an engineering geological characterisation of tropical clays - GBV

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