7.2 OSHA Safety: Excavation & Trenching

7.2 OSHA Safety: Excavation & Trenching (OSHA 1926 Subpart P)

Excavation and trenching are fundamental activities in water distribution, whether installing new mains, repairing leaks, or making taps. However, these activities are also among the most hazardous construction operations. OSHA standard 1926 Subpart P regulates these activities to protect workers from cave-ins and other hazards.

Definitions

• Excavation: Any man-made cut, cavity, trench, or depression in an earth surface formed by earth removal.
• Trench: A narrow excavation (in relation to its length) made below the surface of the ground. In general, the depth is greater than the width, but the width of a trench (measured at the bottom) is not greater than 15 feet.

The most significant hazard in trenching is the risk of a cave-in. Soil is incredibly heavy; a single cubic yard can weigh as much as a car (over 3,000 pounds). When a trench wall collapses, it happens with little or no warning, and the weight of the soil can instantly crush or suffocate a worker.

Soil Classifications

To determine the appropriate protective system, the competent person must classify the soil type. OSHA classifies soils into three categories, plus stable rock.

1. Stable Rock: Natural solid mineral material that can be excavated with vertical sides and remain intact while exposed. (Rarely encountered in typical utility work).
2. Type A Soil: Cohesive soils with an unconfined compressive strength of 1.5 tons per square foot (tsf) or greater. Examples include clay, silty clay, and clay loam. These are the most stable soils. However, a soil cannot be Type A if it is fissured, subject to vibration (like near a roadway), or has been previously disturbed.
3. Type B Soil: Cohesive soils with an unconfined compressive strength greater than 0.5 tsf but less than 1.5 tsf. Examples include angular gravel (similar to crushed rock), silt, silt loam, and soils that are fissured or subject to vibration.
4. Type C Soil: Cohesive soils with an unconfined compressive strength of 0.5 tsf or less. Examples include granular soils like gravel, sand, and loamy sand, submerged soil, or soil from which water is freely seeping. Type C is the least stable soil.

Important Note: Whenever an excavation contains layered soils of different types, the protective system must be designed based on the weakest soil type present. Because previously disturbed soil (which makes up most utility trenches) is automatically downgraded, operators should often treat soil as Type C to ensure maximum safety.

Protective Systems

OSHA requires that workers be protected from cave-ins in all excavations 5 feet deep or greater, or if a competent person determines there is a potential for a cave-in in a shallower trench. There are three main methods of protection: Sloping, Shoring, and Shielding.

1. Sloping and Benching

Sloping involves cutting back the trench wall at an angle inclined away from the excavation. The required angle depends on the soil type.

• Type A: 3/4 to 1 (53 degrees)
• Type B: 1 to 1 (45 degrees)
• Type C: 1 1/2 to 1 (34 degrees) - This requires a very wide trench, often impractical in urban areas.

Benching involves excavating the sides of an excavation to form one or a series of horizontal levels or steps, usually with vertical or near-vertical surfaces between levels. Benching is not permitted in Type C soil.

2. Shoring

Shoring is an active system that supports the sides of an excavation and prevents cave-ins by pressing against the trench walls. OSHA requires shoring for trenches 5 ft or deeper, or other protective systems. Hydraulic shoring is the most common type used in water distribution. It utilizes aluminum or steel cylinders that are pressurized against the trench walls using hydraulic fluid. Shoring systems are preferred over timber because they are lighter, faster to install, and can often be installed from above ground without a worker entering the unprotected trench.

3. Shielding (Trench Boxes)

A trench shield (or trench box) is a passive structure that is able to withstand the forces imposed on it by a cave-in and thereby protect employees within the structure.

• Shields do not prevent a cave-in; they protect the workers if one occurs.
• The space between the outside of the trench box and the trench wall should be backfilled to prevent the box from shifting in the event of a collapse.
• Workers must remain entirely within the confines of the trench box at all times.

Access and Egress

A safe means of entering and exiting the trench is a critical requirement.

• A stairway, ladder, ramp, or other safe means of egress must be located in trench excavations that are 4 feet or deeper.
• Safe access and egress requires ladders every 25 ft of travel in the trench, ensuring workers can quickly evacuate if needed.
• The ladder must extend at least 3 feet above the edge of the trench to provide a secure handhold when transitioning in and out.

Hazardous Atmospheres and Other Trench Hazards

While cave-ins are the primary concern, trenches can also act like confined spaces. Because they are depressions in the ground, heavier-than-air gases (like H2S) can accumulate at the bottom of a trench. Furthermore, water accumulation is a constant threat. Workers must not work in excavations in which there is accumulated water, or in which water is accumulating, unless adequate precautions have been taken to protect employees against the hazards posed by water accumulation.

The Competent Person

Every excavation must be overseen by a "Competent Person." This is an individual who is capable of identifying existing and predictable hazards in the surroundings, or working conditions which are unsanitary, hazardous, or dangerous to employees, and who has the authorization to take prompt corrective measures to eliminate them. The competent person must conduct daily inspections of excavations, the adjacent areas, and protective systems before the start of work and as needed throughout the shift.