CLASSIFICATION OF SOILS FOR EXCAVATIONS


Method Number: ID-194
Sample: Soil that is representative of the walls of an excavation.
Sampling Procedure: Take a representative sample from 1 lb (0.45 Kg) of fine grained soil, such as clay, to 10 lbs (4.5 Kg) of coarse grained soil, such as sand and gravel. A 3 lb or 1L sample of soil is normally satisfactory. Place the sample immediately in a heavy-duty plastic bag and seal it airtight.
OSHA Regualations: Soil classification is used as an index to determine the proper sloping, shoring, and shielding of excavations for construction and safety (29 CFR 1926.650).
Analytical Procedures: Visual observations.
Index tests.
Senior Soil Scientist: Alan Peck
Date: 18 February 1997




Physical Measurements and Analysis Branch
OSHA Salt Lake Technical Center
Salt Lake City, Utah 84165-0200




1. Introduction

This method describes the sample collection, analytical preparation, and the visual and index test techniques used by the Occupational Safety and Health Administration (OSHA) to classify soils from excavation operations. The methodology assumes some familiarity with applicable federal standards and knowledge of principles, equipment, and techniques of physical measurements.

Soils are the most complex of all engineering materials, and the excavation of soil is the most hazardous of all construction occupations. To offset these problems, classification methods are used to simplify the analyses of soil and the engineering calculations required to safely excacate it.

The federal OSHA excavation regulations (6.1.) provide guidelines to classify soils into a hierarchy of three general types called Type A, B, and C for construction and safety purposes. Each type corresponds to specific safety and construction codes required for the proper sloping, shoring, and shielding of the soil in an excavation.

Concepts from soil mechanics, the Unified Soils Classification System (USCS) (6.2.), the American Standard for Testing Materials (ASTM) (6.3.), and the Federal OSHA excavation regulations are used to classify soil at the OSHA Salt Lake City Technical Center (SLTC).

1.1. History

The first federal OSHA regulations (6.4.) governing the construction and safety of excavations were implemented in 1971 and revised in 1987. A more comprehensive revision and amendment of the regulations were made in 1989 to define soil types, terms, and construction requirements more explicitly. Similar concepts were advanced by the U.S. Dept. of Commerce (6.5.) in 1985. Amended revision to the federal regulations were published in the Federal Register in October 1989, and adopted on March 15, 1990.

1.2. Principles

Type A, B, and C soils represent arbitrary ranges of soil strength of high, medium, and low, respectively. These ranges are based on the variable frictional resistance of the coarse grains, of sand and gravel and to the cohesion or chemical bonding of the fine grains of silt and clay. Soils are assigned to each type as determined by the analytical data and the definitions and specifications set forth in the federal OSHA regulations.

Both the USCS and the U.S. Dept. Of Agriculture (USDA) Textural Soil Classification System (6.6.) are complementary methods used to analyze and classify soils. The USDA system is emphasized in the federal OSHA regulations, but the USCS is more consistent with the definitions and engineering terms of the regulatory codes.

1.3. Advantages

Concepts of the USCS are used at OSHA-SLTC in preference to other classical systems of soil classification. Compared to the USDA system, it has the following advantages:

  1. It is cost effective.
  2. It is simple and can be used on-site.
  3. A qualitative analysis of silt and clay is not requied.
  4. Dust is minimized by wet sample break down.
  5. Gravel is treated as a major soil constituent.
  6. It is based on engineerging principles.
  7. Agricultural terms such as loam are not required.
  8. The property of plasticity is more adequately treated.

1.4. Disadvantages

There are no known disadvantages of the USCS compared to other recognized systems of soil classification for excavations.

2. Sampling

Handpick excavated soil to assure a representative sample is taken. Samples are usually taken from the bucket of a back-hoe or from the spoils pile. Do not enter an excavation or stand at the edge of an excavation that is unsafe.

Take a representative sample from 1 lb (0.45 Kg) of fine grained soil, such as clay, to 10 lbs (4.5 Kg) of coarse grained soil, such as sand and gravel. A 3 lb or 1 L sample of soil is the approximate and recommended size of an average sample. Place the sample immediately in a heavy-duty plastic bag and seal it airtight.

