Surface Armor for Erosion Protection
7.4.1 CONCRETE BLOCK MATTRESS
The advantages, disadvantages, and typical applications of concrete blocks as armor
were discussed in Section 7.2.1. Some additional considerations which apply to the use of
concrete blocks in mattress form are as follows:
Mattresses provide a higher degree of safety from progressive failure of the
armor due to displacement of individual blocks from hydraulic or geotechnical
forces or vandalism.
Placing of mattresses is more mechanized and less labor intensive than placing
individual blocks.
Some commercial mattresses incorporate an engineering fabric, which will
eliminate the need for a separate filter layer under some conditions.
Precast concrete blocks can be formed into a flexible mattress in several ways: by
fastening them to engineering fabric, by fastening them together with cable or synthetic rope,
or by forming them in ingenious shapes which are then interlocked. All of these varieties are
commercially available.
7.4.1.1 Design Considerations
Concrete block mattress will usually withstand hydraulic forces greater than an equal
thickness of riprap. However, all designs are not equal, and manufacturers being considered
as a source for a specific project should be asked to furnish evidence of adequacy.
The most conservative design approach, which would be especially appropriate for
areas of high turbulence and areas where waves create the critical loading, is to ignore any
extra uplift resistance which is provided by the blocks being attached together. This extra
resistance would be assumed to be a safety factor, rather than being taken into account when
selecting a block size for hydraulic loading. The rationale is that the "pumping" action
created by even a small amount of uplift of the blocks might result in loss of bank material or
failure of the mattress connections or bonding system.
Anchoring the mats to the bank slope is usually recommended. This should not be
considered as adding to the geotechnical stability of the bank, but rather as providing a margin
of safety from mat displacement if small slope movements occur.
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