EM 1110-2-1100 (Part III)
30 Apr 02
Chapter III-1
Coastal Sediment Properties
III-1-1. Introduction
a. Bases of sediment classification.
(1) Several properties of sediments are important in coastal engineering. Most of these properties can
be placed into one of three groups: the size of the particles making up the sediment, the composition of the
sediment, or bulk characteristics of the sediment mass.
(2) In some cases (in clay, for example) there are strong correlations among the three classification
groups. A clay particle is, in the compositional sense, a mineral whose molecules are arranged in sheets that
feature orderly arrays of silicon, oxygen, aluminum, and other elements (Lambe and Whitman 1969). Clay
particles are small and platey. They are small in part because they originate from the chemical modification
and disintegration of relatively small pre-existing mineral grains and because the sheet-like minerals are not
strong enough to persist in large pieces. The geologist's size classification defines a particle as clay if it is
less than 0.0039 mm. Because a clay particle is so small, it has a large surface area compared to its volume.
This surface area is chemically active and, especially when wet, the aggregate of clay surfaces produces the
cohesive, plastic, and slippery characteristics of its bulk form. Thus, the three classifications each identify
the same material when describing "clay."
(3) On the other hand, most grains of beach sand are quartz, a simpler and chemically more inert material
than clay minerals. In the geologist's size classification, sand grains are at least 16 times larger and may be
more than 500 times larger in diameter than the largest clay particle (4,000 to more than 100 million times
larger in volume). At this size, the force of gravity acting on individual sand grains dwarfs the surface forces
exerted by those sand grains, so the surface properties of sand are far less important than surface properties
of clay particles. Because sand grains do not stick together, a handful of pure dry sand cannot be picked up
in one piece like a chunk of clay. Several differences between clay and sand are summarized on
Table III-1-1. More inclusive discussions of sediment sizes, compositions, and bulk properties are given later
in this chapter.
Table III-1-1
Relations Among Three Classifications for Two Types of Sediment
Bases of Classification
Name of Sediment
Usual Composition
Size Range, Wentworth
Bulk Properties
Clay
Clay Minerals
Less than 0.0039 mm
Cohesive
(sheets of silicates)
Plastic under stress
Slippery
Impermeable
Between 0.0625 mm and
Noncohesive
Sand
Quartz
(SiO2)
2.00 mm
Rigid under stress
Gritty
Permeable
b. Sediment properties important for coastal engineering. Sediment properties of material existing at
the project site, or that might be imported to the site, have important implications for the coastal engineering
project. The following sections briefly discuss several examples of ways that sediment properties affect
coastal engineering projects and illustrate their importance.
Coastal Sediment Properties
III-1-1
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