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HomeMy WebLinkAboutGEO2011-00033 - BLD Engineering / Geo-tech Reports - 5/27/2011 MASON COUNTY DEPARTMENT OF COMMUNITY DEVELOPMENT Planning Division P O Box 279, Shelton, WA 98584 P14 (360)427-9670 Geotechnical Assessment Review Acceptance Letter May 27, 2011 BRIAN ROSS 7402 NE 87TH CIRCLE VANCOUVER WA 98662 Case No.: GE02011-00033 Parcel No.: 121055200067 Project Description: REPLACE CABIN WITH NEW CABIN The Geotechnical Assessment for BRIAN ROSS has been received and reviewed by the Planning Department. The assessment was prepared by wiltsie / Morrissette dated 2/26/2009. Based on the certification provided by the licensed engineer/geologist, the referenced Geological Assessment was prepared in general accordance with the requirements in the Mason County Resource Ordinance, Landslide Hazard Areas 17.01.100.E.4. Mason County considers the review valid until such time as scope of project, site conditions, and/or regulations change. Should the scope of work, site conditions, and/or regulations change after the original review, then an addendum from the original author of the assessment may be required to address these changes. The assessment would only be re-reviewed if a permit for development were submitted after these changes occur. Mason County does not certify the quality of the work done in this Geological Assessment. Please contact me at (360) 427-9670, ext. 365 if you have questions. Sinc ely, Allan Borden Land Use Planner Mason County Planning Department Comments: 5/27/2011 Page 1 of 1 GE02011-00033 C Mason County Review Checklist r � For a Geological Assessment I Instructions: This checklist is intended to assist Staff in the review of a Geological Assessment. The Assessment is reviewed for completeness with respect to the Resource Ordinance. If an item is found to be not applicable,the report should explain the basis for the conclusion. The Assessment is also reviewed for clarity and consistency. If the drawings, discussion, or recommendations are not understandable, they should be clarified. If they do not appear internally consistent or consistent with the application or observations on site, this needs to be corrected or explained. If resolution is not achieved with the author, staff should refer the case to the Planning Manager or Director. Al Applicant's Name: P—" (t-)'Ii0 I t _ (�(�j�`t Permit# Parcel # Date(s)of the Document(s) reviewed: (1) A discussion of geologic conditions in the general vicinity of the proposed development,with geologic unit designation based on referenced maps. � 2 %� OK? L., Comment: ' / (2) (a)A discussion of the ground water conditions at the site, OK? Comment: (b) A discussion of the estimated depth to water OK? Comment: L a'(c) A discussion of the quantity of surface seepage V OK? Comment: (d) A discussion of the upslope geomorphology OK? Comment: (e) A discussion of location of upland waterbodies and wetlands. OK? Comment: (3) The approximate depth to hard or dense competent soil, e.g. glacial till or outwash sand. OK?_tom Comment: (4) A discussion of any geomorphic expression of past slope instability(presence of hummocky ground or ground cracks,terraced topography indicative of landslide block moveTent,,bowed or arched trees indicating downslope movement, etc.). OK? Comment: (5) A discussion of the history of land id ac i ty in the vicinity, as available in the referenced,maps and records. OK? Comment: (6) An opinion on whether the proposed develolpnient 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. OK? Comment: p �� (7) A recommendation by the preparer whether a G otechnical Report should be required to further ev irate site conditions and the proposed dexe�lo ent of the subject property. OK? Comment: I. (8) If the presence of a hazard is determined within 306 fe t of the proposed development, then the following are delineated on a geologic map/site map: (a) the areof the proposed development, OK? Comment: Page 1 of 2 Form Effective June 2008 (b) the boundaries of the landslide hazard area (top, both sides, and