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HomeMy WebLinkAboutRS_TIR_090505.pdf11 BA1MA & HOLMBERG INC. C-'Ity of Rrc,,into a lanr in h;vi. ;icn TECHNICAL INFORMATION REPORT D; ; . ; :• ,: fo r Wilson Park May 5, 2009 1 133 sroNAL Baima & Holmberg, Inc. Job No. 2687-001 Prepared For Robert Wilson 720 South 55th Street Renton, WA 98055 100 FRONT STREET SOUTH • ISSAQUAH • WASHINGTON • 98027-3817 - (425) 392-0250 • (425) 391-3055 TABLE OF CONTENTS SECTION TITLE PROJECT OVERVIEW 2 CONDITIONS AND REQUIREMENTS SUMMARY 3 OFFSITE ANALYSIS 4 FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN 5 CONVEYANCE SYSTEM ANALYSIS AND DESIGN G SPECIAL REPORTS AND STUDIES 7 OTHER PERMITS 8 ESC ANALYSIS AND DESIGN 9 BOND QUANTITIES, FACILITY SUMMARIES, AND DECLARATION OF COVENANT 10 OPERATIONS AND MAINTENANCE MANUAL Street Address 720 South 55th Street King County Tax Parcel No. 312305-9125 Project Overview This project involves developing a 2.5 -acre parcel into 13 single-family lots. The site currently is occupied with a single residence, lawn and wooded areas. The site is located about 250' north of South 55th Street, about 150' east of the east terminus of South 53rd Place, on the slope overlooking SR 167, about %2 mile to the west. The site generally slopes down to the west at an average slope of approximately 20%, Per the SCS soil maps, the site is underlain with Alderwood soil, sandy loam over glacial till. Upstream Tributary Drainage A portion of the parcel to the east of the site drains onto the site. No problems related to this runoff was noted. Level 1 Downstream Drainage Analysis In general, runoff from the site flows west into lots in the adjacent development, Geneva Court (A). The majority of these runoff flows apparently collect in the drain behind an 8'± rockery (B) constructed along the back yards of the west -most lots of said development or in area drains in the back yards, then flow into the storm system in South 53rd Place (C & D). This flow collects in a stormwater pond/bioswale facility located at the intersection of South 53rd Place and Talbot Road South (E), about 750' downstream from the site. This facility outfalls through an 18" pipe to the west side of Talbot Road South into a shallow, poorly defined channel flowing west through the woods (F). The flows pass through a short 12" culvert (H) under a walking path then disappear into thick woods/brush, continuing to flow west to a wooded wetland area (1) beyond'/4 mile downstream from the site. This wetland apparently drains to a 10' X 5'f box culvert crossing under SR -167 (Q), about 1/2 mile downstream from the site. A small area of the south part of the site drains southwest across the south property line of the site into the adjacent parcel to the south (Z). These runoff flows through woods, collecting in a ditches along South 55th Street (K)(about 350' downstream from the site) and/or Talbot Road South (L)( about 800' downstream from the site). Flows into the ditch along South 55th Street (K) continue west in a 6"-12" rock -lined ditch channel OVERW IS 9.56' AT N PROP (INE F LIME, W. 794' 5E 1/4 5.E 1/4. PORTION OF THE SE -114 OF SEC. 31, TWN. 23 N., RNG 5 E., Wil CITY OF RENTON, WASHINGTON iFOUND5/8- PEW & GA SET REB,NrvvR F011M9 5/8- RE84R d a. CR5 2231" 0.14 N 8 r a. 1WE, 5 525. 5.E 1/4, 5 E 1/4 " „jjzB 710 ` CWMCS 75537' 9.39 9,55 F OF PROP COR 588 2578; 526.01 k 015' £ 91 PROP r,OR PER GENc9A GOORT TRACT i 16 A 1 OPFN &cE/ 131212 a 11 s 10 STORM DEiENTKA m 5.,62 Sq EI gi 5.1115q Fk S.t 12 Sq Fl a 5,112 54 FI II 10.494 Sq ES ' O X1(2, 7145, FIIR , I1 I S Q ; a 1A X 4 EAP - - w - FOONU 5/8' R . CAA CF" 22.1,10 0.55 N c4 13 jQ_44' £ OF PROP CGR i, R OVERLAP IS 10.46' AT 5 AAOR UNE d 84' I I I f f I 50' ROAD AND UTUTY EASEMENT -INO209OJ27002018 I I I e I I I I I I I AN8 0639W 11624' 21 2N3 S 55TH ST. pa.h e R L-11.41 R.78.00 o TR D 4 14 TR C 0.319 5q EE fa 1 OPEN SPAEE OPEN SPACE1679 54 Ft 4,321 Sq F1 '. 15' Bs9l 3. 129 \ 3!202' 5,118 x EI - 5.j33 sq Fl 26 PRIVATEhsz.14' AMSS R.sa OO k UTILITY 125 rx NW E V 9E N[NV+FD FT. 60.OG 4300' L-, 2 3 1,77B Sq FI 5 gm 54 FI g 7 g g 5.252 Sq F1 A 105.29' g II c r6,512 5, 1, rr 13E a7' S.SB S"EI T2C72' 4 6.7.4 Sv Ft f -J" 26'17-M 51555 7 LWE. N 201'. 5. S25 5.E f%4, 5.F 1/41RE(`T FFlN11EON5 O TRACT A1 I STORM ORAWAM TRACT, PO551BtE Nlt4¢ A MC 5 FOR MOPERTY TO MONias-OIKJCOMAMIT.NNEO BY MOA 1RACr w , ip• I !, . 111G AREA TRACT 15TEEP 1 i AOR[S}-pMMEOMA,NTntN4Y} BY I n I I MCM I I 1RACT 'C , - r oPEM SRnr - ovR1EaA1A N,tw4Eo- Lor I ;1 BT WA REN7[M' SP 1059-85 18A7r b' C VN SPACE - OFA.'CO/MAIN TAWEO By MOA IL /312.105-9119 30INORES;. 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Os —' Os — fl : Re sQ R°w` AkF ' AgC r AS SCS Soil Map r Ur r Ur9P * ao r I King County Department of Development and, Environmental Services TECHNICAL INFORMATION REPORT (TIR) WORKSHEET Part l ' PROJECT OWNER -AND PROJECT ENGINEER Project Owner Address i m hurt-+ S 5nk c; r Phone Project 5ngineer Company K Address/Phone UYI Tv r APP.UCATION -' c Subdivison Short Subdivision Grading Commercial Other Community Drainage Basin Pert 6 °STTE :CHARACTERISTICS' River Stream Critical Stream Reach Depressions/Swales Lake Steep Slopes Part 2 PROJECT LOCATION AND DESCRIPTlON Project Name wJ .A P IQ F ft — Location Township Range -5 5? .....Section 31 U Part'4 :OTHER R1=ICEEWS ANp PERM]TS ' DFW HPA COE 404 DOE Dam Safety FEMA Floodplain COE Wetlands Shoreline Management Rockery Structural Vaults Other Floodplain Wetlands Seeps/Springs High Groundwater Table Groundwater Recharge Other Part 7 :SOILS Soil Type Slopes Erosion Potential Erosive Velcoties Additional Sheets Attached Part 8: DEVELOOMENT LIMITATIONS REFERENCE LIMITATIONISITE CONSTRAINT Ch. 4 --Downstream Analysis L,a c)I r x5 Additional Sheets Attached MINIMUM ESC REQUIREMENTS DURING CONSTRUCTION Sedimentation Facilities Stabilized Construction Entrance Perimeter Runoff Control Clearing and Graing Restrictions vCover Practices Construction Sequence Other Part 10 : SURFACE 1NATER SYSTEM . Method of Analysis Grass Lined Tank Infiltration K-C-F L) I+ Channel VVault Depression Pipe System Energy Dissapator Flow Dispersal Compensation/Mitigati on of Eliminated Site Open Channel Wetland Waiver Storage Dry Pond Stream Regional Wet Pond Detention Brief Description of System Operation 5 ^8, W"PQ -PF'ri YYf VJff, r t Yt G Facility Related Site Limitations Reference Facility Limitation P,art.i2'..EABEMEKTS,n CTS Drainage Easement Access Easement Native Growth Protection Easement Tract Other Part 113_ :SIGNATURE OF PROFESSIONAi ENGINEER I or a civil engineer under my supervision my supervision have visited the site. Actual site conditions as observed were incorporated into this worksheet and the attachments. To the best of my knowledge the information provided here is accurate. SECTION 2 CONDITIONS AND REQUIRMENTS SUMMARY PREAPPLICATION MEETING FOR WILSON PARK 720 S 55TH ST CITY OF RENTON Department of Community and Economic Development Current Planning Division PRE09-012 February 26, 2009 Contact Information: Planner Gerald Wasser Phone: 425.430.7382 Public Works Reviewer Rick Moreno Phone: 425.430.7278 Fire Prevention Reviewer; Dave Pargas Phone: 425.430.7023 Building Department Reviewer: Craig Burnell Phone: 425.430.7290 Please retain this packet throughout the course of your project as a reference. Consider giving copies of it to any engineers, architects, and contractors who work on the project. You will need to submit a copy of this packet when you apply for land use and/or environmental permits. Pre-screening: When you have the project application ready for submittal, call and schedule an appointment with the project manager to have it pre-screened before making all of the required copies. The pre -application meeting is informal and non-binding. The comments provided on the proposal are based on the codes and policies in effect at the time of review. The applicant is cautioned that the development regulations are regularly amended and the proposal will be formally reviewed under the regulations in effect at the time of project submittal. The information contained in this summary is subject to modification and/or concurrence by official decision -makers (e.g., Nearing Examiner, Planning Director, Development Services Director, Department of Community and Economic Development Administrator, Public Works Administrator and City Council). FIRE & EMERGENCY SERVICES DEPARTMENT 1V0V M E M a R A N D U M DATE: February 24, 2009 TO: Rick Moreno, Plan Reviewer FROM: David Pargas, Assistant Fire Marshal, Community Risk Reduction SUBJECT: PRE -APP 09-012 Wilson Park Project A review of the plans and material regarding the Vasquez Short Plat has been conducted and completed. Please review the Renton Fire & Emergency Services Fire Code and Policy comments and concerns. 1, FIRE FLOW: Structures up to 3,600 square feet (including garage and basement areas) shall require a minimum fire flow of 1,000 gallons per minute for two hours. Structures in excess of 3,600 square feet shall meet a minimum fire flow of 1,500 gallons per minute for two hours. A water availability certificate shall be required. 2. REQUIRED HYDRANTS: As in accordance with Renton Fire & Emergency Services Department standards, one (1) hydrant shall be required for structures up to 3,600 square feet and requiring a minimum fire flow of 1,000 gallons per minute. For structures over 3,600 square feet and requiring a minimum fire flow of 1,500 gallons per minute or more shall require a minimum of two (2) hydrants. The number of hydrants shall also be based on spacing, which shall be in accordance with sound engineering practices. All hydrants servicing the Wilson Park project shall be equipped with five inch Storz fittings on the main ports. 3. HYDRANT SPACING: Residential spacing requires hydrants to be located no greater than 300 feet to the front of any structure. In this case, the existing hydrants are located much greater than the 300 feet to the existing structures, Therefore, a minimum of two (2) new hydrants shall be required in order for the Wilson Park project to meet the 300 foot spacing. 4. FIRE APPARATUS ACCESS: The minimum fire apparatus road access shall be no less than 20 feet wide and on a surface capable of sustaining the weight of a fire apparatus. The turning radius shall be 45 feet to the outside and 25 feet to the inside. Rick Moreno, Plan Reviewer Febntary 24, 2009 Page 2 of 3 At this time, I am concerned about the minimum 20 foot access off of South 192"d as a private access. Private accesses are difficult to regulate and to prevent parking in fire apparatus access roadways that are 20 to 28 feet wide. If this access cannot be designed as a public street, there are some possible options that could be considered, but would need to be reviewed and approved by the Fire Marshal. The options I am proposing as a possibility to mitigate the access issues are as follows: A) The gated access would require it to be the type o erated off a radio frequency and known as..a Click To Enters stent. B) The 20 foot road access would require to be marked on both sides of tlae road as "NO PARKING AT ANY TIME ". The si na e would need to be provided b the developer and maintained by the Home Qwners Association. C) All of the proposed 16 lots would have to be eq ed with a residenticd tre s rinkler s stem. The ire sprinkler systems would not aide in mitigating the stee rade issue. S. FIRE APPARATUS ACCESS — GRADE: It appears that the access into the Wilson Park neighborhood may be greater than a 15 percent grade. It is important to note that we strongly require the grade to be no greater than 15 percent for Renton Fire & Emergency Services to recommend a project. Using fire sprinkler systems to mitigate an access to allow for it to be greater than 15 percent is not an option. 6. FIRE SPRINKLER: Residential fire sprinklers are not applicable to this project unless used as an option to mitigate insufficient fire flow or an access issue other than grade. 