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HomeMy WebLinkAboutBLD2004-00585 Geotechnical Report • ALK CONSULTANTS, LL . AA: ' Environmental Engineering • Geotechnical Engineering • Wetland Consulting March 15, 2005 ACL-05-02-G015 Page No. 1 Mr. Ray Culver 30 Riverdale Drive Quilcene, Washington 98376 GEOLOGIC ASSESSMENT REPORT PROPOSED SINGLE-FAMILY RESIDENCE JEFFERSON COUNTY PARCEL#'S 950-200-625 AND 950-200-626 SOUTH BEACH PARCEL JEFFERSON COUNTY,WASHINGTON This geological report presents the results of a soil and slope assessment directed at evaluating suitability for proposed development at the above referenced site. The scope of this study was developed during our initial site visit on February 3, 2005 and outlined in our proposal to you dated February 23, 2005. The services performed under this authorization were in general compliance with geotechnical report requirements outlined in the Jefferson County Unified Development Code (UDC), section 3.6.10g. Additional fieldwork for this study was undertaken on February 26,2005 and March 1,2005. Site Description The site is located on a primitive road extending off of South Beach Drive as shown on the Vicinity Map, Figure 1. At the time of the site visit building pads for the garage and house had been graded. The subject parcel is bordered by vacant lots to the north and south, a steep ascending slope to the west, and single-family residences with the Hood Canal beyond to the east. Proposed Development Based on our discussions, we understand that the proposed development is to consist of the construction of a wood frame residential structure and garage approximately 30 feet from the toe of the slope to the west. The pads for the proposed buildings were observed at the time of our site visits. It is anticipated/recommended that remaining general site grading for development will be minimal and all disturbed or bare ground areas will be landscaped and/or re-vegetated following construction. Methodology The subject property soil and slope conditions were visually examined and evaluated using available slope exposures in the site vicinity and the site slope itself. This information provided a basis for a general understanding of the local soil and geologic conditions as discussed below. In addition to the 9465 Provost Road NW, Suite 2046 • Silverdale, Washington 98383 • (360) 613-2407 • Fax: (360) 613-2408 05-02-0015 South Beach Para! • ACL05-02-G015 Page No. 2 • March 15, 2005 observation of available soil exposures and reconnaissance of the area the following readily available resources were reviewed. Resources used: • "Soil Survey of Jefferson County Washington",United States Department of Agriculture, 1975. • "Geologic Map of Washington—Northwest Quadrant", Washington State Department of Natural Resources,Division of Geology and Earth Resources, GM—50,2002. • "Geologic Map of East-Central Jefferson County, Washington",R.U. Birdseye, 1976. • U.S.G.S. 7.5 minute series topographic map"Lofall", Washington Quadrangle". • "Coastal Zone Atlas of Washington",Volume 11,Jefferson County, Washington Department of Ecology, 1979. Geologic Setting The subject site lies within the central Puget Lowland. The lowland is part of a regional north-south trending trough that extends from southwestern British Columbia to near Eugene, Oregon. North of Olympia, Washington,this lowland is glacially carved with a depositional and erosional history including at least four separate glacial advance/retreats. The Puget Lowland is bounded on the west by the Olympic Mountains and on the east by the Cascade Range. The lowland is filled with glacial and nonglacial sediments consisting of interbedded gravel, sand, silt,till,and peat lenses. The Washington Division of Geology and Earth Resource (WDGER), Geologic Map of Washington — Northwest Quadrant, dated 2002, indicates that the site is mapped as undifferentiated surficial deposits uc nearglacial till(Qgt). Undiff erentiated) surficial deposits consist of clay, silt, gravel,till,and peat. The term undifferentiated is used because steep slopes preclude differentiation at the map scale. Glacial till consists of an unsorted, unstratified, highly compacted mixture of clay, silt, sand, gravel and boulders deposited by glacial ice. The Geologic Map of East-Central Jefferson County, Washington indicated that the site is mapped as pre- Vashon stratified sediments with beach deposits along the shoreline. The USDA Soil Conservation Service Soil Survey of Jefferson County Area, Washington, 1975, indicates the soils of the site are mapped as Cassolary soil with 0 to 15 and 30 to 50 percent slopes originating in reworked glacial