an engineering geological characterisation of tropical clays - GBV

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For loading conditions equal to swelling pressure, SP, (i.e. P% or P/SP * 100 = 100%), the
percentage swelling is usually expected to be zero, i.e. S% = 0. However, percentage swelling
value given by the Greek method (Eq. 7.51) for these loading conditions is overestimated, i.e.
by substitution,
100 = -17.32Ln(S/SP) + 1,18, or
S/SP = e^-5,706 = 0,0033
For an average swelling pressure of SP = 50,29 kPa for black clays,
S% = 0,0033* 50,29 or
S% = 0,17
In comparison, the second method (Eq. 7.50) developed in this study gives a more reliable
estimation of S% for the same loading conditions above, i.e. S% = 0,01.
On the other hand, estimation of loading pressures necessary to realise permitted percentage
swelling values using the Greek method will require solution of rather cumbersome
mathematical equations of the form
aP + bLn(P) + c = 0,
where P = the loading pressure (kPa)
a, b, c = constants
In addition, the strength of estimating swelling characteristics by the Greek method (Eq. 7.51)
is inferior (R = 0,92) relative to that of the second method (Eq. 7.50) developed in this study
(R = 0,98).
In conclusion, the second method (Eq. 7.50) developed in this study would be preferred and
more reliable than the Greek one in estimating swelling characteristics of black clays.
Abebe Solomon (2002), undertook a laboratory investigation of the swelling characteristics of
partially saturated and remoulded highly expansive clay materials in Germany. He adopted
an improved testing procedure and apparatus in the form of a modified triaxial compression
machine and/ or cell to determine swelling pressure and associated volume change (swelling
strain) simultaneously. The obtained results could be presented in the form of curves of both
swelling pressure and moisture/ water up take variations with time, the interpretation of which
serves to provide more accurate insights regarding swelling behaviour and capability of clays
soils. However, his engineering computations of swelling strain rate (in terms of variations of
swelling stress and moisture) are beyond the scope of the present work.
7.4.8 Evaluation of results of oedometer consolidation, swelling pressure and
percentage swelling
Results of consolidation parameters cv, mv, cc and secondary compression, cα, show the red
soils to be slightly overconsolidated, especially the deeper horizons (Table 7.17), so that they
have in the past been subjected to a pressure greater than the present overburden pressure.
This is most probably due to the clays having been covered by deposits of soil or rock
(volcanic lava), and which were subsequently eroded away in the course of geological time;
leading to a reduction in effective pressure. Preconsolidation effects could have also been