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Surface Armor for Erosion Protection
Design considerations are: Careful attention to geotechnical stability of the
bank, provision for overbank and internal bank drainage, and toe protection
is especially critical for rigid armor. Flexible or self-launching toe protection
is appropriate in many cases, such as in larger channels where dewatering
during construction is impractical and significant toe scour during high flows
is expected.
Asphalt is available in three forms: Pure asphalt, which can be mixed with soil or
other aggregate and spread on the bank; cutback asphalt, which is pure asphalt mixed with
solvent; and asphalt emulsion, which is pure asphalt mixed with water and an emulsifying
agent. The generic term "asphalt" applied to bank stabilization usually infers pure asphalt.
The other two types can be used in the same manner as chemical soil stabilizers; that is, by
being sprayed directly onto the bank and allowed to penetrate the soil before hardening into
a cohesive mass. The properties of an emulsion can be varied by using various emulsifying
Asphalt mixes with a high sand content are sometimes used to retain some
permeability to relieve hydrostatic pressure. However, these mixes have been reported to
become more brittle and less permeable upon long exposure to the elements, and weathering
may result in a slow loss of thickness.
The use of asphalt placed underwater on the Lower Mississippi River was
discontinued because of problems with placement control and inconsistent performance, and
as a result of the development of an efficient and effective articulated concrete mattress.
However, it should be noted that the Lower Mississippi River presents extremely difficult
construction conditions, with high velocities, great depths, and steep underwater slopes.
On slopes above water, concrete can be placed in the conventional manner with forms,
or can be pumped into fabric mattresses which serve as forms for a fine aggregate concrete.
Prefabricated slabs may be the least costly alternative for some sites. An armor of relatively
small slabs would assume some of the characteristics of concrete block armor (see 7.2.1).
Fabric mattresses are the preferred method for underwater placement, and are
available in various configurations. The appropriate design for a given application will depend
on the need for relief of hydrostatic pressure, the design velocity, and the preferred roughness
characteristics. Some mattresses are described as being flexible by the manufacturer, although
this description should be objectively examined by the project engineer if flexibility is a critical
factor for a specific project. Section 7.4.2 below provides further discussion under "Fabric
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