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HomeMy WebLinkAboutBLD2007-00561 Geotechnical Report • „c le 'I 3t` , r • • "M' 'i: . ;L i ,\ � P ' '9 .4 d, 9 f ry./. '' ,; . - r S Geotechnical Assessment of Building Sites Prepared For Forrest Towne May 16, 2007 For the Property Located at 4104 Dosewallips Road, Tax # 602292014 Section 29, Township 26 North, Range 2 West, W.M. Jefferson County, Washington Prepared by RECEIVED NTI Engineering and Surveying OCT 2 6 2007 717 S. Peabody Street Port Angeles, Washington 98362 JEFFERSON COUNn OCO Phone 360-452-8491 Fax 360-452-8498 Web Site wwvv.nti4u.com E-mail info@nti4u.com i 0 Mr. Forrest Towne P.O. Box 694 Brinnon, Washington 98230 May 16, 2007 Subject: Geotechnical Assessment of Building Sites at 4104 Dosewallips Road, Tax Parcel # 602292014 1.0 Forrest Towne Parcel Geotechnical Assessment In April of 2007, Bill Payton, a registered geologist, and Steve S. Luxton, a registered engineer with significant experience in geotechnical engineering, inspected a -20.35-acre tract of land owned by Forrest Towne. The property is located at 4104 Dosewallips Road off of Highway 101, north of Brinnon (Figure 1). cV � 1 - - 1 =� r i � Op'`c.Nt Tonal rest G " yAh�fl � ��4k,Property Vicinity "� r �"��/ 1`1®ya/6pq ♦4R.2 z -_______„ . #4 „/://,-'4:',41;,41.:41r;,,,ti4V: .,i n. _ ` 1.- er P., ar�'' r thy? , as �e. __ l Dosewallips State Park 4 I 5 }j (( 7 iril • 1114 4 do, Hood Canal Figure 1 RF-4-' D OCT 2 6 2007 JEFFERSON COUNTY OCD • e The parcel lies on the northerly and south facing slope of the Dosewallips Valley in the Northwest Quarter of Section 29 of Township 26 North Range 2 West WM, in Jefferson County, Washington. On the north, the parcel is bounded by high ground and the Olympic National Forest. The Dosewallips River lies below, south of the Dosewallips Road, which lies along the south line of the parcel (Figures 2 & 3). e# / , , Iv, Subject Property - 28142w T t , I .: jam, II N jI -j D05EWALLIPS RD11" '1"44 b C® C= 4 .. T GV •� 1 L i t_ i C,v) /—* M''.-, 33 I ( -r:44 1..L.J _ _________ .. ---ITL _4 i ' 3rIt 4 _. Mat ay.*ded Cry,F i rsm Carty Carats Sar ices C'tS i 0 Figure 2 At the time of the inspection, Mr. Towne was planning the development of a house site and a storage/garage building on the parcel. This report provides specific recommendations for the house site and the shop area development 2.0 Geology of the Site The site lies on the north side of the Dosewallips Valley in Jefferson County, Washington. The area is a recently glaciated valley in the Quilcene Range. The parent rock, which rises to over 1500 feet above the property, is a dense basalt mapped in that area as basalt flows and brecciated basalt. The consolidated rock is a part of the Crescent Formation, a volcanic island that collided with the continent and formed much of the Olympic Peninsula (Tabor and Cady, 1974). • - The lower portion of the south-sloping property shows glacial features including a thin colluvium of glacial drift. At least two small, incised drainage channels cross the parcel from north to south. Both are ephemeral and flow only during heavy rain or snowmelt. ' c ' it.it �_ 7 1»__� f, .44 ....E31.... ' :4,.J ,;.- -,iti.,, «►.'=s� iy It. I ' '. 1 .',1 164,' " - ;&M .. M :k fM� yt �.r as �' ar1��y�1�yt. j ,+ ._ r "rA* . t .X•' ♦ .1F al�f' k ;► " ai,41-.3..k it J ''tF`" .it ,,, • .. ~0Ra !t - ` fie , 4r -+ I, * Approximate - r +._ 1 !; }( ii • g is y House Site „, ' b4 "fir 4 •Y '" • Proposed , .r ' )�, . ' c Approximate a Septic y .4. kf.;,,.. ' 4.,.Y►,01. i Equipment ;* Drainfield , i•• Storage/Shop ., i •y Site . �,=,,6 - r � Site �.r•� .