toe), OK? Comment: (c) the associated buffers (top, both sides, and toe) OK? Comment: (d) building or other setbacks (top, both sides, and toe). OK? Comment: (9) A site map drawn to scale showing the property boundaries, scale, north arrow, and the location a re nature of existing and proposed development on t e site. OK? ,_�_Comment: Are the.Document(s)signed and stamped? Type and #of License: If not approved, what is the next action/recommendation for further action? LT Reviewed by , on Time spent in review: SECOND REVIEW/UPDATE: Reviewed by , on Time spent in second review: THIRD REVIEW/UPDATE: Reviewed by , on Time spent in third review: Disclaimer: Mason County does not certify the quality of the work done in this Geological Assessment Page 2 of 2 Form Effective June 2008 Jerome W.Morrissette&Associates Inc., P.S. Civil•Municipal•Geaechnical Engineering and Planning 1700 Cooper Point Road SW,#13-2,Olympia,WA 98502-1110 (360)352-9456 / FAX(360)352-9990 ROSS RESIDENTIAL PROPERTY GEOLOGICAL ASSESSMENT 462 Treasure Island Drive Allyn, Washington 26 February 2009 INTRODUCTION The purpose of this report is to provide geotechnical site evaluation data in accordance with the Mason County Landslide Hazard Areas Ordinance, Section 17.01.100 E for the project site, which is located at 462 Treasure Island Drive,Allyn, Washington. The property consists of Tax Parcel No. 121055200067 (see Figure 1). E%AROM7G8 HILL RD E CRONgUIgT RD E DETROIT OR E DOI/�s FiR LN n 1la,A 1 / W W / FtOiECf Site &' Y _� �.. EECKERTgD E FRONTIER RD C W Amy J W W 4i8ft Figure 1 The investigations and evaluations performed for this report include published regional topography, published soil and geologic investigations and crude site specific topographic measurement of the onsite and nearby slopes. Ross Geological Assessment 1 JWMA N08135 2/26/09 No site specific soil investigations have been performed for this site as the substrata have been observed to be competent and there is no evidence of shallow or deep rotational failure either on or in the immediate vicinity of the project site. The proposed structure would be located on the upper terrace fronting a competent glacial till marine bluff of about 10 ft height and with a slope of about 90%, consisting of a timber bulkhead and timber retaining wall. A preliminary topographic survey has been performed at the site by CSI Surveying. This survey has been used to prepare the proposed development site plan (Figure 10). A final survey will be completed in the near future for the purpose of preparing a final site plan/drainage plan. The project site was visited on 16 October 2008. Measurements of site surface slopes and horizontal offsets were made using a Clineometer and measuring tape. Based on a preliminary evaluation,the subject site falls marginally within the Ordinance definition of a Landslide Hazard Area. A precise topographic survey would likely show the site to have a slope vertical relief of slightly less than 10 ft. However, for the purposes of this site a Geological Assessment has been prepared to justify new construction on this site. The data gathered and investigations performed have been used to prepare general development recommendations. 17.01.100.E.4(1) GEOLOGIC CONDITIONS The project site is shown on the Washington State Coastal Zone Atlas, 1980 (see Figure 2). The site is mapped as Stable with no mapped intermediate or unstable areas within the immediate vicinity or along the shoreline of Treasure Island. • • • 1i © 4 ® ® ® ® © ® O „ rrim .., -------- ---f----"---- — --'-- —"-- Data laws ��abYKdrNaMtat t.ut,mK Wtyaloal r.atutes r 'us�tall - 1 —�lallu:.yna �—Qi�nt to LeG . —Und.4ud 4P£A.M. tneem.d.ee t' nn untatN �'�ir r uMt.4i.P.a.nt\Igdb r lh*&.ad L.4" r wu.a x - OP..— Pro t Site / 0Q.