Used appropriately, a trench box, also known as a trench shield, can help protect workers from serious and sometimes fatal injuries if a cave-in occurs. But too often, contractors misunderstand the right way to build and install a trench box, and some take shortcuts that put workers’ lives at risk.
1. Don’t count on trench boxes to prevent a cave-in. Trench boxes are designed to protect workers from soil movement, not to prevent soil movement. Always backfill the void between the box and the trench wall to stabilize the shield. If working adjacent to a critical structure (roadway, railroad track, or building) and it is important to eliminate soil movement from under that structure, shoring may be a better option.
2. Don’t assume a trench box is the only way to work underground. There are many options for safely working underground. If the soil is weak, flowing, or has high water content, or if the work will be very deep, these are indicators that maybe a trench box may not be the best option. Sliderail systems or braced sheeting systems may be the better option. Know what your options are.
3. Follow the manufacturer’s tabulated data. The biggest issue we see is not using a box within the limitation of its tabulated data. If the tabulated data says you should use eight spreader pins with eight keepers to hold those pins in place, make sure you have installed all of them before anyone gets in the box. Another issue is the placement of a box to a depth greater than the maximum allowable depth, based on the soil type.
4. Watch the bottom and the top. According to OSHA’s Technical Manual, OSHA allows a shield to be left a maximum of two feet off of the bottom of the trench, and that is allowed only as long as soil is not flowing around or below the bottom. Also, the box must be rated for the full depth of the excavation, which would be measured from the deepest part of the trench to the highest side.
OSHA also allows for a vertically sided lower portion of the trench, with a sloped embankment at the top. An 18” minimum roll off protection is required to prevent soil from rolling in over the top.
5. Replace any damaged spreader pipe or trench box panels. One of the responsibilities of a competent person is to perform daily inspections on all shoring equipment. If the competent person is unsure of the ability of the equipment to work properly, the equipment should be removed from service.
6. Understand proper end loading. A traditional trench shield consists of two steel or aluminum sides, with a minimum of two steel spreaders holding them apart on each end. Manufacturers and their engineers intended for the boxes to be open on each end, with the soil to be sloped out appropriately. The instructions for the placement, type of material, and other requirements are typically found on Technical Data Sheets.
7. Stack with care. Stacking pins must be used to ensure safe usage, preventing the upper boxes from shifting. Also, when stacking boxes, make sure that the boxes are rated for the depths at which they will be used.
A seemingly small mistake with a trench box could have huge unforeseen consequences. It is always best practice to contact a qualified and knowledgeable expert before starting any excavation project.
Shane Wareham is a District Sales Manager at United Rentals with over 15 years of experience. Visit unitedrentals.com/solutions for additional guidance to help keep your workers safe.