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HomeMy WebLinkAbout921194012 Geotech Assessment~~ STRATUM GROUP 1451 Grant Street, Bellingham, wA 98225 Phone (360)714-9409 October 3, 2005 David Heid 76 Burns Road Port Ludlow, WA 98365 Re: Geology Assessment --------~ Tax Parcel 921194012 Jefferson County, Washington Dear Mr. Heid: ~d~ ~~ ~v r,v5 ~~ I visited the above-described parcel on Oak Bay Road in July 2005 to evaluate the slope stability of the slopes on the subject property and in the vicinity of the subject property. Based on my observations on the site and vicinity, the upland slope on the subject property within 200 feet of the top of the steep shoreline bluff is subject to periodic sliding towards the top of the shoreline bluff on the east side of the property. Features observed on the site indicate that minor ground movement has taken place in the past on the upland portion of the property within 180 feet ofthe top of the steep shoreline bluff slope. It is my opinion that the risk of a large rapid slide where large portions of the upland area would slide toward the top of the bluff and down the bluff face is very low due to the observed soil conditions on the site and the geologic units involved. However, periodic movement from translational sliding of the upland area could cause foundation support problems for a home built within the area impacted by translational sliding. If a home is built on the site, it should be setback at least 75 feet from the top of the steep shoreline bluff. If a home is built within 250. feet of the top of steep shoreline bluff slope, the home should be designed in such a manner that differential slope movement will not negatively impact the structure of the home. This can be done with a rigid structural support such that if settlement does take place, the home can be easily releveled. In addition to the steep shoreline bluff setbacks, a home should be setback at least 30 feet from the steep slope that slopes down towards the ravine located along south side of the property. This letter provides some background on the general geology of the site, site specific observations made, an assessment of the geologic risk, and recommendations for siting a home on the property. r- ~ .______ ... October 3, 2005 Tax Parce92 1 1 940 1 2, Jefferson County, WA Geology Assessment GENERAL GEOLOGY ... _.~__ J ~1 1 Northwestern Washington has been occupied by continental glaciers at least four times during the Pleistocene Epoch (1.6 million to 10,000 years ago). During these glacial and accompanying interglacial periods, the underlying bedrock was eroded. The Geolo ig c Map of Surficial Deposits in the Seattle 30' x 60' Quadrangle, Washin on (Yount, Minard, and Dembroff, 1993) and the Geolo ig c Map of Northeastern Jefferson County Washin tg_on (Gayer, 1976) indicate that the subject property is underlain by glacial till. Gayer also indicates that bedrock is present along the shoreline of the subject property. The Bedrock Geologic Map of the Seattle 30' by 60' Quadrangle, Washing (Yount and Gower, 1991) indicate the bedrock along the shoreline at the subject property consists of Quimper Sandstone. The Vashon Till consists of silt and clay with sand, cobbles and occasional boulders embedded in the finer matrix of silt and clay. ,The Vashon Till was deposited directly by glacial ice during the last glacial period, which ended approximately 13,000 years ago. The till is very compact and has very low permeability. The Quimper Sandstone consists of late Eocene age sandstone and siltstone and commonly has concretions. The Quimper Sandstone overlies the Crescent Formation basalt exposed along the shoreline south of the subject property. My observations of the subject property and vicinity are consistent with the above-described mapping. However, adeep-seated, old landslide located north of the subject property is not underlain by bedrock as indicated by Gayer and by Yount and Gower. The deep-seated. slide area is underlain by glacial outwash deposits and glacial till. Riprap rock has been placed along the shoreline at the toe of the slide area. The Slope Stability Map of Northeastern Jefferson County, Washington (Gayer, 1976) indicates that the steep shoreline slope of the subject property is a steep slope believed to be stable and the upland area of the subject property is a stable gentle slope. My observations of the steep shoreline slope are consistent with Gayer. However, the upland area above the steep shoreline slope has evidence of past slope movement. In addition, Gayer did not identify the old deep-seated landslide located north of the subject property on his map. This feature is tree and brush covered and would have been difficult to see unless specifically observed at the time of Gayer's mapping. I have been on that particular slide and the old slide area is clearly visible on the LIDAR imagery of the area. SPECIFIC SITE OBSERVATIONS The subject property consists of a gently sloping upland area bound on the east by a very steep shoreline bluff slope. The upland area slopes towards the top of the bluff at an angle of Stratum Group File: 7.8.05C 2 7- `. j j Ct1LV i ~ J ! j _.._._. .._. .._. October 3, 2005 ____~ Tax Parce921194012, Jefferson County, WA - ' " ~`' r Geology Assessment approximately 15 degrees or less. The upland area at the time of my site visit was mostly tree covered. A small cabin and access road are located on the site. Soils consist of silt clay and clayey silt. A few areas of uneven ground and ground breaks are present on the slope between the cabin and the top of the shoreline bluff slope. A possible old ground fracture is located slightly up slope from the cabin south of the cabin. The furthest area of ground that may be related to ground movement from translational sliding is 180 feet from the top. of the steep shoreline slope along the north property boundary. The sloping upland area is bounded on the east by the