+ ..R'Y itii i , a Main vmoded7y xNervn rn Carr,Ces Sercc: L G . s_ ' Figure 3 3.0 Geotechnical Indicators At the Site Most of the soil at the lower portion of the property is gravely silt and gravelly sand that was probably deposited at the site as advance outwash and was over-ridden and formed into glacial till. This material can be embanked and, if properly compacted it can maintain a 1 to 1 slope. The middle portion of the rocky slope where the owner showed us a prospective house-site is overlain with a thin mantle of colluvial glacial sand and gravel. In much of the central section, this slide colluvium soil is only a foot or two in thickness. Inspection showed that the underlying basalt rock (Photo 1) is sound and little affected by weathering, hence the site has little or no potential for deep- ` seated sliding. ECEIVE OCT 2 6 2007 JEFFERSON COUNTY DCD • • The upper portion of the slope may have some loose bounders that could slide or roll down the slope in an earthquake. Thus. the house site should be protected from rock falls. ' ^--5,,V.•k,—. .... iv ,, .. . ., : ..,..4 ,,,, ''',:',' . ' • ‘?, ' 41' r7.,..e.t'' 7,:, ; ' • 4s., tax. A y..:r�x�, ,F: >r `,.e 1'' :e}f � _ _ , .ck ` i 44 46t i r.� � 3 Fr,+ram tC ,,,'. .F- .e.t:r .f S it e j,.. •a s '� ...„ x• Vj' t. cue- ,y}E t. ,.-.-,': . s rx' ,-..;.•.•fir :' it- tx . l -e `�` "r 144t,, _.,z,„,_.--z,'1.4"...,4,'' ''''"1,,,,w4-' ... ..`',..P.:-.'" „,..•, i..,--q- ...".4,,,.,,4.' . * .t 1/4-,S7.•.-.,,AVA1/44.. S.;4".%:.... S .! fjrr •i - I__ ... _ o t, 'F �1F0-i•t-- • i'- �•'''''':,'LW'''' ' + • r .. • `_'If 4 -fir ,,i'\, �x(. -Ae. ' .y .,,�•�w7;i `" P1,:-,:e fir F T.,- '¢. i• �e4 \ "�:> I'r Z EY •'� " • , +fet yes m! \' Photo 1 4.0 Specific Observations and Recommendations for the Proposed House Site During the inspection Mr. Towne showed us the location of a prospective site for a single family residence (Figure 3, Photo 2). The intended structure is a log house about 36 feet in width and 50 to 60 feet in length. We examined the site that lies on an old logging road that ascends the slope about 100 feet above Dosewallips Road and the valley floor below. The area above the proposed house site has shallow outcrops of basalt rock and rock fragments showing at the surface in the proposed house footprint suggesting that the consolidated rock is just below the surface there. R.-4'cCEWVED OCT 2 6 2007 JEFFERSON COUNTY OCO • • R E( RECFAVED p G 2U07 ``II . . 3n.: �,Ty+' TIM Inv Si.w ..yriF-• ,d:. ;1 i • _ - . ,_ .., „.....„.. _ .,,,,._... _ ...„,,, 4,_L � , c. .-..„4..,,,,,,,,,.,,,„,.,,,,,,, v. -„,-,,,,,,,„„,..,-,-.., . - - - '... •-:"7',''.;`','., 1:-.-.r"i--f--1 , y4r x -' .3vI''t.,,,,;,....„;:V71.-.,..,;-,,,-;':,*.,"..4.,.,''...',",i,'-,.. , . . ,, .. .. . .„ . - , , . ;„ .,.. ..;,,_...,..,,, ,,,........„t...v.4,.,izA, _. _ ,.., ...:„........„:„., 4x 4.tom w,. ttra} „„-zie' a — -, s ,. s . r A- .c k`:.� .,,.r.v ,,i .., :;5.,.. ..'yes `��h''p_.., �L " '"� 3r rG,y�3 ,rY* y r?k .. "1.t7 stl i s.'6 a l J *s.Y�44 R"le d-ci- b Photo 2 The road grade is, at present too narrow to site the house. The area must be excavated to provide additional width for the house and toe of slope and slope rim setbacks. The following is recommended: 4.1 House Site Recommendations • Excavate the rock and slope colluvium so that a total width of 52 feet is available from front to back to serve as a house site. If possible remove spoils from the excavation and use the material to fill the low areas below near the shop building. Such an excavation will require that the site be lowered by about 10 feet with a two foot cross slope on the house-site for good drainage. • Steepen the slope above to not more than 1 on 1 so that the excavated slope above the house is not more than 45 degrees from the horizontal in steepness. • Site the house so that there is at least 10 feet from the toe of the slope on the north to the foundation line of the house • Provide at least 6 feet from the front of the house foundation (south side) to the rim of the original slope. Site the house foundation on original rock so that the vertical lines of the foundation and footing are at least 6 feet clear from any of the side cast slope fill material. • • • As side cast material is being placed, do as much as possible to compact it against the original slope. The use of a track excavator is recommended. Thus, the excavator could be used to "pound" the slope fill into place. Complete the slope fill with a slope