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HomeMy WebLinkAboutGEO2021-00123 - GEO Geological Review - 10/29/2021 &60 ao t-- C)O L MASON COUNTY Submitta COMMUNITY SERVICES Geotechnical Report � i onn*oUr Win.Cvm oky e*th Instructions: This checklist must be submitted with a Geotechnical Report and completed, signed, and stamped by the licensed professional(s) who prepared the Geotechnical Report 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 Gary Webb & Nancy Gannon Parcel# 32423-30-90012 P LAN N I N G Site Address N Webb Hill Rd Lilliwaup-Eldon, WA 98555 (1) (a)A discussion of general geologic conditions in the vicinity of the proposed development, Located on page(s) 1 RE CE 1V ED (b) A discussion of specific soil types, Z Located on page(s) 2, 3, 4 &5 (c) A discussion of ground water conditions, £ 15 W. ldf�r fCet Located on page(s) 3 (d) A discussion of the upslope geomorphology, Located on page(s) 2 (e) A discussion of the location of upland waterbodies and wetlands, Located on page(s) 1 (f) A discussion of history of landslide activity in the vicinity, as available in the referenced maps and records. Located on page(s) 3 (2) A site plan which identifies the important development and geologic features. Located on Map(s) Plan Sheets 1 -3 (3) Locations and logs of exploratory holes or probes. Located on Map(s) Plan Sht 2 (4) The area of the proposed development, the boundaries of the hazard, and associated buffers and setbacks shall be delineated (top, both sides, and toe) on a geologic map of the site. Located on Map(s) Plan Sheets 1 & 2 (5) A minimum of one cross section at a scale which adequately depicts the subsurface profile, and which incorporates the details of proposed grade changes. Located on Map(s) Plan Sheet 3 and GALENA Printouts (6) A description and results of slope stability analyses performed for both static and seismic loading conditions. Analysis should examine worst case failures. The analysis should include the Simplified Bishop's Method of Circles. The minimum static safety factor is 1.5, the minimum seismic safety factor is 1.1, and the quasi-static analysis coefficients should be a value of 0.15. Located on page(s) Page 4 (7) (a) Appropriate restrictions on placement of drainage features, Rev. February 2018 Located on page(s) 5 (b) Appropriate restrictions on placement of septic drain fields, Located on page(s) 6 (c) Appropriate restrictions on placement of compacted fills and footings, Located on page(s) 5 (d) Recommended buffers from the landslide hazard areas shoreline bluffs and the tops of other slopes. Located on page(s) 5&6 (e) Recommended setbacks from the landslide hazard areas shoreline bluffs and the tops of other slopes. Located on page(s) 5&6 (8) Recommendations for the preparation of a detailed clearing and grading plan which specifically identifies vegetation to be removed, a schedule for vegetation removal and replanting, and the method of vegetation removal. Located on page(s) 5 (9) Recommendations for the preparation of a detailed temporary erosion control plan which identifies the specific mitigating measures to be implemented during construction to protect the slope from erosion, landslides and harmful construction methods. Located on page(s) 5 (10) An analysis of both on-site and off-site impacts of the proposed development. Located on page(s) 5 (11) Specifications of final development conditions such as, vegetative management, drainage, erosion control, and buffer widths. Located on page(s) 5 - 6 (12) Recommendations for the preparation of structural mitigation or details of other proposed mitigation. Located on page(s) 6 (13) 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) 1 - 3 Alari A_ TaTii a, 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 Geotechnical Report, dated Oct.27t'', 