01 Mar Trenching and Shoring

Trenching and Shoring

Trenching and shoring operations are common when installing buried utilities, pipelines, drainage systems, and other underground infrastructure. These operations can expose workers to cave-ins, falling materials, hazardous atmospheres, underground utilities, water accumulation, and mobile equipment.

A trench cave-in can occur suddenly and without adequate warning. Soil is extremely heavy—a single cubic yard can weigh approximately as much as a small automobile. Workers caught in a collapse may be crushed, immobilized, suffocated, or fatally injured.

Guide for Discussion

Before Work Begins

Before excavation or trench entry:

• Ensure employees involved in trenching operations have received appropriate training.
• Have underground utilities located and marked before excavation begins.
• Determine the location of overhead and underground hazards.
• Evaluate soil conditions and classify the soil as necessary to select an appropriate protective system.
• Identify nearby structures, traffic, equipment, vibration, water, and other conditions that could affect trench stability.
• Determine the appropriate cave-in protective system before employees enter the trench.
• Provide barricades or other warning systems where employees, vehicles, or equipment could approach the excavation.

Soil Classification

Soil conditions are generally classified as Stable Rock, Type A, Type B, or Type C. Classification must be performed by a competent person using appropriate visual and manual tests.

• Type A – Cohesive soil with an unconfined compressive strength of 1.5 tons per square foot (tsf) or greater and meeting the other requirements for Type A classification.
• Type B – Cohesive soil with an unconfined compressive strength greater than 0.5 tsf but less than 1.5 tsf, or soil meeting other Type B criteria.
• Type C – Cohesive soil with an unconfined compressive strength of 0.5 tsf or less, granular soils such as gravel and sand, submerged soil, or other soils meeting Type C criteria.

Previously disturbed soil cannot normally be classified as Type A. When soil conditions change, the classification and protective system must be reevaluated.

Cave-In Protection

Each employee in an excavation must be protected from cave-ins by an adequate protective system unless the excavation is made entirely in stable rock or is less than 5 feet deep and a competent person finds no indication of a potential cave-in.

Protective methods include:

• Sloping.
• Benching where permitted.
• Shoring.
• Shielding with trench boxes or other approved systems.

For excavations 20 feet deep or less, OSHA Appendix B generally permits maximum allowable slopes of:

Type C soil may not be benched under OSHA’s standard configurations.

Excavations more than 20 feet deep require a protective system designed by a registered professional engineer or based on approved tabulated data meeting OSHA requirements.

Shoring and Trench Shields

Shoring supports trench walls to prevent movement and collapse. Systems may include timber, aluminum hydraulic, pneumatic, or other engineered components.

• Install and use shoring according to the manufacturer’s tabulated data or an approved design.
• Inspect components for damage before use.
• Do not subject shoring components to loads greater than those for which they were designed.
• Install and remove support systems in a manner that protects employees from cave-ins and structural failure.
• Employees must not be exposed to an unprotected trench while protective systems are being installed or removed.

A trench shield or trench box is intended primarily to protect workers if a cave-in occurs; it does not necessarily prevent the trench wall from collapsing.

• Workers must remain within the protected area of the shield.
• Do not work outside or above the protection provided by the shield.
• Do not remain inside a trench box while it is being installed, removed, or moved vertically.
• Stack trench boxes only when permitted by the manufacturer or applicable design.

Access and Egress

For trenches 4 feet or deeper, provide a stairway, ladder, ramp, or other safe means of egress.

• Employees must not have to travel more than 25 feet laterally to reach an exit.
• Secure ladders and keep access routes clear.
• Reposition access points as trenching operations progress.
• Do not climb on shoring or trench boxes unless they are specifically designed for access.

Spoil Piles and Equipment

Keep excavated material and equipment at least 2 feet from the edge of the excavation or use retaining devices sufficient to prevent materials from falling or rolling into the trench.

Keep heavy equipment away from trench edges whenever possible. Equipment operating near an excavation can create vibration, surcharge loading, and struck-by hazards that may contribute to a collapse.

Water and Hazardous Atmospheres

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

Where hazardous atmospheres could reasonably be expected:

• Test the atmosphere before employees enter excavations greater than 4 feet deep.
• Test for oxygen deficiency, flammable gases or vapors, and toxic contaminants as appropriate.
• Provide ventilation or other controls when necessary.
• Continue monitoring when conditions could change.
• Provide appropriate emergency and rescue equipment when hazardous atmospheric conditions exist or may reasonably develop.

Competent Person Inspections

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

• Before the start of work and before employees enter.
• Daily throughout the project.
• As needed during the work shift.
• After rainstorms.
• After changes in weather, vibration, water conditions, soil conditions, or other events that could increase hazards.

Employees must immediately leave the trench when the competent person identifies evidence of a possible cave-in, protective-system failure, hazardous atmosphere, water accumulation, or another hazardous condition. Work may resume only after adequate precautions have been taken.

Remember

Never assume a trench is safe because it was safe yesterday—or even earlier in the same shift. Soil, weather, water, vibration, nearby equipment, and changing work activities can quickly affect stability. Proper trenching and shoring requires planning, competent-person inspections, appropriate protective systems, and continuous attention to changing conditions.