Lessons Learned - Tohoku, Japan Earthquake and Tsunami of 2011 - page 66

4.4 Foundations
Foundation failures occurred throughout the area, often in instances where the
tsunami pushed structures off their foundations. Minimal foundation connectors
contributed to a number of the failures, and many gravity structures lacked
mechanical foundation connectors of any kind. The lateral forces from the incoming
waves were sufficient to dislodge segments of barrier walls in several locations; in
the absence of barrier walls, water rushed through the opening and inundated inland
areas. Both lateral and rotational movement occurred in areas that had experienced
scour. At the offshore breakwater at Kamaishi, the tsunami pushed many of the
caisson units from their foundation (through both lateral movement and rotation). The
breakwater continued to provide some protection to the inland areas, but the cost to
repair or rebuild this recently completed $2.5 billion structure (¥200 billion) will
prove extreme.
4.5 Structural Connections
The tsunami leveled many seawalls assembled in sections. The segmented design is
quite useful for a project undertaken in phases or in a multiyear effort. The contractor
can fabricate wall segments off site and bring them to the site or store them near the
wall until equipment is available to assemble a new section. The O'Shaughnessey
Seawall in San Francisco, California, and the Galveston Seawall in Galveston, Texas
are two prominent examples of segmented shore protection. However, each
connection may become the weak link in the overall wall design. Several failures
occurred at the junctions between segments; in some instances, small lengths of
smooth reinforcing bars appeared to serve as the only mechanical connections
between units. Strong segment connections are important if the wall is to function as
a continuous unit.
4.6 Overtopping and Supercritical Flow
The extreme tsunami waves overtopped most coastal barriers along the coast in
Northeast Japan. The height of the structures caused supercritical flow inland of the
structure, with increased velocity and turbulence that accompanies this flow condition.
Collapse of the roadway inland of the Fudai waterwall provided an indication of the
forces that developed on the inland side of this structure. Overtopping is one of the
consequences of an event exceeding the design of the barrier wall or of the
community defensive plan and it should be considered within project design and
contingency planning. If left unaddressed, the resultant supercritical flows and back
scour can add to the inland hazards that result from overtopping alone or from
inundation by structural collapse.
4.7 High Profile and Low Profile Bridge Structures
Low profile and high profile structures responded differently to the tsunami. Low
profile structures consistently featured mechanical attachments to their supports or
abutments —many of these low profile structures remained in place. Also, they
experienced limited, short-term exposure to uplift forces lasting only until the
structures became submerged. Oncoming waves exposed these structures to high
impact loads; but, again, these were most significant while flow levels were close to
the structure elevation. Higher structures, often seemed to be seated into platform