an engineering geological characterisation of tropical clays - GBV

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Primary consolidation is a time-dependent compression due to dissipation of excess pore
pressure under loading. This phase is accounted for by the Terzaghi consolidation theory
(Terzaghi, 1925), and relates closely to the theoretical curve for most clays. It is therefore
used in many applications to make estimation of settlements.
Secondary compression continues after the excess pore pressure of the primary consolidation
phase has virtually dissipated, and is most probably a result of continued movement of
particles as the soil structure adjusts itself to increasing effective stress.
The primary consolidation phase is usually used to determine the coefficient of consolidation,
cv, for each increment of loading through a curve fitting process, which entails comparing a
laboratory consolidation curve with the theoretical curve. In this study, two curve fitting
procedures were adopted and include the log-time method (after Casagrande, 1936), in which
a graphical analysis of log-time/ settlement curves was done (Fig. 7.13); and the square-roottime
method (after Taylor, 1942; Barden, 1965) in which a graphical analysis of square-roottime/
settlement curves was undertaken (Fig. 7.14).
In this study, the log –time plot was used in the analysis of black clays whose settlement
curves were generally of the conventional shape (Fig. 7.16). However, the square-root plot
was used in the analysis of the rather silty red soils, in which otherwise evaluation of initial
consolidation conditions, i.e. the do point of deformation corresponding to U = 0% primary
consolidation using the log-time method was difficult due to the unconventional shape of
settlement curves produced (Fig. 7.17).
7.4.3 Consolidation coefficients and other parameters
7.4.3.1 Coefficient of consolidation (cv)
This parameter relates the change in excess pore pressure with respect to time, to the amount
of water draining out of the voids of a clay layer during the same time, as a result of
consolidation.
The coefficient of consolidation for a given load increment in a consolidation test can be
determined by re-writing Equation (7.28), i.e.
cv = (Tv/t) * h² (mm²/min) (7.30)
where t (min) = time for a given percentage of primary consolidation
h (mm) = length of maximum drainage path
A log-time graphical analysis using time t50 corresponding to 50% primary consolidation (i.e.
U = 50%) and theoretical time factor T50 = 0,197 (Table 7.11) transforms Equation (7.30) into
cv = (T50/t50) * h²
= 0,197 * (h²/t50)
or cv = (0,197 * (h/1000)²)/(t50/(60*24*365,25)
i.e., cv = 0,1036h²/t50 (m²/year) (7.31)
In the standard oedometer consolidation test as used in this study, with double drainage to
both the lower and upper surfaces, the height of the specimen H' (mm) is equal to 2h.
Therefore Equation (7.31) becomes