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Home My WebLink AboutBLD2007-00890 GEO Addendum - BLD Engineering / Geo-tech Reports - 8/3/2007 q,233 ► - 60 -0'W0`1 ALKAI CONSULTANTS, LLC. Environmental Engineering • Geotechnical Engineering • Wetland Consulting TO BE KEPT IN THE August 1, 2007 PARCEL FILE ACL07-06-G179.003 Mason County Department of Community Development i. ) � ' D P.O.Box 279 1 Shelton, WA 98584 Ai.'jr 0 3 2007 Attn: Ms. Tammi Clark IVICCD - PLANNING Geotechnical Report Addendum Review Permit#BLD 2007-00890 Applicant: Ms. Suzanne Washington Dear Ms. Clark, At your request, we have reviewed a Geotechnical Report Addendum for the above referenced permit. The addendum was prepared by Tahja Engineering, Inc. dated July 8, 2007 in response to a review letter prepared by ALKAI Consultants, LLC. (ALKAI). The addendum was signed and stamped by Mr. Alan A. Tahja, PE (License#30784). In our review letter we recommended that Mr. Tahja provide an addendum discussing ground water conditions, amend the stability calculations to include ground water, if appropriate, and extend the limits of the slope stability calculations to the base of the slope or provide a statement that the lowest elevation presented in the calculations is the base of slope. The addendum Mr. Tahja provided addresses all of these issues satisfactorily. We recommend that Mason County accept the report and addendum for this project. Should you have any questions or concerns, or if we may be of additional assistance, please call our office at (360) 613-2407 or contact us by e-mail at Todd@alkai.net. Sincerely, g PA dX ' Todd S. Parkington,P.E. Senior Geotechnical Engineer M SIGNAL��yG Attachments: Geo Tech Work Order EXPinES 5/01/ 7 j Geotechnical Report Addendum by Tahja Engineering, Inc. 9465 Provost Road NW, Suite 202 • Silverdale, Washington 98383 • (360) 613-2407 • Fax: (360) 613-2408 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 Mason County DCD Attention: Tammi Clark,Robert Fink RECEIVED PO Box 279 JUL - 9 2007 Shelton, WA 98584 Suzanne Washington MCCD - PLANNING 1824 SE 56th Avenue Portland, OR 97215-3358 (503) 230-0656 RE: Geotechnical Report Addendum Parcel No. 42331-50-02009 Case No.: BLD2007-00890 Lake Cushman Westside 1, Blk 2, Lot 9 N Cushman Ridge Drive Hoodsport, WA 98548 Dear Tammi and Bob, Attached please find three copies of the geotechnical report addendum I've prepared in response to ALKAI Consultant, LLC's review of my geotechnical report dated 5/11/07. Also enclosed please find a copy of the consultant's review comments. I'm sorry and apologize for my report not meeting the reviewer's standards. As my responses to his inquiries should indicate,the information requested can be considered in determining a site's stability and geological conditions, but I don't feel any of the details requested affect the original report, or my recommendations. As the addendum states, I hope this addendum and the original report are found acceptable to you and my clients. Please feel free to contact me if you have any questions, or feel additional concerns still need further consideration. Sincerely, Alan A. Tahja, P. . File:IC:IProjectslWashingtonlCo Addndm Cvr Page 1 of 1 7/8/2007 t ' '6 TAHJA ENGINEERING, Inc. PO Box 235,Hoodsport, WA 98548 (360) 877-9512 Mason County DCD Planning Division PO Box 279 Shelton, WA 98584 Suzanne Washington 1824 SE 5e Avenue Portland, OR 97215-3358 (503) 230-0656 RE: Geotechnical Report Addendum Case No.: BLD2007-00890 Parcel No. 42331-50-02009 Lake Cushman Westside 1, Blk 2, Lot 9 N Cushman Ridge Drive Hoodsport, WA 98548 INTRODUCTION Review of a geotechnical report prepared by TAHJA ENGINEERING, Inc., dated 5/4/07 by ALKAI CONSULTANTS, LLC. , consultant services to Mason County DCD,resulted in a request for additional information. The following information and materials are offered in response; 1. ALKAI recommended that ground water be identified and used in the slope stability calculations. In my original report dated 5/4/07, I indicated a glacial till/hardpan layer beneath the ground surface at a relatively shallow(-3')depth. This till layer in the Cushman area often extends down to the basalt rock.The hardpan is considered relatively impervious, so the water table can be expected to be beneath the hardpan. The water course shown on the original geotechnical report site plan is scoured down to basalt rock at a depth of approximately 20-feet below the surrounding ground's surface. The slope analysis was"stopped"at elevation 720 because this roughly represents Lake Cushman's low lake level typically held by Tacoma City Light through the fall and spring months. During the summer months,the lake level is raised to elevation 738. It appears evident that the water table in the vicinity of the Washington property is directly related to the Lake Cushman lake level, so a summer time high lake level of 738 is used in modeling the site as the phreatic water surface. Attached please find a GALENATM Stability Analysis printout for the Washington property which includes a phreatic water surface at elevation 738 (worst case condition). As with the original geotechnical report,the required factor of safety is being met. 2. ALKAI recommended that the slope stability analysis be continued further down the File:IC:IProjectslWashingtonlGeo-Rptl4ddndm Page 1 of 2 7/8/2007 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 slope. There appears to be little point in extending the analysis further downslope than the low lake elevation. ALKAI stated that the local anomalies I mentioned in my original report can have a significant effect on the results of slope stability calculations. I agree with this statement, but feel that I have made adequate adjustments, and modeled the slope to address the required stability investigation. The hardpan layers present in the Lake Cushman area typically stand near vertical,and remain stable over long periods of time. The presence of these vertical portions of the slope have already been considered in my approach to modeling them. This concludes my additions or revisions to my original geotechnical report dated 5/4/2007. I hope it meets the needs of the Washington,Mason County, and ALKAI Consultants in planning the property's development. Sincerely, Alan A. Tahja, P.E. Attached: Stability Analysis II—Dynamic analysis printout with phreatic surface included. ALKAI Consultants review letter v�OV WA �0 4E S �SSIONAL EXPIRES File:IC:IProjectslWashingtonlGeo-Rptl4ddndm Page 2 of 2 7/8/2007 t TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 Suzanne Washington Westside 1, Block 2, Lot 9 Stability Analysis GALENA v«.m soo aao pF INA,c ../ azo � � .�� 800 ra�tovrb, 780 30754 Off' �ECISTEg 760 �SSjONAL 740 _ _ `$ no Analysis: t Multiple Stability Analysis Method: Bishop Simpified 700 surface: C—lar ResUifS 680 Crihce'(minimum) Factor of Safety: 1.65 0 20 40 SO 80 100 120 W 180 1. 200 220 240 FaY4 BJJ ani Piow.q BJui Xq] Proied Washington-Cushman Westside 1 Blk 2 Lot 9 Dynamic FJ(g. C.0mumants and sethngs\ownenMy Do menislPro).ck\Washirgton\Stability\Washingten Igmf Tahfa Engineering,Inc. Galena 5.00 Analysis Results Licensee: Tahja Engineering, Inc. DATA: Analysis 1 - Dynamic Material Properties (1 material) ------------------- Material: 1 (Mohr-Coulomb Cu increases with depth) - Cemented Hoodsport Sandy Loamy Gravel Cohesion Phi PI UnitWeight Ru 350.00 36.0 0 125.00 0.00 Material Profiles (1 profile) ----------------- Profile: 1 (2 points) Material beneath: 1 - Cemented Hoodsport Sandy Loamy Gravel 0.00 800.00 240.00 800.00 Slope Surface (6 points) ------------- 0.00 790.00 43.90 782.00 147.70 780.00 177.60 760.00 207.10 740.00 236.30 720.00 Phreatic Surface (2 points) ---------------- File:C.IProjectsI Washington Stability II I of 2 7/8/2007 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 0.00 738.00 250.00 738.00 Failure Surface --------------- Initial circular surface for critical search defined by: XL,XR,R Intersects: XL: 147.70 YL: 780.00 XR: 230.00 YR: 724.32 Centre: XC: 216.48 YC: 793.00 Radius: R: 70.00 Earthquake Force ---------------- Pseudo-static earthquake (seismic) coefficient: 0. 150 Variable Restraints ------------------- Parameter descriptor: XL XR R Range of variation: 55.00 12.60 35.00 Trial positions within range: 10 10 10 ----------------------------------------------------------------------- ----------------------------------------------------- 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: 1.76 There were: 926 successful analyses from a total of 1001 trial circles 75 analyses aborted due to unacceptable geometry Critical (minimum) Factor of Safety: 1.65 Negative normal stresses exist on the base of one or more slices - examine slice data and consult the Galena Users' Guide Circle and Results Summary (Lowest 5 Factor of Safety circles) -------------------------- Circle X-Centre Y-Centre X-Left Y-Left X-Right Y- Right Radius FoS 1 227.15 809.19 144.64 780.06 233.50 721.92 87.50 1.647 2 226.78 810.22 144 . 