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HomeMy WebLinkAboutBLD2006-00078 - 09 Geotechnical Report 111 111110 ALLEN L. HART ENGINEERING GEOLOGIST 1720 North Oakes,Tacoma, Washington 98406 (253) 752-8963 July 7,2005 Project No. 0506042 Rick Davis 280 Whale Road' j913„, Brinnon,Washington 98320 FEB F '‘,.r t@a+ Report of Soil Study and Geological Assessment . :x.00 Proposed Single-Family Residence 162 Dusty Road r is! : prn Brinnon,Jefferson County,Washington Parcel#502-104-037 This report presents the results of a soil and slope review and geologic assessment directed at evaluating potential geologic hazards at the referenced site.The scope of this study was developed during our site visit on June 16,2005 at which time verbal authorization to proceed with the study was received. Fieldwork and data acquisition for this assessment was undertaken between June 29,2005 and July 7, 2005,with both dates being inclusive. As requested a copy of this report has been sent to Jerome Fitzgerald in Shoshone,Idaho. The subject site is located between Dusty Road and U.S.Highway 101,approximately 3/4 mile south of Dosewallips State Park and overlooks Hood Canal to the east,as shown on the Vicinity Map,Figure 1 of this report.The proposed building area is located in the northwest portion of the lot,as shown on Figure 2, in an area which has previously been cleared and terraced.As presently proposed a single-family residence is to be constructed at the north end of the terrace adjacent to the cut slope with a garage located about 80 feet to the south of the residence and about 15 feet from the east edge of the terrace. During previous site grading for development of the terrace cut materials were pushed over the east facing natural slopes and it appears that the top of the slope was extended outward on the order of 10 to 12 feet by filling.Under the current development plan additional site grading is anticipated to be minimal and all foundations will be extended as may be necessary to bear on firm,undisturbed native soils. Following construction,all areas of disturbed and/or bare ground will be re-vegetated, landscaped or otherwise provided with erosion protection such that no erosion or landslide hazards are developed. GEOLOGIC AND GEOTECHNICAL CONSULTING TrA j Project No.0506042 Report of Soil Study and Geological As ment 162 Dusty Road 9 4 Brinnon,Washington t' "` E.aO July 7,2005 Page No.2 Methodology ARM; Wl 3 HO Soil and slope conditions of the subject property were visually examined and evaluated using available slope exposures both on the site and adjacent areas.In addition to the observation of available soil exposures and reconnaissance of the area the following readily available resources were reviewed. Resources used: 1. "Soil Survey of Jefferson County Area,Washington",United States Department of Agriculture, 1975. 2. "Geologic Map of Surfacial Deposits in the Seattle 30' X 60' Quadrangle,Washington", U.S.G.S.Open file report 93-233, 1993. 3. Coastal Zone Atlas of Washington,Jefferson County volume. 4. U.S.G.S. 7.5 minute series topographic map`Brinnon,Washington Quadrangle" 5. Aerial Photographs 6. On-line water well information available from the Washington state Department of Ecology web site. 7. On-line Jefferson County landslide,erosion,and seismic hazard maps. Distance and slope measurements referenced in this report were determined in the field using small, handheld equipment,i.e.,Brunton compass, Suunto inclinometer,tape measure,and optical range finder. The base map for Figure 3 was prepared by Gene Thompson as part of the septic system design and provided by Rick Davis. Site Surface and Subsurface Conditions The proposed development area is located on a natural bench-like feature which has been enhanced by previous grading into a nearly level building site.As part of the previous grading spoil materials were pushed over the east side of the bench as an over slope fill.It appears that this cutting and filling extended the east edge of the bench outward some 10 to 12 feet resulting in the current terrace configuration.At the rear of the terrace is a cut slope ranging from about five to 12 feet in height with slope angles ranging from<40 degrees to near vertical(slope grades<84%to>100%). The cut slope is backed by natural slopes ranging from<20 degrees to near 28 degrees(<36%to 53%slope grades), which extend some 100+feet west,up slope,to Dusty Road,beyond which the slopes steepen.Below the terrace natural slopes range from<20 degrees to about 25 degrees(slope grades of<36%to 47%).The fill slope face of the terrace ranges from about 38 to 40 degrees(slope grades of 78%to 84%). ALLEN L.HART ENGINEERING GEOLOGIST 1720 North Oakes,Tacoma,Washington 98406 F Project No.0506042 Report of Soil Study and Geological Asent 162 Dusty Road ' 4 Brinnon,Washington July 7,2005 Page No.Nowhere 0Y r:; b Tv i1P Nowhere on the property was evidence of past or ongoing slope soil movement � . The Jefferson County landslide Hazard map available on the web indicates that the property is in an area considered to have a slight hazard of landsliding.However,the Coastal Zone Atlas(CZA)indicates that the major