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Home My WebLink About701153016 Geotech Assessment VII.J.~I V""'<W.~U l'^t ... ,)0" 041 ,)144 ~'.n<ATUM ~KUUY ,.....141 01 ."- se-f,o$-140 STRATUM GROUP 1451 Grant Street, Bellingham, WA 91225 PboDe (360) 714-9409 Mmd131, 2004 Micbael Havens 117 Blaine Court Pickerington,OH43147 A#eA+'u-,\: G,e j Bu/tv) (}60) 311- Lfl.(13 Re: Geology Evaluatiou Tax Parcel 701153016, Toandos Road Jefferson County, Washington fr\LA oS p 33"1 Dear Mr. Havens: We are pleased to present the results of our geology evaluation oftbe above referenced property on a shoreline bluff. The purpose of this geology evaluation was to I) determine the suitability of the property for ~ siting of a residence. 2) qualitatively evaluate the risk of slope failures, and 3) provide general site development and mainteuance recommendations for development of the property adjacent to a potentially unstable bluff: This evaluation was limited to a visual inspection of the property and vicinity, a visual inspection of the bluff slopes, review of available geologic mapping in the area, and inspection of the shoreline at the base of the bluff. SCOPE OF SERVICES The scope of our 'services included the following: 1) C9nducted a site visit to vi.mally inspect the subject property including the bluff sl~pe conditions, shoreline conditions. and relevant conditions in the vicinity of the property. 2) Observed surface soil coDditions on the bluff face and on the uplands above the bluffby excavating shallow band dug test pits. . I I 3) Prepared thiS report SUIIIIIl8rizing our findings, including an evaluation of tbt feasibility of building a ~ on the subject~, a qualitative eva1uation of the shoreline bluff stability, reeommendatiolllJ for site development, and ~mmendations for further investiption, if necessary. I .. ---- ....- U, I ~~I Ul>UO: l:O 1'AA..1 3~U "4'/ 3144 S'.l'.KATUIl GROUP ~Ol c' MardI 31, 2004 Tax Pan:el7011530lCi. Toandos Road, Jefferson County. W A GeoIo&Y~ : . GENERAL GEOLOGY Northwestern WaShington has been occupied by continental glaciers at least four times durina the Pleistoceae Epoch (1.6 million to-I 0.000 years ago). During these gJacia1and acc:ompmyina interglacial periods. the underlying bedmck was eroded and a relatively thick layer of glacial related and in1aa1acial fluvial sediments were deposited over the underlying bedrock in the vicinity orthe subject property. The Geolomc Man of Surficial Deposits in the Seattle 30' by 60' Quadranlde. WaClhinDtnn (Yount, Minard and Dembroff 1993) and Geologic Map of East-Central Jefferson County. Washimrton (Birdseye, 1976) indicate the subject property is underlain by glacial till and pre- Fraser glaciation Stratified sediments. Both maps indicate that the upland portions of the subject are underlain by &lacial tilt and the steep slope on the western side of the property and down to the shoreline is underlain by pre-Fraser stratified sediments. In addition to the mapping, a wen log from a site within a half-mile of the subject property indicates the upland area is underlain by approximately 30 feet of glacial tin underlain by at least 300 feet of compact silty sand. Observations OD the subject property and vicinity are consistent with the above-described mapping. Till is plesent only on the most eastern end of the property near the T oandos Road. Most of the upIaod pOrtion of the property is underlain by sand and gravel with sand being the dominant metmaJ towards the bluff slope and in the vicinity of the existing cottage on the pTOperty. The sand exposed on the bluff face consists of cross-bedded stratified deposits with no apparent silt. clay or anwcl1ayers. The b&se of the steep shoreline slope is covered with laDdslide deposits consistiDa of the sand. from the slopes above. No silt Of clay layers could be observed at the base ofthc bluffbecause of the overlying laudslide debris. However. these units are present on the lower slopes in the vicinity and springs and seeps indicate that they may be present on the subject property as well. The Eslativc Slop: S1abilitv in East-Ccntral Jefferson County (Birdseye. 1976) indicates the shoreline bluff slope is area of ~nt active landslides. . A large landslide took place on the propeny approximately 6 yeatS ago. The slide appears to haw been approxillUlfely 20 feet deep. 200 fed wide and 340 feet high. The slide initially created III island of soil. debris ext.eDding approximately 100 feet out into the bay. The debris and soil deposit bas suhecplflntly been reworked by wave action such that there is a sloping ~irMUlalspit extendina out fiom the former shoreline. The deposit oflJuylclidc material is now ~Il v~ wlth grass rmd brash and trees bave begun growing on the s1ope~ SIdtum Clmup Fik:3.II.G4a 2 v.,~.,v~' VV.6' r~ ~ ~QV U., ~,.. ;:)lKA1UJI uKUUY 141 01 MInII 31. 