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b. Sheet Pile Bulkhead and Relieving Platform. The relieving platform is
used in conjunction with a sheet pile bulkhead to reduce the lateral load
on the sheet piling created by heavy surcharges and earth pressures.
See Figure 2-10 . Lateral restraint is provided by the batter piles
supporting the relieving platform. A variation of this type of construction is
to use only vertical piles for the relieving platform and to furnish an
independent anchorage system consisting of tie rods and deadman,
similar to the types specified for sheet pile bulkheads.
c. Cellular Construction Consisting of Sheet Pile Cells. For design
procedures and selection of type, see UFC 3-220-01N. Cellular structures
are gravity retaining structures formed from the interconnection of straight
steel sheet piles into cells. Strength of cellular structures derives from
interlocks and also from the internal shearing resistance of the fill within
the cells. Accordingly, clean granular fill materials such as sand and
gravel are usually used to fill the cells. Extreme care must be exercised in
the construction of cellular structures because excessive driving onto
boulders or uneven bedrock may cause ruptured interlocks, which can
later unzip under hoop tension (from filling) and cause failures of the cell.
Movement and expansion of cells must be compensated for during
construction of the cells and fill placement carefully controlled if
satisfactory alignment of the face of the wharf is to be maintained.
Cellular structures are classified according to the configuration and
arrangement of the cells. Three basic types are discussed below and are
shown on Figure 2-11.
Circular. This type consists of individual large-diameter circles
connected together by arcs of smaller diameter. Each cell may be
completely filled before construction of the next cell is started.
Construction of this type is easier than the diaphragm type because
each cell is stable when filled and thus may be used as a platform
for construction of adjacent cells. Because the individual cells are
self-supporting units, accidental loss of one cell will not necessarily
endanger adjoining cells. Compared to a diaphragm type cellular
structure of equal design, fewer piles per linear foot of structure are
required. The diameter of circular cells is limited by the maximum
allowable stresses in the sheet pile interlocks and, when stresses
are exceeded, cloverleaf cells are used.