load is therefore accounted for almost entirely by mechanism 3 (expulsion
of porewater) as excess porewater pressures dissipate. This hydrodynamic
process is termed consolidationand is mainly relevant to clays and organic
soils in which the volume change process is comparatively slow because of
their very low permeability (Section 2.3.4). The consolidation process is
partly reversible, i.e. compressible soils can swell on removal of load. One-
dimensional vertical consolidation characteristics, determined in laboratory
oedometer tests, are expressed in terms of two principal coefficients.
- The coefficient of volume compressibility, mv, is required to determine
the magnitude of time-dependent primary consolidation settlement:
mv∆ (^) v/∆v (2.7)
where∆ (^) vis the vertical strain increment produced by increment of
vertical stress ∆v, if no lateral yielding is permitted.
- The coefficient of consolidation, cv, is used to establish rates of settle-
ment:
cvk/mv (^) w (2.8)
wherekis a coefficient of permeability.
The stress-dependency of the principal coefficients will be noted.
The coefficient of secondary consolidation, C, is determined in the
same test and is used to describe subsequent and very slow continuing
settlement due to creep of the soil structure under constant effective stress.
2.3.4 Soil permeability
Soil permeability is important to problems of seepage, stability of slopes
and consolidation. It is also important in ground engineering processes,
e.g. in grouting, and in dewatering. Relative permeability of saturated soils
is assessed by the coefficient of permeability, k, expressed in units of veloc-
ity (m s^1 ). It is the most variable of soil properties between the extremes
of a coarse gravel and an intact clay, and even within the confines of a
notionally uniform soil. Illustrative values of kare given in Table 2.3.
The flow of water in a saturated soil can be represented by
k (2.9)
whereis velocity and dh/dli, the hydraulic gradient.
dh
dl