Considerations Before Excavating
An excavator is shown digging a trench in unstable soil.
Date Posted: 05/30/2024
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.
According to data from the Occupational Safety and Health Administration (OSHA), the fatality rate for excavation work is 112% higher than the rate for general construction. 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.
Competent Person
Each trench and excavation must have a “competent” person to evaluate the project site for potential hazards. A competent person is someone who:
Can identify existing and predictable hazards.
Has authority by nature of their position to stop a project and take prompt corrective action to eliminate hazards.
Is knowledgeable in the requirements specified in WAC 296-155-655.
The competent person is required to inspect trench and excavation project sites:
Before the project begins.
Each day before work begins.
As necessary throughout the shift.
After each rainstorm.
Testing the Soil
A competent person must perform both a visual and manual test (plasticity, thumb penetration, or pocket penetrometer) upon breaking ground to help determine factors that could compromise the strength of the soil on the project site. You should be seeking answers to questions such as:
Is the soil clumpy or granular?
Are there sources of heavy vibrations near the excavation or trench site?
Are there signs of previously disturbed soil, such as from utility lines?
Are there signs of water seeping through the soil?
Is the soil fissured or otherwise showing crack-like openings or chunks of soil crumbling off the side of a vertical excavation or trench wall?
Soil Types
Soil can either be cohesive or granular. Cohesive soil contains fine particles and enough clay to stick to itself. More cohesive soil contains more clay and has a reduced chance of cave-in compared to granular soil. Granular soil is more akin to sand or gravel and has coarse particles that are significantly less cohesive. Additional safety precautions must be utilized when excavating in granular soil to prevent a cave-in. Details on the soil classifications are as follows:
Type A Soil: Type A soil is cohesive and has a high unconfined compressive strength of 1.5 tons per square foot or greater. Type A soil includes clay, silty clay, sandy clay, and clay loam. Soil can not be classified as type A if it is fissured, has been previously disturbed, has water seeping through it, or is subject to vibration from sources such as heavy traffic.
Type B Soil: Type B soil is cohesive and has often been cracked or disturbed, with pieces that don't stick together, as well as Type A soil. Type B soil has medium unconfined compressive strength, between 0.5 and 1.5 tons per square foot. Type B soil includes angular gravel, silt, silt loam, and soils that are fissured or near vibration sources but could otherwise be Type A.
Type C Soil: Type C soil is the least stable. Type C includes granular soils in which particles don't stick together and cohesive soils with a low unconfined compressive strength, 0.5 tons per square foot or less. Examples of Type C soil include gravel and sand. Because it is unstable, soil with water seeping through it is also automatically classified as Type C soil, regardless of its other characteristics.
Establish Mean of Exit from the Excavation or Trench
When employees are required to be in excavations or trenches 4 feet deep or more, an adequate means of exit, such as a ladder or steps, must be provided and located within 25 feet of lateral travel. An earth ramp is acceptable, providing all the following are met:
The stability of the earth is adequate for sound footing.
The total travel distance does not exceed 25 feet.
Adequate shoring or equivalent protection is provided for the entire escape route.
Protecting from Hazardous Atmospheres
Atmospheric testing is required before workers enter an excavation or trench greater than 4 feet (1.22 meters) in depth where an oxygen deficiency or a hazardous atmosphere is present or could reasonably be expected, such as in excavations in landfill areas or excavations in areas where dangerous substances are stored nearby.
Call Before You Dig
Utility locates must be completed prior to digging. Calling 811 before you dig is necessary to prevent damage and injuries.
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. Specifically, inspections should help to identify the following:
Unusual Hazardous Conditions: Daily inspections of excavations, adjacent areas, and protective systems will be made by a designated competent person in situations that might result in a cave-in, failure of a protective system, or other hazardous conditions. A competent person will inspect before the day’s work and throughout the shift as conditions require. Inspections will be made after every rainstorm or other hazard-increasing event.
Potential Cave-ins: Whenever evidence of a situation that could result in a possible cave-in, failure of protective systems, or other hazardous conditions is found, exposed employees will be removed from danger until the dangerous conditions have been corrected.
Resources
WAC 296-155-650 - Excavation and Trenching
Soil Classification | Transcript | Occupational Safety and Health Administration (osha.gov)
OSHA Trenching and Excavation Safety
FSP0-912-000 An Unprotected Trench is an Early Grave (wa.gov)
Trenching and Excavation - Overview | Occupational Safety and Health Administration (osha.gov)
