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by Thomas D. O'Rourke
There was severe and widespread liquefaction as a result of the earthquake. Over 50 linear km of quay walls and waterfront retaining structures have been damaged because of liquefaction in Kobe and adjacent artificial islands. Liquefaction-induced damage is prominent on Kobe Port Island and Rokko Island, which cover approximately 10 km2 and 6 km2, respectively. Both islands have been constructed with fill derived from decomposed granite. The average, uncorrected standard penetration resistance of the fill (blow count) is about 6. The grain size of the fill varies from gravel and cobble-sized particles to fine sand. There is less than 10% by weight which is less than silt sized particles, and the mean grain size is approximately 2 mm. The thickness of submerged fill on Port Island is about 12 m, and settlement over a large part of the island is between 0.5 m and 0.75 m. Many of the quay walls are gravity wall-type structures which have displaced laterally by one to several meters. Crane rails have displaced causing severe distortion or collapse of virtually all cranes used to load and unload container vessels. Subsidence behind laterally displaced quay walls is as large as 2 to 3 m. On the north side of Port Island, liquefaction-induced lateral displacement has caused foundation piles to pull away and break beneath the pile caps of warehouses. Lateral ground deformation has caused the piers of the highway bridge and electric rail bridge between Port Island and Kobe to lean between 2° and 3° towards the waterfront. Severe lateral movements were generally confined to a distance of 100 to 200 m from the waterfront. Port facilities were heavily damaged. Approximately 179 of 186 of the heavy shipping berths are inoperable.
|Crane collapse due to ground subsidence caused by
(Photo courtesy M. Hamada.)
There also has been damage to underground rapid transit facilities. The Daikai Station of the Kobe Rapid Transit Railway has collapsed. The station was built by cut-and-cover techniques. The collapsed portion is approximately 100 m long. It is a reinforced concrete box structure, 5.5 m high and 15.3 m wide at a depth of 4.6 m from the street surface to the top of the structure. Preliminary information indicates that the station was constructed in sands underlain by stiff clay. The central reinforced concrete columns of the station failed, apparently by a combination of shear and vertical loading. There was only nominal confining steel (9 mm diameter on 180 mm spacing) for the 20 mm diameter longitudinal rebar on 140 mm centers in the concrete. Collapse of the columns caused subsidence at the street surface of a maximum 2.5 m, with substantial settlement over an area of 100 m by 20 m. Apart from the structural collapse-induced movement, there was no clear evidence of permanent ground deformation at the site induced by other causes, such as liquefaction.
This failure is highly significant because it is the first instance of severe earthquake damage to a modern tunnel for reasons other than fault displacement and instability near the portal. Damage of a less severe nature has been reported for other parts of the Kobe underground rapid transit railways.
Some of the material reported herein is based upon work supported in whole or in part by the National Science Foundation, tbe New York State Science and Technology Foundation, the U.S. Department of Transportation and other sponsors. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of MCEER or its sponsors.