28 July 2005
Dynamic Fill Loads.
In general, piles subjected to seismic forces behave as flexible members and their
behavior is controlled primarily by the surrounding soil. Both vertical and batter piles
move together with the surrounding soil during an earthquake. Provided that shear
failure or liquefaction of the surrounding soil does not occur during ground shaking,
the pile-supported structure will move a limited amount and remain stable after an
earthquake. The magnitude of the horizontal movement depends on the earthquake
magnitude and duration, design details of the platform, flexibility of the piles, and the
subgrade modulus of the foundation soil. If the soil surrounding the piles is
susceptible to liquefaction or if slope failure occurs, the piles will move excessively,
resulting in serious damage to the piles and the structure. For these conditions,
remove and replace unstable materials. When the piles penetrate a deep soft layer
first and then a stiff layer of soil, the soils displace cyclically back and forth during an
earthquake. During the cyclic ground shaking, the piles will move with the ground
and return essentially to their original position if the soil does not fail during these
cyclic displacements. Accordingly, if piles are to continue to safely support loads
after an earthquake, it will be necessary for the piles to have the capability to
withstand the induced curvature without failure.
Embankments and Fills.
For determining the stability of embankments and fills at solid wharves, when
subjected to earthquake forces, refer to UFC 3-220-01N.
Usually, floating structures are not directly affected by seismic events. However,
waves created by offshore seismic activity such as a seiche or tsunami will affect
floating structures. Also, the mooring system employed (spud piles and chain) will
be subjected to the ground motions and should be investigated.
Earth and Water Pressures on Retaining Structures.
Static earth pressures, acting on retaining structures, are determined in accordance
with the criteria detailed in UFC 3-220-01N.
Seismic forces may cause increased lateral earth pressures on earth retaining wharf
structures accompanied by lateral movements of the structure. The degree of
ground shaking that retaining structures will be able to withstand will depend, to a
considerable extent, on the margin of safety provided for static loading conditions.
In general, wharf retaining structures, designed conservatively for static loading
conditions, may have a greater ability to withstand seismic forces than those
designed, more economically, by less conservative procedures. Methods for