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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106<br />
According to Terzaghi <strong>an</strong>d Peck (1967), the cohesion, C, is the component <strong>of</strong> shearing<br />
resist<strong>an</strong>ce due to internal forces holding the particles together in a solid mass; while t<strong>an</strong>φ is<br />
the component <strong>of</strong> shearing resist<strong>an</strong>ce due to interlocking <strong>of</strong> soil particles <strong>an</strong>d friction between<br />
them when subjected to normal stress. The t<strong>an</strong>φ component usually increases with increased<br />
normal stress <strong>an</strong>d disappears under unconfined conditions <strong>of</strong> zero normal loading. On the<br />
other h<strong>an</strong>d, C is independent <strong>of</strong> normal stress <strong>an</strong>d remains const<strong>an</strong>t with increased normal<br />
loading.<br />
7.3.3 Results<br />
The drained shear strength parameters (c´, φ´ ) obtained for the soils in this study are<br />
presented in Table (7.9); with c´ reported to two signific<strong>an</strong>t figures <strong>an</strong>d φ´ to the nearest<br />
degree. The black <strong>clays</strong> exhibit <strong>an</strong>gles <strong>of</strong> shear resist<strong>an</strong>ce (φ´ ) <strong>of</strong> between 11° <strong>an</strong>d 30°, giving<br />
a me<strong>an</strong> value <strong>of</strong> 18°. The red soils are characterised by shear resist<strong>an</strong>ce <strong>an</strong>gles <strong>of</strong> 28° to 29°.<br />
These are all typical values <strong>of</strong> clay soils which are usually less th<strong>an</strong> 28° (Lambe <strong>an</strong>d<br />
Whitm<strong>an</strong>, 1979; Johnson <strong>an</strong>d DeGraff, 1988). The characteristic shear stress/ displacement<br />
curves obtained for the soils are included in Fig. (7.4); with the generally cohesive <strong>an</strong>d high<br />
plasticity black <strong>clays</strong> giving rise to sharp peaks while the loose <strong>an</strong>d low plasticity red soils are<br />
characterised by flattened-out peaks. As a result, the difference between peak strength <strong>an</strong>d<br />
residual strength is large in black <strong>clays</strong> <strong>an</strong>d quite insignific<strong>an</strong>t in red soils. A diagrammatic<br />
representation <strong>of</strong> the strength characteristics <strong>of</strong> these soils is illustrated in the form <strong>of</strong> Mohr-<br />
Coulomb failure envelopes in Fig. (7.5).<br />
The very cohesive nature <strong>of</strong> the black <strong>clays</strong> is evidenced by the relatively large values <strong>of</strong><br />
cohesion (c´ ) obtained, i.e.12 – 48 kN/m², giving a me<strong>an</strong> value <strong>of</strong> 35 kN/m² (Table 7.9; Fig.<br />
7.5). On the contrary, cohesion is insignific<strong>an</strong>t in red soils due to their generally loose <strong>an</strong>d<br />
friable nature.<br />
Typical values <strong>of</strong> the <strong>an</strong>gle <strong>of</strong> shear resist<strong>an</strong>ce for other <strong>geological</strong> materials (quartz grains<br />
<strong>an</strong>d noncohesive soils) are provided in Table (7.8); for comparison purposes.<br />
Table 7.8. Typical values <strong>of</strong> φ for dry noncohesive soils <strong>an</strong>d clay (after Lambe <strong>an</strong>d Whitm<strong>an</strong>,<br />
1979).<br />
Type <strong>of</strong> soil <strong>an</strong>d grading Angle <strong>of</strong> shear resist<strong>an</strong>ce (φ) in degrees<br />
Loose<br />
Dense<br />
Rounded Angular Rounded Angular<br />
Uniform s<strong>an</strong>d- fine to 30 35 37 43<br />
medium<br />
Well-graded s<strong>an</strong>d 34 39 40 45<br />
S<strong>an</strong>d <strong>an</strong>d gravel 36 42 40 48<br />
Gravel 35 40 45 50<br />
Silt 28-32 30-35<br />
Clay < 28 < 30