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HomeMy WebLinkAboutBLD2022-00410_09A- Geotech ReportOctober 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 1 Geotechnical Evaluation of Parcel 962700097, Jefferson County, Washington. David S. Parks M.S. Principal Licensed Engineering Geologist L.G., L.E.G., L.H.G # 533 Crescent Environmental PLLC PO Box 429 Port Angeles WA 98362 360-640-3187 October 31, 2022 October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 2 INTRODUCTION Crescent Environmental PLLC (Crescent) is pleased to present the results of our Geotechnical Evaluation of parcel 962700097, Jefferson County, Washington. (Figure 1. Note: all figures are at the end of the report starting on page 13). The subject property consists of one contiguous parcel (962700097) totaling approximately 0.98 acres at 102 Cleveland Steet, Port Hadlock, Washington. This report is focused on an evaluation of the seismic soil hazards for the area of this parcel for the purpose of siting a single-family residence. This report is an instrument of service subject to the conditions and limitations set forth in Appendix B, and was prepared by an Engineering Geologist and Hydrogeologist licensed in the State of Washington. In preparing this report Crescent Environmental PLLC visited the subject property on August 24 and October 6, 2022. This work was conducted in accordance with the signed scope-of-work between Crescent Environmental PLLC and Tim Murray. Purpose and Scope The purpose of this geotechnical evaluation is to provide a qualitative, reconnaissance- level, analysis of the relative seismic soil and potential earth hazards associated with establishing a residential structure on the subject parcel. Based on information provided by you, it is my understanding that you plan to construct a single-story, single-family residence within 76 horizontal feet of the Ordinary High Water Mark (OHWM) on Oak Bay, a Type S, Shoreline of Statewide Significance. This report first reviews available geologic and critical areas information from a variety of sources including geologic and earth hazard mapping from the Washington Department of Natural Resources, and United States Geologic Survey (USGS) and critical areas mapping from the Jefferson Department of Community Development. Following a review of existing geologic information, this report presents on-site observations of the geologic and vegetative characteristics of the parcels with respect to any potential seismic soil hazards relative to constructing residential structures on-site. Finally, the information presented above is synthesized and summarized to provide an overall evaluation of the long-term stability and suitability of the subject parcel and October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 3 provides recommendations for managing any existing and potential seismic soil and earth hazards. Site Location The subject property is located several miles south and east of Port Hadlock, Jefferson County, Washington, in Section 7, Township 29 North, Range 1 East, W.M., (Figure 1). The subject property consists of one tax parcel comprising approximately 1.0 acres. Site Characteristics The subject property is situated on a gently to moderately sloping glacial till and outwash terrace above Oak Bay (Figure 2). The elevation of the subject property ranges from approximately 50 feet to 7 feet above mean sea level (m.s.l.) and the proposed home site is at approximately 35 feet above sea level. (Figure 2) Precipitation at the site is seasonally variable characterized by wet mild winters and relatively dry cool summers. Annual average precipitation (1981-2010) is 22 inches primarily occurring as rainfall. The majority of the subject property is moderately sloping (0-25%) and the eastern portions of the parcel have an average slope of less than 30% down to the marine shoreline of Oak Bay (Figure 3). The subject property exhibits a mixture of vegetation types consisting of mixed second- growth conifer-deciduous stand composed of Douglas fir, Red alder and Willow. The understory vegetation is dominated by upland plant species such as Salal, Salmonberry, Snowberry, Sword fern, Red alder and Himalayan blackberry. Geologic Conditions Regional The project area is located on the northeast side of the Olympic Mountains, a range comprised of sedimentary and volcanic rocks uplifted as a result of tectonic October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 4 convergence and subduction along the Cascadia margin of North America. The highlands to the south are in Crescent Formation, mostly high-relief terrain formed of volcanic rocks. The lower and less steep lands to the north and east are sedimentary bedrock units overlain by a thick sheet of recessional outwash and till, where ice sheets that occupied the Strait of Juan de Fuca and Admiralty Inlet lapped on the edge of the Olympic Mountains. Surficial Geology On the rolling uplands the land surface is composed of a thick (upwards of 600 feet) of weathered till, outwash and alluvial fan deposits. During major ice ages, continental glaciers pushed south