26 Jun Trenching Safety

Trenching Safety

Trenching is one of the most hazardous activities in construction. A trench is a narrow excavation made below the surface of the ground that is generally deeper than it is wide and no more than 15 feet wide at the bottom. The greatest danger is a cave-in, which can occur suddenly and without warning. Even a relatively shallow trench can expose workers to serious hazards.

Good planning, proper protective systems, and inspections by a competent person are essential before anyone enters a trench.

Guide for Discussion

Know the Hazards

Trenching hazards include:

• Cave-ins – The greatest hazard associated with trenching. Soil can weigh thousands of pounds per cubic yard, and even a small collapse can trap, crush, suffocate, or kill a worker.
• Falling materials – Spoil piles, tools, equipment, and other materials near the edge can fall or roll into the excavation.
• Underground utilities – Electrical, gas, water, sewer, communication, and other utility lines may be present.
• Hazardous atmospheres – Oxygen deficiency, toxic gases, or flammable gases and vapors may accumulate in an excavation.
• Water accumulation – Water can weaken trench walls and increase the likelihood of collapse.
• Mobile equipment and traffic – Vehicles and heavy equipment operating near the excavation can create surcharge loads, vibration, and struck-by hazards.
• Falls – Employees and equipment can fall into unprotected excavations.

Before Digging

Before excavation begins:

• Contact the appropriate utility-location service before digging and comply with applicable state notification requirements.
• Locate and identify underground utilities and installations.
• Evaluate overhead hazards, nearby structures, traffic, equipment, and other conditions that could affect the excavation.
• Have a competent person classify the soil and determine the appropriate protective system.
• Plan safe access and egress.
• Establish barricades, warning systems, or other protection where employees, vehicles, or the public could approach the excavation.

Soil Classification

OSHA classifies soil as Stable Rock, Type A, Type B, or Type C. Type C is the least stable soil classification.

Soil conditions can change because of water, vibration, previously disturbed soil, weather, nearby equipment, and other factors. Soil classification and protective measures must therefore be evaluated by a competent person rather than assumed.

Cave-In Protection

Employees must be protected from cave-ins when working in an excavation 5 feet or more in depth, unless the excavation is made entirely in stable rock.

Acceptable protective systems include:

• Sloping;
• Benching, where permitted by the soil classification;
• Shoring; and
• Shielding, such as a trench box.

Excavations more than 20 feet deep must have a protective system designed by a registered professional engineer or based on data prepared or approved by a registered professional engineer.

Never enter an unprotected trench when a protective system is required.

Spoil Piles and Equipment

Excavated material and equipment can place additional pressure on trench walls and may fall into the excavation.

• Keep spoil piles, materials, and equipment at least 2 feet from the edge of the excavation or use an effective retaining device.
• Keep unnecessary vehicles and heavy equipment away from trench edges.
• Use warning systems when mobile equipment operates near an excavation and the operator does not have a clear and direct view of the edge.

Safe Access and Egress

A stairway, ladder, ramp, or other safe means of egress must be provided in trenches 4 feet or more in depth.

Access must be positioned so employees do not have to travel more than 25 feet laterally to reach it.

Ladders must be secured and extend sufficiently above the landing surface to provide safe access in accordance with applicable ladder requirements.

Hazardous Atmospheres

Where a hazardous atmosphere exists or could reasonably be expected, atmospheric testing must be performed before employees enter excavations greater than 4 feet deep.

Testing and controls may be required for:

• Oxygen deficiency;
• Flammable gases or vapors; and
• Toxic gases, vapors, or contaminants.

Ventilation, respiratory protection, or other controls must be provided when required. Emergency rescue equipment must also be readily available where hazardous atmospheric conditions exist or may reasonably be expected.

Water Accumulation

Employees must not work in excavations where water has accumulated or is accumulating unless adequate precautions have been taken.

Controls may include:

• Pumps or other water-removal equipment;
• Drainage systems;
• Special support or shield systems; and
• Appropriate monitoring by a competent person.

Rainstorms, melting snow, leaking utilities, and groundwater can quickly change trench conditions.

Competent Person Inspections

A competent person must inspect excavations, adjacent areas, and protective systems:

• Daily before work begins;
• As needed throughout the shift;
• After rainstorms or other hazard-increasing events; and
• Whenever conditions change in a way that could increase employee exposure.

The inspection must look for evidence of:

• Possible cave-ins;
• Failure or deterioration of protective systems;
• Hazardous atmospheres;
• Water accumulation;
• Loose material;
• Changing soil conditions; and
• Other hazardous conditions.

If a hazardous condition is identified, exposed employees must be removed from the excavation until the necessary precautions have been taken.

Remember

A trench can collapse without warning. Never assume that because a trench looked safe yesterday—or even earlier in the shift—it is safe now. Never enter a trench until it has been inspected and all required protective systems, access, and other safeguards are in place.