2021 , and entitled Gary Webb&Nancy Gannon Geotechnical Report 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 I©NAL manner as to prevent harm to property and public health and safety. Page 2 of 2 Disclaimer: Mason County does not certify the quality of the work done in this Geotechnical Report. TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 Mason County Community Services 615 W Alder St. Bldg 8 Shelton, WA 98584 Gary Webb &Nancy Gannon 36810 N US Highway 101 Lilliwaup, WA 98555 (360) 877-6745 & (360) 481-5095 Re; Geotechnical Report N Will Webb Road Lilliwaup, WA 98555 Parcel No.: 32423-30-90012 INTRODUCTION I was contacted in October of 2018 by Nancy Gannon who requested my investigation and documentation of slope stability and landslide hazard issues relating to the residential development associated with the above identified Mason County parcel number and site inspection findings. The Webbs wish to develop the property as a residential property. The Gannon-Webb property is located in the rural community of Eldon approximately 2-'/2 miles south of the Mason/Jefferson County Line on Highway 101 North. The Webb property is located approximately an eighth of a mile west of the Eldon Store on the left side of US Highway 101 on the North Will Webb Road, a private road accessed across Highway 101 from the Eldon Store. I've visited the property to collect topographic information and verify measurements and feature locations. Measurements were made using a Sokkia model No. 8047-55 inclinometer, 100 foot and 300 foot long fiberglass surveyor's tapes and other field equipment common to surveying. Software employed in creating a site plan and writing this report included AutoCAD 2006, a Terrain 3-d contour modeling program, GALENA Slope Stability Analysis software, and Microsoft Office Word 2003. This report addresses stability issues relating to the installation of an onsite septic system and residential structures. The property is irregular in shape, generally trapezoidal measuring roughly 172- feet in its east-west direction and roughly 340-feet in the north south orientation. The property's overall slope from north to south has a downslope of roughly 27%with local areas sloping at roughly 42—43%with elevation differences greater then 10 feet. No wetlands or upland water bodies were found on or around the Gannon-Webb property. Hood Canal is located approximately 600 feet southwest of the parcel at its nearest point. A site plan is attached which shows the general locations of these features, as well as US Highway 101 and the N Will Webb Road which generally represents the southern and western boundaries of the property. The sloping areas of the property are vegetated with natural vegetation composed of Douglas Fir, Hemlock trees, Vine and Red Maple Trees with an understory of Oregon Grape, Salal, and Huckleberry plants. Slopes exceeding 40% are identified on the attached site plans as HAZARD AREAs. Because of the steep slopes (>40%under Mason County terminology) on or within 300 feet of the Gannon-Webb's File:C:IProjectslGannon Web Geo-Rpt Prcl 90012 1 of 6 10/27/2021 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 proposed residential development of the parcel, Mason County has requires that a geotechnical investigation be performed and documents prepared to address potential stability issues relating to the property's development. With slopes greater than 40%with more than 10-feet of vertical relief, a geotechnical report meeting the requirements of a Mason County Geotechnical Report is required. The following thirteen elements are required by Mason County to be addressed in Geotechnical Reports prepared to comply with Mason County's Critical Resource Ordinance Sections 17.01.100 through 17.01.104. 