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