EXAMPLE PROBLEM III-1-2

FIND:

The density, specific weight, and specific gravity of a sediment sample.

GIVEN:

18.1 grams of the sample of dry beach sand exactly fills a small container having a volume of

10.0 cm3 after the filled container is strongly vibrated. When this amount of sand is poured into

50.0 ml of water which is subjected to a strong vacuum, the volume of the sand-water mixture is

56.8 ml.

SOLUTION:

It is important to recognize the difference in the volume of the grains themselves, which is

6.8 cm3 (= 56.8 - 50) (a milliliter is a cubic centimeter), and the volume of the aggregate (the grains

plus the void spaces), which is 10 cm3.

Density is the sediment mass divided by its volume:

ρs = 18.1 gm / 6.8 cm3 = 2.66 gm/cm3 = 2,660 kg/m3

If this problem were in English units rather than metric, the sediment weight (in pounds force) would

probably be given, rather than the mass. To obtain the mass, the weight would need to be divided by

the acceleration of gravity (32.2 ft/sec2) (mass = weight / g). After dividing by the volume (in ft3), the

density would be obtained in slugs/ft3.

The specific weight of the sand grains themselves is the density times the acceleration of gravity:

Sp wt of the grains = 2.66 gm/cm3 * 980 cm/sec2

= 2,610 gm/(cm2 * sec2) = 26,100 kg/(m2 * sec2)

The specific gravity of a material has the same value as its density when measured in gm/cm3, because

the density of water at 40 C is 1.00 gm/cm3:

Specific Gravity = 2.66

(1) Grain shape is primarily a function of grain composition, grain size, original shape, and weathering

history. The shape of littoral material ranges from nearly spherical (e.g. quartz grains) to nearly disklike (e.g.

shell fragments, mica flakes) to concave arcs (e.g. shells). Much of the early work on classifying sediment

particle shape divided the problem into three size scales; the sphericity or overall shape of a particle, the

roundness or the amount of abrasion of the corners, and the microtexture or the very fine scale roughness.

These differences can be illustrated by noting that a dodecahedron has high sphericity but low roundness,

while a thin oval has low sphericity but high roundness. A tennis ball has greater micro-texture than a

baseball. More recent approaches to the quantification of grain shape have avoided the artificial division of

shape into sphericity, roundness, and microtexture by characterizing all the wavelengths of the grain's

Coastal Sediment Properties

III-1-19

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