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HomeMy WebLinkAboutGeoTech Assessment Review for BLD2000-00310 - BLD Engineering / Geo-tech Reports - 5/2/2000 MASON COUNTY PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER Shelton,Washington 98584 DATE: May 2, 2000 INTER-DEPARTMENTAL COMMUNICATIONS TO: Shandra O'Haleck, DCD - Planner FROM: Alan A. Tahja, P/W - Co. Hydr. Engr. DCD/Planning WO#00-037 SUBJ: Geo-Tech Assessment Review NAME: Ed Smith 2 Mickkelson Road BLD2000-00310 Shandra, A geotechnical assessment prepared for the proposed Edward Smith single family residence located on lots 3 & 4 of the Smith short plat on the Mickkelson Road has been received and reviewed by Public Works. The geologist indicates in his report that the site is stabile and suitable for the proposed construction. A minimum setback distance of 15 feet has been recommended for structural improvements, and a 25' setback has been recommended as the minimum distance for a drainfield installation. These minimum setback distances should be made conditions for approval. From the information submitted, I concur with the project's geologist/engineer, and feel stability issues have been adequately investigated for this site and proposed level of development. Please feel free to contact me at County extension 461 if you have any questions regarding these comments, or if you feel any features need further discussion or attention. Si cerely, an A. Tahj a File: H:\WP\GEO\ASSESSMENTS\Smith-IDC.doc PERMIT M. Number G�_o37 Requested by: 61tm" rz• - ©1{4 (ed< Authorized by: J e .� Date:. Type of Work: CHARGE TO: NAME AGENCY/COMPANY BILLING ADDRESS PHONE (B) Pub. Works Person In Charge: (C) Project Time Line: (from-to dates Pro}ect SW dab: TO z Esrimated Finish Date: e hocss: ESTIMATED TOTAL$S: COST ESTIMATE (D) Y« £Blt;t H29M Ftir TOTAL$ 0 3�, /O Sty BRIE TOTAL$13 EQUIPMENT USED: MATERIAL USED: ..J"...1.-wwxe..hwii:lit..>...;,:'.'i'•�.,::..<�. Fe...ee YJG VSL� ��`:.3. 'w'�w�4 �cS-,ii ..i.,'� ^C (F) Actual Cost $ BARS: PROD#: DATE Employe YQRKEQHann £ Fps SuNoW FdB4£% TOTAL$ EQUIPMENT USED: Qua q Egi TOTAL f's MATERIAL USED: TOTAL ALL (G) BILLED DATE INV S PAID DATE RECJ CKA FROM• : GEOTESTLAB PHONE NO. : 3607544040 May. 02 2000 12:35PM P2 GEOTECHNICAL TESTING LABORATORY EDWAI213 SMITU 449 t. MIKKEL-SO RD. MELTON, WA 98584 Re: Slope cStability a Setbao-k Analysis Lot Z and 3 (middle lots)of Smith 6horL plat on Miekkelscn Road Mason County, WA Gentlemen: As per your request, we have conducted a slope stability and setback analysis for the above mentioned project. The results of this investigation, together with our recommendations, are to be found in the following report. The site is located on off Mikkelsen Road in Mason County, Washington. SUBSURFACE EXPLORATION PROGRAM The subsurface exploration program consisted of a site reconnaissance and a review of information from our files of similar work performed in this area. The lot was explored by our geologist, who collected samples from the soils exposed along the slope. The soil strata shown on the slope profile were observed at spot locations across the site and on the slope. SITE CONDITIONS The site has a gentle relief toward Oakland Bay to the south. All evergreen trees and shrubs and grasses have been removed from the site. No home currently occupies the site. Cinder the surface, the soil was found to be a sand and gravel mixture. No erosion was observed on the slopes and no deep seated instability was observed. GEOLOGIC CONDITIONS The geology of the area as taken from the "Coastal 'Lone Atlas, Volume 9, Mason County" consists of a Vashon recessional outwash (Qvr). The Vashon outwash is that part of the Vashon Drift consisting of sandy and gravely meltwater, or ablation deposits which were not compacted by the continental ice sheet. On this site there was a consistency of gravels mixed with sand. Test pits dug for septic purposes were used for our observation. The native soils can be considered to be at or near their natural angle of repose, and the factor of safety for most of the slope will be near unity. HYDROLOGIC CONDITIONS The native soils are free draining and we do not expect that these soils will allow for any buildup of porewater pressures, It was observed that the natural ground seepage of water is toward Cranberry Creek. 