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HomeMy WebLinkAbout702243002 Geotech Assessment` ~~ tt ,t ee ~,,~~~''k ~~ LY ~ ~ ~ ~ 3 ~~~$t '~" ._ .~ r", ~ror7 -. __, I ~, ~ ~ ~ i "' ~ ~ r yam' ~ ~ - ~~},5,1 :. ~ } rca2 _ l ,,` y~..,~'~'ti .~.,','r f ~ ~ ' br s .> Muncieyl . a1 , ~ ~_ !!.~ _ tea, ~ a f ~ i~ G ~;', Property ~ _Vr ~;~~ ~ ~ f, rY ~' ~..~~ ~ y~ .. G '4 L _. ,~ 1r^r,~. f• ,,,,~4 ~:f ~~ ~~ ~ ,~ -.,r ~ `•,,.. t \' ,.; j' J 4p} .. s~'f- .._ ;,u ~r ' ~~ ~~, - ~ `, ~ M, r ~ ^ ,~ 1. ,i, ~ 1 ~ ~ :t'. -~vt ~~ ,J F~"'i~E rL'y~r'k^ h I 1 Jr`x ~ y.}7• (__ ~~ _ l; !/rJ ~ ~ ~'1j 11 _,1 ~ ~ 5 4 ~ 5 1 '', ~~I 1 , i ~ s, G I t_ t 1 ~ l`-~-_~-~~'`~ ~3 Syr Ua+s ten; ~ .3cdms.,m Coryfe»~ ~.m~~S,`y , -. r ~ ~^-~jJ _ 1~ r. _ WW' . ~f T. 1,~T ~~}f GEOTECHNICAL REPORT Prepared For Alan Poison July 16, 2008 For the Property Described As Tax Parcel # 702243002 Section 24, Township 27 North, Range 2 West, W.M. Jefferson County, Washington Prepared by NTI ENGINEERING Sz SURVEYING, INC. 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 .~ ~ ~iF~~E~C1~1.1~UU~~~,~ ~~U _~ rr ~ ~, ~ ~~ 4 \ ~•~ 1..~ Geotechnical Report for Tax Parcel # 702243002 ~ ~ t. ~ ~;~ July 16, 2008 Y^~~~~~tqqC~l~,~ ~' ~` rY Alan Polson PO Box 64 Willits, CA 95490 Subject: Geotechnical Report for the Property Described as Tax Parcel # 702243002, Located in Section 24, T27N, R2W, W.M., Jefferson County, WA Dear Mr. Polson: Background At your request, NTI Engineering and Surveying (NTI) conducted a geotechnical evaluation of a portion of the above referenced property where a small cabin and associated septic system is planned. It is our understanding that the cabin will be one bedroom and a single story. The purpose of this evaluation was to examine the site by visual means for any obvious signs of geologic concern, report our findings and provide an opinion regarding the development of the site in accordance with the Jefferson County Critical Areas Ordinance and International Building Code (IBC). The inspection consisted of research of previous geotechnical reports, available geologic data for the area, and a site inspection made on July 3, 2008. Site Description The subject property is located at 262 Claybanks Road in Quilcene, WA (Figure 1). Figure 1: Vicinity Map The property is surrounded by wooded private property and bisected by Big Quilcene River (Figure 2). The south half of the property is hilly and slopes down towards the north; while the north half of the property, on either side of the river, is relatively flat (Figure 3). Figure 2 -- ~: ~-_ ~., ~~ ; ~.. ~_ __- ~ Approximate Approximate --..__ __ - location of location of _ `-~--~ _ = _' J cabin and _ ~-- - - _ upper site - --v -- - ~' septic system _ ~ -~ - - _. _ ~ ~-- (lower site) ---- - ~ ~ r _.~.: x . .~ --- . -- -- f .'~_ _ ~ , r , . _ ~ _ ~ '•~ _ _ ~ ~ ~' ~~ „ ~ - =~~ , - ~~ ~ ; ~' ~ r~ - a A~In Figure 3 The property is mixed wooded and cleared, with a portion having been recently logged. The property is undeveloped except for spur driveways leading to two cleared building sites. The main driveway continues west through the property and onto the adjacent property. The lower site is the location of the proposed cabin, and the upper site is the planned location of a future garage which has reportedly been addressed in a previous. geotechnical report and will not be addressed in this report (Figure 3). The proposed cabin site is a small nearly level cut/fill bench that is approximately 30 feet wide, with the primary drainfield area located north of the cabin site and the reserve area located southwest of the cabin site (Figure 4, Photo 1). ~~~~• ` --_ i ,, Approximate ••'%`a~~ , i Landslide -'' ~ .:. ,' ! ~' ,. Hazard Area i '. / ~ ,`~ ~ Boundary ~ i ~ s, ~~ i ,~~ ~ r _~----tea \ ~ ~ ~~ :. ~ ` ~ a ~._ •....,~~ . ~ ._S . a ro~s~, Cabin. Site _~"~ \~~ ~ ` ~ 4~ 1 -w,ewe: w..Q `` __ i _, N 89d11 "S1 ' W `-- 334.67 +~^ Iw :AUR+c Rpi~'YF OC.