1. SURFACE AND GEOLOGIC CONDITIONS: The property is located in the mountainous foothills of the Olympic Mountain range with moderate slopes (<40%)on the northerly portion of the property with the southerly portion of the parcel containing slopes measured between 40 and 50%. The property has been logged roughly 20 years ago and is generally clear of timber, but has low growing second growth timber and ground cover vegetation over most of the property. The property is located in an area dominated by glacial till soils located above solid basalt rock substrate materials. The glacial till covering the underlying rock has developed a weathered surface layer of soil,typically 3-feet deep or less, covering the strongly cemented underlying glacial till. The trees growing on the gently sloping areas of the property, as well as on the more steeply sloping areas of the property were all observed to be growing vertical and straight, indicating the absence of any recent(last 100 years±) land movements or mass slide activities. Inspection of the portion of Department of Natural Resources) geologic map of the Eldon—Cummings Point vicinity on the following page finds the Gannon-Webb property located in an area between the Olympic Mountain foothills to the west and Hood Canal to the east in an area comprised of a Qapo (Glacial Till) soil designation. The geologic soil description of the area on and surrounding the site is provided on the following page.No upland water bodies or wetlands were found on or near the property. 47'30= y�3 A P tl DNR Geologic Information Portal available at DNR website File: C:(Projects\Gannon Web Geo-Rpt Prcl 90012 2 of 6 10/27/2021 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 Alpine out wash,, pre�1ate Wisconsinan i a (Pleistocene)) Stmtified sand, el,and cobbles; in the Quinault basin,clasts consist of sandstone and less-abundant basalt from the Olympic Mountains ter: and peripheral rocks;in sums draining the southern and southeast em Olympics,clasts consist primarily of Crescent Formation basalt with less-abwidant Olympic-core sandstone; may include peat, silt, and clay,and may be capped by weathered loess;clasts are generally more rounded than those in till and lack facets and striations;poorly to moderately sorted;gray to subtle yellow with wispy orange weath- ering Inspection of the hillside located parallel with highway 101 along the northwesterly side of Highway 101 displays massive basalt rock at the ground's surface, suggesting that the glacial till and weathered till soil layers may range between 5 and 30±feet thick. Inspection of Washington State Ecology's Landslide Hazard Maps reveals that southeasterly portion of the Gannon-Webb property coinciding with the steep slopes on the property is classified as unstable. aaMM +YiCE� JQSA�® Ecology's map legend indicates "Unstable slopes are considered unstable because of geology, groundwater, slope and/or erosion factors". The more gently sloping areas of the Gannon-Webb property are classified as "Stable". The residential development of the property will occur on the stable hillside north of the portion of the property classified as unstable. Aside from clearing areas of the properties in moderately sloping areas, no disturbance of any of the property's steep slopes is proposed. Groundwater at the site generally coincides with the tide elevation of Hood Canal. Shallow groundwater may be expected during the winter above the underlying rock (—30' below the ground's surface. File:C:\Projects\Gannon Web Geo-Rpt Prcl 90012 3 of 6 10/27/2021 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 2. SITE PLAN WITH IMPORTANT DEVELOPMENT AND GEOLOGIC FEATURES: A site plan showing the general topography of the area surrounding the Gannon-Webb property, surrounding area, and existing improvements is attached in this report's appendix. 