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HomeMy WebLinkAboutGEOtechnical Revised Shoreline Stabilization Report Existing 186.64' Bulkhead - GEO General - 12/15/2019 GEOTECHNICAL and SHORELINE STABILIZATION REPORT FUR AN EXISTING 186.64' LONG BULKHEAD 61 Gull Place, Hoodshort, WA 98548 (Parcel #: 42205-52-00051) Prepared for: Scott and Kristin Merritt 410 214"' Ave S.E. Sainmarnish, WA 98074 (206) 214-6042 e-mail: MerrittsCoCikvaboo.com Prepared by: MET Engineering, PLLC 1018 E. Wishkah St. Aberdeen, WA 98520 (360) 3 10-02)70 cell (360) 289-0958 bus (360) 861-8493 fax e-mail: SNIorta(d aol.com Contact: Steven P. Morta, P.E. P.MO July 11, 2019 Rev: Dec. 15, 2019 a RFcIs �SS�ONALE�G 2120 July 25,2019 Rev: Dec. 15, 2019 Page 1 of 7 MET Engineering, PLLC To: Mason County Department of Community Development 615 W. Alder St. Shelton, WA 98584 (360)427-9670 To: Scott and Kristin Merritt 410 24"'Ave S.E. Sammamish, WA 98074 Subject: GEOTECHNICAL AND SHORELINE STABILIZATION REPORT for an Existing 186.54' Long Bulkhead for Property Located at 61 Gull Place, Hoodsport, Mason County,WA 98548 (Parcel #: 42205-52-00051) Introduction The primate purpose of this geotechnical and shoreline stabilization report is to establish the need for an existing bulkhead constructed at the base of a steep vegetated slope to increase long term shoreline stabilization and increase long term slope stability. A detailed slope stability analysis was also completed which takes into account the steepness of slope, the type of soils that makes up the slope material, the cohesion of the soil as well as the cohesion of the soil itself. This geotechnical engineering report provides the rational for the need of a retaining wall, i.e., a bulkhead, to at least significantly slow down the erosional process that is occurring on the shoreline primarily due the fact that Lake Cushman is a reservoir. So, when power is needed there is a drop in the lake level so that the turbines below can generate electricity. And when poser is not needed the lake will have a chance to fill back up. This geotechnical engineering report provides the rational for the need of a bulkhead to protect the residence above as well as protecting the shoreline directly below the residence to increase overall longterm slope stability. Reference is made to Sheet 2.0 for location of existing bulkhead. General Geologic and Site Description The general site is located on a relatively steep hill side with slopes of 30 to 50% near the top of the hillside where the residence is located and then the slope increases to at least one vertical to one horizontal, i.e., over a 100% slope. The placement of a bulkhead at the toe of the slope will reinforce the slope and slox� down the erosional process at the shoreline and also protect the longterm stability of the residence near the top of the slope 1018 E. Wishl.ah St.,Aberdeen,Washington 98520-2937 (300)289-0958 Bus,(360)310-0270 Cell SMorta a atol.com July 25. 2019 Rev: Dec. 15. 2019 Page 2 of 7 AIET Encrinee�inc , PI LC' An existing residence is located at the top of the steep embankment as sho\vn on Sheet 2.0 Nvith a deck that has been constructed nearly to the slope edge. The existing bulkhead below was constructed to help reinforce the toe of the steep slope which in-turn increases the longterm stability of the residence at the top of the residence. It is important that all remaining trees and vegetation remain undisturbed since their root systems help to reinforce the gravelly sandy loam soils and helps to dewater the slope through the process of evapotranspiration where the extensive root systems help to absorb moisture from the slope and evaporate this moisture back into the atmosphere through their limbs. If one of the trees happen to adversely affect the view. it is recommended that the lower limbs of the tree can be removed rather than topping the tree. Delimbing the louver third of the tree is less detrimental to a tree versus topping which can affect the health of the tree. Geology Most of the Pacific Northwest area had been covered with extensive glaciers that advanced and receded at least four times in the last 10,000 to 40.000 years ago leaving sands, silts, and gravels during the last recession. Also. prior to this period there had