Yerba Buena Island Research Articles

The San Francisco–Oakland Bay Bridge, CA, USA – eastern span

With daily traffic of 260 000 vehicles, the San Francisco–Oakland Bay Bridge is a major connection in the San Francisco Bay area (CA, USA). The new bridge is a designated lifeline structure (to remain open for emergency traffic after a major seismic event) with a design life of 150 years. The new bridge is 3.6 km long and consists of four distinct structures: a low-rise post-tensioned concrete box girder near the Oakland shore; a 2.4 km long segmental concrete box girder (skyway); a first-of-its-kind self-anchored suspension (SAS) bridge with a 385 m main span over the navigational channel; and a post-tensioned concrete box girder that connects to the east portal of the Yerba Buena Island tunnel. Opened in 2013, the signature span of the bridge is the SAS bridge with a length of 624 m and a total deck width of 79 m accommodating ten lanes of traffic in addition to a bike/pedestrian path. The US$6.4 billion megaproject was procured under multiple contracts and was delivered using the traditional design–bid–build method. This paper describes the key design innovations and construction methods which address the unique challenges on this project.

Ernest Leslie Ransome

Ernest L. Ransome is a famous but often misunderstood 19th century California engineer and builder. Architectural historians and engineering professionals see him as a central figure in developing reinforced concrete as a usable building material decades before its use became prevalent. He is most commonly recognized as building the first reinforced concrete bridge, San Francisco's Alvord Lake Bridge, which was built in 1890 and is still in use. Historical accounts of his work, however, are based chiefly upon secondary sources and are sometimes incorrect or misleading. This article clarifies Ransome's true role in concrete building in California and debunks misinformation about the famous Alvord Lake Bridge. It traces his career in the United States (he emigrated to California in 1870 at the age of 26), first as a manufacturer of imitation stone and later as a builder of increasingly large and complex buildings and structures. It discusses his work on a series of iconic Northern California buildings and structures: the 1888 Bourn Winery (now the Culinary Institute of America school in St. Helena); the 1890 Torpedo Building, still standing on the Oakland side of Yerba Buena Island; the 1890 Alvord Lake Bridge and its near twin the Conservatory Bridge, both still in use in Golden Gate Park; the 1891 Art Museum, now being used as the Canter Center on the Stanford University campus. It also discusses Ransome's partnership with Sidney Cushing, a railroad magnate in Marin County for whom the Cushing Amphitheater on Mt. Tamalpais was named, and Francis Marion “Borax” Smith, who built the borax industry in Death Valley and who founded and owned the Key System transit in the East Bay. The article concludes with observations about Ransome's true place in the history of concrete engineering in the United States and concrete construction in California.

Bay Bridge anchorage construction: George Booth Post.

Bay Bridge anchorage construction: George Booth Post.

Proof testing in support of the new San Francisco–Oakland Bay Bridge

AbstractFollowing the Loma Prieta earthquake, Caltrans (California Department of Transportation) started a multi‐year seismic vulnerability assessment and retrofit project of all major bridges in California, including the San Francisco–Oakland Bay Bridge (SFOBB) East Bay Span. First, a retrofit of the East Span Steel Truss structure was evaluated. However, owing to the high cost and the questionable reliability in terms of performance of the retrofit and owing to its difficult implementation under full traffic, a replacement structure was determined to provide a seismically more reliable alternative, resulting in the SFOBB East Span Replacement Project.The new bridge consists of four distinct structures: (1) the Oakland landing or touchdown structures; (2) a segmental concrete box girder crossing called the Skyway; (3) a self‐anchored suspension (SAS) signature span; and (4) a series of multi‐cell post‐tensioned concrete box girder bridges providing the transition to the tunnel on Yerba Buena Island. The new bridge will feature parallel roadways and will be built next to the existing bridge, which will be dismantled after the new bridge is opened to traffic.This paper focuses only on the seismic safety aspects in the RC elements of three of the four segments of the new SFOBB East Span Bridge, namely the Skyway, the SAS bridge and the Oakland touchdown. Analytical and numerical models were developed to assess the seismic behavior of the Bay Bridge and a large‐scale proof‐testing program was conducted at the Charles Lee Powell Structural Research Laboratories at the University of California, San Diego. In the framework of this program, two steel shear links at 100% scale were tested, two concrete piers of the Skyway at 25% scale, a 25% scale model of the West Anchor Pier W2, and two column 35% scale piers of the Oakland Touchdown were also tested.The large‐scale tests on all structural elements of the new East Span of the San Francisco–Oakland Bay Bridge with expected inelastic performance characteristics proved that the design fully meets all performance requirements. The large‐ or full‐scale tests were able to identify performance issues, resulting in design improvements and considerable cost savings. Alternative detailing and construction procedures were validated, leading to a more economic construction of the new bridge. Copyright © 2005 John Wiley & Sons, Ltd.

