Loading...
HomeMy WebLinkAboutBLD2006-00405 Geotechnical Report • APPROVED Quadra Engineering 9 9 STORMWATER PLAN 240 W. Cedar St. P.O. Box 2356 Sequim, WA. 98382 SEP 1 9 2006 (360) 683-7019 Project: Niblock Residence File #: 2 5-85 Location: 100 Block of Groves VAy,( i 98365 - to, , AT � G slc;�,�r I 1: E I S . qH0 APN: 721194011 RE cf ,F,P Design: mz (2-Q ,o"t �gs� 1 Z Date: 12/1/05 �' 1 Revised: 6/16/06; 9/6/06 Stornwat r B '�^�s3T1 Require ` ,oyAL tom, EXPIRES JAN 15, 2007 Drainage and Geotechnical Review This report, with the included Drainage & Erosion Control Plan for the subject project, addresses stormwater management and erosion and sediment control as required for the construction of a single-family residence with attached garage. Existing Conditions • The parcel is located 260'± east of Thorndyke Road on the south side of Groves Road, a private 10' wide gravel roadway within a 60' wide easement • Access to the site is via a private 10' wide gravel roadway within a 60' wide ingress/egress easement along the parcels west property line • The parcel is 2.8 acres of forest with moderately dense understory growt • The northerly 62% of the parcel is sloped downhill southerly with&Os0 beginning at 6% and increasing to14% to the top of a bluff. AN" 014 1.3 • The remaining southerly portion of the parcel encompasses: rp `� • The bluff: S)%*N - Is approximately 90-feet high ��5�) - The upper 20 +/- feet is near vertical with areas devoid of vet`ative cover. - This face is broken by a traversing bench that slopes longitudinally west to east at approximately 14% and a width ranging from 5 thru 10 feet. There is a foot path on this bench which winds down the bluff face. Although the bench was originally 10' wide, a small landslide has not limited the width to 5' in one area - The bluff then flattens to a slope of approximately 95% with lig e) �a Y understory growth with a vertical elevation difference of 90'±. 1 �1' d . rl',D • The parcel is bound by residential properties as follows: SEP 0 $ 2006 - North and west by single-family residences - East and south by undeveloped parcels JEFFERSON COUNTY DCD • Zoning is Rural Residential 1 unit/5 acres (RR 1:5) with the following setbacks: - Front: 20 ft E.\_Jobs\05\0585 Niblock\SWM\0585 SWM&Geo Report 2.doc page 1 of 7 • • Quadra Engineering 240 W. Cedar St. P.O. Box 2356 Sequim, WA. 98382 (360) 683-7019 - Side: 5-ft - Rear: 5-ft • The USDA Soils Survey for Jefferson County indicates the soil type in the project building site area is Alderwood gravelly sandy loam (AIC) with the following general characteristics: - permeability: 2.0 to 6.0 in/hr— (moderately rapid) from 0" to 30" deep; <0.06 in/hr — (very slow) through the dense compacted till found below 30" deep - soils log dated 1/27/04 prepared by Soiltech has indicated the till layer is >60" deep in the project site area - shrink-swell potential: low - winter months perched water table 2-3' deep (no evidence found during site visit) - water erosion potential: slight to moderate except when vegetative cover is disturbed • Site precipitation, as found on the isopluvials in NOAA Atlas 2, Volume IX for a 100-yr 24-hr storm is I100 = 4.0-inches • Critical Areas: From the top of the bluff southerly is designated a Geologically Hazardous Area as defined per: - Unified Development Code (UDC) section 18.15.270(2)(a) designates the face of the bluff as an Erosion Hazard Area because the USDA Soils Survey identifies the soil as being severely erosive on slopes greater than 30% - UDC section 18.15.270(2)(b)(iii)(D) designates the face of the bluff as an Landslide Hazard Area because of visual evidence of soil conditions and the recent slide of a portion of the face of the bluff - UDC 18.15.270(2)(b)(i)(B) designates the face of the bluff as an Landslide Hazard Area because the Washington Coastal Atlas has identified the surrounding area as an unstable old landslide • Geologic Conditions at Building Site The soil conditions at the project building site are based on a review of the test pit soils log conducted by Soiltech on 1/27/04, an evaluation of the USDA Soils Survey. data and multiple site visits C:=3 - The gravelly sandy loam and till soil at the project site is suitable for W . `� supporting residential building and vehicle loads, provided that C C recommendations for site preparation included herewith are followed. all °p - The foundation excavation shall have a minimum depth of 12-inches below n