into the lower terrain around the Olympic Mountains and the continental ice sheet pushed southward from the west end of the Puget Lowlands and south from the Juan de Fuca Strait. Glacial drift once formed an irregular blanket throughout the area, but post-glacial incision by streams and creeks have cut through the glacial deposits, leaving drift mantling the rolling uplands and irregular strath terraces within the major canyons. Weathering and mass-wasting (creep and landslide) processes along the new canyons, ravines, and coastal bluffs have continued to erode into the glacial deposits. Locally the project area is mapped with surficial deposits of Vashon age glacial till and outwash overlying Oligocene age marine sandstone and Eocene age volcanic rocks (Figure 3). Geologic mapping of the parcel area includes mapping conducted by the Washington Geologic Survey (WGS) and the United States Geologic Survey (USGS) at differing scales and levels of detail. Geologic mapping of the parcel at the scale of 1:24,000 by Gayer (Gayer, 1976) describes the geology of the parcel as Vashon age lodgment till up to forty-feet thick. Geologic mapping of the parcel by Miller and Pessl, Jr. (USGS, 1986) describes the site as: “Boulders, cobbles, pebble, sand, silt and clay in a non-sorted mixture (glacial till). Larger boulders can be absent; in such places unit can be fine grained. Locally contains beds, layers, lenses and pods of water deposited sand and gravel. Matrix can vary, but generally is a silt, clayey silt, or silty sand. Typically, firm to very firm. Massive, non-stratified, but in places contains sub-horizontal layering; thin, friable separations generally occur in upper 1 meter. Land surface even to very uneven; in places weathered to a loose sandy stony mixture in the upper 1-2 m. Locally called “hardpan”. Thickness varies, ranges from a few October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 5 meters to as much as 40 m, but commonly is between 3 m and 15 m (Miller and Pessl, Jr. 1986). That same year, geologic mapping by the USGS (Pessl, Jr., et al, 1986) described the western three-quarters of the parcel as Vashon age Advance Outwash deposits and the eastern one-quarter of the parcel as Marsh, Bog or Swamp deposits (Holocene- Pleistocene age) consisting of sand, silt and clay mixed with partly decomposed organic matter, deposited in freshwater and saltwater. Includes peat deposits and locally contains interbedded tephra (Pessl, Jr., 1986). Composite mapping by the WDNR at 1:100,000 scale (Figure 3) shows the mapped distribution of these two geologic units on the subject parcel (WDNR, 2015). Soil Textures There are two soil textural series, Tidal Marsh soils and Tukey gravelly loam 0-15 percent slopes mapped on the subject parcel (McCreary, 1979) as shown in Figure 4. Field evaluation of the soils on the parcel indicate that the soil mapping in this area is incorrect because Tidal Marsh soils have not been observed in soil pits excavated on the subject property. On-site evaluation of soils within soil pits dug on the property indicate that the Tukey gravelly loam soil unit is the type of soils comprising the area. The mapped Tidal Marsh soils consist of 0-6 inches of mucky silt loam overlying 6-60 inches of stratified sand to silty clay. These soils were not observed in soil pits excavated on the parcel. Tukey gravelly loam soils consist of 0-8 inches of gravelly loam overlying 8-60 inches of very gravelly loam. Jefferson County has mapped the Tidal Marsh soils which also correspond to a peat deposit mapped by the Washington Geologic Survey and the United States Geologic Survey (Figure 3). Jefferson County has mapped this peat deposit as a seismic soil hazard. October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 6 Review of Existing Geologic Hazard Mapping for the Subject Property Seismic Hazards Western Washington is located in a moderately active seismic zone due to its position at the edge of an active plate margin where subduction is occurring. The historical seismic record (about the last 160 years) is relatively short. Currently, the North American Plate is over-riding the Oceanic, or Juan de Fuca, Plate at a rate of about 1½ inches per year. The converging plates produce typical subduction zone features. These features include a small trench off the west coast of Washington State, where the Juan de Fuca plate begins its descent below the North American plate; and an active volcanic arc in the over-riding plate, where partial melting of the subducted Juan de Fuca Plate generates the geologically young and active volcanoes of the Cascade Mountain Range (HWA, 2012). Researchers identify three earthquake source regions in this tectonic setting. The first source region delineated is within the subducted portion of the Juan de Fuca Plate. These earthquakes are produced by down-dip tensional forces within the subducting plate. A source zone 45 to 60 km deep, underlying the entire Puget Sound, is estimated to be capable of producing earthquakes with magnitudes as great as 7.5. Historic occurrences of such earthquakes include the 6.8 M, 