64 780.06 232.10 722.88 87.50 1.647 3 227.51 808.15 144 . 64 780.06 234.90 720.96 87.50 1.647 4 226.40 811.23 144.64 780.06 230.70 723.84 87.50 1.648 5 227.86 807.09 144.64 780.06 236.30 720.00 87.50 1.648 File:C.-Projectsl Washington lStabilityll 2 of 2 7/8/2007 ALKAI CONSULTANTS, LLC. Environmental Engineering • Geotechnical Engineering • Wetland Consulting June 22, 2007 ACL07-06-G 179.003 Mason County Department of Community Development P.O. Box 279 Shelton, WA 98584 Attn: Ms. Tammi Clark Geotechnical Engineering Report Review Permit#BLD 2007-00890 Applicant: Ms. Suzanne Washington Dear Ms. Clark, At your request, we have reviewed a Geotechnical Engineering Report for the above referenced permit. The report was prepared by Tahja Engineering, Inc. dated May 4, 2007. The report was signed and stamped by Mr. Alan A. Tahja, PE(License# 30784). In our opinion, the referenced geotechnical engineering report was prepared in general accordance with the requirements in the Mason County Resource Ordinance, Geologically Hazardous Areas 17.01.100.E.5 with one exception. In paragraph (1) of the referenced section of the Ordinance it states that "a discussion of ... ground water conditions...". Ground water was not mentioned in the report nor does it appear to have been included in the slope stability calculations. Ground water has a significant effect on the results of slope stability calculations. On the figures and cross sections provided with the report, the base of the slope is not indicated. In the slope stability calculations, the slope stops at elevation 720. However, it is not clear if this is the base of the slope or simply the limit of the topographic map. In the discussion on page 1 of the report, Mr. Tahja indicates that the slope is quite high and varies in steepness from 40 percent to near vertical. The cross section provided indicates a uniform slope. Total slope height and variations in slope inclination can have a significant effect on the results of slope stability calculations We recommend that Mason County request that the engineer (Mr. Tahja) provide an addendum discussing ground water conditions and amend the stability calculations to include ground water, if appropriate. We further recommend that Mason County request that the engineer extend the limits of the slope stability calculations to the base of the slope or provide a statement that the lowest elevation presented in the calculations is the base of slope. 9465 Provost Road NW, Suite 202 • Silverdale,Washington 98383 • (360) 613-2407 • Fax: (360) 613-2408 Project No.ACL07-06-G179-063 Mason County Review June 22,2007 Page No.2 Should you have any questions or concerns, or if we may be of additional assistance, please call our office at (360) 613-2407 or contact us by e-mail at Todd@alkai.net. PA Sincerely, s'F Was CIA J. y Todd S. Parkington, P.E. 360M Senior Geotechnical Engineer sSION� G Attachments: Geo Tech Work Order EXPIRES 5/01/0�) Geotechnical Engineering Report by Tahja Engineering, Inc. 9465 Provost Road NW, Suite 202 9 Silverdale, Washington 98383 9 (360) 613-2407 • Fax: (360) 613-2408 � P TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 Suzanne Washington 1824 SE 56th Avenue Portland, OR 97215-3358 (503) 230-0656 RE: Geotechnical Report Parcel No. 42331-50-02009 Lake Cushman Westside 1, Blk 2, Lot 9 N Cushman Ridge Drive Hoodsport, WA 98548 INTRODUCTION Ms. Suzanne Washington has requested my assistance in investigating and reporting on slope stability and landslide hazard issues relating to the residential development of Mason County Property Tax Parcel Number 42331-50-02009, otherwise known as Lake Cushman Westside 1, Block 2, Lot 9. The lot generally runs west to east along its long dimension(roughly 1320-feet), and is roughly 200-feet in the north-south direction. The attached site plat shows the general orientation of the property along North Cushman Ridge Drive. Ms. Washington wishes to develop the property with a three bedroom residence. An Onsite Septic System(OSS)has been designed for the site and the general drainfield location is shown on the attached site plan. The westerly half of the property climbs to the west at about a 16% slope from Cushman Ridge Drive to the most westerly 130-feet of the property where the slope steepens to about 39%. The easterly half of the property slopes at gentle slopes around 6%or less until the crest of a steep downslope to Lake Cushman is encountered. Slopes from the relatively level upland area of the property to the Lake Cushman elevation vary from 40%+to 100%with isolated areas of near vertical relief. The property's easterly half(east of Cushman Ridge Drive) is bisected from its southwesterly corner to the property's northeast corner by a drainage ravine and road runoff watercourse. Road and surrounding area stormwater runoff are delivered to the Washington property by way of two culverts installed beneath Cushman Ridge Drive. The runoff follows a shallow channel until it drops for a height of approximately 20-feet into the bottom of a cavernous ravine which conveys the runoff to the Lake elevation. The sides of the ravine stand at slopes ranging from 100%to near vertical. The drainage features are shown on the attached site plan. With slopes on the property exceeding 40%and with heights greater than 10-feet, Mason County requires a geotechnical report to be developed to address stability issues relating to the property's development. 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 File:C:ProjectsXashingtoMGeo-Rpt 1 of 5 5/4/2007 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 1. SURFACE AND GEOLOGIC CONDITIONS: The property has been logged in the past, and has medium aged second growth Fir, Hemlock, Cedar, Maple, and Alder trees covering most areas of the property. Besides the conifers and deciduous trees,the property has a moderate covering of Salal, Oregon grape, ferns, and other vegetation. The Series 1950,No. 9 SOIL SURVEY of Mason County indicates the site soils are a Rough Mountainous Land(Rc)derived from the Hoodsport Soil materials. The Hoodsport soil materials are not considered highly erodible, or an erosion hazard, and are commonly described as excessively drained. The Hoodsport soil types are also typically composed of a fairly high percentage of cobbles or gravels and typically a fines content(passing a 200 sieve) at or less than 12%,making the soil a coarse-grained soil classification in the Unified Soil Classification System(USCS). Further consideration of the soils found on site lead to describing the shallower surface soils(0 to 72"±) in terms of the USCS criteria as a gap-graded gravel (GP), with the deeper glacial till soils conforming to a GM(silty gravels, silty sandy gravels) soil type,weak to strongly cemented with fines passing the#200 sieve greater than 15%. Ms. Washington intends to develop an existing leveled portion of the property for a residential structure. Little additional grading will be required to situate a house in the conceptual location shown on the attached plans. 2. DEVELOPMENT AND GEOLOGIC FEATURES: A site plan has been developed for this property and is attached in the appendixes. A potable water source will need to be developed, and a conceptual well location is shown on the attached site plan.Neither electric or telephone service has been extended to the property to date, so electrical power can be expected to be provided with a portable generator. 3. LOCATIONS OF LOGS OF EXPLORATORY HOLES OR PROBES: Inspection of the slopes and cuts in the subject property's vicinity were inspected as well as the soil log holes dug for the septic system design typing. The soil log locations are shown on the attached plans. The cuts in the area confirmed the Mason County Soil Survey description of the soils as generally being medium dense soil horizons of weathered Vashon glacial till to a depth around 3%2- feet where it becomes more dense and can be characterized as hardpan. The soil logs excavated on the site confirm the Soil Survey descriptions. Ms. Washington has staked the location of a proposed residence on the property in the location shown on the attached site plan.Probing along the foundation lines found very dense materials along the southerly and westerly dimensions of the structure. Probing along the foundation lines along the northerly and easterly foundation lines found moderately compacted fill soils to a depth of approximately 24-inches,underlain by dense hardpan. The central area of the proposed building site was found to have very dense soils within a foot or less of the surface. 