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. 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. These 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.
As mentioned earlier, an excavation that has a depth of four feet or more must have a way to enter and exit the excavation. You can use a stairway, ladder, or a ramp. They need to be within 25 feet of employees; their safety may depend on how quickly they can climb out of an excavation. These structural ramps that are used to enter and exit the site must have non-slip surfaces and be designed by a competent person. A competent person must also evaluate ramps made from soil that are used to enter and exit an excavation.
One method of trench protection can be accomplished by sloping the sides of the trench to a safe angle. The trench is sloped on both sides. The safe angle to slope the sides of an excavation varies with different kinds of soil. The safe angle must be determined with each individual project. When an excavation is near water, has silty material or loose boulders, or when it is being dug in areas where erosion, deep frost or sliding is probable, the safe angle is more gradual.
Although sloping and benching is a good method for excavations, there are still some problems you may encounter. For example, wide excavation areas can expose footings or cause damage to the walls of the adjacent structure and pose additional hazards to employees. Wide excavation areas can also expose or place utilities (such as electrical power, water, and gas lines) above the angle for a safe slope. They also require the use of large equipment. There may also be hazards in the movement of the equipment across a larger excavation.
To prevent the collapse of an unsupported bench in an excavation 8 feet or less in depth, the allowable height of a bench at the base of an excavation must be 3 ½ feet or less. The collapse of one bench can, in turn, cause a lower bench to fail in a situation where many benches have been created. For Type A soil, for example, the OSHA standard requires multiple benches to have an overall slope of ¾ inches horizontal to 1 inch vertical.
The contractor needs to make a determination of the soil types at the excavation site using the soil classification system discussed in Module 2. Next, the contractor could consider potential sloping and benching problems, such as those mentioned above. Finally, after considering all other protection that may be necessary to ensure safe working conditions, the contractor can then determine if sloping is the best method to use at the site.
There are two basic types of benching: simple and multiple. The type of soil determines the horizontal to vertical ratio of the benched side. As a general rule, the bottom vertical height of the trench must not exceed 4 feet. However, subsequent benches may be up to a maximum of 5 feet vertical in Type A soil and 4 feet in Type B soil. All subsequent benches must be below the maximum slope allowed for that soil type. Also, in Type B soil, the trench excavation is permitted only in cohesive soil.
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.
This seems to be more of a trend today. 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. In addition, 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!
Whew, that’s a lot of information! Let’s get to the next module quiz!
This excerpt from NAHB Construction Safety & Health's Trenching & Excavation Safety Video highlights the most common hazards associated with residential trenching and excavation.
Before beginning this quiz, we highly recommend you review the module material. This quiz is designed to allow you to self-check your comprehension of the module content, but only focuses on key concepts and ideas.
Read each question carefully. Select the best answer, even if more than one answer seems possible. When done, click on the "Get Quiz Answers" button. If you do not answer all the questions, you will receive an error message.