Place an official sample seal, such as OSHA Form 21, on the bag to identify the sample and to maintain a chain of custody. Enclose and secure the bag in a second bag of plastic, cloth, canvas, or other sturdy material.

Request soil classification and moisture analysis if desired on the sample submission sheets. Put the prepared sample and sample submission reports in a box and ship by mail or personally take it to the SLTC. Do not put any paper in direct contact with the soil.

3. Soil Analysis

The analytical procedures require safety and health precautions, the availability of certain equipment, and a general outline of different soil tests. The tests provide the data required to describe and classify the soil by texture, structure, and type.

3.1. Safety and health

Use general health and safety precautions during the analysis of soils. Respiratory protection is advised when handling dry samples that form dust. Plastic gloves are advised when handling all samples, Dry samples that contain gasoline or other volatile organic compounds in a hood. Wash hands thoroughly after working with soil.

3.2. Equipment

The following equipment is used in analysis:

  1. Bread pans of regular size for drying samples.

  2. Stainless steel bowls of 2 to 4 liter capacities.

  3. A forced air oven that will hold and dry samples at 60C. and an oven that will dry samples at 110C.

  4. U.S.A. Standard 8 in. dry sieving pans #4 (4.75 mm opening), #40 (.425 mm opening), #200 (.075 mm opening) and a catch pan.

  5. A #200 U.S.A. Standard 8 in. wet sieving pan.

  6. A laboratory scale that will read up to 3 Kg or more at plus or minus 0.1 g accuracy.

  7. A standard soil pocket penetrometer (6.7.) and a spatula.

  8. A fine hair bristle brush and a wire bristle brush for cleaning the wire cloth and frame of the test sieves.

  9. Aluminum moisture boxes and cover lids of 50 to 100 g capacity.

3.3. Analytical Procedures

Use the analytical procedures of this section to identify, describe, and determine the properties of the soil. Perform the work in the order given and omit the moisture determination unless it is specifically requested or required. Pay close attention to any time constraints listed for the tests such as for compressive strength (3.3.4.). All tests are based on visual observations or simple index tests that provide information about the engineering properties of the soil. Keep a permanent record of all data.

3.3.1. Sample Identification

Copy all sample identification numbers from the sample submission report form to sample analytical data sheets. For analytical convenience, accountability, and continuity, number and record all containers used in analysis.

3.3.2. General Description of Sample

Open the soil bag and place the soil in a bread pan. Note the presence, number, and size of rock fragments larger than 3 in (7.6 cm). Remove and permanently set aside any rock fragments that are present.

Note if the soil is predominately loose sand and gravel or broken clumps of cohesive clay. Estimate and record the percent of the sample that is in the form of clumps and the corresponding percent of such that are <1 in (2.5 cm) and <0.25 in (0.6 cm) in size.

Note if the soil is unnatural or disturbed as indicated by the presence of construction debris, such as pieces of brick, concrete, wood, and glass. Note other features of the soil that are unusual or of special interest.

3.3.3. Moisture Determination

This test is used to determine the water content as a percent of the dry weight of the soil. Weigh a small sample moisture can and a cover lid (wt.1). Add about 50 g of a sample, and determine the combined weight (wt.2).

Dry the sample at 110C overnight and cover the sample with the lid when it is removed from the oven. Cool the sample and determine the final weight (wt.3). Determine the moisture content in percent on a dry weight basis using the three weights according to the following equation:

Wt.2 - Wt.3
Wt.3 - Wt.1
100 = % Moisture

3.3.4. Compressive Strength

Within five minutes after a sample of broken soil is exposed to the open air, remove one or more of the largest clumps and analyze it with a pocket penetrometer. Slice each clump with a spatula to provide a smooth surface for analysis.

Press the penetrometer cylinder against the sample and compress the soil and a calibrated spring to the marked ring on the cylinder. Read the position of the ring on the calibrated scale of the cylinder. Record the reading of tons per square foot (tsf) or Kg/cm2. Report the average of at least three readings. Note all samples that break apart and do not provide a positive analysis. The cohesive strength of soil is equivalent to of the force, Qu, required to compress it.

