Soil Characteristics

Soil Classifications

We often get asked the question: “Will AirSpade work effectively on my job site?” The answer largely depends upon the type of soil present. Although hundreds of different types of soils exist, the Occupational Health & Safety Administration (OSHA) classifies soils into four general categories (in decreasing order of strength):

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

Relative strengths of these four soil classifications are measured using the metric Unconfined Compressive Strength expressed in tons per square foot. Data on these soils is contained within the following three tables:

Stable Rock
Unconfined Compressive Strength by Rock Type

Rock Type Unconfined Compressive Strength (tons/sq ft)
Minimum Maximum
25Shales 254 2537
Chalk, Limestone, Dolostone 18 1,100
Sandstone 50 2,100
Rock Salt, Potash, Gypsum, Anhydrite 120 1,000
Siltstone 190 870
Igneous / Metamorphic 750 3,400

Cohesive Soils
Unconfined Compressive Strength by Soil Type

OSHA Cohesive
Soil type

Soil Description Unconfined
Compressive Strength
(tons/sq ft)


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


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


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

Unconfined Compressive Strength by Consistency


Unconfined Compressive Strength
(tons/sq ft)

Very Soft

0 - 0.25


0.25 - 0.5


0.5 - 1


1 - 2

Very Stiff

2 - 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 Texturl Soil Classification Chart. This chart plots soil composition as a percentage of sand, silt, and clay and classifies each unique combination. The chart on the right contains the same information with OSHA Cohesive Soil Types superimposed.

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