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HomeMy WebLinkAboutGEO2024-00018 - GEO Geological Review - 2/29/2024 PLANNING FEB 29 2024 D_"" MASON COUNTY-10 W• Alder StreetSubmittal Checklist COMMUNITY SERVICES `� BuIlQirg,Planning,ng,EnvirontnenW Health.Community Health Geological Assessment 4_ Instructions: This checklist must be submitted with a Geological Assessment and completed, signed, and stamped by the licensed professional(s)who prepared the Geological Assessment for review by Mason County pursuant to the Mason County Resource Ordinance. If an item is found not applicable, the report should explain the basis for the conclusion. Note:Unless specifically documented, this report does not provide compliance to the International Residential Code Sections R403.1.7 for foundations on or adjacent to slopes, Section R403.1.8 for expansive soils or section 1808.7.1 of the International Building Code Section for Foundations on or adjacent to slopes. Applicant/Owner Darren Miller Parcel# 4221085500009 Site Address 120 E Olympic Drive (1) A discussion of geologic conditions in the general vicinity of the proposed development,with geologic unit designation based on referenced maps. Located on page(s) 1 & 2 (2) (a)A discussion of the ground water conditions at the site, Located on page(s) 2 (b) A discussion of the estimated depth to water, Located on page(s) 2 (c) A discussion of the quantity of surface seepage, Located on page(s) 2 (d) A discussion of the upslope geomorphology, Located on page(s) 2 (e) A discussion of location of upland waterbodies and wetlands. Located on page(s) 2 (3) The approximate depth to hard or dense competent soil, e.g. glacial till or outwash sand. Located on page(s) 2 (4) A discussion of any geomorphic expression of past slope instability (presence of hummocky ground or ground cracks, terraced topography indicative of landslide block movement, bowed or arched trees indicating downslope movement, etc.). Located on page(s) 2 & 3 (5) A discussion of the history of landslide activity in the vicinity, as available in the referenced maps and records. Located on page(s) 2 8,3 (6) An opinion on whether the proposed development is within the landslide hazard area or its associated buffer or setback and the potential for landslide activity at the site in light of the proposed development. Located on page(s) 3 (7) A recommendation by the preparer whether a Geotechnical Report should be required to further evaluate site conditions and the proposed development of the subject property. Rev. February 2018 Located on page(s) 3 (8) If the presence of a hazard is determined within 300 feet of the proposed development, then the following are delineated on a geologic map/site map: (a) the area of the proposed development, Located on Map(s) N/A (b) the boundaries of the landslide hazard area (top, both sides, and toe), Located on Map(s) N/A (c) the associated buffers(top, both sides, and toe), Located on Map(s) N/A (d) building or other setbacks (top, both sides, and toe). Located on Map(s) N/A (9) A site map drawn to scale showing the property boundaries, scale, north arrow, and the location and nature of existing and proposed development on the site. Located on Map(s) N/A Glen Coad, PE hereby certify under penalty of perjury that I am a civil engineer licensed in the State of Washington with specialized knowledge of geotechnical/geological engineering or a geologist or engineering geologist licensed in the State of Washington with special knowledge of the local conditions. I also certify that the Geological Assessment, dated February 20, 2024 and entitled Geological Assessment: Landslide Hazard Geotechnical Letter meets all the requirements of the Mason County Resource Ordinance, Geologically Hazardous Areas Section, is complete and true, that the assessment demonstrates conclusively that the risks posed by the landslide hazard can be mitigated through the included geotechnical design recommendations, and that all hazards are mitigated in such a manner as to prevent harm to property and public health and safety. r ' Jjaq (Signature and Stamp) Page 2 of 2 Disclaimer. Mason County does not certify the quality of the work done in this Geological Assessment. e5oa0;?_q-- 006 f g DE-CIVIL, PLLC 9601 Key peninsula Hwy NW Gig Harbor,WA 98329 (253)228-0513 www.de-civil.com February 20, 2024 Darren & Kelsie Miller Geologic Assessment: Landslide 5484 Collins Rd Hazard Geotechnical Letter Port Orchard, WA 98366 Proposed Residence 120 E Olympic Drive Mason County,Washington PN:4221085500009 Job: 24-023 Miller.LHGL INTRODUCTION This letter presents the results of our Geological hazard assessment for the proposed residence to be located at120 E Olympic Drive in Mason County, Washington. The general location of the site is shown on the attached Site Vicinity Map, Figure 1. Our understanding of the project is based on our discussions with you and our February 9, 2024 site visit. It is our understanding that you intend to construct a new residence on the partially developed lot. It is anticipated that the residence will be supported on concrete footings and will be constructed of wood framing. Additional site amenities include the drainfield and driveway access. SCOPE The purpose of our services is to evaluate the surface and subsurface conditions at the site as a basis for assessing potential adverse impacts to and from the slopes located within the site area.We understand that because of the height and inclination of the slopes on the site, a geotechnical assessment is being required by Mason County to address geologic hazards at the site. Specifically, our scope of services for the project included the following: 1. Visiting the site and conducting a geologic reconnaissance to assess the site's soil, groundwater and slope conditions. 