Apr 11, 2018
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I am sure you may have heard about trenching incidents near you or reading about them on a national level. Managing safety for these operations is quite simple once you know the requirements and understand some of the nuances that go along with the different soils and protective systems we have.
Cave-ins pose the greatest risk with these activities and are much more likely than other excavation-related accidents to result in worker fatalities. Other potential hazards include falls, falling loads, hazardous atmospheres, and incidents involving mobile equipment. One cubic foot of soil can weigh 100 pounds.
One cubic yard of soil can weigh as much as a car, and the kinetic energy of soil falling 3, 4, 10 feet, and you can see the danger here. It has been said that an unprotected trench is an early grave. So let me start by stating the obvious: do not enter an unprotected trench.
So at what point do we need to protect a trench? According to OSHA, trenches 5 feet (1.5 meters) deep or greater require a protective system unless the excavation is made entirely in stable rock. Oh, and forget about stable rock, I will explain that later. If less than 5 feet deep, a competent person may determine that a protective system is not required.
OSHA standards require that employers inspect trenches daily and as conditions change by a competent person before worker entry to ensure the elimination of excavation hazards.
A competent person is an individual who is capable of identifying existing and predictable hazards or working conditions that are hazardous, unsanitary, or dangerous to workers, soil types and protective systems required, and who is authorized to take prompt corrective measures to eliminate these hazards and conditions.
OSHA standards require safe access and egress to all excavations, including ladders, steps, ramps, or other safe means of exit for employees working in trench excavations 4 feet (1.22 meters) or deeper. These devices must be located within 25 feet (7.6 meters) of all workers. Here are some other requirements to follow:
Keep heavy equipment away from trench edges. In the fire department, we tried to keep all heavy rigs at least 25 feet away. But this was to prevent vibrations to already unstable trenches during rescue operations. But heavy equipment that could pose a hazard of falling into the trench or even knocking materials into the trench has to be located back at a safe distance.
The standard requires that you keep excavated soil (spoils) and other materials (like sand and gravel used for backfill or pipes being installed) at least 2 feet (0.6 meters) from trench edges. Ongoing inspections are needed to ensure this requirement is being met.
Know where underground utilities are located before digging. This is a requirement in the OSHA standards, but it requires you to follow State laws that actually govern locating and marking PUBLIC utilities. Private utilities still need to be located and marked as well. So, familiarize yourself with the 811 "call before you dig" system.
Test for atmospheric hazards such as low oxygen, hazardous fumes, and toxic gases when > 4 feet deep. This is when you suspect there could be a hazardous atmosphere or one could reasonably be expected to exist, such as in excavations in landfill areas or excavations in areas where hazardous substances are stored nearby.
Falls and Falling Objects
Do not work under suspended or raised loads and materials. This means workers down in the trench and the track hoe bucket swinging over their head to drop gravel, sand or pulling the material out.
Also, Walkways have to be provided where workers or equipment are required or permitted to cross over excavations. Guardrails that comply with §1926.502(b) shall be provided where walkways are 6 feet (1.8 m) or more above lower levels. Again, only when workers are to CROSS OVER the open trench of 6 feet deep or more.
Ensure that all personnel has high visibility or other suitable clothing when exposed to vehicular traffic. This is a requirement and you have to reference the Manual on Uniform Traffic Control Devices for minimum requirements for highways as well as flaggers being used. This tells you the ANSI rating needed for certain reflective apparel needed for day and night work.
Also, prepare for an emergency, especially if in an unfamiliar area, a rural area and/or working with a hazardous atmosphere by contacting the local emergency response service and determining whether or not they are equipped and prepared for a potential rescue. This may also drive the need to have someone trained in 1st aid/CPR at the site as well. Also, know where the nearest emergency medical center is located. For job sites, this should ALWAYS be a part of a site-specific safety plan anyway.
Now, let’s get into soil classification. You need to understand what OSHA deems to be stable or unstable soil and how the class of soil drives the protective system you choose. So let’s get into some definitions you need to know and the different types of soil classifications as well as HOW to test soils. Remember, these terms are important to understand as we move forward:
How to Classify Soil
So let’s dig into classifying soil. “Soil classification system" means, for the purpose of this subpart, a method of categorizing soil and rock deposits in a hierarchy of Stable Rock, Type A, Type B, and Type C, in decreasing order of stability. The categories are determined based on an analysis of the properties and performance characteristics of the deposits and the characteristics of the deposits and the environmental conditions of exposure.
