5 Steps to Prevent Trench Collapses

Don’t overlook even the shallowest of trenches. Stay safe on the job site by following this advice.
5 Steps to Prevent Trench Collapses
A steel trench box can be one of the best ways to protect workers depending on the situation.

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Working in trenches and excavations is potentially one of the most hazardous types of work in the construction and utility industries.

One reason? Many people don’t appreciate the enormous weight of soil. One cubic yard of soil can weigh almost 4,000 pounds — the weight of a small car or pickup truck. When a trench or excavation caves in on unprotected workers, it usually means a serious injury or death, even in a very shallow trench. Here are five simple steps you can take to protect your workers.

Step #1 – Have a Trained and Authorized “Competent Person” On Site

In an attempt to reduce the number of deaths and serious injuries that occur in trenches and excavations, OSHA requires that a “competent person” be on site whenever workers are exposed in an excavation. OSHA defines a competent person as one who:

  • is capable of identifying existing or predictable hazards in the surroundings or working conditions that are unsanitary, hazardous, or dangerous to employees, and
  • has the authority to take prompt corrective measures to eliminate them.

OSHA goes on to say that the competent person must have specific training in, and be knowledgeable of, the requirements of the OSHA standard 29 CFR 1926, Subpart P; soil analysis; and the use of protective systems.

The on-site presence of the competent person is crucial in maintaining a safe working environment.

Step #2 – Follow OSHA’s General Requirements

The general requirements section of OSHA’s 29 CFR 1926, Subpart P addresses several common-sense items related to trenches and excavations:

  • Surface Encumbrances — These are described as telephone poles, trees, fire hydrants, street signs, sidewalks, curbs and gutters, and similar objects adjacent to an excavation. They should be removed or supported to ensure their stability and to safeguard workers.
  • Underground Installations — These include underground utilities such as sewer, gas, water, and telephone and electric lines. Prior to the start of actual excavation work, utility companies must be contacted with adequate, established, or customary local lead times, then advised of the proposed work and asked to determine the location of the utilities. All underground utilities must be protected, supported, or removed to protect workers.
  • Access and Egress — These are just fancy words for entering and exiting an excavation. Trenches and excavations deeper than 4 feet require a means of access and egress. Also, each worker must be within 25 feet of a ladder, ramp, or stair. Each means of access and egress must be within a protected area.
  • Vehicular Traffic — Workers exposed to traffic must be provided with, and must wear, warning vests or other highly visible garments. Signs, signals, barricades, and/or flagmen may also be required.
  • Falling Loads — Workers are not permitted underneath overhead loads. In addition, employees must stand away from equipment being loaded or unloaded from vehicles.
  • Warning Systems for Mobile Equipment — When mobile equipment is operated near the edge of an excavation, and the equipment operator does not have a clear and direct view of that edge, warning systems — such as barricades, spotters, or stop logs — are required.
  • Hazardous Atmospheres — This section of the OSHA standard is designed to protect workers from so-called “bad air.” Concerns include too little oxygen, too much oxygen, flammable gases such as methane and natural gas, and toxic gases such as hydrogen sulfide and carbon monoxide. Atmospheric testing, along with the use of ventilation equipment, are two of the most common and important methods of addressing hazardous or potentially hazardous atmospheres.
  • Water Accumulation — Workers must not work in trenches or excavations where there is accumulated water, or where water is accumulating, unless adequate precautions are taken. If the excavation work interrupts the natural flow of surface water, then diversion ditches, dikes, or other means may be required to keep water out.
  • Adjacent Structures — The stability of sidewalks, streets, adjoining buildings, walls, and other structures can be reduced by excavation operations. Specialized shoring systems, bracing, and/or underpinning may be required to ensure the stability of these structures and to protect workers.
  • Loose Soil or Rock — Spoil piles (and equipment) must be set back at least 2 feet from the edge of a trench or excavation.
  • Fall Protection — Walkways with standard guardrails are required when employees or equipment crosses over excavations. Wells, pits, shafts, etc., must be barricaded or covered.

