Trenching and Excavation: Five Feet and What Comes Before It

A cubic yard of soil weighs about as much as a small car. Cave-ins do not injure people so much as bury them.

Excavation work carries a fatality rate well above the construction average, and the reason is physical rather than procedural. A cubic yard of soil weighs roughly 3,000 pounds — about the weight of a small car. A partial cave-in that buries a worker to the waist can be fatal from compression alone, before anything covers the airway.

Trench collapses also happen without warning. There is rarely time to climb out.

The five-foot rule, and its exceptions

Under 29 CFR 1926 Subpart P, a protective system is required for any excavation five feet deep or greater, unless the excavation is made entirely in stable rock.

Two qualifications get missed:

• Trenches less than five feet still require a protective system if a competent person examines the ground and finds indication of a potential cave-in.
• Excavations 20 feet or deeper require a protective system designed by a registered professional engineer.

Choosing a protective system

• Sloping — cutting back the walls to a safe angle based on soil type. Needs room, which urban sites rarely have.
• Benching — a stepped series of horizontal levels. Not permitted in Type C soil.
• Shoring — hydraulic, pneumatic or timber supports that hold the walls in place.
• Shielding — trench boxes. Important distinction: a shield does not prevent a collapse, it protects workers inside it when one happens. Workers must stay inside the box, and it must be used within its tabulated data.

Soil classification is not optional guesswork

A competent person classifies the soil, and the classification drives the allowable slope:

• Stable rock — can be excavated vertically
• Type A — cohesive, unconfined compressive strength of 1.5 tons per square foot or greater; maximum slope 53 degrees
• Type B — cohesive with lower strength, or granular cohesionless; 45 degrees
• Type C — the least stable, including submerged soil and soil with water freely seeping; 34 degrees

Previously disturbed soil cannot be Type A. Any trench that has been dug and backfilled before — which describes most utility work — starts in a lower classification.

The competent person

Subpart P requires a competent person with the training to identify hazards and the authority to take prompt corrective action. Authority is the operative word. Someone who can spot a problem but cannot stop the work does not meet the definition.

Inspections are required:

• Before each shift starts
• As work progresses through the day
• After every rainstorm
• After any event that could increase hazard — vibration, a nearby load, a soil change

Access and egress

For trenches four feet deep or more, a ladder, ramp or stairway must be within 25 feet of lateral travel for every worker. A worker who has to run 60 feet to reach a ladder does not get out.

Ladders must be secured and extend at least three feet above the landing surface.

Spoil piles, water and the atmosphere

• Keep spoil, equipment and material at least two feet back from the edge. A pile at the lip surcharges the wall and is a frequent contributing cause.
• Do not work in accumulated water unless controls are in place, and a competent person monitors them. Water dramatically reduces soil stability.
• Test the atmosphere in excavations deeper than four feet where a hazardous atmosphere could exist. Trenches near landfills, fuel contamination or sewer lines can accumulate oxygen-deficient or toxic air.

Locate before you dig

Underground utilities get located and marked before excavation begins. Strikes on gas, electrical and fiber are among the most common and most expensive incidents in this work — and a gas strike in an open trench puts a crew inside the hazard.

The daily test

Stand at the edge before anyone goes in. Is there a protective system, is the spoil back from the edge, is there a ladder within 25 feet, has a competent person looked at it this shift? If any answer is no, nobody enters.