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112 APPENDIX B-4 Treasure Island Site â Site Characterization General Treasure Island is a man-made island in San Francisco Bay, California. The island is connected by a causeway to the nearby (natural) Yerba Buena Island. Approximate locations of Treasure and Yerba Buena islands with respect to the San Francisco Peninsula are shown in Figure B-1. Figure B-1. Treasure Island (Hydraulic Fill) and Y erba Buena Island (Rock Outcrop) with Indicated Locations of Strong Motion (SM) Recording Stations. Figure B-1 shows the approximate locations of SM recording stations at the Treasure Island (hydraulic fill) and Yerba Buena Island (rock outcrop). Approximate coordinates of these stations are 37.8247 degrees north latitude and 122.3752 west longitude, and 37.8094 degrees north latitude and 122.3632 degrees west longitude, respectively. Loma Prieta Earthquake and Ground Motions Treasure Island (TI) was shaken by the 1989 M 6.9 Loma Prieta earthquake. Numerous sand boils were observed at the TI site during and immediately after the earthquake. Relevant information about the Loma Prieta earthquake, including parameters of ground motions recorded at both islands are tabulated in Table B-1. R e c o r d i n g S t a t i o n S a n F r a n c i s c o Ba y T r e a s u r e I s l a n d Y e r b a Bu e n a I s l a n d R e c o r d i n g S t a t i o n S a n F r a n c i s c o
113 Table B-1. Loma Prieta Earthquake â Earthquake and Strong Motion Parameters at the Site. Earthquake Date M R (km) PGA (N-S) ru Loma Prieta October 17, 1989 6.9 70-75 Yerba Buena Island (Outcrop) Treasure Island (Soft Soil) Hydraulic Fill 0.067 g 0.159 g > 0.95 (Inferred) M = Moment Magnitude; R = Approximate site-to-source distance; PGA = Peak Ground Acceleration (larger of the two horizontal components); ru = Maximum recorded excess pore water pressure normalized by the vertical effective-stress; N-S = North-South (direction). Strong motions recorded at the TI site, including the Yerba Buena rock outcrop and the TI surface motions and the corresponding spectra are shown in Figures B-2 and B-3, respectively. (a) (b) Figure B-2. Acceleration Histories (a) at TI; (b) Y erba Buena Island. Figure B-3. Acceleration Spectra of Acceleration Histories shown in Figure B-2.
114 Geotechnical Information and Representative Soil Profile Geotechnical investigations were conducted at TI after the M 6.9 1989 Loma Prieta earthquake. The results of these investigations are documented in Gibbs et al. (1992), Bennett (1994), and CESMD (2003). The investigations included SPT sounding blow counts, soil sampling and visual observation of samples, geotechnical laboratory testing, and results of site-specific shear wave velocity measurements. The idealized soil profile at the TI site is presented in Figure B-4. The soil profile consists of approximately 11.5 m of fill material (hydraulic fill derived from the ocean bottom sediments), underlain by 17.7 m of Young Bay Mud, 13.1 m of Medium Dense Sand, 33.6 m of Old Bay Mud, 5 m of fine gravelly sand, 7 m of Old Bay Mud, and bedrock. For the Yerba Buena Island, a bedrock shear wave velocity of 2,700 m/s was reported by Gibbs et al. (1992). Figure B-4. Interpreted Soil Profile at TI Site (SM Instrument Locations). Shear wave velocity measurements at the TI recording station from two sources are shown in Figure B-5. Figure B-5 also shows the interpretation of these measurements, i.e., the shear wave velocity model for use in site response analyses.
115 Figure B-5. Recorded and Interpreted Shear Wave Velocity Profiles at the TI Site. The interpretation of SPT sounding in liquefiable hydraulic fill and the corresponding estimates of the relative density (Dr) are shown in Figure B-6(a) and B-6(b), respectively. Raw SPT Blow Counts (N60) are from Bennett (1994) (a) (b) Figure B-6. Interpretation of SPT Sounding in Hydraulic Fill: (a) Raw SPT Blow Counts (N60); (b) Interpreted Values (Relative Density, Dr). Material Properties Hwang and Stokoe (1993) recovered âintactâ samples of the TI Site Fill (mostly Silty Sand), Young Bay Mud, Medium Dense Sand, and Old Bay Mud and performed a series of Resonant
116 Column tests. The results of this testing, interpreted in terms of modulus reduction and damping, are shown in Figures B-7 through B-10. (a) (b) Figure B-7. Modulus Reduction and Damping Curves â Fill (mostly Silty Sand) at the TI Site (Modified after Hwang and Stokoe, 1993). (a) (b) Figure B-8. Modulus Reduction and Damping Curves â Y oung Bay Mud at the TI Site (Modified after Hwang and Stokoe, 1993). (a) (b) Figure B-9. Modulus Reduction and Damping Curves â Medium Dense Sand at the TI Site (Modified after Hwang and Stokoe, 1993).
117 (a) (b) Figure B-10. Modulus Reduction and Damping Curves â Old Bay Mud at the TI Site (Modified after Hwang and Stokoe, 1993). Results of undrained advanced geotechnical laboratory testing are not available. However, this site is relatively well characterized from a geotechnical standpoint and material properties can be selected accordingly. Additionally, results of undrained advanced geotechnical testing (CyDSS) are available for the Medium Dense Sand (MDS) for a site in a relative vicinity. These results are reproduced from Appendix D-1 in Figure B-11. Detailed material characterization and discussion why these test results are applicable is provided in Appendix D-1. (a) (b) Figure B-11. Results of CyDSS Testing of MDS from a site Nearby (see Appendix D-1): (a) Stress-Strain Response; (b) Normalized Excess Pore Water Pressure Response.
118 References Bennett, M. J. (1994), âSubsurface Investigation for Liquefaction Analysis and Piezometer Calibration at Treasure Island Naval Station, California,â Geotechnical Report, US Geological Survey, Menlo Park, California. CESMD (2003), âGeotechnical Investigation at Treasure Island - Fire Station Geotechnical Array,â Geotechnical Report, Center for Engineering Strong Motion Data, USGS, CGS, and ANSS, San Francisco, California. Hwang, S. K., and Stokoe, K. H. (1993). âDynamic Properties of Undisturbed Soil Samples from Treasure Island, California,â Research Report, Geotechnical Engineering Center, Civil Engineering Department, University of Texas at Austin. Gibbs, J. F., Fumal, T. E., Boore, D. M., and Joyner, W. B. (1992). âSeismic Velocities and Geologic Logs From Borehole Measurements at Seven Strong-Motion Stations that Recorded the 1989 Loma Prieta Earthquake,â Report 92-287, US Geological Survey, Menlo Park, California.