With an average of 19 trench fatalities per year, protective systems are essential for trench safety. These fatalities and accidents are preventable. Sloping, shoring, shielding, and benching help prevent the collapse of trenches and other excavated areas.
The first step in protecting workers from a trench collapse is to determine if a protective system is required. From there, analyze the site conditions to determine the type of system that’s best. Protective systems should be inspected and maintained on a daily basis, more often if conditions are constantly changing.
Is a Protective System Required?
OSHA standards for excavation appear in CFR 1926, Subpart P.
- Always err on the side of caution. A competent person should determine the use of cave-in protection.
- A few things to consider when conducting work underground:
- Contact the underground notification center at 811
- Mark utilities prior to work
- Identify whether the excavation is less than 5 feet deep
- Identify whether the excavation is in stable rock
- Have a competent person examine the excavation
A competent person is “an individual who is capable of identifying existing and predictable hazards or working conditions that are hazardous, unsanitary, or dangerous to workers, soil types and protective systems required, and who is authorized to take prompt corrective measures to eliminate these hazards and conditions.”
Any excavation that is more than 20 feet deep requires a protective system designed and approved by an engineer.
Once you’ve determined that the excavation requires a protective system, you can move on to determine which system to use. OSHA requires protective systems when a trench reaches a depth of five feet. However, best practice is to consider some form of protection no matter how deep the trench.
Determining the Protective System Type
The type of protective system you will use depends on five factors:
- soil classification (type of soil)
- depth of excavation
- water content of soil
- weather and climate
- other operations going on nearby
Soil is classified into four basic types: stable rock, Type A, Type B, and Type C. Stable rock is natural solid mineral matter that can be excavated with vertical sides and remain intact. Type A soils are cohesive with an unconfined compressive strength of 1.5 tons per square foot or greater. Examples include clay, silty clay, sandy clay, and clay loam. Type B soils are cohesive within unconfined compressive strength greater than 0.5 tons per square foot but less than 1.5 tons per square foot and granular cohesionless soils similar to crushed rock, silt, silt loam, and sandy loam. Type C soils are cohesive with an unconfined compressive strength of 0.5 tons per square foot or less, granular soils, submerged soil, submerged rock that is not stable, or material in a sloped, layered system where the layers dip into the excavation or with a slope of four horizontal to one vertical or steeper.
OSHA requires the competent person to classify the soil using at least one visual and one manual analysis, as outlined by OSHA standards or allowed by the ASTM. A visual analysis could include examining a spoils pile or trench wall. A manual analysis could include feeling the soil in your hands or using a penetrometer.
If you prefer to skip the soil analysis, you can design your protective systems as if you are in type C soil, which is the least stable.
Workers must monitor activity going on around the area of excavation for hazards as well. Potential hazards include:
- Working with heavy machinery
- Manual handling
- Proximity to traffic
- Electrical hazards
- Underground utilities
- In-rush of water
- Potential confined space hazards
- Undercutting of structures and foundations
Regardless of the type of protective system, the trench should include a means of egress, such as stairs, a ladder, or a ramp.
There are four types of protective systems used for trench safety: sloping, shoring, shielding, and benching.
Sloping for Trench Safety
Sloping involves cutting back the trench wall at an angle that is inclined away from the excavation. The flatter the angle, the more protection the sloping offers. The angle required depends upon soil classification and water content.
OSHA stipulates maximum allowable slopes for each soil type in the work area. For type C soil, a sloping angle of 1 ½:1 is the maximum. That is, for every foot of depth, the trench must be excavated back 1½ feet, resulting in a maximum angle of 34 degrees. The maximum allowable slope is 1:1 (45 degrees) for type B soil, and 3/4:1 (53 degrees) for type A soil.
Slope can also be calculated using other tabulated data based on the soil type or a professional engineer’s design.
Shoring for Trench Safety
Shoring involves installing aluminum hydraulic or other types of supports to prevent the faces of an excavation from a cave-in. The vertical supports are called uprights. The horizontal supports are called walers. When installing shoring be sure to always use engineered shoring materials for the cave in protection.
Shoring systems may also be required to support adjacent structures, like buildings, walls, sidewalks, or pavement. Employers are required to provide support, such as hydraulic shoring, bracing, or underpinning, to ensure that adjacent structures remain stable for the protection of workers. Excavations that would undermine sidewalks, pavements, and appurtenant structures are prohibited unless an appropriately designed support system or another effective method protects workers from the possible collapse of the structures.
Shielding for Trench Safety
Shielding protects workers if a cave-in occurs by preventing debris from falling on them. Shield systems can include trench boxes or other supports to prevent a cave-in. Trench shields are usually constructed with sidewalls held apart by steel or aluminum spreaders. The spreaders are interchangeable to match the width of the trench.
Benching for Trench Safety
Benching is similar to sloping and requires the removal of material from the face of an excavation. The sides of an excavation are dug to form horizontal levels or steps. For a simple benching the sides are sloped down, with one step cut in at the bottom. Multiple benching involves cutting a series of steps into the sides of the excavation.
Benching systems cannot be installed in type C soil.
Reduce Excavation and Trench Incidents
Protective systems for trench safety include sloping, shoring, shielding, and benching. They are used when soil conditions and the depth of the trench present a hazard to workers. Although only required for excavations deeper than five feet, it’s a best practice to provide some form of stabilization in all trenches. Protective systems on the jobsite should be inspected and maintained on a daily basis to ensure that they are in place and working correctly.
Protective systems are just one area to consider for trench safety and excavation. For a more detailed discussion, see Safesite’s article Dig Deep: The Ultimate Excavation Safety Guide.
In that guide, you can learn …
- What Makes a Trench so Dangerous?
- What are the OSHA Trenching and Excavation Standards?
- Before You Dig: Excavation Preplanning and Permitting
- Understanding Excavation Hazards
- Getting Started with Excavation Risk Management
- Scheduling Excavation and Trenching Inspections
Safety management software helps reduce incidents from excavation hazards by simplifying communication and organizing checklists and logs to make information easier to find. Hazards are quickly identified, shared, and resolved. Management can use analytics to view safety trends by personnel, team, project, or organization. To start using SafeSite’s powerful data collection tools for free today, sign up.
