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Home My WebLink AboutCAM2022-00383Online Appointment Scheduling Software :: Free Scheduler for Appoin... https://secure.acuityscheduling.com/appointments/view/887302279?print=1 1 of 5 9/7/2022, 4:29 PM Stratum Group File: 5.11.22 1 PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 June 27, 2022 Isolde and Tony Chatman isoldechatman@msn.com Re: Geologic Hazard Assessment 222 Old Lindsey Hill Road, Quilcene, WA Jefferson County Parcel 701195015 This geologic hazard evaluation was conducted to assess the risk of landslide and erosion hazards on the subject property. It is our understanding that you are planning to construct a new modular home and septic system on the subject property. The Jefferson County Public Land Records landslide hazard map and the Jefferson County shoreline slope stability map indicate that the majority of the subject property is a landslide hazard area. Based on our geologic hazard assessment, the proposed homesite is not a landslide or erosion hazard area. Furthermore, the proposed home will not increase the risk of landslides on or off the site as long as the recommendation of this report are followed. Although the location of the septic system including drainfield was not provided at the time of this assessment, it is our opinion a septic system can be constructed without increasing the risk of landslides or erosion on or off the site as long as the recommendations of this report are followed. Due to the potential for differential settlement, the foundation for the home, septic tank, and septic drainfield must be located at least 15 feet from the top edge of any slope steeper than 30 degrees. This geologic hazard assessment included a field inspection of the subject property and vicinity including all slopes. The evaluation also included review of available geologic mapping, lidar (light detecting and ranging) imagery, and our own notes, photographs, and observations made in the vicinity of the site and at locations with similar geologic conditions. GENERAL GEOLOGY The Geologic map of the Quilcene 7.5-Minute Quadrangle, Jefferson County, Washington (Contreras and others, 2014) indicates that the uppermost portions of the subject property including the proposed homesite are underlain by Vashon ice contact deposits (Figure 1). The majority of the subject property is mapped as being underlain by Possession Drift (Figure 1). June 27, 2022 Jefferson County Parcel 701195015 Geologic Hazard Assessment Stratum Group File: 5.11.22 2 Figure 1. Clipped and annotated geologic map of the subject property and vicinity. Qgic= Vashon ice-contact deposits, Qgdp= Possession drift, Qoaf= old alluvial fan deposits. Vashon ice-contact deposits include sediments that were deposited directly by thin wasting glacial ice as well as sediments deposited by meltwater that was in close proximity to glacial ice. This unit can be highly variable in composition, even within a relatively small area. Contreras and others (2014) describe the unit as loose to compact poorly sorted clay, silt, sand, silty pebble to cobble gravel, and isolated boulders. Possession drift is composed of glacial deposits from the Possession Glaciation that occurred approximately 60,000 to 80,000 years before present. Possession aged deposits are typically overconsolidated due to a combination of several possible processes including contraction due to drying, compaction from later deposition of sediment above the deposit, and compaction from overriding ice during the Possession and/or the Fraser glaciations. Geologic observations were made on the site using hand dug test pits and are somewhat consistent with the above-described mapping. Medium stiff, poorly sorted silt and clay with minor sand and pebble gravel consistent with Vashon ice-contact ablation till was observed in hand dug test pits on the uppermost portion of the property including the proposed homesite (Figure 1). June 27, 2022 Jefferson County Parcel 701195015 Geologic Hazard Assessment Stratum Group File: 5.11.22 3 Glacial till composed of stiff to very stiff, poorly sorted silt, sand, and gravel was observed in hand dug test pits on portions of the subject property (Figure 2). Observed pebbles and cobbles were composed of rock types (lithologies) that are local to the Olympic Mountains. The deposits did not contain significant quantities of granitic rock that is typical of Possession drift and other glacial deposits sourced from large ice lobes that emanated from the Cordilleran ice sheet in Canada and filled the Puget lowlands. Based on the stiffness and composition of the observed till, it is our opinion that the till is sourced from Olympic Mountain alpine glaciers. Figure 2. Vashon ice-contact deposits on the subject property. June 27, 2022 Jefferson County Parcel 701195015 Geologic Hazard Assessment Stratum Group File: 5.11.22 4 Figure 3. Olympic Mountain alpine drift on the subject property SITE SPECIFIC OBSERVATIONS The 9.94-acre subject property is located in an upland area in East Quilcene. Access to the property is via Old Lindsey Hill Road. The site is currently developed with a small cabin in a gentle upland area. Relevant topographic features and the proposed homesite are shown in Figure 4. June 27, 2022 Jefferson County Parcel 701195015 Geologic Hazard Assessment Stratum Group File: 5.11.22 5 Figure 4. Clipped and annotated lidar hillshade image showing the east side of the subject property and vicinity. The proposed new homesite is located on gently sloping ridgeline area on the eastern portion of the property (Figure 4). The ridgetop area consists of primarily gentle slopes between 5 and 20 degrees. The gentle ridgetop is between 50 and 60 feet wide at the homesite. Moderate slopes descending from the homesite to the northeast are between 20 and 30 degrees with limited areas up to 35 degrees (Figures 4 and 5). An old logging road is located on the northeast side of the building pad. Slopes descending to the west and southwest are between 30 and 40 degrees (Figures 4 and 6). Minor soil