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HomeMy WebLinkAbout701201009 Geotech Assessment "''''''':>-701 I :Jet> os- ;t'i"l STRATUM GROUP 1451 Grant Street, Bellingham, W A 98225 Phone (360) 714-9409 January 20, 2006 Henry Boothe th 1005 26 Avenue East Seattle, WA 98112 RECEI\llED Re: Geology Hazard Assessment Tax Parcel 701201009 11111 Piper Road Quilcene, W A JEfffRSuM I'Gllliny' r,,"/,/. ",,"Vii UL_ Dear Mr. Boothe: Stratum Group is pleased to present the results of our geology hazard evaluation of the above referenced property on a shoreline bluff. The purpose of this geology evaluation was to qualitatively evaluate the risk of slope failures impacting the proposed building site 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 on the property and vicinity, review of available geologic mapping in the area, and inspection of the shoreline at the base of the bluff. The property location is indicated on Figure I. The property is located on the east side of the Bolton Peninsula in the northeast quarter of Section 20, Township 27, Range I West W.M. The property extends from the shoreline of Dabob Bay on the east up the slope to an elevation of approximately ISO feet on the west side of the property. SCOPE OF SERVICES The scope of our services included the following: I) Conducted a site visit to visually inspect the subject property including the slope 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 slope stability in regards to the proposed building site on the property, recommendations for site development, and recommendations for further investigation, if necessary. . ! , January 20, 2006 Tax Parcel 701201009, III] I Piper Road, Quilcene, WA Geology Assessment GENERAL GEOLOGY 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 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 Geologic Man of Surficial Deposits in the Seattle 30' bv 60' Ouadrangle. Washington (Yount, Minard, and 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 MaD of East-Central Jefferson County. Washinlrton (Birdseye, 1976) indicates the shoreline and bluff exposed on the subject property and vicinity is underlain Pre- V ashon glaciation stratified deposits. Birdseye indicated that there are three glacial till deposits on the bluff in the vicinity of the property, the Possession Till, the Double Bluff Till and the Vashon Till. The Vashon Till is the youngest of the tills and the Double Bluff Till is the oldest. The till deposits were deposited directly by glacial ice and consist of a mix of rock fragments embedded in a matrix of fine grained material consisting of clay and silt sized rock particles. The Pre- V ashon stratified deposits are described as consisting of sand, gravel, silt and clay deposited prior to the last ice age. These deposits were deposited by glacial melt water and by interglacial streams and rivers. 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. Most of the property is underlain by compact sand and gravel. Glacial till was observed at two locations on the steep slopes of the property and till deposits underlie the lower bluff slopes to the north of the subject property. The proposed building site is located on a bench area that was previously leveled by cutting into the slope on the upper western side of the property. The building site area is underlain by very compacted pre-Vashon glacial deposits consisting of very compact sand and gravel with silt. Compact and gravel deposits underlay most of the steep shoreline slope. Relatively loose sand and gravel deposits are present on portions of the shoreline slope in the vicinity of the subject property. These sand and gravel layers are either pre-Vashon outwash deposits that have not be compacted because of the lack of silt and clay within the unit or they may be a mantle of recessional outwash that was deposited along the ice margins between the compact soils of the bluff and the ice within the present location ofDabob Bay. The Relative Slone Stability in East-Central Jefferson County. Washington (Birdseye, 1976) map Stratum Group File: 12.16.05 2 ! ' , January 20, 2006 Tax Parcel 701201009, 11111 Piper Road, Quilcene, WA Geology Assessment indicates slopes in the vicinity of the subject property are unstable and identifies areas of recent landsliding in the immediate vicinity 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. Unstable slopes are located on the steep slopes west (inland) of the subject property, particularly in areas of convergent topography near the upper slopes of the Bolton Peninsula and along the steep sided ravines on the slopes of the Bolton Peninsula. However, the building site is located in an area of divergent topography and in area of relatively gentle slopes. SPECIFIC SITE OBSERVATIONS A sketch map of the property with pertinent features indicated is provided in Figure 2. The building site is accessed via a spur road off of Piper Road. The level building site essentially was created by cutting into the slope above and using cut soils as fill on the slopes below. The building pad area is approximately 70 feet wide and approximately 105 feet long. The cut soils consist of well-sorted sand and gravel and hence the fill material used to create the building area consists of compact sand and gravel. The cut slope is cut into the slope at an approximately 2: I angle and is