existed an inland sea that existed several million years ago and left compacted silt and sand deposits. The last glaciation advance and recession resulted in the formation of Pu,;et Sowed due to the scouring action of the thick ice. Evidence of these glaciers can be seen in the smooth pebbles, cobbles. and boulders that were ground smooth by the sandpaper-like action of the glacier ice and the broken and fractured rock in the glacier's path. The last glacial advance and recession took place over 11,000 years ago with thicknesses of over half a mile high at present day Olympia area. The geologic unit is defined as a Qv, i.e.. quaternai} volcanic rocks as indicated on Sheet 2.0. Smooth gravels and cobbles in the area are the result of past glacial recessional deposits in the area. Smooth cobble surfaces result from the sandpaper effect that the glacial snow and ice have on the broken rock. Reference is made to Sheet 1.0 for the geologic unit, Qv. Upslope Geomorphology Upslope geomorphology indicates relatively steep heavily treed slopes with Douglas Firs and Pines growing near vertical indicating that the slopes have not significantly moved over the life of these trees. Maple trees were also observed near the toe of the slope. Any downward slope movement due to potentially heavy rainfall and high winds would have caused the tree trunks to curve into a pistol butted handle shape. There were some relatively minor incidences of curved tree hunks upslope of the project site. Groundwater Conditions Little to no surface or subsurface groundwater or surface water drainage was observed on the site. Also, no upland waterbodies from the project site were present when this geotechnical 1018 E.Wisbkah St..Aberdeen.Washington 98520-293° (360)289-0958 Bus,(360)310-0270 Cell S1lortu<daol.com 7/D July 25,2019 Rev: Dec. 15,2019 Page 3 of 7 MET Engineering, PLLC engineer visited the site. Little to no significant signs of seeps or wetland areas along the face of the slope was observed. Upland Wetlands,Waterbodies,and Vegetation This geotechnical engineer did not observe any upland Nvaterbodies or wetlands that could cause problematic slides or unstable areas in the future. It is highly- recommended that remaining trees and vegetation in the area be preserved and remain undisturbed to increase long term slope stabilization. Douglas Fir trees. Alders, and some maple trees were growing on the relatively steep bank and need to remain undisturbed and preserved since their root systems help to reinforce the granular soils especially during periods of heavy rainfall and high winds. Ground cover vegetation were also present such as salal and ferns also need to be preserved for the same reason. Surface Water Drainage and Control Systems Surface water runoff from impermeable surfaces such as roofs and driveways shall be directed into at least two. preferably three 6" diameter black corrugated tightlines that daylights approximately 15 to 20 feet from the shoreline edge into a 5' x 10" x 2' deep energy dissipator filled with 4 to 12" diameter smooth cobbles. This will allow the surface water runoff to infiltrated back into the ground. Geomorphology of the Lake Cushman Shoreline On an annual basis the level of Lake Cushman gets filled due to the Cushman dam that has artificially produced the man-made lake. Then typically, in the fall and winter, when electrical power is required for heating, then the elevated water conditions is used to run the electrical generators within the dam and,as a result,gradually lowers the lake level. This annual rising and lowering of the lake level have a tendency to saturate the otherwise dense granular material that comprises the local soils. The smooth, rounded cobbles and some boulders indicate that this material is glacial deposits as a result of the last major glacial advance and recession. Once this very dense till soils get saturated,there is then a tendency for the binding silt matrix that holds the cobbles and boulders together to loosen and slough away into the lake. This, in turn, can result in the potential erosion of the shoreline and the toe of the surrounding hillsides that can initiate slide conditions which has been observed in some cases especially along the shoreline of Lake Cushman. The existing bulkhead constructed along the shoreline at the base of the steep slope helps to significantly slow down the erosion of sediment and gravels. This increases the longterm stabilization of the steep bank and as well as decrease the downward movement of sediment along the shoreline. MIN 1'..1\khl:ah tit..Aberdeen,Washin"1101, 98520-2937 (300)289-0958 Bus,(360)310-0270 Cell SMortata'aol.cotn July 25, 2019 Rev: Dec. 15. 