Trenching and Excavation Safety
Thursday, January 3rd, 2019
Portions have been abstracted from the OSHA Technical Field Manual
Of all the hazards on a construction site, trenching and excavation activities present one of the greatest dangers. In the U.S., more than 800 construction workers die every year while on the job. Of those, 40 construction workers are killed from trenching and excavation hazards every year. Workers can suffer death or serious injury within minutes of being caught in between collapsing sides of soil where just a cubic yard can weigh as much as a typical car (3,000 pounds). Other hazards are exposure to falling loads, hazardous atmospheres, and hazards from mobile equipment.
An excavation is any man-made cut, cavity, trench, or depression, while a trench is a narrow excavation longer than it is wide measuring no more than 15 feet deep. No worker should ever be in a trench without protective systems. Adding to this disaster is that with proper pre-job planning it is always preventable. This is why OSHA usually classifies citations involving excavation hazards as willful, escalating the penalty to over $129,000 per violation.
Each employee who enters a trench must be protected from cave-ins by a protective system if the excavation is 5 feet or greater in depth, unless it is dug into stable rock. A support system is not required if the trench is less than 5 feet in depth and examination of the ground by a competent person provides no indication of a potential cave-in. When digging a trench, it's important to know the type of soil you're working with, so you know how to properly slope, bench, or shore the trench. This can help prevent a cave-in.
OSHA requires that employers have a competent person to determine the soil type. A competent person is someone who can identify conditions that are hazardous to employees and who also has the authorization to correct these hazards. All trenches that are five feet or deeper must follow OSHA's rules. The appendices of the OSHA Excavation Standard show the various types of support systems that may be used, up to a maximum depth of 20 feet. Any excavation deeper than 20 feet must use a protective system approved by a professional engineer.
The designated competent person should have and be able to demonstrate the following:
- Training, experience, and knowledge of:
- soil analysis;
- use of protective systems; and
- requirements of 29 CFR Part 1926 Subpart P
- 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
For all excavations, a competent person must conduct a full investigation every day, or when any trench conditions change, to identify and remove any potential hazards. The competent person must also conduct a visual inspection of a construction site's soil using the plasticity test, the thumb penetration test, and the pocket penetrometer test. For best results, OSHA recommends that the competent person use more than one of these methods to test the soil. Knowing the type of soil makes it possible to determine the right protective system to keep workers safe when they're working in an excavation.
Soil can either be cohesive or granular. Cohesive soil contains fine particles and enough clay so that the soil will stick to itself. The more cohesive the soil, the more clay it has, and the less likely a cave-in will happen. Granular soils are made of coarse particles, such as sand or gravel. This type of soil will not stick to itself. The less cohesive the soil, the greater the measures needed to prevent a cave-in. OSHA uses a measurement called "unconfined compressive strength" to classify each type of soil. This is the amount of pressure that will cause the soil to collapse. This value is usually reported in units of tons per square foot.
Soils can be classified as Type A, Type B, or Type C. Type A soil is the most stable soil in which to excavate. Type C is the least stable soil. It's important to remember that a trench can be cut through more than one type of soil. Let's look at each type of soil.
- Type A soil is cohesive and has a high unconfined compressive strength; 1.5 tons per square foot or greater. Examples of type A soil include clay, silty clay, sandy clay, and clay loam. Soil cannot be classified as type A if it is fissured, if it has been previously disturbed, if it has water seeping through it, or if it is subject to vibration from sources such as heavy traffic or pile drivers.
- 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. Examples of Type B soil include angular gravel, silt, silt loam, and soils that are fissured or near sources of vibration, but could otherwise be Type A.
- Type C soil is the least stable type of soil. 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 not stable, soil with water seeping through it is also automatically classified as Type C soil, regardless of its other characteristics.
- Before testing the soil, it's useful to perform a visual test of the construction site. This will help determine if there are factors on site that will lower the strength of the soil. Here are some observations to make when performing a preliminary visual test of the soil around an excavation site:
- First, as the soil is being excavated, does it come out in clumps, or is it granular? Clumps mean that the soil is cohesive.
- Are there sources of vibration near the excavation?
- Are there signs of previously disturbed soil, such as utility lines?
- Are there signs of water seeping through the soil?
- Is the soil fissured? Signs of fissuring include crack-like openings, or chunks of soil that crumble off the side of a vertical excavation wall. If any of these conditions are met, the soil cannot be classified as Type A.
When performing a soil test, it's important to choose a good soil sample. Soil samples should be typical of the surrounding soil in the excavation, and additional samples should be taken as the excavation gets deeper. While the excavation wall is one place to take samples, OSHA recommends taking a large clump from the excavated pile, as long as the soil in the pile is fresh and hasn't been compacted. Test results can change as the soil dries up, so for the best results, samples should be taken and tested as soon as practical.
What Constitutes a ‘competent person’: A competent person is an individual, designated by the employer, who is capable of identifying existing and predictable hazards in the surroundings or working conditions which are unsanitary, hazardous or dangerous to workers, and who is authorized to take prompt corrective measures to eliminate them. Tasks performed include: Soil Classification, Protective System Inspection, Structural Ramp Design, Water Removal Equipment Monitoring, and Site Inspection.
Trenching and excavation accidents are one of the leading causes of worksite fatalities. Ensuring your employees have been properly trained on the safe operating procedures for trenching operations is imperative.