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HomeMy WebLinkAboutMLA17-00064 Geology Hazard AssessmentSTRATUM GROUP P.O. Box 2546, Bellingham, WA 98227 September 7, 2017 Steven Barnowe-Meyer 214 Pleasant Valley Road Winlock, WA 98596-9743 Re: Geology Hazard Assessment Parcel 502291005 312403 Highway 101 Brinnon, Washington Dear Mr. Barnowe-Meyer: u1 SEP 152017 JEFFERSON COUNTY QCD Stratum Group is pleased to present the results of our geology hazard assessment of the above referenced property on a shoreline bluff. The purpose of this geology hazard assessment was assess the potential risks of landslides or erosion at the property. It is my understanding that a home and septic system are proposed for the site. Based on my assessment of the geologic conditions at the site, the proposed home, septic system and other site development associated with the home will not be at risk from landslides or erosion. Furthermore, the proposed development will not increase the risk of landslides or erosion on or off the site. This evaluation included a visual inspection of the property and vicinity, the shoreline slopes and the beach area along this reach of shoreline, review of available geologic mapping in the area and a review of available imagery of the site and vicinity including historic survey maps and lidar (light detecting and ranging) imagery. The assessment also included review of proposed stormwater plan and supplemental information for the stormwater plan. GENERAL GEOLOGY The Geologic Map of the Holly 7.5 -minute Quadrangle, Jefferson, Kitsap, and Mason Counties Washington (Contreras and others, 2012) indicates that the subject property is underlain by Crescent Formation basalt. My observations on the site are consistent with the above mapping. SPECIFIC SITE OBSERVATIONS A site plan by Creative Design Solutions provides topography of the site as well as notations of proposed septic drainfields, building envelope and proposed new driveway. The property is located between Highway 101 and Hood Canal. The property slopes down towards the east and south from Highway 101. A driveway and homes are located to the east and the shoreline of Hood Canal is located along the south side of the site. September 7, 2017 Parcel 502291005, Jefferson County, WA Geology Hazard Assessment The subject property is a former rock quarry. As such the slopes of the site are terraced from the former quarry activity and bedrock is exposed on the surface throughout much of the property. A former access drive angles down the upper part of the property from Highway 101. An upper terrace area on the site is proposed as reserve drainfield area. This bedrock bench is covered with brush and loose rock mixed with soil. The slopes above and below are bedrock slopes. The proposed building area is located on a mid slope bench formed by past mining activity. The slopes above and below the terrace areas are bedrock slopes with minor lose rock at the base of the slopes. The bedrock slopes appear stable and though steep are not vertical. The proposed new driveway access to the building area on the mid slope terrace will angle up the lower bedrock slope from the northeast. The height of this slope is approximately 20 feet and the slope is underlain by bedrock. The bedrock underlying the site consists of basalt lava flows of the Crescent Formation. Much of the basalt consists of pillow lava; that is the lava flows were flows on the sea floor. Although bedrock is exposed throughout the site, fractures and joints in the rock as well as a thin accumulation of organic matter has allowed for trees and brush to cover much of the site. Low cliff slopes are competent bedrock. Other than minor ravelling of small rock fragments, no slope movement is present. The shoreline on the south side of the property is a boulder beach with a low bank of bedrock above the beach. A soil horizon of angular rocky soil overlays the low bedrock bank and shows some evidence of past erosion from waves reaching above the bedrock during storm surges combined with high south winds. CONCLUSIONS AND RECOMMENDATIONS Based on my geology assessment of the subject property and vicinity, I conclude that the proposed home site area, septic drainfield areas and proposed new driveway will not be at risk from landslides or erosion and construction of a home on the site will not increase the risk of landslides or erosion on or off the property. Foundation footings for the home should be setback 10 feet from the face of the slope below the home. The 10 -foot setback should be measured horizontally from the bottom of the footing to the Stratum Group File: 3.17.17 2 September 7, 2017 Parcel 502291005, Jefferson County, WA Geology Hazard Assessment face of the slope. In this way the home can be located closer to the edge of the slope tan 10 feet as long as the footings are deep enough. The purpose of this setback is to avoid differential settlement of the foundation due to loads placed near fractured rock on the slope. I reviewed the strormwater plans prepared for the site. Stormwater will be fully dispersed on site according to the plan. It is my opinion that full dispersion as proposed is readily accomplishable and will not alter the hydrology of the site or increase erosion or landsliding on or off the site. Bedrock on the site is highly fractured and jointed and there is little evidence of surface water flow on the site. The forested low bank area on the south portion of the property between the proposed home building area and the shoreline is underlain by fractured weathered bedrock (and possible old fill of gravel waste rock). Stormwater dispersed toward this area will readily infiltrate into the underlying fractured rock soils and forest soils. Sheet flow off the driveway as proposed will readily infiltrate into the fractured bedrock and gravel along the proposed driveway route. The proposed driveway will require a cut and fill along the bedrock slope. Supplemental information in the stormwater plan notes that excess removed rock will be used on a low gradient driveway on a very nearby property and the volume of excess is estimated to be 65 cubic yards. The cut and fill of high quality fractured bedrock will not cause any slope stability issues and can be readily accomplished with standard construction practices. Debris, landscape debris, or any other material should not be disposed of on steep slopes. Vegetation management on the site should follow applicable regulations associated with the site being a shoreline property. However, from a geologic perspective no specific requirements are recommended. The maintenance of trees as proposed along the low bank shore area will minimize potential erosion of the upper non bedrock soil horizon from large periodic storms. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions please contact our office at (360) 714-9409. Sincerely yours, Stratum Group Pan'VShane, M.Sc., L.E.G. Licensed Engineering Geologist Stratum Group File: 3.17 17 z � Ofqp C N O c i 0 SIM WW b c� i � �a - 2 6 O p O Z5 m 0.h LLI � �CY) p -t-op ,y eco ol[ Lam: CN In �N.. Lf) v' 6 O s � l 7 Po 00 z � Ofqp C N O i 0 SIM WW b c� i �a Z�yw O p O Z5 m 0.h LLI wmm N � t�<�Jatyam LU ol[ Lam: i In �N.. 6 s � 7