Page 279 - Geotechnical Engineering Soil and Foundation Principles and Practice
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Soil Consistency and Engineering Classification
274 Geotechnical Engineering
limit, and PI is the plasticity index. When the calculated group index is negative it
is reported as zero (0).
The group index is expressed to the nearest whole number and is written in
parentheses after the group or subgroup designation. A group index should be
given for each soil even if the numerical value is zero, in order to indicate that the
classification has been determined by the AASHTO system instead of the original
Public Roads system. A nomograph has been devised to solve eq. (12.6), but it
now is more conveniently solved with a computer spreadsheet.
12.13 LIMITATIONS AND COMPARISONS OF
SOIL CLASSIFICATION SYSTEMS
The classification systems described above use disturbed soil properties and
therefore do not take into account factors such as geological origin, fabric,
density, or position of a groundwater table. The classifications nevertheless do
provide important information relative to soil behavior so long as the limitations
are recognized. Classification is no substitute for measurements of important soil
properties such as compressibility, shear strength, expandability, permeability,
saturation, pore water pressure, etc.
Boundary lines for fine soils in the Unified and AASHTO classification systems
do not precisely coincide, but the systems are close enough that there is
considerable overlapping of designations, so a familiarity with one system will
present at least a working acquaintance with the other.
Some approximate equivalents that will include most but not all soils are as
follows:
A-1-a or GW
Well-graded free-draining gravel suitable for road bases or foundation support.
A-1-b or SW
Similar to A-1-a except that it is primarily sand.
A-2 or SM or SC
Sand with appreciable fines content. May be moderately frost-susceptible.
A-3 or SP
Sand that is mainly one size.
A-4 or ML
Silt that combines capillarity and permeability so that it is susceptible to frost
heave. Low-density eolian deposits often collapse when wet.
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