Digging Into Trench Hazards and Protections

by Melanie Baravik
June 21, 2018

This week is the 2018 Trench Safety Stand Down — in case you missed it, check out Monday’s piece on the dangers of working in trenches. Today, we’ll take a look at how to prepare for excavating, the different types of trench protection you can use and how to choose the right system for the job.

Before digging

Before you can decide which protective system you’ll be using for the trench — in fact, before you start digging the trench — take stock of the jobsite to identify any potential hazards:

• Heavy moving loads
• Vibrating equipment
• Rain or water runoff
• Hot, dry weather

The presence of any of these risk factors can increase the risk of a trench collapse. That doesn’t necessarily mean you can’t dig at that site, but it does mean you’ll need to be extra cautious.

You’ll need to designate a competent person whose job is to inspect the trench daily or whenever soil or jobsite conditions change to ensure workers are protected. Basically, that person’s word is law when it comes to trenches — they’re trained to recognize potential and existing hazards and to classify soil using a few different methods. The competent person is authorized to take measures to fix any hazard that comes up, even if that means stopping work to address the problem.

 
Soil classification

Next, what type of soil are you excavating? OSHA has four classifications for soil types:

• Stable rock — Solid rock that doesn’t collapse when excavated.
• Type A — Cohesive soils like clay or clay loam with an unconfined compressive strength of 1.5 tons per square foot (tsf) or more.
• Type B — Cohesive soil with an unconfined compressive strength of 0.5 to 1.5 tsf, cohesionless soils like angular gravel or silt, previously disturbed soil or dry, unstable rock.
• Type C — Cohesive soil with an unconfined compressive strength of 0.5 tsf or less and granular soils like sand or submerged or watery soil or rock.

Unconfined compressive strength measures the amount of compressive stress soil can withstand in an unconfined space. Cohesive soils do not crumble and are hard to break up; when submerged, cohesion increases — for example, clay is an extremely cohesive soil. Cohesionless soils like sand don’t bind together well and are free-running.

No soil can be classified as Type A if it meets any of these standards:

• Fissured
• Subject to vibrations from machinery or heavy traffic
• Previously disturbed
• Part of a sloped system
• Presence of any other factors that would make the soil unstable

It’s important to remember that as you dig, you may run into layers of soil that are different types than the top layer. Any time conditions change in a way that could affect the soil, like if it rains heavily or if a new piece of heavy or vibrating equipment is being used near the trench, the competent person needs to take a second look in case the soil needs to be reclassified.

When in doubt, always consult the project’s competent person. It’s better to be overly cautious and take a little extra time to complete the job than to rush it and find yourself dealing with a trench collapse.

Sloping

Sloping involves sloping the walls of the trench out to reduce risk of cave-ins. Different degrees of slope must be used depending on the soil type at the excavation site:

• Stable rock — Vertical 90° slope
• Type A — Nine inches of outward slope for every foot of trench depth
• Type B — One foot of outward slope for every foot of trench depth
• Type C — 18 inches of outward slope for every foot of trench depth

Sloping is appropriate protection only for trenches 20 feet deep or less. Take a look at Figure V:2-13 to see examples of sloped trenches.

Benching

Benching is generally used in Type A and cohesive Type B soils, and can be a single or multiple bench. Benches look like a stair or set of stairs on the sides of the trench. The first bench at the bottom of the trench must be no taller than four feet high, and subsequent benches can be five feet high if the soil is Type A or four feet high if it’s Type B. Benches also need to be below the maximum allowed slope for the soil type. Like sloping, benching can’t be used in trenches deeper than 20 feet.

Shoring

Shoring is a protective system that supports trench faces, made of aluminum or timber and braced with the help of hydraulic or pneumatic (air) pressure. Shoring is usually used in deeper trenches where benching and sloping is impractical or doesn’t provide enough protection. Here’s a typical timber shoring system and its parts:

via OSHA

Hydraulic shoring, however, is becoming more commonly used; it’s a prefab strut and/or wale system made of steel or aluminum, and it’s gauge-regulated so pressure is evenly applied along the trench face. It’s safer than timber shoring, since timber shorings usually use a screw jack system with struts that have to be manually adjusted inside the trench. Hydraulic shoring is also easier and safer to install and remove since workers don’t have to climb in the trench to do so.

Since pneumatic shoring uses air pressure instead of hydraulic, an air compressor does need to be on-site. Hydraulic and pneumatic shoring systems aren’t your only options, but they are the most common. No matter which type of shoring you use, it must be installed from the top down and removed from the bottom up.  

Shielding

Like shoring, shielding is typically used in deeper trenches and is done using a trench box that “shields” workers from cave-ins. When the trench box is installed, the area between the box and the faces of the trench are backfilled, though the box should be as close to the face as possible. You can also use shielding in combination with benching and sloping. With the proper shielding and if there are no cave-in hazards, you can dig up to two feet below the bottom of the trench box.

More trench safety tips

Temporary and permanent spoil

“Spoil” refers to the piles of soil removed from the trench during excavation. If it’s a temporary pile, the spoil must be at least two feet from the edge of the excavation — and that’s measured from the base of the spoil, not the crown. It also needs to be placed so that any water runoff is directed away from the trench.

Permanent spoil must be “some distance” from the excavation site. Since permanent spoil is generally a lot larger than temporary spoil, it can have a large effect on the trench itself — either by disturbing soil or by changing slope requirements if it’s too close. There’s no set distance between a trench and permanent spoil; OSHA rules say that a safe distance can be determined visually.

Workers in the trench

There are a few other jobsite conditions to be aware of when you’re working around excavations. OSHA advises against putting vehicle crossing or walkways across trenches; if it can’t be avoided, vehicle crossings must be engineer-designed and walkways need to meet OSHA safety standards. You’re obligated to protect employees from being hit by vehicles by providing them with appropriate safety gear and employing a flagger and using signs and barriers when necessary. You’ll also need to use barricades or stop logs to warn and prevent vehicles from falling into the trench.

Trenches four feet deep or more need a “fixed means of egress,” a secured ladder that extends at least 36 inches over the edge of the excavation. No worker should have to travel more than 25 feet to reach an exit. Down in the trench, the air can sometimes pose a risk to workers — oxygen levels might be too high or low, or flammable gases may be present. The air in the trench must be tested daily before work starts, with more frequent testing if equipment is being used or if there is any burning or welding down in the trench.

This isn’t meant to be a one-size-fits-all guide to trench safety — working in trenches is dangerous and requires extensive training and proper execution of safety procedures. When it comes to excavation, taking a shortcut can lead to tragedy, so it never hurts to brush up on your knowledge of what can go wrong and how to protect against it. With the help of a competent person and a solid trench safety plan, you can keep your jobsites accident-free.

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