Design of California's New San Francisco—Oakland Bay Self-Anchored Suspension Bridge

The seismically vulnerable east span of California's San Francisco—Oakland Bay Bridge will be replaced with a dual eastbound and westbound parallel structure 3.6 km long. The Bay Bridge lies between the Hayward and the San Andreas Faults, which can generate magnitude 7.5 and 8.1 M earthquakes, respectively. Performance criteria require that the bridge be operational immediately after a 1,500-year-return-period earthquake from either of these two faults. Four distinct structures will make up the bridge crossing: a low-rise posttensioned concrete box girder near the Oakland shore, a segmental concrete box girder 2.4 km long, a self-anchored suspension signature span, and a posttensioned concrete box girder that connects to the east portal of the Yerba Buena Island tunnel. The single-tower asymmetric self-anchored suspension bridge was selected from a total of four design alternatives that were developed for the signature main span; these included two cable-stayed bridges and two self-anchored suspension bri...

Evaluation of Seismic Performance for Tunnel Retrofit Project

A seismic retrofit program at Yerba Buena Island Tunnel is presented as a case history. The studies include evaluations of portal stability under earthquake excitation and performance of the tunnel liner as a result of seismic induced deformation. Due to different potential failure modes at the two portal areas, two separate evaluation techniques were utilized. Key block theory in conjunction with a Newmark type analysis was used to assess movements of a potential failure wedge at the west portal slope, while discontinuous deformation analysis was utilized at the east portal slope to evaluate a rotational mode of failure. To assess the performance of the liner subjected to design earthquakes, a two-step analysis procedure was adopted. The first step was to compute seismic induced deformations of the tunnel subjected to seismic wave propagation through the island rock without the presence of the liner. The second step, not reported in this paper, involved imposing the deformations of the tunnel onto the structural liner through spring elements that accounted for interaction between the liner and the surrounding rock. From the studies, performance of the existing tunnel supports was found to be acceptable.

Site Response at Treasure and Yerba Buena Islands, California

A variety of methods are utilized to reinvestigate the physical relationship between the seismic response of Treasure Island (TI) and Yerba Buena Island (YBI) in California. These islands are a soil (TI) and rock (YBI) site pair separated by 2 km. The site pair has been used previously by researchers to identify soil response to earthquake shaking. Linear regime ground motions (MW4.0-MW4.6 and PGA: 0.014–0.017 g) recorded in the TI vertical array indicate a coherent wavefield in the sediments and an incoherence between the rock and sediments. Our analyses show that the greatest change in the wavefield occurred between the rock and soil layers, corresponding to a significant impedance contrast. The waveforms change very little as they propagate through the sediments, indicating that the site response is a cumulative effect of the entire soil structure and not a result of wave propagation within individual soil layers. In order to highlight the complexity of the site response, correlation analysis was used to demonstrate that the rock and soil ground motions were not highly coherent between the two sites. YBI was, therefore, shown to be an inappropriate reference site for TI. One-dimensional (1D) vertical wave propagation and inverse techniques were used to differentiate between 1D site response and more complex site behavior. Both 1D methods (vertical wave propagation and inverse transfer functions) proved incapable of capturing the site response at TI beyond the initial four seconds of motion. Finite difference waveform modeling, based on a two-dimensional velocity structure of the northern San Francisco Bay was needed to explain the linear site response at TI as horizontally propagating surface waves trapped in the bay sediments. A simplified velocity structure for the San Francisco Bay including a single 100 m basin layer (constant shear-wave velocity of 400 m/s) over a 1.5 km/s layer of Franciscan bedrock was able to trap energy in the basin and produce surface waveform ringing similar to that observed in the TI data. Due to surface waves propagating in the San Francisco Bay sediments, any 1D model will not fully characterize site response at TI. All 1D models will fail to produce the late arriving energy observed in the ground motions.

