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HomeMy WebLinkAbout502112037 2022 Geotech Assessment PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 April 4, 2022 Alec Olsen 253 Weather N Heights Lane Brinnon, WA 98320 alxtrade@gmail.com Re: Geology Hazard Assessment Tax Parcel 502112037 Talons Roost Road Brinnon, Washington Dear Mr. Olsen: This geologic hazard assessment was conducted to assess the risk of landslide and erosion hazards on the above-mentioned property. It is my understanding that you are planning to build a home on the property and install a septic system. The Jefferson County Public Records landslide map maps the site as a ‘moderate’ slide area. The Jefferson County shoreline stability map maps the site as ‘intermediate’. Based on my geologic hazard assessment, it is my opinion that a home can be located on graded level area of the upper site above Talons Roost Road and not be at risk from landslide or erosion and that the proposed septic drainfield on the portion of the property below Talons Roost Road will not be at risk from landslides or erosion. Furthermore, it is my opinion that development of the home site and septic drainfield will not increase the risk of landslides or erosion on or off the site as long as the recommendations in this report are followed. In order to avoid differential settlement, I recommend that the foundation be located at least 15 feet back from the top edge of the rockery slope. This geologic hazard assessment included a field inspection of the subject property and vicinity. The assessment included a review of my previous assessment of the site in 2004 including post post grading inspection after the level bedrock bench was created. The assessment included a review of available geologic mapping, lidar (light detecting and ranging) imagery and our own notes and observations we have made in the vicinity of the site and at locations with similar geologic conditions. April 4, 2022 Tax Parcel 502112037, Brinon, WA Geology Hazard Assessment Stratum Group File: 3.14.22 2 General Geology The Geologic Map of the Brinnon 7.5-minute Quadrangle, Jefferson and Kitsap Counties, Washington (Polenz and others, 2012) indicates that the property is underlain by basalt of the Crescent Formation and glacial till (Figure 1). Figure 1. Clipped and annotated geologic map showing the subject property location. Evc = Crescent Formation basalt, Qgt = glacial till, Qgic = ice contact deposits, Qga = glacial advance outwash, Qam = marine deltaic alluvium, Qb = beach. Geologic observations made on the site were consistent with the above-described mapping. The upper portion of the property above Talons Roost Road consists of basalt bedrock. The slope below Talons Roost Road is underlain by compact glacial drift that consists of poorly sorted silty gravel and cobbles. SPECIFIC SITE OBSERVATIONS The subject property is located on a slope above Hood Canal and U.S. Highway 101 on the west side of Hood Canal within the Eagle View subdivision. The property is accessed via Talons Roost Road; a gravel road crosses the property. The proposed building area is located on the upper portion of the property at an elevation of approximately 360 feet. At the time of my initial site visit to this site in 2004 there was a cleared and level building area had already on the site. That cleared and graded area was expanded later in 2004 and I inspected the expanded level area and the cut slope into the bedrock at that time and concluded that the bedrock cut slope was stable. The bedrock cut slope is approximately 10 feet high but is not vertical and slopes up to the west April 4, 2022 Tax Parcel 502112037, Brinon, WA Geology Hazard Assessment Stratum Group File: 3.14.22 3 with an overall slope of about 45 degrees. The bedrock is relatively massive with no well developed joints or plains of weakness that could lead to large failures. The slope above the cut is a moderate slope of about 20 degrees and is underlain by bedrock or thin glacial drift over bedrock. The level area is gravel covered. A rockery wall was constructed along part of the cut slope near the entrance from Talons Roost Road on the south of the level area. A rockery wall also fronts part of the slope below the level area (Figure 2). This lower rockery wall is about 10 feet high at its highest. It appears some settlement of soil may have taken place along the top edge of the rockery (Figure 3). Figure 2. View of cut slope and low rockery wall along southern portion of cut slope. April 4, 2022 Tax Parcel 502112037, Brinon, WA Geology Hazard Assessment Stratum Group File: 3.14.22 4 Figure 3. Rockery wall below the level area No springs or seeps or indications of surface water flow were observed on the upper part of the site at the time of my site visits in March 2004 or in March 2022. The lower portion of the property is below Talons Roost Road. A switchback of Moji Road crosses the northeast corner of this lower road section. An old overgrown road spurs off the switch back area and crosses the southwest portion of this lower portion of the property. The upper portion of this lower property section slopes down to the east at an angle of between 25 and 30 degrees and is covered with thick brush including Himalayan blackberry. Below this area the slope is more moderate with slopes of approximately 20 degrees. This slope is forested with a mixed forest that includes western hemlock, western red cedar, Douglas fir, madrone and cherry with a thick understory that includes rhododendron, evergreen huckleberry and salal. The slope is underlain by gravel and cobble rich glacial drift. No indications of slope movement were present and no seep or springs were observed or any other indicators of wet ground. CONCLUSIONS AND RECOMMENDATIONS Based on my geology assessment, the proposed building area on the level graded area on the upper part of the property will not be at risk from landslides or erosion. Furthermore, building a home on this area will not increase the risk of landslides or erosion on or off the property. The proposed septic drain field area on the lower portion of the property will not be at risk from April 4, 2022 Tax Parcel 502112037, Brinon, WA Geology Hazard Assessment Stratum Group File: 3.14.22 5 landslides or erosion and will not increase the risk of landslides or erosion on or off the property. Due to potential differential settlement near the outer edge of the level area near the rockery wall, foundations should be located at least 15 feet back from the top edge of the level building area. Because the depth to bedrock is relatively shallow, I suggest that the building be founded on bedrock if feasible. However, on the sites where bedrock is not very near the surface founding on the glacial till or recessional deposits will be acceptable. Storm water run off from the roof gutters and road access should be routed in such a manner as to avoid excessive concentration of water at a single point. Water should be dispersed towards the south end of the level building area where thicker glacial drift soils are present. We appreciate the opportunity to be of service to you. Should you have any questions regarding please contact our office at (360) 714-9409. Sincerely yours, Stratum Group Dan McShane, L.E.G., M.Sc. Licensed Engineering Geologist