2001 Nisqually earthquake; the 6.5 M, 1965 Sea-Tac earthquake; and the 7.1 M, 1949 Olympia earthquake (HWA, 2012). The second source region is located at the interface between the converging plates. Large (M > 8) interplate earthquakes could result from differential movement between these converging plates. The last subduction zone earthquake occurred January 31, 1700, according to geologic evidence and Japanese historical records of tsunami events (HWA, 2012). The third source zone is in the over-riding North American Plate. This source zone produces shallow crustal earthquakes with focal depths less than 30 km. The actual geographic location of this source zone, possible earthquake magnitudes, and probability of occurrence up until just recently were poorly understood. Recent studies indicate that a large earthquake occurred in the Seattle area, on what is now known as the Seattle Fault Zone, between 500 and 1,700 years ago. The amount of deformation in the vicinity, and other related evidence, indicates that the magnitude was probably larger than 7 on the Richter Scale (HWA, 2012). For seismic design, per 2012 International Building Codes, the site is considered Site Class C, as described in Section 1615.1 of the 2012 IBC. The maximum considered earthquake (a 2,475-year return period event) is consistent with a peak ground October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 7 acceleration (PGA) of 0.52g. The design PGA would therefore be 0.35g (2/3 of the MCE PGA) (HWA, 2012). The design earthquake results in an associated 0.2-second spectral acceleration (Ss) of 1.278g and a 1.0-second spectral acceleration (S1) of 0.516g (USGS, 2008). For this site class (Site Class C), Fa = 1.0 and Fv = 1.3. Design Spectral response acceleration parameters are therefore: SDS = 0.852g and SD1 = 0.447g (HWA, 2012). Soil liquefaction is a phenomenon wherein loose to medium dense, saturated granular deposits temporarily lose strength and behave as a liquid in response to earthquake shaking. Two seismic (earthquake) hazard mapping products, Liquefaction Susceptibility and Seismic Design Criteria are available from the Washington Department of Natural Resources, Washington Geologic Survey, and shown in Figures 6 & 7. Liquefaction susceptibility is a rating of the likelihood that a particular soil unit would liquefy during an earthquake of a certain magnitude and duration. The western half of the property and surrounding area have a “Very Low to Low” liquefaction rating. The eastern half of the parcel is not rated for liquefaction because the peat soils mapped in this location require an on-site evaluation. The National Earthquake Hazards Reduction Program (NEHRP) publishes recommended provisions for seismic regulation for new buildings and other structures and defines seismic site classes as a way of evaluating the relative need to employ certain building design criteria for areas experiencing seismic events (e.g. earthquakes). Seismic Design Criteria (SDC) (Figure 7) is a classification assigned to a structure based on its occupancy, use, and level of expected soil modified seismic ground motion. The seismic design criteria for the western half of the subject property is shown in Figure 7 and is rated as “C”. Seismic Design Criteria (SDC) “C” corresponds to buildings and structures in areas expected to experience severe and destructive ground shaking but are not located close to a major fault. The western half of the parcel is mapped as seismic design criteria “F” for soft soils due to the mapped presence of peat soils. Seismic design criteria F corresponds to buildings of Occupancy Group IV (Hospitals, Police Stations, Emergency control centers etc.) near major active faults Seismic Design Criteria F corresponds to soils within the upper 100 feet underlying the site with peat deposits thicker than 3 meters (10 ft) (ASCE, 2005). For structures that have fundamental periods of vibration equal to or less than 0.5 s, site October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 8 response analysis is not required to determine spectral accelerations for liquefiable soils for areas mapped as site class F. Several faults have been identified in the Port Townsend area, but none have been traced to the subject property, and no evidence of Holocene-age faulting has been observed in the area. Therefore, the risk of damage due to ground fault rupture is minimal. Review of Historic Aerial Photography In order to evaluate the geologic context of the property, I reviewed available historic aerial photographs from 1942 to 2021 (Figures 7-16). No landslides, surface ruptures, faulting or surface erosion were identified on the aerial photography. The parcel apparently has been stable during the period of review. A residential structure is observed on the photography in 1951 and persists until 1989. It is my understanding that this residential structure burned down sometime after 1989. On-Site Observations I visited the subject property on August 25 and October 6, 2022, and walked the area of the proposed single family residence and surrounding areas including the area