2019 Page 4 of 7 MET Engineering, PLLC Evaluation of Erosion and/or Flooding Issues As mentioned in the previous section, the seasonal rising and lowering of Lake Cushman results in the saturation of the Hoodspor7 grcn,elly sandy locan Ole) soils dining high lake levels. Once the lake reaches a certain elevation and when hydroelectric power is needed the lake level goes back down taking loose sands and silts along with it. As a result, this gradually loosens up the larger gravels and cobbles since the silt and sand binders have been removed due to the lowering of the lake level as well as the continual wave action especially during storm events. Potential Damage of Residence The existing bulkhead was constructed to significantly slo\a down the shoreline erosional process due the adverse combination of steep slopes and season lake level fluctuations. There's the potential that if the bulkhead had not been constructed that the residence could have been adversely affected in approximately a 3-year time frame especially if there a severe high windstorm event and at the same time high lake levels. This could significantly erode the base of the steep slope and initiate a slide event. Alternate Slope Mitigation Measures Soft solutions such as the planting of shoreline vegetation or the placement of large woody debris would at most be only temporary since the loose gravels and cobbles do not provide a vets good base for the vegetation or woody debris. Soil Exploratory Borings According to the USDA Web Soil Survey. the soils in this Lake Cushman shoreline area have been identified as a Hoodsport gravell'v samly loam (He) per the online Web Soil Survey prepared by the USDA Natural Resources Conservation Service as indicated on Sheet 4.0. The first soil exploratory site was near the face of the slope which indicated smooth gravels embedded in sandy loam soils for a depth of at least 15 feet or more. There was some evidence of slope movement near the toe due to the steepness of the slope and also the scouring wave action of Lake Cushman along the shoreline. The second soil exploration site was located approximately a third of the way up the slope where some slide activity exposed more sandy silty soils with gravels. Reference is made to Engineering Sheet 3.0 for the soil boring locations and a typical cross-section near the base of the slope is also shown. Slope Stability Analysis A detailed slope stability analysis .was completed for the site as indicated on Engineering Sheet 3.0 which takes into account the height of the slope, the density of the insitu till soils, the cohesion of the native soils. and the internal friction angle of the slope material. For the static or non-seismic case. the resulting factor of safety was determined to be 1.061 and since this was less than the normally accepted value of 1.500,the lower third of the slope was considered unstable. 1018 L.Wishkah tit.,Aherdecn, \ba,,hin1tun 98520-293' (360)289-0958 Bus,(360)310-0270 Cell tiMorta�d`aol.coni 6120 July 25, 2019 Rev: Dec. 15, 2019 Page 5 of 7 MET Eray ineerinh, PLLC The quasi-static case or seismic factor of safety was also computed that resulted in a factor of safety of 0.807 and since this was less than the normally accepted value of 1200 this too was considered unsatisfactory. Reference is made to Sheet 4.0 that provides detailed calculations for the factor of safety for both the static as well as the seismic case. Land Slide Activity in the Area The Lake Cushman area was created by the construction of the Cushman Dam that generates power from the Tacoma Power turbines below just off of Hwy 101. As a result, the annual rising and lowering of the lake level has a tendency to saturate the shoreline during summer months and then the water level starts declining during the winter months. This process can result in the removal of fines within the