an engineering geological characterisation of tropical clays - GBV

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The variation of shear strength parameter φ´ for depths of less than 0,5m (mean 21°, std 5,5)
and those greater than 0,5m (mean 17°, std 3,8) in black clays was shown to be generally
similar across the study area. This implies that geo-engineers and planners involved in
planning, design and construction of projected structures could safely treat the black clays as a
single homogeneous layer with respect to their strength characteristics.
Results of correlations between index (Atterberg limits, relative consistency) and shear
strength (c´, φ´ ) parameters have been found to be only weak to moderate in strength. This
implies that index parameters cannot be reliably used to characterise shear strength of soils in
situ. The uncertainy accompanying the estimation and assessment of strength characteristics
could be a result of index tests having been performed on disturbed and/ or fractioned samples
having destroyed/ broken down soil fabric and structure not representative of conditions in
situ.
The results of shear strength parameters (c´, φ´ ) obtained for black clays and red soils in this
study would serve to be useful in computations involving stability analyses of soils when
subjected to imposed loads from projected foundation structures. The parameters would
especially be useful in determining the limiting value of external imposed loads capable of
just mobilising the shear strength of the soils, and thereby cause their failure. In order to limit
deformations to within tolerable limits, foundation design practices and/ or procedures would
require that a suitable factor of safety be adopted and applied so that shear stresses induced in
the soil due to weights of constructed structures are everywhere less than a certain proportion
of its maximum shear strength (Terzaghi and Peck, 1967; Art. 26-32). The drained shear
strength parameters would also serve to aid in the analysis of long-term stability problems that
may arise in a number of projects; whereupon the angle of shear resistance would be useful in
deriving earth pressure coefficients and/ or bearing capacity coefficients for use in relevant
computations.
The analysis of long-term stability problems for the conditions prevailing after construction of
structures may include a number of determinations such as bearing capacity of footings and
foundations for structures on clay soils, earth pressure on a retaining wall, earth pressure
against bracing in temporary excavations, safeguards against heave of the bottom of
temporary and/ or deep open excavations in clays, as well as long-term stability of earth dams
and embankments (Terzaghi & Peck, 1967; Art. 29). In addition, residual shear strength
parameters (cr´, φr´ ) when available, could be useful in long-term stability analyses of slopes
and cuttings, especially in overconsolidated clays (Skempton, 1964; Skempton & La
Rochelle, 1965; Symons, 1968). These parameters are usually obtained by allowing for large
additional horizontal displacements during shearbox testing after the peak strength had been
mobilised. However, their exact and/ or accurate determination in this study was limited by
the capacity of the apparatus. In practice, accurate determination of residual shear strength
parameters is achieved by employing a ring shear apparatus which was, however, unavailable
for the present study.
9.4 Consolidation-settlement characteristics
Results of laboratory consolidation tests (Tables 7.12 & 7.14 ) show that red soils exhibit
relatively larger voids ratios and higher porosity (initial voids ratio, eo: 1,56 at 0,30m depth &
1,16 at 4m depth); with voids ratio, e, varying from 0,59-1,42 for the normal loading range of
25-800 kPa. As a result of the above, together with their generally loose and friable nature,
these soils tend to be more or less free-draining, more permeable (K: 7,57E-11 to 7,33E-9
m/s) and more compressible (mv =0,16-2.94 m²/MN ). They would therefore exhibit faster and