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HomeMy WebLinkAbout901233001 Geotech Assessment (2022)August 20, 2022 To: Tim Lawson Fr: John Bethel, LEG #1300 Re: Proposed shelter construction at the Jefferson Land Trust Valley View site. Introduction Jefferson Land Trust has applied for a building permit (BLD2022-00398) to construct a pole -supported educational shelter on their Valley View Forest site (parcel (901233001). The site is located on the east side of Center Road approximately 1.4 miles south of Chimacum Corner. It is situated near the base of the westerly facing slope of Chimacum Ridge (Figure 1), In a letter dated August 4, 2022 the Jefferson County Department of Community Development informed the Land Trust that this parcel is within a mapped High Landslide Hazard Area and a Seismic Hazard Area. (Note that on the Jefferson County Public Land Records online map (Figure 2) the site is shown as being in an area of Sli&ht Landslide Hazard.) The County has requested that the applicant submit a geological assessment addressing these hazards and assuring that the proposed building can be safely constructed on this site. This memo has been prepared in response to that request. I have reviewed relevant background information and visited the subject site. As a result of this review and site visit I believe that neither the building site nor the surrounding area are located in areas of elevated landslide or seismic hazard as described in Jefferson County Code section 18.22.160. In Jefferson County Code section 18.22.160 Landslide and Seismic Hazard Areas are described as: (b) Landslide Hazard Areas. Areas potentially subject to mass movement due to a combination of geologic, topographic and hydrologic factors including: (i) Areas of historic failures or potentially unstable slopes, such as: (A) Areas described and mapped as having severe or very severe building limitations for dwellings without basements within the United States Department of Agriculture, Soil Conservation Service, Soil Survey for Jefferson County; (B) Areas described and mapped as recent or old landslides or slopes of unstable materials within the Washington State Department of Ecology Coastal Zone Atlas of Jefferson County; and (C) Areas described and mapped as areas of poor natural stability, former landslides and recent landslides by the Washington State Department of Natural Resources, Division of Geology and Earth Resources; (ii) Areas potentially unstable as a result of rapid stream incision, stream bank erosion, or undercutting by wave action; and (iii) Areas with any indications of earth movement, such as: (A) Rockslides; (B) Earthflows; (C) Muciflows; and (D) Landslides. C FEE:OMREE A U G 2 5 2022 (c) Seismic Hazard Areas. Areas subject to severe risk of damage as a result of earthquake - induced ground shaking, slope failure, settlement, soil liquefaction, or surface faulting. These areas are identified by the presence of: poorly drained soils with greater than 50 percent silt and very little coarse material; loose sand or gravel, peat, artificial fill and landslide materials; or soil units with high organic content. Review of Available Information During my investigation of this site I reviewed: • The 2041 USGS 1:24,000 Center topographic map (Figure 1), • Lidar-derived hillshade imagery, available through the Washington State Dept. of Ecology Lidar Portal (Lidar data was collected in 2019, accessed August 2022) (Figure 3), • The Geologic Map of the Center 7.5-minute Quadrangle, Jefferson County, Washington (2014) by the Washington Geological Survey (Figure 4), • Historic aerial photography (from 1990 to 2021) available through Google Earth (accessed August 2022), • A land surface profile produced using Google Earth (accessed August 2022), • The Soil Survey of Jefferson County, Washington (1975) by the Soil Conservation Service, and • Soils information available through the NRCS Web Soil Survey (accessed August 2022). Topographic mapping (Figure 1) shows that the building site is located on a relatively level bench near the base of the west -facing slope of Chimacum Ridge. The steepest section of this slope above the building site is at an inclination of approximately 33%. Lidar-derived hillshade imagery (Figure 3) also shows the building site located on relatively level bench with the steeper slope to the east. Lidar-derived hillshade imagery is especially helpful in identifying areas of former (and likely future) landslide activity. It often clearly shows steep head scarps, hummocky runout debris, depositional fans and other evidence of past slope movement. Examination of hillshade imagery for this site (Figure 3) shows no such evidence at or near the proposed building site. The 2014 geologic mapping of this area (Figure 4) shows the site as being underlain by a unit called Vashon Advance Lacustrine Mud. This is described as "Silt and clay with interbeds of sand, pebbles, diamicton, and rare dropstones: grey to tan, compact; well sorted; laminated to massive." This sediment would have been deposited (likely in lakes or mud -flats) in front of the last ice sheet as it advanced, and was then compacted by this ice mass as the sediment was subsequently overridden. The geologic mapping shows no landslides, areas of mass wasting, or alluvial fans in the vicinity of the proposed structure. No faults were identified in the vicinity of the subject site. The NRCS Web Soil Survey shows soil at the site as "Kitsap Gravelly Loam, 