2. Assessing the landslide hazards in accordance with the current Mason County Title 8 - Environmental ordinance. No subsurface explorations or laboratory testing were completed as part of this assessment however, we we were able to observe subsurface soil conditions in open septic test pits and on other exposed areas of the site. SITE CONDITIONS Surface Conditions The subject parcel is located at 120 E Olympic Drive in Mason County, Washington. The site consists of a single parcel that is rectangular in shape being approximately 114 feet in length west to east by 61 feet in width north to south and encompasses approximately 0.16acres. Pre-developed grades begin off-site sloping down from east to west beginning at grades of 20 to 30 percent but with less than 10 feet of elevation change. Once onsite grades flatten out to approximately 10 percent until reaching the west end of the site where they again reach a 30 percent grade but also with less than 10 feet of elevation change. Based on ortho photos of the site, there was some grading that occurred onsite approximately 3 years February 20, 2024 24-023 Miller.LHGL Page 2 ago. These activities appeared to include the construction of a terraced block wall along the property frontage and the creation of a near vertical cut at the back of the property with a height of approximately 6 feet. The exposed soils of the cut revealed a weathered sol horizon of approximately 2 feet overlying a dense compact soil, typical of a till soil structure. The overall elevation change across the site is on the order of 16 feet. The property is bordered by residential properties on the east, north and south, by E Olympic Dr to the west. No seepage was observed at the time of our site visit. No evidence of deep-seated slope instability was observed at the site or the adjacent areas at the time of our site visit. No erosion or other soil movement was observed at the time of our site visit. Site Soils The USDA Natural Resource Conservation Service Web Soil Survey for Pierce County mapped the soils in area of the site as Alderwood gravelly sandy loam (Ab). The Alderwood soils are derived from glacial till slopes of 8 to 15 percent and are listed as having a "Moderate" erosion potential. Based on our review of the SCS mapping and our observations of the soils exposed on the site the SCS mapping is appropriate for this site. A copy of the SCS map for the site vicinity is attached as Figure 2. Site Geology Based on Geologic Map of the Mason Lake 7.5-minute Quadrangle, Mason County, Washington by Robert E. Derkey, Nicholas J. Heheremann, and Katelin Alldritt (July 2009),the site is underlain by glacial till (Qgt). These glacial soils were deposited during the most recent Vashon Stade of the Fraser Glaciation, approximately 12,000 to 15,000 years ago. The glacial soils consists of a varied mixture of clay, silt, sand, and gravel that was deposited under and at contact points between prehistoric continental glacial ice masses. Generally because the soils have been overridden by glacial ice they can be considered over consolidated and typically exhibit high strength and low compressibility characteristics. An excerpt of the above referenced map in included as Figure 3.. Landslide Hazard Area-Per Mason County Title 8.52.140 The Pierce County Municipal Code, Chapter 18E defines a landslide hazard area as an area potentially subject to mass movement because of a combination of geologic, seismic, topographic, hydrologic, or manmade factors.These areas may be identified by the presence of any of the following indicators: A. The following shall be classified as geologically hazardous areas: (i) Areas with any indications of earth movement such as debris slides, earthflows, slumps and rock falls(see Figure LHA-1). (ii) Areas with artificial over steepened or un-engineered slopes, i.e., cuts or fills. (iii) Areas with slopes containing soft or potentially liquefiable soils. (iv) Areas over steepened or otherwise unstable as a result of stream incision, stream bank erosion, and undercutting by wave action. (v) Slopes greater than fifteen percent (eight and one-half degrees) and having the following: a. Hillsides intersecting geologic contacts with a relatively permeable sediment overlying a relatively impermeable sediment or bedrock(e.g.,sand overlying clay);and b. Springs or groundwater seepage. (vi) Any area with a slope of forty percent or steeper and with a vertical relief of ten or more feet except areas composed of consolidated rock. A slope is delineated by establishing its toe and top and measured by averaging the inclination over at least ten feet of vertical relief. B. The following information may be used as a guide by the county to indicate areas that have a higher likelihood of meeting the classification criteria above: