Environmental Engineering Reference
In-Depth Information
Figure 3.81,
these clays have very high void ratios, often ranging from 8 to 13, indicating
their high compressibility. Time-rate consolidation curves show a high amount of sec-
ondary compression.
Marsh and Swamp Soils
Characteristics of marsh and swamp soils are variable; rootmats and peats can stand in ver-
tical cuts although they are highly compressible, whereas organic silts flow as a fluid when
they are saturated and unconfined. Their low strengths and high compressibility make
them the poorest of foundation soils, and they tend to become weaker with time as decom-
position continues. Usually highly acidic, they are very corrosive to construction materials.
7.4.6
Marine Environment
General
Origin
Marine deposits originate from two general sources: (1) terrestrial sediments from rivers,
glaciers, wind action, and slope failures along the shoreline; and (2) marine deposition
from organic and inorganic remains of dead marine life and by precipitation from over-
saturated solutions.
Deposition
Sediments of terrestrial origin normally decrease in particle size and proportion of the
total sediment with increasing distance from the land, whereas the marine contribution
increases with distance from the land. The selective effect of currents normally produces
well-sorted (uniformly graded) formations.
The typical distribution off the east coast of the United States is as follows:
•
To depths of about 600 ft, in which sea currents are active, deposits include strata
of sand, silt, and clay; depositional characteristics depend on geologic source,
coastal configuration, and the proximity to rivers.
•
Between 600 and 3000 ft depths, silts and clays predominate.
•
Beyond 3000 ft depths are found brown clays of terrestrial origin, calcareous ooze,
and siliceous ooze (sediments with more than 30% material of biotic origin).
Marine Sands
The composition of marine sands is of major significance to offshore engineering projects.
These are normally considered to be composed of quartz grains, which are hard and vir-
tually indestructible, although the deposit may be compressible. Strength is a function of
intergranular friction.
In the warm seas of the middle latitudes, however, sands are often composed of calcium
or other carbonates with soft grains that are weak and readily crushable. These sands can
include ooliths (rounded and highly polished particles of calcium carbonate in the
medium to fine sizes) formed by chemical precipitation in highly agitated waters, oblong
lumps of clay-size particles of calcite (probably originating as fecal pellets), and sands
composed almost entirely of fossil fragments (coral, shells, etc.) (McClelland, 1972). These
“sands” are often found in layers with various degrees of cementation. The general distri-
bution of these materials on continental shelves worldwide is given in
Figure 7.61.
For the design of foundations for offshore structures in calcareous sands, the frictional
resistance common for a quartz sand of a particular gradation is reduced by empirical