the planned bottom of footing elevation. During excavation, careful W observations should be made to identify any unusually soft areas. Sucli E:\_Jobs\05\0585 Niblock\SWM\0585 SWM&Geo Report 2.doc page 2 of 7 • • Quadra Engineering 240 W. Cedar St. P.O. Box 2356 Sequim, WA. 98382 (360) 683-7019 areas shall be over-excavated as directed by the Engineer and filled as described below. - Fill shall be clean well graded granular material free from organics, rocks larger than 6 inch diameter, ice or other deleterious materials. All fill shall be compacted in 8 inch maximum loose thickness lifts to 95% of the maximum Modified Proctor density in accordance with ASTM D1557. - Twelve inches of clean and compacted sand shall be placed under all footings. - The maximum allowable soil bearing value for concrete foundations, placed on compacted sand as described above, is 2000 pounds per square foot (psfl. - Basement foundation walls should have free draining granular backfill or a drainage membrane such as Delta Geo Drain Quattro, available from Blake Sand & Gravel (360) 681-2877. Use a perforated pipe foundation drain for water removal at the bottom of the wall. The perforated pipe should be bedded in drain rock and the rock wrapped with filter fabric. There should be gravity outlets down-slope from the house, conveyed in a closed conduit within and across the bluff face. - An equivalent fluid pressure of 35 pounds per cubic foot (pcf) should be used for active earth pressure in foundation wall lateral load analysis. For sliding analysis use 0.35 as a coefficient of friction and 200 pcf for passive earth pressure. - Concrete floor slabs should be placed on a minimum 4-inch thick uniform blanket of clean, compacted sand. The sand should be on dry glacial till or on 8 to 12 inches of 2-inch crushed rock. A 20-mil visqueen vapor barrier should be placed under the sand. The slab should not be structurally connected to perimeter or interior footings. - For site grading away from the house, cuts in undisturbed soil shall have a maximum slope of 1.5 horizontal to 1.0 vertical. Provide toe of slope ditches to remove water seeping from the cut face. - For site grading away from the house, fill material shall have a maximum slope of 2.0 horizontal to 1.0 vertical. The receiving surface shall have top soil removed. - Finish grading abutting all building structures shall be in conformity with the o International Building Code section1803.3; sloping away from the building @ 5% for a minimum of 10'. o N �= - If the driveways and parking areas are to be paved, use 2 inches of AG- co 0 pavement (WSDOT 5-04; HMA, Class 3/") over 4 inches of crushed surfacing© top course (WSDOT 9-03.9(3)) over compacted subgrade. Four (4") inth.es a. O; of Portland Cement Concrete over 4" of compacted sand may be subs fed c/a W for asphalt pavement. Refer to the Washington State Department of E:\_Jobs\05\0585 Niblock\SWM\0585 SWM&Geo Report 2.doc page 3 of 7 • • Quadra Engineering 240 W. Cedar St. _ P.O. Box 2356 Sequim, WA. 98382 (360) 683-7019 Transportation Standard Specifications for Road, Bridge and Municipal Construction. • Geologic Conditions at Bluff - The soil section, as viewed along the face of the bluff, displays evidence of slowly permeable silt and clay layers interbedded in the dominant sand and gravel layers. This creates instability of the slope by the introduction of layers of greatly differing permeability and planes of weakness. - The bluff shows evidence of a small landslide. The slide is located approximately 25' below the top of bank on a bench. The width is approximately 10-15'. Other areas of the bluff in this area show evidence of older slides. - The bluff face slope is far in excess of any natural angle of repose for this soil type. No analysis was undertaken to determine a natural angle of repose for this bluff. - This soil type's erosive characteristic's becomes severe on slopes greater than 30%. This erosive: action significantly accelerates the deterioration of the bluff face. • Geologically Hazardous Area Mitigation Measures - A standard 30' buffer, measured from the top of the bluff, is required per UDC Section 18.15.275(6)f and is shown on the site plan. No work or alteration is proposed within the landslide hazard area or its buffer. This buffer shall be prominently and permanently delineated with 6" x 6" landscape timbers laid end-to-end (or equivalent delineation). No work shall be permitted in the