This photo shows several OSHA violations pointed out by Basic Training writers Jim Anderson and David Gustafson. Among them, the excavation is not properly sloped and requires the use of a trench box; the escape ladder is more than 3 feet from the worker in the hole; and spoils are placed too close to the excavation. The photo has been altered so the workers cannot be identified.
Recently we were shown a photo of an installation in progress, and one of the workers was standing in an excavated trench where the top of the trench with vertical sidewalls was well over his head. An excavator was continuing to dig the trench. The worker was wearing the required safety equipment including a hard hat and vest. There was a ladder positioned at the other end of the trench.
When the contractor was asked why the worker was allowed in the trench without shoring or other protection, he replied it was because the soil type was being cohesive — that according to OSHA, he did not need further protection and he had a ladder as a means of escape.
DEAD WRONG
We want to be very clear and upfront about this situation. The owner is wrong! And unfortunately for worker safety, he could be dead wrong. Every year we learn of people who have been to our classes or know someone in our classes who have been killed by trench sidewall collapses. These deaths are avoidable if OSHA soil standards are known and followed for excavations.
We have explained these standards in this column and elsewhere in the past, but we thought it would be good to review them again and spend some time in future columns looking closer at some of the requirements.
Any excavation over 5 feet deep requires additional protection based on the OSHA soil classification and other site conditions. If soil classification is required due to an excavation deeper than 5 feet and less than 20 feet, the following requirements must be met:
- Soil is classified by a competent person.
- Classification is based on one visual and one manual analysis.
- The least stable soil layer determines the overall classification.
- The soil must be reclassified when conditions change.
For excavations greater than 20 feet, the safety plan must be prepared by a licensed professional engineer and the signed plan made available for review at the site. Given that we go by the keep-it-shallow principle for onsite systems, hopefully the need for excavations more than 20 feet are minimal.
SOIL CLASSIFICATIONS
Soils are placed into one of three classes — A, B or C — based on their relative stability. Soils 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, or tsf.
Type A soil is the most stable soil to excavate. It has an unconfined compressive strength of 1.5 tsf 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, has been previously disturbed, has water seeping through it, or 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 tsf. 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 of 0.5 tsf 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.
A visual test of the excavation site must be conducted. This will determine if factors are present that will lower the strength of the soil. 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 cracklike 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.
SLOPING STANDARDS
The standard provides several different methods and approaches. The most common for excavations less than 20 feet is to use sloping or benching. The safest approach would be to slope the sides of the excavation at an angle not steeper than 1 1/2 horizontal to 1 foot vertical. This coincides with the requirement for Class C soils, which is the least stable class.
Back to the contractor mentioned earlier. If the excavation is in a soil defined as Type A, the most stable soil type, then the maximum allowable side slope is 3/4 feet horizontal to 1 foot vertical — not vertical as they maintained. If the excavation depth is 7 feet and the bottom of the trench is 3 feet across at the bottom, the slope to the surface will take 5.25 feet on each side so the excavation width at the top will need to be 13.5 feet across to meet the sloping requirement for safety.
In addition, he needs to be aware the spoil pile for the excavation must be back at least 2 feet from the edge of the excavation. The ladder for exit needs to be within 3 feet of the worker. Bottom line here is the worker in the picture is very at risk and the contractor is not following OSHA standards.
