industry over critical velocity because the critical
shear is relatively constant for a product.
Critical velocity varies depending on flow
characteristics (such as depth and turbulence); so
that the critical velocity in the flume test is not
comparable to the required critical velocity in the
field.
The surface void area ratio, DCF, and curvature
radius can be used to specify products best suited
to placement on curved surfaces, and turf
establishment. Some blocks are available with open
cells or solid cores. Most products marketed
specifically for ACB revetments are within
reasonable limits for general use.
Modifying the requirements for a performance
specification may be considered for small low hazard
projects or negotiated contracts, and may be
preferred by some manufacturers. However,
performance requirements for firm fixed price (low
bid) contracts is not recommended for ACB's because
of the price level jumps between product sizes, and
because there is no control in how bidders
incorporate risk for a low frequency design event.
Interface friction for soil/fabric and fabric/blocks
should be considered by the designer. Because
typical design values for soil/fabric are available
in geosynthetic design guides, and because the
Contractor generally has no control over the soil,
the soil/fabric interface friction angle is not
addressed in the specification (although for certain
critical applications, this may need to be added to
verify design assumptions). Interface friction for
fabric/blocks is addressed in the specification
because it is affected by geotextile and block
combinations, it can affect installation, and
because the interface friction for fabric/block is
highly variable dependent on manufacturing
characteristics of both products.
**************************************************************************
The ACB shall meet the following criteria:
TABLE 1.
ACB Requirements
Criteria
Required Value
Test Method
Matrix Assembly: Interlocking Blocks / Cabled System
Thickness, minimum
[100 mm]
N/A
[Net Weight/Area, minimum
1.4 kN/m2
Note a.]
Critical Shear Stress, minimum
[170 N/m2]
FHWA RD-89-199
[Critical Velocity, minimum
5 m/sec
FHWA RD-89-199]
[Curvature Radius, maximum
1 meter
Note b.]
[Surface Void Area Ratio
25% - 40%
Note c.]
[Drainage Correction Factor (DCF)
20% - 35%
Note d.]
Block/Geotextile Interface
Friction Angle
[35 degrees]
Note e.
SECTION 02382
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