5.3 Sloping, Benching, Shoring & Shield Systems

Sloping, Benching, Shoring & Shield Systems

Core Principle: When an excavation reaches 5 feet or deeper, employers must protect employees from cave-ins by selecting, designing, and installing an approved protective system under 29 CFR 1926.652. OSHA recognizes four fundamental protective methodologies: sloping, benching, shoring, and shielding. Each system functions on distinct mechanical principles, carries strict operational limitations, and requires specific geometric configurations based on soil classification.

---

1. Sloping Systems (Appendix B to Subpart P)

Sloping protects employees by cutting back the trench face at an angle inclined away from the excavation, eliminating the gravitational shear stress that causes soil failure.

Maximum Allowable Slopes for Excavations < 20 Feet Deep

OSHA 29 CFR 1926 Subpart P Appendix B (Table B-1) specifies the maximum allowable slope angle (expressed as the ratio of horizontal distance to vertical rise, H:V) for each soil classification:

                     MAXIMUM ALLOWABLE SLOPES (APPENDIX B)

   Stable Rock         Type A (3/4:1)          Type B (1:1)           Type C (1.5:1)
       90°                  53°                     45°                    34°
        │                   ┌─                      ┌──                    ┌───
        │                   │ 3/4                   │ 1                    │ 1.5
        │ 1                 │                       │                      │
        │                   │ 1                     │ 1                    │ 1
        │                   │                       │                      │
   ─────┴─────         ─────┴──────            ─────┴──────           ─────┴──────

The Short-Term Type A Exception

• Under Appendix B, OSHA permits a steeper slope in Type A soil under very narrow operational conditions:
  • Slope Ratio: 1/2 : 1 (horizontal to vertical), which corresponds to an angle of 63 degrees from the horizontal.
  • Depth Limit: The excavation must be 12 feet (3.66 meters) or less in depth.
  • Time Limit: The excavation must remain open for 24 hours or less (short-term unsupported slope).
  • If the excavation will remain open for more than 24 hours, or if it exceeds 12 feet in depth, it must be sloped to the standard 3/4:1 (53°).

Calculating Real-World Surface Cut Widths

Safety professionals must calculate the total top width of a sloped trench to ensure adequate site clearance. Consider a 10-foot deep trench with a 4-foot wide bottom:

• In Type A Soil (3/4:1):
  • Setback per side = 10 ft × 0.75 = 7.5 ft.
  • Total Top Width = 7.5 ft (left) + 4.0 ft (bottom) + 7.5 ft (right) = 19.0 feet.
• In Type B Soil (1:1):
  • Setback per side = 10 ft × 1.0 = 10.0 ft.
  • Total Top Width = 10.0 ft (left) + 4.0 ft (bottom) + 10.0 ft (right) = 24.0 feet.
• In Type C Soil (1.5:1):
  • Setback per side = 10 ft × 1.5 = 15.0 ft.
  • Total Top Width = 15.0 ft (left) + 4.0 ft (bottom) + 15.0 ft (right) = 34.0 feet.

---

2. Benching Systems (Appendix B)

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.

Types of Benching Configurations

• Simple Benching: A single vertical cut at the trench bottom followed by one horizontal step and a sloped upper bank.
• Multiple Benching: A stair-stepped series of horizontal levels and vertical rises descending to the trench floor.

Permissible Soil Conditions and Critical Prohibitions

• Type A Soil: Simple and multiple benching are permitted up to 20 feet in depth. The maximum vertical rise of the bottom bench is 4 feet, and intermediate benches may rise up to 5 feet, provided the overall slope does not exceed 3/4:1.
• Type B Soil:
  • Benching is permitted only in cohesive Type B soils.
  • Maximum Bench Height: The vertical rise between benches cannot exceed 4 feet (1.22 meters).
  • The overall slope of the benched system cannot exceed 1:1 (45 degrees).
• CRITICAL OSHA PROHIBITION: Benching in Type C Soil:
  • BENCHING IS STRICTLY PROHIBITED IN TYPE C SOIL.
  • Granular, cohesionless soils (sand, gravel, submerged material) possess no shear strength to support a vertical bench face. Any attempt to cut steps into Type C soil results in catastrophic sloughing and collapse.

---

3. Shoring Systems: Timber and Aluminum Hydraulic

Unlike sloping—which alters the geometry of the ground—shoring provides structural support against trench faces to prevent soil movement and cave-in initiation.

Structural Mechanics

• Shoring works by exerting active lateral pressure against trench walls.
• By maintaining lateral stress, shoring prevents soil decompaction, tension cracking, and shear plane failure before a collapse can start.

Timber Shoring (Appendix C)

• Composed of wooden cross braces (struts), horizontal wales (stringers), and vertical uprights (sheeting).
• Components must be sized using the engineering tables in Appendix C based on soil type, trench depth, and trench width.
• Today, timber shoring is largely supplanted by modern pre-engineered systems due to labor intensity and timber degradation.

Aluminum Hydraulic Shoring (Appendix D)

• Pre-engineered systems consisting of vertical aluminum rails (shores) equipped with horizontal hydraulic cylinders (cross braces).
• Major Safety Advantage: Hydraulic shores can be installed and removed completely from above ground, outside the trench. Workers never enter an unprotected trench to set or pull shores.
• Operating Principles:
  • Pressurized using a portable hand pump filled with biodegradable fluid (water and water-soluble oil) to a typical gauge pressure of 750 to 1,500 psi.
  • Internal check valves lock hydraulic pressure in the cylinders.
  • Spacing of vertical rails (horizontal spacing) and cylinder spacing (vertical spacing) must strictly adhere to the manufacturer's tabulated data or OSHA Appendix D tables.

---

4. Shield Systems (Trench Boxes)

Shield systems, commonly called trench boxes, function on a fundamentally different principle than shoring:

Shoring vs. Shielding:

• Shoring prevents the trench walls from moving or collapsing by applying active lateral support.
• Shields (Trench Boxes) do NOT prevent trench walls from collapsing; they are engineered structural enclosures designed to withstand the immense dynamic impact forces of a cave-in, protecting the workers inside the protective envelope.

Critical Regulatory Requirements for Trench Boxes

1. Work Within the Shield Envelope:
   • Employees must remain inside the shielded perimeter at all times while in the trench.
   • Workers are strictly prohibited from working outside the shield, in front of or behind the box, or between the exterior shield wall and the unexcavated earth face.
2. Safe Egress Within the Box:
   • In accordance with 1926.651(c)(2), a ladder or safe egress must be located inside the shield so workers do not exit into an unprotected area.
3. Sloping Above the Trench Box (The 18-Inch Rule):
   • When an excavation is sloped above the top of a trench box, the shield must extend at least 18 inches (0.45 meters) above the vertical face / toe of the slope.
   • Rationale: This 18-inch lip acts as a retaining wall to catch rolling stones, boulders, sloughing earth, and spoil clods that tumble down the sloped bank, preventing them from falling into the box onto workers.
4. Unexcavated Earth Below the Shield (The 2-Foot Rule):
   • OSHA permits an excavation to extend a maximum of 2 feet (0.61 meters) below the bottom of a trench box, provided two conditions are met:
     1. The shield is designed and rated to resist the lateral earth forces calculated for the full depth of the trench.
     2. There is no indication of soil loss from behind or below the bottom of the shield (no sloughing, squeezing, boiling, or running soil).
5. Backfilling Void Spaces:
   • The space between the exterior of the trench shield and the trench wall must be backfilled or minimized.
   • If a large gap exists, collapsing soil gains kinetic momentum before striking the shield, generating dynamic impact forces that can shift, tip, or deform the box.