Environmental Engineering Reference
In-Depth Information
FIGURE 7.29
Alluvial fan (Andes Mountains, Peru).
Permeability can be high, particularly in the horizontal direction. In the pastoral zone,
where silty sands are common, high pore-water pressures build up during flood stages,
often resulting in the stability failure of flood-control levees.
Silty sands deposited as valley fill in an arid environment are often lightly cemented
with salts and other agents, and prone to collapse upon saturation following the initial
drying.
Clays
Clay soils are not encountered in the boulder zone and are relatively uncommon in the
floodway zone, except for deposits along the river banks and on the valley floor during
flood stages. In these deposits the soils are usually clayey silts.
In the pastoral zone clays are deposited as clay plugs in oxbow lakes, where they remain
soft, or as backswamp deposits where they are prestressed by desiccation. Even so, they
remain predominantly soft materials. In the backswamp areas, clays are generally
interbedded with silts and sands and are often organic, containing root fibers. The largest
area of fluvial clays in the United States is in the lower portions of the Mississippi alluvial
plain.