3. LOCATIONS OF LOGS OF EXPLORATORY HOLES OR PROBES: The attached site plan shows the locations of soil probing on and around the Gannon-Webb property. Moderate probe resistance was encountered to depths around 3-feet below the ground's surface where the till soils become more strongly cemented. 4. AREA OF PROPOSED DEVELOPMENT &BOUNDARIES OF HAZARDS, BUFFERS, AND SETBACKS: The attached site plan shows the location of the existing N Will Webb Road.No other improvements exist on the Gannon-Webb property. The locations of hazard areas (slopes> 40%), and the hazard areas' 50' buffer lines out from the crests and toes of the steep slopes located on and around the property are displayed.No permanent improvements are proposed on the steep slope portion of the property. A permanent residence may be sited within 10-feet of the crest of the steep slope located along the southerly level building area,provided stormwater runoff from rooftops and the cleared area is dispersed over the crest of the slope and no concentrated stormwater discharges are released within 10-feet of a steep slope's crest. Roof downspouts and parking area runoff may be delivered to level dispersion trenches (3' wide x 2' deep x 3' long per 1000 square feet of collection area) filled with drain gravel or clean pit run gravel. The gravel filled dispersion trenches may be employed to disperse concentrated stormwater flows within 5-feet of a slope's crest. Stormwater modeling indicates that 3- foot long level trench/1000 square feet of impervious area will deliver a sheet flow discharge from the 100 year 24 hour storm event's peak runoff with a depth of less than a quarter inch. 5. CROSS SECTION WITH PROPOSED GRADE CHANGES: A cross section view of the property is provided in the attached plan sheets as well as part of the attached GALENATM Stability Analysis Print-Out. The currently proposed future residential construction will not require the construction of any structural fill sections or changes to the existing slopes or grades. If plans change and development on the steep (>40%) slopes is pursued, engineering specifications for structural fills or cuts on steep slopes should be developed. 6. SLOPE STABILITY ANALYSIS: The property itself does not display any instability indicators. For stability analysis, the hillside soils were investigated and have been characterized as a weakly cemented gravelly sandy loam for a depth extending 3-feet deep below the ground's surface, with an underlying strongly cemented substrate glacial till layer. The weather till soil is modeled with a soil density of 125 lbs/ft3, a phi angle of 35°, and a cohesion value of 200 lbs/ft2. The cemented glacial till soil is modeled with a soil density of 125 lbs/ft3, a phi angle of 35°, and a cohesion value of 400 lbs/ft2. Both soils have been classified as a USCS GW-GC soil category. A phreatic surface was assumed above the ground's surface to mimic a saturated surface soil condition. The slope geometry and soil characteristics provide a factor of safety for the site in a static and a dynamic analysis that exceeds the County's minimum safety requirements. Analysis of the site indicates Factors of Safety of 3_1 in a static condition and 1.6 under seismic conditions are realized on the overall slope. File:C:IProjects\Gannon Web Geo-Rpt Prcl 90012 4 of 6 10/27/2021 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 7. RECOMMENDED SETBACKS &DRAINAGE: Slope stability analysis indicates that the southerly descending slope on Gannon-Webb property is stable in its present geometry and condition and does not represent a landslide or geological hazard to the existing site or potential residence or proposed improvement locations. Specific residence locations are not presently being pursued, but the development of the property