4. AREA OF PROPOSED DEVELOPMENT&BOUNDARY OF HAZARD: The attached site plan shows an approximate location of a proposed residence. The owner has staked the proposed residence location on the ground which provides setbacks from the crest of File:C.IProjectslWashingtoMGeo-Rpt 2 of 5 5/4/2007 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 the steep slopes of roughly 18-feet on the easterly end of the structure and roughly an 8-feet setback between the structure and the crest of the north descending slope. Some settlement has occurred in the area of the north descending slope, but appears to be consolidation of the shallow fill material that was placed in this area during leveling of the proposed building site. The proposed building location provides roughly 5+feet of separation between the settled area and the foundation of the residence which may be considered acceptable. Modeling of the slope descending to the east indicates acceptable factors of safety for structure placement at this location. 5. CROSS SECTION WITH PROPOSED GRADE CHANGES: A slope cross section is included in the appendix as sheet 2 in the plan set, and as part of the slope stability analysis printout. The building area has been leveled in the past, and little additional grading will be needed other than for footing excavations and final leveling and grading of the driveway. 6. SLOPE STABILITY ANALYSIS: The steepest slope(east facing slope measured at—70%)on the property in its present condition contains no obvious recent mass landslides or stability problem indicators though the surface is rough and broken. The level building area in the southeast corner of the property has no discernible irregularities, settlements,fissures,or evidence of consolidation problems,with the exception of the areas described above. This is a localized failure resulting from poor compaction during fill construction and is outside of the foundation bearing area. Unified Soil Classification System typing places the deeper glacial till soils in a fine grained soil (GM)classification with a dry density between 120-135 1bm/ft3 and a typical effective stress friction angle (0)>34°. A soil density of 125 lbm/ft3 will be used in stability calculations. Slope stability calculations are addressed on an analysis program printout following this report.A Phi angle of 36' for the soil internal friction angle was used along with a cohesion value of 350 lbs per W. The glacial till soils can be modeled with cohesion values as high as 1000 lbs per ft2 and greater, with internal friction angles above 40°, so the analysis is based on conservative assumptions. Foundations bearing on the medium dense Hoodsport gravelly sandy loams can expect to realize bearing capacities of the natural soil in the 1000 to 2000 pounds per square foot range,with settlement for residential foundations within half an inch overall. 7. RECOMMENDED SETBACKS & DRAINAGE: A minimal building setback will be needed at this site because of the shallow depth to a hardpan layer. A 5-foot building setback from the face of the footing to the breakout point on the slope (daylight dimension) should be observed in locating the residence.A residence could be located at the crest of the slope, so long as the footing is buried an adequate depth to provide the recommended 5-foot setback from the face-breakout point. The 5-foot building setback recommended applies to standard construction foundations and loads, with footings excavated down to natural undisturbed soils. The residence portrayed on the attached File:C:ProjectslWashingtonlGeo-Rpt 3 of 5 5/4/2007 TAHJA ENGINEERING, Inc. PO Box 235,Hoodsport, WA 98548 (360) 877-9512 plan may be considered in compliance with this requirement,provided bearing footings located along the north and east sides of the residence are installed deep enough to bear on the hardpan layer located roughly 24 inches below ground level. Stormwater runoff from the developed site should be dispersed into the surrounding vegetation as much as possible. Roof runoff may be dispersed at the corners of the residence using downspout splash blocks, or other features which spread the downspout discharge over as large an area as possible before it proceeds in a sheet flow(shallow—unconcentrated)over the steep slopes to Lake Cushman. Future development of the property should avoid the delivery of any collected stormwater discharges within 30-feet of any property boundary. 8. CLEARING, GRADING,AND VEGETATION MANAGEMENT: General site grading and preparation has already been accomplished,and very little additional clearing or grading will be needed to proceed with the residential development of the property. Once a residence is constructed on the property,a greater area of the leveled clearing may be expected to be planted with grass and landscape plantings. The hillsides surrounding the house location should be planted with deep rooting plants of the owner's choice.A good cover of vegetation on the slopes will help reduce or eliminate erosion and sloughing associated with freeze/thaw cycles. Overall, it appears the development of the property will have a minor impact on the property itself,and little or no impact to surrounding properties. A volume of less than 200 cubic yards is expected to be needed to be wasted off site or delivered to the site to accomplish the planned building elevation leveling and other site work. 9. EROSION AND SEDIMENT CONTROL: As indicated earlier in this report,the site's soils are not considered erodible,and minimal Erosion and Sediment(E&S) Control measures should be adequate to protect neighboring properties and State waters.Access to the site already exists off of North Cushman Ridge Drive (private Forest Service Road),and is stable in its present condition. The major earthwork will be leveling the house's building location, and excavating the footing trenches. These development areas drain mainly to the owner's property, and will have approximately 50+-feet of vegetative to pass through before any runoff exits the property at its normal discharge locations. The existing and proposed features should provide adequate protection against adverse stormwater runoff impacts. 10. ON-SITE AND OFF-SITE IMPACTS: Construction and completion of the proposed improvements are not expected to result in any adverse on-site, or off-site impacts. The proposed residential development of the property will not result in adverse impacts to either the subject property or to neighboring properties. File:C. ProjectslWashingtonlGeo-Rpt 4 of 5 5/4/2007 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 11. SPECIFICATIONS OF FINAL DEVELOPMENT CONDITIONS: The completed site will be allowed to return, for the most part,to a naturally vegetated state. The main ground cover will be lawn and medium height shrubbery. In the future development of the property,discharging concentrations of collected Stormwater runoff within 30 feet of a property boundary should be avoided. 12. STRUCTURAL MITIGATION MEASURES RECOMMENDED: Location of a structure sited 5-feet or more "back" from the crest of the steep slope located along the northerly and easterly boundaries of the existing cleared building location should need no further mitigation then the setback distance(s), bearing requirements, and stormwater runoff measures already recommended. 13. SITE MAP WITH EXISTING& PROPOSED DEVELOPMENT: The attached site plan contains this information. The property currently is un-developed with no permanent existing features. Conclusions and Summary; The residential development of the Washington property located on North Cushman Ridge Drive does not appear to present any stability problems that cannot be overcome,and will not result in any undesirable impacts. The property itself provides an adequate vegetated buffer area to protect neighboring properties and State Waters from adverse stormwater runoff impacts. The stormwater related recommendations made in this report should be adequate for both temporary and permanent stormwater runoff management and concerns. I hope this report meets the needs of Ms.Washington and Mason County in planning the property's development. Sincerely, A ?, y F, OF WA Alan A. Tahja, P.E. Attached: Site Plan(Plan Sheet 1) 1o� .E 784 Cross Section(Plan Sheet 2 & Incl Galena print-out) �, VEL 4'GI T E��l Galena Stability Analysis EXPIRES 11/07/08 File:C:ProjectslWarhingtoMGeo-Rpt 5 of 5 5/4/2007 o �• n — --- --- _ rrnsyl �`'a --- G Q �>p g 4 Neighboring ,s SIGNAL E� Foundation \ Emmm 11/07/08 g U : I m Pro ert Line PL 1290.681 ^w to LAKE CUSHMAN o y OSS pe {. v �k,&tr, SWG20 -00198 `o w cm II VIM \ oo (Conte tual) \ ♦ e S ► P 780 os d e id ce _N _100 adiu\s �o ,S��a L ca t18 x PL 6-- _ St mwater View Dir ion .