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).
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.
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:
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:
A competent person should inspect and document an area. The following components specify the frequency and conditions requiring inspections.
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).
Follow these general safety practices below when working in and around excavations:
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.
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.
Maximum allowable slopes for excavations less than 20 ft. (6.09 m) based on soil type and angle to the horizontal are as follows:
|Soil Type||Height/Depth Ratio||Slope Angle|
|Stable Rock||Vertical||90 degrees|
|Type A (Clay)||3/4:1||53 degrees|
|Type B (gravel, silt)||1:1||45 degrees|
|Type C (sand)||1 1/2:1||34 degrees|
|Type A (short term)||1/2:1||63 degrees|
Source: OSHA Technical Manual, Sec V. Chap. 2, Excavations: Hazard Recongition in Trenching and Shoring (Jan 1999).
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.
Employees working in excavations should receive training on the following topics:
Competent person training should also include the following:
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).
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