Soil Definitions

The definitions and examples given below are based in whole or part on the following:

• American Society for Testing Materials (ASTM) Standards D653-85 and D2488
• The Unified Soils Classification System (USCS)
• The U.S. Department of Agriculture (USDA) Textural Classification Scheme
• The National Bureau of Standards Report BSS-121

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

Cohesive soil means clay (fine grained soil), or soil with a high clay content, that has cohesive strength. Cohesive soil does not crumble, can be excavated with vertical side slopes, and is plastic when moist. Cohesive soil is hard to break up when dry and exhibits significant cohesion when submerged. Cohesive soils include clayey silt, sandy clay, silty clay, clay, and organic clay.

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 (coarse grained soil) with little or no clay content. Granular soil has no cohesive strength. Some moist granular soils exhibit apparent cohesion. Granular soil cannot be molded when moist and crumbles easily when dry.

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 that allows the soil to be deformed or molded without cracking or without 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 that is underwater or is free seeping.

Type A

Type A means cohesive soils with an unconfined compressive strength of 1.5 ton per square foot (tsf) (144 kPa) or greater. Examples of 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:

• 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 classification as a less stable material.

Type B

Type B means:

• Cohesive soil with an unconfined compressive strength greater than 0.5 tsf (48 kPa) but less than 1.5 tsf (144 kPa); or
• Granular cohesionless soils, including angular gravel (similar to crushed rock), silt, silt loam, sandy loam, and in some cases, silty clay loam and sandy clay loam
• 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 that is less steep than four horizontal to one vertical (4H:1V), but only if the material would otherwise be classified as Type B

Type C

Type C means:

• Cohesive soil with an unconfined compressive strength of 0.5 tsf (48 kPa) or less; or
• Granular soils including gravel, sand, and loamy 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

Unconfined compressive strength means the load per unit area at which a soil will fail in compression. Several methods can determine unconfined compressive strength, including laboratory testing, in-field estimation using a pocket penetrometer, by thumb penetration tests, and other methods.

Wet soil means soil that contains significantly more moisture than moist soil, but in such a range of values that cohesive material will slump or begin to flow when vibrated. Granular material that would exhibit cohesive properties when moist will lose those cohesive properties when wet.

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