an engineering geological characterisation of tropical clays - GBV

207
Chapter 12
Recommendations
Analysis of short term stability problems during and/ or immediately after construction
involve use of the undrained shear strength or apparent cohesion, cu, in the relevant
computations (Terzaghi and Peck, 1967). The corresponding angle of shear resistance, φ, is
especially useful in the derivation of the necessary earth pressure coefficients and/ or bearing
capacity coefficients. It would be more helpful therefore, that future works involving soils of
the area include laboratory triaxial tests in their studies with the purpose of providing the
necessary undrained shear strength parameters, cu, φ. The triaxial testing procedure is
generally more satisfactory in measuring the shear strength of clay soils in terms of total
stresses, through quick undrained shear tests (Head, 1988). Short-term stability analyses may
include computations related to bearing capacity of footings and foundations for structures on
saturated homogeneous clays, earth pressure on retaining walls, earth pressure against bracing
in temporary excavations, safeguard against heave of the bottom of temporary open
excavations in clays, stability of side-slopes of cuttings, as well as short term stability of
embankments and earth dams.
The black clays in some parts of the study area were found to contain gravelly lateritic
materials of up to 40 mm sizes. Investigation and determination of shear strength
characteristics of such materials by triaxial testing would be impracticable; while use of the
standard shearbox apparatus as employed in this study, would only produce unreliable results.
Future investigations and studies regarding shear strength of these significantly coarse
gravelly clays should therefore employ the large shearbox apparatus such as the one described
by Head (1988). This apparatus could provide more reliable shear strength parameters by
executing slow drained shear tests on large representative and undisturbed block samples of
the clays measuring 305 mm square and 150 mm thick. The results of shear strength
parameters so obtained would be useful at the design stages of a number of structures such as
embankments and earth dams which usually incorporate gravel fill material (Pike, 1973;
Pike, Acott and Leech, 1977). The results would also serve as a basis for discriminating,
subdividing and classifying gravelly soil materials to suit different construction needs and
works (Bishop, 1948; Pike, 1973).
The red soils were found in this study to be generally more permeable and relatively free
draining. They would therefore exhibit rapid drainage and settlement when externally loaded.
The resulting compression/ settlement curves tend to deviate from conventional shapes
derived from one-dimensional theory of consolidation so that determination of relevant
consolidation parameters would also be difficult. Use of a Rowe consolidation cell
(Rowe,1966), capable of accommodating larger soil specimens would be recommended for
oedometer tests on the clayey silt and silty varieties of red soils, so as to obtain a more
definite value of coefficient of consolidation, cv, for use in the more reliable estimation of
rates of settlement under various loading conditions. Alternatively, cv could be derived
empirically from the relation
cv = K/(mv * 0,31*E-9) m²/year
where K (m/s) = coefficient of permeability as measured in situ in the field
mv (MN/m²) = coefficient of volume compressibility derived from laboratory
consolidaton test using the Rowe cell