OSHA classifies soils by type. Type A, B, C-60, and C-80. The characteristics and materials that make up the soil will determine which type you are working with.
OSHA describes the different types of soils as:
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 can not 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.
Most OSHA compliance investigators believe that an excavator working on excavating the soils generates vibration. Thus if any excavation of soils occurs with machinery, it cannot be classified as Type A. So for just about all work, the best soil can be classified at would be Type B. In New England, we assume all soils to be Type C. Soil classifications are made by the Competent Person on site. They are required to perform at least one visual and one manual test.
OSHA provides options for acceptable visual and manual tests. The most common visual tests include inspecting the soil as it is being removed, inspecting the spoil pile, and the color and makeup of the excavation walls. While visually inspecting the soils, the competent person looks to see if the soil is staying clumped together while being excavated. The trench walls can be inspected for signs of layering different soil types, water seeping through, and any fissures present.
The most common manual tests are the plasticity test, the thumb penetration test, and the use of a penetrometer. The thread test, which shows if the soil is cohesive, is when the competent person takes a moist soil sample and rolling it into a thread that is 1/8” thick and 2” long. The competent person then takes that thread holds it at one end. If the soil thread stays together, then it is cohesive. If the soil thread cannot stay together, it is classified as Type C. The thumb penetration test is used to quickly estimate the compressive strength of the soil. To perform this test, the competent person takes a fresh clump of soil in their hands and press your thumb into it. In Type B soils, your thumb will make a sink into the soil to the base of your thumbnail. If your thumb can go in past the end of your nail, then the soil is classified as Type C. A pocket penetrometer can be used for a more numeric version of the thumb penetration test.
Generally speaking, protective systems are the same no matter which type of soil you are encountering. Trench shields and manhole boxes are used in all types of soils. However, the systems present different limitations based on the soil type. The tabulated data that accompanies each trench shield or manhole box states the depths it can be used at based on the type of soil it’s used in. The easiest method of protection from a trench wall collapse is to slope the sides. Type A soil can be sloped to a ¾:1 (53°) angle, Type B can be sloped to a 1:1 (45°) angle, and Type C can be sloped to a 1.5:1 (34°) angle. We rarely, if ever, come across Type A soils. There are times when an operator wants to bench soils; you cannot bench Type C soils. Benching can be done in Type B soils, but to be honest, most attempts at benching are not in compliance with the standard. Remember, as a rule of thumb, always assume the soil is Type C and protect your workers accordingly.
One of the most common misconceptions is that clay soil is good soil and allows for a change in soil type classification. The problem with clay soils is that they are always either gaining or losing moisture. Either situation can end with disastrous consequences. The average weight of a cubic yard of soil is about 3,000 lbs. It takes less than a second for a trench wall to collapse. A collapse with that weight and speed is the equivalent of getting hit by a car that is going 70 mph. Protect yourself, protect your crew; know the soils you are working in, and never get complacent.