seemingly very hard and dense glacial till like soils. Over time, once these binders. that is fine silt has been removed, then there is little left to hold the smooth gravels and cobbles in place. This eventually can result in the gradual unraveling of the shoreline in some places especially where the slopes are steeper along the shoreline. Land Slide Mitigation The existing bulkhead retaining wall provides long teen slope stability since in reinforces the toe of the slope and, thus, significantly slows down shoreline erosion due to the fluctuating lake level and also this bulkhead increases the longterm stability of the residence near the top of the slope. Bio-retention Bulkhead Replacement This -Soft" solution is not applicable for this slope since the base of the slope consists of gravel and cobblers and that any plantings in this area is not feasible. For this steep toe condition, the most effective solution is a mechanically stabilized wood bulkhead. It may be possible to hydroseed the bare slope areas near the bulkhead but would only be temporary in nature. Getting this kind of pumping equipment to spray the bare slope would be problematical at best. Shoreline Stabilization,i.e.,the Bulkhead,is the Minimum Size Necessa►y Very steep slopes are present along the entire shoreline property line and that the existing bulkhead prevents further continual shoreline erosion and also provides longterm slope stabilization. This bulkhead \was constructed as far landward as possible at the toe of the steep slope and,therefore,could not be constructed any further landward. On-Site and Off-Site Impacts(if any) There will be little to no on-site or off-site impacts since all the work was already completed within property boundaries as indicated on Engineering Sheet 2.0. it is to be noted that the existing bulkhead significantly slows down the shoreline erosional sediment process. 1018 E.A\ishl.ah St.._Abc.(Ice,,. A\,rshinhton 98-;20-2937 (300)289-0958 Bus,(360)310-027(1 Cell SmortwWaol.com 7/2c) July 25,2019 Rev: Dec. 15, 2019 Page 6 of 7 MET Engineering, PLLC Final Development Conditions and Structural Mitigation Currently, the Lake Cushman water level is approximately 10 feet below the normal filled lake level height. The existing wood bulkhead increases long term slope stability by reinforcing the toe of the slope which in-turn significantly increases the long term stability of the residence near the top of the steep slope. Structural Mitigation Plan The existing bulkhead retaining wall is already in place and appears to be structurally sound with no signs of tipping or instability. Conclusions and Recommendations This geotechnical engineer concludes that the construction of the wood bulkhead was necessary to increase the long-term stability of the slope by reinforcing its toe. As a result, the shoreline erosion has been significantly slowed down and also, the overall longtenn stability of the residence near top of the slope has increased. This geotechnical engineering report has been prepared per standard accepted engineering practice and that should there be any changes to the site conditions, this geotechnical engineer shall be contacted immediately to make the necessary updates to this geotechnical report, if necessary. it is to be noted that this report applies to this site only and not be used at any other location. Also, this geotechnical engineering report satisfies Mason County's requirements checklist for a geotechnical report which has been attached to this document as well. Any questions or comments regarding this geotechnical engineering report can be addressed by e-mail at SMorta a-aol.coni or by calling this engineer directly at(360)310-0270. Respectfully Submitted, P. 0 h T Steven P. Morta, P.E. y 30738 a �FcrsrE�< �SsfoNALti�G /�/S/Za l 9 1018 E.Wishkah St..Aberdeen,Washington 98520-2937 (360)289-09i8 Bus,(360)310-0270 Cell SVlorta!a aol.com �'l2a July 25,2019 Rev: Dec. 15,2019 Page 7 of 7 MET Engineering, PLLC References: 1. Online Soil Web Survey prepared by the USDA Natural Resource Conservation Service that provides a detailed description of the soils and soil strata as well as its origin. 2. USGS Geological Map for Western Washington. 