an engineering geological characterisation of tropical clays - GBV

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