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	Minimum (tons/sq ft)	Maximum (tons/sq ft)
Schist	84	1729
Sandstone	104	2455
Shale	358	2412
Limestone	369	3894
Granite	505	3383
Gneiss	882	2620

Cohesive Soils
Unconfined Compressive Strength by Soil 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; Some silty clay loam, sandy clay loam	0.5 - 1.5
C	Granula soils such as gravel, sand, loamy sand, submerged soil; Soil from which water is freely seeping	≤0.5

Clays
Unconfined Compressive Strength by Consistency

Consistency	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 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.