February 20, 2024 24-023 Miller.LHGL Page 3 (i) Landslide hazards mapped by the Washington Department of Natural Resources, Division of Geology and Earth Resources (Washington Geological Survey) such as "Landforms and Hazard Ratings—Mason Watershed," Isabelle Sarikhan and Timothy J.Walsh,August 2007. (ii) The areas identified on the Mason County Soil Survey Map as having slopes greater than fifteen percent. (iii) The areas identified on the Coastal Zone Atlas,Volume 9, of Mason County, Washington as: a. Unstable "U"; b. Unstable Old Slides- "UOS"; c. Unstable Recent Slides-"URS"; d. Intermediate Slopes-"I"; e. Modified Slopes-"M." (iv) The areas identified as Class 2, 3, 4, or 5 of the maps: "Relative Slope Stability of the Southern Hood Canal Area, Washington," by M. Smith and R.J. Carson, U.S. Geological Survey, Series Map I- 853-F, 1977 (vii) Areas described and mapped as areas of poor natural stability and historical and recent landslides by the Washington State Department of Natural Resources, Division of Geology and Earth Resources including"The Geological Map of North Central Mason County,Washington," by R.J. Carson, Washington State Department of Natural Resources, Division of Earth Resources, 1975; (viii) Areas mapped as landslide deposits (Map Unit Qls) on the Geologic Maps of Washington 7.5- Minute Quadrangle (Longbranch, Squaxin Island, Shelton, Summit Lake, Vaughn, Lake Wooten, Mason Lake, Belfair,Skokomish Valley and Union, Lilliwaup, Hoodsport, and Holly). CONCLUSIONS Based on our site evaluation, it is our opinion that the site does not include an active landslide hazard area and is suitable for the proposed residential development. Landslide Hazard Indicators—per Mason County Section 8.52.140 Mason County, Chapter 8, Section 52.140 uses the above referenced checklist to define a potential landslide hazard area. Based on our observations of the site and review of published information, we offer the following comments. No evidence of landslide activity or significant erosion was observed at the site at the time of our site visit. No intersecting contacts were observed or are mapped in the site vicinity. Slopes greater than 15 percent exist on the site, but no springs, seepage or intersecting contacts were noted. Provided appropriate soil erosion control, building setbacks, and site stabilizing measures are employed, we do not anticipate that the presence of the slopes will be a limiting feature to site development. No springs or groundwater seepage were observed at the site. No planes of weakness or rockfall hazards were observed at the site. There are no other landslide hazards mapped onsite. No other landslide hazard criteria were observed at the site or the immediate adjacent areas. Based on the above, it appears that there are no landslide hazard areas on or within 300 feet the property. Therefore, no buffer should be required on the property. February 20, 2024 24-023 Miller.LHGL Page 4 LIMITATIONS We have prepared this report for the Miller's and other members of the design team for use in evaluating a portion of this project. Subsurface conditions described herein are based on our observations of exposed soils on the parcel. This report may be made available to regulatory agencies or others, but this report and conclusions should not be construed as a warranty of subsurface conditions. Subsurface conditions can vary over short distances and can change with time. Within the limitations of scope, schedule and budget, our services have been executed in accordance with generally accepted practices in this area at the time this report was prepared. No warranty, express or implied, should be understood. We trust this is sufficient for your current needs. Should you have any questions, or require additional information, please contact us at your earliest convenience. Respectfully submitted, Development Engineering,PLLC 1 � I Glen Coad, PE Owner WGC Doc ID:24-023 Miller.LHGL Attachments: Figure 1:Vicinity Map Figure 2:SCS Soil Survey Figure 3—USGS Geology Map Moson Lake o„w, 04, a c` �2 LL I a `• l I' Lrtt�¢.yHOQ{1 ,, {`p Approximate Site Location Not to Scale DE-CIVIL, PLLC Site Vicinity Map 9601 Key Peninsula HWY NW Miller Residence D Gig Harbor,WA 98329 120 E Olympic Drive Ph(253)228-0513 www.de-civil.com Mason County, Washington Job No:24-023 February 2024 Figure 1 Soil Map—Mason County.Washington ±•S' 5D1pG `L`C z-rcc S.TEi s—rrc s-rc; + !) f 1 r s .. [,•� r y� ti•�,t�tii 4tYr �. �RA- sam � sa® :uc � sn s aver ax smm s� t } - Mip Sak I:Zl4DVpYUdWAler e(11':a.S)~- . NN 0 L m to fE0 0 1tD 2A AM ®0 k MV po)8Mm Nkb Meter Cmvm linif—VKNM ErtRfa:111M Ine ICh VVCM4 �I Natural Resources Web Soil Survey 21232024 r Conservation Service National Cooperative Sal Survey Page 1 of 3 Approximate Site Location h Not to Scale DE-CIVIL, PLLC USDA Web Soil Survey 9601 Key Peninsula HWY NW Miller Residence D Gig Harbor, WA 98329 120 E Olympic Drive Ph (253)228-0513 www.de-civil.com Mason County, Washington Job No:24--023 February 2024 Figure 2 g _y. f r, Qgo Qgt Qga ittle Hoquia_ . f 30 , Qgo Qgt Approximate Site Location Not to Scale DE-CIVIL, PLLC Geological Map 9601 Key Peninsula HWY NW Miller Residence D Gig Harbor, WA98329 120 E Olympic Drive Ph (253)228-0513 www.de-civil.com Mason County, Washington Job No:23-023 February 2024 Figure 3