will likely include encroachment into the Hazard Area's 50-foot Buffer, so a request that the proposed property development be allowed inside the Hazard Area with a 40-foot intrusion into the Hazard Area Buffer has been prepared and attached to this report for County acceptance. Drainage from the present site sheets to its natural drainage directions to the southerly area of the property. Concentrated discharges of collected stormwater runoff over hazard areas are not allowed by Mason County, so stormwater runoff produced by a proposed residence's roof and downspout disposal system should be dispersed over as large an area as possible before it is allowed to proceed over the steep sloping areas of the property. 8. CLEARING GRADING, AND VEGETATION MANAGEMENT: Natural vegetation should be promoted on the steep areas of the property. Felling, limbing or topping of trees located on the steep slopes shouldn't produce any detrimental affects,providing tree stumps are left in place and understory disturbance is minimized. Clearing of areas on the property for residential development can be expected to occur on areas of the property where the ground's slope(s) is less than 40%. 10-foot building setback lines are shown on the attached site plan which will be observed during the property's development. The building setback buffers should be preserved with vegetation to provide stormwater flow attenuation and an absorption area for stormwater infiltration. 9. EROSION AND SEDIMENT CONTROL: Inspection of the Soil Survey of Mason County found the site to be located in an area with a Hoodsport(Hf Gravelly sandy loam) soil type,which is not identified in the County's Critical Resource Ordinance as susceptible to severe erosion when disturbed, and does not require the development of a site specific erosion and sediment control plan. The site already has an access to the property from the N Will Webb Road(private) which is stable in its present condition. 10. ON-SITE AND OFF-SITE IMPACTS: The proposed residential improvement of the property shouldn't result in any off-site impacts. Construction of a single family residences on the property shouldn't cause any offsite impacts, provided stormwater runoff is dispersed as near to its sources as possible and no concentrated stormwater discharges are delivered within 30+feet of any of the property boundaries. 11. SPECIFICATIONS OF FINAL DEVELOPMENT: Residential foundations should bear down on the undisturbed cemented glacial till soil layer underlying the property. Stepped foundations may be required to accomplish bearing on the till layer. Structural fill may be employed to level building areas, but should be placed under the supervision of a licensed professional engineer or a licensed engineering geologist. Building elements of the residence's construction are expected to comply with Mason County's current International Building Code requirements. A specific septic drainfield location is likely to be governed by exploration of File:C:\Projects\Gannon Web Geo-Rpt Prcl 90012 5 of 6 10/27/2021 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 specific site locations for localized pockets of gravel. No slopes on the properties preclude drainfield siting. 12. RECOMMENDATIONS FOR THE PREPARATION OF STRUCTURAL MITIGATION OR DETAILS OF OTHER PROPOSED MITIGATION: Modeling of the slopes in the proximities of the conjectural residence(s) location(s) and amenities indicates that the locations of proposed improvements will be safe if the recommendations of this report are followed. No additional structural measures appear needed. 