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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54<br />
Table 5.8. Results <strong>of</strong> chemical <strong>an</strong>alyses <strong>of</strong> black <strong>clays</strong> obtained in this study.<br />
% SA2- SA2- SA41- SB1- SB1- SB41- SB41- SC17-<br />
content<br />
<strong>of</strong>:<br />
70cm 105cm 50cm 50cm 70cm 30cm 50cm 50cm<br />
SC41-<br />
30cm<br />
SC41-<br />
50cm<br />
SiO2 52,50 51,40 58,35 52,79 53,44 52,64 53,21 56,38 52,06 53,71<br />
Al2O3 14,71 14,93 13,37 12,74 12,84 12,14 12,20 11,87 12,06 11,77<br />
Fe2O3 8,66 8,85 6,98 6,90 6,96 6,67 6,75 6,20 6,49 6,10<br />
FeO - - - - - - - - - -<br />
BaO 0,052 0,052 0,031 0,062 0,062 0,064 0,062 0,085 0,123 0,104<br />
MgO 1,25 1,22 1,13 1,16 1,23 1,43 1,42 1,28 1,34 1,31<br />
CaO 1,28 4,29 1,28 1,54 2,01 2,61 2,32 1,47 1,64 1,48<br />
Na2O 1,06 1,04 0,69 0,71 0,76 0,50 0,56 0,82 0,90 0,89<br />
K2O 1,78 1,73 1,25 1,27 1,34 1,02 1,04 1,41 1,32 1,31<br />
P2O5 0,026 0,024 0,037 0,032 0,038 0,038 0,038 0,047 0,036 0,04<br />
ZrO2 0,092 0,091 0,085 0,091 0,092 0,078 0,078 0,072 0,073 0,072<br />
TiO2 0,96 0,95 0,90 0,76 0,75 0,77 0,77 0,75 0,79 0,77<br />
MnO 0,312 0,363 0,249 0,455 0,439 0,497 0,505 0,488 0,469 0,511<br />
SO3 - - - - - - - 0,011 0,028 0,016<br />
Cl 0,065 0,039 - - - - - - 0,015 0,015<br />
F 0,262 0,362 0,247 0,294 0,325 0,281 0,267 0,311 0,300 0,293<br />
Loss on - - - - - - - - - -<br />
ignition<br />
Total 83,01 85,34 84,60 78,80 80,29 78,74 79,22 81,19 77,64 78,39<br />
SA2-70cm, SA2-105cm, SA41-50cm, SB1-50cm, SB1-70cm, SB41-30cm, SB41-50cm, SC17-50cm, SC41-<br />
30cm <strong>an</strong>d SC41-50cm are black clay samples.<br />
The black to dark grey <strong>clays</strong> are characterised by a number <strong>of</strong> distinctive properties which<br />
serve to point to their possible genetic origin, as discussed below.<br />
Results <strong>of</strong> chemical <strong>an</strong>alyses performed on these soils in this study (Table 5.8) show the<br />
common exch<strong>an</strong>geable bases <strong>of</strong> Ca, Mg, K <strong>an</strong>d Na to be all <strong>of</strong> the same order irrespective <strong>of</strong><br />
the type <strong>of</strong> rock underlying the soil. This serves to show that the black <strong>clays</strong> are most<br />
probably not derived from the underlying rocks by weathering. This is further supported by<br />
results <strong>of</strong> chemical <strong>an</strong>alyses <strong>of</strong> the soils, which generally do not agree with <strong>an</strong>d/ or are not <strong>of</strong><br />
the same order as those for the main underlying rocks <strong>of</strong> Nairobi <strong>an</strong>d Kapiti phonolites<br />
(Tables 3.1, 3.2 <strong>an</strong>d 3.3).<br />
The black <strong>clays</strong> exhibit a generally uniform depth, varying from 0,90 to 1,50m. Depths <strong>of</strong> up<br />
to 1,80 to 2,10m are also registered in the swampy areas <strong>of</strong> Madaraka West <strong>an</strong>d environs. The<br />
<strong>clays</strong> generally show pronounced cracking in the form <strong>of</strong> strong shrinkage cracks on drying<br />
from a wet condition.<br />
The black <strong>clays</strong> were also observed harbouring occasional s<strong>an</strong>dier vari<strong>an</strong>ts in some places.<br />
Results <strong>of</strong> X-ray diffraction <strong>an</strong>alyses performed in this study as well as earlier results <strong>of</strong><br />
chemical/ mineralogical studies by Stephen, Bellis <strong>an</strong>d Muir (1956) show the black <strong>clays</strong> as<br />
containing a number <strong>of</strong> heavy minerals usually characteristic <strong>of</strong> metamorphic rocks <strong>of</strong> the<br />
gneissic type. The heavy minerals include sillim<strong>an</strong>ite, ky<strong>an</strong>ite, garnet <strong>an</strong>d staurolite; <strong>an</strong>d were<br />
traced in soil samples collected from Embakasi area <strong>an</strong>d its environs which are underlain by<br />
Nairobi phonolite. The presence <strong>of</strong> the s<strong>an</strong>dier vari<strong>an</strong>ts <strong>an</strong>d heavy minerals serves to point to<br />
the possibility <strong>of</strong> portions <strong>of</strong> materials forming the black <strong>clays</strong> as having originated from