Paying close attention to each of these important potential dangerous circumstances will help ensure worker safety, as well as help contractors and utilities stay legal with OSHA.

Step #3 – Carefully Analyze the Soil

An important responsibility for the competent person is soil analysis. Even when sloping, soil analysis is the very first step in choosing a protective system.

There are hundreds of different types of soil. OSHA is concerned with only four – Stable Rock, Type A, Type B and Type C.

OSHA says the competent person must perform at least one visual and one manual test of the soil that is excavated. But the competent person can also take a worst-case-scenario approach and assume all the soil is Type C. In fact, many contractors and utilities take this position.

Step #4 – Use a Protective System

With one exception, OSHA requires the use of protective systems in all trenches and excavations that are deeper than 5 feet. The exception involves trenches or excavations in stable rock. In those situations, it’s “legal” to work with vertical walls, but stable rock is extremely rare.

And if a trench or excavation is less than 5 feet deep, a protective system is still required if there’s potential for a cave-in. Note that in some states, local laws require use of protective systems in all trenches that are 4 feet or more deep. Always check your state regulations.

There are five options for protective systems:

  • Sloping and Benching: This procedure calls, first, for classifying the soil, then referring to the charts and illustrations in the OSHA standard to determine the correct angle of the sloping or benching.
    Soil Type Angle of Bank
    Stable Rock Vertical wall
    Type A 53 degrees
    Type B 45 degrees
    Type C 34 degrees
  • Timber Shoring: This is a system of wooden walers, cross braces, and uprights that support the walls of a trench or excavation. The OSHA standard specifies the sizes of timbers, based on the various types of soil, the depth of the trench or excavation, and the type of timber used to build the structure (oak or Douglas fir).
  • Aluminum Hydraulic Shoring: Hydraulic shoring systems have replaced a lot of timber shoring. Developed in California in the late 1950s, there are two main types of hydraulic shoring systems: vertical shores and horizontal walers. Both employ aluminum rails and hydraulic cylinders that apply pressure to the walls of a trench, creating an “arching” effect in the adjacent soil that prevents the walls from collapsing.
  • Trench Shields: Trench shields are aluminum or steel structures designed to protect workers by withstanding the forces of a cave-in. Regardless of the material they are made of, the principles for trench shields are the same.
  • Site-Specific Engineered Systems: Occasionally the charts in the OSHA standard are not appropriate. There may be job site conditions or other factors that prevent the use of all these systems. In that case, a registered professional engineer must design a system specifically for the job. The engineer has to be registered in the state where the system will be used, and will have to “stamp” the design. There are additional requirements, as well, all of which are covered in the OSHA standard.

Step #5 – Inspect the Excavation

The properly trained and authorized competent person must inspect excavations daily for:

  • indications of possible cave-ins,
  • failure of protective systems,
  • hazardous atmospheres, and
  • other hazardous conditions.

Inspections must be conducted prior to the start of work, and also as needed throughout each shift, after rainstorms, and after other potential hazard-increasing events. The competent person must also check adjacent areas and protective systems (before and during use), and always look for indications of possible cave-ins and hazardous or potentially hazardous conditions.

About the Author
David Dow is co-founder of TrenchSafety and Supply — now part of Underground Safety Equipment, LLC — which supplies excavation safety products and services to construction, excavation, and utility companies. From its facilities in Kansas City, Missouri; Lafayette, Colorado; Memphis and Nashville, Tennessee; North Little Rock, Arkansas; Salt Lake City, Utah; and San Antonio, Texas, Underground Safety Equipment provides sales, rental equipment, repair service, and safety training. Dow is also chair of the Training Committee for the North American Excavation Shoring Association (NAXSA).

Visit www.TrenchSafety.com or www.UndergroundSafety.com.


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