Geohazard Assessment for the Frank R. Howard Memorial Hospital, Willits, California Gary D

Geohazard Assessment for the Frank R. Howard Memorial Hospital, Willits, California Gary D. Simpson SHN Consulting Engineers & Geologists, Inc. 812 W. Wabash Eureka, CA 95501 [email protected] ABSTRACT to be a significant hazard at the site. The hazard was mitigated within the proposed hospital foot- This paper describes geologic and geotechnical print by removal of susceptible materials, mixing investigations for the proposed relocation of the (to homogenize previously stratified sandy and Frank R. Howard Memorial Hospital in Willits, clayey materials), and replacement as engineered Mendocino County, California. The proposed hos- fill. This remove/replace mitigation method was pital site is located just east of the active Maacama implemented to a depth of up to 21 feet beneath fault, within the Alquist-Priolo Earthquake Fault the ground surface. Site specific ground response Zone, on the Holocene floodplain of Haehl Creek; was initially calculated following the methods out- therefore, thorough investigations of surface fault lined in the 2001 California Building Code, and rupture hazard, liquefaction potential and other later revised to reflect updated methods presented geotechnical issues were completed. The investi- in the 2007 California Building Code. Geohazard gation of surface fault rupture was complicated by mitigation plans have been reviewed and approved the presence of Holocene sediments underlying the by the California Geological Survey and Office floodplain to depths of up to 25 feet. Therefore, a of Statewide Health Planning and Development, series of large stepped pits were excavated at the and construction of the hospital was completed in site, extending to depths in excess of 30 feet in August 2014. order to expose pre-Holocene strata. Stratigraphic evidence for past surface fault rupture was evaluated in late Pleistocene age, fine-grained alluvial Introduction sediments exposed in the lower portion of the deep The Frank R. Howard (FRH) Memorial Hospital pit exposures, and provided positive evidence has been serving central Mendocino County from for the absence of past surface rupture within the its current location in central Willits since 1928 proposed hospital footprint. Several buried trees (Figure 1). As the region has grown, the existing were encountered deep in the excavations, some facilities have become outdated and no longer meet in growth position, which provided valuable age the community needs nor current seismic code. control. Subsequent collaborative research of the Therefore, the need to modernize and expand the paleoseismic history of the Maacama fault at the existing hospital facilities was identified, and the site with a U.S. Geological Survey-funded team decision was made to develop a new regional hos- resulted in understanding of the precise fault loca- pital complex. Due to a lack of space on the existing tion, preliminary event timing, and minimum hospital campus, however, the new FRH hospital slip rate. Liquefaction potential was determined would have to be re-located to a new site. Property 2 Applied Geology in California Figure 1. Location of the Frank R. Howard Hospital study site in southern Little Lake Valley. offered by a local benefactor seemed ideal; a large Holocene floodplain associated with nearby Haehl undeveloped parcel on flat ground just outside of Creek (Figures 2 and 3). The new hospital was town. This site, however, is located just east of the determined to be economically feasible only if the mapped trace of the active Maacama fault, within donated land could be used, so an extended geologic an Alquist-Priolo Earthquake Fault Zone, on the and geotechnical investigation ensued. This paper Geohazard Assessment for the Frank R. Howard Memorial Hospital, Willits, California 3 describes the efforts related to characterization were fully vetted by the agencies, as were the results and mitigation of surface fault rupture and soil of construction observation and testing during mit- liquefaction, as well as a brief discussion of the igation efforts. This work was completed primarily characterization of strong ground motion. during 2006 and 2007. Following a lengthy design Little was known about the geologic site and review process, additional geotechnical bor- conditions prior to the onset of fault rupture hazard ings were drilled at the site in 2010 to provide con- investigations, which began in 2001 and continued firmation of post-mitigation soil conditions and to through 2004. The fault studies at the hospital develop design-level geotechnical parameters. site were conducted in conjunction with similar investigations on adjacent parcels to the south and southeast, where a residential subdivision Geologic Setting and Senior Assisted Living