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HomeMy WebLinkAbout701205011 Geotech AssessmentSTRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Ph~ne(360:)'1i4-9409 October 13, 2006 Brian Reed 6544 Fautneroy Way SW Seattle, WA 98136 Re: Geology Hazard Evaluation, Shoreline Bluff Tax Parcel 701205011, bast Camp Road Quilcene, WA Dear M.r. Reed: Stratum Group is pleased to present the results of our geology hazard. evaluation of the above re#erenced property. The purpose of this geology evaluation was to qualitatively evaluate the risk o#'slope failures impacting the proposed residence on the subject property and provide general site development and maintenance recommendations. This evaluation included a visual inspection of the property and vicinity, a visual inspection of the slopes an the property and vicinity, review of available geologic mapping in the area, hand dug test holes on the site and inspection of the shoreline at the base of the bluff on the site. "Che property is located on an approximately 200-foot high bluff on the east side of the Bolton Peninsula. The location. of the property is indicated on Figure l . SCOPF: OF SERVICES "I~he scope of our services included the following: 1 } Conducted. a site visit to visually inspect the subject property including the slaps conditions, shoreline conditions, and relevant conditions in the vicinity of the property. 2) Observed surface soil conditions on the slope and on the uplands above the slope. 3) Prepared this report summarizing our findings, including a qualitative evaluation of the shape stability in regards to the proposed residence on the property, recommendations for site development, and recommendations for further investigation, if necessary. ~ _ , A.~~~~'~~~II~ .~1i1~' ~4a#' October 13, 2006 "!"ax Parcel 70120501 I, Last Camp Qu.ilcene, WA Geologic Evaluation of shoreline Bluff GENERAL GEOLOGY Northwestern Washington has been occupied by continental glaciers at least four times during the Pleistocene Epoch (I.6 million to 10,000 years ago). During these glacial and accompanying interglacial periods, the underlying bedrock was eroded and a relatively thick. layer of glacial related and interglacial fluvial. sediments were deposited over the underlying bedrock in the vicinity of the subject property. The Cseologic Map of Surficial Deposits in the Seattle 30' b~ 60' (?uadrangle, Washington (Yount, Minard, anal Dembroff, 1993) indicates the bluff is underlain. by undifferentiated Pleistocene deposits. These deposits are described as consisting of poorly exposed gravel, sand, silt, and clay of uncertain provenance. The Geologic Map of East-Central Jefferson County, Washington (Birdseye, 1976) indicates the shoreline and bluff exposed on the subject property and vicinity is underlain Pre-Vachon glaciation stratifed deposits. Birdseye indicated that there are three glacial till deposits an the bluff in the vicinity of the property, the Possession Till, the Double Bluff Till and the Vachon Till. The Pre-Vachon stratified deposits are described as consisting of sand, gravel, silt and clay deposited prior to the last ice age. Birdseye also indicates that Vashort advance outwash deposits are present on the east slope north of the subject property. Observations on the subject property, slopes in the vicinity of the property and upland areas above the subject property are consistent with the mapping described above. The proposed building site is located on the east side of a north-northeast plunging ridge. "I'he upper portions of the ridge and the building site are underlain by very compacted pre-Vachon glacial deposits consisting of very hard layers of slit. Compact silt deposits are also present at the base of the bluff immediately above the beach. Glacial ice contact/ice wasting deposits are present on the steep shoreline slope below the building area and on the slope north of the building area. These deposits consist of a very chaotic mix of sand and gravel and blocks anal fragments of compact stratified deposits mixed together. 't'opographic features and excellent soil exposures in road cuts south of the subject property are consistent with features found on glacial terrace benches. The ice-contact/ice wasting deposits an the subject property are relatively thin compared to the area to the south. tin expastu~e of glacial till is present on the sleep shoreline slope to the south of the subject property. It is unclear if the till exposure is Vachon Till or an older till. 1:-lowever, the non-glacial silt units exposed along the base of the bluff are extremely hard, and suggest the #iil is an older till as indicated by Birdseye (1976). "lhe Relative Slope Stabilit~r in East-Central Jefferson County. Washington (Birdseye, 1976) map indicates slopes in the vicinity of the subject property are unstable and identifies an area of recent ~~.