Page 182 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Fabric and Structure
Soil Fabric and Structure 177

Macrostructure that is missed by soil borings becomes a matter for speculation,
inference, and expectation. For example, vertical ground cracks that open during
a dry season are macrostructure that will be missed by borings but nevertheless
are readily observable by an experienced field person who knows what to look for.

A failure to recognize megastructure can lead to embarrassment and possible
legal difficulties. The engineer who fails to take notice of shrinkage cracks,
sagging ground, a landslide in the back yard, or distressed structures across
the street, may be forced into a learning experience. Attempts to conceal trouble-
some megastructure such as a landslide at best is gross ignorance, and at worst,
fraud.

Some common descriptive terms for soil macrostructure are listed in Table 8.2.
The engineer will be familiar with these terms and know their significance.

Problems

8.1. Whereas the soil fabrics shown in Figs. 8.1 and 8.3(a) show grains touching,
in an actual cross-section obtained by sawing across a soil specimen only a
few will touch. Explain.
8.2. What is the meaning and significance of thixotropy?
8.3. Give an example of a mechanically dispersed clay and a chemically
dispersed clay.
8.4. What is the X-ray diffraction spacing of a sodium smectite dispersed in
water? Of a flocculated calcium smectite in water?
8.5. What is the difference between a quick clay and a dispersive clay? Which
might be most likely to influence a landslide? The stability of a levee? Of a
retaining wall where pressure is lowered as soil is partly restrained as a
result of its internal shearing resistance?
8.6. Explain the geography of quick clay, taking into account that sea level rose
over 100 m from melting of continental glaciers.
8.7. Explain why the last of a convoy of military vehicles passing over wet soil is
more likely to get stuck than the first vehicles in the convoy.
8.8. What is meant by overcompaction?
8.9. Muddy water is observed flowing from a leak on the downstream face of a
large earth dam. Consider and discuss consequences from the following
options:
(a) Run for high ground.
(b) Try to seal the leak by boring and grouting.
(c) Draw down the reservoir.
(d) Dump soil into the cavity eroded by the leak.
(e) Dump coarse granular material into the cavity.
(f) Inform the authorities and warn of the possible consequences.

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