Stacking Trench Boxes: What You Need to Know
Stacking trench boxes can extend the protected height of a shield system when a single box does not provide the configuration required for the excavation.
But stacking two shields does not automatically increase the allowable excavation depth. The boxes, connection hardware, soil conditions, shield position, and complete stacked configuration must all comply with the applicable manufacturer's current tabulated data.
Why Are Trench Boxes Stacked?
Trench boxes are commonly stacked when the required shielded height cannot be provided by one trench shield alone.
A compatible upper box can extend the shielded area above a lower box while allowing the crew to continue working within the protected area of the shield system.
In product lines designed for stacking, shorter 4-foot and 6-foot shields are often used as upper boxes on taller base shields. Those shorter boxes may also be used independently when their tabulated data and the jobsite conditions permit.
Stacking extends the shielded height of the system. It does not create a new depth rating simply by adding the heights of two boxes together.
Stacked Height vs. Allowable Excavation Depth
Two measurements are easy to confuse when planning a stacked trench-box system.
Stacked Shield Height
This is the physical height created by placing one compatible trench box above another.
It describes the geometry of the stack, not the maximum soil depth the system is permitted to resist.
Allowable Depth Rating
The allowable depth comes from the applicable manufacturer's tabulated data and the conditions under which the shield is being used.
The complete stacked configuration must remain within those requirements.
For example, placing a 4-foot shield above an 8-foot shield creates approximately 12 feet of physical shield height. That does not by itself establish that the combined system is rated for a 12-foot excavation.
Can Any Two Trench Boxes Be Stacked?
No. Trench boxes should only be stacked when the manufacturer permits the particular shields and connection configuration to be used together.
Similar exterior dimensions do not establish compatibility. Different shield models may use different corner posts, stacking-pin locations, connection hardware, panel designs, depth ratings, or engineering requirements.
Compatible Shields
Verify that the upper and lower boxes are permitted to be used together under the applicable manufacturer's information.
Correct Hardware
Use the stacking pins, connectors, and related hardware specified for the shield system rather than improvised field connections.
Approved Configuration
Confirm that the complete stack remains within the manufacturer's tabulated limits for the actual excavation.
The Lower Box and Upper Box Have Different Positions
A stacked trench shield should be evaluated as one complete protective system, but the upper and lower shields occupy different positions within the excavation.
Do not assume that because each box can be used by itself, the two boxes can automatically be combined in any order or at any excavation depth.
The manufacturer's tabulated data or other applicable design documentation should establish which boxes can be used in the stack, their allowable configuration, and any limitations on the system.
How Are Stacked Trench Boxes Connected?
Stackable trench shields typically use manufacturer- specified connection points and stacking hardware to secure the upper box to the lower box.
The exact connection varies by manufacturer and product line. The important point is that the connection should be made using the components intended for that system.
Verify that the selected upper and lower shields are approved for the intended stacked configuration.
Check corner posts, pin locations, hardware, and other connection components for damage or missing parts.
Position the boxes so the designated stacking points align as required by the manufacturer.
Use the required stacking pins or connectors and verify that they are properly installed before the system is placed into service.
What Should Be Checked Before Stacking?
Stacking should begin with the excavation requirements, not simply with the height of the boxes available on the jobsite.
Do Not Forget Working Clearance
A taller protective system still has to provide enough usable space for the actual work being performed.
Spreader locations, spreader width, the upper-box configuration, pipe diameter, fittings, manholes, vaults, and other structures can all affect the usable working area inside the stack.
The dimensions of the pipe, structure, or utility component should therefore be checked against the complete stacked configuration before the equipment is selected.
Installing and Moving a Stacked Shield System
Stacked shields are heavier and create a taller assembly than a single trench box, so lifting, positioning, and jobsite access become important parts of the plan.
The equipment used to handle the stack must have appropriate capacity for the actual shield assembly and the lift being performed. Manufacturer lifting points and handling instructions should be followed.
Employees should remain clear of suspended loads and should not be inside a trench shield while it is being installed, removed, or moved vertically.
A stack that is acceptable from a trench-protection standpoint must also be capable of being safely assembled, lifted, positioned, and moved with the equipment available on the jobsite.
Manufacturer Tabulated Data Controls the Stack
When a manufactured trench shield is being used under manufacturer tabulated data, the system must be used in accordance with the manufacturer's specifications, recommendations, and limitations.
This is particularly important with stacked boxes because the contractor is no longer evaluating only one shield. The complete configuration—including the upper box, lower box, spreaders, connections, and intended excavation depth—must be supported by the applicable documentation.
A field modification or configuration outside the manufacturer's permitted use should not be assumed acceptable simply because the components can be physically connected.
Stay Within the Shield Rating
The shield system must not be subjected to loads greater than those it was designed to withstand.
Follow the Permitted Configuration
Manufacturer specifications, recommendations, and limitations govern when the protective system is selected from manufacturer tabulated data.
Employees Out During Vertical Movement
Employees should not be inside the shield while the system is being installed, removed, or moved vertically.
Inspect Before Use
Damaged or defective protective-system equipment that could impair proper function requires evaluation before continued use.
The Competent Person's Role
The Competent Person should evaluate the actual excavation and protective-system conditions before the stack is placed into service and as work progresses.
That evaluation includes the condition of the shields and stacking hardware as well as changing soil, water, surcharge, excavation, and jobsite conditions.
OSHA requires inspections of excavations, adjacent areas, and protective systems before work, as needed throughout the shift, and after rainstorms or other hazard-increasing events when employee exposure can be reasonably anticipated.
Common Trench Box Stacking Mistakes
- Adding the box heights together and treating the result as the allowable excavation depth.
- Stacking boxes simply because their exterior dimensions appear compatible.
- Mixing shields, pins, spreaders, or other components without confirming manufacturer compatibility.
- Failing to verify the complete stacked configuration in the current tabulated data.
- Using damaged or missing stacking hardware.
- Ignoring the added weight and lifting requirements of the complete assembly.
- Failing to account for working clearance around the pipe, manhole, vault, or other structure.
- Continuing to use the same system without reevaluation after excavation or jobsite conditions change.
Frequently Asked Questions
Can you stack two trench boxes?
Yes, when the manufacturer permits the boxes to be used in the intended stacked configuration and the complete system remains within the applicable tabulated-data requirements.
Does stacking two boxes double the depth rating?
No. Physical shield height and allowable excavation depth are different. The allowable depth must be determined from the manufacturer's tabulated data for the complete configuration and actual excavation conditions.
Can a 4-foot or 6-foot trench box be used on top of a taller box?
Many product lines include shorter shields that can serve as upper boxes, but the exact lower box, upper box, connection hardware, and configuration must be permitted by the manufacturer.
Can a shorter trench box also be used by itself?
Yes, when its manufacturer tabulated data and the actual excavation conditions permit its use as a standalone shield.
Can different brands of trench boxes be stacked together?
Do not assume they can. Compatibility must be supported by the applicable manufacturer documentation or other required engineering approval.
Treat the Stack as One Protective System
The upper box, lower box, spreaders, stacking hardware, excavation depth, soil conditions, surcharge loads, working clearance, equipment capacity, manufacturer tabulated data, and Competent Person review all affect whether a stacked trench-box configuration is appropriate for the project.
Need a Trench Box for an Upcoming Project?
Iron Lot supplies steel trench boxes, aluminum trench boxes, manhole boxes, spreaders, stacking hardware, and related trench-safety equipment for underground utility, municipal, public works, excavation, and construction projects.
Contact us with your excavation dimensions, project requirements, and delivery location for current pricing, availability, tabulated-data information, and freight coordination.