7. FIRE MITIGATION FEES: Fire mitigation fees shall be $488.00 per unit and shall be paid prior to final plat recording. Credit would be given for the one home that has been dedicated to be removed. RECOMMENDATION: LADDER/AERIAL ACCESS: Ladder access for a 35 foot ground ladder set at a 70 degree angle shall be provided on all four sides of any structure two stories or greater in height. DP/kc hAcedV1anninglcurrent planning1preapps109-012.jerr Nfire comments pre09-012 wilson park.doc Rick Moreno, Pian Reviewer February 24, 2409 Page 3 of 3 Jerry Wasser, Associate Planner haceftlanninglcurrent planning1preapps149-012.jerrylflre comments preO9-012 wilson park.doc y DEPARTMENT OF COMMUNITY AND ECONOMIC DEVELOPMENT M E M O R A N D U M DATE; February 23, 2009 T0; Jerry Wasser 4FRONTRickMoreno SUBJECT: Utility and Transportation Comments for Wilson Park Short Plat P RE09-012 NOTE; The applicant is cautioned that information contained in this summary is Preliminary and non-binding and may be subject to modification and/or concurrence by official city decision -makers, Review comments may also need to be revised based oil site planning and other design changes required by City staff or made by the, _aplalicant_ We have completed a preliminary review for the above -referenced development proposal. The following comments are based on the pre -application submittal made to the City of Renton by the applicant. WATER 1. This site is within the Rclaton service area. However, due to elevation of the site, Renton cannot provide sufficient sustainable water pressure. It is recommendcd that the owner receive a certificate of water availability through Soos Creek eater District. The City will be able to release the service area capon evidence that Soos Creek can service this parcel. This certificate will be submitted to the City prior to release of service area. TRANSPORTATION 1. ,Street improvements are required including, but not limited to paving, sidewalks, curb and gutter, storm drainage, street fighting and landscaping along street frontage. 2. Traffic mitigation fees of $75 per additional daily trip shall be assessed per single family home at a rate of 9.57 trips per day. 3. All wire utilities shall be installed underground per City of Renton Ordinance, 4. A dedication of roadway to adjacent northern property is required prior to recording. It appears that Tract A is set up for this application. (See Stoma Utility issues) 5. Traffro , inoI v dyi'g site -distaftOe tvaIuatson-sl>a11 be required for roa why b. The proposed 32 ft. right-of-way with curb, gutter and sidewalk on one side does not meet cuia-ent code. An approved variance is required with this proposal. PILE 09-012 PRCommenis.doc 7. Dedicated right-of-way, meeting City of Renton Street Standards including curb, gutter and sidewalk improvements fronting all lots shall be required prior to recording. Current proposal is not acceptable for lots 6 through 9. 8. Maximum road slopes shail be 15% in all cases. Maximum angle of approach is 8 degrees and maximum angle of departure is 10 degrees. 9. Secondary access roadways are sometimes required as specified by the Fire Marshall_ Secondary access roadways are also required by the City of Renton Street Standards (Title 4, Chapter 6, Section 60G). If streets are over 500 feet, but not over 700 ft. long, dwellings shall either provide a secondary access road or provide an approved residential fire sprinkler system shall be installed, If streets are over 700 ft., a secondary access road is required. Any proposed access -gating system requires approval from the Renton Fire Department. 10. Sidewalk, curb and gutter shall abut S 192'd St with radius curb per City of Renton standards. SANITARY SEWER 1. This site is within the City of Renton Sewer area. The owner small extend an 8 - inch sanitary sewer main within the proposed right -of way from S 55"i St. and extend to the frontage of each proposed lot. A 15 -foot utility easement shall be required to extend to the easterly boundary of lot 6, or lot 9. 2. All system development fees shall apply based on the size of the water meter services. SURFACE WATRER 1. The project is required to do a drainage analysis and meet the design criteria in accordance with the 1990 King County Surface Water Design Manual with conservation flow control criteria or current city standard. The 2005 KCSWDM criteria may be required subject to submitted calculations by the engineer. 2. The Surface Water System Development Charge {SDC) is 51,012 per new single- family residence. This is payable at the time the utility construction permit is issued_ 3. If the proposed Tract A is dedicated for detention, and a night -of -way dedication is used to connect to the northern lot, traffic bearing lid(s) will be required to any proposed storm vault. GENERAL COMMENTS 1. All utility and street improvements will require separate plan submittals prepared according to City of Renton drafting standards by a licensed Civil Engineer. 2_ If fire sprinkler systems are necessary, then a separate fire sprinkler permit will be required. 3. Permit application must include an itemized cost of construction estimate for these improvements. 4. The fee for review and inspection of these improvements is 6% of the first 150,000 of the estimated construction costs 5% of anything over $150,000 but less than $300,000, and 4% of anything over $300,000. Half the fee must be paid upon application. PRE 69.012 PRComments.doc CITY OF RENTON Planning{Building/Public Works MEMORANDUM DATE: February 26, 2009 TO; Pre -Application File No. 09-012 FROM: Gerald Wasser, Associate Planner, (425) 430-7382 SUBJECT: Wilson Park General: We have completed a preliminary review of the pre -application for the above -referenced development proposal. The following comments on development and permitting issues are based on the pre -application submittals made to the City of Renton by the applicant and the codes in effect on the date of review. The applicant is cautioned that information contained in this summary may be subject to modification and/or concurrence by official decision -makers (e.g., Hearing Examiner, Planning Director, Public Works Administrator, Development Services Director and City Council). Review comments may also need to be revised based on site planning and other design changes required by City staff or made by the applicant. The applicant is encouraged to review all applicable sections of the Renton Municipal Code. The Development Regulations are available for purchase for $50.00 plus tax, from the Finance Division on the first floor of City Hall. Project Proposal: The subject property is located north of S. 55'h St_ and east of Talbct Rd. S. at 720S,551h St. The proposal is to subdivide a 108,884 sq. ft. (2.5 acre) parcel into 16 lots and one tract, for future development of detached single-family houses. There is one existing home on the parcel, which would be removed. The site has three different zoning designations, R-1, R-8, and R- 14, and the applicant is proposing a zone change to R-8 for the entire site. The proposed lots range from approximately 4,500 sq. ft. to 6,243 sq. ft. in size. The preliminary plat would be accessed via a 50 ft. wide access easement running from S. 55'h St. over a neighboring parcel to the south. Secondary access would be provided via a 30 ft. ingress, egress, & utility easement over a neighboring property to the south. The site contains protected slopes and a number of significant trees. Zoning/Density Requirements: The subject property encompasses three different zoningdesignations. Staff estimates that approximately 75,300 sq. ft, (1.73 ac) are zoned R-8, 31,200 sq. ft 0.72 ac) are zoned R-1, and 2,384 sq. ft. (0.06 ac) are zoned R-14. These are only approximations. Density is calculated by first subtracting the area of any access easements and critical areas from the gross site acreage, and then dividing the number of units by the net acreage. The applicant did not submit a density worksheet for the proposal. Based on the breakdown of hew much land is located in each zoning designation, staff has estimated that up to 13 units could be built on the R-8 portion of the lot, no units could be built on the 0.72 acre portion in the R-1, and no units could be built on the R-14 portion of the lot. The R-8 zone permits a minimum of 4.0 units per acre and a maximum of 8.0 units per acre. The applicant's submittal is predicated on a rezone of the R-1 and R-14 portions of the site to R-8 zoning. Note: While the applicant may request a rezone to R-8, staff is not supportive of this change of zone. The R-1 zone was applied in this general area to aid in the protection of critical areas. The R-1 zone designation on this property is part of a larger continuous area which includes wetlands, steep slopes, landslide areas, and coal mine areas. This rezone would also require a Comprehensive Plan Amendment from Residential Low Density (RLQ) to Residential Single Family (RSF). Development Standards: Because the only developable part of the lot is the R-8 portion, unless a rezone and Comprehensive Plan Amendment are approved, staff has only reviewed compliance with the development standards for that part of the site. Wilson Park Pre -Application Meeting February 26, 2009 Page 2 of 3 The R-8 zone permits one residential unit per lot. Detached accessory structures are permitted at a maximum number of two per lot at 720 square feet each, or one per lot at 1,000 square feet in size. No accessory structures are allowed on lots without a primary structure. Minimum Lot Size, Width and Depth — The minimum lot size permitted in the R-8 zone is 4,500 square feet for tots greater than 1 acre in size and 5,000 square feet for lots 1 acre or less in size. A minimum lot width of 50 feet for interior lots and 60 feet for corner lots, as well as a minimum lot depth of 65 feet, is also required. The total area of the proposed subdivision is greater than 1 acre, so the minimum lot size is 4,500 square feet. All of the proposed lots in the R-8 portion of the site would far exceed this minimum. Note: Since this may be a Hillside Subdivision, lots may be required to be larger than the minimums prescribed in the zone. Building Standards — The R-8 zone allows a maximum building coverage of 35% of the lot area or 2,500 square feet, whichever is greater for lots over 5,000 square feet in size. Building height is restricted to 30 feet and 2 -stories. Detached accessory structures must be below a height of 15 feet and one-story. The gross floor area must be less than that of the primary structure. Accessory structures are also included in building lot coverage calculations. Given the proposed lot configuration, it appears that building standards could be satisfied. Setbacks — Setbacks are the minimum required distance between the building footprint and the property line or private access easement. The required setbacks in the R-8 zone are 15 feet in front for the primary structure and 20 feet in front for the attached garage, 20 feet in the rear, 5 feet for interior side yards, and 15 feet for side yards along streets (including access easements) for the primary structure and 20 feet for side yards along streets (including access easements) for attached garages. Given the proposed lot configuration, it appears that setbacks could be satisfied. Access/Parking' The site has no street frontage, but has existing access easements to South 55th Street and SE 192nd Street. The applicant is proposing to access the project via 50 -foot road and utility easement. Road A would have to be a dedicated public right-of-way. Secondary access is proposed via a 30 -foot ingress, egress, and utility easement. Additionally, the City would require that the extension of Road A at proposed Lots 8, 9 and 10 be developed to the same standards as the rest of Road A (including width, sidewalk, curb and gutter) to the northern property line. Subdivisions with five or more internal lots must provide a public road to serve the new homes. Curb, gutter and sidewalk improvements would be required. Public rights-of-way must be a minimum of 42 ft. wide. Reductions in public right -of width and street improvements would require a street modification. Sidewalks on both sides of the street within the plat and on one side of the southwest access from South 55th Street would be required. Each lot must allow for the parking of two vehicles on the property. Landscaping: The applicant would be required to plant at least two trees of a City approved species with a minimum caliper of 1 112 inches in the front yard or planting strip of each lot of the short plat and a 5 -foot strip along the new right-of-way. The trees and landscaping must be planted