Fountaingrove Environmental Impact Report - City of Santa Rosa ...

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3.6 GEOLOGY, SOILS, AND SEISMICITY Active and potentially active faults within the Project site limits and immediate vicinity have been mapped and documented by a number of government agencies. The United States Geological Survey and CDMG have published numerous maps and reports on faults of various types, ages, and levels of activity. The faults nearest to the Project site are shown on Figure 3.6-3. Surficial Deposits. Soil deposits found onsite typically comprise a thin layer of brown, porous silt, surficial deposit overlying gravel, cobble, and occasional boulders. Over much of the Project site, a clayey soil was encountered beneath this gravelly deposit (Giblin, 2007). Slope-wash and ravine fill are classified as colluvial-type deposits, which consist of loose sediments at the foot of a slope brought there principally by gravity and slope wash. Substantial accumulations of fill were found in the drainage swales in the northern and south-central portions of the Project site (Giblin, 2006b). The existing fill appears to be related to past grading, filling, and stockpiling of materials in the drainage swales. The fill appears to be about 4 to 5 feet thick and consists of soft to medium-stiff sandy clay with cobbles and occasional boulders. Laboratory tests indicated that the soils exhibit a moderate expansion potential (Giblin, 2006b). Below the existing fill deposits, natural sandy clays and sandy silts were found. These soils were observed to be porous, highly organic, and compressible. They generally exhibited a low to moderate expansion potential. In the swale areas onsite, sandy clays of high expansion potential were observed below the weak porous upper soils (Giblin, 2006b). Bedrock. Published maps indicated that rock in the Project vicinity comprises primarily two geologic units: the Miocene-Pliocene age sedimentary rocks of the Petaluma Formation and the Pliocene age Sonoma Volcanics (Giblin, 2006a). The Petaluma Formation includes poorly indurated to lightly cemented sandstone, siltstone conglomerate, mudstone, and claystone and is reported to include some tuffaceous and diatomaceous beds locally. The Petaluma Formation is considered highly susceptible to landslides and slope instability. Although published maps indicate that the Petaluma Formation is located west and beyond the Project site, volcaniclastic sedimentary rocks encountered in the 2006 geologic hazards field investigation by Giblin Associates could be interpreted to be part of the uppermost portion of the Petaluma Formation, as shown in Figure 3.6-4. Published maps and previous geologic work performed indicate that the bedrock comprises various rock types predominantly within the Sonoma Volcanics unit. This unit is composed primarily of andesitic to basaltic lava flows and, as a result ranges, from relatively weak sediments to hard and strong lava flows. Scattered test pits dug by Giblin Associates showed lava flow rocks, agglomerate, tuffaceous, and sedimentary rocks. Bedding observed indicated that the rocks in the central portion of the property are folded in an anticline (concave downward). Beds are gently folded with dips varying from gently eastward at the higher elevation in the eastern property area to moderately down to the west in proximity to Thomas Lake Harris Drive. A northwest strike interrupts these rocks to form a crude layering or bedding with a moderate westerly dip of about 25 to 30 degrees. The test pits also indicated that rock in the swale areas are generally cemented sands and gravels and tuffaceous siltstone. Surficial geologic units observed in test pits and scattered outcrops in the Project vicinity are described below; see Figure 3.6-4 for the Giblin Associates mapping of geologic units. 3-70 BAO\082970001
3.6 GEOLOGY, SOILS, AND SEISMICITY Special Publication 42, which states that no structure for human occupancy should be located within 50 feet of an active fault. To meet requirements of the Alquist-Priolo Special Studies Zones Act, earthquake fault zone boundaries have generally been established approximately 500 feet on either side of major, active fault traces and approximately 200 to 300 feet on either side of well-defined, minor fault traces. Exceptions to this general pattern of earthquake fault zone delineation periodically occur where faults are obscured, poorly located, locally complex, and/or not vertical. Because of these criteria for determining zone boundaries, an earthquake fault zone designated by CDMG for a particular fault may be wider than the actual fault zone occupied by traces of the fault. Conversely, due to specific zoning criteria, mapped fault traces not shown to be sufficiently active or well-defined may not be included within the designated Alquist-Priolo earthquake fault zone. Therefore, in some cases the actual zone of potential surface rupture may not be entirely included within the CDMG-designated earthquake fault zone. All but the extreme northeast corner of the Project site is located within the Alquist-Priolo earthquake fault zone designated for the Rodgers Creek-Healdsburg fault. California Geological Survey Special Publication 42 requires a site-specific fault study if a site is within an Alquist-Priolo zone. Due to its location within the earthquake fault zone, a number of site-specific fault studies have been performed within and adjacent to the site. Results of these studies are summarized below, without any new interpretations. • Alquist-Priolo Special Studies Map (CDMG, 1976). The 1976 Alquist-Priolo map prepared by CDMG depicted three northwest-trending fault traces located southwest of the Fountaingrove Lake north dam. The locations of these faults were largely drawn by compiling previously published maps with little, if any, field verification. Two fault traces are shown projecting through the Project site, one through the central portion and another through the southern portion. • TMI Property, Fountain Grove Ranch (Harding-Lawson Associates, 1980). Two fault traces were identified during the Fountain Grove Ranch Environmental Impact Report (EIR) studies that project through or immediately adjacent to the Project site. One northwest-trending fault trace passes through the northern portion of the site, and a 50-foot building setback was recommended surrounding this fault trace. The second fault trace, considered to be the main active trace of the Healdsburgh-Rodgers Creek, was estimated to exist in the southernmost end of the Project site with a northwest orientation. A building setback zone was provided in the EIR that encroaches into the Project site. The locations of these traces were estimates at the time and, appropriately, the EIR stated that additional investigation by trenching may modify the setback zones. • Alquist-Priolo Special Studies Zone Map (July 1, 1983). The CGS updated the earthquake fault maps, and the modified data show that an active fault does not pass through either the northern nor southern portions of the Project site. The main fault trace was relocated in the 1983 updated maps about 150 feet further southwest and outside the Project site. Stonefield Development (Moore & Taber, 1983). A geotechnical investigation was performed for a residential development south of the Project site. The investigation 3-84 BAO\082970001
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Aegis Senior Living Continuing Care
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CONTENTS Response to Comment Letter
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ACRONYMS AND ABBREVIATIONS HFC hydr
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