~t n •. >ae Figure 4 North of the drainfield there is a steep slope that leads down to the river. There is also a ravine east of the cabin site and drainfield that runs in a northeast direction. The top of the steep slope to the north and the side slope and head of the ravine constitutes the landslide hazard boundary. The drainfield is approximately 19 feet from the landslide hazard area at its closest point and the cabin site is approximately 30 feet from the landslide hazard area. The location of the landslide boundary (Figure 4) was marked in the field with white flagging tape and also approximately coincides with the timber harvest boundary. ~~~~' ~ .~~8 _r1,~R. , ~~t~t~,:~~ l.Ulitr ~ ~ ,. ~u ~~ ~ ~' ;;x r` t I. { J ~l y ~/F ~ ~ ~ r sky f~ ,Z _ A ~~~ K~W!-~ c~~• ri. .~ .•~'f yi9.Sws ., 4`.i ~`1kFi '7 ~.i ' wl~; .~; ^;,3~e Photo 1: View of cabin site looking east '~ > r ~ a -_ < „~` # 9,' ~. .l~- .. Drainage above the cabin site has been improved with a ditch along the south side of the main driveway and a culvert under the driveway that diverts runoff into the ravine on the east side of the cabin site. There has been a small slide near the intersection of the main driveway and the upper driveway west of the cabin site. It is believed the slide was triggered by a ground water seep. A rock retaining wall has been constructed along the edge of the driveway at the base of the slide area (Photo 2) and the water from the seep flows into the previously mentioned ditch. It appeared that the head of the slide is at the transition of a silt layer and overlying sand layer. Groundwater in the sand layer is forced to move laterally when it hits the silt layer. When this water exits the slope, the saturated sandy soil looses strength and can slough. If this occurs, it is usually in the rainy season when the ground is excessively wet. For this reason it is important to control surface and ground water in and around the building site. 1~ ~~~~~~a~,~ JUL 2 82008 ~~~~~~u~ ~UU~~Y D~ v Y ~ v ~T' ~- ~]~~~l ~ ~'a ~ `~ ~~'~ Ir ~~ JUL 2 8 2008 Regional Geology The Olympic Peninsula is dominated by the high and rugged Olympic Mountains of the interior. The Strait of Juan de Fuca lies to the north, Hood Canal to the east, and the Pacific Ocean to the west. In between the water and mountains lie the plains, foothills and valleys where most of the development occurs. The underlying bedrock geology, that is so magnificently exposed in the Olympic Mountains, is composed predominantly of igneous basalt, and sedimentary sandstone and shale, all of marine origin. The sandstone and shale formed as sand and mud on the ocean floor. The basalt formed as "pillow" basalt on the ocean floor from under sea volcanic eruptions. The pillow basalt gets its name from its pillow-like appearance. Long ago, these rocks were pushed up onto the continental margin forming the Olympic Mountains. This resulted in the rocks being folded and faulted. Later, during the ice age, the Peninsula was mostly overrun by large glaciers coming down from the north, and from smaller local glaciers from the interior Olympics. The glaciers deposited sedimentary till, gravel, sand, silt and clay with occasional "eratics" - Photo 2: View of rock wall at intersection of main driveway and upper driveway, looking west large stones and boulders that had been trapped in the ice and were left behind when the glaciers receded. These glacial deposits represent the majority of the surficial soil deposits on the Peninsula. Site and Vicinity Geology The site and vicinity is in an area dominated by preglacial and glacial sediments, marine deposits, outwash deposits from receding glaciers, and more recent alluvial deposits.. The Soil Survey of Jefferson County Area, Washington (United States Department of Agriculture, 1975) maps two types of soil in the vicinity of the property, Cassolary - Kitsap complex (CkE) and Belfast silt loam (Bg). The Cassolary -Kitsap complex is mapped on the hilly southern portion of the property. This soil is composed of about equal parts of Cassolary and Kitsap soils. The Cassolary soil formed in reworked glacial till and marine sediments and consists of predominantly silty sand with a clay layer about two feet below the surface. The Kitsap soil formed in glacial lacustrine or marine sediments and is made up of silts and clays. The survey predicts that the Kitsap soil is prone to slippage. The Belfast soil formed in alluvium on flood plains and is made up of silt. The Survey predicts that the areas where this sail is mapped are