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HomeMy WebLinkAbout002283032 Geotech Assessment 1 Stratum Group File: 5.11.23 PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 May 25, 2023 Reed Griffin Rjgriff25@gmail.com Re: Geologic Hazard Assessment 30 Sunshine Drive, Sequim, WA Jefferson County Parcel 002283032 Dear Reed Griffin: This geologic hazard assessment was conducted to assess the risk of seismic hazards on the subject property to inform future development of the property. The Jefferson County Public Land Records seismic hazard map indicates that the subject property is a potential seismic hazard area. It is our understanding that there are plans to construct a new septic system, and large shop/garage. Additional development may occur in the future. Based on our geologic hazard assessment, the risk of seismic shaking to the proposed new septic system and shop is low but at an acceptable level for the intended use. The risk of seismic shaking to any future development on the site is low to moderate but at an acceptable level for residential or commercial use. The risk of liquefaction on the subject property is low. There is no risk of seismically induced landslides on the subject property. Development of the site does not warrant additional seismic design mitigation. This assessment included a visual inspection of the property and vicinity, review of available geologic mapping in the area, review of lidar (light distance and ranging) imagery of the vicinity and our own notes and observations we have made in the vicinity of the site and at locations with similar geologic conditions. GENERAL GEOLOGY The Surficial Geologic Map of the Port Townsend 30- by 60- minute Quadrangle, Puget Sound region, Washington (Pessl and others, 1989) indicates that the subject property is underlain by Vashon till (Figure 1). Vashon till is a very poorly sorted mix of clay, silt, sand, and gravel with scattered cobbles and boulders. The unit is highly compacted to the point of resembling concrete and commonly forms steep, nearly vertical coastal bluffs along Puget Sound and the Strait of Juan de Fuca. Vashon till was deposited directly by overlying glacial ice between 18,000 and 14,000 years ago. The 2 May 23, 2023 Jefferson County Parcel 002283032 Geologic Hazard Assessment Stratum Group File: 5.11.23 maximum ice thickness in the area has been estimated to be approximately 4,000 feet thick during the last major regional glaciation (Thorsen, 1981). Figure 1. Clipped and annotated geologic map (Pessl, 1989) showing the subject property location. Geologic observations made on the site were made using hand and machine dug test pits at the proposed septic and shop locations as well as other locations. Our observations were consistent with the above described mapping. Compact, poorly sorted, clay, silt, sand, and gravel with scattered cobbles consistent with Vashon till was observed in all hand and machine dug test pits on the subject property (Figures 2 and 3). 3 May 23, 2023 Jefferson County Parcel 002283032 Geologic Hazard Assessment Stratum Group File: 5.11.23 Figure 2. Vashon till in a machine dug test pit at the proposed septic location. Figure 3. Vashon till at the proposed shop/garage location. 4 May 23, 2023 Jefferson County Parcel 002283032 Geologic Hazard Assessment Stratum Group File: 5.11.23 SITE SPECIFIC OBSERVATIONS The 5.17-acre subject property is located in the central portion of the Miller Peninsula, approximately 10 miles west of Port Townsend. Access to the property is via Sunshine Drive. A home is currently under construction on the site. The subject property including the proposed septic drainfield and shop locations are level to gentle sloping (Figures 4 and 5). A gentle slope between 0 and 8 degrees is present in the northeast corner of the property. Large portion of the property have been logged but remaining vegetation consists of scattered Douglas fir, red alder, and western red cedar trees with an understory of salal, salmonberry, rhododendron, stinging nettle, other brush, and grass. Figure 4. The proposed septic drainfield location. 5 May 23, 2023 Jefferson County Parcel 002283032 Geologic Hazard Assessment Stratum Group File: 5.11.23 Figure 5. The proposed shop/garage location. Seismic Hazard Assessment Seismic Shaking Western Washington is seismically active and seismic shaking may occur on the site due to earthquakes along the Cascadia Subduction Zone (CSZ) or other faults present in the area. The subject property is located approximately 140 miles from the CSZ. The Washington Department of Natural Resources Cascadia Seismic Scenario Shaking intensity map indicates that the subject property along with all of eastern Jefferson County will have very strong shaking intensity with a modified Mercalli intensity level of 7. Expected ground conditions for this scale include negligible damage to well-built structures of good design and construction and slight to moderate damage in well-built ordinary structures. Damage may be considerable in poorly built structures. The subject property is not located in close proximity to any known or mapped seismogenic features including active faults, active folds, paleo-seismic trench locations, earthquake epicenters, or earthquake induced mass movements. However, there are several faults in the Puget and Juan de Fuca lowlands that are poorly constrained that could extend beyond their known locations towards the subject property. This includes the Lofall Fault, the Southern Whidbey Island Fault zone, and possibly other previously unrecognized faults or fault zones. An earthquake along a previously unrecognized fault near the subject property could produce moderate to strong ground shaking. 