not steeper than 1 vertical to 1 horizontal. • If a deck is to be constructed on the area south of the house, provide concrete piers that penetrate into the original slope to support the outboard edge of the deck. These may be built using concrete tube forms and concrete or short steel pipe piles that are driven into the slope fill. • The proposed location of the septic system drainfield (Figure 3) is acceptable from a geotechnical perspective. RECEFVJNaD 4.2 House Site Rock-Fall Barrier Recommendations OCT 2 2007 Due to the potential for rock-falls, the upslope side of the house shomffilSON CORRTI HU protected. The following actions are recommended: • Prior to the construction of the house, search the slope above the site for loose rocks and roll them out or shoot them with explosives. • Construct an "ecology block" barrier that is at least two courses high (5 feet) that protects the entire length of the house. Site the barrier as close to the house as practical so that the space available to capture rocks is as wide as possible. • As an alternative to ecology blocks, gabion baskets may be placed to a height of 5 feet and local rocks from the excavation may be used to fill the baskets. 5.0 The Equipment Storage/Shop Building Site Mr. Towne was actively working on a site for a pre-engineered steel equipment storage/shop building 44 by 60 feet in length with the larger dimension oriented in a north-south direction (Photo 3). The building will normally be unoccupied. The site lies a few feet above Dosewallips Road and is being created by filling the toe of the slope and steepening the grade of the native slope with rises at about 25 degrees from the horizontal (1 vertical to 2 horizontal). The site is suitable for the proposed use. The gravelly sand and rock fragment mixture being used to make • • RECEIVED the fill is good in quality and the side cast fill is unlikely to slide or slump inQ1ie2 6 2007 road below. rr , JEffERSON COUNTY BCD , i r , ; • 5 qYz . . fT-i .. i..- r 4 1 ]►' ry t ;© 59oo . �• • . !I _ S T i Photo 3 The following are recommended • All materials placed in the fill should be compacted in lifts of not more than one foot. • Slope cuts and fills should be compacted as much as possible • The slope above the shop should not be steepened to more than approximately 1 foot vertical to 1.5 feet horizontal to prevent sloughing of the granular fill forming that slope. • Provide 10 feet clear between the toe of the slope and the back wall of the building to provide some protection from rock falls. • Ensure that no part of the building foundation lies on fill materials 6.0 Limitations of This Report • The observations, conclusions and recommendations of this report were based upon observations of the surface soils, shallow excavations and visible conditions at and near the site. No borings or deep test pits were completed, nor were any longer-term measurements made at the site, nor any soil samples tested. Soil conditions at depth could vary from the visible indicators in an unexpected way, thus affecting the recommendations made. Those who require a higher degree of reliability in findings are encouraged to obtain additional observations and testing. The investigation of the site and the recommendations of this report were prepared in accordance to the standard of care of professional engineering and in accordance with the principles and practice of geologic engineering. No other warranty, neither written nor express, is provided herewith. Sincerely yours '� \r �: NTI ENGINEERING & SURVEYING t I t.:1 *) \inQhl�r�b9 0 'rw� I �'��,��!9� ff Bill Payton, LEG William C. Payton Jr. Engineering Geologist 4 S. Ltrr , Steve S. Luxton, MSc. PE co) xI Professional Engineer, NAL EXPIRES: 3/23. I G:\Gen\Bill\Reports\TOWF0701.29(26-2).Dosewallips Rd.SL.doc RE CEIVED OCT 2 6 2007 JEFFERSON COUNTY OCR r,- NTI ENGINEERING & SURV ING 717 SOUTH PEABODY STREET,PORT ANGELES,WA 98362 411, ECEIVE Engineers■Land Surveyors IN Geologists Construction Inspection■Materials Testing NT! (360)452-8491 1-800-654-5545 