Settlement of the Kansai International Airport Islands
settlement;consolidation;marine clay;Geology;compressibility;permeability;construction;instrumentation;numerical modeling;Kansai International Airport;Osaka Port;Osaka Bay;Japan;coefficient of consolidation
The Kansai International Airport was constructed 5km off the coast of Senshu in 18m to 20m deep sea water to avoid noise pollution, and land acquisition disputes that had been experienced at Itami and Narita Airports. Construction of Island I began in 1987 and the Island I runway began operation in 1994. Construction of Island II began in 1999 and the Island II runway began operation in 2007. Utilizing over 2 million vertical sand drains and 400 million cubic meters of reclamation fill material for construction of two airport islands, the project is viewed as an engineering marvel. In fact, the American Society of Civil Engineers named the airport one of the “Civil Engineering Monuments of the Millennium”. As part of the original design considerations, the surface elevation of the airport islands were to remain above +4m Chart Datum Level (CDL) to avoid the erosive action of waves overtopping the seawall (Arai, 1991). However, as of December 2012, the average seabed settlement has exceeded 12.9m and 14.2m, respectively, for Airport Islands I and II (NKIAC, 2012). Much of the surface elevation of Island I is already below +4m CDL, and the surface elevation of part of Island II is predicted to be at +4m CDL in September 2023. Furthermore, Island I is predicted to be at sea level in June 2067, and part of Island II is expected to be at sea level in January 2056.The Kansai Airport project has received considerable attention in geotechnical engineering literature in part because of the scale of the project; Island I is 511ha in approximately 18m deep seawater and Island II is 545ha in approximately 20m deep seawater. The project is also significant because of the sufficiently detailed subseabed information as well as observations of settlement and porewater pressure reaching depths up to 350m below the seabed. The projected or observed consolidation behavior of the subseabed has been arrogated in an ongoing debate on the uniqueness of the end-of-primary void ratio – effective vertical stress relationship of clay and silt deposits. Some have explicity argued against application of the uniqueness principle to settlement and porewater pressure predictions at the Kansai Airport site (Hight and Leroueil, 2003; Mimura and Jang, 2005; Rocchi et al., 2006), while others have made settlement calculations for the Kansai International Airport using rheological models that assume separate and independent effective stress and time compressiblities (Imai et al., 2005; Oda et al., 2005; Tanaka, 2005). Additionally, in connection to secondary compression behavior of Osaka Bay clays, potentially misleading interpretations have been published as a result of possible misinterpretation of laboratory oedometer tests as well as settlement observations at the airport site.This study includes a review of the geologic history of Osaka Bay as well as a careful evaluation of the available subseabed data for 14 marine clay layers, 8 non-marine clay layers, and 20 sand layers that are contained within the up to 350m depth below seabed contributing to settlement of the Kansai Airport islands. Additionally, the reclamation process used to construct the airport islands and the instrumentation installed for settlement and porewater pressure observations were carefully examined as part of this study. Explicit finite difference techniques were developed within the ILLICON framework (Mesri and Rokhsar, 1974; Choi, 1982; Lo, 1991; Mesri et al., 1994) to facilitate settlement and porewater pressure calculations at the Kansai International Airport site. Settlement and porewater pressure calculations were performed at the Kansai International Airport Island I and Island II based on the assumption of uniqueness of end-of-primary void ratio – effective vertical stress relationship, together with the Cα/Cc law of compressibility to compute secondary compression.Settlement calculations for the Holocene marine clay layer (Ma13), which is fully penetrated by forty centimeter diameter displacement type vertical sand drains at the Kansai Airport site, are in good agreement with observations. Therefore, the uniqueness principle satisfactorily explains the magnitude of vertical compression of Ma13 at Kansai Airport. End-of-primary compression for the 17m to 24m Ma13 clay layer ranges from 6.2m to 8.3m at the airport site. End-of-primary compression for Ma13 was reached 2 to 4 years after start of reclamation for Airport Island I and Island II. Therefore, the majority of the post-construction seabed settlement for the Kansai Airport Islands will result from compression of the Pleistocene clay layers underlying Ma13.The ILLICON program was used together with settlement and porewater pressure observations to back-calculate equivalent permeabilities (keq) for the Pleistocene sand layers to compute the post-construction settlement projections of the Pleistocene layers. The permeability values used in the ILLICON analysis are comparable to values used by Mimura and Jang (2005) and Shibata and Karube (2005) in their settlement analysis for Kansai Airport. Calculated settlements and excess porewater pressures for the Pleistocene layers are in good agreement with observations that are available up to 2011. Seabed settlement, including contributions from the Holocene clay layer and the Pleistocene clay layers, is expected to be 15.7m at Island I and 24.9m at Island II in the year 2100.The uniqueness principle together with the Cα/Cc law of compressibility sufficiently explains settlement and porewater pressure observations reported for the Kansai International Airport project.
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