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Course 802 - Trench and Excavation Safety

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

Soil Characteristics and Types

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.

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

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:

  1. Fine-grained soils - silt and clay; and
  2. Coarse-grained soils - gravel and sand.

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.

1. Soils are classified into two general categories based on _____.

a. hardness
b. weight
c. grain size
d. cohesion

Next Section

Soil Size
Size difference for soil types.

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:

  1. Loose soil is any type of soil that has been worked or disturbed so that it is not compacted.
  2. Sand is a granular soil. The shape of individual grains may be round or angular. Sandy soil tends to have large pores, allowing faster ground absorption. In most situations, sandy soil is the easiest to compact with vibration.
  3. USDA Soil Triangle
    USDA Soil Triangle.
  4. 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.
  5. 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.
  6. Loam is a combination of sand, silt, and clay such that the beneficial properties from each is included.

Click on the buttons below to see definitions.

Definitions

Cemented soil means a soil in which the particles are held together by a chemical agent, such as calcium carbonate, such that a hand-size sample cannot be crushed into powder or individual soil particles by finger pressure.

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.

Dry soil means soil that does not exhibit visible signs of moisture content.

Fissured means a soil material that has a tendency to break along definite planes of fracture with little resistance, or a material that exhibits open cracks, such as tension cracks, in an exposed surface.

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.

Layered system means two or more distinctly different soil or rock types arranged in layers. Micaceous seams or weakened planes in rock or shale are considered layered.

Moist soil means a condition in which a soil looks and feels damp. Moist cohesive soil can easily be shaped into a ball and rolled into small diameter threads before crumbling. Moist granular soil that contains some cohesive material will exhibit signs of cohesion between particles.

Plastic means a property of a soil which allows the soil to be deformed or molded without cracking, or appreciable volume change.

Saturated soil means a soil in which the voids are filled with water. Saturation does not require flow. Saturation, or near saturation, is necessary for the proper use of instruments such as a pocket penetrometer or sheer vane.

Soil classification system means, for the purpose of this subpart, a method of categorizing soil and rock deposits in a hierarchy of Stable Rock, Type A, Type B, and Type C, in decreasing order of stability. The categories are determined based on an analysis of the properties and performance characteristics of the deposits and the environmental conditions of exposure.

Stable rock means natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed.

Submerged soil means soil which is underwater or is free seeping.

2. Which soil type is a granular, has large pores, and is easy to compact?

a. Gravel
b. Clay
c. Silt
d. Sand

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Unit Weight of Soils

The unit weight of soils refers to the weight of one unit of a particular soil. The weight of soil varies with type and moisture content. One cubic foot of soil can weigh from 110 pounds to 140 pounds or more, and one cubic meter (35.3 cubic feet) of soil can weigh more than 3,000 pounds.

The maximum weights will be reached when the soil is wet. See examples of the weight of one cubic meter for the five types soils:

  • Loose dirt can weigh up to 2690 lb (1220 kg);
  • Loam can weigh up to 2800 lb (1270 kg);
  • Sand can weigh up to 3428 lb (1555 kg);
  • Clay can weigh up to 3527 lb (1600 kg); and
  • Gravel can weigh up to 3704 lb (1680 kg).

3. Which soil type has the greatest maximum weight per cubic meter?

a. Sand
b. Gravel
c. Clay
d. Loam

Next Section

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: Stable rock, Type A soils, Type B soils, and Type C soils.

Click on the buttons below to see the characteristics of the OSHA Soils Types.

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.

Type B Soils

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 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 soil is the least stable type of soil and include 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.

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;
  • Soils that have been disturbed;
  • Submerged soil or soil from which water is freely seeping;
  • 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.

Click on the buttons below to see a flow diagram that used to determine soil type and a chart showing how to downgrade soil types based upon soil conditions.

4. Which of the following is considered Type A cohesive soil?

a. Clay
b. Sand
c. Silt
d. Loam

Next Section

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.
  2. 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.
  3. 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.