subject to overflow about once every 10-30 years. The Department of Ecology's "Geology and Ground-Water Resources of Eastern Jefferson County, Washington" maps four soil types in the vicinity of the property. See Figure 5 for a geologic map of the area. The map comes from another source but is consistent with Ecology's map. Post glacial alluvial deposits mapped .along Big Quilcene River are composed of gravel, sand, silt, clay and peat. Near the south side of the property, glacial till is mapped. Till was deposited by a glacier and is usually very compact and is composed of a mix of boulders, cobbles, and pebbles in a matrix of sand, silt and clay. The east side of the property is mapped as recessional outwash and ice-contact stratified drift. This unit was deposited by streams from receding ice lobes of the Fraser Glaciation. It consists primarly of gravel with some sand, silt, and clay. Towards the west side of the property, a unit of undifferentiated sediments is mapped. This unit consists of sediments resulting from pre-Fraser Glaciations, interglaciation, and advance outwash sands of the. Fraser Glaciation as well as sediments from non-glacial sources. Visual observations made at the site are generally consistent with the above soil descriptions. Test pits in the proposed drainfield area reveal silty soil with some sand and gravel. Above the main driveway, there is a change to sandy soil. ~~~~~~ JUL 2 8 20Qfl ,~~$~111'~~~~i~ ~4~D ~. ~~~~~~vl~a. JUL 2 8 2008 Conclusions and Recommendations Development of the site seems feasible from a geotechnical perspective. We recommend that the landslide hazard area buffer be reduced to 19 feet in order to accommodate the drainfield as shown on Figure 4. The bench on which the cabin will be constructed is believed to be composed of fill material on the north side. There may be on the order of 4 to 6 feet of fill at the north side of the bench. The fill likely gets thinner towards the south side of the bench. We recommend that the cabin not be founded in non-structural fill material. The foundation could be extended through the fill and bear on native undisturbed soil below the fill, or the fill could be dug out and replaced with properly compacted structural fill. We recommend also that the cabin floor be supported by the foundation elements and not by the fill material. Possible foundations could consist of footings in native soil with extended stem walls or pin piles driven through the fill. There is currently a log retaining the fill along the north side of the bench. We recommend that this log be replaced with a retaining wall similar to the rock wall mentioned above. The wall should also wrap around the east side of the bench. We also recommend a retaining wall along the cut slope on the south side of the bench. NTI can assist with foundation and retaining wall design if requested. Figure 5: Informational geologic map (Source: Jefferson I.D.M.S.) ~ w L..1 ,'~ The following recommendations should also be considered with regards to development of the subject property: ~~ ,f ~',j ~ +r~,! nil ~K~6d!'~~'~ 1. It will be important to control drainage and runoff above the site. All runoff from above the site should be routed around the site. It appears that this has already largely been accomplished with ditches and a culvert along the main driveway. These will need to be properly maintained. If it is noticed that these existing features do not adequately control runoff, additional diversion structures will be necessary. 2. I# will also 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. Heavy irrigation or other activities that would contribute large quantities of water to the sail should be avoided. One cause of slope instability is the presence of excessive groundwater. 4. Surface runoff from hard surfaces such as roofs, driveways, walkways and. patios should be controlled and routed to a drainage control device such that surface water discharge to adjacent properties does not exceed predevelopment conditions. At this site, standard drywells are not recommended. We recommend either detention with controlled release or dispersion trenches. If trenches .are used, they should be as long as practical so that the water is dispersed as much as possible. The drainage control system should be properly constructed and comply with all applicable regulations. NTI can assist with this. if requested. 