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98 APPENDIX B-2 Owi Island Site â Site Characterization General Owi Island (i.e., Owi Island No. 1) is a man-made (hydraulic fill) island in Tokyo Bay, Japan. Approximate locations of the Owi Island and its downhole array are shown in Figure B-1. The Owi Island geotechnical instrument array (site) is located approximately 1 km from the shore. Approximate site coordinates are 35.5862 degrees north latitude and 139.7625 east longitude. Figure B-1. Location of the Owi Island Downhole Array with respect to the Epicenter of the 1985 M 6.2 Chiba-Ibaragi Earthquake and the Port of Y okohama Facilities. S i t e S i t e E p i c e n t e r o f t h e 1 9 8 5 M 6 . 2 Ch i b a - I b a r a g i E q . O w i I s l a n d Do w n h o l e Ar r a y H a n e d a Ai r p o r t O w i I s l a n dT o k y o / Y o k o h a m a 5 0 k m
99 Geotechnical Instrumentation The Owi Island site was instrumented by Ishihara (1981). Layout of geotechnical instrumentation within idealized profile is shown in Figure B-2. Figure B-2. Interpreted Soil Profile and Geotechnical Instrumentation at the Owi Island Site. As shown in Figure B-2, the Strong Motion (SM) instruments were installed at the ground surface (SM1) and at a depth of 10 m b.g.s. (SM2). The P2 pore pressure transducer that is relevant for site response analysis was installed at 6 m b.g.s. Chiba-Ibaragi Earthquake, Ground Motions, and Pore Water Pressure Records The epicenter of the 1985 M 6.2 Chiba-Ibaragi earthquake was approximately 50 km from the site (Figure B-1). Relevant information is summarized in Table B-1. The earthquake induced relatively weak ground motions and pore water pressure (PWP) response at the site. Table B-1. Chiba-Ibaragi Earthquake â Earthquake and Strong Motion Parameters at the Site. Earthquake Date M R (km) PGA (N-S) ru Chiba-Ibaragi October 4, 1985 6.2 50 Surface (SM2) 10 m b.g.s. (downhole; SM1) 6 m 14 m 0.072 g 0.043 g 2.6% 3.0% M = Moment Magnitude; R = Approximate site-to-source distance; PGA = Peak Ground Acceleration; ru = Maximum recorded excess PWP normalized by the vertical effective-stress; N-S = North-South (direction).
100 Figure B-3 shows the acceleration histories recorded at Owi Island in the 1985 M 6.2 Chiba- Ibaragi earthquake in the North-South (N-S) direction. Both the free-field (i.e., ground surface) record and the record at 10 m b.g.s. are shown. The corresponding acceleration response spectra are shown in Figure B-4. (a) (b) Figure B-3. Acceleration Histories from the 1985 Chiba-Ibaragi Earthquake Recorded at the Owi Island Site: (a) Ground Surface; (b) 10 m Downhole. Figure B-4. Surface and Downhole Spectra (N-S) in the 1985 M 6.2 Chiba-Ibaragi Earthquake. Figure B-5(a) shows the time history recorded at a depth of 6 m. For comparison, the acceleration time history recorded at a depth of 10 m is shown in Figure B-5(b). The peak = 0.026. The value fluctuation around zero from at the onset of shaking (before the seismic motion reached to the transducer at 7 seconds) is an index of transducer accuracy.
101 (a) (b) Figure B-5. Response within the Profile in the 1985 M 6.2 Chiba-Ibaragi Earthquake (a) Excess PWP History; (b) Acceleration History. Site Characterization and Representative Soil Profile The conventional site exploration program was performed in 1981 (Ishihara et al., 1981). The program included drilling, SPT sounding, soil sampling, and CPT sounding. Geotechnical instrumentation was installed shortly thereafter. Shear wave velocity was measured after the 1985 Chiba-Ibaragi earthquake (in 1987; see Ishihara et al., 1987). Figure B-2 above shows the interpreted soil profile at the recording station, i.e., soil profile as used for site response analyses documented herein. Figure B-5 shows interpreted results of the two site exploration programs in profile view. The SPT blow counts are as reported by Ishihara et al. (1981) (no conversion to US (N1)60 was made).
102 (a) (b) Figure B-5. Summary and Interpretation of the Ishihara et al. (1987) site Exploration Program: (a) SPT sounding; (b) Shear Wave Velocity Measurements. The results of site exploration program revealed that the thickness of reclaimed land was approximately 10.5 m (both before and after the Chiba-Ibaragi event). The reclaimed land is layered horizontally with interbedded layers of sand, silt, and gravel. The underlying natural deposit consists of clay, silt, and sand. The depth of the groundwater was recorded as approximately 1 m below ground surface (b.g.s.) in 1981 and approximately 1.4 m b.g.s. in 1987 (Ishihara et al. 1981; 1987). The recorded depth of groundwater does not reflect groundwater elevation at the time of the earthquake, which was influenced by tidal changes. Based upon the interpretation of recorded PWP at the onset of shaking, representative groundwater level was established at approximately 1.5 m b.g.s. Material Properties Dynamic properties of silty sand [modulus reduction, only were measured on "undisturbed" specimens by Ishihara et al. (1987)]. Dynamic properties of silt were not measured. Instead, they were "estimated" by Ishihara et al. (1987) based upon testing of similar materials in the area. Ishihara et al. (1987) postulated that other materials in the profile, including gravel and various gradations of sand, respond to strong ground shaking as silty sand. Measured and estimated modulus reduction and damping curves are shown in Figure B-6.
103 (a) (b) Figure B-6. Owi Island Silty Sand: (a) Modulus Reduction Curve; (b) Damping Curve (Ishihara et al., 1987; Measured). (a) (b) Figure B-7. Owi Island Silt: (a) Modulus Reduction Curve; (b) Damping Curve (Ishihara et al., 1987; Estimated). Interpretation of Recorded Strong Motion Information Figure B-8 shows interpretation of site information in terms of ârecordedâ stress-strain loops. In particular, the loops were developed from ground surface and in-hole SM records and idealized stratification of the site. Shear stress was calculated following the principles of basic physics. In particular, from Newton's law (Force = mass x Acceleration), shear stress (i.e., force) was calculated from the mass (total unit weight times distance between two SM instruments) and acceleration (average acceleration between two SM instruments). The shear strain was calculated from displacement time histories at two SM instruments and the distance between these two instruments.
104 Figure B-8. Stress-Strain Loops in the 1985 M 6.2 Chiba-Ibaragi Earthquake (Evaluated for an Approximate Depth of 5.9 m b.g.s.). References Ishihara, K., Anazawa, Y., and Kuwano, J. (1987), "Pore Water Pressures and Ground Motions Monitored during the 1985 Chiba-Ibaragi Earthquake," S oi l s an d f oun d ati on s, The Japanese Geotechnical Society, 27(3), 13â30. Ishihara, K., Shimizu K., and Yamada, Y. (1981), "Pore Water Pressures Measured in Sand Deposits During an Earthquake," S oi l s an d F oun d ati on s, The Japanese Geotechnical Society, 21(4), 85â100. Ziotopoulou, A. (2010), "Evaluating Model Uncertainty against Strong Motion Records at Liquefiable Sites," M aster T hesi s, University of California, Davis, Department of Civil and Environmental Engineering, Davis, California.