mapped as a seismic soil hazard. The home site is located on a gently sloping 0-25% hillslope adjacent to the marine shoreline of Oak Bay. The majority of the subject parcel is composed of Vashon age glacial outwash and till deposits. The lower, eastern area of the parcel exhibits a relatively thin < 1 meter-thick deposit of peat underlying the glacial outwash just above sea level. (Photograph 1). October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 9 Photograph 1: Thin peat deposit underlying the eastern portion of the subject parcel. Drain pipe is 6 inches in diameter. October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 10 The thin peat deposit observed on the eastern portion of the parcel just above sea level appears to be composed of fibric (undecomposed) organic matter mixed with fine sand. Qualitative Seismic Soil Assessment The soils and underlying glacial outwash supporting the proposed residential house on the subject property have relatively high values for soil cohesion, ranging in value from 1500-5000 pounds per square foot (psf) (Koloski, Schwarz, and Tubbs, 1989). The mapped peat deposit in the eastern portion of the parcel is relatively thin (<1 meter- - thick) and appears to be an interbed within the Vashon age lodgment till deposit. The American Society of Civil Engineers (ASCE) procedure for assessing seismic site class provides an exemption to the requirement of conducting ground response evaluations for soils with peat horizons less than 3 meters thick (10 ft.) (ASCE 7-05, 2005). Based on ASCE guidance, the seismic site class for the subject parcel would be assigned to Site Class C, because of the < 1-meter thick peat horizon within the lodgment till on the parcel precludes the assignment of Site Class F. Recommendations for Seismic Soil and Earth Hazard Mitigation In order to minimize the uncertainty of the risk to the home from being located adjacent to a mapped seismic soil hazard area, the following mitigation actions should be considered: 1. A qualified design professional should be consulted when designing the residential structure so that the structure has a fundamental period of vibration equal to or less than 0.5 s. Most single and two-story residential buildings have a fundamental period of vibration less than 0.12 s, so this should entail verification that the selected design falls within the stated fundamental period of vibration. Conclusion Although Jefferson County has mapped a seismic soil hazard (Seismic Site Class F) on the subject parcel, on-site investigation has determined that the parcel should be considered Seismic Site Class C. October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 11 The Vashon lodgment till soils comprising the parcel have adequate bearing strength for typical residential construction methods for areas mapped as Seismic Site Class C. In my opinion, the parcel is suitable for siting a single-family residence within 76 feet of the ordinary high water mark of the shoreline on Oak Bay as proposed. Limitations This report is subject to the limitations set forth in Appendix B. Sincerely, David S. Parks M.S. Licensed Engineering Geologist, Hydrogeologist L.G., L.E.G., L.H.G. #533 Crescent Environmental PLLC PO Box 429 Port Angeles, WA 98362 360-640-3187 Email: crescentenvironmental@gmail.com October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 12 References ASCE, 2005. American Society of Civil Engineers and Structural Engineering Institute, Minimum Design Loads for Buildings and Other Structures Including Supplement No. 1 (ASCE 7-05). Geyer, M.J., 1976. Geologic Map of Northeastern Jefferson County, 1:24,000-scale. Open File Report 76-21. Washington Division of Geology and Earth Resources. Washington Department of Natural Resources. HWA Geosciences, 2012. GEOTECHNICAL REPORT, Port Hadlock Treatment Facility, Port Hadlock, Washington, HWA Project No. 2011-150-21 Task 400. Koloski, J.W., S.D. Schwarz, and D. W. Tubbs, 1989. Geotechnical Properties of Geologic Materials. Engineering Geology in Washington, Volume 1. Washington Division of Geology and Earth Resources Bulletin 78. McCreary, F. R., 1975. Soil Survey of Jefferson County Area, Washington. United States Department of Agriculture, Soil Conservation Service. Washington D.C. Miller, R.D. and F. Pessl, Jr. 1986. Map Showing Unconsolidated Deposits Grouped on the Basis of Texture, Port Townsend 30’ x 60’ Quadrangle, Puget Sound Region, Washington. United States Geological Survey (USGS). Pessl, Jr., F., D.P. Dethier, D.B. Booth, and J. P. Minard. 1986. Surficial Geologic Map of the Port Townsend 30’ x 60’ Quadrangle, Puget Sound Region, Washington. USGS Map I-1198 F. United States Geologic Survey, Reston, VA. Polenz, M., J. Faria, I.J. Huber, G.L. Paulin, and R. Cakir. 