and marine sediments. Permeability is moderately slow, runoff is slow, the erosion hazard is slight to moderate, and building limitation for dwellings without basements is moderate for 0 to 15 percent slopes. Runoff is rapid, the erosion hazard is severe, and the building limitation for dwellings without basements is severe for 30 to 50 percent slopes (more specifically, slopes greater than 15 percent). Alkai Consultants • • ACL05-02-G015 • Page No. 3 March 15,2005 Surface and Subsurface Conditions The site slopes upward toward the west. Building pads have been graded in the eastern portion of the site for a house and garage, a fish pond approximately 4 feet deep is located uphill from the building pads, and an existing storage structure occupies the central area of the site as illustrated on Figure 2, Slope Profile. Existing shallow excavations on the property were observed. Vegetation consists mostly of grass on the east side of the site with swordfern and brush such as salal on the slope area to the west. There are a few mature maple trees throughout the site. No evidence of surficial or deep-seated or rotational landsliding was observed on the property. Based on observed soil exposures and surficial observations,the native soils are relatively firm/dense at shallow depths(approximately 2 to 3 feet)and fine grained soils. The building pads may have some small areas of fill toward the west(downhill)sides of the pads due to grading. The upper 2 to 3 feet of soil over most of the parcel appeared to be moist to wet at the time of our site visits,except for under the existing storage building where the soils were more dry. Based on the soils exposed on and around the property it is our opinion that the site soil conditions are generally consistent with those shown on the geologic and SCS maps of the area. Generally differences between the geologic and soil maps and observed soil conditions occur as a result of mapping scale, availability of exposures,and intended map usage. These differences are generally minor and not unexpected. The "Coastal Zone Atlas of Washington", Volume 11, Jefferson County, Washington Department of Ecology, 1979 identifies slopes in the site vicinity as stable (S) in the lower portion to the east, unstable (U) in the slope area to the west, and Modified(M) in the flat area near the shoreline. An area described as Unstable Recent Slide (Urs) is located to the south of the subject site. Stable slopes are generally less than 15 percent but can include areas of steeper slopes that are stable due to low groundwater concentration or competent bedrock. The stable slope designation also includes areas underlain by weak material such as peat, which are stable because they have no significant slope. Unstable refers to slopes that are considered unstable due to factors such as steepness, groundwater or erosion concerns. Modified slopes include areas that have been highly impacted by human activities such as excavation and fill and performance can be difficult to predict. Unstable recent landslide includes areas of recent or historically active slides. It should be noted that the mapping was performed in the late 1970's and does not reflect more recent activity. There was no visible evidence of ground water, springs or seeps observed on the slope face or within 1000 feet of the subject site at the time of the site visit. Our reconnaissance off the subject site was limited by private property access. Water seepage could occur during periods of heavy rainfall. It should Alkai Consultants • • • ACL05-02-G015 Page No. 4 March 15, 2005 be noted that the pond located on the property is man made and did not appear to be spring fed at the time of the site visits. Conclusions and Recommendations The proposed building footprint appears to be feasible for the proposed development as long as the foundation walls are adequately designed to retain the cut area where the garage and house are joined and the footings bear on firm native soil. It may be necessary to deepen footings in order to bear on firm native soil. Footing drains are also recommended. The house site will be approximately 30 horizontal feet from slopes greater than 15 percent (at the fish pond area) and approximately 120 horizontal feet from slopes greater than 30 percent. The shed structure does not appear to be designed as a permanent or dwelling structure. It is located on a slope with approximately 27% slope and approximately 25 horizontal feet from slopes greater than 30 percent. As previously stated, surficial soils around the shed appeared to be saturated, but the soils under the shed appeared to be more dry suggesting