13. SITE PLAN WITH THE LOCATION OF EXISTING AND PROPOSED DEVELOPMENT ON THE SITE: A site plan is attached as an appendix to this report and is hereby made a part of this report along with the cross section view of the property and the stability analysis printout. Summary& Conclusions: The proposed Gannon-Webb residential development of the Gannon-Webb property can be accomplished according to the owner's conjectural plans provided recommendations made in this report are implemented and incorporated into the site's development plans. Please feel free to contact me if there are any questions or concerns regarding this report or my findings. Sincerely, o WA$kf �s'�IONAL E��1 Alan A. Tahja, P Attached: 2 Plan Views & 1 Cross Section Sheet GALENATM Stability Analysis Printout Hazard Area Buffer Reduction Request File:C:IProjects\Gannon Web Geo-Rpt Prcl 90012 6 of 6 10/27/2021 Rev# Description Da e TAIIJA ENGINEERING,INC. SITE PLAN Alan Tahja,P.E. Nancy Gannon&Gary Webb Webb-Gannon PO Box 235 36810 N US Highway 101 ,, HoodspoA WA 98548 Lilliwaup, WA 98555 1 of3 Mason Co.Prc1 No.: 32423-30-90012 (360)877-9512 (360)292-3545 (360)481-5095 \\ �{ �`tx 3 �� t�� �� � � r � , R f O rr r r s # % �, £ 3 R tV �. S � r £ t i � � 1/ ,s1 # 13 �; / { 1 f / f/ g£ } s�; £ £v Br raj r� �2# ` � }� 1. x£ . st /4 ' S 11 �{ �� ' ;�,# � ,,, € ## {`}/ # 3I£� 1 r �� rl 3 # # ### t�� tf#7JI II J i # p #{ � fir r�• /''"j/f/f 3 { E33I t � t £ 1/17Y'( 1 p r r ' ,� €i#I q# tlt jJJ 1 d j � r. ��fi/si � j £ a� 1 1rs r rllill'r rr rr II # ,.£ rr7(Itr U �„✓"r �� ! z r V Ir rI {s z �f r /� t s#{t �r o rflr £ Rev# Description bate TAHJA ENGINEERING,INC. SITE PLAN Alan Tahja,P.E. Nancy Gannon&Gary Webb Webb-Gannon PO Boa 235 36810 N US Highway 101 Hoodspo4 WA 98548 Lilliwaup, WA 98555 2 of3 Mason Co.Pi vi No.: 32423-30-90012 (360)877-9512 (360)292-3545 (360)481-5095 t f { iJH N f grope Lfine 3 x:.644'1 / ) / / / /// r ' ; / // ' r' / fp0 i '2 r �ixldi aLn / L : •.m / / ti ;'CQ7 0 j 1 Q um t'{ mm } / / /, / ii ' ' 1111111 ) o C� r ' /5( f r ebb Road; f 1 �"YYYY f / / / / / /. / / . / / / / / //// /// f r j ao a _-O / r I C7 /` / r r Rev# Description Date TAHJA ENGINEERING,INC. CROSS SECTION "A" Alan Tahja,P.E. Nancy Gannon&Gary Webb Webb-Gannon PO Box 235 36810 N US Highway 101 Hoodsport, WA 98548 Lilliwaup, WA 98555 3 of 3 Mason Co.Prel No.: 32423-30-90012 (360)877-9512 (360)292-3545 (360)481-5095 � ww p w ci o O 06Z _ O - ff w A U o 990 f � � O ti \ QO- M + O O M - = (+f) �i f t A�44 o O p � Jf f}y f b O TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 Webb-Gannon 90012 X-Section A-A Static Stability Analysis GALENA verton 6.02 450 Ph re t c Surface �� A ?' ,S'cy rr Conjectural Residence Loci ��� `�. Ssi AL 300 Consolidated Glacial Till __.. N E Analysis: 1 250 Multiple Stability Analysis Method: Bishop Simplified Surface: Circular Results 200 Critical(minimum) Factor of Safety. 3.12 0 50 100 150 200 250 300 350 400 Edited:26 Oct 2021 Processed'26 Oct 2021 Project: Webb-Gannon Prcel 90012 Static File: C:\Documents and Settings\Owner\My Documents\Galena Files\Models\Webb-Gannon 90012.gmf Tahja Engineering,Inc. Galena 5.02 Analysis Results Licensee: Tahja Engineering, Inc. DATA: Analysis 1 - Static Material Properties (2 materials) ------------------- Material: 1 (Mohr-Coulomb Isotropic) - Weathreed Surface Soil Cohesion Phi UnitWeight Ru 200.00 35.0 123.00 1.10 Material: 2 (Mohr-Coulomb Cu increases with depth) - Consolidated Till Soil Cohesion Phi PI UnitWeight Ru 400.00 35.0 0 125.00 1.10 Water Properties ---------------- Unit weight of water: 62.40 Unit weight of water/medium above ground: 62.40 Material Profiles (2 profiles) ----------------- Profile: 1 (10 points) Material beneath: 1 - Weathreed Surface Soil 0.00 357.00 17.30 361.00 59.30 361.00 71.50 357.00 96.40 347.00 118.00 337.00 151.20 327.00 239.60 307.00 340.00 287.00 387.00 279.00 Profile: 2 (2 points) Material beneath: 1 - Weathreed Surface Soil 0.00 380.00 400.00 380.00 Slope Surface (10 points) ------------- 0.00 360.00 17.30 364 .00 59.30 364.00 71.50 360.00 96.40 350.00 118.00 340.00 151.20 330.00 239. 