Environmental Engineering Reference
In-Depth Information
case, the underlying soft gray organic silt with shell fragments
(Figure 7.14)
(alluvium) is
relatively underconsolidated because of artesian pressures in the underlying sand stra-
tum. Foundations supported in the upper strong soils could be expected to undergo severe
settlements, and possibly even failure by the rupture of the soft soils. The hills at this par-
ticular location are over 1500 ft distant, making surface detection very difficult. These con-
ditions are common in Brazil along the coastal mountain ranges. It is also common to find
large boulders buried in the alluvium along the coast adjacent to granite mountains.
Another case in different regional conditions is described by D'Appolonia et al. (1969).
The colluvium originated as a residual soil derived from horizontally bedded shales and
claystones in Weirton, West Virginia. Carbon dating of the underlying soils yielded an age
of 40,000 years. The ancient slide was determined to be 2.5 mi in length, rising 200 ft in ele-
vation, and extending laterally for about 1000 ft. Stabilization was required to permit exca-
vations at the toe of the slope to heights as great as 60 ft (see
Figure 9.109).
7.4
Alluvial Deposits
7.4.1
Fluvial Environment
General
Fluvial refers to river or stream activity. Alluvia are the materials carried and deposited by
streams. The stream channel is the normal extent of the flow confined within banks, and
the floodplain is the area adjacent to the channel which is covered by overflow during
periods of high runoff. It is often defined by a second level of stream banks. Intermittent
streams flow periodically, and a wash, wadi, or arroyo is a normally dry stream channel
in an arid climate.
Stream Activity
The elements of stream activity are erosion, transport, and deposition.
Erosion
Erosion is caused by hydraulic action, abrasion, solution, and transport. It occurs most sig-
nificantly during flood stages; banks are widened and the channel is deepened by scouring.
Transport
The greater the stream velocity, the greater is its capacity to move materials. Under most
conditions the coarser particles (cobbles, gravel, and sand) are moved along the bottom
where velocities are highest, forming the bed load materials. The finer materials (clay, silt,
and sand, depending on velocities) are carried in the stream body and are referred to as
suspended load materials.
Deposition
As velocities subside, the coarser particles come to rest at the bottom or settle out of sus-
pension. The finer particles continue to be carried until a quiet water condition is reached
where they settle slowly out of suspension. Relationships between erosion, transport, and
deposition in terms of flow velocity and particle size are given in
Figure 7.15.
