D 0 + D10
Di (i = 1) =
(3.11)
2
D10 + D 20
Di (i = 2) =
2
D90 + D100
Di (i = 10) =
2
percent, ..., 100 percent of the material (by weight) is finer. Stevens' equation is the
equivalent to utilizing the arithmetic average of the sum of the weights of the individual
particles.
The effective diameter Dm is also used in sediment studies. It is defined by the following
equation:
n
i=1 pi D si
Dm =
(3.12)
100
Where pI is the percentage by weight of that fraction of the sediment with geometric mean
size Dsi. The geometric mean size is the square root of the product of the end points of a
given size range.
3.2.5 Specific Weight
Specific weight is weight per unit volume. In the English system of units, specific weight is
usually expressed in units of pounds per cubic foot and in the metric system, in grams per
cubic centimeter. In connection with granular materials such as soils, sediment deposits, or
water sediment mixtures, the specific weight is the weight of solids per unit volume of the
material including its voids. The measurement of the specific weight of sediment deposits is
determined simply by measuring the dry weight of a known volume of the undisturbed
material.
3.2.6 Porosity
The porosity of granular materials is the ratio of the volume of void space to the total volume
of an undisturbed sample. To determine porosity, the volume of the sample must be
obtained in an undisturbed condition. Next, the volume of solids is determined either by
liquid displacement or indirectly from the weight of the sample and the specific gravity of
material. The void volume is then obtained by subtracting the volume of solids from the total
volume.
3.11
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