One cubic yard of dirt can fit neatly in the bucket of a 100-hp excavator. And while it may not look like much, that dirt weighs on average 3,000 lbs. or, as Steve Winter at ProTec relates, as much as a Volkswagon. Since trenching involves a lot more than a single bucket load, protecting workers in the trench - who are surrounded by thousands of pounds of soil - is crucial.
The potential hazards of working in deep trenches are fairly obvious. But even a cave-in in a relatively shallow trench can be dangerous.
"It takes only a small amount of soil to create enough pressure to asphyxiate a worker, and a person does not need to be completely covered in soil to die," says Paul McDonnell, vice president - trench safety, United Rentals. "Furthermore, it only takes seconds for a cave-in to become deadly."
The federal standard (OSHA 29 CRF, Part 1926 Subpart P) requires any excavation deeper than 5 ft. to have some type of protective system. However, some states, cities and municipalities have more stringent guidelines, and require protection in trenches at shallower depths.
"There is simply so much weight," says Winter. "Even in a shallow trench, a worker's legs could get caught. And the weight and pressure of the dirt can cut off circulation."
Thanks to modern shoring and shielding devices, you can easily create a safe and productive environment, in which the number of injuries and deaths can be minimized and even eliminated.
The duties of a competent person
Mike West recalls that in the mid-1980s, when he started working for Efficiency Production, Inc., a few hundred deaths occurred each year due to trenching accidents. He believes that today there need not be any if proper shoring and/or shielding are used. "There is no excuse for not being safe," he insists. "Information, training and equipment are readily available."
Training is offered through most manufacturers, many distributors and organizations such as the National Utility Contractors Association, the Trench Shoring and Shielding Association, as well as several rental companies, such as United Rentals' 60 trench safety rental branches.
That puts the ball in the contractor's court. It's up to you, or another "competent person" you deem responsible for the project (as required by the OSHA standard), to implement the right system. The competent person must be capable of identifying existing and predictable hazards in the surroundings, or working conditions that are unsanitary, hazardous or dangerous to employees, and have the authorization to take the proper corrective measures to eliminate them.
"That competent person is responsible for selecting a [trench safety] system," says West. "That person needs to know the standards and is responsible for everything in it. That person needs to know the tabulated data and interpret it correctly. If that person doesn't understand it, it's that person's responsibility to call the manufacturer or distributor and go through the data to make sure the right equipment is being used properly."
One of the first tasks the competent person will need to complete is determining the soil type.
"In understanding trench safety, it is imperative that we be knowledgeable about the soils and rock formations we may encounter, and accept that they both have typically unpredictable behaviors," says Keith Lamberson, Trench Shoring Systems, and chairman of the Trench Shoring and Shielding Association. With that in mind, understand that a trench dug into one area of clay may stand for several weeks without significant deterioration, while a similar trench cut in the same general area can collapse within minutes of being dug.
To assist in determining soil types, OSHA has outlined a soil classification system (outlined in Appendix A of Subpart P) that categorizes soil and rock deposits in a hierarchy of stable rock (vertical sides remain intact when exposed) and types A (clay, silty clay, etc.), B (silt, sandy loam, etc.) and C (gravel, sand, etc.), in decreasing order of stability.