~~ ~ ~i'~, ~~~~~ ~~~~~~~ ~ 9~ ~~ Stratum Grcwp ~'j'f~: C(}.g (}f~ 2 ~r~ ~~ ~ ~ ~~~, October 13, 2046 Tax Parcel 70120501 1, Last Camp Quilcene, WA Geologic Evaluation of Shoreline Bbff landsliding in the immediate ~Jicinity of the subject property. (:Observations confirm. that there are potential unstable slopes on the subject property and the vicinity. The shoreline at the base of the bluff is erosional and thus the slopes above the shore are potentially unstable and in some places are unstable. Old landslide scarps are present on the steep shoreline slope within the ice-margin sand and gravel. UTnstable slopes are located on the steep slopes west of the subject property, part.ieularly in areas of convergent topography near the upper slopes of the Bolton Peninsula. However, no convergent topography is present near the proposed bu.i.l.ding site area. SPECIFIC SITF. UBSEKVATIUNS The subject property straddles anorth-northeast plunging ridge on the east side of the Bolton :Peninsula. A sketch rnap and cross section of the property with pertinent features indicated are provided in Figures 2 and 3. The building site is located on the east side of the ridge and is accessed via a spur road off of Last Camp Road. The building site is located on a bench area that has been partially cut into the side of the slope. The relatively flat building area is approximately 94 feet long by 59 feet wide. This building area is located approximately 160 feet above the shoreline to the east. `l'he building site itself and the slope above the building site are underlain by very hard silt preglacial deposit. Four slopes bind the Level building area. The slope to the west of the building area slopes upward to the top of the ridge west of the building site. "1'he access road to the building site angles down across this slope in a full bench cut into the hillside and then reaches the building area from the south. T'he slope immediately above the building area has been cut into. Alder trees have become established on the cut slope. 'The cut slope is approximately 20 feet high and slopes up from. the building site at an approximately 50 degree angle for approximately 15 feet with the upper 5 feet being on the order of 65 degrees. The slope above the cut is approximately 35 to 40 degrees. "1'he slope northeast of the building site slopes down to the shoreline at an angle of 45 degrees with the upper most I S feet being about 55 degrees. `Phis slope is tree-covered w7th mature straight fir trees and an understory of salal and ferns and. other brush. Soils on the slope consist almost entirely of sand and gravel. Dld landslide scarps where the slope fractures and. a slab failure nyoved part way down the slope are present at three locations on the slope. However, based on the estimated age of the straight mature trees on the slope, no failures have taken place over the past 130 years. The slope east of'the building area slopes down towards the shore at an a ~a ~ , >>~ ,~ ~. Stratum Circwp (=ile: 50.9.(3(1 ,,; . ~~~~E~~f~ ~~~~~~ ~~~~ October 13.2006 `Tax Parcel 70124501 I, Last Camp Quilcene, WA Geologic ~vatuatian of Shoreline Bluff approximately 40 degrees and is plainer with no indications of past slope movement except near the very base of the bluff. This slope is similar to the slope northeast of the building site, but is less steep and the sand and gravel deposits do not extend as far up the slope. `T'he slope to the southeast of the building area is on the order of 25 degrees for a distance of approximately 125 feet before becoming steeper aver the lower portion of the bluff. Sand and gravel underlie the slope. No indications of slope movement are present with the exception that the lower portion appears to have moved sometime in the past. Along the beach a few slope failures are apparent along the shoreline of the property and in the vicinity of the property. All of these the slope failures appear to be relatively shallow soil failures due to the steepness of the slope. The primary cause of slope failures i s tom undermining of the slope by wave action at the beach. As the slope is undermined the lower portions ofthe slope fail first and the slope failures progress up the slope. Lltimately the slope failures will progress up to the top of the bluff. However, no slides that involved the entire bluff slope were observed anywhere on the bluff in the vicinity of the subject property. It also appears that slope failures are relatively shallow slab like failures that have moved only partially down the slope before stabilizing again. The landslide debris from these slides at the base of the bluff acts as a natural protection for the base of the bluff until over time the debris is washed away and the process of progressive slope failures up the shoreline bluff repeats itself. Approximately half of the soil exposed at the base of the bluff along the shoreline in the vicinity of the subject property appeared to consist of landslide failures. Mature trees are growing on the landslide wedges at the top of the beach.. At locations not covered by landslide slabs, the exposed soil consists of'very compact silt that is relatively resistant to erosion. The time period between the slope failures has been generally sufficient to allow for the growth of mature trees on the landslide soils. 