Quantcast Design Considerations (Cont.)

Surface Armor for Erosion Protection
would also be suitable. Simplicity and economy of construction will be
enhanced if the same material is used for the connection as for the rest of the
upper bank. An overlap should be provided to ensure that any downslope
movement of the wooden mattress after placement will not result in an
unprotected area of bank.
If significant toe scour is expected, then a wooden mattress should be
supplemented by separate toe protection measures.
Fasteners and connectors should be of materials which are resistant to
corrosion, abrasion, and failure from fatigue due to flexing of the mattress
when subjected to hydraulic forces. Synthetic materials, stainless steel, or
heavily coated metallic components are therefore advisable.
Other major considerations for design are:
An overlap should be provided between adjacent mattresses in order to
compensate for uncertainties in underwater placement and future differential
displacement of the mattresses by hydraulic or geotechnical forces. As an
example, individual wooden mattresses on the lower Mississippi River were
overlapped from a minimum of 5 feet to a maximum of 15 feet with the
adjacent mats. The individual mats were laid from downstream progressing
upstream so that the downstream edge of each mat lay over the upstream edge
of the adjacent mat, so that the upstream edges were not exposed to the flow.
Because wooden mattresses are relatively inflexible, and because shaping them
to irregularities in the bankline is difficult, protruding points and other
irregularities should be removed or smoothed as much as possible during bank
preparation operations, and sunken debris that would interfere with the
mattress making contact with the underwater slope should be removed. This
requirement presents a dichotomy which is a major obstacle to the use of
wooden mattress, since the fact that wooden mattress is durable only when
permanently submerged restricts its use to the subaqueous bank, where
removal of bankline irregularities and debris is most difficult, and in fact is
likely to be impractical at depths greater than ten feet with standard
construction equipment, even if barge mounted.
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