prior to final inspection of the new homes. to addition, the stormwater pond must also be landscaped. Critical Areas: The site contains two small areas of protected steep slopes. There is also an area of high landslide hazard on the parcel. A geotechnical report will be required at the time of submittal for a preliminary plat which should include recommendations by the geotechnical engineer on location and footings of future homes on the site. Due to the fact that there are known wetlands in the area, a wetlands study must also be submitted. The wetlands study must be conducted according to the City of Renton Critical Areas regulations, a copy of which is included in the packet of information given to the applicant at the pre -application meeting. Secondary review of the wetlands study, at the applicant's expense, may be required. if wetlands are found, appropriate buffers and Natural Growth Protection Easements must be established. Hillside Subdivisions: Hillside Subdivisions are subdivisions in which the average slope is 20% or in which any street in the subdivision has grades greater than 15% at any point. RMC 4-7-220 PRE09-012 Wilson Park PP (R-8, 16-1ot, access,new street, slopes),doc Wilson Park Pre -Application Meeting February 26, 2009 Page 3 of 3 establishes the regulations for Hillside Subdivisions including application requirements, grading information, street requirements, lot size and erosion control requirements. Significant Tree Retention: A tree inventory and a tree retention plan shall be provided with the formal land use application. The tree retention plan must show preservation of at least 30% of significant trees (6 -inches and greater). A tree retention worksheet will also be required at the time of formal application. The submitted tree plan must indicate how proposed building footprints will be sited to accommodate preservation of significant trees that will be retained, Consistency with the Comprehensive Plan: The R-8 portion of the site is located within the Residential Single Family (RSF) Comprehensive Plan Land Use designation, The following proposed policy is applicable to the proposal: Land Use Element Policy LU -158. Net development densities should fall within a range of 4.0 to 8.0 dwelling units per acre in Residential Single Family neighborhoods. Permit Requirements: Because staff is not supportive of the proposed Comprehensive Plan Amendment and rezone, the applicant may wish to redesign the project utilizing the R-8 portion of the site only for residential lots. A redesign of the project would affect permit requirements. The following permit fees assume that a rezone will not be pursued by the applicant. To subdivide the parcel, the applicant must make formal submittal for Hearing Examiner Preliminary Plat Review. A submittal checklist, listing the items that must be included in the submittal packet to the City, is included in the packet of information given the applicant at the pre - application meeting. The applicant will be required to install a public information sign on the property and the City will send out information on the plat to neighboring property owners. There is a $2,000.00 tee for preliminary plat review. The plat will also require Environmental Review with an additional fee of $500.00. The time frame for processing the environmental review and preliminary plat is approximately 12 weeks to the end of the appeal period. Once approval is received, the applicant must complete any required improvements, such as sewer and water stubs, surface water management facilities, fire hydrant and grading. A separate utilities construction permit is required for these improvements. The applicant must also satisfy any conditions of the preliminary approval from the Hearing Examiner before the plat can be recorded. The newly created lots may be sold only after the plat has been recorded. The applicant may submit for building permit review for the new houses before the plat is recorded, but the City can only issue the building permits when the plat has been recorded. A demolition permit would be required for the demolition of the existing house on the subject property. Fees: In addition to the fees for review of the land -use, construction and building permits, the following mitigation fees would be required prior to recording of the short plat. A Parks Mitigation Fee based on $530,76 per new single family residence; A Transportation Mitigation Fee based on $75.00 per each new average daily trip attributable to the project; and, A Fire Mitigation Fee based on $488,00 per new single-family residence. A handout listing all of the City's Development related fees in attached for your review. Expiration: Upon preliminary plat approval, the preliminary plat approval is valid for 5 years with a possible one-year extension. cc: Jennifer Henning PRE09-012 Wilson Park PP (M, 16 -fat, access,new street, slopes).doc I - ov IY.01V m0rL r JUL ZONING PW TECHNICALSKMICE5 07!15M 33 - 46 T22N R5E E 112 13 rrr 20D 400 31 T23N R5E E 112 Feel 1:4,8D0 5331 SECTION 3 OFFSITE ANALYSIS BAIMA & HOLMBERG INC. LEVEL I DOWNSTREAM DRAINAGE REPORT for Wilson Plat January 23, 2009 Baima & Holmberg, Inc..Job No. 2687-001 Prepared For Robert Wilson 720 South 55th Street Renton., WA 98055 100 FRONT STREET SOUTH • ISSAQUAH • WASHINGTON - 98027-3817 • (425) 392-0250 • (425) 391-3055 Street Address 720 South 55th Street King County Tax Parcel No. 312305-9125 Project Overview This project involves developing a 2.5 -acre parcel into 13 single-family lots. The site currently is occupied with a single residence, lawn and wooded areas. The site is located about 250' north of South 55` x' Street, about 150' east of the east terminus of South 53rd Place, on the slope overlooking SR 167, about Yz mile to the west. The site generally slopes down to the west at an average slope of approximately 20%. Per the SGS soil maps, the site is underlain with Alderwood soil, sandy loam over glacial till. Upstream Tributary Drainage A portion of the parcel to the east of the site drains onto the site. No problems related to this runoff was noted. Level 1 Downstream Drainage Analysis In general, runoff from the site flows west into lots in the adjacent development, Geneva Court (A). The majority of these runoff flows apparently collect in the drain behind an 8'± rockery (B) constructed along the back yards of the west -most lots of said development or in area drains in the back yards, then now into the storm system in South 53`d Place (C & D). This flow collects in a stormwater pond/bioswale facility located at the intersection of South 53`d Place and Talbot Road South (E), about 750' downstream from the site. This facility outfalls through an 18" pipe to the west side of Talbot Road South into a shallow, poorly def ned channel flowing west through the woods (F). The flows pass through a short 12" culvert (H) under a walking path then disappear into thick woods/brush, continuing to flow west to a wooded wetland area (I) beyond'/4 mile downstream from the site. This wetland apparently drains to a 10' X 5'± box culvert crossing under SR -167 (Q), about 1/2 mile downstream from the site. A small area of the south part of the site drains southwest across the south property line of the site into the adjacent parcel to the south (Z). These runoff flows through woods, collecting in a ditches along South 55th Street (K)(about 350' downstream from the site) and/or Talbot Road South (L)( about 800' downstream from the site). Flows into the ditch along South 55th Street (K) continue west in a 6"-12" rock -lined ditch channel along the north side of the street, passing through a couple of 12" driveway culverts on the way to the intersection of South 55th Street and Talbot Road South (M). This ditch is eroded in areas and shows signs of overtopping and flowing into the street, presumably during the recent storms that caused widespread flooding in Western Washington. Flows from this ditch collect in an 18" storm system (M). Flows from the ditch along Talbot Road South (L) collect in the same storm system, about 850' downstream from the site. The 18" storm system continues west under Talbot Road South and west along the north side of South 192"a Street (N) to about 1200' downstream from the site where it apparently discharges into another shallow roadside ditch (0)(because of heavy vegetation the pipe outlet was not located). This ditch is poorly defined with flows drifting west along the road, where they enter Springbrook Creels at about 1800' downstream from the site (P). Springbrook Creek, with a channel generally about 5' wide and 3' deep (water depth of about 1') continues west to about '/2 mile downstream from the site (J) where it enters the aforementioned 10' X 5'± box culvert crossing under SR-167 (Q). The creek continues plowing west on the west side of SR-167. Other than the overtopping ditch noted along South 55th Street, there were no apparent downstream drainage problems. A check of King County drainage complaints on their IMAP website yielded no reported complaints along this downstream drainage path. S 4dill S Vicinity Map TOWJt rT Vicinity Map 1682D3 AgC 1'L* I Age 491 I Wo Ur o :AgC u 1PA I • A tl J ITu e• I • ' Ami iy -rr 4 . o r Inc ° Agt3 y so Kt 7 A e M 4 rOB . ,a rr17 Py PLS ------ .- I • II'• .r , Wo 194I'. R I Agg. -• I,` 1 I i • I . ii ' •0 - ' Amar jk AgC I Ur I' ° yet., l •11,tA11 a I p. r G ,•9 . •. Ur r Ng e _ ='— go 32 - d. WO•I a AgC` A f e -I S,a° M& '.. AgB 4 :964* ;. Ng a Br Re' an a Wad, elp Ur + N10 C 4 i 'A Tank.. `7' 6-*• Ur Py Os V ~ f AgC m1 O Sa So Ff f 45 f•; '•' • AgC 1:..• •• r AgE r:. HAYE,9 Ng Ng A I u •m. Pu O I AmC ,,- 4 AtUri " o AkF AgC gB a Ur Ar er 4 PJs Ng Pc M R1gB 1• -A:B 5'k -• Al f •Wo ' _ AgC ,' I T] • — 4 • • ... •. Wo UrsUra Rey w Us r AmC AmC Tvo Ln wc Agp 15r _ Agc i _u\ lt+o • - — - . - r1mC lAle I ' 1 7 •. f• S f •- Q «,183 _ { ti f 9' 4 • vC AmC AmC 11 as Ur Ur I m q . { Agp . L '<= 4T• AgC AmC . y..... _- AmC AgC .. {= 1 Pu Am • - %• .. 1 +Alau i AgC v AmC• Jr as ii I Br N° _ No 12 ` Pk AY I II • II 411 31 - x O • ti •- Amc Re 7 N r n r • AmC, Wo `e, Ur -raise• a Akf Agl9• 15 - r • F •InC m Ag( Pe uIt n 4 1 4 Re 4 AkF 'AgC'+ fo • F Os I as : Re o pRgw. Akf AgC dS Ag a SCS Soil MapUrIUrrr : r 4 r ' AgD Y S yPu9mAkF { ' SECTION 4 FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN BAIMA & HOLMBERG, INC. 100 Front Steet South ISSAQUAH, WASHINGTON 98027-3817 425) 392-0250 FAX (425) 391-3055 JOB 4 ( 5 _490 SHEET NO. OF CALCULATED 6Y 1 --- DATE CHECKED BY DATE SCALE BAIMA & HOLMBERG, INC. 100 Front Steet South ISSAQUAH, WASHINGTON 98027-3817 425) 392-0250 FAX (425) 391-3055 JOB Z (0 SHEET NO. OF CALCULATED BY f DATE CHECKED BY DATE crAI G 5 4.. _.. rT1l.Q L.I._ , ..6t G (2'. Pondcale Worksheet 1 Enter site information in the yellow highlighted cells 2 Verify no error message is displayed 3 Results are displayed in Green Box ote: pondcalc will not work for negative landcover conversions. pondcalc does not handle existing EI or TG very well. Disclaimer: This spreadsheet is provided without warranty of any kind. Use this spreadsheet at your own risk. All facility sizes should be verified using KCRTS software. Rainfall Region ST Scale Factor: 1.00 FC Level: 2 acres f 205 TF= till forest, T either ST or LA see rainfall regions map} 0.8 - 1.2 see rainfall regions map} 1, 2, or 3 see flow control app map} type acres TF z TP rsh il+ I N If i§ TG 0.8 EI 1.7 till pasture, TG = till grass, Adjusted Acres Error Messages converted cover 2.5 0.8 1.7 1 = effective impervious Acreage Check: post pre gross 2.5 2.5 adjusted 2.5 2.5 Existing Conditions 100 -yr SBUH Peak (offsite Roadway) S.C.S. TYPE -1A DISTRIBUTION 100 -YEAR 29 -HOUR STORM **** 3.90" TOTAL PRECIP. ********* DATA PRINT-OUT: AREA(ACRES) PERVIOUS IMPERVIOUS TC(MINUTES) A CN A CN 3 .3 81.0 .0 98.0 20.0 PEAK-Q(CFS) T-PEAK(HRS) VOL(CU-FT) 14 7.83 2576 Developed Conditions 100 -yr SBUH Peak (offsite Roadway) S.C.S. TYPE -1A DISTRIBUTION ******************** 100 -YEAR 24-HOUR STORM **** 3.90" TOTAL PRECIP. ********* DATA PRINT-OUT: AREA(ACRES) PERVIOUS IMPERVIOUS TC(MINUTES) A CN A CN 3 .0 86.0 .3 98.0 2.0 PEAK-Q(CFS) T-PEAK(HRS) VOL(CU-FT) 41 7.67 4658 BAIMA & HOLMBERG, INC. 100 Front Steet South ISSAQUAH, WASHINGTON 98027-3817 425) 392-0250 FAX (425) 391-3055 J013 SHEET NO. OF CALCULATED BY DATE 41 CHECKED BY DATE SCALE