Page 303 - Geotechnical Engineering Soil and Foundation Principles and Practice

Compaction
298 Geotechnical Engineering

Figure 13.15
Controlling the
volume change of
expansive clay by
adjusting the
compaction
moisture content.
(After Seed and
Chan, 1959, with
permission of the
American Society
of Civil
Engineers.)

Seasonal moisture changes in expansive clay can be reduced by (a) periodically
watering around the outer perimeter of a house or other structure, (b) extending a
concrete slab or membrane horizontally outside of the structurally loaded areas,
or (c) installing a vertical moisture barrier around the perimeter. Complete
isolation is difficult because of the high suction forces exerted by dry expansive
clay and the disruption caused by shrinkage cracking. In cooler climates such as in
Canada, interior heating may drive moisture out of expansive clay during the
winter, causing shrinkage that is very difficult to control.
Because some variations in moisture content are unavoidable, another option
that may be economical for small structures is to support them on a structurally
designed slab-on-grade that can bridge between support areas. Larger structures
can be supported on deep foundations that extend below the active zone
of the clay, in which case floor joists are supported on bails of straw or cardboard
‘‘crush boxes’’ so that the beams will not be lifted off the foundations, and
floors are structurally supported high enough off the ground to allow for clay
expansion.

A procedure that should not be overlooked is to replace the upper layer
of expansive clay with a soil that is nonexpansive and treat it with hydrated lime.
Treatment of the upper 1 ft (0.3 m) can reduce expansion, of the upper 2 ft (0.6 m)
can reduce it even further, and of the upper 3 ft (1 m) may bring it under control.

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