60 310.00 340.00 290.00 387.00 282.00 File:C:\Projects\Webb-Gannon X-Sec A-A Analysis Printout 1 of 4 10/27/2021 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 Phreatic Surface (2 points) ---- 0.00 400.00 400.00 400.00 Failure Surface --------------- Initial circular surface for critical search defined by: XL,XR,R Intersects: XL: 40.00 YL: 364.00 XR: 150.00 YR: 330.36 Centre: XC: 106.67 YC: 385.34 Radius: R: 70.00 Variable Restraints ------------------- Parameter descriptor: XL XR R Range of variation: 75.00 125.00 60.00 Trial positions within range: 12 12 12 RESULTS: Analysis 1 - Static Bishop Simplified Method of Analysis - Circular Failure Surface ----------------- Critical Failure Circle Search using Multiple Circle Generation Techniques Factor of Safety for initial failure circle approximation: 3.76 There were: 1137 successful analyses from a total of 1729 trial circles 592 analyses terminated due to unacceptable geometry Critical (minimum) Factor of Safety: 3.12 Circle and Results Summary (Lowest 17 Factor of Safety circles) ------------------- Circle X-Centre Y-Centre X-Left Y-Left X-Right Y-Right Radius FoS 1 129.16 425.40 50.23 364.00 155.68 328.99 100.00 3.125 2 127.06 419.10 50.23 364.00 155. 68 328. 99 94 .55 3.131 3 121.04 418.07 50.23 364.00 144 .32 332.07 89.09 3.145 4 123.12 424.21 50.23 364.00 144.32 332.07 94.55 3.148 5 124.90 412.59 50.23 364.00 155.68 328.99 89.09 3.151 6 118.89 411.75 50.23 364.00 144.32 332.07 83.64 3.154 7 125.16 430.22 50.23 364.00 144.32 332.07 100.00 3.160 8 132.82 420.38 50.23 364.00 167.05 326.42 100.00 3.163 9 130.48 414 .44 57.05 364 .00 155. 68 328.99 89.09 3.164 10 132.70 420.70 57.05 364.00 155. 68 328. 99 94 .55 3.171 11 128.19 407.97 57.05 364 .00 155.68 328.99 83.64 3. 172 12 122.28 407.09 57.05 364.00 144.32 332.07 78.18 3.172 13 123.63 423.71 43.41 364.00 155. 68 328. 99 100.00 3.174 14 124.57 413.35 57.05 364.00 144 .32 332.07 83.64 3.179 15 116.67 405.20 50.23 364.00 144 .32 332.07 78.18 3.181 16 119. 90 400.58 57.05 364 .00 144.32 332.07 72.73 3.184 17 134 .87 426.80 57.05 364.00 155.68 328. 99 100.00 3.189 File:C:\Projects\Webb-Gannon X-Sec A-A Analysis Printout 2 of 4 10/27/2021 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 Webb-Gannon 90012 X-Section A-A Dynamic Stability Analysis GALENA Verebn 502 450 Pnreaue Surface 400 Conjectural Residence Loci im aZattl 350 300 ..._..__.. Consolidated Glacial Till Analysis: 1 250 Multiple Stability Analysis Method: Bishop Simplified Surface: Circular Results 200 Critical(minimum) Factor of Safety:: 1.62 0 50 100 150 200 250 300 350 400 Edded 26 Oct 2021 Processed-2600 2021 Project: Webb-Gannon Prcel 90012 Dynamic File: C:\Documents and Settings\Owner\My Documents\Galena Files Models\Webb-Gannon 90012.gmf Tahja Engineering,Inc. Galena 5.02 Analysis Results Licensee: Tahja Engineering, Inc. DATA: Analysis 1 - Dynamic Material Properties (2 materials) ------------------- Material: 1 (Mohr-Coulomb Isotropic) - Weathreed Surface Soil Cohesion Phi UnitWeight Ru 200.00 35.0 123.00 1.10 Material: 2 (Mohr-Coulomb Cu increases with depth) - Consolidated Till Soil Cohesion Phi PI UnitWeight Ru 400.00 35.0 0 125.00 1.10 Water Properties ----- Unit weight of water: 62.40 Unit weight of water/medium above ground: 62.40 Material Profiles (2 profiles) ----------------- Profile: 1 (10 points) Material beneath: 1 - Weathreed Surface Soil 0.00 357.00 17.30 361.00 59.30 361.00 71.50 357.00 96.40 347.00 118.00 337.00 151.20 327.00 239.60 307.00 340.00 287.00 387.00 279.00 Profile: 2 (2 points) Material beneath: 1 - Weathreed Surface Soil 0.00 380.00 400.00 380.00 Slope Surface (10 points) ------------- 0.00 360.00 17.30 364 .00 59.30 364.00 71.50 360.00 96.40 350.00 118.00 340.00 