B – Soil Characteristics

SOIL CLASSIFICATIONS

The effectiveness of airspading largely depends upon
the type of soil present. Although hundreds of soil
types exist, the Occupational Health & Safety
Administration (OSHA) classifies soils into four
categories (in decreasing order of strength):

  • Stable Rock
  • Cohesive Soil Type A
  • Cohesive Soil Type B
  • Cohesive Soil Type C

Relative strengths of the four soil classifications are
measured using the metric Unconfined Compression
Strength, expressed in tons per square foot. Data on
these soils are contained within the three tables to
the right.

STABLE ROCK
Unconfined Compressive Strength by Rock Type

Rock Type

Unconfined Compressive Strength (tons/sq ft)

Minimum

Maximum
Schist 84 1,729
Sandstone 104 2,455
Shale 358 2,412
Limestone 369 3,894
Granite 505 3,383
Gneiss 882 2,620

COHESIVE SOILS
Unconfined Compressive Strength by Rock Type

OSHA Cohesive
Soil Type

Soil Description Unconfined Compressive Strength (tons/sq ft)

A

 Clay, silty clay, sandy clay, clay loam, caliche, hardpan Some silty clay loam, sandy clay loam ≥1.5

B

 Granular cohesive soils such as angular gravel, silt, silt loam, sandy loam 0.5 - 1.5

C

 Granular soils such as gravel, sand, loamy sand, submerged soil
Soil from which water is freely seeping		 ≤0.5

CLAYS
Unconfined Compressive Strength by Consistency

OSHA

Unconfined Compressive Strength
(tons/sq ft)

Very Soft

 0 - 0.25

Soft

 0.25 - 0.5

Medium

 0.5 - 1

Stiff

 1 - 2

Very Stiff

 2 - 4

Hard

 >4

SOIL TEXTURES

OSHA Cohesive Soil Types (A, B, or C) can be identified by the texture of a given soil. Shown below on the left is the U.S.Department of Agriculture Textural Soil Classification Chart. This chart plots soil classification as a percentage of sand, silt,and clay and classifies each unique combination. The chart on the right contains the same information with OSHA CohesiveSoil Types superimposed.

In summary, understanding the soils at a given jobsite is the key to determining the effectiveness of digging with the Utility AirSpade.