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HomeMy WebLinkAbout983300002 Geotech Assessment (1997)September 26, 1997 4181 Saltsprings Drive · Ferndale, WA 98248 Ms. Beverly Hanson Phone (360) 380-2507 ° Fax (360) 380-2507 883 Park Ave. NE Banbridge Island, WA 981 I0 Re: Report Geological & Geotechnical Site Reconnaissance Blue Jay Lane, E. Jefferson County Parcel No. 983300002 SW 1/4, Sec. 9, T27N, RIE.~, W.M. OCT 2 8 jE'FFEP, qr~ ,,: ~ Western Geotechnical Consultant, Inc. & Coastal Geologic Services, Inc. is pleased to present the results of our site reconnaissance, conducted at the above referenced property.. On September 12, 1997, Mr. Jim Johannessen of Coastal Geologic Services, Inc. and Mr. Ted Hammer of Western Geoteehnical Consultants, Inc. visited the site to evaluate (1) potential safe locations for siting a home and (2) to address special report information requirements required by Jefferson County Critical Areas Ordinance. The property occupies approximately 2 acres. A road easement bisects the property (Linda Lane), in an east-west turning north-south direction and a utility easement runs north through the center of lot 2. (Figure 1 is a general site plan showing the site relative to surrounding properties). Figure 2 is a site plan showing the property boundaries and associated easements. Figure 2 also contains topographic lines with 5 foot contour interval resolution. We evaluated two potential home sites, both located on lot 2. Figure 1 also shows the two potential home sites together with the approximate locations of test holes advanced to evaluate subsurface soil and groundwater conditions. This report addresses information required by the Jefferson County Permit center. Specifically we have addressed the following: 1. Geologic setting of the region 2. Potential landslide & erosion hazards on the site 3. Potential impacts of proposal on existing geologic hazards 4. Recommendations for minimizing the risk of erosion and landslides beverly Hanson-Site Reconnaissance Western Geotechnicai Consultants, Inc. September 26, 1997 (page 2 of 7) Site Features and Geolo~ Topography and Site Features: The above-referenced property is located at the northern .end of Blue Jay Lane, adjacent to the Bridgehaven and the Admiral's Row areas (see Figure 1, location map). The property is located landward of South Point Road, and is approximately 450 feet west of the marine coast. The coast at.the closest location to the property is a protected coast which is located behind the large spit extending from South Point, in the Bridgehaven development. The portion of the lot containing the two building sites (the northern half of the 1.9 acres) consists of a cleared, gently sloping area (11-15%) between 85 and 110 feet in elevation. A hillslope is located west of the two potential building areas, with a slope of approximately 42% at the area nearest to the proposed upper house site. The slope is between 55-65 % (measured with an inclinometer) to the west of, and upslope of, the proposed upper house site. A 1-2 foot deep cut is present across approximately 100 feet of the base of the slope. The hill slope is well vegetated with second growth forest cover. Species include a mix of deciduous and evergreen trees, including abundant large bigleaf maple, western redcedar, Douglas fir, red alder, sword fern, and other ground cover plants. Geolokm: Field reconnaissance geology was determined by close examination and hand excavation of the subject property, which included shallow test pits and cores retrieved fi'om the uplands. Geologic reconnaissance extended beyond the western property boundary to sites which could help define the geology in the broader context. Our geologic evaluation also included the examination of bluffs south and northeast of the site, including the Termination Point area, review of the Coastal Zone Atlas of Washington for Jefferson County (WA Dept. of Ecology 1979), and from professional experience with Pleistocene sediments of the region. Geologic deposits around the site consist of Pleistocene glacial and non-glacial (interglacial) sediments. The uppermost soil in the vicinity of Thorndyke Road is lodgment till, that is interpreted to be of Vashon age, deposited between approximately 20 and 14 thousand years ago. Till here is composed of pebbly, clayey sand. Beverly Hanson-Site Reconnaissance Western Geotechnical Consultants, Inc. September 26, 1997 (page 3 of 7) Lodgment till was deposited directly beneath the ice sheet. Compaction beneath an estimated 3,500 t~. of ice resulted in till of moderate to high strength. The lodgment till examined was dry. Lodgment till extends most of the way across a very gently sloping pasture area immediately west of the subject property, above the steepest slope. Soils present on the slope consist primarily of silt and fine sand, deposited in a sequence of silt, silty sand, and sandy silt deposits. Individual beds within these deposits appear to be discontinuous laterally. The entire slope directly upslope of the proposed building site was dry at the time of examination (9/12/97). Our site reconnaissance followed periodic rains. Deposits here are interpreted to be outwash deposited in advance of the Fraser Glaciation, contemporaneous with the Esperanee sand and Lawton clay, over "pre-Fraser deposits" (Washington DOE 1979), interpreted to be Possession Drift and primarily Whidbey Formation interglacial deposits. Geology of the western-central portion of the site are well-exposed _' r~ _ --- . ........ e, p~sing t o ' - ~ easement on the property, at greater amount of silt and clayey silt deposits occur here, as compared'to the hillslope directly west of the potential building site. The test pits excavated between the base of the hillslope and the extension of Blue Jay Lane (Figure 2, also discussed in the following section). Soils underlying the potential building sites consist of stiff silt with minor clay content. This deposit is a continuation of the sequence that occurs on the slope. Presence of Sorings: The water line running down the slope to the southwest of the potential building sites contains a spring area approximately 20-30 feet below the slope crest (Figure 2). A 2-10 foot deep cut is present at the upper portion of the slope here. Apparently, the bedding material in the water line trench is transporting water from further upslope to this location. The er flow. Wetland indicator plants are presen ere. Surface water flow continues down the easement to the east, as discussed below. No other springs were identifies