CETN-III-7
3/81
Cantilever bulkheads are the most susceptible to failure by toe scour
because of their sole dependence on ground penetration for lateral support.
Anchored and braced bulkheads, because they are better supported and better
able to resist lateral earth pressures, are the type most commonly used in
the coastal zone. Anchors may be buried horizontal beams or plates, as
shown in Figure 2, or vertical piles, as shown in Figure 3. Cells and
rock-filled cribs may be used where foundation conditions will not eco-
nomically permit adequate penetration to develop support for anchored or
braced sheet-pile bulkheads.
Both require banks that are stable with the
gravity structure in place, and foundation soil that is safe against un-
acceptable settlement.
Cells and cribs must be designed to prevent racking.
(the flattening of the structure under lateral forces). Cribs, with limited
resistance to rotation and sliding, usually are restricted to use where
the level of fill behind the bulkhead is about equal to the height of the
bulkhead.
All three types of bulkheads both reflect wave energy and increase the
water velocities, which may increase the scour at the toe.
Scour wi11 re-
duce the piling's depth in the ground and effective soil support, and
undermine the base of gravity structures; therefore, toe protection should
be an integral part of bulkheads. Typical toe protection, shown in
Figures 2, 3, and 4, consists of quarrystone large enough to resist move-
ment by wave forces, with an underlying layer of granular material and/or
geotechnical filter fabric to prevent soil from being washed through
voids in the scour apron.
Sheet-pile structures must be designed to have
adequate penetration both for the expected lateral loads and any expected
scour. To prevent undermining, gravity structures may require a sheet-
pile cutoff wall as well as an apron, as shown in Figure 4.
Tides, storm surges, and overtopping can rapidly raise the ground water
This head drop across the bulkhead wi11 in-
level in the retained soil.
crease the hydrostatic force on the bulkhead, and induce seepage under
the bulkhead which may contribute to scour of the soil at the Toe. Addi-
tional drainage through the face of the bulkhead is provided with naturally
permeable rock-filled cribs, but, with pile structures, drainage holes
should be added. The pore water pressure due to excess accumulation of
water can be controlled by paving the ground surface behind the bulkhead,
5
Previous Page
