an engineering geological characterisation of tropical clays - GBV

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