EM 1110-2-1100 (Part III)
30 Apr 02
porosity decreases as the sand becomes better graded. An increase in the irregularity of the grain shapes tends
to decrease the volume concentration. That is, an irregular grain of longest dimension D usually occupies
almost as much space as a sphere of the same diameter D.
(4) For uniform spheres the loosest packing arrangement is cubic, yielding P = 0.476, while the densest
is rhombohedral, yielding P = 0.260. Natural sands have porosities in the range 0.25 < P < .0.50. Blatt,
Middleton, and Murray (1980, Chapter 12) and Terzaghi and Peck (1967) showed that for laboratory studies
of sands with gaussian size distributions, porosity decreased from 42.4 percent for extremely well-sorted
sands to 27.9 percent for very poorly sorted sands, with no clay matrix. Chamberlain (1960) and Dill (1964)
measured volume concentration of the fine sand on the beach and around the head of Scripps Canyon in
Southern California. When compacted by vibration to its densest packing, the sand had a porosity of 0.40.
In situ measurements from the beach yielded P = 0.42, while measurements from offshore near the canyon
head yielded P = 0.50, and some micaceous (very irregular) sands from the canyon yielded P = 0.62.
(5) Good porosity data are not often available. The standard assumption in longshore transport
computations is that sand has a porosity of 0.40, although there are likely to be significant variations from
that figure, as discussed by Galvin (1979).
b. Bulk density. Density was defined in Part III-1-3-b as relating to a particle itself. Bulk density refers
to a group of particles. Dry bulk density is the mass of an aggregation of grains divided by the volume of the
grains (solids) plus the volume of the pore spaces. That is:
Dry Bulk Density ' N ρs
(III-1-14)
Saturated bulk density is the mass of an aggregation of grains plus the mass of the interstitial water divided
by the volume of the sample. That is:
Saturated Bulk Density ' (N ρs) % (P ρ)
(III-1-15)
The dry bulk density is never greater than the grain density, and the saturated bulk density is only greater than
the grain density if the interstitial fluid is more dense than the grains (if the grains float). Table III-1-6 lists
typical bulk quantities for several sediments that are useful for coastal engineering computations. Table III-1-
6 contains three parts: A typical engineering data; B saturated densities of naturally occurring surficial
soils, along with porosity information; and C dry densities of synthetic laboratory soils. Comparison of the
two columns of data in Part C of Table III-1-6 gives an idea of the consolidation to be expected from settling,
and a minimum estimate of the "bulking" of newly placed dry material.
c. Permeability. Permeability is the ability of water to flow through a sediment bed, and is largely a
function of the size and shape of the pore spaces. Several aspects of this flow are of interest to coastal
engineers. Flow into and out of the bed is one source of energy dissipation for waves traveling in shallow
water (see Reid and Kajiura (1957), Packwood and Peregrine (1980)). Permeability is also a major factor in
determining the steepness of the foreshore. Sediment is carried shoreward during the wave uprush in the
swash zone. The permeability of the swash zone helps control how much of this water returns to the sea on
the surface (above the bed) and how much returns through the bed. The surface backrush will transport
sediment seaward, decreasing the equilibrium foreshore slope (Savage 1958; McLean and Kirk 1969;
Packwood 1983). Just seaward of the breaker zone recent studies have suggested that even small amounts
of wave-induced flow into and out of the bed have a major effect on the bottom boundary layer and the
resulting sediment transport (Conley and Inman 1992).
Coastal Sediment Properties
III-1-27
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