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Course 802 - Trench and Excavation Safety

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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 fine-grained soil
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 above) 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.

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.

OSHA categorizes soil and rock deposits into four types:

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

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

These are cohesive soils with an unconfined compressive strength of 1.5 tons per square foot (144 kPa) or greater. 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. (No soil is Type A if it is fissured, is subject to vibration of any type, has previously been disturbed, is part of a sloped, layered system where the layers dip into the excavation on a slope of four horizontal to one vertical or greater, or has seeping water.)

Determination of Soil Type (continued...)

Type B Soils

These are cohesive soils with an unconfined compressive strength greater than 0.5 tons per square foot (48 kPa) but less than 1.5 (144 kPa). Examples of Type B soils are angular gravel, silt, silt loam, previously disturbed soils unless otherwise classified as Type C, soils that meet the unconfined compressive strength or cementation requirements of Type A soils but are fissured or subject to vibration, dry unstable rock, and layered systems sloping into the trench at a slope less than four horizontal to one vertical (only if the material would be classified as a Type B soil).

Type C Soils

These are cohesive soils with an unconfined compressive strength of 0.5 tons per square foot (48 kPa) or less. Type C soils include granular soils such as gravel, sand and loamy sand, submerged soil, soil from which water is freely seeping, and submerged rock that is not stable. Also included in this classification is material in a sloped, layered system where the layers dip into the excavation or have a slope of four horizontal to one vertical or greater.

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.

Penetrometers are direct-reading, spring-operated instruments used to determine the unconfined compressive strength of saturated cohesive soils
Penetrometers are direct-reading, spring-operated instruments used to determine the unconfined compressive strength of saturated cohesive soils

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.