an engineering geological characterisation of tropical clays - GBV

84
7.1.3 Atterberg limits and consistency of clay soils
7.1.3.1 Scope
The Atterberg limits serve to define the range of moisture contents within which a clay soil
exhibits a plastic consistency by measuring and describing the plasticity range in numerical
terms (Nelson and Miller, 1992).
The Atterberg limits of measurement adopted in this study to describe the consistency of soils
include the liquid limit (LL) and the plastic limit (PL). The liquid limit is the moisture content
at which a soil passes from the plastic to the liquid state, while the plastic limit represents the
moisture content at which a soil passes from the plastic to the solid state and becomes too dry
to be in a plastic condition. The plasticity index (PI) is the moisture content range between the
plastic and liquid limits, and serves as a measure of the plasticity of the clay soil (Nelson and
Miller, 1992).
In this study, the state of consistency of a clay soil in its natural state was numerically
described by relating its natural moisture content to the plastic and liquid limits using two
parameters, i.e. the relative consistency (Cr) and the liquidity index (LI). This is because in
general, clay soils occur as a slurry (liquid state) when moisture contents exceed their liquid
limits, while they exhibit a firm and plastic consistency with moisture contents lying within
the plasticity range, i.e. between plastic and liquid limits. On the other hand, clay soils with
moisture contents below their plastic limits would usually exhibit a hard to stiff consistency
(Head, 1984). According to Terzaghi and Peck (1948), relative consistency, Cr, is the ratio of
the difference between liquid limit and moisture content to the plasticity index, i.e.
Cr = (LL-W)/(LL-PL), or
Cr = (LL-W)/PI (7.2)
where W = moisture content
Lambe and Whitman (1969) defined the liquidity index, LI, as the ratio of the difference
between moisture content and plastic limit to the plasticity index, i.e.
LI = W-PL/PI (7.3)
According to Skempton and Northey (1953) and Wood and Wroth (1978), clay soils usually
exhibit increasing shear strength values as their state of consistency varies with progressive
decrease of moisture content, starting from the liquid limit down to the plastic limit. This
study tries to investigate and establish any possible consistent form of relationship between
shear strength parameters and liquidity index and/ or relative consistency for the clay soils
found.
Skempton (1953) showed that the Atterberg limits of clays are related to the particle size and
clay mineralogical composition of the soils. He noted that the the plasticity index, PI, is
dependent on the proportion of clay fraction, and defined colloidal activity (A) of a clay soil
as the ratio between these two parameters, i.e.
activity A = PI/ clay fraction (7.4)