The Effect of Shallow San Francisco Bay Sediments on Waveforms Recorded during the MW 4.6 Bolinas, California, Earthquake

To investigate the effect of the shallow, low-velocity sediments on the seismic wave field in the northern San Francisco Bay, we modeled tangential component displacement seismograms recorded during the 18 August 1999 M W 4.6 Bolinas, California, earthquake. The modeling indicates that the velocity structure of Pleistocene horizons in the San Francisco Bay is important for simulations of weak ground motions for Bay Area earthquakes. Models including the Pleistocene sediments generate the 1-sec-period surface waves observed at several stations. Modeling of Treasure and Yerba Buena Island records requires structures approximately an order of magnitude higher in spatial resolution than the current 3D velocity models for the region. This pair of sites, located only 2 km apart in the bay, records a sixfold difference in peak ground acceleration during the Bolinas earthquake. Three transects are forward modeled using 1D frequency-wavenumber integration and 2D finite-difference methods. Generally the ground motions are characterized by a direct shear wave ( S ), a midcrustal reflection ( S 1), a near-receiver multiple ( S 2), and surface waves. The direct S arrival at all six stations requires a faster model than GIL7, the model routinely used to estimate earthquake source parameters using the Berkeley Digital Seismic Network. In addition, the timing of S 1 indicates the possibility of a dipping midcrustal interface. S 2 can be matched with a single strong impedance contrast at 3 km depth. A thin (200-m) surface layer of weathered rock and sediments simulates the surface waves that follow S 2 at the Richmond Field Station site. However, the surface waves at Treasure Island and the Berkeley sites are longer in duration and higher amplitude than at Richmond and require 2D structure. A simple shallow uniform basin model for the San Francisco Bay consisting of stiff sediments (shear-wave velocity, β = 400 m/sec; thickness ∼100 m) over weathered rock ( β = 1.5 km/sec) of the Franciscan assemblage produces surface waves in the 0.02-2 Hz passband at Treasure Island and the Berkeley sites. Manuscript received 27 July 2001.

Site Response at Treasure and Yerba Buena Islands, California

A variety of methods are utilized to reinvestigate the physical relationship between the seismic response of Treasure Island (TI) and Yerba Buena Island (YBI) in California. These islands are a soil (TI) and rock (YBI) site pair separated by 2 km. The site pair has been used previously by researchers to identify soil response to earthquake shaking. Linear regime ground motions (MW4.0-MW4.6 and PGA: 0.014–0.017 g) recorded in the TI vertical array indicate a coherent wavefield in the sediments and an incoherence between the rock and sediments. Our analyses show that the greatest change in the wavefield occurred between the rock and soil layers, corresponding to a significant impedance contrast. The waveforms change very little as they propagate through the sediments, indicating that the site response is a cumulative effect of the entire soil structure and not a result of wave propagation within individual soil layers. In order to highlight the complexity of the site response, correlation analysis was used to demonstrate that the rock and soil ground motions were not highly coherent between the two sites. YBI was, therefore, shown to be an inappropriate reference site for TI. One-dimensional (1D) vertical wave propagation and inverse techniques were used to differentiate between 1D site response and more complex site behavior. Both 1D methods (vertical wave propagation and inverse transfer functions) proved incapable of capturing the site response at TI beyond the initial four seconds of motion. Finite difference waveform modeling, based on a two-dimensional velocity structure of the northern San Francisco Bay was needed to explain the linear site response at TI as horizontally propagating surface waves trapped in the bay sediments. A simplified velocity structure for the San Francisco Bay including a single 100 m basin layer (constant shear-wave velocity of 400 m/s) over a 1.5 km/s layer of Franciscan bedrock was able to trap energy in the basin and produce surface waveform ringing similar to that observed in the TI data. Due to surface waves propagating in the San Francisco Bay sediments, any 1D model will not fully characterize site response at TI. All 1D models will fail to produce the late arriving energy observed in the ground motions.

