Airport Liquefaction Susceptibility Analysis Report - ABAG ...

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Fugro Consultants, Inc. Project No. 04.79221200 EXECUTIVE SUMMARY The Association of Bay Area Governments (ABAG), under a grant issued by the California Department of Transportation (CalTrans), is revising and updating their 1999 earthquakeinduced liquefaction susceptibility assessments of San Francisco International Airport (SFO) and Oakland International Airport (OAK), as well as performing new assessments at three smaller general aviation regional airports; Moffett Federal Airfield (Moffett), located in Sunnyvale; Buchanan Field Airport (Buchanan), located in Concord; and Livermore Municipal Airport (Livermore), located in Livermore. These assessments are part of a larger planning study to evaluate the use of and access to these facilities following a hypothetical largemagnitude earthquake. Fugro Consultants, Inc. (FCL) is collaborating with ABAG to assess the susceptibility to liquefaction at each of these five airports. Liquefaction is the transformation of a granular material from a solid state into a liquefied state as a consequence of increased pore pressure and decreased effective stress (Youd, 1973). Increased pore pressures in unconsolidated sediment, especially in west-central California, are typically earthquake-induced (co-seismic). Liquefaction related ground failure can result in settlement, foundation (bearing) failure, lateral movement (lateral spread), and general ground cracking. Airport runway surfaces are susceptible to damage and closure following liquefactionrelated ground failure. In this study, FCL develops earthquake-induced liquefaction hazard maps following the general liquefaction susceptibility hazard analyses and mapping approach of the California Geological Survey and based on current and recent research studies performed by FCL and former William Lettis and Associates (WLA) in California. Eathquake-induced liquefaction-related ground failures historically have caused extensive structural and lifeline damage in urbanized areas around the world. Recent examples of these effects include damage produced during the 1994 Northridge (California), 1995 Kobe (Japan), 1999 Izmit (Turkey), 2001 India earthquakes, and also the 2010-2011 Christchurch earthquake sequence (New Zealand). Damaging liquefaction occurred in the San Francisco Bay Area during the 1868 Hayward earthquake, the 1906 San Francisco earthquake, the 1957 Daly City earthquake, and the 1989 Loma Prieta earthquake (Youd and Hoose, 1978; Holzer, 1998). Studies of historical earthquakes show that the areal distribution of liquefaction-related damage is generally restricted to alluviated basins and filled land areas that contain shallow layers of low density, saturated, granular sediment (e.g. Youd and Hoose, 1978; Holzer, 1998; Tinsley et al., 1985). In the areas around San Francisco Bay, the geologic materials with a history of liquefaction include Holocene-aged estuarine deposits, Holocene stream and alluvial fan deposits, Holocene and Pleistocene eolian deposits, and artificially emplaced fill materials. Understanding the distribution and extent of these materials is critical to understanding the potential hazard posed by co-seismically induced liquefaction. All five of the subject airports are sited on young unconsolidated sediments and artificial fill, materials considered to have moderate to very high susceptibility to liquefaction (Knudsen et al., ii May 31, 2013
Fugro Consultants, Inc. Project No. 04.79221200 2000, Witter et al., 2006). The San Francisco and Oakland airports sit on artificial fill placed over tidal marsh deposits and bay mud. Most of the fill is non-engineered, emplaced before 1970, and may include loose sandy material highly susceptible to liquefaction. Moffett, Buchannan, and Livermore airports are built over young alluvial fan sediments, considered to have moderate susceptibility to liquefaction. There are numerous faults in the San Francisco Bay Area that are capable of producing largemagnitude earthquakes and strong ground shaking that can induce liquefaction. Ground shaking from an earthquake is measured in units of gravitational acceleration (g), and one “g” is about 9.8 m/sec 2 , or 32 ft/sec 2 . For a given location and earthquake scenario, the strongest acceleration anticipated to occur is called peak ground acceleration (PGA). Each earthquake scenario, deterministic or probabilistic, generates a grid of PGA values that spans the area of influence. For this study, we consider ground motions from three likely earthquake scenarios (deterministic), and for two different probability levels (probabilistic) based on the United States National Seismic Hazard Maps working group (Petersen et al., 2008). Earthquakes on the following fault segments are considered for the deterministic events: 1) 1906 San Andreas Fault scenario, 2) Hayward-Rodgers Creek Fault scenario, and 3) Concord-Green Valley Fault scenario (Figure 1). The two probabilistic ground motions consider the contribution of all the faults in the Bay Area, and use the compiled probabilities assigned by the Working Group on California Earthquake Probabilities (WGCEP, 2008). For this study, we considered the 2% chance of excedance in 50 years scenario, and the 10% chance of exceedance in 50 years scenario. Fugro compiled existing borehole and cone penetration test (CPT) data for each airport to perform quantitative liquefaction potential assessments. A liquefaction analysis using the Simplified Procedure was performed for San Francisco, Oakland, Moffett, and Buchanan airports, and yield ranges of settlement values between 0 and 4 inches, 2 and 9 inches, less than one to 3 inches, and 1 to 4 inches, respectively. A liquefaction analysis was not performed for Livermore because the available subsurface data did not extend below the historical high water table. The geologic units found to be most susceptible to liquefaction include coarsegrained artificial fill, artificial fill over estuarine mud, the Merritt Sand and coarse grained lenses within the Young Bay Mud, Old Bay Mud, and Holocene alluvial fan deposits. Given the limitation in data density and extent, we recommend that a liquefaction specific investigation be conducted for Buchanan and Livermore airports to better characterize the liquefaction hazard. To address the gaps in subsurface data coverage at Moffett and Oakland, we recommend additional subsurface studies. A comprehensive investigation to determine the characteristics of different fill types (type, distribution, and thickness), and the potential for differential settlement should be performed at Moffett, Oakland, and San Francisco. In addition, we recommend future evaluations should specifically address perimeter dikes and levees at iii May 31, 2013
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