconfined compressive strength 0.5–1.5 tsf. Examples: angular gravel, silt, previously disturbed soils, soils subject to vibration.

Type C— Least Stable

Cohesive soils with unconfined compressive strength < 0.5 tsf, granular soils, submerged soil, or soil from which water is freely seeping. Examples: gravel, sand, loamy sand.

Protective Systems

Required for excavations 5 feet or deeper (or shallower if the competent person determines a hazard exists). Excavations 20 feet or deeper require a registered professional engineer (PE) to design the protective system.

Sloping

Cutting back the trench wall at an angle. Slope ratios vary by soil type: Type A = ¾:1 (53°), Type B = 1:1 (45°), Type C = 1½:1 (34°).

Shoring

Installing supports (timber, hydraulic, or pneumatic) to prevent soil movement. Includes trench boxes (shields) which protect workers but do not prevent cave-in.

Benching

Cutting the trench wall into a series of steps. Simple benching allowed in Type A soil only. Not permitted in Type C soil.

Competent Person Requirements

OSHA requires a competent person to be present at all excavation operations. The competent person must:

Classify soil type before and during excavation
Inspect excavations daily and after any hazard-increasing event (rain, freeze/thaw, vibration)
Identify and evaluate hazardous atmospheres (test for oxygen deficiency, flammable gases, toxic substances in excavations > 4 ft)
Ensure adequate means of egress (ladder, ramp, or stairway within 25 ft of workers)
Keep spoil piles at least 2 feet from the edge of the excavation
Ensure surface encumbrances (trees, boulders, utilities) are removed or supported

Additional Hazards & Requirements

Utilities

Contact utility companies before digging. Utilities must be located and protected. Hand-dig within 18–24 inches of marked utilities.

Water Accumulation

Workers must not work in excavations with accumulated water unless adequate precautions are taken. Competent person must monitor water removal.

Egress

Ladders, steps, ramps, or other safe means of egress must be located within 25 lateral feet of workers in trenches 4 ft or deeper.

Atmospheric Testing

Excavations > 4 ft deep where hazardous atmospheres exist or could reasonably be expected must be tested before entry.

Vehicular Traffic

Workers exposed to vehicular traffic must wear high-visibility vests. Warning systems (barricades, hand signals, stop logs) required.

Walkways

Walkways or bridges with standard guardrails must be provided where employees or equipment cross over excavations.

Key Numbers to Know

5 ft

Depth requiring protective system

20 ft

Depth requiring PE-designed system

4 ft

Depth requiring egress and atm. testing

25 ft

Max lateral distance to egress

2 ft

Min spoil pile setback from edge

15 ft

Max width for trench classification

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