3. Stability Charts for Uniform Slopes by Radowslaw L. Michalowski, P.ASCE from the Journal of Geotechnica) and Geoenvironmental engineering,April 2002. 1018 E.Wishkah tit.,Aberdeen.Washington 98520-2937 (360)289-0958 Bus,(360)310-(1270 Cell SMortara aoi.com 12/14/2019 Mason County WA GIS /O Mason County • rr 422055200051 X Q Show search results for 42205... i ® I f 6 S4ox� f 1q, rY,1f ( try It '� � `'�:• 600ft �ffifuF�7iiiil� https://gis.co.mason.wa.us/mason/?find=422055200051 1M GEOLOGIC UNIT FOR THE LAKE CUSHMAN REGION NORTH W E 91 GULL PLACE S HOODSPORT,WA 98548 O Is Z oq •. J/� s a ` eTv v 9 a s Qls ' o 't �'` .la spom Y Tvc d 106 L � t Oad Tm v Q9d f,. Volcanic Rocks and Deposits ' l Qv Quaternary volcanic rocks GEOLOGY OF THE LAKE CUSHMAN AREA Prepared by: MET Engineering,PLLC,1018 E.V1Ashkah St.Aberdeen,WA 98520-2937 (360)289-0958 bus.,(360)310-0270 text.SL1oaa i +_ c Prepared for:SCOTT MERRITT Jul 25.2019 SHEET 1.0 Y Site Visit on Tue,2-27-2018 AERIAL VIEW OF THE PROPERTY INCLUDING BULKHEAD 21? NORTH The existing retaining wall is comprised of 31 sections of 6 x 6 pressure treated lumber laid horizontally in steel H-beams and capped with 8"X 2"boards at the top. The sections are varying lengths and heights. The H-beams are set in concrete at the base and by wires at the top, tied back to dead heads in the hillside. The total length of the wall measured across the face of the sections is approximately 186.64 feet and the vertical face of the wall varies in height, but is 15.6'at its highest and averages 14.25'. The northern most w E section measures 65.5"in length and is angled toward the hillside at approximately a 45 degree angle. The southern most section measures 27"and is angled toward the hillside at approximately a 45 degree angle. Moving north-to-south the wall angles toward the hillside about 15 degrees after the 15th section(about 90 degrees from the northern most point as measured across the face of the wall)and continues at that angle S through the 20th section or approximately 29'. At that point the wall continues south parallel to the hillside. r ""'" "mow�':• .0001, low 41 f.. 4•-fir r� i. � �1�� �{:. w ir_ M .� r 11 s� e - 1r y ` • : , ti 5! '� i h. '; -+•� x•� it �.�"���' r ' 1e • 1 "�` rw �. �. Fes. y,4g• .t '` -;, 4 r +�S -^'y1y < �i7,.-; .' �+ ��� ,�,. '�,+4 e+"fi:�.1"''� _ ,�"^�"'A • .��,r _ ,•` 'yew , - "r •a7� AERIAL VIEW OF 61 GULL PLACE Prepared by: MET Engineering,PLLC,1018 E.Wishkah St,Aberdeen,WA 98520-2937 Hoodsport,WA 98548 (360)289-0958,(360)310-0270 text,SMorta_eaol.corn July 25,2019 Prepared for.SCOTT MERRITT Rev: Oct.17,2019 Site Visit on Fri.2-23-2018 SHEET 2.0 Rev: Dec.14,2019 1 2/1 51201 9 Web Soil Survey J? / ,. /2 Mason County,Washington(WA645) Mason County, Washington (WA645) Map Acres percent Unit Map Unit Name in of AOI Symbol AOI Gm Grove gravelly 2.1 17.5% sandy loam, 15 to 30 percent slopes Hd Hoodsport gravelly 4.5 37.8% sandy loam, 5 to 15 percent slopes He Hoodsport gravelly 2.1 17.4% sandy loam, 15 to 30 percent slopes W Water 3.3 27.3% Totals for Area of 12.0 100.0% Interest FGIA i Accessibility Statement I Privacy Policy I Non-D;scnmination Statement 1 Information quality I USA.gov i V.hde House :,Mqn!t Description X Printable version Report—Map Unit Description (� Mason County,Washington Hd—Hoodsport gravelly sandy loam,5 to 15 percent slopes Map Unit Setting National map unit symbol:2hkx Elevation: 160 to 980 feet Mean annual precipitation:60 inches Mean annual air temperature: 50 degrees F Frost-free period: 150 to 175 days Farmland classification:Not prime farmland Map Unit Composition Hoodsport and similar soils: 100 percent Estimates are based on observations,descriptions,and transects of the mapunit. Description of Hoodsport Setting Landform:Till plains Parent material:Basal till Typical profile H1 -0 to 5 inches:gravelly medial sandy loam H2-5 to 24 inches very gravelly medial sandy loam H3-24 to 60 inches:gravelly sandy loam Properties and qualities Slope:5 to 15 percent Depth to restrictive feature: 20 to 40 inches to densic material Natural drainage class:Moderately well drained Capacity of the most limiting layer to transmit water(Ksat):Very low to moderately low(0.00 to 0.06 in/hr) Depth to water table:About 18 to 36 inches Frequency of flooding:None Frequency of ponding:None Available water storage in profile:Very