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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rapid dissipation <strong>of</strong> pore water pressures <strong>an</strong>d faster consolidation settlements (cv = 1,18-12,14<br />
m²/year) when externally loaded. Constructed structures located on red soils are therefore<br />
expected to undergo faster consolidation –settlements, much <strong>of</strong> them during the construction<br />
stage, with minimum long-term instability implications after construction.<br />
On the other h<strong>an</strong>d, the black <strong>clays</strong> have been found to be characterised by relatively smaller<br />
voids ratios <strong>an</strong>d lower porosity (eo: 0,709-1,098; e = 0,23-1,06 over 25-800 kPa loading<br />
r<strong>an</strong>ge). These <strong>clays</strong> are generally more compact, dense <strong>an</strong>d cohesive, <strong>an</strong>d therefore less<br />
permeable (K = 1,67E-12 to 1,20E-9 m/s), less compressible (mv = 0,03-1,62 m²/MN) <strong>an</strong>d<br />
would exhibit slower dissipation <strong>of</strong> pore water pressures <strong>an</strong>d, therefore, slower consolidation<br />
settlements (cv = 0,05-5,08 m²/year) on external loading. The low rates <strong>of</strong> consolidationsettlement<br />
exhibited by the black <strong>clays</strong> would also me<strong>an</strong> structural settlements occurring<br />
beyond the construction stage. Long-term instability problems would therefore be expected<br />
for structures constructed on black <strong>clays</strong>.<br />
Results <strong>of</strong> oedometer consolidation tests obtained in this study could be used to solve<br />
problems related to foundations for light structures in the current area. The consolidation data<br />
<strong>an</strong>d parameters could be applied in conjuction with classification data, as well as knowledge<br />
<strong>of</strong> the loading history <strong>of</strong> the soils to make estimates <strong>an</strong>d predictions as regards probable<br />
behaviour <strong>of</strong> foundations under various loading conditions. The results would specifically<br />
assist in calculating the amount <strong>of</strong> settlement which would ultimately take place for the<br />
structure as a whole, while variations in long-term settlements <strong>an</strong>d/ or differential settlements<br />
between individual footings could also be estimated; by applying methods described by<br />
Terzaghi (1939), MacDonald <strong>an</strong>d Skempton (1955) <strong>an</strong>d Skempton <strong>an</strong>d Bjerum (1957).<br />
According to Skempton (1956), Terzaghi (1934), Mitchell, Vivatrat & Lambe (1977),<br />
differential settlements are usually more critical th<strong>an</strong> overall settlement, <strong>an</strong>d c<strong>an</strong> cause tilting<br />
<strong>of</strong> a structure as a whole <strong>an</strong>d/or distortions within the structure; <strong>an</strong>d should therefore be kept<br />
within acceptable limits to avoid possible structural deterioration <strong>an</strong>d damage. In addition,<br />
possible settlement <strong>of</strong> piled foundations as a result <strong>of</strong> the presence <strong>of</strong> deep-seated weak<br />
compressible clay layers, especially in red soils, could also be estimated.<br />
On the other h<strong>an</strong>d, estimates <strong>of</strong> the rate <strong>of</strong> consolidation could be further used to give the<br />
duration <strong>of</strong> time within which structural settlements would be completed, either during or<br />
after construction. In situations <strong>of</strong> long-term settlements as may occur in black <strong>clays</strong>,<br />
settlement/ time graphs could serve to show the duration <strong>of</strong> the most signific<strong>an</strong>t part <strong>of</strong><br />
settlements, <strong>an</strong>d this could be compared with the economic life <strong>of</strong> the structure. The<br />
settlement/ time relationship would also assist in ascertaining possible development <strong>of</strong><br />
unacceptable differential settlements in the long-term, after construction.<br />
Areas <strong>of</strong> s<strong>of</strong>t ground <strong>an</strong>d <strong>clays</strong> in the form <strong>of</strong> alluvial <strong>an</strong>d / or swampy environments are<br />
common in the present study area. The shear strength <strong>an</strong>d capacity <strong>of</strong> these grounds to support<br />
increased foundation loads could be increased <strong>an</strong>d improved by consolidation through preloading<br />
<strong>of</strong> the ground with a surcharge <strong>of</strong> temporary fill. In this case, results <strong>of</strong> laboratory<br />
consolidation tests would be useful in estimating the extent <strong>an</strong>d rate <strong>of</strong> the resulting<br />
settlement; <strong>an</strong>d thereby indicate whether a provision for accelerating the consolidation, such<br />
as installation <strong>of</strong> s<strong>an</strong>d drains, is justifiable.<br />
In situations where piles could be driven through the pre-consolidated s<strong>of</strong>t strata to tr<strong>an</strong>smit<br />
structural loads to a deeper firm stratum, the tendency <strong>of</strong> continued settlement <strong>of</strong> the fill <strong>an</strong>d<br />
s<strong>of</strong>t strata would have the effect <strong>of</strong> throwing additional loads onto the piles. Here also,