I..s....... .' ..... SECTION 5 CONVEYANCE SYSTEM ANALYSIS AND DESIGN SECTION 6 SPECIAL REPORTS AND STUDIES Altmann Oliver Associates, LLC PO Kox 57.4 (..111,11is,ii, 11.198011 0(hcc ( [23) 313-15,0 ELN January 21, 2009 Steve Beck 4735 NE 4th Renton, WA 98059 1: nvironnwmal Planning Landscape ArchiiC_C.'Lur AOA -3796 SUBJECT: Wetland Reconnaissance for Wilson Property (Parcel 31{2305-9125) 720 So. Se Street, Renton, WA Dear Steve: On January 15, 2009 1 conducted a wetland and stream reconnaissance on the subject property and the adjacent new 50 -foot wide access easement utilizing the methodology outlined in the 1997 Washington State Wetlands Identification and Delineation Manual. No wetlands or streams were identified on or immediately adjacent to the properly or the new access easement during the reconnaissance. The south-central portion of the subject property is currently developed with an existing single-family residence and associated maintained lawn area_ The remainder of the property consists of a mostly coniferous forest that generally slopes moderately down from east to west. Vegetation on the site included western red cedar (Thuja plicata), Douglas fir Pseudotsuga menziesfi), big -leaf maple (Acer macrophyllum), hazelnut (Corylus cornuta), Indian plum (Oemleda cerasiformis), snowberry (Symphoricarpos albus), tall Oregongrape (Mahonia aquifolium), Himalayan blackberry (Rebus discolor), trailing blackberry (Rubus ursinus), sword fern (Polystichum munitum), and English ivy (Hedera helix). The plant community within the undeveloped new access easement consisted of a mix of trees and brush including black cottonwood (Populus trichocarpa), big -leaf maple, Himalayan blackberry, snowberry, sword fern, and English ivy. No hydrophytic plant communities were observed on the site. Borings taken throughout the property revealed high chroma non -hydric soils and there was no observation or evidence of prolonged soil saturation. Steve Beck January 21, 2009 Page 2 If you have any questions regarding the reconnaissance, please give me a call. Sincerely, ALTMANN OLIVER ASSOCIATES, LLC John Altmann Ecologist LIU & ASSOCIATES, INC. Geotechnical Engineering Engineering Geology Earth science February 20, 2009 Mr. Robert Wilson 720 South 55`h Street Renton, WA 98055 Dear Mr. Wilson: Subject: Geotechnical Engineering Study Wilson Plat 720 South 551h Street Renton, Washington L&A Job No. 9AO06 INTRODUCTION We have completed a geotechnical engineering study for the site of the subject plat, located at the above address in Renton, Washington. The general location of the project site is shown on Plate 1 — Vicinity Map. We understand that the proposed development for the site is to plat it into 16 single-family residential lots, with supporting infrastructure. The purpose of this study is to characterize the subsurface conditions of the site and provide geotechnical recommendations for grading, slope stabilization, erosion mitigation, surface and ground water drainage control, foundation design and construction, etc., for the proposed development of the site. Presented in this report are our findings, conclusions and recommendations. PROJECT DESCRIPTION For our use in this study, you provided us with a set of topographic survey and plat plan of the proposed development for the site. According to this plan, the subject site is an rectangle -shaped land about 526 feet wide (east -west) by about 207 feet deep (north -south), lying to the north of South 55th Street. The platted building lots are to be accessed from 55Eh Avenue South to the south side of the site via a new roadway entering the site near its southwest corner and an existing gravel driveway at about 120 feet west of the southeast corner. These roadway/driveway 19213 Kenlake Place NE - Kenmore, Washington 98028 Phone (425) 483-9134 - Fax (426) 486-2746 February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 2 are to connect a paved road running mainly east -west down the middle of the site. kdivi" t t }a r qR! ¢ < : x cu'y ! ".5": a, Y,.. t ;M ui:"'s i 'S4 .. }*_... is to be located at the northwest corner of the site to store storm runoff collected over the impervious surfaces of the proposed development. We understand that grading for the proposed development of the site willUMMyt; im 1.ls . Retaining walls and/or rockery walls may be required to support or line the cut banks and/or fill embankments along the cast, west and part of the south boundaries of the site. SCOPE OF SERVICES Our scope of services for this study comprises specifically the following: 1 Review the geologic and soil conditions at the site based on a published geologic map. 2. Explore the site for subsurface conditions with backhoe test pits to a firm bearing soil stratum or to the maximum depth (about 12 feet) capable by the backhoe used in excavating the test pits, whichever occurs first. 3. Perform necessary geotechnical analyses and provide geotechnical recommendations for grading, slope stabilization., erosion mitigation, surface and ground water drainage control, design and construction for building foundations and stormwater detention vault, etc., based on subsurface conditions encountered in the test pits and results of our geotechnical analyses. 4. Prepare a written report to present our findings, conclusions, and recommendations for the proposed development of the subject plat site. SURFACE CONDITIONS SITE CONDITIONS LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AOO6 Page 3 The subject site is situated on a broad, moderate to steep, westerly -declining hillside. It is backed into undeveloped wooded lands to the north and east, and adjoined by a single-family residence and undeveloped land to the south and a subdivision to the west. The site generally slopes down t portions of the site generallyliewesterlatgradesfrom13topercent. The '*"` y within the eastern 100 to 120 feet and the western 150 to 200 feet of the site, while the more moderate area lies in about the middle third of the site where the building pad and the yard of an existing residence are located. The existing residence is accessed from South 55th Street via a long gravel driveway. The unpaved area around the existing house is covered with lawn grass. The g 4 eastern and western areas of the site is dotted with tall, mature evergreen trees with few mature deciduous trees mixed in between. The eastern steeper area is covered with thick brush, while the western steeper area is covered by sporadic brush and dense ivy. GEOLOGIC SETTING The Geologic Map of the Lake Stevens Quadrangle, Snohomish County, Washington, by James P. Minard, published by U. S. Geological Survey in 1985, was referenced for the geologic and soil conditions of the subject site. According to this publication, the surficial soil units at and in the vicinity of the site are mapped as Kame Terrace (Qit) deposits underlain by Ground Moraine Qgt). The geology of the Puget Sound Lowland has been modified by the advance and retreat of several glaciers in the past and subsequent deposits and erosion. The latest glacier advanced to the Puget Sound Lowland is referred to as the Vashon Stade of the Fraser Glaciation, which has occurred during the later stages of the Pleistocene Epoch and retreated from the region some 14,500 years ago. LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 4 The Kame terrace deposits were laid down by ice -marginal streams flowing between higher ground on one side and an ice margin on the other side during the last glaciation. They consist mostly of silty sand and gravel to cobble. The Kame terrace deposits also contain lenses and pods of till and beds of sand, silt and clay locally. The Kame terrace deposits are of moderately - high to high permeability and can provide good foundation support to structures in their native undisturbed state. The ground moraine soil unit, underlying the Kame deposits, is composed of a thin layer of ablation till over lodgmont till sediments, deposited by Puget glacial lobe of the Vashon Stade of the Fraser Glaciation. The lodgmont till is generally a compact mixture of unsorted clay, silt, sand, gravel and cobble, commonly referred to as "hard pan". The ablation till is similar to lodgmont till, but is much less compact and coherent. The thickness generally varies from 2 to 4 feet for the ablation till deposit, and 5 to 30 feet for the lodgmont till deposit. The lodgmont till deposit is practically impervious, except local lenses of sand and gravel. It has a compressive strength comparable to that of low-grade concrete and can stand in steep natural or cut slopes for a long period. The lodgmont till can provide excellent foundation support with little settlement expected to structures. The overlying ablation till is generally in a loose to medium -dense state, and is more compressible and permeable. SOIL CONDITIONS Subsurface conditions of the site were explored with seven test pits excavated within the site. These test pits were excavated on February 4, 2009, with a tract -mounted backhoe to depths from 6.5 to 9.5 feet. The approximate locations of the test pits are shown on Plate 2 - Site and Exploration Location Plan. The test pits were located witb either a tape measure or by visual reference to existing topographic features in the field and on the topographic survey map, and their locations should be considered only accurate to the measuring method used. LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 5 A geotechnical engineer from our office was present during subsurface exploration, who examined the soil and geologic conditions encountered and completed logs of test pits. Soil samples obtained from each soil unit in the test pits were visually classified in general accordance with United Soil Classification System, a copy of which is presented on Plate 3. Detailed descriptions of soils encountered during site exploration are presented in test pit logs on Plates 4 through 7. The test pits encountered a layer of loose, organic topsoil, from about 10 to 24 inches thick, mantling the site. In Test Pits 1 and 2, located in the eastern high ground area, the layer of topsoil was found underlain by a layer of ablation till of brown, medium -dense, silty, fine to medium sand with a trace of gravel, from 2.3 to 3.5 feet thick. This ablation till is underlain to the depths explored by a lodgmont till deposit of brown to brown -gray, dense, silty fine sand, with some gravel and occasional cobble. Test Pits 3 and 4, located at the downhill fringe of the moderately -sloped middle section of the site, encountered a Kame terrace deposit underlying the topsoil. The top 1.7 to 2.0 feet of this Kame terrace deposit is weathered to a loose to medium -dense state and is composed of brown, fine to medium sand, locally with some silt and/or gravel. The fresh Kane terrace deposit underneath is medium -dense to dense, and is composed of brown -gray to light -gray, fine to medium sand with a trace of gravel. This clean sandy deposit should be of high permeability. Test Pits 5, 6 and 7. Located near at the west end of the site, encountered a weathered and/or fresh Kaxne terrace deposit, up to about 9.1 feet thick, underlying the topsoil. The Kame terrace deposits were found underlain by a lodgmont till deposit in Test Pits 5 and 7 at a depth of 5.0 and 9.0 feet, respectively. GROUNDWATER CONDITION LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 6 Groundwater seepage was not encountered by any of the seven test pits excavated on the subject plat site. The topsoil, ablation till, and weathered and fresh Kame terrace deposits are loose to medium -dense, and would allow some storm runoff to seep into the ground. The underlying dense to weakly -cemented lodgmont till deposit is of extremely low permeability and would perch stormwater infiltrating into the more permeable surficial soils. The amount of and the depth to this perched groundwater would fluctuate seasonally, depending on precipitation, surface runoff, ground vegetation cover, site utilization, and other factors. The perched groundwater may dry up completely during the dryer summer months and accumulate and rise in the wet winter months. The test pits excavated in the heart of winter did not encountered any groundwater. Therefore, it is our opinion that groundwater "' K. " under the subjectgp3' 1 J plat site _ : 1 should be minimal and minor