"Type A" means:
Cohesive soils with an unconfined, compressive strength of 1.5 ton per square foot (tsf) (144 kPa) or greater. Examples of cohesive soils are clay, silty clay, sandy clay, clay loam and, in some cases, silty clay loam and sandy clay loam. Cemented soils such as caliche and hardpan are also considered Type-A.
However, no soil is Type-A if:
"Type B" means:
"Type C" means:
In order to properly classify the soil, OSHA requires at least on visual analysis and one manual test be performed on the soil.
Visual analysis is conducted to determine qualitative information about the excavation site. In general, you need to consider the soil adjacent to the excavation, the soil forming the sides of the open excavation, and soil samples taken from excavated material. Here are the steps:
OSHA also requires at least 1 manual test to be performed in order to determine quantitative as well as qualitative properties of soil. This provides more information in order to classify soil properly so as to get us to the next step; selecting appropriate protective measures. Let’s run down the options for manual testing:
Some of these tests are more for soil engineering and designing complex protective systems. Your average worker involved in excavation activities is probably going to use the visual analysis along with the plasticity test or thumb penetration test. Just getting your hands on a sample and trying to mold it, feel it, see how it behaves is really a good way to determine what you are dealing with.
ProTip: Forget solid rock; it can have any fractures/fissures, if you are scraping, blasting, hydraulic fracturing, pile-driving, drilling, then you are creating this condition. Which means pieces of varying size could potentially come loose and fall into the trench. Therefore you move to the next level: Type A soil. Well, if it is fissured or subject to vibration, etc. then it cannot be considered type A. And let’s face it, most excavations will be subject to varying degrees of activities that meet this criterion.
So, we are left with Type B soil and C. Cohesive or non-cohesive? (ok, there is a gray area known as non-cohesive type B, but honestly, just call it non-cohesive and get to work!). So is it Clay or Sand? It really is that simple. Once you know the soil type, you can determine what protective system to use.
Here your options are sloping, benching or shoring of some kind. The maximum allowable slope is as follows:
Notice I said MAXIMUM allowable slope! This means you may need to make it “flatter” if it is needed. Sloped means the angle at which it will lie and NO LONGER MOVE! So keep that in mind. If you have Type B soil based on the visual and manual test and slope it to a 45-degree angle and you get sloughing then you need to dress that slope back more.
Benching is the same; the angle of the bench as measured from the TOE of the trench (nearest bottom side). So you can tell how many steps will be needed to achieve the 45-degree angle. You are only allowed a 4-foot maximum face for the first bench step you make, then a 5-foot maximum face thereafter.
You can also use a single bench; make a 4’ maximum face, cut it back then go to the slope needed for the soil type. Or an unsupported vertically sided lower portion; you can come straight up no more than 3 1/2 feet then hit your slope. But keep in mind, Type C soil according to OSHA CANNOT be benched! Only sloped or shored.
And, these are ONLY allowed for trenches up to 20 feet deep. Trenches 20 feet (6.1 meters) deep or greater require that the protective system be designed by a registered professional engineer or be based on tabulated data prepared and/or approved by a registered professional engineer.
As for shoring; like trench boxes and speed shoring - you have to make sure you have what is called the tabulated datasheet for that system ONSITE. This tells you the depth limits based on soil type as well as width limitations, whether you can stack 2 or more trench boxes on top of one another, things like that. This gets tricky as there are all sorts of systems so I won’t get into specifics here. Just keep in mind, if your box or shield is below grade at any point, you must slope or bench as allowed per the soil type for any soil above the top of the device and you need at least an 18” lip to catch any rolling materials from going into the trench.
Same for the bottom; you can only raise the box or shield off the bottom of the trench up to 18” if conditions allow. So there are some little rules that go along with these systems; but again, you need to know the system requirements BEFORE beginning work and have that data onsite while work is taking place.
I hope you got some good info from this episode. Please follow up and seek more formal training for yourself and your co-workers on this topic. If you are overseeing work at your facility that involves this type of activity then I hope I gave you some good tips so that you can begin to go out and look at this work and assess whether or not things are compliant. Please let me know what you think, share your thoughts by emailing me at firstname.lastname@example.org.
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