Excavation and Trenching

Introduction

Two workers are killed every month in trench collapses. An excavation is any man-made cut, cavity, trench, or depression in an earth surface formed by earth removal.

Trench (Trench excavation) means a narrow excavation (in relation to its length) made below the surface of the ground. In general, the depth is greater than the width, but the width of a trench (measured at the bottom) is not greater than 15 feet (4.6 meters).

Dangers of Trenching and Excavation

Cave-ins pose the greatest risk and are much more likely than other excavation-related accidents to result in worker fatalities. Other potential hazards include falls, falling loads, hazardous atmospheres, and incidents involving mobile equipment. One cubic yard of soil can weigh as much as a car. An unprotected trench is an early grave. Do not enter an unprotected trench.

Competent Person

Employers should inspect trenches daily, before each shift, and as conditions change by a competent person before worker entry to ensure elimination of excavation hazards. The designated competent person should have and be able to demonstrate the following:

• Training, experience, and knowledge of:
• soil analysis
• use of protective systems
• requirements of 29 CFR Part 1926 Subpart P

An Oregon OSHA compliance officer stopped by an excavation where two workers were partially buried. They were unable to move until co-workers (also in danger) dug them out. The question is, was there a competent person for this worksite?
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• Ability to detect:
• conditions that could result in cave-ins
• failures in protective systems
• hazardous atmospheres; and
• other hazards including those associated with confined spaces

• Authority to take prompt corrective measures to eliminate existing and predictable hazards and to stop work when required.

Access and Egress

Inadequate shoring and access or egress
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To avoid fall injuries during normal entry and exit of a trench or excavation at your job site, ladders, stairways, or ramps are required. In some circumstances, when conditions in a trench or excavation become hazardous, survival may even depend on how quickly you can climb out. Use the following guidelines and safe practices to avoid excavation access-egress hazards:

• Provide stairways, ladders, ramps, or other safe means of egress in all trenches that are 4 feet deep or more.
• Position means of egress within 25 lateral feet of workers.
• Structural ramps that are used solely for access or egress from excavations must be designed by a competent person.
• When two or more components form a ramp or runway, they must be connected to prevent displacement, and be of uniform thickness.
• Cleats or other means of connecting runway components must be attached in a way that would not cause tripping (e.g., to the bottom of the structure).
• Structural ramps used in place of steps must have a non-slip surface.
• Use earthen ramps as a means of egress only if a worker can walk them in an upright position, and only if they have been evaluated and approved by a competent person.

Inspections

What would you do here as an inspector?
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A competent person should inspect and document an area. The following components specify the frequency and conditions requiring inspections.

• daily and before the start of each shift
• as dictated by the work being done in the trench
• after every rainstorm
• after other events that could increase hazards, e.g. snowstorm, windstorm, thaw, earthquake, etc.
• when fissures, tension cracks, sloughing, undercutting, water seepage, bulging at the bottom, or other similar conditions occur
• when there is a change in the size, location, or placement of the spoil pile
• when there is any indication of change or movement in adjacent structures

Trench Safety Measures

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Trenches 5 feet (1.5 meters) deep or greater require a protective system unless the excavation is made entirely in stable rock. If less than 5 feet deep, a competent person may determine that a protective system is not required.

Trenches 20 feet (6.1 meters) deep or greater require that the protective system be designed by a registered professional engineer or be based on tabulated data prepared and/or approved by a registered professional engineer in accordance with 1926.652(b) and (c).

General Safety Practices

Follow these general safety practices below when working in and around excavations:

• Keep heavy equipment away from trench edges.
• Identify other sources that might affect trench stability.

• Keep excavated soil (spoils) and other materials at least 2 feet (0.6 meters) from trench edges.
• Know where underground utilities are located before digging.
• Test for atmospheric hazards such as low oxygen, hazardous fumes and toxic gases when greater than 4 feet deep.
• Do not work under suspended or raised loads and materials.
• Ensure that personnel wear high visibility or other suitable clothing when exposed to vehicular traffic.
• Do not cross over an excavation unless approved walkways or bridges are provided. Do not use ladders to cross over excavations.
• Check for the possibility of hazardous atmospheres such as a lack of oxygen or the accumulation of combustible gas concentrations.
• Do not let standing water accumulate in or around the trench.

Protective Systems

The basic methods for protection from cave-ins are sloping, benching, shoring, and shielding. The method you should use depends on factors such as soil type and water content, excavation depth and width, the nature of the work, and nearby activities that could increase the risk of a cave-in.

The competent person has the responsibility for considering these factors and for determining the appropriate protective system. A registered professional engineer must design protective systems for all excavations that are more than 20 feet deep.

Benching is a method of protecting workers from cave-ins by excavating the sides of an excavation to form one or a series of horizontal levels or steps, usually with vertical or near- vertical surfaces between levels. There are two basic types of benching, simple and multiple. Benching cannot be done in Type C soil.

Spoils less than 2 feet from the collapsed edge caused a fatality.
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Sloping to protect workers can be done by cutting back the trench wall at an angle inclined away from the excavation not steeper than a height/depth ratio of 1.5 :1, according to the sloping requirements for the type of soil.

• Always provide a way to exit a trench—such as a ladder, stairway or ramp—no more than 25 feet of lateral travel for employees in the trench.
• Keep spoils at least two feet back from the edge of a trench.
• Make sure that trenches are inspected by a competent person prior to entry and after any hazard-increasing event such as a rainstorm, vibrations or excessive surcharge loads.

Maximum allowable slopes for excavations less than 20 ft. (6.09 m) based on soil type and angle to the horizontal are as follows:

Allowable Slopes

Source: OSHA Technical Manual, Sec V. Chap. 2, Excavations: Hazard Recongition in Trenching and Shoring (Jan 1999).

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Shoring is the provision of a support system for trench faces 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 systems consist of posts, wales, struts, and sheeting. There are two basic types of shoring, timber and aluminum.

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 caused by weather or climate, surcharge loads (e.g., spoil, other materials to be used in the trench) and other operations in the vicinity.

Training

Train about the dangers of hazardous atmospheres in trenches.
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Employees working in excavations should receive training on the following topics:

• surface encumbrances
• underground installations
• access and egress
• exposure to vehicular traffic
• exposure to falling loads
• warning system for mobile equipment
• hazardous atmospheres
• water accumulation
• stability of adjacent structures
• loose rock and soil
• daily inspections
• fall protection

Competent person training should also include the following:

• protection of employees in excavations
• design of sloping and benching systems
• design of support systems, shield systems, and other protective systems
• materials and equipment
• installation and removal of support
• sloping and benching systems
• shield systems

Real World Accident

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A 24-year-old worker died when he was buried under a wall of the trench he was working in. The excavation wall and part of the sidewalk next to the concrete garage floor collapsed onto him while he was attempting to attach the new PVC pipe he had installed to the main sewer. One of the decedent’s coworkers was also caught in the collapse. Firefighters who arrived on the scene were able to extricate the decedent’s coworker from the excavation. He was transported to a hospital and recovered. The decedent’s body was recovered from the excavation approximately 8 hours after the wall collapsed.

For more information on excavation safety be sure to take OSHAcademy Course 802, Trench and Excavation Safety, and refer to DHHS (NIOSH) Publication 2011-208, Preventing Worker Deaths from Trench Cave-ins and the OSHA Technical Manual (OTM).

Instructions

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