The OSHA excavations standard recognizes and allows a variety of soil classification systems under certain conditions. A special simple soil classification system used by OSHA for excavation planning and protection is included in the standard. If that classification system is strictly followed, trench protection systems can be designed for many situations without the approval of a registered professional engineer.
In the soil classification system used by OSHA, the terms used to identify soil types are drawn largely from another system, commonly used for construction, called the Unified Soils Classification System. Both systems are based upon the engineering properties of soils and are concise and easily associated with actual soil behavior.
The OSHA system can be applied in the laboratory or the field. The terms used for classification are based upon the soil particles, including the quantity of the various sizes of soil particles and the qualities or characteristics of the very fine grains.
The principal types of soil may be divided into two general classes according to grain size. Coarse-grained soils are gravel and sand. Fine-grained soils are silt and clay.
The composition or texture of a soil is a critical factor in its stability. The more cohesive the soil particles; the more the entire soil mass tends to stick together rather than crumble.
However, it is important to remember the time element involved in cuts. If an excavated cut is to be left open for long periods of time, cohesive forces may not withstand exposure to weather conditions.
When fresh fill dirt is not properly compacted, subsequent excavations in the same area result in almost no cohesion properties; thus, a greater width may be required to maintain a stable slope.
The soil found at a site is usually a mixture of one or more of the basic types listed below. From the amounts of each soil type blended together to form the actual soil conditions, descriptive soil terms are combined in the order of lowest content to highest content. For example, soil classified as “silty clay” is a mixture of mostly clay with noticeable but lesser amounts of silt. The single term “loam” is used to describe a mixture of clay, sand and silt.
The types of soil found most often include:
By grouping different types of soils (described in the previous sections of this module) according to requirements for safe excavation, the excavation standard has defined four soil classifications (provided below). For a detailed explanation of OSHA classification system, please see Appendix A of the excavation standard.
OSHA groups soil and rock deposits into four classifications:
Cohesive soil means clay, or soil with a high clay content, which has cohesive strength. Cohesive soil does not crumble, can be excavated with vertical sideslopes, and is plastic when moist.
Granular soil means gravel, sand, or silt with little or no clay content. Granular soil has no cohesive strength, though some moist granular soils exhibit apparent cohesion. Granular soil cannot be molded when moist and crumbles easily when dry.
Granular cohesionless soil means soil that contains less than 85% sand and gravel but does not contain enough clay to be molded.
Cemented soil means a soil in which the particles are held together by a chemical agent, such that a hand-size sample cannot be crushed into powder or individual soil particles by finger pressure. Cemented soils are a special case. They are typically too dry to test for unconfined compressive strength, but if they are cemented and not fissured, and not subject to other factors that would require them to be classified as a less stable material, they are classified as Type A.
Stable Rock is natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed. It is usually identified by a rock name such as granite or sandstone. Determining whether a deposit is of this type may be difficult unless it is known whether cracks exist and whether or not the cracks run into or away from the excavation.
Examples of Type A cohesive soils are clay, silty clay, sandy clay, clay loam and, in some cases, silty clay loam and sandy clay loam. Cemented soils such as caliche and hardpan are also considered Type A.
However, no soil is Type A if:
Type B soils are defined as meeting any of the following:
Type C soils are defined as meeting any of the following:
If soils are configured in layers, the soil must be classified on the basis of the soil classification of the weakest soil layer. Each layer may be classified individually if a more stable layer lies below a less stable layer, (for example, where a Type C soil rests on top of stable rock.)
The standard also contains other important criteria that must be examined to classify soils properly.
Many kinds of equipment and methods are used to determine the type of soil prevailing in an area, as described below.
Penetrometers are direct-reading, spring-operated instruments used to determine the unconfined compressive strength of saturated cohesive soils. Once pushed into the soil, an indicator sleeve displays the reading. The instrument is calibrated in either tons per square foot or kilograms per square centimeter. However, penetrometers have error rates in the range of 20-40 percent.
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