15 to 30 percent slopes". In the 1975 Jefferson County Soils survey these soils are identified as having "Severe Slippage hazard on slopes: low strength". This characterization is not carried forward in the current Web Soil Survey. Site Visit On August 15, 2022 1 visited the subject site with Tim Lawson, a Jefferson Land Trust volunteer and coordinator for this construction project. I returned for a second visit on August 16th. The building site is located in an existing clearing and is relatively level with a slight slope to the west. An abandoned outbuilding is located at the approximate location of the proposed shelter. Vegetation in the clearing consists of horsetail, reed canary grass, blackberry, buttercup, with areas of rush and salmonberry. Soils in the immediate vicinity of the proposed construction were explored using both a hand auger and tee probe. These soils consist of a variable thickness of moist to wet organic rich sandy silt over loose wet grey -rust mottled sandy silt and silty sand. Auger and tee post penetration indicate soft soils present to depths of between 6 and 18 inches, with increasingly dense mineral soil encountered below that depth. The wet soils observed at the building site ended at the base of the steeper slope east of the clearing. Soil on the slope above the building site was loose dry silt. Observations on the slope above the proposed construction showed no evidence of spring discharge, or active or relict landsliding. Vegetation on the slope above the building site (Douglas fir, big leaf maple, sword fern) gave no indication of seasonal saturation. Soils observed on site were generally consistent with the geologic mapping. The loose soils at the building site are likely a thin layer of colluvium derived from the underlying glacially consolidated fine-grained sediment. Wet conditions at the building site are likely an example of the common phenomenon of groundwater emergence where topography flattens at the base of a slope. Discussion Landslide Hazard The 2014 geologic map of this area (Figure 4) shows no areas of landsliding, mass wasting or alluvial fan deposition on or near the proposed building site. This is consistent with evidence from hillshade imagery and observations during the site visit. The building site is not located in or adjacent to a drainage that could serve as a conduit for debris flows. Slopes across the site are less than those typically associated with shallow debris avalanches. There are no rock faces capable of producing rockfall in the vicinity. Observations during the site visit on and above the proposed construction were consistent with the information from the hillshade imagery and geologic mapping. No indications of either active or relict landsliding were observed at the building site or on the slope above. Seismic Hazard The seismic concerns identified in the Jefferson County Critical Areas code are: • ground shaking, • slope failure, • settlement, • soil liquefaction, and • Surface faulting Of these, locally amplified ground shaking, settlement, and soil liquefaction are concerns where soft soils are present below the footings. While a shallow thickness of loose wet soils is present at the building site, probing indicates that dense mineral soil is located at shallow depth. Footing excavations should extend through any soft surficial soil and bear on dense mineral soil below. As discussed above there is no evidence of existing or past slope instability (which might be exacerbated by seismic shaking) in the vicinity of the proposed structure. Finally, recent geologic mapping did not find any surface faulting in the vicinity of the proposed structure. Provided footings bear on dense mineral soil there are no indications that the proposed structure is at an elevated level of seismic hazard. Conclusions Based on my review of the available geologic and topographic information, as well as observations during a site visit, I believe that neither the building site nor the surrounding area are at an elevated risk from landslides or earthquake damage. Building footings should extend through any loose soil present to hear on dense mineral soil. Beyond that I do not believe any additional measures are required to mitigate these hazards. 71 11 516 517 1170000 FEET 1 122*45'48*00' hi ac�m Y ley 19 370000 FEET l 5316 a ,.-ice'-•�i 22 fi 23 1 r24 5315 �. 1000 0 1000 2000 FEET Figure 1, A portion of the 2014 USGS Center Quadrangle with the approximate location of the building shown with a black X. Figure 2, Landslide hazard areas identified on the Jefferson County Public Land Records online map. Approximate location of the building site is shown with a black X. Figure 3, Lidar-derived hillshade image showing a portion of the west -facing slope of Chimacum Ridge with the approximate location of the proposed building site marked with a black X. This image was copied from the Washington State Lidar Portal. This Lidar data was collected for the USGS in 2019. 47'30" ag;� Qgos "r Qaf k y f + 1 �Qgic wti. r • .' dgiC� 4� l h Ogir ' :i ,• oaf QGOS oaf. k . _ •i� Qgaa �r Op, .,.' r Qgic' ti +Qaf 122°45'00" +48°00'00" Up Qmw Ug° [gyp 7 1 l: oa". [)a 7 �Qoaf O g g Figure 4, A portion of the Geologic Map of the Center Quadrangle, Jefferson County, Washington, Published by the Washington State DNR, Geological Survey in 2014. The approximate location of the building site is marked with a black X. The site is shown as being underlain by Qgaf (Vashon Advance Lacustrine Mud).