shoreline bluff. The bluff is approximately 50 feet high. The uppermost portion of the bluff slopes down at an angle of approximately 40 degrees and is tree and brush covered. This portion of the slope is underlain by silty clay and clayey silt glacial till and by weathered bedrock. The slope then becomes nearly vertical for a height of 25 feet to the beach below. The vertical portion of the bluff is underlain by bedrock consisting of sandstone and mudstone. The sedimentary beds strike towards the north and dip to the east between 5 and 10 degrees. The beach consists of agravel-covered wave cut platform beach. Erosion at the base of the bluff is minimal due to the competent bedrock at the base of the bluff. Approximately 350 feet north of the subject property there is an ancient deep-seated, rotational landslide. T'he toe area of this slide has been armored with rock rip in the past to prevent erosion. Fractures are apparent in the ground on the lower portions of the slide. 'The head wall scarp of the slide is approximately 50 feet high. Mature trees are growing on the headwall scarp area. The headwall scarp is underlain by compact advance outwash sand and gravel. No bedrock is present within the slide area. An approximately 40-foot deep ravine is located along the south boundary of the subject property. A small seasonal stream with some seeps of water was present near the lower end of the ravine at the time of my site visit. The ravine drains water from the uplands to the west. It appears that the stream has eroded down through the glacial sediments to the underlying bedrock. Stratum Group File: 7.8.OSC 3 October 3, 2005 Tax Parce92 1 1 940 1 2, Jefferson County, WA Geology Assessment ASSESSMENT OF GEOLOGIC RISK Based on my observations on the subject property and in the vicinity of the property, it is my opinion that the upland area near the top of the steep shoreline bluff overlies a translational landslide. The failure plain likely is within the contact zone between the overlying compact glacial sediments and the underlying bedrock. The slide involves the upper slab consisting primarily of compact glacial sediments sliding slowly as a slab over the underlying bedrock. Weathered bedrock may be a factor in the sliding. There is no evidence of deep-seated rotational failures. The conditions that triggered the deep-seated landslide to the north of the subject property are not present at the subject property as the subject property is underlain by bedrock at depth. No evidence of deep-seated rotational features is present on the subject property. It is my opinion that the deep-seated slide north of the subject property does not pose a risk to the subject property. It is also my opinion that the geologic conditions at the subject property are such that the site is not at risk of adeep-seated rotational slide. Slide movement on the translational slide at the site has been fairly minor as evidenced by mature trees on the steep shoreline bluff slopes and the lack of evidence of large scale slide blocks having slid off the bluff along the areas that may be effected by past slide activity. Based on these observations, it is my opinion that the risk of a large rapid slope failure on the upland portion of the property is very low. The following recommendations for the site are made to minimize the geologic risks to a home built on the site. No building should take place within 75 feet from the top edge of the steep shoreline slope. For purposes of this report the top edge of the steep slope is the location where the slope angle begins to exceed 20 degrees. If the home is built between 75 feet and 250 feet of the top edge of the steep slope, periodic minor ground movement and ground settlement from periodic translational sliding should be anticipated. If the home is constructed within this area construction should be such that. minor ground movement or differential settlement of the ground surface will not harm the structural integrity of the home. The foundation supporting the home should allow for correcting any differential settlement of the ground surface. The design should be done in consultation with a structural engineer. Minor ground movement and ground settlement from periodic settlement can be avoided ~,. Stratum Group ____ :.. _ _ ___ File _7 8 OSC ' , ~sl F ~~. I L-_: October 3, 2005 Tax Parce921194012, Jefferson County, WA Geology Assessment by siting the home at least 250 feet back from the top of the steep slope. Native vegetation and trees should be disturbed as little as possible on the steep slope above the shoreline bluff, on the bluff itself, or on the steep slope on the ravine on the south side of the property. Limbing and topping of trees. for view purposes is acceptable. Maintaining as much of the upland area within native evergreen tree species will greatly reduce the volume of water impacting the site from rainfall. At least 65% of the upland area should be left in native evergreen forest cover. No debris or yard waste should be placed on the steep slope. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions regarding our reconnaissance please contact our office at (360) 714-9409. Sincerely yours, Stratum Group Dan McShane, L.E.G., M.Sc. Licensed Engineering Geologist Stratum Group ~`~"`.~' ~_r ~ ~ ~, y ~, '. v. 0 ~~ ~ ~ ~ 1 1 i 1 --~ ~ '~ ~1 ~ ' ~` ~* i `l ~ \ \ o ~,~~ ~ ~ ~ I~j~ }: ~I~ lily (~~ I~ I o~~ ~ ~ ~3~ G ~ ~n~ ~ G N ~v, 1'' ` ,~l\ s ~ ~ ~ `, .r , ~ ~" ~ ~ Leo O ~ , -. ~ .f't `` s ~- ~ ~', 7 '~ -, 'si fir/ i~ ~ -, ,3, -~ ~~ ~ ~~ '~, ~~~ ~ ~ :~ o C~ `~ ~,• X u: ~ `z . ~ . ~~ '"`f ~ \ :~ .. \ .,+ ` ~ ~q a '~ -~ 9 ~ ...g A '~ 4 X_ ~ 3` ~ ;~- b `„~, c~ ~_ T` ~ //~ ~ yl ~ t7 ~' r-F~ -D .. - tQ Q ~ ~ ~ ~ ~ Irn N ~, r-+- ~n I~ ~' N ~ ~~ ~ ~ ~^b ~ ~~ ~ ~ ~ ~."'-`~' any, ~ ~ d ~" a t~~- ~K' -'' "~ " ~~sS~ ~ >'a~ '~ tom;, ~ ,.r'i ~ ~77~ `r'` 9 .e^. ~.>• ~ ~ ~ .fi" ~~ s~ ~~ ~ ~ ~ ~,~ °~ ~~,, ~ ~~ ~~~ ~~ .., ~~ .~~-° ~ -_;_ -_.__ _ _ -ice.. , ~ ... -::