3.3.5. Plasticity

Plasticity is defined as the ability of a soil sample to mold and roll between the palms of the hands into a stable thread or ribbon 1/8 in (0.3 cm) in diameter and the tensile strength to support a 5 cm section when held at one end. To possess plasticity, a soil must satisfy these conditions and contain at least 12% silt and clay.

Analyze the sample in the as-received state and record if it satisfies the conditions of plasticity. If it does not, analyze the sample again after it is dried. Use material that passes the #40 sieve. If necessary, break the soil with a rubber pestle to provide at least 10 g of material. Add water in different amounts to obtain a wide range of water content for analysis. If the clay content is low, plasticity is not an inherent property of the soil, and it will not be identified at any water content. Note the tendency of the soil to satisfy the condition of plasticity.

3.3.6. Fissures

The structural strength of fissured soil is substantially less than that of intact soil. Identify fissured soil by the appearance of cracks in the soil or the presence of small clumps of soil from 0.25 to 1 in size. Small clumps are evidences of cohesive soil with a network of microscopic and submicroscopic fissures that has been disturbed. Common disturbances that expose this property include shipping, excavation, and dropping. Drop clumps of the soil to the floor and note if the soil breaks into small pieces. Examine both wet and dry samples to determine this property.

3.3.7. Sample Drying

Dry samples in a bread-pan and an oven at 60C for two days. Samples dried at this temperature simulate a natural dry state. Dry soil is convenient to store and analyze at any time. It is required for certain tests in this method.

3.3.8. Wet Break-down

Use ordinary tap water to break down dry soil completely into individual grains and to identify transitional soil (earth material intermediate to soil and rock) in the following manner:

Tare a bowl to read zero on a laboratory balance. Add at least 150 g of a dry sample of fine-grained soil or up to at least 1 Kg of coarse-grained soil to the bowl. If that amount is not available, use as much as possible. Record the weight and submerge each sample in water, and let it stand for at least two hours and up to 24 hours.

If the sample only partially breaks-down in 24 hours, call the sample transitional soil. Assign a descriptive name to it, such as caliche (hard calcareous earth deposit) or hardpan (hard, dense clay).

3.3.9. Gradation Analysis

This test is used to determine the texture or the quantitative distribution of the sand, gravel, and silt and clay in a given weight of dry soil.

Break down the soil as described in Section 3.3.8. and transfer it to a #200 wet sieving pan. Wash the fine grains of silt and clay through the sieve with running water until it is visibly clear. Wash the material that is retained on this sieve back into the bowl and decant the water and any supernatant. Place the bowl and the contents in an oven at 60C for at least eight hours to dry the coarse grains.

Place the dried soil onto a nest of pans that has a #4 sieve at the top, a #200 sieve in the middle, and a pan at the bottom to catch any residual silt and clay. Tap the pans manually on a table top 20 times to separate the grains by size.

Report the total gravel as the weight of material retained on the #4 sieve, and report the total sand as the weight of material retained on the #200 sieve. Silt and clay are identified as a composite weight equal to the difference between the combined weight of the sand and gravel and the amount of material used in the wet break down. Convert all weights to dry weight percent.

Compose the exact textural name of the gradated soil with the most important or predominant constituent written last. Modify that name with the names of the other important constituents written first according to the following instructions and example:

Use clay as the last word in the textural name of soils that possess plasticity, Use gravel in the textural name if it represents at least 30% of the dry weight of the soil; use sand in the textural name if it represents at least 10% of the dry weight. Correspondingly, call a soil gravelly sandy clay if it possesses plasticity and contains at least 30% gravel and more sand than gravel.

Clay may be used as a subordinate term in the textural name if the soil partially satisfies the conditions required for plasticity.

4. Classification

This section deals with the sorting and selection of the analytical data as required to determine soil type. The determination of soil type is set by the structure of granular and granular cohesionless soil. Soil type for cohesive soils is set by the value of Qu and the identification of fissures.

Texturally classify all soil samples according to the preceding section 3.3.9. Structurally classify the soil granular, cohesive, or granular cohesionless and Type A, Type B, or Type C as follows:

Call all soils that possess plasticity cohesive. Classify cohesive soil Type C if Qu is <0.5, Type B if Qu is 0.5 to 1.5, and Type A if Qu is >1.5 and not fissured. Classify all fissured cohesive soils Type B unless the Qu dictates that it is Type C.