facility were proposed The project site is located within the Coast Ranges (the individual study areas are shown on Figure 3; physiographic province of northern California, in SHN, 2004a, 2004b, 2003). These studies resulted an area sometimes referred to as the “Mendocino in an awareness that the site conditions offered a Highlands” (Figure 1). This area is characterized unique opportunity to evaluate fault history and by a distinct north-northwest trending structural seismic potential, which led to a collaborative grain, which is manifested in a regional pattern research-level study involving geologists from of subparallel ridges and valleys. Post-Pliocene SHN and the U.S. Geological Survey (USGS), northward migration of the Mendocino triple junc- URS Corporation, and Humboldt State University tion and subsequent right-lateral strike-slip faulting (Fenton et al., 2002; Larsen et al., 2005; Prentice associated with development of the San Andreas et al., 2014). In all, these investigations resulted in transform fault system is largely responsible for nearly 4000 feet of paleoseismic trenching, includ- this structural pattern. Structural development ing at least 13 fault crossings (Figure 3). associated with the active transform faulting has Geotechnical site conditions, to evaluate resulted in the formation of a series of large down- liquefaction potential and other soil engineering dropped valleys across the Mendocino Highlands concerns, were evaluated through an extensive (for example, Ukiah valley, Little Lake valley, program of subsurface investigation and labora- Laytonville valley; Crowell, 1974 and McLaughlin tory testing (SHN, 2004). The site was found to be and Nilsen, 1980, in Pampeyan et al., 1981); the underlain by potentially liquefiable soils within the subject site is located in the southern end of the upper 17 to 21 feet. A variety of soil improvement Little Lake valley. Little Lake valley is a deep, and deep foundation options were considered and sediment-filled basin, the stratigraphy of which is a large-scale mitigation effort ensued. described in Woolace (2005). The investigations described in this paper generally followed the requirements described in Site Geology the currently available version of the California The new FRH hospital site is located on the Geological Survey’s Note 48, the “Checklist for the Holocene floodplain of Haehl Creek (Figure 3). Review of Engineering Geology and Seismology Topography of the site is essentially flat (there is a Reports for Public Schools, Hospitals, and Essential very slight northward slope). A 15- to 20-foot-high Services Buildings” (note that over the duration of pressure ridge located along the Maacama fault this investigation significant evolution of Note 48 west of the hospital site, and the Haehl Creek chan- occurred). The results of the geologic and geotech- nel, are the only topographic deviations in the proj- nical investigations were rigorously reviewed by ect area. Haehl Creek drains the upland areas south the lead regulatory agency, the Office of Statewide of the valley, flows northward across the Little Health Planning and Development (OSHPD), Lake valley floor in a 20- to 25-foot-deep arroyo with collaboration from the California Geological (within about 400 to 500 feet west of the site), and Survey (CGS). Geohazard mitigation strategies joins Baechtel Creek north of the site. 4 Applied Geology in California Figure 2. Site map showing the hospital study area relative to adjacent sites, and the location of paleoseismic (USGS) and consultant (SHN) trenches. The northwest trending dashed line is the Maacama fault. Stream valley walls along Haehl Creek (described foot thick deposit of young, unconsolidated granu- in Woolace, 2005), as well as exploratory trenches lar alluvium overlying broad tabular (1 to 3 foot and borings at the site, have exposed a consistent thick) beds of predominantly fine-grained silty clay stratigraphic condition in the study area: a 15 to 25 deposits (distinctly gray and blue-gray soil colors). Geohazard Assessment for the Frank R. Howard Memorial Hospital, Willits, California 5 Figure 3. Geologic map of the Little Lake valley region, Mendocino County (Jennings and Strand, 1960). Radiocarbon dating of charcoal samples collected overbank deposits with intermittent coarse sand in trenches and test pits at the hospital site, in the and gravel channels) is Holocene in age, with some USGS paleoseismic research trenches, and at the near-surface channels as young as a few hundred Senior Assisted Living facility site, indicates that years old (see the compilation of radiocarbon dates the surficial Haehl Creek alluvium (mostly silty in Tables 1 and 2 in Prentice et al., 2014). 6 Applied Geology in California The underlying silty clay deposits,

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