low(about 1.6 inches) Interpretive groups Land capability classification(irrigated):None specified Land capability classification(nonirrigated):6s Hydrologic Soil Group:B/D Forage suitability group: Limited Depth Soils(G002XF303WA) Hydnc soil rating:No Description—Map Unit Description ,J https://websoilsurvey.sc.egov.usda.gov/AppfWebSoi]Survey.aspx 2/2 12/15/2019 Web Soil Survey ?E I SI)1 z _ i ar 4VVII Contact Us Subscribe Archived Soil Surveys Soil Survey Status Glossary Preferences Link Logout Help A Area of Interest(AOI) Soil Data Explorer 1 Download Soils Data I Shopping Cart(Free) Printable Version] Add to Shopping Gartl �a If �S r T� Y t f �0 ;t Warning:Soil Map may not be valid at this scale. Map Unit Legend https://websoitsurvey.sc.egov.usda.gov/App[WebSoi]Survey.aspx 112 NORTH LOCATIONS OF SOIL EXPLORATORY BORINGS /s o W E S »syl..w c:ccs:an: +Soil Exploratory#1 +Soil Exploratory*2 BULKHEAD REINFORCES TOE OF SLOPEAND INCREASES LONG TERM SLOPE STe&QTY Sal PRESERVE ALL REMAINING TREES Exploration AND VEGETATION ON SLOPE TO #2 INCREASE LONG TERIvt SLOPE SOH 780 STABILTY TO PROTECT SHORELINE Exploration AND RESIDENCE aK1 'o 760' 186.6'LONG BULKHEAD CONSISTING OF H-BEAMS PLACED IN CONCRETE AND SECURED WTH DEADMAN INTO THE HILL SIDE. 6 x 6 P.T TIMBER WAS THEN PLACED o o�0 74(Y — 742' INBETWEEN THE STEEL H-BEAMS ° 738' 0 LAKE CUSS-WAN 80' 60' 40' 21Y 0' JtiN P.MO wnsy��c�� e �O BROWN SANDY LOAM WITH SOME 6"MINUS GRAVELS, 8 TO 10%MOISTURE,1600 PSF ALLOWABLE BEARING VALUEPC 1058 k7'G STIE tit` ® DENSE GLACIAL TILL SOILS WITH AN ALLOWABLE f�'S7UNAt, a SOIL BEARING CAPACITY OF 2200 PSF AND A 9 TO 12% MOISTURE CONTENT EXPLORATORY SOIL BORINGS Prepared by. MET Engineering,PLLC,1018 E.Wishkah St,Aberdeen,WA 98520-2937 (360)289-0958,(360)310-0270 text,SMorta@aol com July 25,2019 1 Site Visit on Fri.2-23-2018 Prepared for.SCOTT MERRITT Rev: Dec 15.2019 SHEET 3.0 Slope Stability,Analysis (Simplified Bishop's Method of Circles) A�*/,?d 61 Gull PLace, Hoodsport, WA 98548 Potential Failure Plane 45 ft 1� i Not to scale(NTS) Alpha=43.5 deg (95%slope) 1. Compute factor of safety(FS)against sliding-Static Case(Ref: "Stability Charts for Uniform Slopes'by Radoslaw L. Michalowski, F.ASCE) - "Journal of Geotechnical and Geoenvironmental Engineering',April 2002. From Figure 3"Stability charts for uniform slopes"the following parameters are known for Hoodsport gravelly sandy loam soils(He) from the online USDA Soil Web Survey: c(cohesion)=200 psf,gamma(insitu unit density)= 136 pcf, H(height of slope)=35 ft, phi(internal friction angle)=23 deg, then c/[gamma x H x tan(phi)]=200/(136 x 35 ft x tan 23)=0,0990 Then for a 43.5 deg(95%)slope from Fig. 3: FS/tan(phi)=FS/tan(23)=2.5 or, FS=Factor of Safety Against Sliding= 1.061 <1,500 NOT OKAY 2. Compute factor of safety(FS)against sliding-Quasi-Static(seismic)Case(Ref: "Stability Charts for Uniform Slopes' by Radoslaw L. Michalowski, F.ASCE) - "Journal of Geotechnical and Geoenvironmental Engineering",April 2002, From Figure 4"Stability charts for uniform slopes"the following parameters are known for Hoodsport gravelly sandy loam soils(He): c(cohesion)=200 psf,gamma(insitu unit density)= 136 pcf, H(height of slope)=35 ft, phi(internal friction angle)=23 deg, then BEN P.M WASy�'Q�9 c/[gamma x H x tan(phi)]=200/(136 x 35 ft x tan 23)=0.0990 Then for a 43.5 deg slope(95%slope)from Fig.4 for a quasi-static analysis coefficient of 0.1: F/tan(23)=2.2 or FS=Factor of Safety=0.934 1 � 0758 p `�4 'roc FcrsTE�� � For a quasi-static analysis coefficient of 0.2: SS�ONAI.ti�G F/tan(23)=1.9 or FS=Factor of Safety=0.807 Asa result,for a quasi static analysis coefficient of 0.15,the Factor of Safety, FS=(0.934+0.807)/2=0.870<1.100 NOT OKAY MITIGATION MEASURE. CONSTRUCTION OF A BULKHEAD RETAINING WALL AT THE TOE OF THE SLOPE WILL INCREASE LONG TERM SLOPE STABILITY BY REINFORCING THE TOE OF THE SLOPE. THIS HELPS TO CONTROL SHORELINE EROSION AND PRESERVE THE STABILITY OF THE RESIDENCE SLOPE STABILITY ANALYSIS Prepared by: MET Engineering,PLLC,1018 E.Wishkah St,Aberdeen,WA 98520-2937 (360)289-0958, (360)310-0270 text,(360)861-8493 fax.SI:Ic i,a a- of Prepared for:SCOTT MERRITT July 25,2019 Site Visit on Fri.2-23-2018 SHEET 4.0 171b MASON COUNTY COMMUNITY SERVICES Bill,1-1,PL,nningkn•rroiwnlalHv.11lll.CommunityH­II, Shoreline