�1�y —I, t A Cross Section W A Sur \ o� A View Direction b Pond ^ �, r \ OD b I � o --- --- --- -- 6100 1900 1&0 1;10 SC81e„1 to=100, °� 0 5O 100 Revision# Description Date TAWA ENGINEERING,Inc. CROSS SECTION SHEET "''v° Alan Tahja,P.E. Suzanne Washington Washington PO Box 235 1824 SE 56th Avenue Hoodspor4 WA 98548 Portland, OR 97215-3358 ae. Mason County Prd No.:42331-50-02009 (360)877-9512 (503) 230-0656 2 of 2 w - � O LO 61 10 f0 _ hj 1 _ O � i n ( I O CO i O Uco / O � O / 00 - o x T ►"'q 3 U i Q i C. R H co 780 z ° v S N � O O TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 Suzanne Washington Westside 1, Block 2, Lot 9 Stability Analysis cALE1VA V.ti s. ago 800 780 r0� 37784 GISTS IV 760 SSZONAL 740 E PUM 1 ii07i08 720 Anatysis: 1 Multiple Stability Analysis Method: Bishop Simplified 700 Surface: Circular Results 680 Critical(minimum) Factor of Safety 2.11 0 20 40 60 80 100 120 140 -160 160 200 220 240 •Eare.-.Ma aor aaosn.¢. W aom Protect: Washington-Cushman Westside 1 BIk 2 Lot 9 Static EI& C.%Documents and Setange\Oomerft Do merits\Projects\Washington\Stabilrty\Washington 1.pmf Tahia Engineering,Inc. Galena 5.00 Analysis Results Licensee: Tahja Engineering, Inc. Project: Washington - Cushman Westside 1 Blk 2 Lot 9 DATA: Analysis 1 - Static Material Properties (1 material) ------------------- Material: 1 (Mohr-Coulomb Cu increases with depth) - Cemented Hoodsport Sandy Loamy Gravel Cohesion Phi PI UnitWeight Ru 350.00 36.0 0 125.00 0.00 Material Profiles (1 profile) ----------------- Profile: 1 (2 points) Material beneath: 1 - Cemented Hoodsport Sandy Loamy Gravel 0.00 800.00 240.00 800.00 Slope Surface (6 points) ------------- 0.00 790.00 43.90 782.00 147.70 780.00 177.60 760.00 207.10 740.00 236.30 720.00 Failure Surface File:C.11 rojecislWashingtonStability I 1 of 4 5/4/2007 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 --------------- Initial circular surface for critical search defined by: XL,XR,R Intersects: XL: 147.70 YL: 780.00 XR: 230.00 YR: 724.32 Centre: XC: 216.48 YC: 793.00 Radius: R: 70.00 Variable Restraints ------------------- Parameter descriptor: XL XR R Range of variation: 55.00 12.60 35.00 Trial positions within range: 10 10 10 ----------------------------------------------------------------------- ----------------------------------------------------- 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: 2.25 There were: 926 successful analyses from a total of 1001 trial circles 75 analyses aborted due to unacceptable geometry Critical (minimum) Factor of Safety: 2.11 Negative normal stresses exist on the base of one or more slices - examine slice data and consult the Galena Users' Guide Circle and Results Summary (Lowest 9 Factor of Safety circles) -------------------------- Circle X-Centre Y-Centre X-Left Y-Left X-Right Y- Right Radius FoS 1 227.15 809.19 144.64 780.06 233.50 721.92 87.50 2.115 2 227.51 808.15 144. 64 780.06 234.90 720. 96 87.50 2.115 3 226.78 810.22 144 . 64 780.06 232.10 722.88 87.50 2.116 4 227.86 807.09 144. 64 780.06 236.30 720.00 87.50 2.116 5 226.40 811.23 144.64 780.06 230.70 723.84 87.50 2.118 6 233.14 807.44 150.76 777. 96 236.30 720.00 87.50 2.119 7 226.01 812.23 144.64 780.06 229.30 724.79 87.50 2.121 8 232.78 808.43 150.76 777. 96 234.90 720.96 87.50 2.123 9 225.62 813.22 144.64 780.06 227.90 725.75 87.50 2.125 File:C.IProjectslWashingtonLStability1 2 of 4 5/4/2007 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 Suzanne Washington Westside 1, Block 2, Lot 9 Stability Analysis GALENA Me.e.eao 840 820 0 Soo HW...um 780 760 740 720 Analysis: 1 Multiple Stability Analysis Method Bishop Simplified 700 Surface: Circular Results 680 Critical(min um) Factor o/SaIAW, 1.65 0 20 40 60 80 100 120 140 160 180 200 220 240 EfMlfl. Mry aw] Ci.on....M.300) Protect: Washington-Cushman Westside 1 Blk 2 Lot 9 Dynamic Ede C'\Documents and Settings\OanaMly Documents\Projects\Washington\Stabilay\Washington 1.gm/ Tahja Engineering,Inc. Galena 5.00 Analysis Results Licensee: Tahja Engineering, Inc. Project: Washington - Cushman Westside 1 Blk 2 Lot 9 DATA: Analysis 1 - Dynamic Material Properties (1 material) ------------------- Material: 1 (Mohr-Coulomb Cu increases with depth) - Cemented Hoodsport Sandy Loamy Gravel Cohesion Phi PI UnitWeight Ru 350.00 36.0 0 125.00 0.00 Material Profiles (1 profile) ----------------- Profile: 1 (2 points) Material beneath: 1 - Cemented Hoodsport Sandy Loamy Gravel 0.00 800.00 240.00 800.00 Slope Surface (6 points) ------------- 0.00 790.00 43.90 782.00 147 .70 780.00 177.60 760.00 207.10 740.00 236.30 720.00 File:C:Wrojects l Washington Stability 1 3 of 4 5/4/2007 TAHJA ENGINEERING, Inc. PO Box 235,Hoodsport, WA 98548 (360) 877-9512 Failure Surface --------------- Initial circular surface for critical search defined by: XL,XR,R Intersects: XL: 147.70 YL: 780.00 XR: 230.00 YR: 724.32 Centre: XC: 216.48 YC: 793.00 Radius: R: 70.00 Earthquake Force ---------------- Pseudo-static earthquake (seismic) coefficient: 0.150 Variable Restraints ------------------- Parameter descriptor: XL XR R Range of variation: 55.00 12.60 35.00 Trial positions within range: 10 10 10 ----------------------------------------------------------------------- ----------------------------------------------------- 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: 1.76 There were: 926 successful analyses from a total of 1001 trial circles 75 analyses aborted due to unacceptable geometry Critical (minimum) Factor of Safety: 1.65 Negative normal stresses exist on the base of one or more slices - examine slice data and consult the Galena Users' Guide Circle and Results Summary (Lowest 6 Factor of Safety circles) -------------------------- Circle X-Centre Y-Centre X-Left Y-Left X-Right Y- Right Radius FoS 1 227.15 809.19 144.64 780.06 233.50 721.92 87.50 1.647 2 226.78 810.22 144.64 780.06 232.10 722.88 87.50 1.647 3 227.51 808.15 144. 64 780.06 234.90 720.96 87.50 1.647 4 226.40 811.23 144. 64 780.06 230.70 723.84 87.50 1.648 5 227.86 807.09 144 . 64 780.06 236.30 720.00 87.50 1.648 6 226.01 812.23 144 .64 780.06 229.30 724.79 87.50 1.650 File:C.IProjectsl WashingtonlStability 1 4 of 4 5/4/2007 ALKAI CONSULTANTS, LLC . Environmental Engineering • Geotechnical Engineering • Wetland Consulting June 22, 2007 ACL07-06-G 179.003 Mason County Department of Community Development P.O. Box 279 Shelton,WA 98584 Attn: Ms.Tammi Clark Geotechnical Engineering Report Review Permit#BLD 2007-00890 Applicant: Ms. Suzanne Washington Dear Ms. Clark, At your request, we have reviewed a Geotechnical Engineering Report for the above referenced permit. The report was prepared by Tahja Engineering, Inc. dated May 4, 2007. The report was signed and stamped by Mr. Alan A. Tahja,PE(License#30784). In our opinion, the referenced geotechnical engineering report was prepared in general accordance with the requirements in the Mason County Resource Ordinance, Geologically Hazardous Areas 17.01.100.E.5 with one exception. In paragraph (1) of the referenced section of the Ordinance it states that "a discussion of... ground water conditions...". Ground water was not mentioned in the report nor does it appear to have been included in the slope stability calculations. Ground water has a significant effect on the results of slope stability calculations. On the figures and cross sections provided with the report, the base of the slope is not indicated. In the slope stability calculations, the slope stops at elevation 720. However, it is not clear if this is the base of the slope or simply the limit of the topographic map. In the discussion on page 1 of the report, Mr. Tahja indicates that the slope is quite high and varies in steepness from 40 percent to near vertical. The cross section provided indicates a uniform slope. Total slope height and variations in slope inclination can have a significant effect on the results of slope stability calculations We recommend that Mason County request that the engineer (Mr. Tahja) provide an addendum discussing ground water conditions and amend the stability calculations to include ground water, if appropriate. We further recommend that Mason County request that the engineer extend the limits of the slope stability calculations to the base of the slope or provide a statement that the lowest elevation presented in the calculations is the base of slope. 