buffer. Signage stating "Landslide Hazard Area & Buffer — No work permitted in this area" shall be displayed during construction activity. - We recommend a 100' building setback from the top of the bluff for all building structures. This is based on a setback approximately equal to the vertical height of bluff which is a common standard utilized among local public agencies and geotechnical professionals. - Restrict recreational and casual use of bluff and buffer as much as practical - Monitor / repair all surface erosion and slope damage. - Owner shall inspect and repair any rivulets that may appear in the buffer on a yearly basis. Immediately repair any concentrated stormwater flows and cA o disperse the flows to as broad an area as possible. Utilize least invasive v equipment possible (i.e. hand tools) to minimize soil compaction. co o - If concentrated stormwater flows must be directed towards or down the bluff, o a closed conduit must be utilized across the entire bluff face, with a energy W dissipation device at the outlet. ci) - Avoid destructive thinning of trees and understory E:\_Jobs\05\0585 Niblock\SWM\0585 SWM&Geo Report 2.doc page 4 of 7 • • Quadra Engineering 240 W. Cedar St. _ P.O. Box 2356 Sequim, WA. 98382 (360) 683-7019 - Maintain and promote growth of trees and understory within the buffer • Geotechnical Conclusion - This project will not: - alter a landslide hazard area or its buffer - create a new parcel within a known landslide hazard area - construct a publicly owned facility which are the activities specified in UDC 18.15.275(9)(a) and is therefore exempt from the requirement of a Geotechnical Report. Drainage Design Stormwater management will be accomplished by collecting as much runoff from new impervious surfaces as practical and conveying it to a Stormwater Management Facility (SWMF) designed to promote infiltration. The SWMF shall be located so as to minimize the impacts to the Landslide Hazard Area. • Quality - Permanent ground cover shall be established on all disturbed surfaces - Collect and convey roof surface stormwater directly to a pump chamber where accumulated flows will be pumped to the SWMF - Slope the driveway to abutting landscaping and convey via surface flow to the storm drain pump chamber. The drain rock surrounding the pump chamber and the perimeter holes drilled into the chamber will allow infiltration into the pump chamber where the accumulated flows will be pumped to the SWMF for infiltration far from the Landslide Hazard Area • Quantity - Route / convey runoff from new impervious surfaces to a SWMF of sufficient volume to retain and infiltrate the stormwater resulting from runoff during a 100-yr 24-hr storm (Q1oo)• - Permit relocation and adjustment of the shape of the SWMF to account for existing topography. Require the contractor to verify that the SWMF has the minimum designed volume. - Limit the infiltration design rate to '/z the median rate of permeability • Erosion Control: - During construction and until final stabilization of disturbed areas is achieved, the contractor shall be responsible for preventing the conveyance of soil, silt and dust from the leaving the building site. Methods to be used will depend cn on seasonal and site conditions, but could include maintenance of disturbed 2 areas with use of temporary / permanent ground cover and sprinkling for.dust control. - All exposed areas shall be seeded as specified in the construction notes'' o_. following completion of construction activities. wi� w W E:\_Jobs\05\0585 Niblock\SWM\0585 SWM&Geo Report 2.doc page 5 of 7 • • Quadra Engineering 240 W. Cedar St. _ P.O. Box 2356 Sequim, WA. 98382 (360) 683-7019 - Avoid grading activities prior to and during heavy rainfall as specified in the construction notes - Temporary filter fabric fence will be used to protect off-site properties and critical areas as needed. Calculations • Software: Calculations are preformed using HydroCAD® by Applied Microcomputer Systems. The following are discussions / pre-calculations / adjustments that are applied to address requirements and/or variable limitations of the program: - Infiltration Rate: The infiltration is identified in the calculations as Discarded Outflow. Rates less than 0.01-cfs are shown as 0.00-cfs due to a display limitation of the significant digits used by the software, not computation limit. The lower rates are calculated as can be seen in Outlet Devices column where the velocity is