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 inspected at least once a year. 7. The requirements of the International Building Code (IBC) that regulates construction near slopes should be followed. This may require a deeper than normal foundation for the cabin (See Appendix for an excerpt from the Code). Limitations This report has been prepared for your exclusive use in conjunction with the above referenced project. The report has not been prepared for use by others or for other locations. 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. 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 laboratory tests were performed. Soil and geologic conditions can vary significantly between test holes and/or surface outcrops.. If there is a substantial lapse of time, canditions 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 J~E.uc_ S , Lim ~~ Steve S. Luxton, MSc. PE Professional Engineer ~ ~~ Bill Payton, L.E.G. Engineering Geologist B:\Reports\POLS0801geotech.24(27-2).Ouilcene.doc ~ ~-®~, JUL L 81~ JEFFERSON COUNTY DCD ~~s: t~ ~~ Appendix JUL 2 8 20~ ~tttci~3UN UUU~iY DCD SOILS AND FOUNDATIONS 1805:3 Footings an or adjacent to slopes. The placement of buildings and structures on or adjacent to slopes steeper than one unit vertical in three units horizontal (33.3-percent slope) shall conform to Sections 1805.3.1 through 1805.3.5. 1805.3.1 Building clearance from ascending slopes. In general, buildutgs below slopes shall be set a sufficient tiis- tancefrom the slope to provide protection from slope drain- age, erosion and shallow failures. Except as provided foz in Section 1805.3.5 and Figure 1805.3.1, the fallowing criteria will be assumed tD provide this protection. Where the exist- ing slope is steeper than one unit vertical in one unit hori- zontal (100-percent slope), the toe of the slope shall be assumed to be at the inteeseetion of a horizontal plane drawn from the top of the foundation and a plane drawn tangent to the slope at an angle of 45 degrees (0.79 rod) to thr horizon- tal. Where a retaining wall is constructed at the roe of the slope, the height of the slope shall be measured from the top of the wall to the top of the slope. 18053.2 Footing setback from descending slope sur- face. Footings on or adjacent tb slope surfaces shall be founded in firm material with an embedment and set back from the slope surface sufficientto provide vertical. and lat- eralsupport 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 be measared from an imaginary plane 45 degrees (0.79 rod) to the horizontal, projected upwazd from the toe of the slope. 1805.33 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 wail within a horizontal distance of 7 feet {2134 mm} fromthe top of the slope shall becapable ofsup- porting the water in the pool without soil support. 1805.3.4 Foundation elevation. On graded sites, the top of any exterior foundation shall extend. above the elevation of the street gutter at point of discharge or the inlet of an approved drainage device a minimum of 12 inches (305 mm) plus 2 percent. Alternate elevations are permitted sub- ject to the approval of the. building official, provided it can be demonstrated that required drainage to the point of dis- chazge and away from the structure is provided at all loca- tions on the site. 1805.35 Alternate setback and clearance. Alternate set- backsand clearances are permitted, subject to the approval of the building official. The building official is permitted to require an investigation and recommendation of a registered design professional to demonstrate that the intent of this sec- tion has been satisfied. Such an investigation shall include consideration of material, height of slope, slope gradient, load intensity and et~ion characteristics of slope material. r~ACEO~ wonNc mP aF sr..orE FACEDF /srAUCruRE roE of sEOae For SI: 1 foot = 304.8 mm M2 8UT NEED NOT EXCEED 15 Fl: MAX: FIGURE 1803.3.7 FOUNDATION CLEARANCES FROM SLOPES 2008 INTERNATIONAL BUILDING CODE® tvseurNEEDNOr ExcEEaxoFr. 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