Engineering Geology

Chapter 3
Figure 3.4
The slake-durability apparatus.
Intraparticle swelling (i.e. swelling due to the take up of water – not only between particles of
clay minerals but also within them – into the weakly bonded layers between molecular units)
of clay minerals on saturation can cause mudrocks to break down where the proportion of
such minerals constitutes more than 50% of the rock. The expansive clay minerals such as
montmorillonite can expand many times their original volume.
Failure of consolidated and poorly cemented rocks occurs during saturation when the
swelling pressure or internal saturation swelling stress, s s ’ developed by capillary suction
pressures exceeds their tensile strength. An estimate of s s can be obtained from the modulus
of deformation, E:
E = s / e
S
D
(3.1)
where e D is the free-swelling coefficient. The latter is determined by a sensitive dial gauge that
records the amount of swelling of an oven-dried core specimen per unit height, along the
vertical axis during saturation in water for 12 h, e D being obtained as follows:
e D
=
Change in length after swelling
Initial length
(3.2)
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