shown. - Calculate median velocity Calculate median velocity of water through soil and convert to feet per minute. V= '/s x (2.0 + 6.0)"'/1 c x 1 ft/12 , x 1 4/60min) = 0.0056ft/min. - Adjust Infiltration Rate Decrease the median infiltration velocity per the Stormwater Management Manual for the Puget Sound Basin to '/z by setting the exfiltration outlet "Discharge Multiplier" in the software to 0.50. This adjustment is shown in the Outlet Devices column by the equation: V fpm Exfiltration X 0.50 over entire Surface area (or Wetted area) Conclusion Per the HydroCAD® calculation data shown hereafter, a SWMF (retention / infiltration pond) 2.0' deep with a 40 square foot base (2'x20') and 3:1 side slopes with a volume = 389 cubic feet is more than sufficient to manage the stormwater generated by this projects new improvements for the Q100 design storm (fills to 1.19'±). R SEP 08205 JEFEERSON MINTY rICh E:\_Jobs\05\0585 Niblock\SWM\0585 SWM&Geo Report 2.doc page 6 of 7 • 411 Quadra Engineering 240 W. Cedar St. _ P.O. Box 2356 Sequim, WA. 98382 (360) 683-7019 Calculation data from HydroCAD Software ( s) D 1P New Impervious Retention / Infiltration Surfaces Pond Stormwater generated by new impervious surfaces Summary for Subcatctment 1 S:New Impervious Surfaces New Imperious Surfaces Runoff = 0.05 cfs @ 7.88 hrs, Volume= 785 cf, Depth= 3.76" Runoff by SBUH method,Time Span=0.01-24.00 hrs,dt=0.01 hrs Type IA24-hr 100yr-24hr Rainfall=4.00" Area(sf) CN Description 1,155 98 home&garage roof 1,350 98 drivewaylparking 2,505 98 V\Aighted Average Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 5.0 Direct Entry,Direct Row SWMF — Retention/Infiltration Pond Summary for Pond 1 P:Retention/Infiltration Pond Retention/Infiltration Pond Inflow Area= 2,505 sf, Inflow Depth= 3.76" for 100w-24hr event Inflow = 0.05 cfs @ 7.88 hrs, Volume= 785 cf Outflow = 0.01 cfs @ 9.41 hrs, Volume= 689 cf, Atten=75%, Lag=92.0 min Discarded = 0.01 cfs @ 9.41 hrs, Volume= 689 cf Routing by Stor-Ind method,Time Span=0.01-24.00 hrs,dt=0.01 hrs Peak Elev=1.19'@ 9.41 hrs Surf.Area=264 sf Storage=232 cf Plug-Row detention time=241.0 min calculated for 689 d(88%of inflow) Center-of-Mass det.time=154.9 min(813.1 -658.2) # Invert Aveil.Storage Storage Description 1 0.00' 389 cf Custom Stage Data(Irregular)Listed below Elevation Surf.Area Perim. Inc.Store Cum.Store V\kt.Area (feet) (sq-ft) (feet) (cubic-feet) (cubic-feet) (sq-ft) t 0.00 39 42.0 0 0 39 2.00 417 82.0 389 389 453 , # Routing Invert Outlet Devices 1RJEc1_,A ` 1 Discarded 0.00' 0.005600 fpm Infiltration X 0.50 over entire Wetted area RSEP082u:,F Discarded Outflow tax 0.01 cfs©9.41 hrs HVV--1.19' (Free Discharge) L1Infittrafion (Efiiltration Controls 0.01 cfs) JEFFERSON COUni E:\_Jobs\05\0585 NiblocF\SWM\0585 SWM&Geo Report 2.doc page 7 of 7 v �r GROVES WAY a rn cn cN ry 1 t.� 12. t 1 ,� 1ik �;f o 2� o �o �� a b � zo . , a1 'fir �,� �I a1 �a1 '� r'1 H r*7 p a b^� t p C v1 r+� • A T tom �,' 22� a a p mo� � 2-, a oa �x a 2r ba i, b N 2 O k nibg2 cn ,? bat cc�y b S- �a rs ch. � fj1, I Cr i y 9s (n cn cn�n i• I I �� pp� 2ak b b C y aQ ' b a nm i"� C� n� wrcc moo 'a �l ; g a r- lD �m NIPa '8 n ry � � a P 2b 000 nn yOo- , ^t 1 /.\/ n C o� a 2= � Oo y �' a. 666,6 a n _o �'� o ff' �' -c i g^ � T ' m N IK 2a �; vaio L30. Ili JO"--I ti� Odd �' -O ! i I n ~� N 20 iy 3 na b �U O g 2 � a� 3 rr9 �O • NORTH 1�- �t r ga 2 w I^ Cpsp � ^bi y' � o a Co 0 � 2 , �' �� ;' �z GROVES WAY ► .aro a 143-55^.._.._ t. o c� olio x 11 aft 2 • Q, �� poi � � 5 c�S�n� r I g ='wJJ� • r �a ' boa tsa r-/ LI ni I , 10' O d••� + t o / F S .I ram- q �u o ,6 CD I n� Z C) J a rT-1 0,3 4,, Crn� z ry p o0 r'na o E,. a irri 6 _ ri) oho �oX � !� _ = (' 1 \ r,i�2 `� o mom / ~ ✓ alke • crtl f ' I a / O(,)/ , / o � DO KELLY DRIVE /et c, /„, Aor 444 zt • 4PfC\� �O • •tie, ' �0/ • C7 GAF 90 Cr) / i A'V ifs Ladd 40 �0� ro s `� r ti 4,-do/ C it. rri AZ /1111 043 I ` / `l`P / gyp r— 9/6/06 REVISION: ADDED 50' SETBACK FROM TOB TO DRAINFIELD; REVISED v) HOME SIZE; ADDED WELL AS-BUILT LOCATION; REVISED SWMF SIZE c � ry Preqxred G-' kr: DANIEL NIBLOCK D c 0 NIBLOCK RESIDENa =� 612 HEWITT AVENUE GROVES WAY, BREIMERTON, WA 98337 ENGINEERING, INC. DRAINAGE & EROSION CONTROL PLAN PHONE: (206) 634-0628 FAX:: 206 632-1280 P.O. BOX 2356 TEL (360) 683-7019 SITE PLAN 260 240 W. CEDAR STREET FAX: (360) 683-7087 DATE:12�02/05 JOB No: 05-85 SCALE:AS NOTED SHEET: 1 of 2 DRAWN: MZ DESIGNED: MZ CHECKED: JMW SEQUTAI, WA 98382 quadra@olypen.com