3.7 Summary

Deleterious effects of liquefaction were major contributors to the vast amount of damage inflicted by the 1995 Hyogoken-Nanbu earthquake. In particular, port facilities were severely impacted by liquefaction-induced ground displacements. These ground movements pervasively displaced quay walls seaward by an average of 2 m to 3 m and created grabens 2 m to 4 m deep landward from the walls. The damage generated from these ground disturbances effectively shut down the Port of Kobe to shipping; only 6 of about 187 shipping berths were operational after the earthquake. Most of these berths were located around the margins of Port and Rokko islands (filled islands within Osaka Bay) and along filled segments of the Kobe shoreline. Most of the fill in these areas consisted of poorly compacted decomposed granite that was excavated from borrow areas in the Rokko Mountains. This material was transported to the fill sites and loosely dumped in water locations to form the "made-ground." Compaction was generally only applied to materials placed above water level. Liquefaction occurred pervasively within underwater segments of these poorly compacted fills. Around the exteriors of islands and along the Kobe waterfront, liquefaction generated lateral displacements that devastated the port facilities as noted above. This destruction graphically illustrates the vulnerability of waterfront structures constructed on loose, saturated soils to the severe consequences of liquefaction, as well as the need to improve the ground under such important facilities to prevent these deleterious effects.

Within the interiors of the man-made islands, liquefaction led to pervasive eruption of sand boils and to rather uniform ground settlement averaging about 0.5 m. The ground settlement caused surprisingly little damage to high- and low-rise buildings, bridges, tanks and other structures supported on deep foundations. These foundations, including piles and piers, performed very well in supporting superstructures where ground settlement was the principal effect of liquefaction. Where liquefaction generated lateral ground displacements, such as near island edges and in other waterfront areas, foundation performance was typically poor. Lateral displacements fractured piles and displaced pile caps, causing structural distress to several bridges, a few of which collapsed. In a few instances, such as the Port Island Ferry Terminal, strong foundations withstood the lateral ground displacement with little damage to the foundation or the superstructure.

Shallow foundations consisting of a grid of interconnected perimeter-wall footings and grade beams performed well in several areas subjected to minor effects of liquefaction, such as eruption of sand boils, ground oscillation, and minor lateral spread. In these instances, the foundation behaved as a diaphragm, preventing differential ground displacements from propagating upward into the superstructure. Most of these structures were small buildings, such as houses and low-rise commercial and industrial facilities. Where foundation elements were not well tied together, differential ground displacements pulled apart overlying structures at points of weakness, such as joints and doorways.

Although hidden from view, and thus not widely examined during this reconnaissance, damage to pipelines and other buried utilities was apparently widespread in areas affected by liquefaction, both in areas of pronounced lateral ground displacement and in areas of ground settlement, ground oscillation and minor lateral spreading. The information collected in this study is insufficient to delineate the extent of damage or the performance of various types of pipe and embedment conditions. These topics, along with more quantitative evaluation of ground and structural response, provide an extraordinary opportunity to evaluate the performance of modern urban and industrial infrastructure in areas subjected to moderate to severe effects of liquefaction.

Chapter Four
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