approximately 5 feet high. The fill slopes are also at approximately 2: I slopes and are approximately 5 feet high as well. The slopes above and below the building pad area are relatively gentle. The slope above the cut slope slopes up from the building area at an angle of approximately 8 degrees. The slope to east below the fill area slopes down at an angle of approximately 30 degrees over a distance of approximately 130 feet to the top of the steep shoreline slope. The shoreline slope slopes down to the beach below at an average angle of approximately 45 degrees. The slope to the south of the building area slopes down towards a ravine that cuts across the property from west to east. The slope initially slopes to the south at an angle of 15 degrees over a distance of 70 feet to a slope break where the slope steepens to 35 degrees down to the bottom of the ravine. The slopes on the subject property are primarily tree covered with mature Douglas fir as the primary species. The understory consists primarily of salal and bracken ferns. Except for the very steep slopes along the shoreline and on the steep slopes of the ravine, the slopes on the property appear to be stable with very straight trees and no evidence of deep-seated landslides. The ravine that bisects the property from west to east was mostly dry at the time of my site visit in January 2006 despite three weeks of daily rainfall in the region and very heavy rainfall at the time of my site visit. Water was flowing onto the surface from an area of cobbles in the bottom of the ravine south southeast of the building area. This surface flow went subsurface again within approximately 20 feet downstream of the spring. Below this location the stream has sharply Stratum Group File: 12.16.05 3 ? ! , Janumy 20, 2006 Tax Parcel 70]201009, 11111 Piper Road, Quilcene, WA Geology Assessment incised downward and appears to have very high flows on a rare basis. These high flows have caused severe erosion and undermined the slopes of the lower part of the ravine down to the shoreline. Several shallow soil failures have slid down the side slopes of the ravine from the undermined slopes above. A small alluvial fan is located at the end of the ravine creating a small gentle sloping bench above the beach. The steep shoreline slope down to the beach has had shallow surface soil failures in the past. This slope is primarily tree--covered but areas of bare soil are present on portions of the very steep slope where slides have taken place in the past. An approximately 4-foot thick layer of soil and logs from a past slide is located on the upper beach at one location of the subject property. This slide deposit has trees and brush growing on it. No springs or seeps of water were present anywhere on the steep shoreline bluff slope of the property despite the very wet weather the previous few weeks and there were no indications of seasonal wet areas on the bluff slope on the subject property. Seeps and springs of water are present on the bluff slope to the north of the subject property and some of the upper portions of those areas above the bluff may be less stable due to perched water. No perched water is present above the steep slope of the subject property. CONCLUSIONS AND RECOMMENDATIONS Based on my inspection of the subject property and vicinity, I conclude that a residence can be located on the 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 (75 years) as long as our recommendations are followed. No debris should be placed on the top of the bluff or any of the slopes that are greater than 30 degrees. Thinning and trimming of trees on the bluff slope should be limited to topping of alders and maples and limbing for view corridors of Douglas fir. Any debris from thinning on the bluff slope itself should be removed from the slope. Storm water run off from the roof gutters should be routed into dispersion trenches to the east and west of the building area. Water should be able to readily infiltrate into the ground at these sites. The septic drain field may be located will not cause any stability problems on the steep slopes as long as the drain field is located at least 50 feet back from the steep shoreline bluff slope and at least 50 feet back from the steep slopes in the ravine. CLOSURE Please note that there are inherent risks associated with lots near or adjacent to steep slopes. StralUmGroop File: 12.16.05 4 ~ ~ , January 20, 2006 TaxParce] 701201009, 1111] Piper Road, Quilcene, WA Geology Assessment 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 concerns arise, we request that we be notified so 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 (360) 714-9409. Sincerely yours, ~:~ Dan McShane, M.Sc., L.E.G. Licensed Engineering Geologist Stratum Group File: 12.16.05 5 ! . J 2 b o o ci III o l"- V Agure 1. Sit Vldnlty Map 122050.000' W 122049.000' W WGS84 122048.000' W 2 b o o o III o ~ 2 b ~ ~ o ~ 2 b o o ~ o ~ Z b o o ~ o ~ 2 b o ~ CD V o l"- V 122051.000' W TNT !.MN VIS" 122050.000' W 122"49.000' W WGSB4 122048.000' W D 5 I~L~ o 1<<10 1m 0 !QL _ .OOD MEWlS Mop cIIOled with TOPO'. 0DJ3 NobJIlI1 0e0gJapIlic (WW\I'~'" <ODlIIopo) . /f.p F'..~~ . "'-. '- "- '\ \ \ -----_._\" -\..-- ---. \ \ \ \ \ \ 1 I' I I I I ~ $feep ~~Ore'illt Slope. :::;:-LfSQ ~ g....IIJ((I~ 3()il CO -1 --.. 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