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HomeMy WebLinkAbout983300002 Geotech Assessment (1999)4181 Saltsprings Drive · Ferndale, WA 98248 Phone (360) 380-2507 · Fax (360) 380-2507 November 17, 1999 Mr. Xrmce Richey 25035 108th Ave. SE Apartment F-209 Kent, WA 98031 Re: Report Geological & Geotechnical Site Reconnaissance Blue Jay Lane, E. Jefferson County Parcel No. 983300002 SW 1/4, Sec. 9, T27N, RIE, W.M. Western Geotechnical Consultant, Inc. & Coastal Geologic Services, Inc. are pleased to present the results of our site'reconnaissance, conducted at the above referenced property. This site inspection supplements a reconnaissance level site inspection conducted by the undersigned on September 12, 1997. This initial investigation was performed for the previous owner of the property. On November 9, 1999, Mr. Jim Johannessen of Coastal Geologic Services, Inc. and Mr. Ted Hammer of Western Geotechnical Consultants, Inc. visited the site to evaluate (1) potential safe locations for siting a home and (2) to address special report information requkements 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. Two potential home sites were evaluated in our initial assessment of the site. This report evaluates only the upper site (westerly site) since that is the preferred building site. Figure 2 shows the approximate location of the proposed building site. 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 Site Reconnaissance-Tax Parcel # 983300002 Vince Richey Property November 17, 1999 (Page 2 of 7) Western Geotechnical Consultants, Inc. Coastal Geologic Services, Inc. Job #991591 Site Features and Geology 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 building site (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 building area, with a slope of approximately 42% at the area nearest to the proposed upper house site. The slope is between 55-65 % (measured with a Suunto brand inclinometer) to the west of, and upslope of, the proposed 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 bigleafmaple, western red cedar, Douglas fir, red alder, sword fern, and other ground cover plants. Geology. Field reconnaissance geology was determined by close examination and hand excavation of the subject property, which included shallow test pits and cores retrieved from 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 1978), 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 Thomdyke Road is lodgment till, that is interpreted to be of Vashon age, deposited between approximately 20 and 14 thousand years ago. The till here is composed of pebbly, clayey sand. Lodgment till was deposited directly beneath the ice sheet. Compaction beneath an estimated 3,500 ft. 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 The first site visit (9/12/97). That site reconnaissance followed periodic rains. Portions of the upper slope showed minor seepage at the second site visit (11/9/99). Deposits 2 Sit~e Reconnaissance-Tax Parcel # 983300002 Western Geotechnical Consultants, Inc. Vince Richey Property Coastal Geologic Services, Inc. November 17, 1999 Job #991591 (Page 3 of 7) here are interpreted to be outwash deposited in advance of the Fraser Glaciation, contemporaneous with the Esperance sand and Lawton clay, over "pre-Fraser deposits" (Washington DOE 1978), interpreted to be Possession Drift and primarily Whidbey Formation interglacial deposits. Geology of the western-central portion of the site are well-exposed where it appears that a water line runs down the slope from Thorndyke Road to the north end of Blue Jay Lane, passing through the east-west easement on the property. A somewhat 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. A small debris slide occurred on the upper portion of the slope southwest of the potential building area following the 1997 site visit, probably in the winter of 1997-98. The loss of bank material appeared to have been limited to several feet deep across an appro,-fimately 25 foot wide area. The slide debris was deposited on the lower half of the steep slope. The path of the slide debris, had it continued much further downslope, would have been south of and over the east- west portion of Linda Lane. Presence of Springs The water line that appears to run down the slope to the southwest of the potential building sites contains a spring area approximately 20-40 feet above the base of the slope (Figure 2). A 2-10 foot deep cut is present at the upper portion of the slope here. The bedding material in the apparent water line trench may be transporting water from further upslope to this location. The spring daylights above the easement, and heads east as surface water flow. Wetland indicator plants are present here. 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. Erosion & Land Slide ]lazard The property contains a steep slope along the west boundary. This area should not be disturbed other than the possible removal of hazard trees. The locations where a house would be sited are mildly sloping ground (less than 15%) and should not pose an erosion hazard provided adequate erosion control facilities are utilized during construction and provided adequate long term erosion control measures are performed. The steep slope to the west of the building site was inspected from the base of the slope up to Thorndyke Road, located on the upslope side of a mild sloping bench. The slope angle is up to 55-65%. A mature forest of maple, fir, and cedar is present on the slope. Very large old cedar 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 3 Site Reconnaissance-Tax Parcel # 983300002 Western Geotechnical Consultants, Inc. Vince Richey Property Coastal Geologic Services, Inc. November 17, 1999 Job #991591 (Page 4 of 7) has been relatively stable for at least 600 years. The trees on the slope are straight and appear stable. To the southwest of the building site, 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 and on the steeper slope to the west-northwest of the easement. Figure 2 shows the approximate location of the waterline easement springs and downgradiant seepage channels and the location ora recent shallow slope failure. The building site is sufficiently north of the water line easement and slide area to b.e away from the unstable portion of the 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 below. The construction ora house and garage within the 36 foot by 75 foot building footprint area would not significantly change the physical characteristics of the lot or surrounding areas. Increased runoff, associated with the construction of impervious surfaces (house, garage, etc.) will be collected and transmitted to the community drainage system located along Blue Jay Lane. House construction would not require the removal of any existing trees (except for trees that are deemed to be a direct fall hazard to the home, as determined by a reputable tree hazard appraiser). The proposed building site is approximately 30 feet from the base of the steep slope and approximately 70 feet from the steeper (55-65%) portion of the slope. Standard excavation (less than 5 feet in depth) should