Design of Looping Cable Anchorage System for New San Francisco–Oakland Bay Bridge Main Suspension Span

Located at the rocky edge of the Yerba Buena Island, the west anchorage of the San Francisco–Oakland Bay Bridge suspension span serves as the anchor for this single tower self-anchored suspension bridge. With extensive comparative studies on numerous alternatives, the new looping cable anchorage system is recommended for the final design of the west anchorage of the self-anchored suspension span. The looping cable anchorage system essentially consists of a prestressed concrete portal frame, a looping anchorage cable, deviation saddles, a jacking saddle, independent tie-down systems, and gravity reinforced-concrete foundations. This anchorage system is chosen for its structural efficiency and dimensional compactness. This paper describes major design issues, design philosophy, concept development, and key structural elements and details of this innovative suspension cable anchorage system.

Analysis of strong and weak ground motions recorded at two sites during Loma Prieta earthquake by wavelet transform

This paper explores the applicability of wavelet transform for the analysis of strong and weak ground motions recorded on rock at Yerba Buena Island and on dredged sandfill at nearby Treasure Island about 2 km away during the 1989 Loma Prieta earthquake in California. An interpretation of the results from this analysis is presented and discussed. The major important feature of this data set is that liquefaction occurred at the Treasure Island site during the strong shaking. The Daubechies 20-coefficient wavelet transform is utilized in this study. Results of the application of the wavelet transform can be summarized as follows: most of the energy of the earthquake at the Treasure Island site is in the frequency range of 0.58–1.77 Hz and arrives in the time interval between 11 and 15 s. For the Yerba Buena Island site, most of the energy lies within the frequency range of 1.15–3.53 Hz and arrives between 8 and 13 s. The analysis indicates that there are some large peaks at the highest resolution in the wavelet transforms at the Treasure Island site, which can be attributed to a progressive change in the stiffness characteristics of the soil.Key words: wavelet transform, Fourier transform, frequency content, wavelet scale, site response.

Pharmacy Officer Support of U.S. Coast Guard Cutters

U.S. Public Health Service commissioned officers serve in 16 pharmacy billets with the U.S. Coast Guard (USCG). Thirteen of these billets involve serving as points of contact for, and providing logistical, materiel, and educational support to, USCG cutters. USCG instructions have solidified the role of pharmacy officers in the support of USCG afloat units. This article describes one USCG pharmacy officer's experience in providing pharmacy service support to USCG cutters based in Alameda, California, Yerba Buena Island (San Francisco), California, and Honolulu, Hawaii.

Centimeter-Level Positioning of a U. S. Coast Guard Buoy Tender

With the availability of high-accuracy, differential global positioning system (GPS) results in real-time, there is a new opportunity to use GPS to accurately measure a marine vessel's dynamic draft (settlement and squat) and 3D attitude (roll, pitch, and heading). The National Geodetic Survey (NGS) and the Coast Survey (CS), offices of the National Ocean Service (NOS), National Oceanic and Atmospheric Administration (NOAA), propose to transfer this technology to the shipping industry. The overall goal of this project is to provide the position of a vessel's keel in real time to within 10 cm (about 4 inches) relative to the bottom of the shipping channel. In support of this phase of the project, there were three meetings hosted by the Port of Oakland, California and NOS to discuss the real-time positioning of vessels project. On December 3 and 4, 1996, CS, NGS, Trimble Navigation Ltd., and the U. S. Coast Guard (USCG) performed GPS tests on a USCG buoy-tender ship. GPS data were used to compute the vessel's dynamic draft and 3D attitude. During the test, five receivers continually collected data; one receiver was located at a base station on the USCG pier on Yerba Buena Island, and four were on the ship: two on the stern and two on the bow. CS installed a TSS-335B vertical reference unit (to measure heave, pitch, and roll) in the engine room of the ship. NOS processed the GPS data and computed the vessel's dynamic draft and 3D attitude. The results indicate that the linear equivalent to the vessel's dynamic draft and 3D attitude were accurate to the 10-cm level using GPS. It was also demonstrated how a ship can be used to measure local water-level changes and actual water-level values everywhere it travels. © 1999 John Wiley & Sons, Inc.