Structurally classify all soils that are not cohesive either granular or granular cohesionless. Classify it granular if the silt and clay content is <12%. Classify it granular cohesionless if the silt and clay content is >12%. If it contains at least 75% angular gravel, call it granular cohesionless regardless of the silt and clay content.

Classify all-granular soils Type C and all granular cohesionless soils Type B.

The above specifications are summarized in Chart 1. Use this chart for a comparative study of granular, cohesive, and granular cohesionless soil. These three structural classifications are at the top of the chart. Corresponding soil types are directly below at the bottom of the chart. The common textures for each are found on line 4.

Classify rock fragments and broken or fissured transitional soil Type B. If these structural materials are intact at the work site, the transitional soil must be called Type A, and the rock must be called stable by a field engineer.

Report the textural, structural, and type classifications and note if any environmental factors are used in the assignments.

Field personnel are required to reclassify Type A soil to either Type B or Type C and Type B soil to Type C under certain conditions. Water seepage, earth vibrations, and tension cracks are three common environmental conditions that weaken the structure or shear strength of the soil. Invariably, a soil must be classified Type C if water is observed in an excavation. The detailed instructions for soil reclassification are given in the Code of Federal Regulations, Title 29, Part 1926, Subpart P, Appendix A.

Chart 1: SOIL CLASSIFICATION SPECIFICATIONS

Soil Classification Specifications

5. Applications

Soil type is used by construction workers to properly excavate the ground for utility and other purposes. Different methods of construction are permitted that include sloping, shoring, shielding, and benching of the soil for protection. The following paragraphs treat this practice in general. Detailed instructions in the use of soil type are given in the Code of Federal Regulations, Title 29, Part 1926, Subpart P, Appendices B, C, D, and E.

5.1. Sloping

The federal excavation regulations relate soil type to maximum allowable or permitted slopes of the sidewalls of excavations, The slope can be expressed in terms of the tangent or degrees of the angle that the walls form with the horizontal. Imminent danger and slope failure is anticipated if the actual slope of the walls exceeds the allowed maximum.

Soils that are Type C must not be sloped to >34, and Type B soils must not be sloped to >45. Type A soils must not be sloped >63; if the wall is >12 ft. high, it must not be sloped >53. Type C represents the lowest strength soil, and it requires a flatter slope than Type A and Type B soil to attain the same stability.

5.2. Shoring

Shoring is a viable alternative to sloping. Timber and aluminum are commonly used for this purpose as outlined in the regulations. Shoring includes the cross braces, sheeting, uprights, and other structural members used for support. The minimum dimensions and requirements are given for each. Soil type, the dimensions of the excavation, and the type and spacing of the material are used to determine these specifications.

5.3. Shielding and Benching

Support or shield systems are allowed in the lower or vertical portion of excavations that are sloped at the upper level. Benching may be constructed with single or multiple terraces. The design and specifications for these procedures are based on soil type, soil structure, and the slope and depth of the excavation.

6. References

6.1. Code of Federal Regulations, Title 29, Part 1926, Subpart P, 1994, pp 453-491. U.S. Government Printing Office, Washington, D.C. 20402-9328.

6.2. Earth Manual, U.S. Dept. of the Interior, Water and Power Resources Service, Denver, CO: 1980, pp. 2-27.

6.3. Soil and Rock; Building Stones. 1983 Annual Book of ASTM Standards, American Society for Testing and Materials, (Section 4, Vol. 04.08). Philadelphia, PA, 1983.

6.4. Excavations, Code of Federal Regulations, Title 29, Part 1926, Subpart P, 1987, p.201. U.S. Government Printing Office, Washington, D.C. 20402-9328.

6.5. Yokel, F.Y., Soil Classification for Construction Practice in Shallow Trenching, U.S. Department of Commerce, Washington, D.C. Government Printing Office, 1980.

6.6. National Soils Survey Handbook, Series 430, Part 618, Edition VI, 1993, U.S. Dept. of Agriculture, National Resources Service, Lincoln, Nebraska.

6.7. Materials Testing Catalogue, Soiltest Inc., Corporate Headquarters, 86 Albrecht Drive, Lake Bluff, Illinois, 60044-8004.