not affect slope stability at the planned distance from the toe of the slope slope. However, deeper excavation for a basement should be avoided at the building site. The ground surface for approximately 150 feet east of the subject property is sloped at approximately 10-15%. This relatively gentle slope should preclude large deep-seated failures from occurring. Building Site Evaluation A total of three shallow hand excavated test pits were dug during our initial site evaluation on Septemberl2, 1997 and three additional test pits were excavated during this current investigation. The test pits revealed an orange gray mottled silt grading to a clayey silt below 1 foot, with occasional fine sand layers. Pocket penetrometer tests and hand push probe tests were conducted on in-situ soils in the test holes and the Soils were determined to be stiff to hard in consistency at foundation level (1-1/2 to 2 feet), which means that the soils will support a house founded on conventional spread footings. Site Reconnaissance-Tax Parcel # 983300002 Vince Richey Property November 17, 1999 (Page 5 of 7) Western Geotechnical Consultants, Inc. Coastal Geologic Services, Inc. Job #991591 Conclusions and Recommendations We conclude that 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 site will be close enough to the base of the slope where maintaining vegetation on the slope for slope stability 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 good construction and .maintenance practices are followed and provided normal precautionary measures are taken to maintain the stability of the slope. There are a number of reasons why we r~ach this conclusion, (1) the fact that there is not a marine coast present adjacent to the property (lack of wave attack), (2) there is no evidence in the immediate area of deep-seated failures (such as at Termination Point), (3) excessive seepage does not appear to be present, and (4) the slope behind the potential building sites is well vegetated with diverse native tree and bush species with moderate to high erosion control qualities. We recommend the slope directly behind the potential building sites (to the west) be left in its present vegetated condition with the exception that some of the larger trees that are dose to the proposed building may have to be removed [fit is determined that they present a fall hazard. If no 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. We recommend that a hazard tree assessment be performed before construction begins. If selective trees are removed, every effort should be made to minimize disturbance 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 grow up and add to the root mat stability. Any cut tree trunks should be removed from the slope. No excavation into the toe: of the hillside should occur, as such action could help destabilize the slope. 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. Siltation devices should be placed downslope of all construction areas and cleared areas to provide siltation control during construction. All siltation control devices should be maintained Site Reconnaissance-Tax Parcel # 983300002 Western Geotechnical Consultants, Inc. Vince Richey Property Coastal Geologic Services, Inc. November 17, 1999 Job #991591 (Page 6 of 7) during construction, and lef~ in operable condition until the site has been revegetated and siltation is no longer a threat. At that time the siltation facilities should be removed. Foundation Support & Drainage We recommend that all building foundations be founded on the stiffto hard native soils or on structural fill placed on the native soils. Structural fill is defined as any non-organic predominately granular fill that is placed and compacted to a minimum 95 percent of maximum dry density or as determined by ASTM D-1557 test procedure. Any fill should be placed in thin lifts (8 inch maximum), on a horizontal surface and each lift should be compacted. Structural fill placed beneath footings should extend beyond the edges of the footings a distance equal to the thickness of the fill beneath the footings. Continuous and spread footings should have minimum dimensions of 16 and 24 inches respectively, and should be founded a minimum of 18 inches below final grade for frost protection. If large column footings are planned the soil bearing pressure should be limited to 2,000 psf. This value may be increased by 1/3 for transient loads (wind & seismic). Based on our site evaluation we recommend that a footing drain system be installed around the entire perimeter of the foundation. The footing drain system shoul'd consist ora 4-inch diameter perforated pipe surrounded by washed rock. Based on soil conditions observed at the site, we recommend NOT using a drainage geotextile around the footing drain system. The silty site soils could clog a geotextile, thus making the footing drain system inoperable. The footing drain system should discharge directly into the county storm drain system at Blue Jay Lane by tightline. We recommend that a roof gutter and drain system be constructed at the proposed house. The downspouts should connect to a minimum 4-inch diameter rigid smooth wall pipe that transmits roof water directly to the county drainage system. It is important that this pipe goes directly to the county drain system and not into the footing drain system as this could cause backup of water into the footing drain. All earth supported floor slabs be underlain by a minimum of 4 inches of free draining gravel with a sand or gravel covered vapor barrier on top. Limitations of this Report This report was prepared for the specific conditions present at the subject property to meet the needs of specific individuals. No one other than the client should apply this report for any purposes other than that originally contemplated without first conferring with the geologist and geotechnical engineer who prepared this report. The findings and recommendations presented in this report were reached based on two brief field visits. The report does not reflect detailed examination of deeper subsurface conditions present at the site. It is based on examination of surface features, bank exposures, shallow hand-excavated test pits, and soil characteristics. In addition, conditions may change at the site due to human influences, floods, groundwater regime changes, or other factors. Great care must be exercised when working on or near steep slopes: 6 Si,te Reconnaissance-Tax Parcel # 983300002 Vince Richey Property November 17, 1999 (Page 7 of 7) Western Geotechnical Consultants, Inc. Coastal Geologic Services, Inc. Job #991591 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. References Washington Department of Ecology, 1978, Coastal Zone Atlas of Washington; Jefferson County, Publication No. DOE. 77-21-11, Olympia. Sincerely, Western ~t~techn~al4~onsultants, Inc. Theodore A. Hammer, P.E. Geoteclmical Engineer Coastal Geologic Services, Inc. Senior Geologist Attachments: Figure 1 & 2 Typical Erosion Control Facilities fiN:991591xep EXPIRES 7 4'o00 TAX le 8.00 e. LANE o Illll I1.[$1 I]lll ~llll It Il ti 'Il I I/I / LOT C