an engineering geological characterisation of tropical clays - GBV

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