creep was observed on slopes steeper than 30 degrees. The slopes descending northeast from the proposed homesite form the southwest side of prominent plunging valley (gully-like) feature located to the northeast. The valley bottom is located on the adjacent property to the northeast (parcel 701191014). Areas of wet muddy ground from saturated soils are present at the valley bottom. Seeps and springs near the head of the valley feed a small stream that flows to the northwest towards Maury Anderson Road. The stream is well incised with steep eroded banks between 2 and 6 feet high (Figure 7). June 27, 2022 Jefferson County Parcel 701195015 Geologic Hazard Assessment Stratum Group File: 5.11.22 6 Figure 5. Moderate slopes descending from the proposed homesite to the northeast. Figure 6. Moderately steep slopes to the west of the proposed homesite. June 27, 2022 Jefferson County Parcel 701195015 Geologic Hazard Assessment Stratum Group File: 5.11.22 7 Figure 7. Sharply incised stream channel in the stream channel on the adjacent property to the northeast of the subject property. An old logging landing forms a level pad located on the opposite (west) side of Silver Marmot Drive to the proposed homesite (Figure 4). The pad is approximately 65 feet by 60 feet. An older RV is currently located on the pad. Slopes directly above and below the pad are between 25 and 35 degrees. Soil creep was observed on slopes steeper than 30 degrees. The subject property west of the proposed homesite consists of moderate to moderately steep slopes between 25 and 35 degrees with limited steeper areas up to 40 degrees (Shown as “Possible Drain Field Location” in Figure 4). Soil creep was observed on slopes steeper than 30 degrees. Silver Marmot Drive (private road) switchbacks down the subject property and adjacent properties. Several old logging roads also traverse the moderate to moderately steep slopes on the property. Vegetation on the subject property consists of mixed forest stands that include western red cedar, Douglas fir, big leaf maple, and red alder with an understory of sword fern, salal, Oregon grape, salmonberry, Himalayan blackberry, stinging nettle, and horsetail, and other brush. Stinging June 27, 2022 Jefferson County Parcel 701195015 Geologic Hazard Assessment Stratum Group File: 5.11.22 8 nettle and horsetails are abundant in the valley to the northeast of the subject property due to saturated soils at the surface and at depth. GEOLOGIC HAZARD ASSESSMENT Areas of wet ground and saturated soils were observed on slopes to the northeast of the proposed homesite, particularly in the stream valley to the northeast of the proposed homesite. The soil saturation is likely caused by groundwater perching within weathered, relatively permeable ice contact deposits overlying low permeability Olympic Mountain alpine drift. The groundwater perching causes seasonal springs and seeps. In sloped areas with saturated soils, observed slope activity was limited to slow soil creep on slopes steeper than 30 degrees. The soil creep is also leading to top of slope settlement, including in the vicinity of the proposed homesite. Based on our observations and interpretations, it is our opinion that the soil saturation is driving soil creep on the slopes but is not causing landsliding. This is due to the relatively moderate slope angles on the site that are not steep enough to be subject to landsliding. No evidence of previous, ongoing, or incipient landslides were observed. No conditions that indicate the potential future landslides were observed on the subject property. The proposed homesite is located over 90 feet from the stream to the northeast. The stream has a well incised channel and does not pose an erosion risk to the proposed homesite. CONCLUSIONS AND RECOMMENDATIONS Based on our geologic hazard assessment, the proposed homesite is not a landslide or erosion hazard area. Furthermore, the proposed home will not increase the risk of landslides on or off the site as long as the recommendation of this report are followed. Although the location of the septic system including drainfield was not provided at the time of this assessment, it is our opinion a septic system can be constructed without increasing the risk of landslides or erosion on or off the site as long as the recommendations of this report are followed. Geotechnical Considerations: Due to the potential for top of slope differential settlement, the foundation for the home, septic tank, and septic drainfield must be located at least 15 feet from the top edge of any slope steeper than 30 degrees. Due to the sensitive nature of the site soils, no grading, excavation, or filling should occur on the slopes below the proposed homesite. Cut slopes on the site higher than 3 feet should be supported with an engineered retaining wall. Stormwater Management: Stormwater from the home can be dispersed or infiltrated onsite consistent the Department of Ecology Stormwater Management Manual for Western Washington. Stormwater should not be discharged on or within 12 feet of slopes steeper than 30 June 27, 2022 Jefferson County Parcel 701195015 Geologic Hazard Assessment Stratum Group File: 5.11.22 9 degrees. If space is not available to infiltrate or disperse in the vicinity of the homesite, stormwater should be tightlined to a gentle area in or near the valley bottom to the northeast of homesite. If the driveway on the site is located in the swale directly west of the homesite, stormwater from the driveway should be collected in a ditch and routed to Silver Marmot Drive. Vegetation Management: Due to the potential to exacerbate soils creep on the site, vegetation clearing should not occur on slopes steeper than 30 degrees on the subject property. Yard Waste and Debris: No debris or yard waste should be placed on the slopes greater than 30 degrees on the property. Yard waste can damage underlying root structure and eventually builds up and to form a wet unstable mass that will slide down the slope and exacerbate soil creep. 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