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HomeMy WebLinkAboutBLD2010-00228 Geotechnical Report 7. • • 4. 144 • 4.401 • f .y � •°c • • • • Y e - t GEOTECHNICAL REPORT Prepared For the C. Green Trust Property October 10, 2007 JUN 2 8 2C Tax Parcel # 502104052 Section 10, Township 25 North, Range 2 West, W.M. Jefferson County, Washington Prepared by NTI Engineering and Surveying 717 S. Peabody Street Port Angeles, Washington 98362 Phone 360-452-8491 Fax 360-452-8498 Web Site www.nti4u.com E-mail info@nti4u.com C. GREEN TRUST PARCEL NO. 502104052 211 PLEASANT HARBOR RD, BRINNON WA 98320 MBP-MAIN RESIDENCE • NTI ENGINEERING & SURVEYING 717 SOUTH PEABODY STREET,PORT ANGELES,WA 98362 IS-17 Engineers , Land Surveyors L Geologists Construction Inspection'J Materials Testing arr— (360)452-8491 1-800-654-5545 FAX 452-8498 E-Mail:infoOnti4u.com NT1 www.nti4u.corn October 10, 2007 Gazinta LLC, Trustee Attn: Kathleen Brace, Manager PO Box 749 Brinnon, WA 98320 Subject: Geotechnical Report for the Property Described as Tax# 502104052, Located in Section 10, T25N, R2W, W.M., Jefferson County, WA Dear Mrs. Brace: Background At your request, NTI Engineering and Surveying (NTI) conducted a bluff stability inspection at the above referenced property. The purpose of this inspection was to examine the bluff at the subject site by visual means in order to determine the relative stability of the bluff and to make recommendations in regards to the proposed construction of a single family residence in general accordance with Section 3.6.10 of the Jefferson County, WA Unified Development Code (UDC) (See Appendix for Partial Site Plan). The inspection consisted of office research of available literature, previous reports completed by NTI, and geologic maps of the area and a site visit for visual observations made on September 24, 2007. Site Description The subject property is located on Pleasant Harbor Road off of US 101, south of Brinnon (Figure 1). The property is bounded on the north by developed residential property (161 Pleasant Harbor Road), on the south by residential property under construction, on the west by US Highway 101, and on the east by Hood Canal. Pleasant Harbor Rd bisects the property from north to south (Figure 2). The upland portion of the property is wooded and is vegetated with young to mature trees and brush (Photo 1). The property slopes eastward toward the bluff at approximately 10° (18%) in the area of the proposed building area that lies east of Pleasant Harbor Rd. Elsewhere on the upland, the slope angle varies from 8° (14%)to 20° (36%) The bluff height is about 40' in elevation with an average slope angle of g approximately 65° (214%), although some portions are less steep and others are near • • vertical (Figure 3). The bluff is vegetated with predominantly young alder trees and brush (Photo 2). Some of the trees have curved trunks, indicating that down-slope creep of the soil is occurring (Photo 3). There is a slide scar on the bluff face south of the proposed building area which appears to have removed a few feet of the bluff. No seeps were observed on the bluff at the subject property. However, there is a culvert under Pleasant Harbor Rd south of the proposed building area. Reportedly, this drainage is dry most of the time. W A S H I N G T O I N - 1 f t ICLALL M a I4 — J. j " - iaMount � Townsend \.504.--, i )) (— r JEFFEftO I i 4 Buckhom r� r � I cl Mountain , 1_ Quulcene ' 'k �° 1 I Buckhorn / fiyWilderness ---14 wk._ Tunnel Creek I5 1 Mount on:tame . Mount 7743R WalkerP Walker 07 4Pn R ; 0?bob Bay K I J.S A P t •t f -5 am * 3 l r } LosewalliPs : ) Suquamish •:-PPlet •Hdod Bangor r ``� Y ioil a Port Madison Mount �I it 1 an Radon Jupiter Olympic Property Crai z National Vicinityi 3os i Duckabus. Forest enemas . -1 - 4 State Park s ' —,I The Brothers a fd t'. / l� �, o Wilderness rinnon I m. 1 Silverdale 1 Bainbridge II !� h� 1 , •Seabeck I.., F 10 I. l- I$land "I- ,, it, + + 03n -� ra�o � ' � COI tW61 r j Erlands MASON 1 1 1 iPoint ` , . _ ,,18L I Figure 1 Mapped Geology of the Site and Vicinity The Soil Survey of Jefferson County Area, Washington (United States Department of Agriculture, 1975) maps the soil in the vicinity of the property as Hoodsport very gravelly sandy loam (HoD). This soil formed in glacial till and is cemented at a depth of 20" to 36". Runoff potential is listed as medium and erosion as moderate. In the Unified Soil Classification System (USCS), this soil is listed as silty gravel and/or poorly graded gravel (GM, GM-GP) with 20%-30% of the material being larger than 3" diameter. • • . The Washington State Department of Ecology's Coastal Zone Atlas — 1978, maps the property and vicinity as Vashon lodgement till (Qvt1). This soil is described as a compact mixture of boulders, cobbles, pebbles, sand, silt, and clay. The Atlas maps the stability of the upland and bluff as Intermediate. The Department of Ecology's "Geology and Ground-Water Resources of Eastern Jefferson County, Washington" — 1981, also maps the property as lodgement till. Visual observations made at the site are generally consistent with the above descriptions. However, in the vicinity of the recent slide, the exposed soil profile contained layers of sand and gravel which may be Vashon recessional outwash in an old glacial meltwater channel. ,,/,,,t. .,, it. ,,,k„Awr, , 7.