Preface

Jonathan D. Bray - University of California, Berkeley
Ross W. Boulanger - University of California, Davis
John T. Christian - Consulting Engineer, Boston, MA
W.D. Liam Finn - University of British Columbia (also on the Canadian team)
Leslie F. Harder, Jr. - Calif. Dept. of Water Resources, Sacramento, CA
Izzat M. Idriss - University of California, Davis
James K. Mitchell - Virginia Polytechnic Institute
Yoshiharu Moriwaki - Woodward-Clyde Consultants, Inc., Santa Ana, CA
Thomas D. O'Rourke - Cornell University
Raymond B. Seed - University of California, Berkeley
Nicholas Sitar - University of California, Berkeley
Kenichi Soga - Cambridge University, U.K.
T. Leslie Youd - Brigham Young University

The reconnaissance team's visit was officially sponsored by the Japanese Society of Soil Mechanics and Foundation Engineering, and the Japanese hosts were led by Professor Koichi Akai, Professor Emeritus of the Kyoto University, Professor Kenji Ishihara of the University of Tokyo, Professor Koichi Nakagawa of the Osaka City University, and Dr. Yoshinori T. Iwasaki, Director of the Geo-Research Institute in Kobe.

Due to the extensive disruption to the transportation network, the investigation within Kobe was performed principally on foot, the port facilities were inspected by boat and on foot, and a couple of helicopter overflights were made. Thus, while effort has been made to provide a balanced overview, the limited ground time, on the order of less than a week for the U.S. members of the team, necessarily limits the scope and substance of this report. Consequently, the observations and opinions presented here are intended to serve as a guidance to further in depth assessment and not as a final, definitive evaluation of the specific events and phenomena.

A combination of several factors contributed significantly to the severity of much of the damage: the area had been previously considered to have relatively low seismic risk, the projected location of the release of energy along the earthquake fault was almost immediately below a densely developed urban area, and the geologic setting of the region, on the shores of a large embayment, provided for a substantial thickness and areal distribution of soft and liquefiable sediments. The principal geotechnical aspects of the Hyogoken-Nanbu Earthquake include the following:

• Extensive liquefaction of natural and artificial fill deposits occurred along much of the shoreline on the north side of the Osaka Bay. Probably the most notable were the liquefaction failures of relatively modern fills on the Rokko and Port Islands. On the Kobe mainland, evidence of liquefaction extended along the entire length of the waterfront, east and west of Kobe, for a distance of about 20 km. Overall, liquefaction was a principal factor in the extensive damage experienced by the port facilities in the affected region.
• Liquefaction caused disruption of underground utilities, including gas, water, and sewer systems. In fact, the pervasive damage to the underground utilities throughout the region affected by liquefaction may yet prove to be a major, and as yet not fully appreciated, aspect of the earthquake damage.
• Notably, areas of fills that have been densified using fairly conventional techniques performed satisfactorily. Similarly, mechanically stabilized soil walls also performed very well.
• While numerous pile foundations for various types of structures apparently performed very well, there were several failures of large pile-supported bridge piers. These cases provide a unique opportunity for further studies of lateral pile capacity and of the overall seismic performance of piles.
• Shallow foundations for most modern buildings consisting of reinforced mats or, more commonly for houses, reinforced perimeter and wall footings that are strongly tied together performed quite well. These types of foundations acted as diaphragms, holding structures together and preventing differential ground movements from fracturing the foundations and superstructures.
• Although the region immediately north of downtown Kobe is relatively hilly, landsliding was minimal, mostly limited to shallow sliding and raveling of boulders. However, one large flow slide occurred in decomposed granite in a residential area in the Nigawa district, northeast of Kobe. This slide destroyed several houses and killed 34 people.
• Movement of fills and retaining walls was evident in many parts of the residential area in the hills northeast of Kobe. In some locations, the displacement of side-hill and valley bottom fills was sufficiently large to cause structural damage, and to damage the utilities and disrupt service. This type of damage was also observed in the 1994 Northridge Earthquake.
• While underground structures are generally considered relatively safe from seismically induced damage, a failure of a cut-and-cover underground metro station did occur.
• A number of relatively modern dams of various types within the region of intense shaking seemed to survive the earthquake with little or no damage. However, three small embankment dams, the Niteko Dams, more than 100 years old, failed by lateral spreading and liquefaction. The damage, as a result of these failures, appeared to be minimal because of the initially low water level and low overall height of the dams ((12 m).
• So far, the ground rupture due to right lateral strike slip faulting has been reported only on the Awaji Island, to the southwest of the epicenter. However, the north and south towers of the Akashi Kaikyo Bridge, currently under construction, are known to have been displaced relative to each other laterally and vertically. This is the first time that a structure of this size has been known to be offset by a fault rupture.