Page 418 - Corrosion Engineering Principles and Practice

386 C h a p t e r 1 0 C o r r o s i o n i n S o i l s a n d M i c r o b i o l o g i c a l l y I n f l u e n c e d C o r r o s i o n 387

• Road culverts
• Steel piling for the foundations of bridges and buildings
• Well and shaft casings.
The equipment in these systems is typically expected to function
reliably and continuously over several decades.

10.2.1 Soil Classification
Soil is an aggregate of minerals, organic matter, water, and gases
(mostly air). It is formed by the combined weathering action of wind
and water, and also organic decay. The proportions of the basic
constituents vary greatly in different soil types. For example, humus
has very high organic matter content, whereas the organic content of
beach sand is practically nil. The properties and characteristics of soil
obviously vary as a function of depth. A vertical cross-section taken
through the soil is known as a soil profile, and the different layers of
soil are known as soil horizons. The following soil horizons have
been classified:

A: Surface soil (usually dark in color due to organic matter)
O: Organic horizon (decaying plant residues)
E: Eluviation horizon (light color, leached)
B: Accumulation horizon (rich in certain metal oxides)
C: Parent material (largely nonweathered bedrock)
The distribution and size of mineral particles in a soil is described
by its texture. Sand (rated from coarse to very fine), silt, and clay refer
to textures of decreasing particle coarseness (Table 10.1). Soils with a
high proportion of sand have very limited storage capacity for water,
whereas clays are excellent in retaining water. A popular soil

Category Diameter (mm)
Sand (very coarse) 1.00–2.00
Sand (coarse) 0.50–1.00
Sand (medium) 0.25–0.50
Sand (fine) 0.10–0.25
Sand (very fine) 0.05–0.10
Silt 0.002–0.05
Clay < 0.002

TABLE 10.1 Particle Sizes in Soil Texture