Course 157 Excavation Safety: Basic

Protection Systems

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Competent person conducting pre-job planning.

Pre-Job Planning

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:

  • Evaluate soil conditions and select appropriate protective systems.
  • Construct protective systems in accordance with the standard requirements.
  • 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.

1. What is always required because safety cannot be improvised as part of the excavation work process?

a. Conduct pre-job planning
b. Removal of all archaeological artifacts
c. Report all accidents to OSHA within 24 hours
d. Continuous improvement processes
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Do we need a protective system here?

Protective Systems

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 (For example, spoil and other materials to be used in the trench) and other operations in the vicinity.

2. Each of the following is one of the three primary protective systems for excavations, EXCEPT _____.

a. sloping
b. shaking
c. shoring
d. shielding
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.
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 section, you'll learn more about safely installing and removing protective systems.

3. What does OSHA require to make sure buildings, walls, sidewalks and pavements remain stable?

a. Ensure the excavation is no closer than 50 feet to walls
b. Provide warning notices to all building occupants
c. Install guardrails around all excavations
d. Provide shoring, bracing, or underpinning

Installation and Removal of Protective Systems

soil weight
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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:

  • Connect members of the support systems securely.
  • Install support systems safely.
  • Avoid overloading members of support systems.
  • Install other structural members to carry loads imposed on the support system when you need to remove individual members temporarily.

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.

4. All of the following steps are required by OSHA when installing or removing a protective system, EXCEPT _____.

a. avoid overloading members of support systems
b. connect members of the support systems securely
c. limit excavations below shield system to three feet
d. install structural members to carry sufficient loads

Appropriate Protective System Designs

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Example of sloping.

Designing a protective system can be complex. You must consider many factors, including:

  1. soil classification
  2. depth of cut
  3. water content of soil
  4. changes due to weather and climate
  5. other operations in the vicinity

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.

Method 1: Sloping

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.

Simple Slope Excavation of Type C Soil

5. To achieve a slope of 34 degrees, for every one foot in depth, excavate the sides back _____.

a. 1 foot
b. 1½ feet
c. 2 feet
d. 2½ feet

Appropriate Protective System Designs (continued...)

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Trench Boxes

Method 2: Design Using Data

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.

Method 3: Trench Box or Shield

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.

Example of Trench Shields
Example of Trench Shields

6. OSHA requires a protective system unless an excavation is _____.

a. made entirely in stable rock or less than five feet deep
b. made in Type A soil or under six feet in depth
c. designed by a registered engineer or made in rocky soil
d. approved by local utilities or made entirely in stable rock

Benching

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Example of benching.

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.
  • 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.
  • Type C soil is not stable enough for benching as a protective system: use sloping instead.
Type A Soil Benching
Type B Soil Benching

7. Benching may be used as a protective system in which two soil types?

a. Soil types A, B, or C
b. Soil type A only
c. Soil type B only
d. Soils types A and B

Shoring Types

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
Hydraulic Shoring Types
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Hydraulic Shoring

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:

  • light enough to be installed by one worker
  • gauge-regulated to ensure even distribution of pressure along the trench line
  • can have their trench faces "pre-loaded" to use the soil’s natural cohesion to prevent movement
  • can be adapted easily to various trench depths and widths

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

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.

8. Why does hydraulic shoring provide a critical safety advantage over timber shoring?

a. Hydraulic shoring is less expensive to use
b. Hydraulic shoring can be loaded with any kind of hydraulic fluid
c. Workers do not have to enter the trench to install or remove shoring
d. Timber shoring is heavier and less stable long-term
When a trench is excavated, employees who work in the area MUST be protected from cave-ins.

Other Protective Systems

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!

Ingress and Egress

Access to and exit from the trench require the following conditions:

  • Trenches 4 ft or more in depth should be provided with a fixed means of egress.
  • Spacing between ladders or other means of egress must be such that a worker will not have to travel more than 25 ft laterally to the nearest means of egress.
  • Ladders must be secured and extend a minimum of 36 in (0.9 m) above the landing.
  • Metal ladders should be used with caution, particularly when electric utilities are present.

9. When in a trench, workers should be no farther from a ladder than _____.

a. 15 feet
b. 25 feet
c. 30 feet
d. 40 feet

Additional Hazards and Protections

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Many potential hazards exist when trenching.

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:

  • Protect workers from excavated or other materials or equipment that could pose a hazard by falling or rolling inside the excavation by placing and keeping such materials or equipment at least 2 feet (0.61 meters) from the edge and/or by using a retaining device to keep the materials or equipment from falling or rolling into the excavation.
  • Provide a warning system (such as barricades, hand or mechanical signals, or stop logs) when mobile equipment is operated adjacent to an excavation, or when such equipment must approach the edge of an excavation, and the operator does not have a clear and direct view of the edge.
  • Protect workers from loose rock or soil that could fall or roll from an excavation face by scaling to remove loose material, installing protective barricades at appropriate intervals, or using other equivalent forms of protection.
  • Institute and enforce work rules prohibiting workers from working on faces of sloped or benched excavations at levels above other workers unless the workers at the lower levels are adequately protected from the hazards of falling, rolling, or sliding material or equipment.
  • Institute and enforce work rules prohibiting workers from standing or working under loads being handled by lifting or digging equipment.
  • Require workers to stand away from vehicles being loaded or unloaded to protect them from being struck by any spillage or falling materials. (Operators may remain inside the cab of a vehicle being loaded or unloaded if the vehicle is equipped, in accord with 29 CFR 1926.601(b)(6), to provide adequate protection for the operator.)

10. Which of the following is NOT allowed when working in a trench?

a. Provide a warning system when equipment is operating by the excavation
b. Keep materials at least 2 feet from the edge of the excavation
c. Workers may work under loads only if a spotter is close by
d. Require workers to stand away from vehicles loading or unloading

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