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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40<br />
Table 4.1 Classification <strong>of</strong> s<strong>of</strong>t soils based on v<strong>an</strong>e shear strength (BS 1377: 1975).<br />
General descriptive term<br />
Maximum shear stress<br />
for strength<br />
(kN/m²)<br />
Very s<strong>of</strong>t 20<br />
S<strong>of</strong>t 40<br />
S<strong>of</strong>t to firm 60<br />
Firm 90<br />
4.4.3 Results <strong>of</strong> study<br />
Results <strong>of</strong> field v<strong>an</strong>e shear tests performed in this study are summarised in Tables 4.2, 4.3 <strong>an</strong>d<br />
4.4 as shown below. The variations <strong>of</strong> v<strong>an</strong>e shear strength with soil depth are presented in<br />
Figure 4.4.<br />
Table 4.2 Results <strong>of</strong> field v<strong>an</strong>e shear tests in s<strong>of</strong>t black <strong>clays</strong> at Madaraka West, Nairobi.<br />
Date: 14.01.2001 Spring = Tr/ θf i.e. K = 1333,33<br />
Location: Madaraka West Const<strong>an</strong>t, K Nmm/deg.<br />
Soil type: Black <strong>clays</strong><br />
Depth, Moisture Bulk Angle <strong>of</strong> Torque, V<strong>an</strong>e shear Soil<br />
Dp (m) Content, density Rotation, θf Tr (Nmm) Strength, strength<br />
Wn (%) (g/cm³) (degrees) c (kPa) description<br />
0,3 40,53 1,56 10,6 14166,67 30,94 S<strong>of</strong>t<br />
0,6 40,46 1,74 14,3 19000,00 41,50 s<strong>of</strong>t to firm<br />
1,0 24,68 1,78 45,6 60833,33 132,86 firm to stiff<br />
1,5 28,23 1,78 60,6 80833,33 176,53 Stiff<br />
The s<strong>of</strong>t <strong>an</strong>d sensitive clay soils are characterised by a very s<strong>of</strong>t to firm consistency, up to<br />
depths <strong>of</strong> 1 m for black <strong>clays</strong> <strong>an</strong>d 3 m for red soils. The black clay vari<strong>an</strong>ts exhibit undrained<br />
v<strong>an</strong>e shear strength <strong>of</strong> 30,94 – 41,50 kPa; while the red soil equivalents have values <strong>of</strong> 10,92<br />
– 87,36 kPa. There is a general increase in the undrained shear strength with depth, in both<br />
soil types. This is probably due to preconsolidation <strong>of</strong> deeper soil horizons by overburden<br />
material above, so that bulk density <strong>of</strong> the soils generally increases with depth (Tables 4.2, 4.3<br />
& 4.4).<br />
Table 4.3 Results <strong>of</strong> field v<strong>an</strong>e shear tests in s<strong>of</strong>t red soils at Arboretum, Nairobi.<br />
Date: 11.01.2001 Spring = Tr/ θf i.e. K = 1333,33<br />
Location: Arboretum Const<strong>an</strong>t, K Nmm/deg.<br />
Soil type: Red friable <strong>clays</strong><br />
Depth, Moisture Bulk Angle <strong>of</strong> Torque, V<strong>an</strong>e shear Soil<br />
Dp (m) Content, density Rotation, θf Tr (Nmm) Strength, c strength<br />
Wn (%) (g/cm³) (degrees) (kPa) description<br />
0,3 39,44 1,37 3,8 5000,00 10,92 very s<strong>of</strong>t<br />
1,0 24,03 1,41 30,0 40000,00 87,36 Firm<br />
2,0 24,25 1,41 36,5 48616,47 106,18 firm to stiff<br />
3,0 25,07 1,52 38,3 51020,39 111,43 firm to stiff
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