Seismic Retrofit of San Francisco-Oakland Bay Bridge West Crossing

From the toll plaza on the Oakland shores through the approach structures in San Francisco, the San Francisco—Oakland Bay Bridge, built at a cost of $78,000,000 in 1937, is an engineering marvel that carries over a quarter-million vehicles per day. Because of the different segments of the bridge and their inherent vulnerabilities, retrofitting was assigned to several groups within the California Department of Transportation. Briefly described are the analysis and design of the seismic retrofit of the West Crossing, which spans San Francisco Bay between the city of San Francisco and Yerba Buena Island. From the outset of this project, the goal was to keep the bridge in service following a magnitude 8.0 earthquake from the nearby San Andreas fault. Included in this discussion is a summary of analytical and engineering procedures used to model the seismic behavior and the performance of this complex, important structure. In addition there is some insight into the various levels of analysis that were utilized so that the project-specific performance-based design criteria could be met. Furthermore, there is discussion of how energy dissipation through foundation rocking and the use of viscous dampening devices made the overall design objective obtainable. Finally, there is some discussion on the retrofit details used to ensure compliance with the design criteria.

Analysis of ground motions at Treasure Island site during the 1989 Loma Prieta earthquake

Site response analyses and coherence studies were conducted at the Treasure Island site where surface motions were recorded during the Loma Prieta earthquake. The analyses were conducted using a nonlinear dynamic effective stress method which took into account the effects of the liquefaction that occured at the site. The rock motions recorded at nearby Yerba Buena Island were used as input motions. Computed and recorded ground motions transverse to the direction of wave propagation and associated response spectra were in good agreement. Agreement was also good in the radial direction, except in certain frequency bands higher than 1·25 Hz. Coherence studies showed that some of these discrepancies may be due to low coherence between the Treasure Island and Yerba Buena motions in these same frequency bands.

The Loma Prieta earthquake, ground motion, and damage in Oakland, Treasure Island, and San Francisco

AbstractThe basis of this study is the acceleration, velocity, and displacement wave-forms of the Loma Prieta earthquake (18 October 1989; M = 7.0) at two rock sites in San Francisco, a rock site on Yerba Buena Island, an artificial-fill site on Treasure Island, and three sites in Oakland underlain by thick sections of poorly consolidated Pleistocene sediments. The waveforms at the three rock sites display a strong coherence, as do the three sedimentary sites in Oakland. The duration of strong motion at the rock sites is very brief, suggestive of an unusually short source duration for an earthquake of this size, while the records in Oakland show strong amplification effects due to site geology. The S-wave group at Treasure Island is phase coherent with the Oakland records, but at somewhat diminished amplitudes, until the steps in acceleration at approximately 15 sec, apparently signaling the onset of liquefaction. All seven records clearly show shear-wave first motion opposite to that expected for the mainshock radiation pattern and peak amplitudes greater than expected for sites at these distances (95 ± 3 km) from an earthquake of this magnitude.While the association between these ground motion records and related damage patterns in nearby areas has been easily and eagerly accepted by seismological and engineering observers of them, we have had some difficulty in making such relationships quantitative or even just clear. The three Oakland records, from sites that form a nearly equilateral triangle about the Cypress Street viaduct collapse, are dominated by a long-period resonance (≃ 1 1/2-sec period) far removed from the natural frequency of the structure to transverse motion (2.5 Hz) or from high-frequency amplification bands observed in aftershock studies. A spectral ratio arbiter of this discrepancy confuses it further. The failure of the East Bay crossing of the San Francisco-Oakland Bay Bridge cannot be attributed to relative displacements of the abutments in Oakland and Yerba Buena Island, but the motions of the Bay Bridge causing failure remain unknown. The steps in acceleration at Treasure Island present unusual strong-motion accelerogram processing problems, and modeling suggests that the velocity and displacement waveforms are contaminated by a spurious response of the filtering operations to the acceleration steps. A variety of coincidences suggests that the Treasure island accelerogram is the most likely strong-motion surrogate for the filled areas of the Marina District, for which no mainshock records are available, but the relative contributions of bad ground, poor construction and truly strong ground motion to damage in the Marina District will never by known in any quantitative way. The principal lesson of all of this is that until a concerted effort is mounted to instrument ground and structures that are likely to fail during earthquakes, our understanding of the very complex relationships between strong ground motion and earthquake damage will, in general, remain rudimentary, imprecise, and vague.

