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HomeMy WebLinkAbout802261006 Geotech Assessment 1 Stratum Group File: 2.15.22 PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 March 18, 2022 Jim Munn munnrealestate@gmail.com Re: Geology Hazard Assessment Jefferson County Parcel 802261006 It is my understanding that you are planning on installing a new septic system on the property and will later add a home to the property. Jefferson County Public Records seismic hazard map includes a portion of the eastern part of the property within the seismic hazard area. The Jefferson County landslide map indicates a ‘slight’ landslide hazard area to the immediately to the west. Based on my geologic hazard assessment, it is my opinion that the risk of seismic hazards at the subject property is no greater than the regional hazard and there is no risk of potential liquefaction. The slopes to the west of the property do not pose a landslide risk to the property. The property is located on an alluvial fan associated with a creek that flows from west to east across the northern portion of the property. Based on my assessment of stream conditions and geomorphic processes the property is not at risk from debris flood or debris flows or stream avulsions associated with the creek and the risk of shallow sheet wash flooding is minimal. Furthermore, it is my opinion that the locating of a septic system and later a home on the site will not increase the geologic hazards on or off the site. This geologic hazard assessment included a visual inspection of the property and vicinity including the stream and areas on the property and areas upstream of the property and excavation of test pits on the site. The assessment also included a review of available geologic mapping, liquefaction susceptibility mapping, historical aerial photographs, and lidar (light detecting and ranging) imagery of the site and vicinity. Past observations at sites with similar geologic conditions including areas in the vicinity of the subject property also aided in my interpretations. SITE GEOLOGY The Lidar-revised Geologic Map of the Uncas 7.5’ Quadrangle, Clallam and Jefferson Counties, Washington (Tabor and others, 2011) indicates that the property is underlain by alluvial fan sediment of Holocene unit 5. 2 March 18, 2022 Jefferson County Parcel 882261006 Geologic Hazard Assessment Stratum Group File: 2.15.22 Figure 1. Clipped and annotated geologic map showing the subject property location. Qf5 = alluvial fan deposits (Holocene unit 5), Qw = wetland, af = artificial fill, prevt = pre Vashon stade till deposits, Qls = landslide deposits (Holocene and upper Pleistocene), Qvt = Vashon age till, Qrg = gravel dominated recessional outwash, Qvas = sand dominated advanced outwash, Tss = Sandstone of Snow Creek. Geologic observations made on the site were consistent with the above-described mapping. Most of alluvial fan sediment consists of angular fragments of sandstone and siltstone gravel that is derived from the Snow Creek Sandstone that underlies the stream valley up slope from the site. A few pebbles of glacial derived sediment are present as well. Figure 2. Sandstone pebbles and gravel in test pit on site Site 3 March 18, 2022 Jefferson County Parcel 882261006 Geologic Hazard Assessment Stratum Group File: 2.15.22 Figure 3. Bank exposure of sandstone/siltstone gravel that underlies the property Figure 4. Alluvial sediment in stream on the subject property. SITE SPECIFIC OBSERVATIONS The property is located at the end of Munn Road on the west shore of Leland Lake. A cabin is located on the site and much of the site is grass covered with some landscaped areas and a tree lined stream. The very gentle sloped alluvia fan covers essentially the entire property. The creek is incised approximately 5 to 8 feet down into the fan. The stream flows into Lake Leland just 4 March 18, 2022 Jefferson County Parcel 882261006 Geologic Hazard Assessment Stratum Group File: 2.15.22 north of the northeast corner of the site. The stream banks and stream bed consist of gravel derived from sandstone and siltstone with very minor glacial components. There is no indication of past flooding on the creek banks and no evidence of overtopping of the incised channel. It does not appear on the ground or in lidar bare earth imagery that dredging has been done to maintain the stream route. Back erosion is very minor and only present in a few locations. Test pits excavated on the site all encountered gravel derived primarily from sandstone and siltstone. No old buried to soil horizons were encountered to the depth of my excavations at 4 to 5 feet. 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 150 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 to poorly built structures. Other faults are located in the area, but the potential hazard associated with these faults is poorly understood with very few constraints exist for previous seismic events. Future earthquakes along these local faults zone could produce moderate to strong ground shaking. 