EM 1110-2-1100 (Part III)
30 Apr 02
e. Bulk properties of different sediments.
(1) Clays, silts, and muds.
(a) Coastal engineers typically encounter clay as a foundation material or as a material to be dredged.
The flat topography of coastal plains and the quiet waters of bays and lagoons are often underlain by clay.
Some older clays are consolidated and can stand with near-vertical slopes when eroded. For example, the
deepest parts of tidal inlets may have steep sides cut into stiff clay. Many eroding coastal flats contain much
clay. Revetments laid on these clays, or sheet piles driven into them, need particular attention; such design
may require the advice of a geotechnical engineer.
(b) A silt-sized particle is intermediate between sand and clay. Most silt is produced by the gradual
chemical weathering of rocks, but some silt is rock flour ground out by glaciers. Sediments consisting mostly
of silt are common in deltas, estuaries, and glacial lakes, but are relatively uncommon on beaches where
waves drive the dominant processes. Silt remains in suspension far longer than sand grains, so it is easily
removed from any shore where wave action is moderate or severe. A mass of pure silt differs from a mass
of clay in that, when dry, the silt has very little cohesion and will easily fall apart, whereas clay will cohere
like a brick.
(c) Muds are watery mixtures of clay and silt, typically in approximately equal proportions, often with
minor amounts of sand and organic material. Muds act more as a viscous fluid than as a cohesive solid. They
have coastal engineering importance because muds often accumulate in dredged channels where the upper
and lower boundaries of the mud layer can be difficult to determine. Maintenance dredging of channels that
accumulate mud requires particular attention, especially a clear definition of the material to be dredged.
(d) Some deposits of mud lie offshore of the coast, where the mud is believed to modify the transmission
of water waves. See, for example, Jiang and Zhao (1989); Shen, Isobe, and Watanabe (1994); and deWit,
Kranenburg, and Batljes (1994) for introduction to recent work. Such mud deposits have been reported at
the mouth of most large Asian rivers, off the Gulf Coast of Louisiana, northeast of the mouth of the Amazon
River, and seaward of the Kerala district in India. These mud deposits are believed to oscillate with the
passage of water waves above them, absorbing energy from the waves and reducing their height.
(2) Organically bound sediment.
(a) Marsh grasses growing in back bays and other tidal wetlands bind sands, silts, and clays with their
root systems to form an organically bound sediment, sometimes called peat (typically, organic silt in the
engineer's soil classification). These sediments are very compressible; when overlain by a barrier island, field
evidence indicates that the compression of organic matter by the weight of the sand results in subsidence of
the barrier island. Cores illustrating the vertical sequence of organic sediment overlain by barrier island sands
have been shown in many geological studies (e.g., Figures 9 and 15 of Kraft et al. (1979)). Shore erosion can
expose the organically bound sediment to ocean waves. Erosion by ocean waves produces pillow- or
cobble-shaped fragments of organically bound sediment, often found on barrier island coasts after storms.
Coastal Sediment Properties
III-1-31
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