fine-grained - grain diameter = 0.002 - 0.06mm
very fine-grained - grain diameter <0.002mm
Use lOX hand lens if necessary to examine rock sample.
d. Hardness Classification. Describe as very soft, soft, etc. in
accordance with Table 9 (from Reference 5), which shows range of strength
values of intact rock associated with hardness classes.
e. Geological Classification. Identify the rock by geologic name and
local name (if any). A simplified classification is given in Table 10.
Identify subordinate constituents in rock sample such as seams or bands of
other type of minerals, e.g., dolomitic limestone, calcareous sandstone,
sandy limestone, mica schist. Example of typical description:
Fresh gray coarse moderately close fractured Mica Schist.
CLASSIFICATION BY FIELD MEASUREMENTS AND STRENGTH TESTS.
Classification by Rock Quality Designation and Velocity Index.
(1) The Rock Quality Designation (RQD) is only for NX size core
samples and is computed by summing the lengths of all pieces of core equal
to or longer than 4 inches and dividing by the total length of the coring
run. The resultant is multiplied by 100 to get RQD in percent. It is
necessary to distinguish between natural fractures and those caused by the
drilling or recovery operations. The fresh, irregular breaks should be
ignored and the pieces counted as intact lengths. Depending on the
engineering requirements of the project, breaks induced along highly
anisotropic planes, such as foliation or bedding, may be counted as natural
fractures. A qualitative relationship between RQD, velocity index and rock
mass quality is presented in Table 11 (Reference 8, Predicting Insitu
Modulus of Deformation Using Rock Quality Indexes, by Coon and Merritt).
(2) The velocity index is defined as the square of the ratio of the
field compressional wave velocity to the laboratory compressional wave
velocity. The velocity index is typically used to determine rock quality
using geophysical surveys. For further guidance see Reference 9, Design
of Surface and Near Surface Construction in Rock, by Deere, et al.
Classification by Strength.
(1) Uniaxial Compressive Strength and Modulus Ratio. Determine the
uniaxial compressive strength in accordance with ASTM Standard D2938,
Unconfined Compressive Strength of Intact Rock Core Specimens. Describe the
strength of intact sample tested as weak, strong, etc., in accordance with
Figure 3 (Reference 10, The Point Load Strength Test, by Broch and
(2) Point Load Strength. Describe the point load strength of
specimen tested as low, medium, etc. in accordance with Figure 3. Point
load strength tests are sometimes performed in the field for larger projects
where rippability and rock strength are critical design factors. This
simple field test can be performed on core samples and irregular rock
specimens. The point