Proximity of Parallel Trenches
Hi everyone,
I’m currently planning a project that involves digging multiple parallel trenches, all expected to be around 5 feet in depth. Given the depth, I’m aware of OSHA’s requirements for trench protective systems, which can include sloping, benching, shoring, or shielding, depending on factors like soil type and trench depth. My question centers on how these safety requirements influence the recommended spacing between parallel trenches to maintain both safety and structural integrity. Specifically, how might the proximity of another trench affect the soil stability of neighboring trenches and, by extension, the overall safety of the worksite?
I’m seeking guidance or any shared experiences on the best practices for determining the spacing between such trenches, especially in conditions involving less stable soil or projects with complex requirements involving multiple trenches. Insights on planning and assessing these factors to ensure the safety of workers around these trenches would be greatly appreciated.
Thank you in advance for your advice and expertise!
Comments (7)
There are a lot of factors that go into this issue, such as the following:
- How far apart are these trenches? If they are close, is the wall in-between serving as a face for both trenches?
- Is there equipment involved that is in close proximity to each trench? If so, what is the weight of each piece of equipment? The weight and vibration of this equipment can cause potential cave-ins.
- Where are the spoil piles located? Are they located between the trenches so that you have two spoil piles located in close proximity to each other? Both of these would apply additional weight.
- What type of soil is this? As you may know, you have to conduct at least one manual test (e.g., penetrometer, plasticity test, etc.) and one visual test on the soil. The total amount of unconfined compressive strenth (expressed in TSF) will help determine how stable the soil is.
If there's ever any question, I would always default to a trench shield so that you don't have to worry about benching or sloping. May require more money to rent or purchase these, especially if it's a lengthy trench, but if used properly, it provides you some pretty solid piece of mind.
As noted in 1926 Subpart P, Appendix C, they define the term “adjacent” as used meaning the area within a horizontal distance from the edge of the trench equal to the depth of the trench. This can be used as a general rule of thumb as far as how far out to evaluate adjacent areas. If you have a trench that's 5 feet deep, I would evaluate and consider all loads, surcharges, etc. that are within 5 feet horizontally of each trench wall. One cubic yard of soil can weigh as much as 3,000 lbs., so how much do you have in your spoil pile? Where is the spoil pile located? Etc.
Overall, there's not a specified distance between adjacent trenches that is considered "safe" because there's so many variables involved, which is why a Competent Person is involved in the inspection process (and you may even need to involve an RPE).
When I would watch over trenching operations, we also considered it class C, even in straight rock, and put our guys in trench boxes at all times.
They company I worked with had an outstanding safety record for the hours they worked, and I believe reasons as this is why. They were no nonsense and protected people no matter the cost.