scope:

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.