9465 Provost Road NW, Suite 202 • Silverdale, Washington 98383 • (360) 613-2407 • Fax: (360) 613-2408 1, Project No.ACL07-06-G179-003 Mason County Review June 22,2007 Page No.2 Should you have any questions or concerns, or if we may be of additional assistance, please call our office at (360)613-2407 or contact us by e-mail at Todd@alkai.net. S, PA Sincerely, WAS �r Ny � � Todd S. Parkington, P.E. , ?; Senior Geotechnical EngineerIONAL Attachments: Geo Tech Work Order EXPIRES 5/01/� Geotechnical Engineering Report by Tahja Engineering, Inc. 9465 Provost Road NW, Suite 202 9 Silverdale, Washington 98383 • (360) 613-2407 9 Fax: (360) 613-2408 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 Mason County DCD Attention: Tammi Clark. Robert Fink RECEIVED PO Box 279 Shelton, WA 98584 J U L - 9 2007 Suzanne Washington MCCD - PLANNING 1824 SE 56`h Avenue Portland, OR 97215-3358 (503) 230-0656 RE: Geotechnical Report Addendum Parcel No. 42331-50-02009 Case No.: BLD2007-00890 Lake Cushman Westside 1, Blk 2, Lot 9 N Cushman Ridge Drive Hoodsport, WA 98548 Dear Tammi and Bob, Attached please find three copies of the geotechnical report addendum I've prepared in response to ALKAI Consultant, LLC's review of my geotechnical report dated 5/11/07.Also enclosed please find a copy of the consultant's review comments. I'm sorry and apologize for my report not meeting the reviewer's standards.As my responses to his inquiries should indicate,the information requested can be considered in determining a site's stability and geological conditions,but I don't feel any of the details requested affect the original report, or my recommendations. As the addendum states, I hope this addendum and the original report are found acceptable to you and my clients. Please feel free to contact me if you have any questions,or feel additional concerns still need further consideration. Sincerely, 1 Alan A. Tahja, P. . File:IC:IProjectsl WashingtonlCo Addndm Cvr Page 1 of 1 7/8/2007 ALKAI CONSULTANTS, LLC. Environmental Engineering • Geotechnical Engineering • Wetland Consulting June 22, 2007 ACL07-06-G 179.003 Mason County Department of Community Development P.O. Box 279 Shelton, WA 98584 Attn: Ms. Tammi Clark Geotechnical Engineering Report Review Permit#BLD 2007-00890 Applicant: Ms. Suzanne Washington Dear Ms. Clark, At your request, we have reviewed a Geotechnical Engineering Report for the above referenced permit. The report was prepared by Tahja Engineering, Inc. dated May 4, 2007. The report was signed and stamped by Mr. Alan A. Tahja, PE(License# 30784). In our opinion, the referenced geotechnical engineering report was prepared in general accordance with the requirements in the Mason County Resource Ordinance, Geologically Hazardous Areas 17.01.100.E.5 with one exception. In paragraph (1) of the referenced section of the Ordinance it states that "a discussion of... ground water conditions...". Ground water was not mentioned in the report nor does it appear to have been included in the slope stability calculations. Ground water has a significant effect on the results of slope stability calculations. On the figures and cross sections provided with the report, the base of the slope is not indicated. In the slope stability calculations, the slope stops at elevation 720. However, it is not clear if this is the base of the slope or simply the limit of the topographic map. In the discussion on page I of the report, Mr. Tahja indicates that the slope is quite high and varies in steepness from 40 percent to near vertical. The cross section provided indicates a uniform slope. Total slope height and variations in slope inclination can have a significant effect on the results of slope stability calculations We recommend that Mason County request that the engineer (Mr. Tahja) provide an addendum discussing ground water conditions and amend the stability calculations to include ground water, if appropriate. We further recommend that Mason County request that the engineer extend the limits of the slope stability calculations to the base of the slope or provide a statement that the lowest elevation presented in the calculations is the base of slope. 9465 Provost Road NW, Suite 202 •Silverdale.Washington 98383 • (360) 613-2407 • Fax: (360) 613-2408 1 Project No.ACL07-06-G179-003 Mason County Review June 22,2007 Page No.2 Should you have any questions or concerns, or if we may be of additional assistance, please call our office at (360) 613-2407 or contact us by e-mail at Todd@alkai.net. PA Sincerely, 5'F WAS CIA . Todd S. Parkington, P.E. O , 71 Senior Geotechnical Engineer �SfONAL ti�G Attachments: Geo Tech Work Order EXPIRES 5/01 Geotechnical Engineering Report by Tahja Engineering, Inc. 9465 Provost Road NW, Suite 202 • Silverdale, Washington 98383 9 (360) 613-2407 • Fax: (360) 613-2408 6LDXo 0off TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 Suzanne Washington 1824 SE 50h Avenue Portland, OR 97215-3358 (503) 230-0656 RE: Geotechnical Report Parcel No. 42331-50-02009 Lake Cushman Westside 1, Blk 2, Lot 9 N Cushman Ridge Drive Hoodsport, WA 98548 INTRODUCTION Ms. Suzanne Washington has requested my assistance in investigating and reporting on slope stability and landslide hazard issues relating to the residential development of Mason County Property Tax Parcel Number 42331-50-02009, otherwise known as Lake Cushman Westside 1, Block 2, Lot 9. The lot generally runs west to east along its long dimension(roughly 1320-feet), and is roughly 200-feet in the north-south direction. The attached site plat shows the general orientation of the property along North Cushman Ridge Drive. Ms. Washington wishes to develop the property with a three bedroom residence. An Onsite Septic System(OSS)has been designed for the site and the general drainfield location is shown on the attached site plan. The westerly half of the property climbs to the west at about a 16% slope from Cushman Ridge Drive to the most westerly 130-feet of the property where the slope steepens to about 39%. The easterly half of the property slopes at gentle slopes around 6%or less until the crest of a steep downslope to Lake Cushman is encountered. Slopes from the relatively level upland area of the property to the Lake Cushman elevation vary from 40%+to 100%with isolated areas of near vertical relief. The property's easterly half(east of Cushman Ridge Drive)is bisected from its southwesterly corner to the property's northeast corner by a drainage ravine and road runoff watercourse. Road and surrounding area stormwater runoff are delivered to the Washington property by way of two culverts installed beneath Cushman Ridge Drive. The runoff follows a shallow channel until it drops for a height of approximately 20-feet into the bottom of a cavernous ravine which conveys the runoff to the Lake elevation. The sides of the ravine stand at slopes ranging from 100%to near vertical. The drainage features are shown on the attached site plan. With slopes on the property exceeding 40%and with heights greater than 10-feet, Mason County requires a geotechnical report to be developed to address stability issues relating to the property's development. 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 File:C:IProjectsl Washington lGeo-Rpt I of 5 5/4/2007 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 1. SURFACE AND GEOLOGIC CONDITIONS: The property has been logged in the past, and has medium aged second growth Fir, Hemlock, Cedar, Maple, and Alder trees covering most areas of the property. Besides the conifers and deciduous trees,the property has a moderate covering of Salal, Oregon grape, ferns, and other vegetation. The Series 1950,No. 9 SOIL SURVEY of Mason County indicates the site soils are a Rough Mountainous Land(Rc)derived from the Hoodsport Soil materials. The Hoodsport soil materials are not considered highly erodible, or an erosion hazard, and are commonly described as excessively drained. The Hoodsport soil types are also typically composed of a fairly high percentage of cobbles or gravels and typically a fines content(passing a 200 sieve) at or less than 12%, making the soil a coarse-grained soil classification in the Unified Soil Classification System(USCS). Further consideration of the soils found on site lead to describing the shallower surface soils (0 to 72"±)in terms of the USCS criteria as a gap-graded gravel (GP),with the deeper glacial till soils conforming to a GM(silty gravels, silty sandy gravels) soil type, weak to strongly cemented with fines passing the#200 sieve greater than 15%. Ms. Washington intends to develop an existing leveled portion of the property for a residential structure. Little additional grading will be required to situate a house in the conceptual location shown on the attached plans. 