Lessons Learned - Tohoku, Japan Earthquake and Tsunami of 2011 - page 67

holds, without adequate connections to allow structures to resist vertical uplift or
buoyant forces. This lack of connection, combined with the increased potential uplift
force for high structures, led to large losses of elevated bridges.
4.8 Vertical Uplift Forces
Vertical uplift causes a great deal of structural damage. Large forces from flow under
bridges caused widespread damage in places that lacked adequate connectivity
between cross members and supports. In addition, many of the shoreline protection
walls used concrete panels over compacted earth fill, and a buildup of excess pore
pressure in the earthen core caused unit displacement. Such displacement likely
resulted from inadequate drainage through the structure and poor connections
between armor units.
4.9 Armor Units
Shore and scour protection in Japan relies heavily on concrete armor units. Only a
small percentage of the little quarry stone available in Japan seems to go to the
creation of revetment structures. The wide variety of concrete units provides an
opportunity to compare the performance of different armor unit designs under similar,
real-world conditions. At Oya Station, the tsunami dislodged tetrapod units from a
revetment structure while “beehive” units in an adjacent revetment remained in place.
The performance difference warrants investigation into the stability coefficients of
various armor units as well as their performance under controlled conditions in a
wave tank or through monitored field investigations.
4.10 Subsidence
Subsidence occurred on several scales. On a local scale, subsidence — most
noticeable on large expanses of paved area, such as in ports and parking lots — was
due to soil liquefaction, differential and lateral spreading. Adding soil amendments,
removing and re-compacting the substrate, surcharging the area, and providing
adequate drainage can remedy local subsidence. If left uncorrected, similar
subsidence problems can occur in the future.
On a regional scale, long sections of the Northestern Japanese coast dropped in
elevation due to seismic activity. This large-scale subsidence converted the tidal flats
at Gamo to an open water area. The City of Onagawa now regularly experiences
overwash of the harbor quays during daily high tides. Such subsidence provides a
preview of the future coastal problems from rising sea level. Unfortunately,
subsidence areas will remain subjected to inundation from rising sea level — co-
seismic subsidence does not replace but rather adds to rising sea levels. Regional
subsidence or uplift requires a reassessment of flood and inundation risks throughout
the region. Actions to address the consequences of regional subsidence can draw
from the options for adapting to rising sea level. At Goma, options could include
augmenting the natural sediment supply to enable the flooded tidal flats to gain
elevation so that the land area is again within the intertidal zone. For communities
such as Onagawa, options would include elevating key roadways and utility corridors
above the reach of daily or maximum tides, changing land use practices to conform to
the newer land elevation, elevating the floor level of new structures, or building