Page 181 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Fabric and Structure
176 Geotechnical Engineering

that can be dispersed without the addition of a dispersing agent is assumed to be
dispersive (ASTM Designation D-4221).

8.4.6 Dispersive Silt

The pinhole test also determines erosion-prone silty soils that strictly speaking are
not dispersive. Tests of the loess soil used for the core of the ill-fated Teton Dam
did not pass the pinhole test, but did pass the double-hydrometer test (Sherard,
1987). The dam failed by piping, that is, by enlargement of seepage channels under
and through the dam until the flow reached the category of a deluge shown in
Fig. 8.8. Warnings were given in time to evacuate most of the people from the area
downstream but nevertheless 14 lives were lost. This tragic misadventure cast a
long shadow over the future of big dam building in the U.S., but earth dams
continue to be built in developing countries of the world.

8.5 SOIL MACROSTRUCTURE

8.5.1 Overview
Macro means large, and mega implies larger by a factor of a million or so. For the
present purpose macrostructure may be defined as soil or rock structure that is
large but far enough apart that it may be missed by soil borings. Pockets of clay
such as shown in Fig. 3.3 come under this definition of macrostructure, whereas
the sink in Fig. 3.4 may be considered megastructure. Megastructure such as a
cavern or landslide should not be missed by borings so long as they are correctly
interpreted.

Figure 8.8
Teton Dam,
Idaho, 5 June
1976, the highest
dam in the world
to fail, so far.
(Photo ß Mrs.
Eunice Olson,
used with
permission.)

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