The site response of two rock and soil station pairs to strong and weak ground motion

Abstract The site response to strong and weak ground motion depends largely on the subsurface conditions at the soil site for the two rock-soil station pairs studied. The first station pair consists of a soft-soil site (Treasure Island) and a sandstone and shale site (Yerba Buena Island). These stations recorded strong ground shaking from the Loma Prieta mainshock and weak ground motion from four aftershocks. The range of peak ground acceleration is from approximately 0.00006 to 0.07 g at the rock site. Compared to the rock site, the strong ground motion at the soft-soil site is amplified by a factor of about 3 over a frequency range from 0.5 to 2.0 Hz. The amplification is much higher for weak motion and suggests a dependence on signal amplitude. For example, near 1 Hz, the site response shows an increasing amplification as magnitude (and the peak velocity at the rock site) decreases. For events of local magnitude 7.0, 4.3, 4.1, 3.5, and 3.3, the maximum soil-site amplifications are 4, 12, 17, 19, and 25, respectively. A second station pair consisting of a stiff-soil site (Gilroy #2) and a sandstone site (Gilroy #1) was also studied with contrasting results. These two stations recorded strong ground shaking from the 1979 Coyote Lake, 1984 Morgan Hill, and 1989 Loma Prieta mainshocks. Weak ground motion was recorded at these stations after the Loma Prieta mainshock. The range of peak ground acceleration is from 0.006 to 0.43 g at the rock site. Unlike the results for the soft-soil study above, the estimated stiff-soil site responses are not significantly different for strong and weak motion from 0.5 to 2.0 Hz. Near 0.7 Hz, the stiff-soil site responses range from 2.5 to 4.5 for strong ground shaking from three mainshocks and from 1.5 to 4.0 for weak ground shaking from thirteen aftershocks.

Selected Strong and Weak-Motion Data From the Loma Prieta Earthquake Sequence

Abstract The purpose of this paper is to document the strong- and weak-motion seismic data from the Loma Prieta earthquake and its aftershocks obtained by Lawrence Livermore National Laboratory (LLNL), and to present some analysis of the spectral seismic response using both weak- and strong-motion recordings. LLNL operates six free-field, digitally recorded, triaxial, strongmotion accelerographs in the vicinity of LLNL; five of these were operating during the Loma Prieta earthquake. Two days after the main event, LLNL initiated a field deployment of 3-component weak-motion instruments to record aftershocks at three LLNL sites and four California Strong Motion Instrumentation Program (CSMIP) sites that recorded strong-motion from the main event. Spectral ratios of strong- and weak-motion recordings are computed for two pairs of rock and soil sites. One pair of stations is in the vicinity of LLNL, and the other pair is Treasure Island TRI (fill) and Yerba Buena Island YBI (rock) in San Francisco Bay near the Bay Bridge. For the first pair, the weak-motion spectral ratios predict the strong-motion amplification, within 95% confidence limits, for frequencies from 3 to 12 Hz. For TRI and YBI, the strong-motion spectral ratio is much lower than the weak-motion 95% confidence region for frequencies from 1 to 7 Hz. The strong-motion ratio, however, still suggests that the soil underlying TRI resulted in a factor of 3 amplification of energy between 1 and 4 Hz. This is in contrast to the factor of 8 amplification of the weak-motion energy, derived from the spectral ratios of 7 Loma Prieta aftershocks. The large difference between the weak-motion and strong-motion spectral ratios reinforces the limitation that weak-motion cannot be used to directly predict strong-motion amplification at sites underlain by soils that may respond non-linearly at high strain levels. A further examination of weak-motion recordings indicates that the source effect can be removed and the propagation path effects approximated so that the site response can be isolated. Resulting site specific spectral amplifications reveal that the spectral ratio method can lead to erroneous conclusions if the “rock” site has a complicated geology. At two sites near LLNL the apparent diminishing of spectral amplitudes below 5 Hz observed in the spectral ratios was actually due to amplification of spectral response at the rock site. It appears that the reference site spectral ratios at low frequencies may have been influenced by topography or near-surface geologic features. For the other pair of sites, the spectrum at YBI, the rock site, was flat, so that the features in the spectral ratios are due to the seismic response of the soil at TRI.