151.20 330.00 239.60 310.00 340.00 290.00 387.00 282.00 File:C:(Projects\Webb-Gannon X-Sec A-A Analysis Printout 3 of 4 10/27/2021 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 Phreatic Surface (2 points) ----- 0.00 400.00 400.00 400.00 Failure Surface --------------- Initial circular surface for critical search defined by: XL,XR,R Intersects: XL: 40.00 YL: 364.00 XR: 150.00 YR: 330.36 Centre: XC: 106.67 YC: 385.34 Radius: R: 70.00 Earthquake Force ---------------- Pseudo-static earthquake (seismic) coefficient: 0.150 Variable Restraints ------------------- Parameter descriptor: XL XR R Range of variation: 75.00 125.00 60.00 Trial positions within range: 12 12 12 RESULTS: Analysis 1 - Dynamic Bishop Simplified Method of Analysis - Circular Failure Surface ------------- Critical Failure Circle Search using Multiple Circle Generation Techniques Factor of Safety for initial failure circle approximation: 1. 94 There were: 1137 successful analyses from a total of 1729 trial circles 592 analyses terminated due to unacceptable geometry Critical (minimum) Factor of Safety: 1.62 Circle and Results Summary (Lowest 16 Factor of Safety circles) ----------------- Circle X-Centre Y-Centre X-Left Y-Left X-Right Y-Right Radius FoS 1 123. 63 423.71 43.41 364 .00 155.68 328. 99 100.00 1.621 2 129.16 425.40 50.23 364.00 155.68 328.99 100.00 1. 621 3 132.82 420.38 50.23 364 .00 167.05 326.42 100.00 1.626 4 127.06 419.10 50.23 364.00 155.68 328.99 94.55 1. 627 5 119.46 428. 93 43.41 364.00 144 .32 332.07 100.00 1. 633 6 121.59 417.17 43.41 364.00 155.68 328.99 94 .55 1.638 7 117.50 422.73 43.41 364.00 144.32 332.07 94.55 1. 639 8 124.90 412.59 50.23 364.00 155.68 328. 99 89.09 1.640 9 127.43 418.23 43.41 364.00 167.05 326.42 100.00 1.640 10 130.67 413.68 50.23 364 .00 167.05 326.42 94.55 1. 644 11 123.12 424.21 50.23 364.00 144.32 332.07 94.55 1.646 12 121.04 418.07 50.23 364 .00 144.32 332.07 89.09 1. 646 13 118.25 421.72 36.59 364.00 155. 68 328. 99 100.00 1.650 14 125.16 430.22 50.23 364 .00 144.32 332.07 100.00 1. 651 15 113.96 427.35 36.59 364.00 144 .32 332.07 100.00 1.651 16 115.48 416.37 43.41 364 .00 144.32 332.07 89.09 1. 651 File:C:\Projects\Webb-Gannon X-Sec A-A Analysis Printout 4 of 4 10/27/2021 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 Mason County Community Services 615 W Alder St. Bldg 8 A T Shelton, WA 98584 � + f Gary &Nancy Webb vw 36810 NUS Highway 101 Lilliwaup, WA 98555 'Z x)784 w (360) 877-6745 °� GFSTS ' IONALE Re; Hazard Area Buffer Reduction Request and Authorization to perform work on a Hazard Area. N Will Webb Road Lilliwaup, WA 98555 Parcel No.: 32423-30-90012 Mason County's geotechnical document review process will determine that the proposed Gary Webb and Nancy Gannon residential development plans will result in encroachment inside the required 50-foot Vegetated Hazard Area Buffer required under Resource Ordinance 17.01.100. Stability analysis of the slope(s)below the proposed residential improvements indicates that the structure(s) will satisfy Mason County's minimum landslide hazard safety requirements, and will not result in any adverse effects. On behalf of the Gannon-Webbs, I request that the proposed property improvements be allowed to be sighted inside the landslide hazard area buffer as promoted in my site plans and geotechnical report dated 10/27/21 with a 10-foot building setback from the crest of slopes exceeding 40%. This reduction request result in an intrusion of 40-feet into the lower slope's 50-foot vegetated buffer by the residence. The Webbs intend to construct a single family residence inside the Hazard Area's buffer. Please feel free to contact me if there are any questions or concerns regarding these requests. Sincerely, t Alan A. Tahja, P.E. File:C:\ProjectsWebb\HA Buffer Reduction Rqst 1 of 1 10/27/2021