Evaluation of short-term stability for sea-wall structure at Kobe Airport(2008)

KSCE Journal of Civil Engineering (2008) 12(3):155-163DOI 10.1007/s12205-008-0155-4www.springer.com/12205

Geotechnical Engineering

󰀂155󰀂*Professor, Graduate School of Civil Engineering, Kobe University, Kobe 657-8501, Japan (Corresponding Author, E-mail: sshibuya@kobe-u.ac.jp)**Doctoral Student, Graduate School of Natural Science, Kobe University, Kobe 657-8501, Japan (E-mail: jungmica@yahoo.co.jp)***Research Associate, Graduate School of Civil Engineering, Kobe University, Kobe 657-8501, Japan (E-mail: chaie1976@yahoo.co.jp)****Doctoral Student, Graduate School of Natural Science, Kobe University, Geo-research Institute, Kobe 657-8501, Japan (E-mail: geor5@diary.ocn.ne.jp)

Evaluation of Short-term Stability for Sea-wall Structure at Kobe AirportSatoru Shibuya*, Minsu Jung**, Jonggil Chae***, and Teruyuki Fujiwara****Received January 23, 2008/Accepted March 12, 2008··································································································································································································································· Abstract

Prior to the huge reclamation work for constructing Kobe Airport, the sea-wall structure resting on a 30m-thick soft Holocene claylayer in the 16m-deep sea was constructed at stages over the period of three years. Regarding the design of short-term stability of thesea-wall structure, the ratio of undrained shear strength of the normally-consolidated seabed clay to in-situ yield stress was estimatedto take the value of 0.35 in order to satisfy the factor of safety in excess of 1.3. This design strength in the clay foundation waschecked with the unconfined compression strength of the fresh clay samples retrieved from boreholes during the construction. In thispaper, the short-term stability of the embankment in this well-documented case history was re-evaluated after-the-event with theresults of comprehensive series of constant-volume direct shear box tests in which the effects of strength anisotropy as well as theshearing rate on undrained shear strength were both examined. Discussion is made how to estimate the short-term stability of theembankment by considering the effects of strength anisotropy and shearing speed on undrained shear strength and how it is linkedwith the current design method in Japan in which a half of the undrained compression from unconfined compression test isemployed.Keywords: embankment, foundation, clay, reclamation, stability analysis, laboratory test

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1. IntroductionKobe airport welcomed the first flight in February 2006. Theoffshore airport was constructed by means of land reclamation overa relatively short period of seven years. Prior to the reclamationwork, the sea-wall structure, i.e., an embankment with a concretewall, resting on a 30m-thick soft Holocene clay layer in the 16m-deep sea was constructed at three stages over the period of threeyears (refer to Hasegawa et al., 2007). In the original design, theratio of undrained shear strength of the normally-consolidatedseabed clay, Su, to in-situ yield stress Vvy was considered the valueof 0.35 in order to satisfy the factor of safety, Fs, in excess of 1.3throughout the stage construction. This design strength fromunconfined compression test (UCT) was also checked against theunconfined compression strength, qu, of the fresh clay samplesretrieved from three boreholes in the foundation clay.Embankment on soft ground is a classical problem in geotechnicalengineering. Several engineering approaches have been proposed toestimate the short-term stability of the embankment. Bjerrum(1972), for example, proposed an empirical method to correlatethe vane shear strength to in-situ strength by considering theanisotropy as well as the rate effect on Su. The SHANSEPmethod proposed by Ladd and Foott (1974) postulates the claystrength that can fully be normalized by the over-consolidationratio (OCR). Hanzawa (1979) proposed so-called modifiedBjerrum method utilizing the direct shear box (DSB) test.Conversely in Japan, the design is based on the unconfinedcompression strength, i.e., Su=qu/2.In this paper, the short-term stability of the embankment in thiswell-documented case history was re-evaluated after-the-event. Inso doing, the strength anisotropy of the foundation clay wasmanifested by performing a series of DSB test in which thespecimens prepared by trimming them at different angles relativeto the horizontal bedding plane were all sheared under constant-volume (i.e., undrained) conditions. The shearing rate effects on Su