•‘ •/ , — If ---- lit i R st"41-'41*ci"1., . , CI. is ' if ii.i *. 2 , '}!_j jL r io. p 9q j y zs ' li' '1ff ,it,-. , 1 . ,..,‘ ,t _... . cif, , , 1.:74 7Rslt ; 2 F I` ' / ++�� �' n n. ' Property �; `" # . i .-at ,' �Id III, ' r!i o: , t. t P, s:j ..' 1 -._- . cir a v., 1 1.1,le z . (., 4 a , _., ysil, .z., 4 4, „,,-.. , . i -.11. • --. iir il I I i , - ‘ ' 6- IV 4 I ' . :: -y-r- ,, ....:_ 060,4. if. . '':, ' "Illkikt $t 16 ' _ 2 e 12T8ft J d /0[11srs:r ..±' 7. <.c• n`:ur.._`.r r _..,. ..�.. Figure 2 • • Bluff Recession As with most of the shoreline of western Washington, the bluff at the subject property is undergoing bluff recession. The bluff has been formed by marine erosion and undercutting along the shoreline, resulting in oversteepening and subsequent periodic slides on the bluff face. Typically, these periodic slides are shallow, and only involve the outer few feet of bluff soil. This ongoing bluff recession results in a slow landward migration of the bluff. . A w 4.4 A' i d , 4a •l'`y t 03,:., a '�, ',,+ +�I:. ♦; • .icet i,g ,011 •: .SS.µ , .•.•, ' 4Lr dl .... .r,✓,.t {} ` ., , •' , �p w i '• • ' �..t. .a q sa •r • , 0 � " r -mk io„.....3,,,, „ , ....-: --',,•7. -;. ‘,,--.",it....::,4 ..-• -..-A: ,LI1,; , '',. , ; ;ii, • - _ c-44',.., `.,,—,y,:::-.•,-;;k• + * r o e • :• air. c' I Y� "`i 4•,ri i-.7 4`°' i • Yam".. '' a. .. 0 1ifr." -0: .u, I e; r F.,. .a 4:', `a'■■ . Q'Fi' •�t` I� i-- ,., •tG" 1....4 w • f,� 1, ....e400„.. s. Photo 1: View of property looking north from Pleasant Harbor Rd It is very difficult, if not impossible, to predict rates of bluff recession or if or when a landslide may occur. Bluff recession can occur in several ways and is generally cyclic in • • nature. The bluff may weather slowly over time and recede very little for several years. Then, a portion of the bluff may slough off suddenly. Average rates of bluff recession in the greater Puget Sound area have been reported to be on the order of an inch or two a year, but in some areas may average as much as 6 inches per year. At any particular location, landslides typically occur infrequently, often decades apart. The bluff at the subject property is situated such that it is relatively well protected from large storm waves that can erode the toe of the bluff such as along the Strait of Juan de Fuca or elsewhere in Hood Canal. This is evidenced by the vegetation that exists along the base of the bluff(Photo 2). The relatively protected location, along with the apparent lack of seeps on the bluff at the subject property suggests that the average rate of bluff recession here may be towards the low end of the 4 • scale. �t .its iris' / / r.. f ., f , ,,1 II) _..__ JYYLi - v.YI .,a N Y .r Subject Property t ` t 40,. , ))it a J ipp#4 .jYl+u � �4 �� _ 1 I_ :N , �� r 1r 4%, , r/ s �'` I I f J! Jse r - `may li Uax�u:cicdl,_ere:m::u�r';G¢r.i Se-'ems G7S . 41 11:%811 Figure 3 Conclusions and Recommendations As mentioned above, the bluff at the subject property is in a relatively protected location, and slide activity appears to be infrequent, however, the bluff at the subject property is undergoing bluff recession, and periodic slides should be expected. The average bluff slope angle of 65° is over the typical "angle of repose", which is defined as the • • extend below the angle of repose. It is estimated that the projection of a 35° angle of repose from the toe of the bluff, in the vicinity of the proposed building area east of Pleasant Harbor Rd, would surface 51' back from the top edge of the bluff. At a distance of 30' back from the top edge of the bluff, piles/piers would need to be approximately 12' deep to be below a 35° angle of repose. If you chose to pursue this option, we can provide geotechnical assistance for pile/pier foundation design. ilik, \. K" ,1. s �r 4. _,. II l;. 1 VC — , .s,��• ` ,� Yam; �1 ) •., . , u. ..sf • . .: - t .r 4 A' ) ,..Y C 2 , 'c: },� :�`.� - 0 a•'r( fin 2,1\ ,�_ ;`r�twyT, l;i, ') -t f A.-„m 4 , i a , t �K'+ .