2015. Geologic Map of the Port Ludlow and Southern Half of the Hansville 7.5 minute quadrangles, Kitsap and Jefferson County, Washington. Washington Division of Geology and Earth Resources Map Series 2015-2. Washington Department of Natural Resources, Olympia, Washington. Washington Geological Survey (WGS) 2015. Composite Geologic Mapping of the Port Townsend 30 ‘x 60’ Quadrangle, Washington. Washington Department of Natural Resources, Olympia, Washington. October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 13 FIGURES Figure 1: Location map including the Subject Property located at 102 Cleveland Street, Port Hadlock, in Jefferson County, Washington. October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 14 Figure 2: Map showing parcel boundaries and LiDAR topography contours on the Subject Property (parcel 962700097) in Jefferson County, Washington. October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 15 Figure 3: Geologic Mapping of the subject parcel (WDNR, 2015). October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 16 Figure 4: NRCS Soil Mapping of the Subject Parcel (McCreary, 1979) October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 17 Figure 5: Earthquake liquefaction susceptibility rating map (WDNR). October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 18 Figure 6: Seismic Design Site Class Rating for the Subject Parcel. October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 19 Figure 7: Orthophotograph from 1942 for the subject property. October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 20 Figure 8: Orthophotograph from 1951 for the subject property. October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 21 Figure 9: Orthophotograph from 1989 for the subject property. October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 22 Figure 10: Orthophotograph from 2006 for the subject property. October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 23 Figure 11: Orthophotograph from 2009 for the subject property. October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 24 Figure 12: Orthophotograph from 2011 for the subject property. October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 25 Figure 13: Orthophotograph from 2013 for the subject property. October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 26 Figure 14: 2015 Aerial photography of the subject parcel. October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 27 Figure 15: 2017 Aerial photography of the subject parcel. October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 28 Figure 16: 2021 Aerial photograph of the subject parcel. October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 29 Appendix B REPORT LIMITATIONS AND GUIDELINES FOR USE This attachment provides information to help you manage your risks with respect to the use of this report. Geotechnical Services Are Performed for Specific Purposes, Persons and Projects This report has been prepared for use by the client Tim Murray. This report is not intended for use by others, and the information contained herein is not applicable to other sites. Crescent Environmental PLLC structures our services to meet the specific needs of our clients. For example, a geotechnical or geologic study conducted for a civil engineer or architect may not fulfill the needs of a construction contractor or even another civil engineer or architect that are involved in the same project. Because each geotechnical or geologic study is unique, each geotechnical engineering or geologic report is unique, prepared solely for the specific client and project site. This report should not be applied for any purpose except the one originally contemplated. A Geotechnical Engineering or Geologic Report is Based on a Unique Set of Project-Specific Factors This report has been prepared for the Tim Murray Property located in Jefferson County, Washington. Crescent Environmental PLLC considered a number of unique, project-specific factors when establishing the scope of services for this project and report. Unless Crescent Environmental PLLC specifically indicates otherwise, do not rely on this report if it was: 1. Not prepared for you. 2. Not prepared for your project. 3. Not prepared for the specific site explored. 4. Completed before important changes were made. For example, changes that can affect the applicability of this report include those that affect: a. Elevation, configuration, location, or orientation of activities and structures. b. Project ownership. October 31, 2022 GEOTECHNICAL EVALUATION OF PARCEL 962700097, JEFFERSON COUNTY, WASHINGTON 30 Within the limitations of scope, schedule, and budget, our services have been executed in accordance with the standard of care in this area at the time this report was prepared. All features displayed on figures and maps in this report are approximately located and do not and are not intended to represent a legal survey. Users of this report are responsible for independently determining the location and distances of features portrayed on maps and figures. No subsurface investigations other than those described in this report were carried out as part of this study. It is not currently possible with existing scientific methodology to predict the time or place of earthquakes and resulting damage from ground motion beyond general recommendations of relative, qualitative, hazard. No warranty or other conditions, expressed or implied, should be understood.