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Course 802 - Trench and Excavation Safety

Safety guides and audits to make your job as a safety professional easier

Soil Types and Pressures

Example of coarse-grained soil
Example of coarse-grained soil

Soil Characteristics

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.

Example of fine-grained soil
Example of fine-grained soil

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.

Example of the size difference between gravel, sand, silt, and clay particles

Soil Types

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:

  • Clay
    • Clay is a very, very fine-grained soil. In general, water moves slowly through clay. Large amounts of rainfall may pond on the surface and evaporate before being absorbed.
  • Sand
    • Sand is a granular soil. The shape of individual grains may be round or angular. Sandy soil tends to have large pores, allowing faster groundwater absorption. In most situations, sandy soil is the easiest to compact with vibration.
  • Silt
    • Silt has properties intermediate between fine sand and clay. Silt is the most sensitive to changes in soil moisture content. Silt tends to crumble with drying.

Determination of Soil Type

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:

  1. Stable Rock
  2. Type A soils
  3. Type B soils
  4. Type C soils

Definitions

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.

Determination of Soil Type (continued...)

Stable Rock

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.

Type A Soils

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:

  • Cohesive soils with an unconfined compressive strength of 1.5 tons per square foot (144 kPa) or greater.
  • The soil is fissured; or
  • The soil is subject to vibration from heavy traffic, pile driving, or similar effects; or
  • The soil has been previously disturbed; or
  • The soil is part of a sloped, layered system where the layers dip into the excavation on a slope of four horizontal to one vertical (4H:1V) or greater; or
  • The material is subject to other factors that would require it to be classified as a less stable material.

Determination of Soil Type (continued...)

Type B Soils

Type B soils are defined as meeting any of the following:

  • Cohesive soils with an unconfined compressive strength greater than 0.5 tons per square foot (48 kPa) but less than 1.5 (144 kPa)
  • Angular cohesionless soils including: angular gravel (similar to crushed rock).
  • Previously disturbed soils except those which would otherwise be classed as Type C soil.
  • Soil that meets the unconfined compressive strength or cementation requirements for Type A, but is fissured or subject to vibration; or
  • Dry rock that is not stable; or
  • Material that is part of a sloped, layered system where the layers dip into the excavation on a slope less steep than four horizontal to one vertical (4H:1V), but only if the material would otherwise be classified as Type B.

Type C Soils

Type C soils are defined as meeting any of the following:

  • Cohesive soils with an unconfined compressive strength of 0.5 tsf (48 kPa) or less>
  • Granular soils including gravel and sand; or
  • Submerged soil or soil from which water is freely seeping; or
  • Submerged rock that is not stable, or
  • Material in a sloped, layered system where the layers dip into the excavation or a slope of four horizontal to one vertical (4H:1V) or steeper.
If soils are configured in layers, the soil must be classified on the basis of the soil classification of the weakest soil layer.
If soils are configured in layers, the soil must be classified on the basis of the soil classification of the weakest soil layer.

Layered Geological Strata

Accident Prevention Recommendations

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.

Important:

  1. The laboratory testing process and compressive strength calculations must be conducted under the direction of a registered professional engineer. The OSHA standard requires that the excavation protection system be designed by a registered professional engineer when the depth of the excavation exceeds 20 feet or where unusual site conditions exist.
  2. The manual field testing alternative permitted under the standard does not require the approval of a registered professional engineer under certain specific conditions. However, at least one visual test and one manual test are required to classify soil according to the OSHA method. The specific manual and visual field tests are listed and described in the standard.

Test Equipment and Methods for Evaluating Soil Type

Many kinds of equipment and methods are used to determine the type of soil prevailing in an area, as described below.

Penetrometer testing

Pocket Penetrometer

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.

  1. Shearvane (Torvane): To determine the unconfined compressive strength of the soil with a shearvane, the blades of the vane are pressed into a level section of undisturbed soil, and the torsional knob is slowly turned until soil failure occurs. The direct instrument reading must be multiplied by 2 to provide results in tons per square foot or kilograms per square centimeter.
  2. Thumb Penetration Test: The thumb penetration procedure involves an attempt to press the thumb firmly into the soil in question. If the thumb makes an indentation in the soil only with great difficulty, the soil is probably Type A. If the thumb penetrates no further than the length of the thumb nail, it is probably Type B soil, and if the thumb penetrates the full length of the thumb, it is Type C soil. The thumb test is subjective and is therefore the least accurate of the three methods.
  3. Dry Strength Test: Dry soil that crumbles freely or with moderate pressure into individual grains is granular. Dry soil that falls into clumps that subsequently break into smaller clumps (and the smaller clumps can be broken only with difficulty) is probably clay in combination with gravel, sand or silt. If the soil breaks into clumps that do not break into smaller clumps (and the soil can be broken only with difficulty), the soil is considered non-fissured unless there is visual indication of fissuring.

VIDEO

Instructions

Before beginning this quiz, we highly recommend you review the module material. This quiz is designed to allow you to self-check your comprehension of the module content, but only focuses on key concepts and ideas.

Read each question carefully. Select the best answer, even if more than one answer seems possible. When done, click on the "Get Quiz Answers" button. If you do not answer all the questions, you will receive an error message.

Good luck!

1. According to the text, ________ is a mixture of mostly clay with noticeable but lesser amounts of silt.

2. Which of the following is a characteristic of clay?

3. Silt is the most sensitive to changes in soil moisture content.

4. Penetrometers have error rates in what range?

5. According to the text, the OSHA standard requires the excavation protection system be designed by a registered professional engineer when the depth of the excavation exceeds ________.


Have a safe day!

Important! You will receive an "error" message unless all questions are answered.