Engineering Geology

Chapter 5
Table 5.3b. Plasticity according to liquid limit
Description Plasticity Range of liquid limit
Lean or silty Low plasticity Less than 35
Intermediate Intermediate plasticity 35–50
Fat High plasticity 50–70
Very fat Very high plasticity 70–90
Extra fat Extra high plasticity Over 90
important second constituent in the very coarse fraction. If over half of the very coarse material
is of cobble size, then it is described as cobbles. Similarly, it may be described as bouldery
cobbles if boulders are an important second constituent in the very coarse fraction.
Mixtures of the very coarse material and soil can be described by combining the terms for the
very coarse constituent and the soil constituent as shown in Table 5.4.
According to Anon (1999), further factors that should be incorporated in a soil description, to
help identification of soil, include the following:
1. Mass characteristics
(a) Field strength or compactness and indication of moisture condition. A scale for estimating
the strength of clays is given in Table 5.5, however, where assessment of
strength is important, appropriate testing should be undertaken. The relative densities
of sands and gravels may be determined by the standard penetration test (see
Chapter 7) that, in turn, can be related to their angle of friction (Table 7.3).
(b) The thickness of the bedding should be described as indicated in Table 2.1.
Where beds are too thin to be described individually, they may be referred to as
interbedded or interlaminated.
(c) Discontinuities include joints, fissures, and shear surfaces. Their orientation,
spacing, persistence, openness and surface texture (i.e. rough, smooth, polished,
striated) should be described (see Chapter 2).
Table 5.4. Mixtures of very coarse materials and soil
Description
Boulders (or cobbles) with a little finer material*
Boulders (or cobbles) with some finer material*
Boulders (or cobbles) with much finer material*
Finer material* with many boulders (or cobbles)
Finer material* with some boulders (or cobbles)
Finer material* with occasional boulders (or cobbles)
Composition
Up to 5% finer material
5–20% finer material
20–50% finer material
50–20% boulders (or cobbles)
20–5% boulders (or cobbles)
Up to 5% boulders (or cobbles)
*Give the name of the finer material, e.g. Gravel with occasional boulders; cobbly boulders with some
finer material (sand with some fines).
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