1-fence, most of the slopes above the beach on the subject property and vicinity are heavily wooded. This section of shoreline is well protected from waves due to the presence of ~3road Spit extending out into the bay south of the subject property. "Phis spit effectively blocks waves generated by south winds blowing up Hood Canal. The fetch direction to the north and east is too short to generate significant waves. Based on the presence of 80 year old or older Douglas firs and madrones on most of the bluff slope on this stretch of coast line, and the consistent geology along this portion of the coast, it appears that the frequency of slope failures that. remove all the trees has been Tess frequent than 80 years. y17/~1Q~ ~. e& ~~.t ~i 1f Stratum C7roup File: it).4.E36 C)ctober 13, 2006 Tax Parcel 70 1 2050 1 1, Last Camp Quilcene, WA geologic Evaluation of Shoreline Sluff CONCLUSIONS AND RF,COMMENDATIONS Based on my inspection of the subject property and vicinity,l conclude that a residence can be located on the relatively level building area as indicated in the site plan sketch. A residence located at this site should not be at risk from bluff erosion or landsliding that would threaten the structure within the expected life of the structure (100 years) as long as the recommendations presented below are followed. As described above there are four slopes abutting the level. building area. ~f these slopes, only the slope abutting the northeast side ofthe building area is potentially hazardous to the building site. As such, a home built on the site should be set back at least I S feet back from the slope on the northeast side of the building area. The hazardous portions of the slopes east and southeast of the building site area are well down slope from the building site and do not pose a risk to the building area. within the expected life of the structure (100 years). A deck may be constructed within. the setback from the top of the steep slope as long as it is not an integral part of the structure of the residence. That is if a slope failure occurs, the deck will detach from the house. "I~e upper S feet of the cut slope is steep enough that ravelling of compact silt fragments shauld be expected. This steep section of slope poses a law risk to the level building area. Setting the home 15 feet horizontally back. from this slope will provide adequate protection from the risk oi' failed soil building up against the back. of the home. This distance corresponds to a set back from the toe of the slope of 5 feet. Alternatively, the home may be constructed against the tae of the slope as Long as the back wall is engineered to hold back soil to a depth of 5 feet. No debris should be placed on the top of the bluff. Thinning and trimming of trees on the bluff slope should be limited to topping of alders anal maples and limbing for view corridors of Douglas lir. Any debris from thinning an the bluff slope itself should be removed from the slope. Storm water run off from the roof guttexs should be routed towards the south and should. be infiltrated into the ground in a dispersed manner south of the building. The septic drain field is Located within a communal drainfield adjacent to the access road south of the subject property. The septic tanks may be located south of the proposed building site near the building. The wastewater will be pumped from the tanks up the slope to the drainfield. The drainfield is adequately sized and located such that the addition of water at this location does not pose a risk of causing slope stability problems. Stratum Gmu~ k sl r, i9 r* ^~ ~~~ ~ ~. __ File: 10.~).l?G 5 ~~~~~~1~~]~~I~ ~~~~~ October 13, 2006 Tax Parcel 701205011, last Camp Quilcene, WA Geologic Evaluation of Shoreline Bluff CLOSURE Please note that there are inherent risks associated with lots near ar adjacent to steep slopes. `These are risks that the building owner should recagni~e and be willing to accept. If conditions appear different than those described in this report, ar other concerns arise, we request that we be notified sa we can. review those areas and modify our recommendations as required. We appreciate the opportunity to be of service to you. Should you have any questions regarding please contact our office at (350) 714-9409. Sincerely yours, Stratam Group ~.,.. Dan McShane, L.E.G., M.Sc. licensed Engineering Geologist Y f ~ ~ ~ ~~~ .~~,~ 1 f ~~ ~.' 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