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 D-E. The map indicates that “Site classes C, D, and E represent increasingly softer soil conditions which result in a progressively increasing amplification of ground shaking.” The NEHRP designation indicates that soil conditions may amplify ground shaking at subject the property. However, based on our observation of medium dense to dense coarse gravel soils underlying the site, the ground shaking amplification is likely to be minimal. It is our opinion that subject property is not prone to severe ground-motion amplification due to the soils underlying the site. Based on my assessment, it is my opinion that the seismic shaking hazard at the site is moderate due to the potential for damaging shaking to occur during a Cascadia Subduction Zone, the 5 March 18, 2022 Jefferson County Parcel 882261006 Geologic Hazard Assessment Stratum Group File: 2.15.22 Dabob Bay Fault Zone, and possible other unknown nearby faults. However, the site is no worse than any other location with similar soils in the area of eastern Jefferson County. Liquefaction Liquefaction is a process whereby saturated, unconsolidated sandy soils temporarily become liquefied and loose 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 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 subject property have a moderate to high liquefaction susceptibility. However, it is our opinion that the subject property has no liquefaction susceptibility due to the coarse gravel alluvium underlying the site. I cannot rule out the possibility that fine sand layers exist at depth below the subject property; however, any earthquake induced deformation within a deeper fine sand layer would likely have negligible effect on the surface. This is because movement deep below ground would produce a cone of influence on the overlying sediments and distribute the displacement or settlement over a large area on the surface. Furthermore, deeper fine sand units if present would have already undergone multiple seismic events and compaction based on the age of the alluvial fan sediment. The soils underlying the site are composed of medium dense, well sorted (poorly-graded) pebble to small cobble gravel (GP). These types of soils have a negligible liquefaction potential because their capacity to hold water is lower and they do not dramatically compress under seismic loads like loose fine sand. Furthermore, the relict alluvial soils underlying the proposed homesite have undergone multiple previous seismic events with resulting compaction and densification. Based on my assessment, it is my opinion that the liquefaction hazard at the subject property is low. Seismically Induced Landslides The subject property is located on very gentle sloping ground and is not susceptible to slope failures. While there is a mapped land slide deposit within the upper stream valley, the stream valley gradient west of the property is very low and the valley is not tightly confined. As such it is my opinion that the risk of landslide run out reaching the property is negligible. Alluvial Fan Hazard Assessment The subject property is located on a broad low slope angle alluvial fan. There is no evidence of past debris flows or debris floods evident on the site or along the stream both on the ground and 6 March 18, 2022 Jefferson County Parcel 882261006 Geologic Hazard Assessment Stratum Group File: 2.15.22 via lidar bare earth imagery or historic aerial photographs dating back to the late 1940s. There is also no evidence of historic flooding or channel migration and the stream has not required dredging. The stream channel is incised into the fan surface throughout the entire fan. Incision alone should not be used as a means to determine stability of the channel, but there is no evidence in historic photographs, on the ground or in the lidar bare earth imagery of any channel change and there are no locations on the stream channel that appear to be at risk of channel movement. CONCLUSIONS Based on my geologic hazard assessment, it is my opinion that the risk of seismic hazards at the subject property is no different than other sites in the area. The site is not subject to enhanced seismic movement and is not at risk from soil liquefaction. Furthermore, the proposed development will not result in an increase in seismic hazards at the site. No landslide or alluvial fan hazards were identified on the site. 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 Dan McShane, L.E.G., M.Sc. Licensed Engineering Geologist 7 March 18, 2022 Jefferson County Parcel 882261006 Geologic Hazard Assessment Stratum Group File: 2.15.22