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Why is Excavation Safety Important?
Hundreds of workers face unsafe trench conditions daily. Imagine being trapped in a trench with tons of dirt bearing down on you. This terrifying scenario is a reality faced by many workers who are injured or killed in trench collapses each year. Excavation safety is a critical issue that demands attention, as even a seemingly stable trench can become a deadly hazard. It’s important to be educated on what is considered a stable trench by OSHA. As a disclaimer, this article is not guaranteed as fact, not intended as a thorough safety guide, and should be verified against all relevant federal regulations.
Do you need Shoring for Excavation Safety?
This is a question you may have asked yourself. Does my excavation need shoring? The answer isn’t straightforward, but we’ll outline some times that may need shoring. Excavations with walls greater than five-feet deep need shoring, unless it’s sufficiently sloped, or in competent rock.
What is considered sufficient sloping? What is considered competent rock? We will dive into some of these definitions below and what they mean for you as someone planning an excavation.
Slope and Soil Definitions
OSHA Classifies soils based on stability, and it’s worth visiting their website for more info. Click on the soil types below to understand their ramifications.
Stable Rock – vertical excavation slopes.
Stable rock will not collapse when excavated, and therefore it’s acceptable to excavate it at very steep side slopes. However, rock is also much more difficult to excavate, and may require excavation types such as blasting or a hydraulic hammer. Additionally, for excavation safety it’s critical that any loose rock is scaled from the walls of the trench and that hardhats are worn when inside.
Type A – 0.75:1 Excavation slopes
Type A are Highly Cohesive soils, usually clays with tensile strength. If you think about most soil you’ve held, it’ll just crumble. Highly cohesive clay can be rolled into a ball at the right moisture content. OSHA requires it to have an unconfined compressive strength of greater than 1.5 tons per square foot to allow a max of 0.75:1 side slope. However, this test data isn’t always given in geotechnical reports, and OSHA requires “no vibration of any kind”, which is usually unrealistic. Therefore, Type A is rare, and it’s more likely that excavations are Type B.
Type B – 1:1 Excavation Slopes
Type A soils become type B soils if the slopes are subject to fissuring, vibration, or is the unconfined compressive strength of the soil is between 1.5 and 0.5 tons per square foot. 1:1 (45 degree angle) is a safe side slope if you think it might be Type A soil but you’re unsure.
Type C – 1.5:1 Excavation Slopes
Cohesive soils are non-cohesive, or weakly cohesive (less than 0.5 tons per sf of unconfined compressive strength) soils. If uncertain whether a soil is Type A or B, excavate the trench as a Type C, at 1.5:1 side slopes.
Finally, it’s important to realize that for excavation safety in a layered soil system, the shallowest soil slope layer governs all the slopes above it. For example, you can’t have a 1:1 over a 1.5:1 slope even if the soil strata changes from Type C to Type B. Refer to the OSHA diagrams below for illustrated examples.
More Excavation Safety Rules
Stockpiles
When you excavate a trench, the soils will create a pile known as a stockpile, or an excavation spoils pile. These are regulated as well. OSHA requires that the toe of the stockpile is a minimum of two feet away from the top of the excavation. This helps prevent stockpiled material from rolling into a trench and potentially impacting workers below.
Trench Shield Heights
Similar to the above, another excavation safety measure – when you’re using a trench-box, the lip of the trench box needs to be at least 18″ above the lip of the excavation if it transitions into a sloped excavation. This is common when installing deeper stretches of pipe.
Excavations Requiring a Professional Engineer
There are a few instances where a third-party or internal professional engineer are required to ensure compliance with excavation safety, as outlined by OSHA.
- Trench crossings – on constrained sites, such as needing an open trench under an in-service road, a professional engineer is required to design and oversee installation of the trench crossing.
- Underpinning – in the case where an excavation is near an existing structure, where the structure’s stability may be affected by the excavation, underpinning may be required to ensure the building does not shift as its bearing soil is taken away.
- Excavations greater than 20 feet – OSHA describes that if excavation depths exceed 20-feet, a registered professional engineer is required to approve the means and methods of excavation.
- Custom shoring – It’s true that commercially available trench boxes are designed by professional engineers, but if you need a custom solution for a complex excavation, a professional engineer is required to oversee and approve the design for the shoring system.
Conclusion
There are a lot of rules to remember when planning to keep an excavation safe. It’s important to hire a qualified contractor for the work, and when in doubt, ask a civil engineer to help make sure that your excavation is possible. Contact 633 Construction if ever in need of a second opinion.
How do I make sure my excavation is safe?
If your excavation is greater than 5-feet deep, slope it per OSHA’s outlined soil type. But when in doubt, shore it or slope at a 1.5:1 side slope
How do I know what type of soil is in a trench?
Refer to your geotechnical report, assess the cohesion of the soils, and when in doubt, ask a licensed professional engineer.
How do I know when I need a professional engineer for excavation safety?
If the trench is greater than 20-ft deep, needs a trench crossing, requires assessment of building stability, or needs a custom shoring system.