Excavations and Trenches
An excavator is shown digging with two trench shields placed.
Date Posted: 05/29/2024
WAC 296-155-650 defines an excavation as “Any person-made cut, cavity, trench, or depression in the earth's surface, formed by earth removal.” A trench is “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. If forms or other structures are installed or constructed in an excavation so as to reduce the dimension measured from the forms or structure to the side of the excavation to 15 feet (4.6 m) or less (measured at the bottom of the excavation), the excavation is also considered to be a trench.” One cubic yard of soil can weigh up to 3000 pounds – approximately the weight of a small car.
The fatality rate for excavation work is 112% higher than the rate for general construction, according to data from the Occupational Safety and Health Administration (OSHA). The agency lists “employee injury from collapse” as the primary hazard of excavation work and includes “no protective system” among the leading causes of worker injuries.
Excavation and Trenching Hazards
Trenching and excavation work presents serious hazards to all workers involved. Cave-ins pose the most significant risk and are more likely than other excavation-related incidents to result in worker fatalities. One cubic yard of soil can weigh as much as a car.
Collapse of soils and materials (cave-in) due to lack of, inadequate, or weak protective systems
The striking or damaging of underground utility services
Persons falling into excavations due to a lack of barriers or inadequate fencing
Asphyxiation from toxic gases which have collected in the excavation/trench
Spoil (soil from excavation/trench) or materials/equipment being placed too close to the sides of the excavation/trench, which then become overloaded and collapse
Failure to check and maintain shoring, particularly after inclement weather
Water accumulation or seepage
Moisture in the soil
Falls from unsafe means of access/egress into or out of the excavation/ trench
Employees being struck by excavation machinery
Vehicles being driven into the excavation due to driving errors, inadequate barriers, or the absence of stop blocks or adequate control procedures
Changing weather conditions
Lack of training or technical knowledge
Trench Excavation Egress
A stairway, ladder, ramp, or other safe means of egress must be provided in trench excavations that are 4 feet or more in-depth within 25 feet of lateral travel for all workers.
Protective Methods
Before excavation or trenching work starts, a competent person shall identify and assess all existing and potential hazards. Cave-ins are the leading cause of fatalities in excavation and trenching accidents. To prevent these incidents, mining companies should implement appropriate protective measures, such as:
Sloping: The process of cutting back the trench wall at an angle inclined away from the excavation. Sloping is typically used to improve stability and prevent collapse.
Sloping is an integral part of trenching operations and should be carried out carefully to ensure workers' safety and the trench's stability. Sloping can be a complex process, and several factors need to be considered before starting work, including:
The angle of the slope.
The type of soil.
The depth of the trench.
The weather conditions.
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. Generally, the bottom vertical height of the trench must not exceed 4 feet. However, subsequent benches may be up to a maximum of 4 feet vertical in Type A soil and 4 feet in Type B soil. In addition, all subsequent benches must be below the maximum slope allowed for that soil type.
Shoring: The process of installing aluminum hydraulic or other types of supports to prevent soil movement and cave-ins. This is often necessary when excavating a trench or working where the ground may give way. Shoring can also stabilize a structure damaged by an earthquake or other disaster.
Shoring is a critical part of construction and should only be done by trained professionals. Improper shoring can lead to severe injuries or even death.
Shielding: This method is the most common system and protects workers with trench boxes or other supports to prevent soil cave-ins. This method uses a trench box or shield designed or approved by a registered professional engineer (RPE).
Three trench boxes are shown beside one another.
Note: Excavations that are less than 4 feet in depth may still require protection for workers to enter if the competent person believes that a cave-in may occur.
Inspections
WAC 296-155-655 requires that a competent person inspect excavations, adjacent areas, and protective systems daily for evidence of a situation that could result in possible cave-ins, indications of failure of protective systems, hazardous atmospheres, or other hazardous conditions. The competent person must inspect before the start of work and as needed throughout the shift. Inspections must also be made after every rainstorm or other hazard-increasing occurrence.
Resources
WAC 296-155-650 - Excavation and Trenching
FSP0-912-000 An Unprotected Trench is an Early Grave (wa.gov)
Trenching and Excavation - Overview | Occupational Safety and Health Administration (osha.gov)