2. DEVELOPMENT AND GEOLOGIC FEATURES: A site plan has been developed for this property and is attached in the appendixes. A potable water source will need to be developed, and a conceptual well location is shown on the attached site plan. Neither electric or telephone service has been extended to the property to date, so electrical power can be expected to be provided with a portable generator. 3. LOCATIONS OF LOGS OF EXPLORATORY HOLES OR PROBES: Inspection of the slopes and cuts in the subject property's vicinity were inspected as well as the soil log holes dug for the septic system design typing. The soil log locations are shown on the attached plans. The cuts in the area confirmed the Mason County Soil Survey description of the soils as generally being medium dense soil horizons of weathered Vashon glacial till to a depth around 3%z- feet where it becomes more dense and can be characterized as hardpan. The soil logs excavated on the site confirm the Soil Survey descriptions. Ms. Washington has staked the location of a proposed residence on the property in the location shown on the attached site plan. Probing along the foundation lines found very dense materials along the southerly and westerly dimensions of the structure. Probing along the foundation lines along the northerly and easterly foundation lines found moderately compacted fill soils to a depth of approximately 24-inches,underlain by dense hardpan. The central area of the proposed building site was found to have very dense soils within a foot or less of the surface. 4. AREA OF PROPOSED DEVELOPMENT&BOUNDARY OF HAZARD: The attached site plan shows an approximate location of a proposed residence. The owner has staked the proposed residence location on the ground which provides setbacks from the crest of File:C.Trojectsl WashingtonlGeo-Rpt 2 of 5 5/4/2007 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 the steep slopes of roughly 18-feet on the easterly end of the structure and roughly an 8-feet setback between the structure and the crest of the north descending slope. Some settlement has occurred in the area of the north descending slope, but appears to be consolidation of the shallow fill material that was placed in this area during leveling of the proposed building site. The proposed building location provides roughly 5+feet of separation between the settled area and the foundation of the residence which may be considered acceptable. Modeling of the slope descending to the east indicates acceptable factors of safety for structure placement at this location. 5. CROSS SECTION WITH PROPOSED GRADE CHANGES: A slope cross section is included in the appendix as sheet 2 in the plan set, and as part of the slope stability analysis printout. The building area has been leveled in the past, and little additional grading will be needed other than for footing excavations and final leveling and grading of the driveway. 6. SLOPE STABILITY ANALYSIS: The steepest slope (east facing slope measured at—70%)on the property in its present condition contains no obvious recent mass landslides or stability problem indicators though the surface is rough and broken. The level building area in the southeast corner of the property has no discernible irregularities, settlements, fissures,or evidence of consolidation problems,with the exception of the areas described above. This is a localized failure resulting from poor compaction during fill construction and is outside of the foundation bearing area. Unified Soil Classification System typing places the deeper glacial till soils in a fine grained soil (GM)classification with a dry density between 120-135 lbm/ft3 and a typical effective stress friction angle(0)>34°. A soil density of 125 lbm/ft3 will be used in stability calculations. Slope stability calculations are addressed on an analysis program printout following this report.A Phi angle of 36' for the soil internal friction angle was used along with a cohesion value of 350 lbs per W. The glacial till soils can be modeled with cohesion values as high as 1000 lbs per W and greater, with internal friction angles above 40°, so the analysis is based on conservative assumptions. Foundations bearing on the medium dense Hoodsport gravelly sandy loams can expect to realize bearing capacities of the natural soil in the 1000 to 2000 pounds per square foot range,with settlement for residential foundations within half an inch overall. 7. RECOMMENDED SETBACKS & DRAINAGE: A minimal building setback will be needed at this site because of the shallow depth to a hardpan layer. A 5-foot building setback from the face of the footing to the breakout point on the slope (daylight dimension)should be observed in locating the residence. A residence could be located at the crest of the slope, so long as the footing is buried an adequate depth to provide the recommended 5-foot setback from the face-breakout point. The 5-foot building setback recommended applies to standard construction foundations and loads, with footings excavated down to natural undisturbed soils. The residence portrayed on the attached File:C.IProjectslWashingtonlGeo-Rpt 3 of 5 5/4/2007 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 plan may be considered in compliance with this requirement,provided bearing footings located along the north and east sides of the residence are installed deep enough to bear on the hardpan layer located roughly 24 inches below ground level. Stormwater runoff from the developed site should be dispersed into the surrounding vegetation as much as possible. Roof runoff may be dispersed at the corners of the residence using downspout splash blocks, or other features which spread the downspout discharge over as large an area as possible before it proceeds in a sheet flow(shallow—unconcentrated)over the steep slopes to Lake Cushman. Future development of the property should avoid the delivery of any collected stormwater discharges within 30-feet of any property boundary. 8. CLEARING, GRADING,AND VEGETATION MANAGEMENT: General site grading and preparation has already been accomplished,and very little additional clearing or grading will be needed to proceed with the residential development of the property. Once a residence is constructed on the property,a greater area of the leveled clearing may be expected to be planted with grass and landscape plantings. The hillsides surrounding the house location should be planted with deep rooting plants of the owner's choice.A good cover of vegetation on the slopes will help reduce or eliminate erosion and sloughing associated with freeze/thaw cycles. Overall, it appears the development of the property will have a minor impact on the property itself, and little or no impact to surrounding properties. A volume of less than 200 cubic yards is expected to be needed to be wasted off site or delivered to the site to accomplish the planned building elevation leveling and other site work. 9. EROSION AND SEDIMENT CONTROL: As indicated earlier in this report,the site's soils are not considered erodible, and minimal Erosion and Sediment(E&S)Control measures should be adequate to protect neighboring properties and State waters. Access to the site already exists off of North Cushman Ridge Drive (private Forest Service Road),and is stable in its present condition. The major earthwork will be leveling the house's building location, and excavating the footing trenches. These development areas drain mainly to the owner's property, and will have approximately 50+-feet of vegetative to pass through before any runoff exits the property at its normal discharge locations. The existing and proposed features should provide adequate protection against adverse stormwater runoff impacts. 