5. If soils are configured in layers, the soil must be classified on the basis of the soil classification of _____.

a. the top soil layer
b. the weakest soil layer
c. the base soil layer
d. the most cohesive layer

Next Section

Methods for Evaluating Soil Type

Penetrometer testing

Site Soil Analysis

An analysis of conditions around the site is conducted by observing soil adjacent to the site and the soil being excavated. If the soil remains in clumps, it is cohesive; if it appears to be coarse-grained sand or gravel, it is considered granular. The evaluator also checks for any signs of vibration.

During the analysis, the evaluator should check for crack-line openings along the failure zone that would indicate tension cracks, look for existing utilities that indicate that the soil has previously been disturbed, and observe the open side of the excavation for indications of layered geologic structuring.

The evaluator should also look for signs of bulging, boiling, or sluffing, as well as for signs of surface water seeping from the sides of the excavation or from the water table. The area adjacent to the excavation should be checked for signs of foundations or other intrusions into the failure zone, and the evaluator should check for surcharging and the spoil distance from the edge of the excavation.

Test Methods

When performing a soil test, it's important to choose a good soil sample. Soil samples should be typical of the surrounding soil in the excavation, and additional samples should be taken as the excavation gets deeper. While the excavation wall is one place to take samples, OSHA recommends taking a large clump from the excavated pile, as long as the soil in the pile is fresh and hasn't been compacted. Test results can change as the soil dries up, so for the best results, samples should be taken and tested as soon as practical. Let's look at three main types of soil tests.

Click on the buttons below to see the four basic methods for evaluating soil types.

Plasticity or Wet Thread Test

The plasticity (cohesiveness) test, which is sometimes called the pencil test, is used to determine if the soil is cohesive.
  • This test is performed by rolling a moist soil sample into a thread that's one-eighth of an inch (3 mm) thick and two inches (5 cm) long, resembling a short, slim pencil.
  • If the sample can be held at one end without breaking, it is cohesive.
  • Any soil that is not cohesive is automatically classified as Type C, although some Type C soils are cohesive.

Thumb Penetration Test

The thumb penetration test is used to quickly estimate the compressive strength of a cohesive soil sample. To perform the thumb penetration test, simply press the end of your thumb into a fresh clump of soil.

  • If the soil sample is Type A, your thumb will only make an indentation in the soil with great effort, as you can see demonstrated here.
  • If the soil sample is Type B, your thumb will sink into the soil up to the end of your thumbnail, just like this.
  • If the soil sample is Type C, your thumb will sink all the way into the soil clump, as you can see here.

Your results for this test will probably be somewhere in between these results.

Dry Strength Test

This test is usually conducted on a small sample of dried soil 1/2 inch think and 1 1/2 in diameter. After the sample is thorough dry, try to break it using the thumbs and forefingers of both hands.
  • 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.

Pocket Penetrometer Test

For a more numeric measurement, the pocket penetrometer test can be used. A soil's compressive strength can be given a numeric value by using the pocket penetrometer test. There can be some variability (+ 20-40%) in these results, so it's a good idea to run this test on a few soil samples from the same part of the excavation, just to make sure your results are consistent.

  • A pocket penetrometer works much like a tire pressure gauge.
  • A thin, metal piston is pushed into a soil sample, and the penetrometer records the compressive strength of the soil.
  • Be sure that the scale indicator is inserted into the penetrometer body until only the "zero" mark is showing.
  • To conduct the test, push the piston into the soil until it reaches the engraved line. Then, simply take the reading from the scale indicator.
  • It's important to recognize that a penetrometer may give false results if the soil contains rocks or pebbles, which won't compress.

6. OSHA does NOT recommend taking soil to be tested from _____.

a. a large clump of soil
b. soil from an excavated pile
c. soil that has been compacted
d. soil that is fresh

Check your Work

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