level of sand on either side of the groin, or ice loading, timber and
generally steel sheet-pile groins require the structural support of longer,
round timber piles driven to a greater depth.
Where ground penetration would be inadequate to support piling, or where
piling is uneconomical, gravity structures must be used. The most common
type is the rubble-mound groin consisting of an impermeable core protected
by armor stone and sometimes sealed with concrete or asphalt grout. Rubble-
mound structures are discussed in detail in the SPM. Another type is the
cellular sheet-pile groin which is a series of chambers built of steel
sheet-piles driven to shallow penetration, then filled with sand or rock
topped by wave-protection armor stone or paving, Cellular sheet-pile
structures are discussed in CETN-III-7 (Bulkheads).
Scour is usually
Scour of the bottom alongside a groin must be controlled.
concentrated at the seaward end and on the downdrift side.
Scour can re-
duce the effective soil support for pile groins until they tilt, and scour
can also undermine the toe of gravity structures until the armored surface
The effects of erosion at the landward end
unravels and exposes the core.
can be reduced by tying the groin into the dune, bank, or bulkhead.
The
seaward end can be protected with quarrystone scour aprons placed on geo-
technical filter fabric, crushed stone, or coarse sand.
MATERIALS:
Rubble-mound groins are relatively damage resistant and easily
repaired; but, being the heaviest of the types, may produce foundation set-
Sheet-pile cells
tlement leading to unacceptable loss of structure height.
Sheet-pile groins of wood
are also heavy structures subject to settlement.
require a supporting framework of wales and structural piles; whereas,
groins of steel and concrete generally do not. Timber must be pressure-
treated with preservatives to prevent rot in freshwater, and fouling and
Steel piling, in the form of cells, are
other infestations in saltwater.
used where the soil is dense or contains rocks or other hard materials. To
prevent the corrosion of steel in seawater, appropriate protective
coating
can be applied or special corrosion resistant steel can be used.
DESIGN
CONSIDERATIONS:
1 . A groin, or a groin system, can provide a recreational and/or protective
beach only if waves and currents transport sufficient quantities of sand
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