even in the winter months of the year. P $i " " " Y DISCUSSIONS AND RECOMMENDATIONS 11401` '7 11 Based on the soil conditions encountered by the test pits excavated on the site, it is our opinion that the site is suitable for the proposed development from the geotechnical engineering viewpoint, provided that the recommendations in this report are fully implemented and observed during construction. Due to moisture -sensitive fine-grained soils mantling the site and the local areas within the site, we recommend that grading and foundation construction work for the proposed development be carried out and completed during the dryer period from April 1 through October 31. If grading work has to proceed beyond the above dryer period, the measures for slope stabilization, erosion mitigation, and surface and ground water drainage control recommended in this report should be in place and operational on a daily basis during construction. Surficial unsuitable soils, including topsoil and loose to medium -dense weathered soils mantling the site, should be stripped down to the medium -dense to dense, fresh Kame terrace and/or lodgmont till soils within the building pads of the lots and the roadways. The underlying fresh LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 7 Kame terrace and/or lodgmont till soils are of high to moderately -high to high strength and are capable of providing adequate foundation support to the proposed residential buildings, roadways and stormwater detention vault. Conventional footing foundations constructed on or into the above competent basal soils may be used to support the proposed residential building and stormwater detention vault. Structural fill, if required for site grading, should be placed on proof -rolled, underlying undisturbed, competent basal soils following the stripping of the surficial unsuitable soils. GEOLOGIC HAZARDS AND REMEDIATIN Landslide Hazard The medium -dense to dense, fresh Kame terrace and lodgmont till soils underlying the site at shallow depth are of moderately -high to high shear strength and have good to excellent resistance against slope failures. Therefore, it is our opinion that the potential for deep-seated landslides to occur on the site should be minimal, provided the recommendations in this report are fully implemented and complied with during construction. Erosion Hazard The topsoil and loose to medium -dense weathered soils are of low resistance against erosion. Erosion may occur in the weaker surficial soils over the 1 4_ areas of the site if they are devoid of vegetation cover and overly saturated. Progressive erosion can lead to shallow, skin -type mudflows in the 64%4 f areas of the site. To mitigate such erosion hazard, vegetation outside of construction limits should be preserved and maintained. Unpaved exposed ground within the site resulted from construction activities should be re -seeded and re -vegetated as soon as possible. Concentrated stormwater should not be discharged uncontrolled onto the ground within the site. Stormwater over impervious surfaces, such as roofs and paved roadways and driveways, should be captured by underground drain line systems connected to roof downspouts or by catch basins installed in paved roadways and driveways. Water collected by these drain line systems should be tightlined to discharge into a storm sewer or a suitable LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&.A Job No. 9AO06 Page 8 stormwater disposal facility. Areas devoid of vegetation cover should be re -seeded and re - vegetated as soon as possible, and should be covered with clear plastic sheets until the vegetation is fully established. Seismic Hazard and Design Consideration The Puget Sound region is in an active seismic zone. The subject site is underlain at shallow depth by medium -dense to dense, fresh Kame terrace and lodgmont till soils of moderately -high to high shear strength. There is a lack of continuous, extensive, static groundwater table at shallow depth under the site. The combination of the above makes it rather unlikely for seismic hazards, such as landslides, liquefaction or soil lateral spreading, to occur on the site during strong earthquakes. Therefore, the seismic hazard of the site should be minimal. The residential buildings to be constructed on the site, however, should be designed to withstand seismic forces induced by strong earthquakes. Based on the soil conditions encountered by the test pits, it is our opinion that Seismic Use Group I and Site Class D should be used in the seismic design of the proposed residential buildings in accordance with the 2006 International Building Code (IBC). SITE PREPARATION AND GENERAL GRADING Vegetation within construction limits should be cleared and grubbed. Loose surficial soils, including topsoil and loose weathered soils, should be completely stripped down to the medium - dense to dense, fresh Kame terrace and/or lodgmont till soils within the building pads of the proposed residential buildings and roadways. The exposed soils should be compacted to a non - yielding state with a vibratory compactor and proof -rolled with a piece of heavy earthwork equipment. TEMPORARY EROSION CONTROL The onsite surficial soils contain a high percentage of fines, and are sensitive to moisture and can be disturbed easily by construction traffic when saturated. A layer of clean, 2 -to -4 -inch, quarry LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 9 spalls should be placed over excavated areas and areas of frequent traffic, such as the entrance to the site, as required, to protect the subgrade soils from disturbance by construction traffic. Silt fences should be installed along the downhill sides of construction areas to prevent sediment from being transported onto adjoining properties or streets by storm runoff. The bottom edge of silt fences should be folded inward and ballasted with onsite soils. Ditches or interceptor trench drains should be installed on the uphill sides of construction areas, as required, to intercept and drain away storm runoff and near -surface groundwater seepage. Water captured by such ditches or interceptor trench drains should be discharged into onsite detention/settling ponds or nearby storm inlets. The storm inlets, if into which stormwater is to be to be discharged, should be covered with a filter sock to prevent sediment from entering the storm sewer system. The filter socks should be cleaned frequently during construction to prevent clogging, and should be removed after completion of construction. EXCAVATION AND FILL SLOPES Under no circumstance should excavation slopes be steeper than the limits specified by local, state and federal safety regulations if workers have to perform construction work in excavated areas. Unsupported temporary cuts greater than 4 feet in height should be no steeper than 1 H:1 V in topsoil, loose to medium -dense weathered Kame terrace and ablation till soils, no steeper than 3/4H:1 V in medium -dense to dense Kame terrace deposits and may be vertical in lodgment till soils if the overall depth of excavation is no more than 15 feet. Otherwise, temporary cut in lodgment till should be no steeper than 1/2H:IV. Unsupported permanent cuts should be no steeper than 211: I V in topsoil, loose to medium -dense weathered Kame terrace and ablation till soils, no steeper than 1-3/414:1V in medium -dense to dense Kame terrace deposits, and no steeper than 1-1/211:1 V in lodgment till soils. A 5 -foot -wide level bench should be built into temporary or permanent cut slopes to keep the vertical rise between the benches no more than 15 feet. The soil units into which cut slopes and LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No, 9A006 Page 10 the stability of the cut slopes are to be made should be verified by a geotechnical engineer during excavation. Permanent fill embankments required to support structural or traffic load should be constructed with compacted structural fill placed over undisturbed, proof -rolled, firm, native, fresh Kame terrace and/or lodgment till soils after the surficial unsuitable soils are completely stripped. Permanent fill to be placed over slopes steeper than 15 percent grade should be retained structurally. The exposed ground exceeding 15 percent grade should be benched with vertical steps not exceeding 4 feet tall after stripping of surficial unsuitable soils and prior to placing structural fill. The slope of permanent rill embankments should be no steeper than 2H:1 V. Upon completion, the sloping face of permanent fill embankments should be thoroughly compacted to a non -yielding state with a hoe -pack. The above recommended cut and fill slopes are under the assumption that groundwater seepage would not be encountered during construction. If groundwater is encountered, the cut and fill earthwork should be immediately halted and the slope stability re-evaluated. The slopes may have to be flattened and other measures taken to stabilize the slopes. Stormwater should not allowed to flow uncontrolled over cut and fill slopes. Permanent cut slopes or fill embankments should be seeded and vegetated as soon as possible for erosion protection and long-term stability, and should be covered with clear plastic sheets, as required, to protect them from erosion until the vegetation is fully established. STRUCTURAL FILL Structural fill is the fill that supports structural or traffic load. Structural fill should consist of clean granular soils free of organic, debris and other deleterious substances and with particles not larger than three inches. Structural fill should have a moisture content within one percent of its optimum moisture content at the time of placement. The optimum moisture content is the water LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 11 content in the soils that enable the soils to be compacted to the highest dry density for a given compaction effort. Onsite clean silty sand to gravelly sand soils, meeting the above requirements, may be used as structural fill. Imported material to be used as structural fill should be clean, free -draining, granular soils containing no more than 5 percent by weight finer than the No. 200 sieve based on the fraction of the material passing No. 4 sieve, and should have individual particles not larger than three inches. The ground over which structural fill is to be placed should be prepared in accordance with recommendations in the SITE PREPARATION AND GENERAL GRADING and EXCAVATION AND FILL SLOPES sections of this report. Structural fill should be placed in lifts no more than 10 inches thick in its loose state, with each lift compacted to a minimum percentage of the maximum dry density determined by ASTM D1557 (Modified Proctor Method) as follows: Application Within building pads and under foundations Roadway/driveway subgrade Retaining/foundation wall backfill Utility trench backfill STORMWATER DETENTION VAULT of Maximum Dry Density 95% 95% for top 3 feet and 90% below 92% 95% for top 4 feet and 90% below An underground concrete detention vault ed e43 pgspXtt= is to be constructed at the northwest corner of the site to store stormwater collected over impervious surfaces of the proposed development of the site. Two test pits (Test Pits 5 and 6) were excavated within the footprint of the proposed vault, and these test pits encountered dense LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9A006 Page 12 lodgment till soils and/or medium -dense to dense fresh Kame terrace deposit at depths about 4.0 to 5.0 feet. The vault may be supported on footings founded on these competent basal soils. An allowable soil bearing pressure not exceeding 3,500 psf may be used for the design of the vault footing foundations. A drain line consisting of perforated, rigid PVC, drain pipe or slotted, corrugated ADS, drain pipe, at least 6 inches in diameter, should be installed at a few inches below bottom of the perimeter footings of the vault walls to intercept and drain away groundwater which may flow towards the vault. The drain line should have sufficient slope (0.5% minimum) to generate flow by gravity, and water collected in the drain line should be tightlined to discharge into a storm sewer or a suitable stormwater disposal facility. The vault footing drain line should be completely embedded in washed gravel wrapped in a layer of non -woven filter fabric, such as 140N by Miraf Inc. or approved equal. A vertical drainage blanket at least 12 inches thick horizontally, consisting of clean 3/4 to 1 -1/2 -inch washed gravel or crushed rock, should be placed against the perimeter vault walls. The remaining backfill should be constructed of structural fill. Alternatively, a vertical drain mat, such as Miradrain 6000 by Mirafi Inc. or equivalent, may be placed against the perimeter vault walls as the vertical drainage blanket. The vertical drainage blanket or drain mat should be hydraulically connected to the drain line at the base of the vault perimeter walls. Sufficient number of cleanouts at strategic locations should be installed for periodical cleaning of the vault wall drain line to prevent clogging. The perimeter walls of the detention vault would also serve as retaining walls to support cut banks and backfill. The perimeter walls of the vault capped with a lid would be restrained at their top from horizontal movement and should be designed for at -rest lateral soil pressure. For the condition that groundwater behind the perimeter vault walls can be fully drained by the drain line provided at the base of the walls, we recommend an at -rest soil pressure of 50 pcf equivalent fluid density (EFD) be used for the design of vault perimeter walls. To counter the at -rest soil pressure, a passive lateral soil pressure of 375 pcf EFD may be used, except that the passive LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L.