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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Results <strong>of</strong> chemical <strong>an</strong>alyses <strong>of</strong> the red soils performed in the current study (Table 5.4) show<br />
them as lacking <strong>an</strong>d deficient in magnesia (MgO: 0,097-0,14 %). Chemical <strong>an</strong>alyses <strong>of</strong> the<br />
rocks from which these soils are derived also indicate their relatively very low magnesia<br />
content (MgO: 0,30-0,89 %), compared with the basic lavas <strong>an</strong>d other volc<strong>an</strong>ic rocks <strong>of</strong> the<br />
Nairobi region (Table 3.4). Farmers in the c<strong>of</strong>fee-growing estates found in the areas <strong>of</strong> red<br />
soils tackle this problem through frequent artificial application <strong>of</strong> magnesite-rich fertilisers.<br />
The red soils generally exhibit lower contents <strong>of</strong> SiO2, MgO, CaO, Na2O <strong>an</strong>d K2O th<strong>an</strong> the<br />
volc<strong>an</strong>ic materials from which they are derived (Tables 3.3 <strong>an</strong>d 5.4). This could also be a<br />
result <strong>of</strong> the good drainage conditions <strong>of</strong> the soils as well as the high rainfall which favoured<br />
leaching <strong>an</strong>d removal <strong>of</strong> the soluble bases <strong>an</strong>d silica, thereby leaving the soils relatively<br />
enriched in Fe2O3 <strong>an</strong>d Al2O3 minerals.<br />
Table 5.4. Results <strong>of</strong> chemical <strong>an</strong>alyses <strong>of</strong> red soils obtained in this study.<br />
% content <strong>of</strong>: Rd1-30cm Rd1-100cm Rd1-200cm Rd1-400cm<br />
SiO2 47,50 47,10 47,20 47,6<br />
Al2O3 32,20 33,40 33,20 33,30<br />
Fe2O3 15,30 15,50 15,50 15,20<br />
FeO - - - -<br />
BaO 0,062 0,061 0,071 0,064<br />
MgO 0,12 0,097 0,14 0,13<br />
CaO 0,26 0,24 0,16 0,26<br />
Na2O ‹ ‹ ‹ ‹<br />
K2O 0,96 0,66 0,68 0,60<br />
P2O5 0,19 0,086 0,096 0,062<br />
ZrO2 0,37 0,37 0,37 0,36<br />
TiO2 1,57 1,51 1,51 1,46<br />
MnO 0,96 0,53 0,61 0,52<br />
Loss on ignition - - - -<br />
Total 99,49 99,55 99,54 99,56<br />
Rd1-30cm, Rd1-100cm, Rd1-200cm, Rd1-400cm. Samples <strong>of</strong> red soils collected from Arboretum (Nairobi), at<br />
depths <strong>of</strong> 30cm, 100cm, 200cm <strong>an</strong>d 400cm, respectively.<br />
‹: Concentration less th<strong>an</strong> 0,04 % (400 ppm).<br />
Similar results <strong>of</strong> previous chemical <strong>an</strong>alyses <strong>of</strong> a number <strong>of</strong> <strong>clays</strong> <strong>an</strong>d red soils <strong>of</strong> Nairobi<br />
area are also given in Table 5.5, for comparison purposes. A relation <strong>of</strong> the results <strong>of</strong> Tables<br />
5.4 <strong>an</strong>d 5.5 reveals a depletion in the amounts <strong>of</strong> soluble bases (MgO, CaO, NaO) to have<br />
taken place with time, most probably due to effects <strong>of</strong> leaching in the red soils.<br />
Correspondingly, the proportionate amounts <strong>of</strong> Fe2O3 (free iron oxide, haematite, goethite)<br />
<strong>an</strong>d Al2O3 (clay minerals) have increased with time.<br />
The more easily leached components from areas <strong>of</strong> red soils persist or accumulate on lower<br />
ground <strong>of</strong> depressions between ridges <strong>an</strong>d/ or the flat plains, where they contribute to the<br />
formation <strong>of</strong> black soils.