FAX 452-8498 E-Mail:info@dti4u.com OCT 2 6 www.nti4u.com JEFFERSON COUNTY UCO Addendum to NTI Geoteehnical Assessment Report of May 16, 2007 Prepared for Forrest Towne By NTI Engineering &Land Surveying 717 South Peabody Street Port Angeles,WA. 98382 In May of 2007 NTI Engineering issued a geotechnical assessment report regarding the proposed site development of land belonging to Mr. Forrest Towne. The subject 20-acre tract lies in the Dosewallips drainage at 4104 Dosewallips Road in Jefferson County. Most of the site lies on the uniformly-sloped side of a rocky ridge. Mr.Towne intends to construct a single family residence on a prepared house pad within that property,Tax Parcel Number 602292014, carved into the underlying bedrock of the uniform slope. The original geotechnical investigation found that the south facing slope is principally competent basalt bedrock overlain by a thin cladding of glacial drift and partially cemented alluvial debris. Based upon the strength of the underlying rock, we agreed that the site selected by Mr.Towne is appropriate for a single family residence. Several recommendations were made in the May 16`h report including widening the house platform so that favorable setbacks could be obtained from the building lines to the break lines of the descending and ascending slope. Subsequently, Mr. Towne attempted to complete,the excavation to the limits suggested in the May 16t'report. The occurrence of dense underlying basalt thwarted efforts to widen the house platform and ultimately the site was widened to only about 45 feet. As a result of this difficulty,NTI's geotechnical engineer revisited the site to discuss how toplace the house on the narrower platform. Characteristics of the House,Site Examination of the excavated site showed outcrops of hard ledge basalt beneath cemented alluvium across all of the site. Our original inspection of the site and the adjoining.geologically-similar region showed that the underlying,rock material has no V tendency to form landslides. We also concluded that the proposed action does not destabilize the downslope or upslope areas. Due to the recent logging of the site prior to Mr. Towne's ownership and the inherent stability of the bedrock, there is no requirement for a vegetated buffer. 07 - 0rr. r AFFEE SON • Concern About House Foundation Mitigated Due to the narrower-than-expected width of the house site, the southerly building line lies within about 5 feet of the rim of the descending slope. A portion of the deck piers and their footing must be embedded into the near-break area of the slope. At the follow-up visit we inspected the rock outcrops and estimated their locations in the subsurface. We determined that due to the presence of the rock, an excellent house foundation could be constructed at the site. We felt,however that the foundation work, and the pier footing installation in particular, should be the subject of special design requirements and inspection by a geotechnical engineer. Hence,NTI Engineering&Land Surveying reviewed the house building plans.and developed the foundation plan in collaboration with the Precision Craft designers and the structural engineer. Thus,the construction plans now call for all piers near the slope to be founded on keyed bedrock. The plans also require that the pier installation be inspected by the geotechnical —foundation engineer. Once construction of the foundation is completed, Steve Luxton MSc. PE, the geotechnical-foundation engineer of NTI Engineering&Land.Surveying, will provide.an inspection certificate to the Jefferson County Building Department i certifying thatthe foundation is constructed as specified and to:his satisfaction. It is recommended that the certificate be.a necessary condition of the occupancy permit. Sincerely, �EIV i NTI ENGINEERING &LAND SURVEYING � 10`E 'E�-) s, /-u-1-e t.s. ttrl, OCT 2 6 2007 4 o Steve S. Luxton MSc. PE ,�, w�+sN1r Geotechnical Engineer 6, 'o� y `" JFER, !! 9 CO DCD ^. 23 19 -.>ItAI. 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