&A Job No. 9AO06 Page 13 pressure within the top 12 inches of the finish subgrade should be ignored. The above passive pressure assumes the backslope of the walls is level or ascending away from the walls. The at - rest soil pressure may also be resisted by the friction force between the footings and the subgrade soils based on a coefficient of friction of 0.55. If the site grades are such that it is not feasible to completely drain groundwater behind the vault walls with a gravity drain line system, the hydrostatic pressure on the perimeter vault walls should also be taken into consideration for the design of the vault perimeter walls. For the condition that a perimeter drain line has to be placed higher than the footing level, the perimeter vault walls should be designed for a lateral soils pressure of 50 pcf EFD above the drain line level and a combined lateral soil and hydrostatic pressure of 80 pcf EFD below the drain line level. The above lateral pressures on the walls may be countered by a passive soil pressure of 375 pcf EFD above the drain line and 210 pcf EFD below. The detention vault should also be designed for seismic loading based on a 100 -year seismic event. For seismic design of the detention vault walls, a pseudo static soil pressure diagram of inverted triangle from the finished ground level to the bottom of the foundations should be used. Based on the soil conditions in the detention vault area, we recommend the lateral soil pressure at the top of the triangle be 8H psf for a 100 -year seismic event, where H is the height from finish grade over top of the vault to bottom of footings in feet. A one-third increase in the above recommended allowable soil bearing pressure may be used when considering the seismic loading condition. The above design parameters are unfactored ultimate values. Proper factors of safety should be applied for the design of the vault walls against sliding and overturning failures. PAVED ROADWAYS AND DRIVEWAYS LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9A006 Page 14 Performance of roadway and driveway pavement is critically related to the conditions of the underlying subgrade soils. We recommend that the subgrade soils under the roadways/driveways be treated and prepared as described in the SITE PREPARATION AND GENERAL EARTHWORK section of this report. Prior to placing base material, the subgrade soils should be compacted to a non -yielding state with a vibratory roller compactor and proof -rolled with a piece of heavy construction equipment, such as a fully -loaded dump truck. Any areas with excessive flexing or pumping should be over -excavated and re -compacted or replaced with structural fill or crushed rock placed and compacted in accordance with the recommendations provided in the STRUCTURAL FILL section of this report. We recommend that a layer of compacted, 7/8 -inch crushed rock base (CRB), be placed for the roadways/driveways. This crushed rock base should be at least 6 inches for the public roadways and 4 inches for the private driveways. The crushed rock base should be overlain with a 3 -inch asphalt treated base (AT13) topped by a 2 -inch -thick Class B asphalt concrete (AC) surficial course for public roadways and overlain by a 3 -inch -thick Class B asphalt concrete (AC) surficial course for the private driveways. BUILDING SETBACK The purpose of building setback from the top or toe or an overly steep portion of a slope is to establish a safe buffer such that if a slope failure should occur the stability of the structure can be maintained and damages to the structure minimized. In general, the greater the setback, the lower the risk for the structure to sustain damages from a slope failure. To maintain stability, the residential buildings to be constructed on the site should be sufficiently setback from the top or toe of slopes of 40% gradient or more. We recommend the buildings be set back at least 15 feet from top or toe of slopes with grades 40% or more. If footing foundations are used to support the new residences of future development, the footing foundations should be embedded at least one foot into the underlying, medium -dense to dense, LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A .lob No. 9AOO6 Page 15 fresh Kame terrace or lodgment till soils. if the buildings are to be constructed on the slopes with grades 40% or more, the footing foundations should be extended downward to such elevation that the horizontal distance from the edge of footings to the face of slopes should be at least 15 feet, and that a plane drawn from the edge of footings to the toe of slopes of 40% or more gradient should be no steeper than 2.5H:1 V. BUILDING FOUNDATIONS Conventional footing foundations may be used to support the residential buildings to be constructed on the site. The footing foundations should be placed on or into the underlying, medium -dense to dense, fresh Kame terrace or lodgmont soils, or on structural fill placed over these undisturbed competent basal soils. Water should not be allowed to accumulate in excavated footing trenches. Disturbed soils in footing trenches should be completely removed down to undisturbed, competent basal soils and the basal soils should be thoroughly compacted to a non -yielding state with a vibratory mechanical compactor prior to pouring concrete for the footings. If the above recommendations are followed, our recommended design criteria for footing foundations are as follows: The allowable soil bearing pressure for design of footing foundations, including dead and live loads, should be no greater than 3,000 psf if constructed on or into native, undisturbed, competent basal soils, and no greater than 2,500 psf if constructed on structural fill placed over competent basal soils. The footing bearing soils should be verified by a geotechnical engineer after the footing trenches are excavated and before the footings poured. LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 16 The minimum depth to bottom of perimeter footings below adjacent final exterior grade should be no less than 18 inches. The minimum depth to bottom of the interior footings below top of floor slab should be no less than 12 inches. The minimum width should be no less than 16 inches for continuous footings, and no less than 24 inches for individual footings, except those footings supporting light -weight decks or porches. A one-third increase in the above recommended allowable soil bearing pressure may be used when considering short-term, transitory, wind or seismic loads. For footing foundations designed and constructed per recommendations above, we estimate that the maximum total post - construction settlement of the buildings should be 3/4 inch or less and the differential settlement across building width should be 1/2 inch or less. Lateral loads on the proposed residential buildings may be resisted by the friction force between the foundations and the subgrade soils or the passive earth pressure acting on the below -grade portion of the foundations. For the latter, the foundations must be poured "neat" against undisturbed soils or backfilled with a clean, free -draining, compacted structural fill. We recommend that an equivalent fluid density (EFD) of 325 pcf (pounds per cubic foot) for the passive earth pressure be used for lateral resistance. The above passive pressure assumes that the backfill is level or inclines upward away from the foundations for a horizontal distance at least 1.5 times the depth of the foundations below the final grade. A coefficient of friction of 0.55 between the foundations and the subgrade soils may be used. The above soil parameters are unfactored values, and a proper factor of safety should be used in calculating the resisting forces against lateral loads on the new garage. SLAB -ON -GRADE FLOORS Slab -on -grade floors, if used for the residential buildings to be constructed on the site, should be placed on firm subgrade soils prepared as outlined in the SITE PREPARATION AND LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9A006 Page 17 GENERAL EARTHWORK and the STRUCTURAL FILL sections of this report. Where moisture control is critical, the slab -on -grade floors should be placed on a capillary break which is in turn placed on the compacted subgrade. The capillary break should consist of a minimum four -inch -thick layer of clean, free -draining, 718 -inch crushed rock, containing no more than 5 percent by weight passing the No. 4 sieve. A vapor barrier, such as a 6 -rail plastic membrane, may be placed over the capillary break, as required, to keep moisture from migrating upwards. BASEMENT AND CIP CONCRETE RETAINING WALLS Building basement walls would be required to support backfill. Cast -in-place concrete walls may be used to retain fill embankments along the west and part of the south boundaries of the site. Basement walls restrained horizontally at the top are considered unyielding and should be designed for a lateral soil pressure under the at -rest condition; while cast -in-place concrete retaining walls free to move at the top should be designed for active lateral soil pressure. We recommend that a lateral soil pressure of 55 put EFD be used for the design of basement walls restrained at the top; and 40 pef EFD for retaining walls unrestrained at the top. These lateral soils pressures are applicable to walls with level backslope. With a backslope rising away from the walls, an additional pressure of 0.75 pcf per degree of angle of the backslope above horizontal should be added to the above pressures. To counter the active soil or at -rest pressure, a passive lateral soil pressure of 300 pcf EFD may be used, except that the passive pressure within the top 12 inches of the finish subgrade should be ignored. The above passive pressure is applicable to walls with level backslope. The above lateral soil pressures are under the assumption that groundwater behind the walls is fully drained. To resist against sliding, the friction force between the footings and the subgrade soils may be calculated based on a coefficient of friction of 0.55. The above soil parameters are ultimate values, and proper factors of safety should be used in the design of the basement and retaining walls against sliding and overturning failures. Basement walls or retaining walls may be supported on footing foundations seated on or into the underlying, medium -dense to dense, fresh Kame terrace or lodgment till soils, with an allowable soil bearing pressure not to exceed 3,000 psf. LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 18 A drain line consisting of perforated, rigid PVC, drain pipe or slotted, corrugated ADS, drain pipe, at least 4 inches in diameter, should be installed at a few inches below bottom of basement or retaining walls to intercept and drain away groundwater flowing towards the walls. The drain lines should have