Environmental Engineering Reference
In-Depth Information
Uplands
Coastal plain
Fall line
Cuestas
Barrier beach
Terraces
Tidal marsh
Ocean
Precambrian
crystalline
rock
Tertiary and
cretaceous
sediments
FIGURE 7.50
Schematic of coastal plain in mature stage of development, generally representative of the Atlantic coastal plain.
older beds of Cretaceous and Tertiary periods to surface as long narrow bands trending in
a northeast-southwest direction. These older beds include alternating thick strata of clays,
marls, and sands, with clayey soils and marls the dominant materials near the surface.
The term marls is used by geologists to refer to loose earthy materials that engineers call
soils. They consist chiefly of calcium carbonate mixed with clay. The carbonate content is
readily detectable by its effervescence in hydrochloric acid. Sandy marl contains grains
of quartz sand or other minerals; shell marl is a whitish material containing shells of organ-
isms mixed with clay; greensand marl contains many grains of glauconite (a hydrous sili-
cate of iron and potassium colored bright green on freshly exposed nodule surfaces).
Tertiary soils overlie the Cretaceous clays and often consist of well-defined interbedded
layers of brightly colored sands and clays (
Figure 7.51
and
Figure 7.52)
.
Late Tertiary and Quaternary sands and gravels cover much of the coastal plain region.
They are the most recent deposits (except for recent alluvium and swamp soils).
Composed chiefly of gravelly soils and more resistant to erosion than sands and clayey
sands, they cap the hills that are characteristic of the seaward portions of the region. There
are remnants of wave-cut terraces in New Jersey, but terrace features are much more evi-
dent in the coastal regions of Maryland, Virginia, and North Carolina.
Engineering Characteristics
Selected boring logs from several locations are given in
Figure 7.53.
In general, the soils of
the Atlantic coastal plain do not present difficult foundation conditions. The clays are
overconsolidated and of low activity, and the Quaternary formations provide sources of
sand and gravel borrow.
Gulf Coastal Plain
Soil conditions can be troublesome to construction in the East Gulf and West Gulf sections.
Cretaceous formations, including the Selma chalk of Alabama and the Austin chalk and
Taylor marl of Texas, have weathered to produce black plastic clays. The marine clays of
the outer coastal plain in Texas and Louisiana are of the Beaumont formation.
The most important characteristics of these clays are their high activity and tendency to
swell. Particularly in the western portions of the region, where seasonal dry periods are
common, these clays are active, undergoing large volume changes by shrinking and
