TM 5-818-1 / AFM 88-3, Chap. 7
Table 2-2. Determination of the Consistency of Clays
Unconfined
Compressive
Strength, qu
tsf
Field Identification
Consistency
Less than 0.25
Easily penetrated several inches by fist
Very soft
0.25 - 0.5
Easily penetrated several inches by
Soft
thumb
0.5 - 1.0
Can be penetrated several inches by
Medium
thumb with moderate effort
1.0 - 2.0
Readily indented by thumb but pene-
Stiff
trated only with great effort
2.0 - 4.0
Readily indented by thumbnail
Very stiff
Over 4.0
Indented with difficulty by thumbnail
Hard
classification, as shown in table 2-5, will be adequate.
U. S. Army Corps of Engineers
For any in-depth geology study, proper stratigraphic
classification by a qualified geologist should be made to
h. Consistency.
The consistency of an
ensure that proper interpretation of profiles is being
made. All the rock types in table 2-5 may exist in a
undisturbed cohesive soil may be expressed
sound condition or may be fissured, jointed, or altered by
quantitatively by the unconfined compressive strength
weathering to an extent that will affect their engineering
behavior. Descriptive criteria for the field classification of
terms of q are given in table 2-2. If equipment for
rock is contained in table 2-6.
making unconfined compression tests is not available, a
b. Classification of intact rock. An engineering
rough estimate can be based on the simple field
classification of intact rock is contained in table 2-7. The
identification suggested in the table; various small
classification is based on the uniaxial compressive
penetration or vane devices are also helpful.
strength and the tangent modulus.
2-4.
Soil classification. The Unified Soil Classi-
2-6.
Rock properties for foundation design.
fication System, based on identification of soils ac-
a. The principal rock properties of concern for
cording to their grain-size distribution, their plasticity
foundation design are the structural features and shear
characteristics, and their grouping with respect to be-
strength. Strength properties of rock are discussed in
havior, should be used to classify soils in connection with
chapter 3. Structural features include-
foundation design. Table 2-3 summarizes the Unified
(1) Types and patterns for rock defects (table
Soil Classification System and also presents field
2-6)-cracks, joints, fissures, etc.
identification procedures for fine-grained soils or soil
(2) Bedding planes-stratification and slope
fractions. It is generally advantageous to include with the
(strike and dip).
soil classification anv regional or locally accepted
(3) Foliation-a general term for a planar
terminology as well as the soil name (table 2-4).
arrange-
2-5.
Rock classification.
a. Geological classification. The geological
classification of rock is complex, and for most
engineering applications a simplified system of
2-5
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