an engineering geological characterisation of tropical clays - GBV

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Relationships between values of cc and liquid limit for the soils in this study are discussed in a
later section and represented in Table (7.15) and Figs. (7.23 & 7.24). Measured values of cc
obtained from consolidation tests in this study are also compared and correlated with
calculated values, both from the new relationship established in this study as well as
Skempton´s relationship (Table 7.16).
7.4.3.4 Swell index (cs)
This is the slope of the swelling or unloading curve of e plotted against log p (Fig. 7.18). It is
the change in voids ratio, e, over one log cycle of pressure change in the linear section of the
swelling/ unloading curve of e/ log p. The straight-line section of the curve is given
approximately by the equation
e = eo ± cslog10 (po+δp)/po (7.44)
where eo and po refer to initial conditions of voids ratio and loading pressure.
Correlations between swell index and index properties are discussed in a later chapter.
7.4.3.5 Coefficient of permeability (K)
The coefficient of permeability, K, is usually computed from Equation (7.25), i.e.
cv = K/(ρwgmv ), or by re-writing
K = cvmvρwg (mm/s), or in practical units
K = cv/(365,25*24*3600) * (mv*10^¯6) * (1*10³) * 9,81 (m/s), or
K = cvmv * 0,31*10^¯9 (m/s) (7.45)
where cv (m²/year) and mv (m²/MN) are consolidation parameters whose values are known,
g (m/s²) = acceleration due to gravity
ρw (kg/m³) = mass density of water
Values of K calculated for black clays and red soils of this study using the above equation are
given in Table (7.12).
7.4.3.6 Coefficient of secondary compression (cα)
This is the compression which continues after primary consolidation has virtually finished,
and is time dependent. According to Head (Head, 1988), it is a significant factor in the
settlement of soft soils, especially organic clays and peats where it usually increases with
increased load application.
Secondary compression is equal to the slope of the linear portion of the secondary
compression part of log-time/ settlement plot, in terms of strain per log cycle of time (Fig.
7.15b), i.e.
cα = (δHs/Ho)/ δlogt, or for one log cycle of time change