Pre-job planning is very important to prevent accidents when trenching. In other words, safety cannot be improvised as the work progresses.
The following concerns must be addressed by a competent person:
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.
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 (For example, spoil and other materials to be used in the trench) and other operations in the vicinity.
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:
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 section, you'll learn more about safely installing and removing protective systems.
You must take the necessary steps to protect yourself and your employees when installing and removing a protective system. The OSHA standard requires you to take the following steps to protect your employees:
In addition, the standard permits excavation of two feet or less below the members of a support or shield system of a trench if the system is designed to resist the forces calculated for the full trench depth.
Designing a protective system can be complex. You must consider many factors, including:
Once you have selected an approach, however, the system must meet the required OSHA performance criteria.
The OSHA standard describes methods and approaches for designing protective equipment. Let’s discuss the different methods to designing protective equipment.
Slope the sides to an angle that isn't steeper than 1½:1. (34 degrees measured from the horizontal) For example, for every foot of depth, the trench must be excavated back 1½ feet. All simple slope excavations 20 feet or less deep should have a maximum allowable slope of 1½:1. These slopes must be excavated to form configurations similar to those for Type C soil. A slope of this gradation or less is safe for any type of soil.
Use tabulated data such as tables and charts approved by a registered professional engineer to design excavation. This data must be in writing and must include enough explanatory information, including the criteria for making a selection and the limits on the use of the data, for the user to make a selection.
At least one copy of the data, including the identity of the registered professional engineer who approved it, must be kept at the worksite during the construction of the protective system.
After the system is completed, the data can then be stored away from the jobsite. However, a copy must be provided upon request to the Assistant Secretary of Labor for OSHA.
In this method, you would use a trench box or shield designed or approved by a registered professional engineer. Timber, aluminum, or other suitable material may also be used in the construction. OSHA standards permit the use of a trench shield if it provides the same level of protection as the appropriate shoring system.
Employers can choose the most practical method for the particular circumstance, but that system must meet the required performance criteria. The standard doesn't require a protective system when an excavation is made entirely in stable rock or is less than five feet deep. However, in this case, a competent person must examine the ground and find no indication of a potential cave-in.
There are two basic types of benching: simple and multiple. The type of soil determines the horizontal to vertical ratio of the benched side.
Shoring is the part of a support system for trench faces. It is used to prevent movement of soil, underground utilities, roadways and foundations. Shoring or shielding is used when the location or depth of the cut makes sloping back to the maximum allowable slope impractical. Shoring consists of posts, struts and sheeting. There are two types of shoring: timber and aluminum hydraulic.
Hydraulic shoring, a pre-fabricated strut and/or wale system made from aluminum or steel. Hydraulic shoring provides a critical safety advantage over timber shoring because workers do NOT have to enter the trench to install or remove hydraulic shoring.
Other advantages to most hydraulic systems include:
All shoring should be installed from the top down and removed from the bottom up. Hydraulic shoring should be checked at least once per shift for leaking hoses and/or cylinders, broken connections, cracked nipples, bent bases, and other damaged or defective parts.
Pneumatic shoring works in a manner similar to hydraulic shoring. The primary difference is pneumatic shoring uses air pressure in place of hydraulic pressure. However, you need to have an air compressor on site when using pneumatic shoring. Air shoring involves using compressed air instead of hydraulic fluid to expand the trench jacks into position. Using the air type of system, pins are put in place to lock the jacks when a desired level of stability is achieved. To remove this type of trenching system, air is injected into the jacks to extend them. This allows the pin to be removed. These types of jacks are popular since they are cleaner than hydraulic jacks and there isn't a danger from the leakage of fluids or other lubrication.
As mentioned earlier, when a trench is excavated, employees who work in the area must be protected from cave-ins. Therefore, the contractor should consider excavating a wider area than the necessary minimum. When this is done, it provides a more comfortable working environment for your employees in the trench. This extra working area may provide a way for workers to escape an unexpected crisis, such as falling objects or debris.
Contractors should also reduce risk by limiting the number of workers in the trench at all times. The only workers allowed in the trench should be those who are absolutely needed to perform the task at hand.
As the trench is backfilled, the braces and planks can be removed to be used at another site. If installed and removed correctly, vertical planks and trench braces may be used several times!
Access to and exit from the trench require the following conditions:
In addition to cave-ins and related hazards, workers involved in excavation work are exposed to hazards involving falling loads and mobile equipment. To protect workers from these hazards, OSHA requires employers to take certain precautions. For example, employers must:
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