Trenching, Scaffolding, and Struck-By / Caught-In Hazards

Why Trenching, Scaffolding, and Struck-By Hazards Matter

Excavations, scaffolds, and moving objects are consistently among the leading causes of construction fatalities in the United States. OSHA's construction safety standards — primarily found in 29 CFR 1926 — set minimum requirements that protect workers every day. Understanding these rules is essential for the OSHA 10 credential and for working safely on any job site.

Soil Classification and Excavation Hazards

Before any trench or excavation is dug, a competent person must classify the soil. A competent person is someone trained and authorized to identify hazards and take corrective action. Soil classification determines which protective system is required.

OSHA defines three soil types under Subpart P:

• Type A is the most stable soil. Examples include hard clay and cemented soils. Type A soil has an unconfined compressive strength of 1.5 tons per square foot (tsf) or greater. It has not been fissured, is not subject to vibration, and has not been previously disturbed.
• Type B is moderately stable. Examples include angular gravel, silt, and previously disturbed soils that do not meet Type A criteria. Unconfined compressive strength is between 0.5 and 1.5 tsf.
• Type C is the least stable. Examples include granular soils such as gravel and sand, submerged soil, and soil from which water is seeping. Unconfined compressive strength is 0.5 tsf or less. Type C is the most dangerous and requires the most protective measures.

Field tests a competent person can use include the thumb penetration test, the pocket penetrometer, and observing whether the soil is fissured or layered.

Protective Systems for Excavations

For excavations 5 feet or deeper, a protective system is required unless the excavation is entirely in stable rock. For excavations 20 feet or deeper, a registered professional engineer must design the protective system.

Three protective systems are permitted:

1. Sloping — cutting the trench walls back at a safe angle. Required slope ratios are: Type A = ¾:1 (horizontal to vertical), Type B = 1:1, Type C = 1½:1. Sloping requires the most horizontal space but no equipment beyond the excavator.

2. Shoring — installing timber, hydraulic, or pneumatic supports against the trench walls to resist soil pressure. Shoring is used when space is limited and soil is unstable.

3. Shielding (trench box) — placing a manufactured steel or aluminum box inside the trench to protect workers. The shield does not prevent collapse; it protects workers inside it. Workers must stay inside the shield's protection zone.

Additional excavation rules: spoil piles must be kept at least 2 feet from the trench edge. A means of egress (ladder, ramp, or stairway) must be within 25 lateral feet of any worker in a trench 4 feet or deeper. Atmospheric testing is required in any excavation where hazardous atmospheres may exist.

Scaffold Safety — Subpart L

Scaffolds must be erected, moved, dismantled, and altered by a qualified person and supervised by a competent person. Key requirements include:

• Capacity: Scaffolds must support their own weight plus at least four times the intended load.
• Guardrails: Required on all open sides and ends of scaffold platforms 10 feet or more above the lower level. The top rail must be between 38 and 45 inches high. A mid-rail must be positioned at approximately the midpoint between the top rail and the platform surface. Toeboards are required to prevent falling objects.
• Platform planking: Platforms must be fully planked or decked. The gap between planks and between the platform edge and the structure being worked on must not exceed 1 inch (with exceptions for certain work).
• Access: A safe means of access — such as a portable ladder, hook-on ladder, or stair tower — must be provided. Climbing cross-braces is prohibited.
• Inspection: Scaffolds must be inspected by a competent person before each work shift and after any event that could affect structural integrity.

Struck-By and Caught-In/Between Hazards

Struck-by hazards occur when a worker is hit by a moving object. Common examples: vehicles, swinging crane loads, falling tools or materials, and projectiles from power tools.

Controls for struck-by hazards:

• Establish exclusion zones around crane swing radii and vehicle paths.
• Use hard hats (head protection) and high-visibility vests in vehicle traffic areas.
• Tether tools when working at elevation — tool lanyards attach to the worker's belt or wrist and prevent dropped tools from becoming projectiles.
• Use barricades and warning tape to keep non-essential workers out of hazard zones.
• Secure loads before lifting; never work or walk under a suspended load.

Caught-in/between hazards occur when a worker is caught, compressed, or crushed between objects. Examples include cave-ins (a leading cause of excavation fatalities), unguarded rotating equipment, and equipment pinch points.

Controls:

• Always use the required protective system in excavations.
• Machine guards must remain in place during operation — never bypass or remove them.
• Maintain safe distances from equipment swing areas.
• Apply lockout/tagout (LOTO) procedures before servicing any machinery with stored or hazardous energy.

Key Takeaways

Every worker on a construction site is responsible for recognizing these hazards. Soil type determines the protective system. Scaffold height determines guardrail requirements. Struck-by and caught-in hazards are controlled through engineering, administrative, and PPE measures — in that order of preference. A competent person must be present whenever excavation or scaffold work is underway.