Soil is a mixture of sand, gravel, silts, clay, water, and air. The amounts of these ingredients which determine its "cohesiveness", or how well a soil will hold together. Cohesive soil does not crumble. It can be molded easily when wet, and is hard to break up when dry. Clay is a very fine grained soil, and is very cohesive. Sand and gravel are course grained soils, having little cohesiveness and often called granular. Generally speaking, the more clay that is in the soil being excavated, the better the trench walls will hold up.
Another factor in soil cohesiveness is water. Soil that is filled with water is termed saturated. Saturated soil does not hold together well, and is particularly dangerous in excavation work. However, the opposite can also be true. Soil that has little or no water in it, or oven-dry, can crumble easily, and will not hold together when excavated.
Soil is heavy. A cubic foot can weigh as much as 114 pounds, and a cubic yard can weigh over 3,000 lb. — as much as a pick-up truck! Most workers don’t realize the force that will hit them when a cave in occurs. A person buried under only a few feet of soil can experience enough pressure in the chest area to prevent the lungs from expanding. Suffocation can take place in as little as three minutes. Heavier soils can crush and distort the body in a matter of seconds. It’s no wonder trench accidents involve so many deaths and permanently disabling injuries.
OSHA classifies soils into four categories: Solid Rock, Type A, Type B, and Type C. Solid Rock is the most stable, and Type C soil is the least stable. Soils are typed not only by how cohesive they are, but also by the conditions in which they are found. Stable rock is practically unachievable in the excavation of a trench. This is because the excavation of rock typically requires drilling and blasting, which fractures the rock, making it less stable.
Type A soil can be clay, silty clay, or sandy clay.
A soil cannot be considered Type A if it is fissured (cracks) or other conditions exist that can adversely affect it, such as:
- subject to vibration from heavy traffic, pile driving, or similar effects
- having been previously disturbed/excavated
- where it is part of a layered system, where less stable soil is near the bottom of the excavation, with the more stable soils on top.
- subject to other factors which would make it unstable-such as the presence of ground water, or freezing and thawing conditions.
Many OSHA compliance personnel believe that construction equipment at the site create enough vibrations to prevent any soil from being typed as "A". If vibrations can be felt while standing next to an excavation, the competent person should consider downgrading Type A soil to Type B or C.
Type B soils include both cohesive and non-cohesive soils. They include silts, sandy loams, medium clays, and unstable rock. Soils that might be classified as A, but have fissures, or are subject to vibration, may also be classified as "B" soils.
Type C soils are the most unstable (and therefore most dangerous) of the four soil types. They are easily recognized by the continual sloughing of the sides of the walls of excavation. If soil is submerged, or water is seeping from the sides of an excavation, it’s probably "C" soil. Soil may be classified as Type C if an excavation is dug in "layered" soils, where different soil types lay on top of each other. When an unstable soil type is underneath a stable soil type in an excavation, the "weakest link" will soon give way.
In many construction projects, the soil that is being excavated has been previously disturbed. This means the soil has been dug up or moved in the past. This is another factor a competent person must consider when typing soils. Previously disturbed soils are rarely as strong as undisturbed soils, and are usually typed as "C" soil. Previously disturbed soil is commonly found above existing utilities, such as water, sewer, electrical and gas lines. This makes work around these utilities more dangerous due to the unstable nature of the soil. Much of the work performed in excavations are along right-of-ways, where the soil is almost always likely to be Type C. Because of where we dig, it’s important to understand that once soil has been excavated, it will never be returned to the way it was naturally formed.
Per 1926 Subpart P, Appendix A (c)(2), a competent person must type soils by using at least one visualand one manual test. A visual test can include inspecting the soil as it is being removed, and examining the spoil pile and the color and make-up of the excavation walls. A manual test means working with the soil with either your hands or with an instrument designed to measure soil strength. For example, if you can roll the soil in your hands into a long "worm" or ribbon, the soil is cohesive and may be classified as A or B, depending on other conditions. One useful instrument for measuring soil strength is a penetrometer. When you press this instrument into a soil sample, it measures its unconfined compressive strength in tons per square foot (tsf).
Regardless of the methods used, the typing of soils must be done by the competent person prior anyone entering the excavation. The weaker the soil, the greater the need for protective systems.
Note: If you are uncertain of the soil type, ALWAYS assume Type C soil.