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