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HomeMy WebLinkAboutBLD2008-00048 Geotechnical Report •j • STRATUM GROUP P.O. Box 2546,Bellingham,WA 98227 Phone(360)714-9409 February 8, 2008 RE cp,IvED Colleen Scott FE3 1 3 2��3 10258 Battle Point Drive NE Bainbridge Island, WA 98110-1490 Pry fSflN(fllN/ DCD Re: Geology Hazard Assessment Parcel # 601324007 Thousand Trails Road Quilcene, Washington Dear Ms Scott: Stratum Group is pleased to present the results of our geology assessment of the above referenced property adjacent on a shoreline bluff. A yurt is already located on the property and a new septic system is proposed for the property. The purpose of this geology assessment was to 1) evaluate the potential risk of slope failures impacting the existing yurt site and the proposed septic system, 2) evaluate the risk posed by the construction of the new septic system to the stability of the slope, and 3)provide general site development and maintenance recommendations for the property adjacent to a potentially unstable bluff. Based on my investigation of the site and vicinity, it is my opinion that the yurt is not at risk from slope failures over the expected life of the structure (25 years) and that construction of the proposed septic system will not be at risk nor with construction of the system increase the risk of slope failures at the site or off site provided the recommendations presented in this report are followed. This assessment included a visual inspection of the property, slopes on and in the vicinity of the property, available geologic mapping in the area, and investigation of the shoreline at the base of the bluff. SCOPE OF SERVICES The scope of our services included the following: 1) Conducted a site visit to visually inspect the subject property and vicinity including the bluff slope conditions, slope conditions on and in the vicinity of the property and shoreline conditions. 2) Observed surface soil conditions on the shoreline slope and upland areas by excavating shallow hand dug test pits. 3) Prepared this report summarizing our findings, including an evaluation of the risk of slope failures impacting the residence on the subject property and the proposed • February 8,2008 Parcel 601324007,Jefferson County,WA Geology Assessment u ' 1 i V septic system, a qualitative evaluation of the shoreline bluff stability, recommendations for site maintenance, and recommendations for further investigation, if necessary. 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 Map of Surficial Deposits in the Seattle 30' x 60' Quadrangle, Washington (Yount, Minard and Dembrof, 1993) and the Geologic Map of West-Central Jefferson County, Washington (Birdseye, 1976) indicate the subject property is underlain by Vashon till at on the upland area of the property, Vashon advance outwash and the preglacial stratified deposits with an older till at the base of the bluff in the vicinity of the subject property. The till units consist of a poorly sorted mix of clay, silt sand and gravel with occasional boulders. The preglacial stratified sediments consist of interbedded oxidized brown, red-brown, and gray gravel, sand, silt, and clay. The layers are moderately to well bedded and the unit contains minor amounts of ice- contact deposits and outwash gravel and sand. Generally the unit is nonglacial and has abundant peat and woody debris. These sediments were deposited sometime prior to the last glacial event and include deposits of the Whidbey and Olympia nonglacial periods. The sediments have been overridden and consolidated by glacial ice. The Vashon advance outwash was deposited by melt water streams from the advancing Vashon ice sheet. The unit generally consists of sand and gravel with a coarsening upward within the unit. My observations on the bluff face and uplands of the subject property and vicinity are consistent with the mapping described above. The Vashon till does not appear to be present along the very top of the bluff where the yurt is located. This area is underlain by well rounded gravel with sand and is consistent with the upper part of the Vashon advance outwash. SPECIFIC SITE OBSERVATIONS A site plan sketch showing pertinent features is provided in Figure 2. The subject property consists of a nearly level upland area on the east that slopes back away from the top of the steep slope on the west side of the property such that the upland area actually slopes towards the east away from the steep shoreline bluff. Except for the cleared area immediately adjacent to the yurt site and Sierra Trails Road that angles across the upland portion of the subject property, the upland area is tree-covered primarily with Douglas fir and with an understory of salal and Stratum Group File:11.15.07A 2 • RECEI\ J 1_ F L 3 13 200,3 February 8,2008 Parcel 601324007,Jefferson County,WA Geology Assessment -;-C ra f 1i "FPt evergreen huckleberry. The Yurt is supported on concrete blocks. The closest block to the top of the steep slope to the southwest is slightly more than 15 feet from the top of the steep slope. The proposed drainfield is located to the east of the yurt. The upland is bounded on the west by southwest facing slope that slopes downward towards the very steep shoreline