2004 Tax Plnel7011S3016. TC8Idos Ro.d, JeIfenon County, W A C.d..,. E.....tioa SPECifIC SITE OBSERVATIONS The _location map is provided on Figure 1 and a general site plan sketch is provided on Figure 2. The JKOperly is approximately 9.94 acres in size. The property is accessed from T oandos Road via a eravel driveway. An A..1iamecottage is located on the site. The lot is located at the top of a hiahs west facina shoreline bluff above Dabob Bay. The uppermost portion of the property is at an 'elevation of approximately 525 feel The property J)rim8rily slopes .towards the west towards Dabob Bay. However, the property also slopes to the nmtb. and south as the property consists of a broad ridge with the broad ridae top sloP1na towards the west and sides slopina to the south and north. The uppermost portion of the J)roperty is essentially level. The slope to the west gradually steepens to approximately 20 degrees. The 20- ~ sloJ)e end abruptly at the top of the essentially vertical headwall of the landslide area. The headwall of the landslide area is approximately 350 feet at its highest point I The north and south facing slopes slope down into draws. These slopes gradually steepen to as much as 40 degrees. No indication of flowing water is present in the bottom of the draws. The draws are truncated on the west by the headwall of the landslide area. The elevation above the shoreline at the ~ of the draws is approximately 300 feet The upland and slopes down to the headwall of the landslide area are well veaetated with second growth Douglas.fir and a few western red cedar and madrona tees. The understory consists primarily of sal8:1 and evergreen bucklebeny. No evidence of fractures or impe.ndina upper slope failures are present on the slopes above the headwall area of the hmdslide. The slide area and adjoining slide areas appear to have involved approximately 2P-foot sections of the slope in broad slab-type failures. The exposed very steep compact sand sl9pe5 have subsequently raveled and been eroded by wiDd. I DDtc:d sand covering mucb of the ground and plants on the slopes immediately above landslide headwall area. A IIqe landslide deposit is present along the shoreline at the base of the bluff from the last .... slide IIpPI'Oximately 6 years ago. This slide has sicnificantly altered the shoreline at the base of the slope such that the base of the slope is well protected from shoreline erosion and willlibly be tbal way for many years. CONCLUSIONS AND RECOMMENDATIONS BaRd OIl my inspection of the subject property and vicinity, I conclude 1bat a structDre can be built em the subjFct property such that it will not be at risk from future hmdsJictinc within the expected life of any struo1ure as long as the structure is built at least 150 :feet &om the top of the s...o-p fiII:3.lI.IMB 3 --...- - Oolf' . --r- .. II' 11-' " . A '~.---, I .~... I V"~~/VUVO.40 rAA L ~OV 04' ~,~~ ~nl<JUU. GKUUl' Ila 02 M.-ch 31. 2004 T8 PIRe170llS3016. T.udos RoId. Jefferson eou..y, WA GeoIDIJ E....... steep landslide hadwaIl area and the following recom~aVO.DS are followed. ~ ~ Ground cover vqetation within the 1 SO-foot landslide buffer should not be disturbed. Trees within the buffer ID8Y be tmnlV'.d or limbed for view purposes. However. if the 1reeS are tbiDned, trees should be left on at least 30-foot centeno The mature trees on the slope are hydrologaUy mature and as sudl will areatly reduce the risk of the upper slope from becoming saturated. The trees also play a sipifiamt role in lawyi"lw deposit stability. That is Jarge wood debris gratJy slows the nde at which erosion of a landslide debris pile at the base of the bluff in the event that another 11lM~lide takes place. Typical construction methods are acceptable on the gentle sloping areas on the subject property. If a structure is to be built on the steeper north or south sloping sections of the property on the slopes 30 degrees' or steeper, a geotechnical engineer should review and approve any foundation or retaining wall structures. The development of the subject property will not cause any negative impactS on the stability of the slopes or the shoJetine bluff as long as our recommendations are followed. Soils on the uplaDd portion of the property are well drained. Roof drains and any other d:raiDage should be discbarpd to a water infiltration system that will distribute water into the ground at the site. Please note that tbae are inherent risks associated with building on lots near or adjacent to steep slopes. These are risks that the building owner should recognize and be willing to accept. If conditions appear different than those described in this report, or other coucems arise. we request that we be notified so we can review those areas and modify our recommendations as zequired. We appreciate the opportunity to be of savicc to you. Should you have any questions rqardiD& our rcconnai$~ please contact our office at (360) 714-9409. Sincerely y~ . SInUD CiIoDp File:3J 1.1MB ()tuDlG"'" J(J&-~ Dan McShane, Mkt LE.G. LiN'mged R~-iD8 Geologist 4