Engineering Geology

Chapter 5
Table 5.19. Some properties of varved and laminated clays
Varved clays, Elk
Valley, British Columbia*
Laminated clays,
Teesside, England**
Moisture content (%) 35 25–35 (30)
Plastic limit (%) 22 18–31 (26)
Liquid limit (%) 34 29–78 (56)
Plasticity index (%) 15.5 19–49 (33)
Liquidity index 0.36 -
0.12–0.35 (0.15)
Activity 0.36 0.47–0.65 (0.54)
Linear shrinkage 9–14 (11)
Compression index 0.405–0.587 (0.496) 0.55
Undrained shear strength (kPa) 20–102 (62)
Note: Range with average value in brackets.
*After George, 1986.
**After Bell and Coulthard, 1997.
be classified as inorganic silty clay of medium to high plasticity or compressibility. In some
varved clay, the natural moisture content is near the liquid limit. Consequently, these clays
are soft and frequently have sensitivities around 4 (Bell and Coulthard, 1997). Their activity
tends to range between active and normal, and some may be expansive. The average
strength of some varved clays, for example, from Ontario, is about 40 kPa, with a range of
24–49 kPa. The effective stress parameters of apparent cohesion and angle of shearing
resistance range from 0.7 to 19.5 kPa, and 22–25∞, respectively.
The material of which quick clays are composed is predominantly smaller than 0.002 mm
(Geertsema and Torrance, 2005). Many deposits, however, seem to be very poor in clay
minerals, containing a high proportion of ground-down, fine quartz. The fabric of these
soils contains aggregations. Granular particles, whether aggregations or primary minerals,
are rarely in direct contact, being linked generally by bridges of fine particles. Clay
minerals usually are non-oriented, and clay coatings on primary minerals tend to be
uncommon, as are cemented junctions. Networks of platelets occur in some soils. Quick
clays generally exhibit little plasticity, their plasticity index usually varying from 8 to 12%.
Their liquidity index normally exceeds 1, and their liquid limits are often less than 40%.
Quick clays usually are inactive, their activity frequently being less than 0.5. The most
extraordinary property possessed by quick clays is their very high sensitivity. In other
words, a large proportion of their undisturbed strength is permanently lost following shear
(Fig. 5.13). The small fraction of the original strength regained after remoulding may be
attributable to the development of some different form of interparticle bonding. The reason
why only a small fraction of the original strength is recovered is because the rate at which
it develops is so slow.
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