bluff slope. This slope slopes downward at an angle of between 35 and 40 degrees. The top of the slope is at an elevation of approximately 420 feet. The slope is plainer with a gradual curve to the south off of the subject property such that the slope faces directly west south of the subject property. The slope is tree-covered primarily with Douglas fir with an understory of salal and evergreen huckleberry. The trees on the slope are straight and there is no indication of recent slope movement or slides. The slope is underlain primarily with sand and gravel in a very compact condition and appears to be well drained. Numerous old skid/logging roads cut across the slope in the vicinity of the property. Except for minor raveling along road cut slopes, the roads and past harvest activities do not appear to have caused any slope stability problems and no landslide areas were observed. The slope described above is bounded on the west by the steep shoreline bluff slope. The bluff slope is much steeper and is vertical in many locations. The shoreline bluff slope is approximately 80 feet high. The steep shoreline bluff is underlain by very compact sand, gravel and silt and is very resistant to erosion due to the very compact nature of the soils. No springs or seeps or seasonally wet areas are present on the bluff slope. The time of my site visit was a few days after an extreme heavy rain on snow event where as much as 16 inches of rain was recorded over a two day period a few miles south of the subject property. Despite this recent very high rain fall event I observed no indications of surface water flow on the property. A narrow drainage swale that ends along the top of the steep shoreline bluff appeared to have had a small flow of water from the recent heavy rainfall event as the soil at very top of the steep shoreline bluff contains silt layers. No significant erosion was caused by the water flow and the drainage was dry at the time of my site visit indicating that water flow in this drainage is very rare. I would note that other seasonal drainages in the vicinity including one a half mile from the subject property had very high damaging water flows during the storm event of early December 2007. The shoreline bluff is eroded by a combination of wave action that undermines the base of the bluff, ravelling of material off the exposed soils on the unvegetated portions of the bluff, and periodic shallow soil failures. The material eroded from the bluff face is removed from the base of the bluff and transported along the shore generally towards the north by prevailing wave Stratum Group Hie.1 I.15.07A 3 • RECEIVE! February 8,2008 1`3 13 2008 Parcel 601324007,Jefferson County,WA Geology Assessment i f iEBSO ; if t ll action. The beach was in overall good condition and consists of sand and gravel. Erosion at the base of the bluff appears to happen only during very high tides/storm events as the top of the beach appears to be inundated infrequently. CONCLUSIONS AND RECOMMENDATIONS Based on my visual inspection of the subject property and vicinity, I conclude that the existing yurt and the proposed septic system are not at risk from potential landslides or erosion induced slope failures within the expected life of the yurt structure (25 years) as long as my recommendations are followed. The very steep shoreline bluff slope on the western most portion of the subject property is a geologically hazardous landslide slope. The yurt is located approximately 500 feet east of this steep shoreline bluff slope. The 35 to 40 degree slope between the level to gently sloping upland area and the very steep shoreline bluff slope is stable and is not subject to landslides or other slope movement beyond very minor soil creep. This interpretation is not consistent with the Washington Coastal Atlas which is based on the Relative Slope Stability in East-Central Jefferson County, Washington by Birdseye (1976). Birdseye (1976) identified this feature as a former slide area with unknown stability. LIDAR imagery as well as topographic mapping indicates a broad convergent area on this slope; however, there is no evidence of slope failures or landslide deposits anywhere on the slope or below the slope. The entire slope as well as the shoreline bluff slope to west is underlain by undisturbed preglacial deposits. This feature may be associated with a very ancient past failure, but the feature is not definitive and there is no clear field evidence of this feature being a slide area. If the feature is a slide area, it is clearly a very old feature and may have been associated with slope erosion that would have taken place shortly after the glacial ice had retreated out of the area and relative sea-level was higher than at present prior to the region rebounding from the removal of the glacial ice through isostatic rebound. Relative sea level is estimated to have been up to 600 feet above the current sea level shortly after the glacial retreat (Geologic Map of the Freeland and Northern Part of the Hansville 7.5-minute Quadrangle, Island County, Washington.,Polenz, Schasse and Peterson, 2006). Based on the location of the site, relative sea level may have been on the order 100 to 200 feet in the area. The former higher relative