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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141<br />
Black <strong>clays</strong><br />
Swelling pressure SP (kPa)<br />
120<br />
n = 7<br />
100<br />
80<br />
60<br />
40<br />
20<br />
0<br />
10 15 20 25 30 35 40<br />
Natural moisture Wn (%)<br />
Wn vs SP<br />
Figure 7.33. Correlation between Wn <strong>an</strong>d SP.<br />
7.4.7.2 Swelling test<br />
Swelling is the increase in volume <strong>of</strong> a soil due to absorption <strong>of</strong> water within the voids when<br />
the applied stress is reduced. It is therefore the reverse <strong>an</strong>d opposite process <strong>of</strong> consolidation,<br />
<strong>an</strong>d occurs when overconsolidated <strong>clays</strong> are allowed free access to water (Nelson <strong>an</strong>d Miller,<br />
1992). In consolidation tests, swelling is represented by the unloading portion <strong>of</strong> the e/ log p<br />
curve (Fig. 7.18 ). The mech<strong>an</strong>ism <strong>of</strong> swelling is explained by <strong>an</strong> overconsolidated soil<br />
possessing high suction tensions within its skeleton on unloading, thereby drawing water into<br />
the voids. The resulting increase in volume <strong>of</strong> voids causes the soil to swell <strong>an</strong>d eventually<br />
disintegrate.<br />
Testing procedure involved use <strong>of</strong> same apparatus as in the st<strong>an</strong>dard oedometer consolidation<br />
test. However, the height <strong>of</strong> soil specimen was 3 mm less th<strong>an</strong> the height <strong>of</strong> consolidation<br />
ring; so that the specimen was laterally confined at all times during swelling. The space in the<br />
ring above test specimen was taken up by a flat metal disc <strong>of</strong> 3 mm thickness <strong>an</strong>d diameter<br />
about 1 mm less th<strong>an</strong> the internal diameter <strong>of</strong> ring. The specimen in the ring was weighed,<br />
assembled in oedometer cell <strong>an</strong>d mounted in the load frame <strong>of</strong> consolidation press. Initial<br />
height <strong>of</strong> specimen was given by height <strong>of</strong> ring less the average disc thickness.<br />
Swelling pressure, SP, was determined as described above. After establishment <strong>of</strong><br />
equilibrium, the swelling test was performed by unloading the specimen in stages from the<br />
swelling pressure, SP, as the starting point; <strong>an</strong>d by following <strong>an</strong> unloading sequence <strong>of</strong><br />
halving the loads at each successive decrement, i.e. 1/2SP, 1/4SP, 1/8SP, <strong>an</strong>d so on down to<br />
the smallest load required. Each unloading stage was maintained until equilibrium had been<br />
reached <strong>an</strong>d no further ch<strong>an</strong>ge in compression gauge reading was observed. During<br />
unloading, cumulative amount <strong>of</strong> swell (mm) undergone by the specimen for successive<br />
decrements <strong>of</strong> load was recorded by noting the ch<strong>an</strong>ge in compression gauge reading from its<br />
initial position.<br />
The cumulative swell (S mm) under each load decrement was recorded <strong>an</strong>d expressed as a<br />
percentage <strong>of</strong> the ultimate amount <strong>of</strong> swell (Smax mm) which was registered with specimen<br />
under zero load. Results <strong>of</strong> cumulative percentage swelling (S %) recorded against