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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98<br />
The percentage by mass <strong>of</strong> soil retained on each <strong>of</strong> the sieves was calculated, <strong>an</strong>d from this<br />
the cumulative percentage passing (P) each sieve size was then calculated by subtracting the<br />
corresponding summation from 100, i.e.<br />
Pi(%) = 100 - (Σmi/M1) *100 (7.11)<br />
where M1 = total initial dry mass <strong>of</strong> soil<br />
Σmi = cumulative mass retained on upper sieves, down to <strong>an</strong>d including sieve<br />
in question<br />
Pi (%) = cumulative percentage passing sieve size in question<br />
The total mass <strong>of</strong> fines (Mf) <strong>an</strong>d corresponding percentage fines (Pf) representing clay <strong>an</strong>d silt<br />
sizes which pass through the bottom-most (0,063mm) sieve was given by<br />
<strong>an</strong>d<br />
Mf = M1 – Σmi (7.12)<br />
Pf (%)<br />
= ((M1- Σmi)/M1 )*100, or<br />
= 100 – ((Σmi/M1)*100) (7.13)<br />
where M1, Mf <strong>an</strong>d Σmi are as defined above.<br />
The cumulative percentage passing (P%) <strong>of</strong> each sieve size was plotted against corresponding<br />
sieve size (D mm) on a semi-logarithmic chart to give grain size distribution curves <strong>of</strong><br />
<strong>an</strong>alysed soils in the coarse fraction r<strong>an</strong>ge (s<strong>an</strong>d, gravel).<br />
The sedimentation <strong>an</strong>alysis was carried out according to British St<strong>an</strong>dard (BS 1377: 1975,<br />
Test 7(D)), <strong>an</strong>d aimed at investigating the distribution <strong>of</strong> particles in the finer fraction (silt<br />
<strong>an</strong>d clay) <strong>of</strong> soils, by using a suitably calibrated specific gravity hydrometer to monitor <strong>an</strong>d<br />
measure the density <strong>of</strong> a suspension <strong>of</strong> the fraction in water with time (Plate 7.6). The <strong>an</strong>alysis<br />
essentially facilitated the distribution <strong>of</strong> particles in the silt r<strong>an</strong>ge (0,060 – 0,0020 mm) to be<br />
assessed. The maximum diameter <strong>of</strong> particles remaining above a particular depth, H, in the<br />
suspension at <strong>an</strong>y time, t, from the start <strong>of</strong> the sedimentation test was calculated by applying<br />
Stokes´ law (after Sir George Stokes, 1891), i.e.<br />
D = 0,005 531* [ηH/(t(Gs- 1))]^½ (7.14)<br />
where<br />
D = particle diameter (mm)<br />
η = dynamic viscosity <strong>of</strong> water (mPas)<br />
g = acceleration due to gravity (i.e. 9,81m/s²)<br />
H (mm) = effective depth<br />
t (min) = elapsed time<br />
ρw = mass density <strong>of</strong> liquid water (i.e. 1,000 Mg/m³)<br />
Values <strong>of</strong> η <strong>an</strong>d ρw for water temperatures <strong>of</strong> 0-40 °C are given in Table (7.6), i.e. after Kaye<br />
<strong>an</strong>d Laby (1973), while intermediate values may be obtained by either arithmetic or graphical<br />
interpolation.