10011 Blomberg Street SW,Olympia, WA 98512 Phone#(360)754-4612 Fax#(360)754.4848 FROM- : GEOTESTLRB PHONE NO. : 3607544848 May. 02 2000 12:35PM P3 GEOTECHNICAL TESTING LABORATORY SLOPE STABILITY MODELING & ANALYSIS Slopes may fail by a number of mechanisms, depending on the nature of the soil involved and the arrangement of natural earth materials at the site in question. Slope failures occur because forces tending to cause instability exceed those tending to resist it. Generally, the driving forces are represented by a component of soil weight downslope, and the resisting forces are represented by the soil strength acting in the opposite direction. The factor of safety (FS) of the slope is expressed as the ratio of the resisting forces or "moments" to the driving forces or "moments". When the FS is 1 or less, the slope must fail. When the FS exceeds 1, the slope is theoretically stable. When estimating the FS for the slopes encountered, we considered the following types of information: 1) The soil and water profiles. 2) The kinematics of the potential slope failure. 3) The strength and weight of the soil. 4) The proposed slope geometry. 5) Static and seismic (dynamic) loading conditions. The site was modeled using a computer generated slope program and we have included one profile of the model later in this report. The native soils were modeled using 129.4 lbs./ft3 unit weight with a phi angle of 37.2° with no cohesion. We have calculated various scenarios of unsaturated and saturated conditions and have found that the site shows no indication of any deep seated instability of the slope under either static or dynamic loading. Our analysis indicates that the total slope in the saturated and unsaturated condition is safe and that the total factor of safety exceeds one. RECOMMENDATIONS We recommend that the proposed structure be built using a minimum setback of 15 feet from the top of the downhill slope. The on-site drainage (roof drains & catch basins for pavement areas) should be collected and directed into the existing roadside ditch along the north side of the property. The septic drainfield should be set back at least 25 feet from the top of the downhill driveway and preferably 50 feet, to substantially reduce any chances of untreated effluent seeping from the face of the slope with water from the drainfield during periods of extremely heavy rainfall. IMPACT ANALYSIS We do not anticipate that the proposed residential structure and construction of the project will have any off-site impact. On-site care should be taken during construction to make sure that runoff caused by wet weather be directed away from all open excavations. The site is stable enough to support the proposed residential construction providing the 15 foot setback is followed. 10011 Blomberg Street SW,Olympia, 'WA,98512 Phone#(360)7544612 Fax#(360)754-4848 FROM- : GEOTESTLAB PHONE NO. : 360754484E May. 02 2000 12:36PM P4 GEOTECHNICAL TESTING LABORATORY SUMMARY construction. However, the setback guidelines stated herein should Overall, the site is suitable for the proposed be followed. Drainage should be directed away from the slope as much as feasible. We do have some concerns with the septic drainfield, specifically that it is not placed within 25 feet of the top of the slope, and it would be better if a setback of 50 feet was useld,d for e either setbacks However houldbe sg ot' 360 uitab