Due to a rise in excavation and trench safety related deaths in early 2022, OSHA stepped up its emphasis on excavation and trench safety. Even a small amount of dirt, like one square yard, can weigh more than 3,000 pounds, which is the weight of a compact car. With this in mind, it always pays to review safety topics with workers so that they are top of mind during potentially dangerous construction activities. The OSHA standards are clear about the trench safety requirements that must be followed to keep workers safe.
OSHA requirements for trench and excavation safety
Excavations over twenty feet deep must have a protective system designed by a registered professional engineer.
For a trench to be considered safe to enter, it must meet five conditions:
- Have a safe way to enter and exit.
- Have cave-in protection (sloping, benching, shoring, or shielding).
- Be free of materials at the edge of the trench.
- No standing water or other hazards.
- The trench must be inspected at least once a day by a competent person trained in excavation safety.
A competent person is someone who has completed a trench safety training course or has on the job experience with trench safety.
Soil types
Before determining the technique to be used to stabilize the excavation or trench, the soil or rock type must be determined. OSHA recognizes four types of soil or rock: stable rock and soil types A, B, and C. Soil type C is the loosest type, and includes gravels and sandy soils. In areas with layered rock or soil types, the overall soil is classified based on the weakest, or loosest, soil layer.
Soil types are determined based on performing two of several testing methods. There are tools, like a penetrometer, that can help contractors accurately determine the type of soil or rock they are working with. On the other side of the spectrum, workers can also use a simple thumb test or a visual test to determine how loose the soil or rock is.
Based on the depth of the excavation and the soil type, the contractor can then choose the best method for maintaining safety.
Shoring
Shoring uses a support system to hold soil intact and is commonly used when the location or depth makes sloping impractical. There are two main types of shoring systems: hydraulic and pneumatic. The main difference is that pneumatic shoring uses air pressure instead of hydraulic pressure to keep the sides stabilized. Both of these systems can be installed from outside the trench, which improves safety for workers.
Shielding
The most common form of shielding is the trench box. The box is inserted in the trench or excavated area and protects workers from cave-ins and similar hazards. These boxes can be used in combination with sloping and benching or can be stacked on top of each other, but the boxes must extend at least eighteen inches above the surrounding area. A ladder or other means of egress must be located within twenty-five feet of all workers and must extend beyond the trench by three feet.
Trench boxes are made out of aluminum or steel and have varying capacities according to the thickness of the materials. They are designed to protect workers from the pressure of cave-ins, not to prevent them.
Sloping
The angle of the sloping cut depends on the soil type. Stable rock can be cut at a 90-degree angle, while type A soil must be at a 53-degree angle, type B soil at a 45-degree angle, and type C soil at a 34-degree angle.
Benching
The dimensions of the steps, or benches, are dependent on the soil type, as well. As a general rule, the bottom vertical height of a trench must not exceed four feet for the first bench. After that, subsequent benches can be up to five feet high in type A soil and four feet high in type B soil, up to a total trench depth of twenty feet.
Trench safety tips
- Know where underground utilities are located and make sure that excavations are at least five feet away from utilities, when possible.
- Don’t store materials at the edge of or above a trench.
- Test for atmospheric dangers, such as low oxygen or hazardous vapors or gases.
- Consider alternatives to trenches, like directional boring, tunneling, or pipe ramming.
- Install visible signage around the trench or site to warn workers and visitors of the dangers.
