Page 180 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Fabric and Structure
Soil Fabric and Structure 175

Figure 8.7
Mudslide in quick
clay annexing the
town of Nicolet,
Quebec. (National
Research Council,
Canada.)

unstable. When such a structure is stressed, as by a tremor or other vibration, the
clay can suddenly turn into a liquid with devastating consequences. Quick clays
occur in terraces along the St. Laurence River in Canada and in Norway, partly as
a result of rebound of the earth’s crust following melting of continental glaciers.

A quick clay can be stable for decades and then, when sharply vibrated, as by an
explosion, ground tremor, or when a truck goes by, can suddenly become a
rapidly flowing liquid mudslide, as shown in Fig. 8.7.

8.4.5 Dispersive Clays

Weakly flocculated soils can become dispersed from the action of flowing water.
This is particularly important in earth dams and levees, where a small leak can
quickly become larger and cause eventual failure by piping.

A ‘‘pinhole test’’ was developed by engineers of the USDA Soil Conservation
Service to detect dispersive clays. The test involves drilling a 1 mm diameter hole
through a compacted soil specimen and running water through the hole under
standardized conditions. The hole enlargement after a particular time is a measure
of the soil dispersivity (Sherard et al., 1976). Extensive testing has shown that clay
minerals in dispersive soils usually are smectites, with over 60 percent of their
exchangeable cations being sodium.

Another test for dispersive clay uses the hydrometer test described in the
preceding chapter both with and without a chemical dispersing agent. The soil

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