• .' -•� '4 it ss •fi v, y�ylyw J ` .r;'t k`c�\.y �y; ��:".� k _ � y • R 't � • • tee i' ,k ':.► It. -f'.. la /l �, -- Photo 3: View of upland at bluff edge. Note that the fir tree at the bluff edge is curved,while the other fir tree just a few feet back is straight. This suggests that the steep bluff face soils may be creeping and the flatter upland soils are not. For further information please review the three enclosed publications published by the Washington State Department of Ecology (DOE) entitled: "Slope Stabilization and Erosion Control Using Vegetation", "Vegetation Management: A Guide for Puget Sound Bluff Property Owners" and "Surface Water and Groundwater on Coastal Bluffs". These publications can also be viewed at the DOE website at: http://www.ecy.wa.gov/biblio/sea.html under the 1993 and 1994 year heading. The DOE website also contains more useful information regarding slope stability and site development; this reference is highly recommended. • • Based upon our inspection and the above assumptions, we recommend a minimum building setback of 30 feet from the foundation of permanent structures to the top of the bluff, not including decks or other lightweight temporary structures such as gazebos. Septic systems and drainfields should also be behind the 30' building setback. We also recommend that the Landslide Hazard Vegetative Buffer be reduced to 15 feet, measured from the top of the bluff. This buffer is within the 30' building setback, not in addition to it. The following recommendations should also be considered with regards to development of the subject property: 1. For buildings and structures on or adjacent to slopes steeper than one unit vertical in three units horizontal (33.3% slope, -18°), we recommend that Section 1805.3 of the International Building Code (IBC) be followed (See Appendix). 2. It will be necessary to maintain ground cover to reduce erosion from surface runoff. Any bare areas that develop should be revegetated. Native deep-rooted vegetation that requires little or no irrigation would be the most beneficial. 3. One cause of slope instability is the presence of excessive groundwater. Heavy irrigation or other activities that would contribute large quantities of water to the soil should be avoided. 4. Surface runoff from hard surfaces such as roofs, driveways, walkways and patios should be controlled and routed to a drainage control system such that surface water discharge to adjacent properties does not exceed predevelopment conditions. From a bluff stability standpoint, the best option would be to route the drainage to the base of the bluff via tightline drain. The drainage control system should comply with all applicable regulations. 5. Silt fences or other sediment control devices may be needed during construction such that sedimentation to adjacent properties does not exceed predevelopment conditions. 6. All drainage control devices should be maintained in good working order and be inspected at least once a year. Optional Considerations Since there is no way to accurately predict the bluff erosion rate, an owner might choose an increased setback which may be more beneficial for long term asset planning. For example, it may be possible that a large magnitude earthquake could trigger a landslide on the bluff. There are so many variables to consider that an accurate analysis of this potential and how much the bluff could be expected to recede if an earthquake triggered slide did occur would be very difficult if not impossible. In a worst case scenario, one assumption might be that the bluff would slough back to the angle of repose. If you wanted to attempt to mitigate this, the house could be located beyond the projection of the angle of repose or founded on piles/piers that • S Limitations This report has been prepared exclusively for use by you and your agents in conjunction with the above referenced project. The report has not been prepared for use by others or for other locations. Copies of this report should be provided to future owners of the property. Others may use it only with the expressed written permission of the Engineer. Within the limits of scope, schedule and budget, this report was prepared in general accordance with accepted professional engineering and geological principles and practices in this or similar localities at the time the report was prepared. An analysis of the potential for large scale earth movements was not performed. If you desire a more detailed analysis of your site, you are encouraged to obtain such services. No other warranty, expressed or implied, is made as to the conclusions and professional advice included in this report. The observations, conclusions and recommendations presented in this report were based on our visual observations of the subject property at the time of our site visit; no subsurface exploration or laboratory testing was performed. Soil and geologic conditions can vary significantly between test holes and/or surface outcrops. If there is a substantial lapse of time, conditions at the site have changed or appear different than those described in this report, we should be contacted and retained to evaluate the changed conditions and make modifications to our report if necessary. Sincerely, NTI ENGINEERING & SURVEYING ,Ro was F9C' Q �� yiy,y lil.