Stabilization Note:This checklist must be submitted with a Shoreline Geotechnical Assessment and completed, signed, and stamped by the licensed professional(s) who prepared it for review pursuant to the Mason County Shoreline Master Program. If an item is found not applicable, the Assessment should explain the basis for the conclusion. MCC 17.50.340 B.2. New structural stabilization measures shall not be allowed except as follows: a. To protect existing primary structures: i. New or enlarged structural shoreline stabilization measures for an existing primary structure, including residences, shall not be allowed unless there is conclusive evidence documented by a Shoreline Geotechnical Assessment* that the structure is in danger from shoreline erosion caused by tidal action, currents, waves, or sea level rise. Normal sloughing, erosion of steep bluffs, or shoreline erosion itself, without a scientific or geotechnical analysis, is not demonstration of need. The geotechnical assessment shall evaluate on-site drainage issues and address drainage problems away from the shoreline edge before considering structural shoreline stabilization. ii. The erosion control structure will not result in a net loss of shoreline ecological functions. This shall be demonstrated in a Habitat Management Plan as required in MCC 8.52.170(J). iii. Primary structure means the structure or the only access associated with the principal use of the property that cannot feasibly be relocated. It may also include single family residential appurtenant structures that cannot feasibly be relocated. Applicant/Owner Srnff /��rr�' Parcel Site Address �� 6u�� Plae- Items to be included in assessment Located on page(s): Scope of review,including landowner concerns and site assessment objectives. Discussion of site development, including the location and setback of important primary and appurtenant structures, roads and utilities. Z Assessment of relevant wetlands and surface water drainage and control systems. ✓ Assessment of site vegetation, including species and communities present. Assessment of regional/site geology, including relevant rock/soil unit descriptions,geologic history and stratigraphic correlation. 2- Assessment of regional/site geomorphology,including the influence of wave or tidal processes in controlling sediment transport and the evolution of the site's landform. Include assessment of the drift cell sediment budget and local beach conditions. May 2018 Evaluation of erosion and/or flooding issues,including assessment of causes, and rates or timeframes associated with each issue. Include evaluation of how historic shoreline J? modifications near the site may have impacted patterns of erosion. Evaluation of the amount of time until primary structures or major utilities will be damaged by wave or tidal action,based on evidence of erosion rates, a slope stability analysis or other geotechnical considerations if no action is taken on the site. Discussion of alternative approaches to reduce risk from erosion on the site,including the use of nonstructural measures such as drainage and vegetation,and soft shoreline techniques such as gravel berms, large woody debris,or other measures. Explain why these techniques would be S infeasible or insufficient to reduce the threat from erosion. Discussion describing the proposed shoreline stabilization is the minimum size necessary, placed as far landward as possible and will not result in a net loss of shoreline ecological functions. s A site map drawn to scale which identifies property boundaries,scale, north arrow and the location of proposed development,geologic features and ordinary high water mark(OHWM).All development setbacks should be delineated and marked. Cross-section(s),identified on site map,showing both horizontal and vertical scales, and identifying important features such as: MHHW and OHWM,top and bottom of bluff, location of Sflee'f 3. upland structures and utilities, and geologic units where relevant. Supplementary photographs. _ 1, �reU� P/+�Orfa 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 Assessment,dated !JW,lS ZU Q and entitled W . 