egprovided an Cthatted fldra n eow �nfrom gallons per day from the drainfie the downspouts and paved areas is not combined with the drainage from the drain Meld. Thank you for this opportunity to be of service to you. If you have any questions concerning these recommendations or if we can be of further assistance, please contact us at (360) 754-4612. Respectfully Submitted, GEOTECHNIrC-a R TING LABOATORY Harold Parks, Engineering Geologist 10011 Blomberg Street SW,Olympia,WA 98512 Phone#(360)754-4612 Fax#(360)754-4848 m m 0 Grid Line 2 Chart 1 c� m 0 m Slope Profile rl 160.00 Lot .2 D m 155.00 150.00 --- --- - .- S • m Z 0 Elevation (feet) 140.00 •----- -------- .. W m W A 135.00 � CD 3 130.00 - r 125A0 m I N N 120.00 m 0 100 200 m Horizontal Distance(Feet) N W Ql • 3 Page I O cl m O Grid Line 2 ILo t 7 Mi kke l s e n Rd . Grid Line 2 m m CO r 4 angle: 37.2° T) Unit Weight: 12.9.4 Critical Slope Angle,Saturated: 21.5° Horizontal Sectional Sectiooaf Angle of Total Saturated Saturated Unsaturated Unsaturated R Y Z Elevation Distance % Sloe F r t I Internal Factor Section To1a1 Section Total Sectional p acto Total Total Tota Tofu Grid Station Elevation Change From Last Slope Angle of Sateh Horizontal Change Slope Angle Friction of Safely 11> p> P> > Factor t. ft p ( ) 0 F Feet Feet % Q V F Critical Critical ol5afet?¢ Q From Line Point Feat Feet Sta.1 0 156.00 0.00 0.00 0 0.01 WA 0 0 0 0.0" 18.6 Wa F Ste. l 1 J 56.00 0.00 100.00 0.0% 0-0° n/a 100.00 0 0.0°0 0.01 18.6 rda We Safe Safe Safe nla 0 1 2 154.00 2.00 9.00 25.0% 14.0° 1.35 108.00 2 1.9% 1.1 1 l 8.6 19.17 Safe Safe Safe Safe 1.35 0 1 3 132.00 2.00 8.00 25.0% J 4.0° 1.35 116.00 4 3.4% 2.0° 18.6 9.76 Safe Safe Safe Safe 1.35 0 1 4 130.00 2.00 7.00 28.6% 15.9° 1.18 123.00 6 4.9% 2.8° 18.6 6.90 Safe Safe Safe Safe L 18 0 1 5 148.00 2.00 7.00 28.6% 15.9` 1.18 130.00 8 6.2"a 3.5° 19.6 5.47 Safe Safe Safe Safe 1.19 0 1 6 146.00 2.00 7.00 28.6% 15.9° 1.19 137.00 l0 7.3% 4.2° 19.6 4.61 Safe Safe Safe Sate 1.18 0 1 7 144.00 2.00 7.00 28.6% 13.9° 1.18 144.00 l2 8.1% 4.9° 18.6 4.04 Safe Safe Safe Safe LIS 0 m ] 8 143.00 1.00 7.00 14.3% 9.1 ° 2.36 151.00 13 9.6% 4.9° 18.6 3.91 Safe Safe Safe Safe 1.31 4 i 9 142.00 1.00 30.00 3.3% 1.9° 10.10 181.00 14 7.7% 4.4° 19.6 4.35 Safe Safe Safe Safe 1.95 2 Z O 1 10 140.00 2.00 7.00 28.6% 159° 1.19 188.00 16 8.5% 4.9° 18.6 3.95 Safe Safe Safe Safe 1.19 0 1 11 138.00 2.00 7.00 23.6% 15.9° 1.18 195.00 18 92% 5.3° 18.6 3.65 Safe Safe we Safe L18 0 1 12 116.00 2.00 7.00 28.6% 15.9° 1.18 202.00 20 9.9% 5.7° 18.6 3.413 Safe Safe Sate Safe 1.18 0 W 1 13 134.00 2.00 6.(!0 33.3% 18.4° 1.01 2U8.00 22 10.6% 6.0` 18.6 3.18 Sate Safe Safe Safe 1.03 0 lT m 1 14 132.00 2.00 3.50 36.4% 20.0° 0.93 213.50 24 11.2% 6.4` 18.6 2.99 Safe Safe Safe Safe 0.93 0 t 1 1 15 130.00 2.00 5.00 40.0% 21.8 0.84 219.50 26 11.9% 6.9° 19.6 2.93 Yes Safe Safe Safe 0.84 0 � ]. 16 128.00 '<.00 5.50 36.4°k 20.01 0.93 2214.00 28 12.5°ro 7.1 ° 19.6 2.69 Safe Safe Sate Safe 0.88 15 0D 1 17 126.00 2.00 6.00 33_,% 18.4° 1.01 230.00 30 13.0% 7.4° 19.6 2.58 Safe Safe Safe Safe 0.93 14 1' O 1 19 124.00 2.00 6.00 33.3% 18.4° 1.01 236.00 32 13.6% 7.7° 19.6 2.48 Safe Safe Safe Safe 0.9.4 14 I 19 122..00 2.00 6.00 33.:% 19.4^ 1.01 242.00 34 14.0% 8.0' 18.6 2.40 Safe Safc Safe Safe 0.95 14 1 20 120.00 2.00 6.00 33.3% 19.4' 1.411 248.00 36 14.5% 8.3' 18.6 L32 Safe Safe Sale Safe 0.96 14 3 . 1L �C t9 hJ IV (9 CD fV W "U 3 CD m 0 Grid Line 2 Chart 1 m 0 m CO Slope Profile D 160.00 Lot .3. m 155.00 150.00—; ..... .. ......... f IE 145.00-------------- --._. m Z 0 Elevation (feet) _. .._. m A A 135.00 ------------ ---- --- -- --.----._.... ..---------- .................. .-- -------- CD A m 130.00 ---••--- - - --- --- ----------•--- -- -------- ------ ------ --._..-- ----- --- 3 " 125.00 _........... .. ..._. .. . ........ .. . 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