� ��?��4. ' CTo2.. 4 . 4 Robert A. Leach, P.E., MBA �o�F,fe 13 eel5:1 e`er /o//7/©7 Principal Engineer ss!auAL ENS EXPIRES 12/30/2008 Bill Payton, L.E.G. .f c. +tt;},0 Engineering Geologist '1.;*0".%* B:\Reports\BRAK0701.10(25-2).bluff stability.Brinnon.doc f"O� 191 d 7,0 d c 40t�$ William C.Payton Jr. • : .,„ ..,,,,.._____ ."•: , • '-'1:7?__, •••: 'c - - ... (-,,..-.t„. • ..-,,,,,L. ::,,f,s-•,. :•.o. .11)-"---- r--- ".••.,t;.::.(.•:..rtYL-'`, :•;•-'°;••6 AIL 7.•...:•. -_,. ON98 1.0•FEN.AIP"rp.:., ... ...1 . ,.. ci b-:4 • -- • ,•--c,, ..zi,,,, .1,00.1.,Ici'Fl. 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'17_i I I 0 I '-',-:2,, •I ..7. . 1 • . ..1- ?.E.--.. •.2::t :..!.., .z-. i....:4' ;...I,.= . -17 . i -.... • -.. :". •,CR 1111-.1 ..: • 411 SOILS AND FOUNDATIONS 1805.3 Footings on or adjacent to slopes.The placement of (2134 nun)from the top of the slope shall be capable of sup- buildings and structures on or adjacent to slopes steeper than porting the water in the pool without soil support. one unit vertical in three units horizontal(33.3-percent slope) 1805.3.4 Foundation elevation.On graded sites,the top of shall conform to Sections 1805.3.1 through 1805.3.5. any exterior foundation shall extend above the elevation of 1805.3.1 Building clearance from ascending slopes.In the street gutter at point of discharge or the inlet of an general,buildings below slopes shall be set a sufficient dis- approved drainage device a minimum of 12 inches(305 tance from the slope to provide protection from slope drain- mm)plus 2 percent.Alternate elevations are permitted sub- age,erosion and shallow failures.Except as provided for in ject to the approval of the building official,provided it can Section 1805.3.5 and Figure 1805.3.1,the following criteria be demonstrated that required drainage to the point of dis- will be assumed to provide this protection.Where the exist- charge and away from the structure is provided at all loca- ing slope is steeper than one unit vertical in one unit hori- tions on the site. zontal (100-percent slope), the toe of the slope shall be 1805.3.5 Alternate setback and clearance.Alternate set- assumed to be at the intersection of a horizontal plane drawn backs and clearances are permitted,subject to the approval of from the top of the foundation and a plane drawn tangent to the building official. The building official is permitted to the slope at an angle of 45 degrees(0.79 rad)to the horizon- require an investigation and recommendation of a registered tal.Where a retaining wall is constructed at the roe of the design professional to demonstrate that the intent of this sec- slope,the height of the slope shall be measured from the top tion has been satisfied.Such an investigation shall include of the wall to the top of the slope. consideration of material, height of slope, slope gradient, 1805.3.2 Footing setback from descending slope sur- load intensity and erosion characteristics of slope material. face. Footings on or adjacent to slope surfaces shall be founded in firm material with an embedment and set back from the slope surface sufficient to provide vertical and lat- eral support for the footing without detrimental settlement. Except as provided for in Section 1805.3.5 and Figure 1805.3.1, the following setback is deemed adequate to meet the criteria.Where the slope is steeper than 1 unit ver- tical in 1 unit horizontal(100-percent slope),the required setback shall he measured from an imaginary plane 45 degrees (0.79 rad) to the horizontal, projected upward from the toe of the slope. 1805.3.3 Pools.The setback between pools regulated by this code and slopes shall be equal to one-half the building footing setback distance required by this section.That por- tion of the pool wall within a horizontal distance of 7 feet FACE OF FOOTING TOP OF SLOPE FACE OF r • �1 1 ��STftUCT IRE / \ SLOPEBUT NEED NOT fl /...�`v... \ EXCEED 40 FT. MAX. 4 NH/2 BUT NEED NOT EXCEED 15 FT MAX. For St I foot=304.8 mm. FIGURE 1805.3.1 FOUNDATION CLEARANCES FROM SLOPES 2006 INTERNATIONAL BUILDING CODE® 347