10. ON-SITE AND OFF-SITE IMPACTS: Construction and completion of the proposed improvements are not expected to result in any adverse on-site,or off-site impacts. The proposed residential development of the property will not result in adverse impacts to either the subject property or to neighboring properties. File:C:IProjects l Washington lGeo-Rpt 4 of 5 .5/4/2007 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 11. SPECIFICATIONS OF FINAL DEVELOPMENT CONDITIONS: The completed site will be allowed to return, for the most part,to a naturally vegetated state. The main ground cover will be lawn and medium height shrubbery. In the future development of the property,discharging concentrations of collected Stormwater runoff within 30 feet of a property boundary should be avoided. 12. STRUCTURAL MITIGATION MEASURES RECOMMENDED: Location of a structure sited 5-feet or more "back" from the crest of the steep slope located along the northerly and easterly boundaries of the existing cleared building location should need no further mitigation then the setback distance(s), bearing requirements, and stormwater runoff measures already recommended. 13. SITE MAP WITH EXISTING&PROPOSED DEVELOPMENT: The attached site plan contains this information. The property currently is un-developed with no permanent existing features. Conclusions and Summary; The residential development of the Washington property located on North Cushman Ridge Drive does not appear to present any stability problems that cannot be overcome, and will not result in any undesirable impacts. The property itself provides an adequate vegetated buffer area to protect neighboring properties and State Waters from adverse stormwater runoff impacts. The stormwater related recommendations made in this report should be adequate for both temporary and permanent stormwater runoff management and concerns. I hope this report meets the needs of Ms.Washington and Mason County in planning the property's development. Sincerely, A y ti�F WAS +� Alan A. Tahja, P.E. Attached: Site Plan(Plan Sheet 1) `��'G gTER�9 ww� Cross Section(Plan Sheet 2 & Incl Galena print-out) �SSioNAL Galena Stability Analysis EXPIRES 11/07/08 File:C.kProjectslWashingtoMGeo-Rpt 5 of 5 5/4/2007 x �v---- — pF 00 WA o �O --- --- --- - - ell o y �o A . A � 3D784 .Cy4 w \\ �+ OA�S�CISTEgF'�� ' �. Neighboring N Foundation o notes 11i07i0e 8 U I � o z I H Pro e Line PL 1290.68, ` x LAKE CUSHMAN OSS pe f �fr ® II e 00 (Conce tual) ♦ l 780 os d e id ' ce � _100, adiu\s �O� L ca io 18 x 6 P L 1�'t--- _ _ St mwater n View Dir ion 11� " 1 - 1 e Cross Section Sur \\ °jam i L ,View Direction Pond \ OD C W I 00 -- O - --- --- ego 8b'O 011,10 (sro Scale"1„=100, ° N 0 SO 100 et Revision# Description Dare TAHJA ENGINEERING,Inc. CROSS SECTION SHEET "a187s Alan Tahja,P.E. Suzanne Washington Washington PO Box 235 1824 SE 56th Avenue Am Hoodspor4 WA 98548 Portland, OR 97215-3358 2 of 2 Mason County PrW No.:42331-50-02009 (360)877-9512 (503) 230-0656 � B dFl � - LO �% N ___ ___ ____ O a 00 o o x I --- ---tl--- --- --- --- c w U1 T I � � I d Iwo �W 780 Z 4w �~ o - M � o 0 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 Suzanne Washington Westside 1, Block 2, Lot 9 Stability Analysis caiEJvw y.Ma sro eao L 800 760 784 0+4' �ECISTER 760 �`r`pIONAL F, 740 inn 11/07/08 720 Analysis: 1 Multiple Stability Analysis Methot! Bishop Simplifier 700 Surface: Circular Results 680 Cntical(mmlmum) Factor of Safety, 2.11 0 20 40 80 60 100 120 140 160 180 200 220 240 EJMrt.Mq Sal] Pmcrtep.Mry Hq) ProieO. Washington-Cushman Westside 1 BIk 2 Lot 9 Static E/JL C:1Documenb and Settings\O~My DmumentslProjKts%Washington%StabilitylWashington 1.gmf Tahja Engineering,Inc. Galena 5.00 Analysis Results Licensee: Tahja Engineering, Inc. Project: Washington - Cushman Westside 1 Blk 2 Lot 9 DATA: Analysis 1 - Static Material Properties (1 material) ------------------- Material: 1 (Mohr-Coulomb Cu increases with depth) - Cemented Hoodsport Sandy Loamy Gravel Cohesion Phi PI UnitWeight Ru 350.00 36.0 0 125.00 0.00 Material Profiles (1 profile) ----------------- Profile: 1 (2 points) Material beneath: 1 - Cemented Hoodsport Sandy Loamy Gravel 0.00 800.00 240.00 800.00 Slope Surface (6 points) ------------- 0.00 790.00 43.90 782.00 147.70 780.00 177.60 760.00 207.10 740.00 236.30 720.00 Failure Surface File:C.IProjectsl WashingtonlStability I 1 of 4 5/4/2007 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 --------------- Initial circular surface for critical search defined by: XL,XR,R Intersects: XL: 147.70 YL: 780.00 XR: 230.00 YR: 724.32 Centre: XC: 216.48 YC: 793.00 Radius: R: 70.00 Variable Restraints ------------------- Parameter descriptor: XL XR R Range of variation: 55.00 12. 60 35.00 Trial positions within range: 10 10 10 ----------------------------------------------------------------------- ----------------------------------------------------- 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: 2.25 There were: 926 successful analyses from a total of 1001 trial circles 75 analyses aborted due to unacceptable geometry Critical (minimum) Factor of Safety: 2.11 Negative normal stresses exist on the base of one or more slices - examine slice data and consult the Galena Users' Guide Circle and Results Summary (Lowest 9 Factor of Safety circles) -------------------------- Circle, X-Centre Y-Centre X-Left Y-Left X-Right Y- Right Radius FoS 1 227.15 809.19 144 . 64 780.06 233.50 721.92 87.50 2.115 2 227.51 808.15 144 . 64 780.06 234.90 720.96 87.50 2.115 3 226.78 810.22 144 . 64 780.06 232.10 722.88 87.50 2.116 4 227.86 807.09 144.64 780.06 236.30 720.00 87.50 2.116 5 226.40 811.23 144.64 780.06 230.70 723.84 87.50 2.118 6 233.14 807.44 150.76 777.96 236.30 720.00 87.50 2.119 7 226.01 812.23 144.64 780.06 229.30 724.79 87.50 2.121 8 232.78 808.43 150.76 777.96 234.90 720.96 87.50 2.123 9 225.62 813.22 144.64 780.06 227.90 725.75 87.50 2.125 File:C.IProjecWWashingtoMStability 1 2 of 4 5/4/2007 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 Suzanne Washington Westside 1, Block 2, Lot 9 Stability Analysis GALENA e4o fi2o o�s 800 780 760 740 720 Analysis: 1 Multiple Stability Analysis Method Baihop simplified 700 Surface Circular Results 680 C,#,car(minimum) Factor of Safety: 1.65 0 20 40 60 80 100 120 140 160 180 200 220 240 Eeee:.Mry]W] Pglwt•4.M.Y]W] Pirgioct Washington-Cushman Westside 1 Blk 2 Lot 9 Dynamic E& C\Documents and Sathnge\Ow r\My D=ments\ProjeCte\Washington\Stabil"Wmhirgton 1.gmf Tahja Engineering,Inc. Galena 5.00 Analysis Results Licensee: Tahja Engineering, Inc. Project: Washington - Cushman Westside 1 Blk 2 Lot 9 DATA: Analysis 1 - Dynamic Material Properties (1 material) ------------------- Material: 1 (Mohr-Coulomb Cu increases with depth) - Cemented Hoodsport Sandy Loamy Gravel Cohesion Phi PI UnitWeight Ru 350.00 36.0 0 125.00 0.00 Material Profiles (1 profile) ----------------- Profile: 1 (2 points) Material beneath: 1 - Cemented Hoodsport Sandy Loamy Gravel 0.00 800.00 240.00 800.00 Slope Surface (6 points) ------------- 0.00 790.00 43.90 782.00 147.70 780.00 177. 60 760.00 207.10 740. 00 236.30 720.00 File:C.IProjectslWashingtonlStability 1 3 of 4 5/4/2007 J V TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 Failure Surface --------------- Initial circular surface for critical search defined by: XL,XR,R Intersects: XL: 147.70 YL: 780.00 XR: 230.00 YR: 724.32 Centre: XC: 216.48 YC: 793.00 Radius: R: 70.00 Earthquake Force ---------------- Pseudo-static earthquake (seismic) coefficient: 0.150 Variable Restraints ------------------- Parameter descriptor: XL XR R Range of variation: 55.00 12. 60 35.00 Trial positions within range: 10 10 10 ----------------------------------------------------------------------- ----------------------------------------------------- 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: 1.76 There were: 926 successful analyses from a total of 1001 trial circles 75 analyses aborted due to unacceptable geometry Critical (minimum) Factor of Safety: 1.65 Negative normal stresses exist on the base of one or more slices - examine slice data and consult the Galena Users' Guide Circle and Results Summary (Lowest 6 Factor of Safety circles) -------------------------- Circle X-Centre Y-Centre X-Left Y-Left X-Right Y- Right Radius FoS 1 227.15 809.19 144.64 780.06 233.50 721.92 87.50 1.647 2 226.78 810.22 144.64 780.06 232.10 722.88 87.50 1.647 3 227.51 808.15 144.64 780.06 234.90 720.96 87.50 1.647 4 226.40 811.23 144.64 780.06 230.70 723.84 87.50 1.648 5 227.86 807.09 144 . 64 780.06 236.30 720.00 87.50 1.648 6 226.01 812.23 144 . 