sea level has formed a variety of elevated shoreline features around Puget Sound and may be the cause of the broad convergent landforms in this area. Regardless of the feature observed, there is no evidence in the area of recent slope movement with the exception of the steep shoreline bluff itself and there are no landslide deposits anywhere on the slope. Stratum Group File I.15.07A 4 • � February 8,2008 Parcel 601324007,Jefferson County,WA „i , Geology Assessment My investigation and observations of the slope and soils on the slope indicate that the slope is stable and is not a landslide hazard. Evidence observed on the site to support this conclusion: The slope angle of 35 to 40 degrees is stable due to the fact that the slope is underlain by very well drained and very compact sand and gravel. Given the compact nature of the preglacial sand and gravel underlying the slope, this slope angle is stable. All trees on the slope and stumps on the slope are very straight indicating minimal soil creep. Past logging of the slope did not cause any slope failures. Road cuts on the slope from past logging activity have been stable with the exception of minor raveling. No springs or seeps are present anywhere on the slope. Building Setbacks It is my opinion that the steep slope to the southwest of the yurt is stable and there is no risk of landslides on this slope under present conditions. The steep slope is stable and no specific setback is required from this slope. However, if a home were to built on the site, I recommend that base of the home foundation be set back from the slope 7.5 feet as measured horizontally from the base of the foundation to the intersection of the slope in order to minimize the risk of differential settlement along the outer edge of the slope. No fill soils are present along the outer edge of the slope. The very steep shoreline bluff slope is a geologically hazardous landslide slope due to erosion at the toe of the bluff and the very steep slope conditions. The yurt is located approximately 500 feet back from this steep slope. This distance is well back from the slope and will provide adequate protection to the yurt. Shoreline Because the shoreline bluff is an eroding bluff, slope failures should be expected to occur on a periodic basis along the bluff shoreline. The only way to prevent continued erosion of the base of the bluff is to construct hard armoring at the shoreline at the base of the bluff. However, the eroding bluff acts as a feeder bluff(erosion of the bluff provides sediment) for the beaches east and west of the subject property, and any shoreline armoring will have a negative impact on properties down drift from the bluff. The construction of shoreline armoring on other properties Stratum Group File:11.15.07A 5 • • February 8,2008 Parcel 601324007,Jefferson County,WA Geology Assessment - i c P. ' ; ri.r may cause an increase of erosion of the subject property. The owner of the subject property should contact Jefferson County officials regarding any proposed shoreline protection projects along the bluff or any shoreline protection construction. There are currently no shoreline- armored areas along this stretch of coastline. Storm and Septic Drainage Soils on the upland portion of the property are well drained. The soils appear to consist primarily of coarse grained deposits. The present manner of dispersed drainage should be maintained. Any additional buildings if constructed should have roof drains and any other drainage should be discharged into a level dispersion/infiltration trench located to the east of the structure. The proposed septic drain field should not cause any problems for slope stability as long as the drain field is located at least 30 feet back from the top of the steep shoreline slope. Vegetation Site grading soils or debris, landscape debris, or any other material should not be disposed of over any slopes greater than 30 degrees. Over time debris will break down and form an unstable mass that will slide damaging the slopes below. A 50-foot buffer of trees and native vegetation should be maintained along the top of the steep shoreline bluff slope. If trees are harvested or thinned or limbed for view purposes on the slope between the buffer and the upland area, all debris should be removed from the slope. Please note that there 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 concerns arise, we request that we be notified so we can review those areas and modify our recommendations as required. 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. '' as Sincerely yours, ��O — 4, Str turn Group �'►� �p )21./k---- an • ' J McShane, M.Sc., L.E.G. .. Licensed Engineering Geologist f `- . . r " :/ o,a 1 378 04, o 4;1` ,/C)Z Stratum Group File:1 1.I j.07A DDaniel'McShane I • Figure 1.• Location 47°42,101' N, 122°49.494' Va.S84 122°51,000' W 122°50.000' W 122°49.000' W WGS84122°48.000' W fit" :ait .I r:i..::'?.::::::„..::...-- N'ctl.:41'( 2 .. 4 s "jr= I 1P .� 0 k` O / K' •," 1• .e.: - ' .. ``o O 06-44 r. cr:,h ' •/ ,'- -3.- ri. ,, , t,) ,y,."--7,-,,, % , 1 \ - 1 - , i&11.-- / / • Y . wirr . _ \ /, + ,/ /N 1:f' lif 1\ii) )M-7,::1 / ( / • /27 ' -.!i ' I 17- \ I ..: :\Y 1\' . 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