Page 280 - Geotechnical Engineering Soil and Foundation Principles and Practice
P. 280
Soil Consistency and Engineering Classification
Soil Consistency and Engineering Classification 275
A-5 or Low-Plasticity MH
Includes micaceous silts that are difficult to compact.
A-6 or CL
Moderately plastic clay that has a moderate susceptibility to frost heave and is
likely to be moderately expansive. All A-6 is CL but not all CL is A-6.
A-7-5 or Most MH
Silty clay soils with a high liquid limit, often from a high mica content.
A-7-6 or CH
Highly plastic clay that is likely to be expansive. Low permeability reduces frost
heave. All A-7-6 soils also classify as CH.
A-8 and Pt
Peat and muck.
12.14 OTHER DESCRIPTIVE LIMITS
Other tests and descriptive terms have been devised or defined that are not as
widely used or have fallen into disuse. Some are as follows:
12.14.1 Toughness
Toughness is defined as the flow index from the liquid limit test, which is the
change in moisture content required to change the blow count by a factor of 10,
divided by the plasticity index.
12.14.2 Shrinkage Limit
The shrinkage limit test was suggested by Atterberg and has been used as a
criterion for identifying expansive clay soils. However, the test involves complete
destruction of the soil structure and drying from a wet mud, which makes
correlations less reliable. The shrinkage limit generally is lower than the plastic
limit, and the transition from the intermediate semisolid state to a solid is
accompanied by a noticeably lighter shade of color due to the entry of air.
The shrinkage limit test also fell into disfavor because it used a mercury
displacement method to measure the volume of the dried soil pat. An alternative
method now coats the soil pat with wax for immersion in water (ASTM D-4943).
In order to perform a shrinkage limit test a soil-water mixture is prepared as for
the liquid limit but with a moisture content that is considerably above the liquid
limit, and the moisture content is measured. A sample is placed in a shallow dish
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