6 May 23, 2023 Jefferson County Parcel 002283032 Geologic Hazard Assessment Stratum Group File: 5.11.23 National Earthquake Hazard Reduction Program (NEHRP) seismic site class mapping has been completed by the Washington state Department of Natural Resources (WA DNR). This mapping provides guidance as to the potential ground-motion amplifying effects of soft soils during an earthquake. The NEHRP mapping indicates that the subject property is located within site class C (Figure 6). The map indicates that C soils are medium to relatively soft soils potentially subject to ground shaking amplification. Figure 6. Clipped and annotated NEHRP Site Class Map of Jefferson County (Palmer and others, 2004) showing the subject property location. Based on our assessment, it is our opinion that the seismic shaking hazard at the site is low to moderate due to the potential for damaging shaking to occur during a Cascadia Subduction Zone or other faults in the area. However, the site is no worse than any other location with similar soils in eastern Jefferson County. It is our opinion that subject property is not prone to severe ground- motion amplification due to the compact till soils underlying the site. Liquefaction Liquefaction is a process whereby saturated, unconsolidated sandy soils temporarily become liquefied and lose strength due to strong ground shaking. Liquefaction can result in differential settlement and/or lateral displacement. The soils found to be most susceptible to liquefaction are loose, fine sands containing very little silt or clay. Potential liquefaction decreases with coarser grained sand or higher silt content or if the fine sand is denser. Higher ground shaking accelerations or long periods of shaking can increase the potential for soil to liquefy. Hence, a 7 May 23, 2023 Jefferson County Parcel 002283032 Geologic Hazard Assessment Stratum Group File: 5.11.23 close proximity quake may cause high accelerations and liquefaction and a distant quake with lower site acceleration but long time periods of shaking can also cause soils to liquefy. The Liquefaction Susceptibility Map of Jefferson County, Washington (Palmer and others, 2004) indicates that the liquefaction susceptibility at the subject property is very low. Figure 7. Clipped and annotated Liquefaction Susceptibility Map of Jefferson County (Palmer and others, 2004) showing the subject property location. The soils underlying the site are composed of compact, poorly sorted (well graded) clay, silt, sand, and gravel with scattered cobbles. These soils have a low liquefaction potential because they are very compact, well sorted, and moderately well drained. Furthermore, these soils are glacial sediments that would have undergone multiple previous seismic events with resulting compaction and densification. Therefore, they would not be subject to dramatic compression from seismic loads during an earthquake. Based on our assessment, it is our opinion that the liquefaction hazard at the subject property is low. Seismically Induced Landslides The subject property is gently sloping and not at risk of seismically induced landslides. Risk 8 May 23, 2023 Jefferson County Parcel 002283032 Geologic Hazard Assessment Stratum Group File: 5.11.23 Risk is a measure of potential harm determined by combining the degree of hazard and the degree of exposure. Based on our geologic hazard assessment, the seismic shaking hazard at the site is low to moderate and the liquefaction hazard at the site is low. Human exposure to potential seismic hazards (shaking and liquefaction) at a shop is low due to the fact that people will occupy the shop for shorter periods of time when compared to a house. Human exposure to potential seismic hazards (shaking and liquefaction) at a residence is low to moderate due to the fact that people occupy a home for extended periods of time. However, the risk at the site is an acceptable level for both commercial and residential use. Development of the site does not warrant additional seismic design mitigation. CONCLUSIONS Based on our geologic hazard assessment, the risk of seismic shaking to the proposed new septic system and shop/garage is low but at an acceptable level for the intended use. The risk of seismic shaking to future development on the site is low to moderate but at an acceptable level for residential or commercial use. The risk of liquefaction on the subject property is low. There is no risk of seismically induced landslides on the subject property. Development of the site does not warrant additional seismic design mitigation. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions regarding this assessment please contact our office at (360) 714-9409. Sincerely yours, Stratum Group Geoff Malick, L.G., M.Sc. Licensed Geologist