0'Pr jmeets all the requirements of the Mason County Shoreline Master Program,Shoreline O .pFC30758�0 �c+a Stabilization Section(MCC 17.S0.340),and is complete and true. S7E1� � ALG (Signature and Stamp) •MCC17.50.020 Definitions:Shoreline Geotechnical Assessment. A scientific study or evaluation conducted by a qualified expert that includes a description of the ground and surface hydrology and geology,the affected land form and its susceptibility to mass wasting,erosion,and other geologic hazards or processes,conclusions and recommendations regarding the effect of the proposed development on geologic conditions,the adequacy of the site to be developed,the impacts of the proposed development, alternative approaches to the proposed development,and measures to mitigate potential site-specific and cumulative geological and hydrological impacts of the proposed development,including the potential adverse impacts to adjacent and down-current properties. Geotechnical assessments shall conform to accepted technical standards and must be prepared by qualified professional engineers or geologists who have professional expertise about the regional and local shoreline geology and processes. Page 2 of 2 Disclaimer.Mason County does not certify the quality of the work done in this Geotechnical Assessment. �?c �• MASON COUNTY COMMUNITY SERVICES Geotechnical Reort 6r0dety.Vl.wintng.f-u- rntal Wafth Cam ty H-1th f 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 �D7/ OZPZ- le Parcel# Zla Z(, Site Address �� S�el P/are r1,�� U/�4 ��sq�d (1) (a) A discussion of general geologic conditions in the vicinity of the proposed development, Located on page(s) / e-� 7 (b) A discussion of specific soil types, Located on page(s) !y 0 7 (c) A discussion of ground water conditions,Located on page(s) Z, �3 ,I /�7 (d) A discussion of the upslope geomorphology, Located on page(s) Z Df '? (e) A discussion of the location of upland waterbodies and wetlands, Located on page(s) 3 of (f) A discussion of history of landslide activity in the vicinity, as available in the referenced maps and records. Located on page(s) 3 0 f 7 (2) A site plan which identifies the important development and geologic features. Located on Map(s) e f 2_ D (3) Locations and logs of exploratory holes or probes. Located on Map(s) J/ee 7"-. ;? d (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) f Z D (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) SXee-f �� Q (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) e f q D (7) (a) Appropriate restrictions on placement of drainage features, Rev. February 2018 2U�2a Located on page(s d 6:� (b) Appropriate restrictions on placement of septic drain fields, Located on page(s) /V IA (c) Appropriate restrictions on placement of compacted fills and footings, Located on page(s) & M (d) Recommended buffers from the landslide hazard areas shoreline bluffs and the tops of other slopes. Located on page(s) /vZ,4 ue-Lre 6 Ire 4 td I /�q ol-kcC) (e) Recommended setbacks from the landslide hazard areas shoreline bluffs and the tops of other slopes. Located on page(s) /yf/.J '; Uf-d G/heatA, ;Lt /IlCla-) (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) N1 4u,-C' //'t/JIuCe (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) N ZA �. 7h .ee_,4 o e is/vlaee (10) An analysis of both on-site and off-site impacts of the proposed development. Located on page(s) `- O T 7 (11) Specifications of final development conditions such as, vegetative management, drainage, erosion control, and buffer widths. / L Located on page(s) Co 47 (12) Recommendations for the preparation of structural mitigation or details of other proposed mitigation. Located on page(s) t; O:� 7 (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) ,s6e" ' - Z, U I, S-�e eye P &1 or¢a 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 P.MO Geotechnical Report, dated /2�/.��ZV/ and entitled ti AS R 0�y rbc�ir�:car 2.ra(�I�orelyi��S �h,/� ,fs'an /�i�vrfi 117 �o meets all the requirements of the Mason County Resource Ordinance, Geologically Hazardous Areas Section, is complete and true, that the 5� 30758 ��%1ST6R�G assessment demonstrates conclusively that the risks posed by the S/0N. ►- landslide hazard can be mitigated through the included geotechnical /2. ?�/9 design recommendations, and that all hazards are mitigated in such a (Signature and Stamp) 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.