CETN-III-4
Revised
3/85
ARMOR UNIT PLACEMENT METHOD
VERSUS
STABILITY COEFFICIENTS
PURPOSE:
This Technical Note provides clarification of the relationship of
placement method to the selection of the stability coefficient KD used in
determining the weight of armor units.
BACKGROUND: A rubble structure is made up of several layers of randomly-
shaped, randomly-placed stones , protected with a cover layer of selected armor
units of either quarrystone or specially shaped concrete units. The stability
coefficient KD varies directly with several of the armor unit characteris-
tics, including roughness of the armor unit surface, angular shape, and degree
of interlocking between units. Of these, the degree of interlocking, which is
of special interest in this note, is the variable most easily altered by the
placement technique.
PLACEMENT METHOD CLARIFICATION: The Shore Protection Manual (SPM 1984) shows
placement as either uniform, special, or random.
Uniform placement is applicable only to concrete armor units and cut or
dressed quarrystones in that they are of a uniform size and shape, and, thus,
lend themselves to a precise, orderly, placement pattern. Since quarrystone
(as opposed to cut stone) is of random size and shape, uniform placement of
quarrystone is impracticable.
Special placement is applicable only to parallel pipe-shaped stone, and
it involves the longest axis being placed perpendicular to the slope of the
structure face. This type of placement was originally used in the Buffalo
District and was subsequently adopted for use in the Pacific Northwest. The
stone produced by local quarries for the Portland District projects have a
unique shape in that they are rectangular solids having a very distinctive
U. S. Army Engineer Waterways Experiment Station, Coastal Engineering Research Center
P. 0. Box 631, Vicksburg, Mississippi 39180
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