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HomeMy WebLinkAbout996400008 Geotech Assessment STRATUM GROUP P.O. Box 2546, Bellingham, WA 98227 Phone (360) 714-9409 July 19, 2017 David Whitney 9817 Mona Lisa Lane Bakersfield, CA 93312 Re: Geology Hazard Assessment Proposed Building Site Tax Parcel 996400008 Jefferson County, Washington Dear Mr. Whitney: It is my understanding that you are proposing to build a building on the level bench area just west of Whitney Road on parcel 996400008. I visited the site to assess the potential landslide hazard at the site. Based on my assessment the proposed building site, the building site is not located within a landslide hazard area and development of the site with a building will not increase the risk of landslides on or off the site. I do recommend that drainage be installed to minimize the wet ground at the base of the slope from impacting the foundation area and no cutting into the base of slope should take place. This geology hazard assessment included a visual inspection of the property and vicinity, a visual inspection of the slope on the property as well as the slopes to the west of the property, hand dug test pits and other observation of soils and geologic units on the site, review of available geologic mapping in the area, and review of lidar (light detecting and ranging) imagery of the properties and vicinity. The location of the proposed building site is indicated on the lidar derived image and aerial image on the following page. DCD REC'D 10/13/2022 2 July 19, 2017 Proposed Building Site, Parcel 996400008 Geology Hazard Assessment Stratum Group File: 8.24.16 Lidar imagery showing level area where proposed home is planned Aerial view of site Proposed building site Proposed building site DCD REC'D 10/13/2022 3 July 19, 2017 Proposed Building Site, Parcel 996400008 Geology Hazard Assessment Stratum Group File: 8.24.16 GENERAL GEOLOGY The Geologic Map of the Seabeck and Poulsbo 7.5-minute Quadrangles, Kitsap and Jefferson Counties, Washington (Polenz and others 2013), indicates that most of the subject property is underlain by pre-Vashon alluvial sediments of the Olympia non glacial interval. Pre-Vashon alpine drift is mapped on the upper slopes just to the west the subject property and Vashon drift covering the uplands further up slope and to the west. Portion of geologic map (Polenz and others, 2013) Qgt = Vashon drift, Qad = alpine drift, Qco = Olympia non glacial interval sediments, Qgdp = Possession Drift, Pink hachure line is identified as an incipient landslide scarp, blue line is a strand line Subject Property DCD REC'D 10/13/2022 4 July 19, 2017 Proposed Building Site, Parcel 996400008 Geology Hazard Assessment Stratum Group File: 8.24.16 The Vashon age drift located to the west and up slope from the property was deposited directly by glacial ice during the last glacial period approximately 18,000 to 14,000 years ago. The pre- Vashon alpine drift consists of sediment derived from alpine glaciation in the Olympic Mountains prior to the Vashon ice arrival. The unit includes glacial outwash and stratified sediments. It is distinguished from the Vashon age glacial sediments due to the preponderance of Olympic Mountains derived sediment. The property itself is underlain by pre Vashon alluvial sediment correlated with the Olympia non glacial interval. The unit was deposited during a non glacial period prior to the last glaciation including prior to the alpine glaciation from the Olympic Mountains. My site observations are generally consistent with Polenz and others (2013). Site observations are consistent with the site being underlain mostly with alluvial non glacial sediments in a compact condition. Units vary between gravel and silt units consistent with a river alluvial environment and the deposits were deposited prior to the last glacial period and hence highly compacted. A portion of the lower slope of the site east of the proposed building area may be partially underlain by post glacial alluvial sediments derived from the slope above. The general topography of the site and other areas in similar settings is incised relict stream valleys along with active streams that have subsequently down cut into the older alluvial surfaces. Polenz and others (2013) also indicate the presence of what they termed on the map as an incipient landslide scarp on the slops near the west edge of the property. There is no further explanation of the feature in the text of report accompanying the map. I did observe and inspect this feature. The feature is not as continuous as the map suggests, but on the west portion of the site there is a distinct bench-like area. Based on my site observations this feature is not a drop down deep-seated landslide feature. The bench area is underlain by silt/clay with seeps of water that likely led to past shallow sliding and differential erosion creating the bench landform. Hence, the feature referenced as an incipient landslide scarp is not a landslide scarp. SPECIFIC SITE OBSERVATIONS The proposed building site is located on a level bench area that was graded may years ago. A small shed is located on the bench. The level graded bench area is approximately 100 feet long from north to south and 40 feet wide from east to west. The bench has been cut slightly into the base of the slope to the west. The slope to the west of the level building area slopes up to at an angle of 23 degrees and rises up DCD REC'D 10/13/2022 5 July 19, 2017 Proposed Building Site, Parcel 996400008 Geology Hazard Assessment Stratum Group File: 8.24.16 approximately 55 feet. The slope is covered with a mixed stand of trees and brush. No indications of slope movement is present on the slope other than some minor soil creep where an old skid logging road traverses the slope. Above this slope the land levels out to a broad bench area that is approximately 200 feet wide before rising up again. This upper slope does have some minor soil creep and shallow sliding on the lower slopes where the area is underlain by a silt/clay unit and small areas of accumulated slope colluvium are located along the base of slope from these shallow slides. The slope above the silt/clay unit appears stable and is well drained. CONCLUSIONS AND RECOMMENDATIONS Based on my inspection of the subject property and vicinity, I conclude that the proposed home site on the level bench area is not located in a landslide hazard area. Development of a home at the site will not increase the risk of landslides on or off the subject property. However, no further cutting into the slope to the west of the proposed home should be done without engineering the slower slope area. Based on my assessment of the slopes to the west of the subject property, upslope from the west property boundary, there is a low risk of minor shallow slope movement on the steeper slope to the west. This unstable slope does not pose a risk of slide delivery to the proposed home site and any small scale slides that may take place on this slope are anticipated to be restricted to the very west edge of the property and will have no impact on the proposed home site. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions regarding our reconnaissance please contact our office at (360) 714-9409. Sincerely yours, Stratum Group Dan McShane, M.Sc., L.E.G. Licensed Engineering Geologist DCD REC'D 10/13/2022 LOT 7 6.78 AC LOT 8 5.46 AC Tract A Open Space Uplands 5.5850 AC Whitn e y R o a d Hood CanalTract A Open Space Tidelands (NOT AFFECTED BY THIS ALTERATION) Lagoon D NORTHWESTERN TERRITORIES, INC. NTI VEYORNALD RS U FORP NOI LA E SS DE SHAW ONNI T G RETEREATTSOF GISSCOTT G . HARKS E LL32430 Lagoon View Long Plat Alterationof Tract A along with Lots 7, and 8,as per Volume 7 of Long Plats, Pages 84-86,being a portion the of Northeast Quarter of Section 34,Township 26 North, Range 1 West, W.M.,Jefferson County, Washington DCD REC'D 10/13/2022