Foraminiferal Distributions and Environmental History of Quaternary San Francisco Bay: ABSTRACT

The Holocene San Francisco Bay and adjacent ocean-shelf foraminiferal fauna have been divided into five principal biofacies: brackish-water nearshore mudflat, marsh, normal bay, open bay, and open ocean. The open-ocean biofacies probably is restricted to the area outside the Golden Gate; at some localities individual specimens were found to occur within the open bay, but they were thought to have been current transported. By analogy, these biofacies provided the basis necessary for recognizing Pleistocene paleoenvironments in two bay cores. Cold-water, open-ocean influence existed at the onset of Pleistocene deposition within the bay, reaching inland as far as the east side of Yerba Buena Island (core 88), and at times as far southeast as the Oakland Estuary (core 484). The degree of periodic open-ocean influence, both in its geographic extent and its duration, was far greater than anticipated. Fluctuations ranging from open-ocean to marsh conditions persisted throughout the Pleistocene and correlate with Pleistocene and Holocene periods of glacial and interglacial eustatic sea-level changes. No faunal evidence was found to indicate the existence of Pleistocene nearshore mudflat biofacies. This may have been indicative of either lack of preservation or rapidly changing environments. The core samples contained five specie not found within the Holocene bay: Bulimina marginata, Elphidium albiumbilicatum, Lagena filicosta, Oolina melo, and Globigerina quinqueloba. End_of_Article - Last_Page 1702------------

Site Response At Treasure Island

The reliability of site response analysis procedures is assessed in terms of strengths and weaknesses of the available models. The site response analysis was performed for Treasure Island, in San Fracisco Bay, using the procedures incorporated in computer programs CHARSOIL (Streeter, et al. [1]), MASH (Martin and Seed [2]) and SHAKE (Schnabel, et al. [3]). The base motion for this site was estimated from the nearby rock-site at Yerba Buena Island as recorded during the 1989 Loma Prieta earthquake. The computed peak horizontal accelerations compared well with the recorded data but the peak spectral accelerations were underestimated. The pore water pressure computation predicted liquefaction for the event. INTRODUCTION During an earthquake, ground motions generated at the surface of soil deposits are considerably different from the motions that may occur on outcropping bedrock. This difference is attributed to the impedance contrast between the softer, upper soil layers and the relatively harder, underlying base rock layer. If ground motions are to be estimated at soft-soil sites, a site response analysis is performed to investigate the propagation of seismic waves from the base-rock level to the surface of the soil profile. For such analyses, the soil profile is either idealized as an assemblage of lumped masses, springs and dash-pots, or it is viewed as a combination of layers of viscoelastic media. Typically, the response is calculated using a one-dimensional analysis that assumes that the incoming seismic energy can be reasonably modeled by vertically propagating shear waves. As the earthquake loading can be viewed as a uni-dimensional propagation of simple shear disturbance through the profile, the soil model need to reliably simulate the shear stress-strain characteristics of soil. In a cyclic loading scenario, the shear stress-shear strain characteristic in virgin loading is usually simulated by closed form mathematical functions. The Davidenkov, and Ramberg-Osgood can be cited as examples of shear stress-shear strain model which have been implemented in computational procedures. The unload-reload behavior of soils are simulated by empirical schemes proposed by Masing. This scheme, commonly known as the Mas ing rule, obtains the unload reload curve from a given virgin loading curve by Transactions on the Built Environment vol 3, © 1993 WIT Press, www.witpress.com, ISSN 1743-3509