— r ®«: rotuees �tl � Nxleratenns �/ •� ttt�t.nw IMi..i+.t.n.. T—ponaWn reatwe Nxipround I..,-y Figure 2 Ross Geological Assessment 2 JWMA N08135 2/26/2009 Geologic mapping available from the Washington State Department of Natural Resources (see Figure 3) shows the site to be located on the westerly margin of Treasure Island, a remnant of an ancient glacial till plane. The site is mapped as Qgt—Vashon Till, which is defined as "Unstrat fed to moderately stratified, compact, unsorted mixture of clay, silt, sand, and gravel deposited directly by glacial ice; gray to tan; nearly everywhere in sharp contact with underlying units; does not drain well as permeability and porosity are low; sand and finer grains in matrix are very angular;pebble- to boulder-size clasts are commonly striated and faceted, having angular and(or) rounded edges; boulders are generally disseminated and relatively rare; may contain interbeds of sand and gravel. The surface of this unit is characterized by streamlined drumlins, striations, and flutes that are generally hundreds to thousands of feet long. Angular to subrounded glacial erratic boulders, consisting mostly ofplutonic or metamorphic rock, are common but disseminated on the surface of this unit. Unit may be capped by a few feet of unsorted and stained ablation sand and gravel or by unit Qgos. Vashon till locally crosscuts older sediments,forming angular unconformities. Drag folding and horizontal shearing may occur at the base of the till or internally between layers. " &e, . w NORTH w s � � Protect Site r A,* Geologic Map of the%anghn 7.5-minute Quadrant le. Pierce anti Mason Counties.Washington IXcnlib YMI? xft, Figure 3 Figure 4 shows the Earthquake Class mapping for the project site. The project site is listed as Class C, which is one step below soft rock where there is no essentially no amplification or reduction of shaking due to the subsurface sediments, As such, the site subgrade conditions may be considered relatively stable as compared to other subgrade soil conditions under seismic excitation. Ross Geological Assessment 3 JWMA N08135 2/26/2009 ,tart aNA'r1Uv f , in the methodology presented by E3S5C (t997), site class t; 7- retxesents a soft rock conditionu where earthquake shaking is .. - so.,rt - neither asnphfied or reduced by the near-surface geology. Site classes C.D,and F represent increasingly softer soil conditions "n11cn'x which result in a progressively increasing arnphBcabon ofground .cNr.rse..n ^Y shaking.Site class F 1s reserved fix unusual soil co"nons whore prediction of the amphfication of earthquake shaking can only he detrntntted by a slte•spes'lfie evahsatix on p this map wedelineaterea as of peat sod as site class F.LiqueliaMe soils tiro faU uuo sne ut w class F.be have not included them on tbat map:please refer to the liquefaction suskeptnbihty maps in this series for more innfpornation. / project Site P 'Site Class Ni;a;Tt of�lasua('uuni�. �4'a�1>tin„t(►n by SWI"Y.UJ---SO--Udfre l..btac u u lli: tliWabwl.Jmina t.r..et4-Lk3.13.Fotra'.eui Hrbaw A.+fiu,•ema>tu Scdcd.a J1Ut IN <'ewl... .... Figure 4 Figure 5 shows the liquefaction mapping for the project site. The site is mapped as having very low liquefaction potential. Ross Geological Assessment 4 JWMA N08135 2/26/2009 EXPLANATION ` L Liquefaction susceptibility:HIGH Athn� . Liquefaction Susceptibility:MODERATE to HIGH Li Liquefaction w5ccptlbili[y:MODERATE { '{ Liquefaction susceptibility:LOW to MODERATE r e ° Liquefaction susceptibility:LOW l; oject Liquefaction susec7>tibility:VERY LOW to LOW ,n Site sr Liquctctction ukcptihility:VERY LOW aBedrock µti d Y i;1'cat ifioot suw;cptible is liytxfa,tHm but ncty floatdeposi[ undcegnpernuncatdiviaccmattorlossof b�g,.t a s' t, ♦...r' �stmtgth as a resolt of corthginl.c slinking_ Ice T his expL•nwtion is standarJved for thiii strics of count}-bascv! hytttrluction nmps:w Me caugories may trot appear on this imp, Liquefaction Susceptibilith MUP(of Mason Counh,Washington b5 titmhcn P.PaMer,s+ninundu t..M•K�a.,tare;.L.Pus I+tra.Fne L HAkih.:t.Ihrxt a folp,aid ltdw ca:A.