Place and Time - Sustainable Design: The Science of Sustainability and Green Engineering - page 162

concerned principally with soil mechanics, particularly such aspects as gel strength
and stability, so that it serves a sufficient underpinning for structural foundations
and footings. They are also concerned about drainage, compaction, shrink-swell
characteristics, and other features that may affect building site selection.
Soil is classified into various types. For many decades, soil scientists have strug-
gled with uniformity in the classification and taxonomy of soil. Much of the rich
history and foundation of soil scientists has been associated with agricultural pro-
ductivity. The very essence of a soil's “value” has been its capacity to support plant
life, especially crops. Even forest soil knowledge owes much to the agricultural
perspective, since much of the reason for investing in forests has been monetary.
A stand of trees are seen by many to be a standing crop . In the United States,
for example, the National Forest Service is an agency of the U.S. Department
of Agriculture. Engineers have been concerned about the statics and dynam-
ics of soil systems, improving the understanding of soil mechanics so that they
may support, literally and figuratively, the built environment. The agricultural
and engineering perspectives have provided valuable information about soil that
green designers can put to use. The information is certainly necessary, but not
completely sufficient, to understand how pollutants move through soils, how the
soils themselves are affected by the pollutants (e.g., loss of productivity, diversity
of soil microbes), and how the soils and contaminants interact chemically (e.g.,
changes in soil pH will change the chemical and biochemical transformation of
organic compounds). At a minimum, environmental scientists must understand
and classify soils according to their texture or grain size (see Table 4.1), ion-
exchange capacities, ionic strength, pH, microbial populations, and soil organic
matter content.
Whereas air and water are fluids (see Chapter 2), soil is a matrix made up
of various components, including organic matter and unconsolidated material.

Table 4.1 Commonly Used Soil Texture Classifications

Name Size Range (mm)

Gravel > 2.0

Very coarse sand 1.0-1.999

Coarse sand 0.500-0.999

Medium sand 0.250-0.499

Fine sand 0.100-0.249

Very fine sand 0.050-0.099

Silt 0.002-0.049

Clay < 0.002

Source : T. Loxnachar, K. Brown, T. Cooper, and M. Milford,
Sustaining Our Soils and Society , American Geological Institute,
Soil Science Society of America, and USDA Natural Resource
Conservation Service, Washington, DC, 1999.