on the property or within the immediate, surrounding area. Beverly Hanson-Site Reconnaissance Western Oeotechnical Consultants, Inc. September 26, 1997 (page 4 of 7) Erosion & Land Slide Hazard Erosion Hazard' The rot~ertv contains a steep slol~e along the west boundary ~ e locations where a house wou are mildly sloping ground (less than 15%) and should not pose an erosion hazard provided a _ r ~ _ .. ~ H ' 'ng construction. The steep slope to the west of one of the potential building sites was inspected fi'om the base of the slope up to Thorndyke Road, located on the upslope side of a mild sloping bench. The slope angle is roughly 55-65%. A mature forest of maple, fir, and cedar is present on the slope. Very large old tree stumps from when the area was originally logged are also present on the slope. These stumps had 5-7 foot diameters, and are present at the original ground surface level, indicating that the slope has been stable for at least 400 years. The trees on the slope are straight and appear stable upslope. To the southwest of the potential building sites, considerable erosion and minor slumping has occurred along the waterline trench easement. The springs have created erosion channels down the pipe alignment to a slope break where the water splits into two channels, with one channel extending to the north of the waterline easement. The waterline cut and associated spring water has caused instability along the slope. This instability is for the most part contained within the waterline easement. Figure 2 shows the approximate location of the waterline easement springs and downgradiant seepage channels. The two potential building sites on Lot 2 are sufficiently north of the water line easement and northern most seepage channel to be away fi.om the unstable portion of tho slope. (see Conclusions and Recommendations for recommended action here). Potential Impacts of Building On Existing Geologic Hazards The existing risk of landslide at the site is deemed to be moderately low, as discussed belo~ ~ --- _'.: _ ' '. _~-- ' ~ _~ 6footby75 foot potential building square feet), would not significantly change the physical characteristics of the lot or surrounding areas. Construction would not change ~s~u~rfaco water drainage, except to collect any sheet flow and direct precipitation into a I~~tl/ll~ouse construction would not require the removal of any existing trees. (except for trees t~'h-~f'~3~emed to be a direct fall hazard to the home, as determined by a reputable tree hazard appraiser). Beverly Hanson-Site Reconnaissance Western Geote~hnieal Consultants, Inc. September 26, 1997 (page 5 of 7) The western proposed building site is approximately 30 feet from the base of the slope and approximately 70 feet from the steeper (55-65%) portion of the slope. Standard excavation (less than 5 feet in depth) should not,~_e_ct slope ~! at the pla~ ~.i$~g~9~ ~gg~g~, The ground surface for approximately 150 feet east of the SUbject property is sloped at approximately 15-25%. This relatively gentle slope should preclude large deep-seated failures from occurring. Building Site Evaluation The two sites were evaluated on Lot 2 at the approximate locations shown on the site plan, Figure 2. A total of three shallow hand excavated test pits were advanced to 1.1- 2.8 feet in depth. The sand layers. Test p~ ~ght seepage in the bottom of the hole (2.8 feet). Pocket penetrometer tests were conducted on in-sim soils in the three test holes and the soils were determined to be stiffto hard in consistency, which means that the soils will support a house Conclusions and Recommendations We conclude that either of the home site locations can be built on without significantly impacting the stability or erosion potential of the site provided our recommendations are followed. The lower site is far enough from tho slope where it will not have any impact on slope stability and only our comments regarding erosion control should apply to this site. The upper site will be dose enough to the base of the slope where maintaining vegetation and stability of the hillslope should be addressed. Landslide Hazard: Our reconnaissance level observations of the slope behind the upper potential home site location indicates that the landslide potential of the site is low provided normal precautionary measures are taken to maintain the stability of the slope. There are a number of reasons why we reach this conclusion, (1) the fact that there !~s not a marine coast p~sent a~d'acent to the property (lack of wave attack), (2~'~ ~' ' : '~ '~ ~ (such as at Termination Poi-~)~ :~ ~~i--:~ present, ~d (4) t Beverly Hanson-Site Reconnaissance Western Geotechnical Consultants, Inc. September 26, 1997 (page 6 of 7) · - ...... fno satisfactory binding agreement can be reached with the owner of the majority of the adjacent slope to maintain the large majority of the existing large trees, then we recommend that only the lower potential house site be used. f selective trees are remove , every effort should be made to ~~ of the slope. Where trees are removed, we recommend that the stumps be left in the ground to provide root strength while surrounding vegetation is allowed to gro~ Erosion Control: The site soils consist of silts, sandy silts and clayey silts. Such materials are susceptible to erosion if exposed to precipitation and/or runoff. Erosion control during construction of proposed facilities can be accomplished through placement of proper sedimentation control facilities. We recommend that siltation control facilities, consisting of either hay bales or Silt fences, be fabricated around the construction area. Typical details for siltation control using either hay bales or silt fences are attached to this report. ed ~tion until the site has been revegetated and siltation is no longer a threat. At that time the siltation facilities should be removed. Beverly Hanson-Site Reconnaissance Western Geotechnical Consultants, Inc. September 26, 1997 (page 7 of 7) -- percent-of maximum dry density or as determined by ASTM D-1557 test procedure. Any fill should be placed in thin lffis (8 inch maximum), on a horizontal surface. We appreciate the opportunity to be of assistance to you on this project. If you have any questions regarding the contents of this report, or if we can be of further assistance, please contact our office. Sincerely, Western Geotechnical Consultants, Inc. Theodore A. Hammer, P.E. Geotechnical Engineer Coastal Geologic Services, Inc. Senior Geologist Attachments: Figure 1 & 2 Typical Erosion Control Facilities file:97741.rep