that the surfical moisture does not migrate quickly through the soil. We recommend that roof drains for any structure be tightlined to the toe of the slope near the primitive roadway and an energy dissipater such as quarry spalls be utilized to reduce erosion. Vegetation should be maintained as currently established outside the building and driveway areas. In our opinion, maintaining vegetation on the steeper slopes will help maintain to the stability of the moist to wet surface soils which are underlain by more firm fine grained soil. At the time of this letter report no building plans were available for review. It is recommended that a representative of Alkai view building plans prior to construction and also the excavated footings to verify that the building is bearing on firm native soil. REPORT LIMITATIONS This report has been prepared for the exclusive use of Mr. Ray Culver and his agents for use in planning of the referenced development. The conclusions and recommendations in this report are based on our interpretation of site conditions, as they presently exist, anticipated future construction activities, and the expectation that the exploratory efforts adequately define the subsurface conditions across the site. The soil conditions described in this report and the conclusions and recommendations contained in this report are provided for this specific site only and should not be expanded for use on adjacent properties without additional exploration and review of those sites by our firm. The data and report should be provided to prospective contractors for their bidding or estimating purposes, but the report conclusions and interpretations should not be construed as a warranty of the subsurface conditions. There are possible variations in subsurface conditions. In the event that the scope or location of the project should change or subsurface conditions different from those encountered during this study be observed or suspected, we Alkai Consultants S • ACL05-02-G015 Page No. 5 March 15, 2005 should be advised. At that time a review of the changed conditions will be made, and alternative or remedial recommendations given as required. NOTE: We have not explored subsurface conditions as part of this study, nor have we evaluated the site for the potential presence of contaminated soil, and have not evaluated or addressed ground water conditions or concerns except as noted in this report. The evaluation of possible environmental or geo- environmental considerations is beyond the scope of this report. The owner and the contractor should make themselves aware of and become familiar with applicable local, state, and federal safety regulations, including current OSHA excavation and trench safety standards. Construction site safety generally is the sole responsibility of the contractor. The contractor shall also be solely responsible for the means, method, techniques, sequences, and operations of construction operations. The firm, Alkai Consultants, LLC is providing the preceding information and recommendations solely as a service to Mr. Ray Culver. Under no circumstances should the provision of this information or recommendations be construed to mean that the firm Alkai Consultants, LLC (or subcontractors) are assuming responsibility for construction site safety or the contractor's activities; such responsibility is not implied and should not be inferred. Within the limitations of scope, schedule, and budget for this work, it is warranted that the work has been done in accordance with generally accepted practices followed in this area at the time this is report was made. No other warranty,expressed or implied is made. Should you have any questions or concerns, which have not been addressed, or if we may be of additional assistance,please call our office at(360) 613-2407. Sincerely, oq Washy David Hurum, L.G. , 'i) * Senior Staff Geologist • e,,s�d Ge° DAVID C. HURUM- g i Alkai Consultants ! • J r;., •( I - 1 ` • . il‘. 1\, • I ` • :i f II i r .141 \`" ,. \` f - ,IY\a ! I 1 iI1'111) r _ - „ ' •'h• Ali j� ' ' - a� , ``- � ._ \ I �,jT, \•ic4, 11j ( :, , , SMIre 1..4 i I l� r ,s\ ` •,..: x�`'3`-�, I '- •>` � kt 11 t' rr .Shin .,!I c ult' :if;e'' I C V:\ !, 1' ti11It ' —_ tI " lilt 111 J/f • '� 1 1 Ir'��'.'�r'!4 4 �^ Nfis ti,4 j '`.,�,4, ``,` y�\ �{ " ,� �l+: f}'r � r 7 lj:h /r. i y '' �tr St ''.J f! i:. 11 ,; rlyY.ir/. 1 i .I Jf r I. j ' f y— ..�i -t- 1. tfir r r r - •' I 1 r- 1 1 ! 1, • •4 'I `4 i 1 . of +• I I�r / 5 1 j ' 1l•. y. �• •, l + r n / j I / :, t^�`C r fl r -t� �f ll tXl al i r /' � - 4 1'• , I / / i 1 tyf\•` ., 1 + . r t _ I i �. 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