sufficient slope (0.5 percent minimum) to generate flow by gravity, and water collected in the drain line should be tightlined to discharge into a storm sewer or a suitable stormwater disposal facility. The drain lines should be completely embedded in washed gravel wrapped in a layer of non -woven filter fabric, such as 140N by Mirafi Inc. or approved equal. A vertical drainage blanket at least 12 inches thick, consisting of clean, 3/4 to 1 -112 -inch, washed gravel or crushed rock, should be placed against the walls. Alternatively, a vertical drain mat, such as Miradrain 6000 by Mirafi Inc. or equivalent, may be placed against the walls as the vertical drainage blanket. The vertical drainage blanket or drain mat should be hydraulically connected to the drain lines at the base of the perimeter walls. ROCKERY WALLS General Rockery walls may be used to line the cut banks along the east boundary of the site. Rockery by nature is not an engineered retaining wall, such as a reinforced concrete wall. Although a rockery wall can provide some degree of retention capability, its main function is to serve as a protective facing to retard weathering and erosion process to the earth bank behind the rockery. To achieve a satisfactory rockery wall, the earth bank behind the wall must be stable by itself on a long-term basis. In addition, the rockery wall should be constructed in a proper way to assure long -terra stability of the cut bank or fill embankment. The medium -dense to dense, fresh Kame terrace and/or lodgment till deposit underlying the site at shallow depth is of moderately -high to high shear strength. It is our opinion that cut banks in these soil deposits will be able to maintain long-term stability if lined by properly constructed rockery. Our design of rockery walls lining cut banks in these competent deposits is shown on Plate 8 attached hereto. LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9A006 Page 19 Rockery Material and Construction Rock material used for rockery construction should be hard, well -cemented, sound, durable and free of cracks, fissures, joints, air holes and other defects. Construction of rockery walls should be in compliance with the Standard Rockery Construction Guidelines published by the Association of the Rockery Contractors. Subgrade and Keyway Trench Preparation Construction of the rockery walls should start immediately following completion of cut banks and keyway trenches. The keyway trenches should be cut into medium -dense to dense, fresh Kame terrace and/or lodgment till soils capable of rendering an allowable bearing capacity of at least 3,000 psf. The keyway trenches should be at least 12 inches deep below the finish grade in front of the rockery wall and should be wide enough such that the heel of the keyway trenches would be at least 12 inches from the back of the base -course facial stones. Keyway trenches should be free of loose disturbed soils or standing water and the exposed soils at bottom of keyway trenches should be compacted to a non -yielding state with a vibratory mechanical compactor prior to rockery installation. Groundwater Drainage Control A drain line should be placed in the keyway trenches behind the base -course facial stones to collect and drain away groundwater flowing toward the rockery walls. The drain lines should consist of minimum 6 -inch, perforated, rigid, PVC pipes, wrapped in a filter fabric sock. The bottom of the keyway trench and the drain line should have sufficient slope (0.5 percent minimum) to generate flow by gravity. Water collected in the drain lines should be tightlined to discharge into a storm sewer or a suitable stormwater disposal facility. Facial Stones Facial stones of the rockery walls should be as nearly rectangular as possible with the long dimension of the stones placed perpendicular to the wall alignment. Facial stones should be LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 20 stacked tightly against one another to minimize voids between the stones. Excessive openings between the facial stones should be chinked with smaller rock from behind. The rockery walls should be constructed such that the facial stones of each successive course would be staggered over and firmly supported on stones of the previous course. Facial stones of the rockery walls should be tilted back at an inclination no steeper than 6V:1 H. Drain Rock Course A drain rock course, consisting of 2 -to -4 -inch rock spalls, should be installed between the facial stones and the cut bank in lifts as each course of the facial stones is completed. The horizontal thickness of the drain rock course should be at least 12 inches. The purpose of the drain rock course is to retain soils in place while allowing groundwater to bleed out. PRECAST CONCRETE BLOCK WALLS General Precast concrete block walls may be used to support fill embankments to be constructed along the west and part of the south boundaries of the site. These walls may be constructed with commercially available precast concrete blocks, such as by Keystone, Lock Block or Redi Rock. Loose topsoil and loose to medium -dense weathered soils within the keyway trenches of the walls should be over -excavated down to medium -dense to dense, fresh Kame terrace and/or lodgment till soils. The block walls should be constructed with geogrid mesh reinforcement anchored to the block walls and embedded in the wall backfill. The wall should be founded on a minimum 4 -inch layer, 718 -inch crushed rock, leveling base, placed over firm undisturbed bearing soils capable of rendering an allowable bearing capacity of at least 3,000 psf. A vertical drainage blanket should be placed against the back of the concrete block facing and hydraulically connected to the drain lines at the base of the walls. The remaining wall backfill behind the vertical drainage blanket should consist of compacted structural fill consisting of clean granular soils. LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 21 Design Soil Parameters We recommend that the precast concrete block walls to be constructed to support fill embankments be designed for a fully drained condition in accordance with the following soil perimeters: The block walls should be designed for a minimum factor of 1.5 against sliding failure and 1.7 against overturning failure under the static loading condition. The Puget Sound region is in an active seismic zone and the block walls should also be designed under the seismic loading condition for a 100 -year seismic event. The peak ground acceleration is about 0.3g (g — gravity force) for such an event in the Puget Sound region. The block walls, however, are built with interlocking concrete blocks with relatively high flexibility, and the blocks do not move in unison during earthquakes. Therefore, for design of the block wail under the seismic loading condition, the ground acceleration may be reduced to 0.2g. The block walls should designed for a factor of safety of at least 1.15 against sliding and overturning failures under the seismic loading condition. Heavy equipment or material storage should not be allowed within 10 feet of the block wails; otherwise, the walls should be designed for 250 psf of uniform load. Construction of Precast Concrete Block Walls Vegetation within construction limits of the block walls and its backfill should be cleared and roots thoroughly grubbed. Unsuitable surficial soils, such as topsoil and loose to medium -dense weathered sand soils within the block walls and backfill footprint should be stripped down to the LIU & ASSOCIATES, INC. Reinforced Retained Foundation Leveling Soils Soils Soils Rock Base Unit Weight, r, pef 130 120 135 135 Angle of Internal Friction, 36 33 36 40 O, degrees Cohesion, c, psf 0 0 0 0 The block walls should be designed for a minimum factor of 1.5 against sliding failure and 1.7 against overturning failure under the static loading condition. The Puget Sound region is in an active seismic zone and the block walls should also be designed under the seismic loading condition for a 100 -year seismic event. The peak ground acceleration is about 0.3g (g — gravity force) for such an event in the Puget Sound region. The block walls, however, are built with interlocking concrete blocks with relatively high flexibility, and the blocks do not move in unison during earthquakes. Therefore, for design of the block wail under the seismic loading condition, the ground acceleration may be reduced to 0.2g. The block walls should designed for a factor of safety of at least 1.15 against sliding and overturning failures under the seismic loading condition. Heavy equipment or material storage should not be allowed within 10 feet of the block wails; otherwise, the walls should be designed for 250 psf of uniform load. Construction of Precast Concrete Block Walls Vegetation within construction limits of the block walls and its backfill should be cleared and roots thoroughly grubbed. Unsuitable surficial soils, such as topsoil and loose to medium -dense weathered sand soils within the block walls and backfill footprint should be stripped down to the LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 22 medium -dense to dense, fresh Kame terrace and/or lodgment till soils. Over -excavation down to these competent basal soils should be backfilled with structural fill. The keyway trench for the block walls should be cut into native, undisturbed, medium -dense to dense, fresh Kame terrace and/or lodgment till soils, capable of rendering an allowable bearing pressure of at least 3,000 psf. The soils exposed at bottom of the keyway trench should be compacted to a non -yielding state with a vibratory compactor. A minimum 4 -inch layer of 7/8 - inch -minus crushed rock leveling base, compacted to a noxi -yielding state, should be placed over firm subgrade soils supporting the block walls. The base -course blocks are to be placed on this crushed rock base with an embedment at least 10 inches below the adjacent finish grade in front of the block wall. The precast concrete blocks should be stacked tightly against one another. A minimum 6 -inch perforated, rigid, PVC drain line fitted in a non -woven filter fabric sock should be laid in the keyway trenches behind the base -course blocks. The bottom of keyway trenches and the drain line should have sufficient slope (0.5 percent minimum) to generate flow by gravity. The drain pipes should be tightlined to discharge into a storm sewer or a suitable stormwater disposal facility. A minimum 10 -inch -thick (horizontally) vertical drainage blanket, constructed of clean 3/4 to 1 -1/2 -inch washed gravel crushed rock, should be placed against the back of the block wall facing. The vertical drainage blankets should be hydraulically connected to the drain lines at the base behind the block walls. The wall backfill behind the vertical drainage blankets should consist of structural fill. The vertical drainage blanket and structural fill should be constructed in lifts after each course of blocks is completed. Each layer of geogrid mesh should be laid on level backfill surface, with one end securely anchored between two rows of concrete blocks, stretched tight, and the other end staked down prior to the placement of the next lift of wall backfill. Overlaps of geogrid mesh in the direction of the wall alignments should be at least 12 inches, overlaps in the direction perpendicular to the wall alignments should not be allowed. LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 23 DRAINAGE CONTROL Onsite Stormwater Disposal The moderately -sloped mid-section and the low-lying western portion of the subject plat site are underlain by fresh Kame terrace deposit of clean, gravelly, fine to medium sand deposit and infiltration of stormwater into this soil stratum is feasible for the residential buildings to be constructed in these areas. The impervious lodgmont till deposit, however, was found underlying Tract A", located at the northwest corner of the site, at a depth as less as 5 feet below the existing grade. Therefore, we do not recommend a concentrated infiltration facility in "Tract A" to dispose a large amount of stormwater into the ground. This is because the large amount of concentrated disposed stormwater in this tract may increase the potential of groundwater seepage downhill off the subject site where the lodgmont till may be exposed or near the ground surface. Increased groundwater seepage may elevate the possibility of erosion, sloughing and landlside. Appendix C - Small Project Drainage Requirements, of the King County, Washington Surface Water Design Manual, January 24, 2005, allows stormwater collected on 2,000 or more but not more than 10,000 square feet of impervious surfaces of small residential projects to be disposed onsite with infiltration or dispersion method. The "full infiltration" method specified in Section C.2.2 —.Full Infiltration of the above design manual can be used on sites underlain by medium sand or coarse sand or gravel and cobble. The moderately -sloped mid-section and the low-lying western portion of the subject plat site are underlain by fresh Kame terrace deposit of clean gravelly, fine to medium sand deposit and are qualified for using Full Infiltration method to dispose stormwater for the residential buildings in these areas. According to the above design manual, infiltration trenches installed into clean gravelly sand soils should be designed to be at least 30 feet long for every 1000 s.