Methods of Protection
This is a dangerous example of what you should not do.
All excavations are hazardous because they are inherently unstable. If they are restricted spaces, they present the additional risks of oxygen depletion, toxic fumes, and water accumulation. If you are not using protective systems or equipment while working in trenches or excavations at your site, you are in danger of suffocating, inhaling toxic materials, fire, drowning, or being crushed by a cave-in.
There are different types of protective systems.
- Sloping involves cutting back the trench wall at an angle inclined away from the excavation.
- Shoring requires installing aluminum hydraulic or other types of supports to prevent soil movement and cave-ins.
- Shielding protects workers by using trench boxes or other types of supports to prevent soil cave-ins.
Designing a protective system can be complex because you must consider many factors: soil classification, depth of cut, water content of soil, changes due to weather or climate, surcharge loads such as spoil and other materials used in the trench, and other nearby operations such as heavy equipment and vehicle traffic.
Check out this short audio clip by Dan Clark of the theSafetyBrief.com describes trenching protective systems.
The following case reports of trenching accidents investigated by OSHA only illustrate how seemingly innocent workplace activities can have deadly consequence, especially when it comes to excavation work.
- Two employees were installing 6" PVC pipe in a trench 40' long x 9' deep x 2' wide. No means of protection was provided in the vertical wall trench. A cave-in occurred, fatally injuring one employee and causing serious facial injuries to the other.
- An inadequately protected trench wall collapsed, killing one employee who had just gotten into the trench to check grade for installation of an 8" sewer line. The trench was 20-25 feet deep and had been benched about one bucket-width (4 feet) on each side. At the time of the collapse, a backhoe was still extracting soil from the trench.
- Four employees were in an excavation 32' long x 7' deep x 9' wide boring a hole under a road. Eight-foot steel plates used as shoring were placed against the side walls of the excavation at about 30-degree angles. No horizontal bracing was used. One of the plates tipped over, crushing an employee.
On March 8, 2011, an OSHA investigator was performing a worksite inspection on a trench being dug by Trimat Construction in Mercerville, Ohio. He directed an employee to exit the trench believing collapse was imminent. Within five minutes, the collapse occurred and could have buried the worker under six to seven feet of soil. Workers were ordered out of the trench just moments before a portion collapsed avoiding possible injury or loss of life.
OSHA standards mandate that all excavations 5 feet or deeper be protected against collapse. The employee was working in a trench at a depth greater than 10 feet without cave-in protection. OSHA's investigation is ongoing. The agency will determine why the job was being done in this fashion and what, if any, violations occurred.
Pre-job planning must to be conducted by a competent person or an engineer.
Pre-job planning is very important to prevent these types of accidents when trenching. In other words, safety cannot be improvised as the work progresses.
The following concerns must be addressed by a competent person:
- Evaluate soil conditions and select appropriate protective systems
- Construct protective systems in accordance with the standard requirements
- Pre-Plan: contact utilities (gas, electric) to locate underground lines
- Plan for traffic control, if necessary
- Determine proximity to structures that could affect your choice of protective system
- Test for low-oxygen, hazardous fumes and toxic gas, especially when gasoline engine-driven equipment is running, or the dirt has been contaminated by leaking lines or storage tanks.
- Provide safe access into and out of the excavation
- Inspect the site daily at the start of each shift, following a rainstorm, or after any other hazard-increasing event
Example of no support systems in a trench next to an adjacent structure. The worker is buried up to his waist beneath collapsing concrete and brick veneer.
Other Safety Precautions
The OSHA standard requires you to provide support systems such as shoring, bracing, or underpinning to ensure that adjacent structures such as buildings, walls, sidewalks, or pavements remain stable. The standard also prohibits excavation below the base or footing of any foundation or retaining wall unless:
- You provide a support system such as underpinning,
- The excavation is in stable rock, or
- A registered professional engineer determines the structure is far enough away from the excavation and the excavation will not pose a hazard to employees.
Excavations under sidewalks and pavements are prohibited unless you provide an appropriately designed support system or another effective means of support. There must not be any indications of a possible cave-in (while the trench is open) below the bottom of the support system. Also, you must coordinate the installation of support systems closely with the excavation work.
Once the work is finished, you are required to backfill the excavation when you take apart the protective system. After the excavation is cleared, remove the protective system from the bottom up. Make sure you are careful! In the next module, you’ll learn more about safely installing and removing protective systems.
Example of a shield that has not been maintained
Maintaining Materials and Equipment for Protective Systems
You are responsible for maintaining materials and equipment used for protective equipment. Defective and damaged materials and equipment can cause failure of a protective system and other excavation hazards.
To avoid possible failure of a protective system, you must make sure that:
- materials or equipment is not damaged or defective;
- manufactured materials or equipment is used and maintained consistent with the manufacturer's recommendations;
- a competent person examines all damaged materials and equipment; and
- unsafe materials or equipment is removed from service until a registered professional engineer evaluates and approves it for use
Residential Contractors and the Excavations Standard
OSHA recognizes that residential construction sites can be very different from commercial sites as they relate to excavations. OSHA's requirements in 29 CFR 1926.652 are not applicable to house foundation/basement excavations if all the following conditions are met. However, OSHA 1910.652 will apply if any of the following conditions are not met:
- The house foundation/basement excavation is less than 7½ feet in depth or is benched for at least 2 feet horizontally for every 5 feet or less of vertical height.
- The minimum horizontal width at the bottom of the excavation is as wide as needed, but not less than 2 feet.
- There is no water, surface tension cracks or other environmental conditions present that reduce the excavation stability.
- There is no heavy equipment operating in the vicinity that causes vibration to the excavation while employees are in the excavation.
- Work crews in the excavation are the minimum number needed to perform the work.
- The work has been planned and is carried out in a manner to minimize the time employees are in the excavation.
This policy applies to all house foundation/basement excavations including those which become trenches by definition when formwork, foundations, or walls are constructed. This policy does not apply to utility excavations (trenches) where 29 CFR 1926.652 remains applicable.
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