Care must be taken in the interpretation of Table 2.3, as the meas-
ured properties of notionally homogeneous and uniform soils are subject
to very considerable variation. The ranges of values quoted should there-
fore be regarded as purely illustrative. They may be compared with the
corresponding figures for compacted fills given in Table 2.6 (Section 2.5.1
should also be referred to).
2.3.7 Partially-saturated soils and the embankment dam
Engineering soils have to this point been considered as existing in the fully
saturated state, i.e. all inter-particle void space is completely filled with
pore water. An appreciation of the influence of degree of saturation of a
soil,Sr, upon engineering behaviour is of considerable importance to the
dam engineer, as compacted fine-grained soils are initially in a partially-
saturated state. The soil is then a three-phase particulate material, with the
solid particles forming the soil skeleton in point contact and the interstitial
void spaces containing both pore water and compressible pore air.
It should be appreciated that the pore water within a fine-grained
soil is a dilute electrolyte, the engineering characteristics of clayey soils
reflecting their mineralogy and the concentration and nature of the ‘free’
ions present in the pore water. By the same token pore air should be cor-
rectly seen as embracing air, gases, and/or water vapour.
In the unsaturated state a meniscus is formed at the pore water/pore
air interfaces. These menisci are responsible for the local pore water pres-
sure, uw, always being less than pore air pressure, ua. The difference
between pore air and pore water pressures is responsible for generating
considerable intergranular contact forces between the soil particles. In
fine-grained soils, especially those with an appreciable clay content, the
matric suction, defined as (ua–uw), diminishes dramatically with increasing
degree of saturation, Sr, and plays a very significant role in determining
important aspects of mechanical behaviour.
Specific soils display a quite unique and therefore characteristic rela-
tionship between matric suction and degree of saturation; the shape of the
(ua–uw).Srcurve is therefore a characteristic related to soil type, well-
graded soils displaying a smoothly curving relationship with matric suction
diminishing at a reducing rate as saturation level increases. More uni-
formly graded soils, on the other hand, display a much flatter curve,
showing comparatively little diminution in matric suction over an appre-
ciable range of increasing degree of saturation.
The increased matric suction associated with diminishing degree of
saturation for a fine-grained soil will effect significant changes in several
engineering characteristics of the soil of primary concern to the dam
engineer, i.e. shear strength, collapse behaviour and permeability.
58 EMBANKMENT DAM ENGINEERING
