Last month I outlined confined-space safety issues encountered when septic service contractors find it necessary to enter sewage tanks of any type and septic tanks specifically. Many people are not aware that excavations more than 4 feet deep are also considered a confined space and have a separate set of rules to follow.

The Occupational Safety and Health Administration sets the standards along with any additional state-specific requirements. It is important that all employees working in excavations receive proper training to identify potential hazards and how to reduce those hazards.

One of the requirements is designating a “competent person” on the site. This person must have a higher level of training and be able to make decisions about the site and put in place the appropriate safety precautions. One of the responsibilities of the competent person is to classify the soil at the site. Usually in the onsite system world this means identifying one of the 12 major soil textural classifications according to the United States Department of Agriculture classification system.

It is important that the competent person and others understand the differences in classification. The USDA method relates specifically to the size of soil particles less than 2 millimeters in diameter. The OSHA classification system relates not only to particle size but also to soil strength and stability features.

In an excavation deeper than 4 feet, these 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.

THE CLASSES

Soils are placed into 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 tons per square foot.

Type A soil is the most stable soil for excavation. It has an unconfined compressive strength of 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, 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 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 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.

VISUAL TESTS

A visual test of the excavation site must be conducted. This will determine if there are factors on site 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 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 the 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. 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, so for best results take and test samples as soon as practical.

There are three main types of soil tests. The plasticity test, sometimes called the pencil test, is used to determine if the soil is cohesive. This test is performed by rolling a moist soil sample into a thread 1/8-inch thick and 2 inches long, resembling a short, slim pencil. If the sample can be held at one end without breaking, it is cohesive. This is the same test used by installers to determine whether the soil is dry enough to excavate.

The thumb penetration test is used to quickly estimate the compressive strength of a cohesive soil sample. Simply press the end of your thumb into a fresh clump of soil. If the soil sample is Type A, your thumb will only make an indentation in the soil with great effort. If the soil sample is Type B, your thumb will sink into the soil up to the end of your thumbnail. If the soil sample is Type C, your thumb will sink all the way into the soil clump.

NUMERIC MEASURE

For a more numeric measurement, a pocket penetrometer can be used. There can be some variability in these results, so it’s a good idea to run this test on a few soil samples from the same part of the excavation to make sure your results are consistent. A pocket penetrometer works like a tire pressure gauge. The scale reads in units of tons per square foot. It’s important to recognize that a penetrometer may give false results if the soil contains rocks or pebbles, which won’t compress.

Determining the type of soil on a site will help the competent person decide which methods of sloping, benching or shoring are needed to prevent cave-ins. For more information, visit the OSHA website at www.osha.gov, where you can read the OSHA Excavation Standard, or use OSHA’s Construction e-tool for Trenching and Excavation. You can also contact OSHA at 800/321-6742.