£ of impervious surface served (with a 2 -foot -wide trench). The length of infiltration trenches may be shortened proportionally with increased trench width. LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9A006 Page 24 The infiltration trenches should be set back at least 10 feet from adjacent buildings and 5 feet from property lines. The schematic presentation of an infiltration trench with a single dispersion pipe to serve the residential buildings is shown on Plate 9. The infiltration trenches should be at least 24 inches wide. The bottom of infiltration trenches should be excavated at least 6 inches into the underlying clean, fresh, gravelly sand Kame terrace deposit. The side walls of the trenches should be lined with a layer of non -woven filter fabric. The trench is then filled with clean 3/4 to 1 -112 -inch washed gravel or crushed rock to within about 8 inches of the finish grade. The' dispersion pipes should be constructed of 4 -inch rigid or 6 -inch flexible perforated PVC pipes and laid level in the gravel or crushed rock filled trenches at about 16 inches below the top of trenches. The top of the gravel or crushed rock fill should also be covered with the filter fabric liner. The remaining trench should then be filled with compacted on-site soils. Stormwater captured over impervious surfaces should be routed into a sediment control structure/oil-water separator structure before being released into the infiltration trenches. Building Footprint Excavation Building footprint excavation for the proposed residential buildings, if encountering groundwater seepage, should have the bottom of excavation sloped and ditches excavated along bases of the cut banks to direct collected groundwater into sump pits from which water can be pumped out. A layer of 2 -inch crushed rock should be placed over footing bearing subgrade soils, as required, to protect the soils from disturbance by construction traffic. This crushed rock base should be built to a few inches above groundwater level, but not less than 6 inches thick. The crush rock base should be compacted in 12 -inch lifts to a non -yielding state with a vibratory mechanical compactor. Runoff over Impervious Surfaces Storm runoff over impervious surfaces, such as roofs and paved roadways/driveways, should be collected by underground drain line systems connected to downspouts and by catch basins LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AOO6 Page 25 installed in paved roadways/driveways. Stormwater thus collected should be tightlined to discharge into a storm sewer or suitable stormwater disposal facilities. Building Footing Drains A subdrain should be installed, around the perimeter footings of each of the residential buildings. The subdrains should consist of a 4 -inch -minimum -diameter, perforated, rigid, drain pipe, laid a few inches below bottom of the perimeter footings of the buildings. The trenches and the drain lines should have a sufficient gradient (0.5% minimum) to generate flow by gravity. The drain lines should be wrapped in a non -woven filter fabric sock and completely enclosed in clean washed gravel. The remaining trenches may be backfilled with clean onsite soils. Water collected by the perimeter footing subdrain systems should be tightlined, separately from the roof and surface stormwater drain lines, to discharge into a storm sewer or suitable stormwater disposal facility. Surface Drainage Water should not be allowed to stand in any areas where footings, on -grade slabs, or pavement is to be constructed. Finish ground surface should be graded to direct surface runoff away from the residential buildings. We recommend the finish ground be sloped at a gradient of 3 percent minimum for a distance of at least 10 feet away from the buildings, except in the areas to be paved. Cleanouts Sufficient number of cleanouts at strategic locations should be provided for underground drain lines. The underground drain lines should be cleaned and maintained periodically to prevent clogging. LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat I..&A Job No. 9AO06 Page 26 RISK EVALUATION STATEMENT The subject site is underlain at shallow depth by medium -dense to dense Kame terrace and/or lodgmont till soils. These soils are of moderately -high to high shear strength and gutaler was not Jenctiun a etl . iy the lest ;fits excav to l;vn lute 11.1 site r Therefore, the bite should be quite stable. The key to maintain site stability during and after completion of construction is to have proper and adequate erosion and drainage controls. It is our judgment that provided the recommendations in this report are fully implemented and observed during construction, the areas disturbed by construction will be stabilized and will not increase the potential for soil movement. In our opinion, the risk of damage to the proposed development and from the development to adjacent properties due to soil instability should be minimal. LIMITATIONS This report has been prepared for the specific application to this project for the exclusive use by Mr. ].t6`bert Nilson `incl his associates, consultants and contractors. We recommend that this report, in its entirety, be included in the project contract documents for the information of the prospective contractors for their estimating and bidding purposes and for compliance with the recommendations in this report during construction. The conclusions and interpretations in this report, however, should not be construed as a warranty of the subsurface conditions. The scope of this study does riot include services related to construction safety precautions and our recommendations are not intended to direct the contractor's methods, techniques, sequences or procedures, except as specifically described in this report for design considerations. Our recommendations and conclusions are based on the geologic and soil conditions encountered in the test pits, and our experience and engineering judgment. The conclusions and recommendations are professional opinions derived in a manner consistent with the level of care and skill ordinarily exercised by other members of the profession currently practicing under similar conditions in this area. No warranty, expressed or implied, is made. LIU & ASSOCIATES, INC. February 20, 2009 Wilson Plat L&A Job No. 9AO06 Page 27 The actual subsurface conditions of the site may vary from those encountered by the test pits excavated on the site. The nature and extent of such variations may not become evident until construction starts. If variations appear then, we should be retained to re-evaluate the recommendations of this report, and to verify or modify them in writing prior to proceeding further with the construction of the proposed development of the site. CLOSURE We are pleased to be of service to you on this project. Please feel free to call us if you have any questions regarding this report or need further consultation. Nine plates attached Yours very truly, LIU & ASSOCIATES, INC. J. S. {Julian) Liu, MD., P.E. Consulting Geotechnical Engineer LIU & ASSOCIATES, INC, SECTION 7 OTHER PERMITS SECTION 8 ESC ANALYSIS AND DESIGN SECTION 9 BOND QUANTITIES, FACILITY SUMMARIES, AND DECLARATION OF COVENANT KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL STORMWATER FACILITY SUMMARY SHEET Development Date Location ENGINEER DEVELOPER Name Name Firm Firm Address Address Phone Phone Developed Site: Acres Number of lots Number of detention facilities on site: Number of infiltration facilities on site: ponds ponds vaults vaults tanks tanks Flow control provided in regional facility (give location) No flow control required Exemption number JV rii.]L14Q1L1 LiSLlLlu r iauua.a Immediate Major Basin Basin A Basin B Basin C Basin D. Number & type of water quality facilities on site: biofiltration swale (regular/wet/ or continuous inflow?) combined detentionfWQ pond WQ portion basic ar large?) combined detention/wetvault compost filter filter strip flow dispersion farm management plan landscape management plan oil/water separator (baffle or coalescing plate?) catch basin inserts: Manufacturer pre -settling pond pre -settling structure: Manufacturer flow -splitter catchbasin sand filter (basic or large?) sand filter, linear (basic or large?) sand filter vault (basic or large?) stormwater wetland wetpond (basic or large?.) wetvault 1998 Surface Water Design Manual 91119 ICING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL DESIGN INFORMATION, cont'd TOTAL INDIVIDUAL BASIN Drainage basin(s) Onsite area A 13 C D Offsite area 1)2e of Storage Facility Live Storage Volume PredeveIo ed Runoff Rate 2 -year 10 -year 100 -year Developed runoff rate 2 -year 10- year Type of restrictor 100 -year Size of orificehestriction No. I No. 2 No. 3 No. 4 FLOW CONTROL & WATER QUALITY FACILITY SUMMARY SHEET SKETCH All detention, infiltration and water quality facilities must include a sketch per the following criteria: 1. Heading for the drawings should be located at the top of the sketch (top right-hand corner). The heading should contain: North arrow (point up or to left) * D9# Plat name or short plat number • Address (nearest) Date drawn (or updated) • Thomas Brothers page, grid number 2. Label CBs and MHs with the plan and profile designation. Label the control structure in writing or abbreviate with C -S. Indicate which structures provide spill control. 3. Pipes --indicate: Pipe size Pipe length Flow direction Use s single heavyweight line 4. Tanks-- use a double, heavyweight line and indicate size (diameter) 5. Access roads Outline the limits of the road Fill the outline with dots if the road is gravel. Label in writing if another surface. b. Other Standard Symbols: Bollards: 0 0 0! 12 r d E Rip rap 000000 000000 Fences --x---x---x---x---x---x--- Ditches—D----}p_--D_--D 7. Label trash racks in writing. S. Label all streets with the actual street sign designation. If you don't know the actual street name, consult the plat map. 9. Include easements and lot lines or tract limits when possible. 10. Arrange all the labeling or writing to read from left to right or from bottom to top with reference to a properly oriented heading. 11. Indicate driveways or features that may impact access, maintenance or replacement. 911199 1998 Surface Water Design Manual 2 SECTION 10 OPERATIONS AND MAINTENANCE MANUAL OJ i SS 9':'41Ni3 - Y,,79 -3 -.1 C) -- I 7VM yi,)O ra 313 J -i 9,164+/—SQ. F 99 285.86' Cr-- CD Q) tl 55.00' 55,00' 66,41' Ln CL C) n uj LL - CKD < t- Ln C- r -n Lo oC Cl- LQ c) c N L.Lj Rei cn CC D CD n CD CD CD C) -- I 7VM yi,)O ra 313 J -i 9,164+/—SQ. F 99 285.86' C14 ROAD TO BE CONSTRUCTED PROPOSED As PART 4 OF WILSON PARK - LUA-09-140 IN 1 6-.d 55,44' 55.00' 55.00' 55,00' 66,41' C) LL - CKD < Ln C- r -n Lo oC r-- LQ c) c N L.Lj cj Ln cn CC D CD n CD CD CD 00 u) m m Ti C) + r-- C) + CD D + 1 (7) cc OD Ln 57-47' Lc') Inu r-- -C::) Lc) 57.60' Lc,) Ln c IY 55.05' 55.00' 55.00' 55,00' 31.01' C14 ROAD TO BE CONSTRUCTED PROPOSED As PART 4 OF WILSON PARK - LUA-09-140 IN 1 6-.d 57.74' N00 58'48"E 288.01' ro ro C) II Yi CKD < Ln C- r -n Lo oC r-- LQ c) c N L.Lj CD CD L.Li Of C) CL> CD CD CD m m Ti Ln L n 57-47' 57.60' 57.60' Lmummmmmum 57.60' 57.74' 30 N00 58'48"E 288.01' C) CKD < LLJ — C-) n-- c) c N L.Lj L.Li Of C) CL> Cf') -cr CD 30