�i�tnuiai 4n�+:1rr;Mi Figure 5 17.01.100.E.4(2) GROUNDWATER CONDITIONS Figure 6 shows the Mason County Map Viewer aerial photograph of the project site. The property drops approximately 30 ft from the edge of pavement on Treasure Island Drive to the toe of bulkhead of the waterfront. There are no water bodies or wetlands located inland/upslope from the bluff area. No evidence of seepage was observed on the project site above the beach level. Ross Geological Assessment 5 JWMA N08135 2/26/2009 Figure 7 shows the NRCS surficial soil mapping for the project site, which indicates Soil Series Ab, Alderwood gravelly sandy loam, 5 to 15%. This soil type generally consists of a thin mantle (2 to 3 ft of lose gravelly sandy loam) underlain by glacial till, grading from weathered to competent in 1 to 2 ft of depth. Site observations confirm the Alderwood soil type and firm subgrade at about 3 ft depth on the upper site with granular backfill behind the bulkheads at the water front. 17.01.100.E.4(4) GEOMORPHOLOGY& PAST SLOPE INSTABILITY Nearly the entire waterfront along Treasure Island has been developed with residential improvements. The waterfront in the vicinity of the project site is all developed with bulkheads and earth retaining structures (see Figures 8 and 9). As such,there are no indications of past slope failures in the vicinity of the project site. The general land form consists of a modest bluff rise of 8 to 12 ft along this shore front with a moderately 17 -J t; F.i d ry 'F• z f,b Looking South along Bulkhead Face -Figure 8 Ross Geological Assessment 7 JWMA N08135 2/26/2009 r F r. AW s3� Looking North along Bulkhead Face-Figure 9 sloped inland area that rises about 20 ft to the edge of road at the property street frontage. Beyond the property street frontage the ground surface rises about 1 ft to the local high point on the island from which point the surface begins to fall again toward the waterfront on the east side of the island. 17.01.100.E.4(5) SITE LANDSLIDE HISTORY Based on available mapping and historical documentation, there have been no recent or near-past landslide on or in the vicinity of the project site. 17.01.100.E.4(6) SITE DEVELOPMENT LANDSLIDE CONTEXT Based on the data gathered and site specific observations, it is clear that the subject site does not contain nor is it in the vicinity of landslide hazard areas. Ross Geological Assessment 8 JWMA N08135 2/26/2009 17.01.100.E.4(7) GEOTECHNICAL REPORT RECOMMENDATION Based on the above, the project site does not require the preparation of a Geotechnical Report as described in the Mason County Landslide Hazard Ordinance. 17.01.100.E.4(8) LANDSLIDE HAZARD BOUNDARIES,BUFFERS AND SETBACKS Not applicable. 17.01.100.E.4(9) PRELIMINARY SITE DEVELOPMENT PLAN Figure 10 shows the conceptual site development plan for the project site as prepared by CSI Surveying. The figure shows the 2 horizontal to 1 vertical slope intercept line, which should be a conservative of a maximum slope failure environment for this site, give the compact gravelly sand subgrade conditions. Based on competent site conditions and exposures, and local area development, it is recommended that the minimum setback from the slope break be reduced to 25 ft as shown on Figure 10. As such, it is recommended that structural development inshore of the 25 ft setback line be permitted. UUMMR �. FJ;RTFf (EiddngBuf Sbpe Bre* / ! 2H:tv Slope Inmroept Lim j f 11 r 5 Se bw*•SlapE& . Proceed Llammum Fm�of New De(y I I I 1) 1 } \ } RWmed 1.1s)dm7 Nss Dxellina Footprint f qJA I 4 I .N,, i 16 r � JW r � r I Apam 50' r ! Figure 10 Ross Geological Assessment 9 JWMA N08135 2/26/2009 SPECIAL NOTES WSDOE has published three short manuals relating to site development and maintenance on marine bluff sites that provide a good reference source for maintaining the stability of your site. The WSDOE site can be reached via the internet at http://Yvw%v.ecy.wa.gov/t)rograms/sea/shorelan.htmi. Look in the subsection titled Property Owner Guides. These manuals may also be obtained by calling the WSDOE publications center via(360)407-6000. Should you have any questions or require additional information, please contact me at(360)352-9456. Yours sincerely, J.W. Morrissette & Associates Inc.,P.S. 9 A- Wr $ of �AS+�L�t1� �tgti�► l��,o�,� x 0 SIGNAL Edward A. Wiltsie, P.E. Sr. Geotechnical Engineer/Associate Ross Geological Assessment 10 JWMA N08135 2/26/2009