64 780.06 229.30 724.79 87.50 1.650 File:C:IProjectslWashingtonlStability 1 4 of 4 5/4/2007 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 Mason County DCD Planning Division PO Box 279 Shelton, WA 98584 Suzanne Washington 1824 SE 56a' Avenue Portland, OR 97215-3358 (503) 230-0656 RE: Geotechnical Report Addendum Case No.: BLD2007-00890 Parcel No. 42331-50-02009 Lake Cushman Westside 1, Blk 2, Lot 9 N Cushman Ridge Drive Hoodsport, WA 98548 INTRODUCTION Review of a geotechnical report prepared by TAHJA ENGINEERING, Inc., dated 5/4/07 by ALKAI CONSULTANTS, LLC. , consultant services to Mason County DCD,resulted in a request for additional information. The following information and materials are offered in response; 1. ALKAI recommended that ground water be identified and used in the slope stability calculations. In my original report dated 5/4/07, I indicated a glacial till/hardpan layer beneath the ground surface at a relatively shallow(-3')depth. This till layer in the Cushman area often extends down to the basalt rock. The hardpan is considered relatively impervious, so the water table can be expected to be beneath the hardpan. The water course shown on the original geotechnical report site plan is scoured down to basalt rock at a depth of approximately 20-feet below the surrounding ground's surface. The slope analysis was"stopped"at elevation 720 because this roughly represents Lake Cushman's low lake level typically held by Tacoma City Light through the fall and spring months. During the summer months,the lake level is raised to elevation 738. It appears evident that the water table in the vicinity of the Washington property is directly related to the Lake Cushman lake level, so a summer time high lake level of 738 is used in modeling the site as the phreatic water surface. Attached please find a GALENATM Stability Analysis printout for the Washington property which includes a phreatic water surface at elevation 738 (worst case condition). As with the original geotechnical report,the required factor of safety is being met. 2. ALKAI recommended that the slope stability analysis be continued further down the File:IC.lProjectslWashingtonlGeo-RptUddndm Page 1 of 2 7/8/2007 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 slope. There appears to be little point in extending the analysis further downslope than the low lake elevation. ALKAI stated that the local anomalies I mentioned in my original report can have a significant effect on the results of slope stability calculations. I agree with this statement, but feel that I have made adequate adjustments, and modeled the slope to address the required stability investigation. The hardpan layers present in the Lake Cushman area typically stand near vertical, and remain stable over long periods of time. The presence of these vertical portions of the slope have already been considered in my approach to modeling them. This concludes my additions or revisions to my original geotechnical report dated 5/4/2007. I hope it meets the needs of the Washington, Mason County, and ALKAI Consultants in planning the property's development. Sincerely, Alan A. Tahja,P.E. Attached: Stability Analysis II—Dynamic analysis printout with phreatic surface included. ALKAI Consultants review letter v�OF A�T N Ir a w �784 00' �EG1 TER�9 �`SSIONAL Elmm File:IC:Wrojectsl WashingtonlGeo-RpMddndm Page 2 of 2 7/8/2007 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 Suzanne Washington Westside 1, Block 2, Lot 9 Stability Analysis GALENA 840 WA 800 / 760 � 30784 w� �+�'�,REGISTER 911Z� 760 s`SIONAL E C~ 740 EXPIIM.S 720 Analysis: 1 Multiple Stability Analysis Method: Biehop Simplified 700 Surface Circular Results 680 CrKical(mmrmum) Factor or Safety: 1.65 o 20 40 Bo Bo too 120 140 16D 18o 2o0 220 240 nu 8-m 7 i1W1 Project: Washington-Cushman Westside 1 Blk 2 Lot 9 Dynamic Efts: C:\Documents and Settings\Ow ar\My Documents\Projects\Washington\Stadlity\Weehington 2.gm1 Tahja Engineering,Inc. Galena 5.00 Analysis Results Licensee: Tahja Engineering, Inc. DATA: Analysis 1 - Dynamic Material Properties (1 material) ------------------- Material: 1 (Mohr-Coulomb Cu increases with depth) - Cemented Hoodsport Sandy Loamy Gravel Cohesion Phi PI UnitWeight Ru 350.00 36.0 0 125.00 0.00 Material Profiles (1 profile) ----------------- Profile: 1 (2 points) Material beneath: 1 - Cemented Hoodsport Sandy Loamy Gravel 0.00 800.00 240.00 800.00 Slope Surface (6 points) ------------- 0.00 790.00 43.90 782.00 147.70 780.00 177. 60 760.00 207.10 740.00 236.30 720.00 Phreatic Surface (2 points) ---------------- File:C:IProjectsl Washington lStabdityII I of 2 7/8/2007 TAHJA ENGINEERING, Inc. PO Box 235, Hoodsport, WA 98548 (360) 877-9512 0.00 738.00 250.00 738.00 Failure Surface --------------- Initial circular surface for critical search defined by: XL,XR,R Intersects: XL: 147.70 YL: 780.00 XR: 230.00 YR: 724.32 Centre: XC: 216.48 YC: 793.00 Radius: R: 70.00 Earthquake Force ---------------- Pseudo-static earthquake (seismic) coefficient: 0.150 Variable Restraints ------------------- Parameter descriptor: XL XR R Range of variation: 55.00 12.60 35.00 Trial positions within range: 10 10 10 ----------------------------------------------------------------------- ----------------------------------------------------- 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: 1.76 There were: 926 successful analyses from a total of 1001 trial circles 75 analyses aborted due to unacceptable geometry Critical (minimum) Factor of Safety: 1. 65 Negative normal stresses exist on the base of one or more slices - examine slice data and consult the Galena Users' Guide Circle and Results Summary (Lowest 5 Factor of Safety circles) -------------------------- Circle X-Centre Y-Centre X-Left Y-Left X-Right Y- Right Radius FoS 1 227.15 809.19 144.64 780.06 233.50 721.92 87.50 1.647 2 226.78 810.22 144 .64 780.06 232.10 722.88 87.50 1.647 3 227.51 808.15 144 . 64 780.06 234.90 720.96 87.50 1. 647 4 226.40 811.23 144 . 64 780.06 230.70 723.84 87.50 1.648 5 227.86 807.09 144 . 64 780.06 236.30 720.00 87.50 1.648 File:C.IProjectsl Washington lStability11 2 of 2 7/8/2007 ALKAI CONSULTANTS, LLC. Environmental Engineering • Geotechnical Engineering • Wetland Consulting June 22, 2007 ACL07-06-G 179.003 Mason County Department of Community Development P.O. Box 279 Shelton, WA 98584 Attn: Ms. Tammi Clark Geotechnical Engineering Report Review Permit#BLD 2007-00890 Applicant: Ms. Suzanne Washington Dear Ms. Clark, At your request, we have reviewed a Geotechnical Engineering Report for the above referenced permit. The report was prepared by Tahja Engineering, Inc. dated May 4, 2007. The report was signed and stamped by Mr. Alan A. Tahja, PE(License#30784). In our opinion, the referenced geotechnical engineering report was prepared in general accordance with the requirements in the Mason County Resource Ordinance, Geologically Hazardous Areas 17.01.100.E.5 with one exception. In paragraph (I) of the referenced section of the Ordinance it states that "a discussion of... ground water conditions...". Ground water was not mentioned in the report nor does it appear to have been included in the slope stability calculations. Ground water has a significant effect on the results of slope stability calculations. On the figures and cross sections provided with the report, the base of the slope is not indicated. In the slope stability calculations, the slope stops at elevation 720. However, it is not clear if this is the base of the slope or simply the limit of the topographic map. In the discussion on page 1 of the report, Mr. Tahja indicates that the slope is quite high and varies in steepness from 40 percent to near vertical. The cross section provided indicates a uniform slope. Total slope height and variations in slope inclination can have a significant effect on the results of slope stability calculations We recommend that Mason County request that the engineer (Mr. Tahja) provide an addendum discussing ground water conditions and amend the stability calculations to include ground water, if appropriate. We further recommend that Mason County request that the engineer extend the limits of the slope stability calculations to the base of the slope or provide a statement that the lowest elevation presented in the calculations is the base of slope. 9465 Provost Road NW, Suite 202 •Silverdale,Washington 98383 • (360) 613-2407 • Fax: (360) 613-2408 1 Project No.ACL07-06-G179-003 Mason County Review June 22,2007 Page No.2 Should you have any questions or concerns, or if we may be of additional assistance, please call our office at (360) 613-2407 or contact us by e-mail at Todd@alkai.net. PA Sincerely, 5 F WAS Todd S. Parkington, P.E. Senior Geotechnical Engineer SIGNAL�� EXPIRES S�O�� Attachments: Geo Tech Work Order Geotechnical Engineering Report by Tahja Engineering, Inc. 9465 Provost Road NW, Suite 202 • Silverdale, Washington 98383 9 (360) 613-2407 • Fax: (360) 613-2408