Page 273 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Consistency and Engineering Classification
268 Geotechnical Engineering

reaction is typical of nonplastic, uniform fine sand, of silty sand (SP or SM), of
inorganic silt (ML), particularly the rock-flour type, and of diatomaceous earth
(MH). The reaction becomes more sluggish as the uniformity of gradation
decreases and the plasticity increases, up to a certain degree. Even a small amount
of colloidal clay will impart some plasticity to the soil and will materially slow
the reaction to the shaking test. Soils that react in this manner are somewhat
plastic inorganic and organic silts (ML or OL), very lean clays (CL), and some
kaolin-type clays (ML or MH). Extremely slow reaction or no reaction to the
shaking test is characteristic of typical clays (CL or CH) and of highly plastic
organic clays (OH).

Field Estimate of Plasticity
The plasticity of a fine-grained soil or the binder fraction of a coarse-grained soil
may be estimated by rolling a small sample of minus-40 material between the
palms of the hand in a manner similar to the standard plastic limit test.
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The sample should be fairly wet, but not sticky. As it is rolled into -inch threads,
8
folded and re-rolled, the stiffness of the threads should be observed. The higher
the soil above the A-line on the plasticity chart (CL or CH), the stiffer the threads.
Then as the water content approaches the plastic limit, the tougher are the lumps
after crumbling and remolding. Soils slightly above the A-line (CL or CH) form a
medium-tough thread that can be rolled easily as the plastic limit is approached,
but when the soil is kneaded below the plastic limit, it crumbles.

Soils below the A-line (ML, NH, OL, or OH) form a weak thread and with the
exception of an OH soil, such a soil cannot be lumped into a coherent mass below
the plastic limit. Plastic soils containing organic material or much mica form
threads that are very soft and spongy near the plastic limit.

In general, the binder fraction of a coarse-grained soil with silty fines (GM or SM)
will exhibit plasticity characteristics similar to those of ML soils. The binder fraction
of a coarse-grained soil with clayey fines (GC or SC) will be similar to CL soils.

Field Estimate of Dry Strength
Dry strength is determined from a pat of minus-40 soil that is moistened and
molded to the consistency of putty, and allowed to dry in an oven or in the sun
and air. When dry the pat should be crumbled between the fingers. ML or MH
soils have a low dry strength and crumble with very little finger pressure. Also,
organic siIts and lean organic clays of low plasticity (OL) and very fine sandy soils
(SM) also have low dry strength.

Most clays of the CL group and some OH soils, as well as the binder fraction of
gravelly and sandy clays (GC or SC), have medium dry strength and require
considerable finger pressure to crumble the sample. Most CH clays and some
organic clays (OH) having high liquid limits and located near the A-line have high
dry strength, and the test pat can be broken with the fingers but cannot be
crumbled.

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