6.1 Protective Systems: Sloping, Shoring, & Shielding

6.1 Protective Systems: Sloping, Shoring, & Shielding

Under 29 CFR 1926.652, employers are required to protect employees from cave-ins by using an adequate protective system in all excavations. The only exceptions are when the excavation is made entirely in stable rock, or if the excavation is less than 5 feet (1.52 m) in depth and the Competent Person has examined the ground and found no indication of a potential cave-in.

If the excavation is 5 feet or deeper, or if the Competent Person identifies cave-in hazards in an excavation less than 5 feet deep, one of three primary protective systems must be implemented: sloping (or benching), shoring, or shielding. For any excavation exceeding 20 feet (6.1 m) in depth, the protective system must be designed and approved by a Registered Professional Engineer (RPE), and the design documents must be kept at the job site.

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1. Sloping and Benching Systems

Sloping involves cutting the walls of the excavation back at an angle inclined away from the excavation. Benching is a method of protecting employees from cave-ins by 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.

The maximum allowable angle of a slope depends directly on the classification of the soil as determined by the Competent Person. These requirements are outlined in Appendix B to Subpart P:

Sloping and Benching Configurations

• Type A Soils: A maximum slope of 3/4:1 is permitted. However, if the excavation is open for a short duration (24 hours or less) and is 12 feet (3.66 m) or less in depth, a maximum slope of 1/2:1 (63°) is permitted. Benching is allowed in Type A soil, with the maximum rise of any single bench limited to 4 feet (1.22 m).
• Type B Soils: A maximum slope of 1:1 is permitted. Benching is allowed in Type B soil, with the maximum bench height restricted to 4 feet (1.22 m).
• Type C Soils: Benching is never permitted in Type C soils due to their lack of cohesion and high risk of sloughing. The only allowable configuration is a continuous slope of 1-1/2:1.
• Layered Soils: When soils are layered (e.g., Type B over Type C), the slope of the lower layer must meet the requirements of its classification, and the slope of the upper layer must not exceed the slope allowed for its classification. If a weaker layer is below a stronger layer (e.g., Type C below Type A), the entire excavation must be sloped to the requirements of the weaker soil (1-1/2:1).

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2. Shoring Systems

Shoring is an active support system that applies positive lateral pressure against the walls of an excavation to prevent the soil from moving. It consists of vertical uprights (sheeting), horizontal stringers (wales), and crossbraces (struts) that span the trench.

OSHA recognizes two primary types of shoring:

1. Timber Shoring: Uses wood members (such as Douglas Fir or Southern Yellow Pine) sized and spaced according to the tables in Appendix C to Subpart P.
2. Aluminum Hydraulic Shoring: Uses hydraulic cylinders that are pressurized with a pump (usually hand-operated) to expand vertical rails against the trench walls. This is widely preferred because it can be installed and removed from the surface, eliminating the need for workers to enter an unprotected trench. Spacing and cylinder sizes are governed by Appendix D to Subpart P.

Design Options for Shoring

Under 1926.652(c), employers must select a shoring system design using one of the following four options:

• Option 1 (Designs using Appendices A, C, and D): Timber or aluminum hydraulic shoring designed using the tables and charts provided in OSHA's Subpart P.
• Option 2 (Designs using Manufacturer's Tabulated Data): Shoring systems selected and installed in strict accordance with written specifications, recommendations, and limitations provided by the manufacturer (e.g., pre-engineered hydraulic shores). The manufacturer's tabulated data must be in written form and kept at the job site.
• Option 3 (Designs using other Tabulated Data): Systems designed using alternative tabulated data (such as industry-standard manuals), which must be approved by an RPE and kept on site.
• Option 4 (Design by an RPE): A custom-designed shoring system engineered specifically for the project by a licensed RPE.

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3. Shielding Systems (Trench Boxes)

Shielding systems, commonly called trench boxes or trench shields, do not prevent the soil from moving or cave-ins from occurring. Instead, they are designed to withstand the forces of a cave-in and protect the workers inside the shield.

Key safety rules for using shielding systems under 29 CFR 1926.652(g) include:

• Worker Protection: Workers must remain within the confines of the trench box while working. They are strictly prohibited from entering or leaving the shield while inside an unprotected portion of the trench.
• Trench Box Installation: The space between the outer sides of the shield and the excavation walls must be minimized. If the gap is significant, it must be backfilled to prevent lateral movement of the shield in the event of a soil collapse.
• Sloping Above the Shield: If the excavation is sloped above the top of the trench box, the shield must extend at least 18 inches (0.46 m) above the vertical walls of the trench to prevent sloughed soil from rolling or falling into the box.
• Excavation Below the Shield: Excavations are permitted up to 2 feet (0.61 m) below the bottom of a shield, but only if the shield is designed to resist the forces calculated for the full depth of the excavation, and there are no indications of loss of soil from behind or below the bottom of the shield.

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4. Practical Traps and Exam Scenarios

A common trap on the OSHA 510 exam involves confusing shoring and shielding. Shoring is a preventative system (it exerts pressure to keep the soil in place), while shielding is a protective system (it lets the soil cave in but protects the worker inside).

Another trap involves the depth threshold: while protective systems are required at 5 feet, the 20-foot limit is a hard ceiling where standard OSHA appendices and manufacturer tabulated data no longer apply. Any excavation deeper than 20 feet must have its protective system custom-designed by an RPE.