Safe Use of Trench Boxes for Large Diameter Flexible Pipe Installation

Introduction

Trench boxes provide essential worker safety when installing pipe in deep trenches or unstable soils. However, their use with large diameter flexible pipes presents a critical challenge: maintaining compacted embedment integrity during trench box movement. Improper technique can create voids alongside the pipe or reduce backfill density adjacent to the pipe, eliminating the lateral support that flexible pipes require to control deflection.

The Risk of Embedment Disturbance

Flexible pipes mobilize support through interaction with surrounding soil. When backfill density drops from specified levels (typically ≥ 90% Standard Proctor) to loose or disturbed conditions, the backfill material cannot properly support the loads from the flexible pipe. This loss of supporting strength can lead to deflections beyond acceptable limits—and can lead to issues with joint separation, wall stress, and long-term system performance.

The Subtrench Method

The most common approach is constructing a subtrench within the main excavation. This creates a protected zone for pipe and embedment below the trench box elevation, completely isolating them from box movement.

OSHA requires trench box bottoms remain within 24 inches of the work surface. Position the box bottom at least 12 inches above the installed pipe crown. After placing and compacting embedment to the specified height above the pipe (minimum 6-12 inches depending on pipe size), pull the trench box along the subtrench edge without contacting the pipe zone. This method eliminates void creation and density loss. For larger diameter pipes, thinner steel extension plates may be placed below the bottom of the trench box to allow the box to remain at an elevation above the crown of the pipe.

Alternative Methods When Subtrench Is Not Feasible

Modified Trench Box Procedure: Excavate below pipe crown elevation from inside the shield, forcing it down as soil is removed. This maintains contact between the shield exterior and undisturbed native soil. After placing pipe and bedding, install embedment in lifts with proper compaction. After each lift, lift the shield vertically and shovel embedment material under the sides to fill voids. Only advance the shield after it clears the pipe zone elevation. Never drag the shield through embedment.

Lift-and-Place Method: Vertically lift and reposition the box rather than dragging it. This eliminates lateral forces that disturb embedment but requires adequate equipment.

Critical Requirements

• Never allow trench box walls to contact the pipe or compress embedment material
• Maintain a minimum 15-18 inches clearance between pipe springline and trench walls for compaction equipment access
• Place embedment in specified lifts: typically 12" maximum for Class I and 8" for Class II
• Work material carefully into haunch zones beneath the pipe—use hand tampers for pipes 42" and larger
• Shovel-slice backfill under haunches to eliminate voids
• Bring lifts up evenly on both sides of the pipe following manufacturer’s guidelines

Conclusion

Successful trench box use with flexible pipes requires planning to assure that the compacted fill placed beside the pipe isn’t disturbed as the box is advanced during construction. The subtrench method offers the most reliable protection against embedment disturbance. When alternatives are necessary, procedures must maintain the specified embedment density throughout the pipe zone as is essential for long-term system integrity.