portion of the property is considered to be of intermediate stability with slopes considered to be unstable extending about 100 feet into the property from the west-southwest.Based on my observations, it is my opinion that as mapped the CZA unstable slopes do not extend to cover the proposed building site,and the mapped area of unstable slopes should not extend beyond the west side of Dusty Road adjacent to the site.It is suspected that differences between mapped and observed conditions are a result of map scale and gross topographic mapping combined with outcrop availability. As exposed in cuts along Dusty Road,the cut at the rear of the terrace,and in the site water well,below a thin layer of forest duff the upper most site soils are comprised of a brown to tan,loose to dense, silty, gravelly sand which is underlain at a depth of about three to six feet by gray,very dense, silty,gravelly sand.The looser soils appear to be a combination of weathered and root disturbed glacial outwash and glacial till while the gray,denser materials appear to be an unweathered glacial till.Based on the site water well soil log the till underlies the site to a depth of 65 feet,below which is a ten foot thick layer of gravel overlying basalt bedrock.Up slope of the property the near surface soils are comprised of bedded outwash sands and gravels. The Soil Conservation Service(SCS)mapping indicates that the property is located on or very near the contact between the soil units Hoodsport very gravelly sandy loam, 15%to 30%slopes(SCS soil type HoD)and the Hoodsport-Grove very gravelly sandy loam,0%to 30%slopes(SCS soil type HrD).The SCS indicates that the parent material for the Hoodsport soils is a glacial till while that of the Grove is a glacial outwash. Soils of the Hoodsport-Grove series are a combination of the two soil types as would be typically found near a contact or transition zone between materials. Based on the soils exposed on and around the property,it is my opinion that the site soil conditions are generally consistent with those shown on the geologic and soil maps of the area.It is suspected that any differences between the mapped and observed soil conditions are a result of mapping scale,availability of exposures,and intended map usage. Vegetation around the terrace area is comprised of a mixed age and species woods with a moderately dense to dense understory of brush and shrubs.Across the terrace flat there is no vegetation while on the ALLEN L.HART ENGINEERING GEOLOGIST 1720 North Oakes,Tacoma,Washington 98406 11 l t Q Project No.0506042 J. A � 1 Report of Soil Study and Geological As ent 162 Dusty Road y 9 ) CS Brinnon,Washington + s 13 t> July 7,2005 Page No.4 terrace slope f a c e and across the cleared area below vegetation is a m i x ' '; ' # "c ss and weeds. At the north end of the terrace ground water daylights from the cut slope at the rear of the terrace.This water is intercepted in a shallow French drain and directed around the north end of the terrace and then down slope for disposal where down slope slopes and properties are not jeopardized.This water appears to flow atop the dense till layer and/or through sandier seams within the till and because of its apparent shallow depth may be a seasonal flow. Based on the site water well log it appears that the static water level at the well site is about 23 feet below the ground surface. Conclusions Based on my site observations,reconnaissance and review of the materials noted above, it is my opinion that: • No landslide hazards are present on the natural site slopes. • The east-facing fill slopes on edge of the terrace are steep but do not show evidence of slope soil movement,distress,or excessive erosion.Landscaping and vegetating of the slope as planned will aid minimizing the potential for slope erosion. • The proposed building area does not meet the requirements for identification as a landslide, erosion,or seismic hazard area. • A top of slope building setback(buffer)of 15 feet from the top of the east-facing slope at the edge of the terrace may be used provided that all foundations are extended as necessary to bear on firm,undisturbed native soils. • Due to the potential for long-termed raveling or sloughing of materials from the face of the cut slope at the rear of the terrace,a minimum setback of 15 feet is recommended. • All runoff from roofs,patios,driveway and parking areas,and similar hard surfaced areas should be intercepted and directed away from slopes and to appropriate disposal areas.The free flow of water over the terrace fill slope face should not be allowed due to the potential for erosion and slope damage. Aids for Runoff Control and Landscape Planning: Maintenance of slope vegetative can aid in slowing the overall rate of erosion and slope degradation. Although not written specifically for this type of site,the Washington State Department of Ecology has ALLEN L.HART ENGINEERING GEOLOGIST 1720 North Oakes,Tacoma,Washington 98406 Project No.0506042 Report of Soil Study and Geological A ent • 162 Dusty Road FEB 2 4 2C16 Brinnon,Washington July 7,2005 Page No.5 e. r. igof E�y� three publications,which may be helpful in developing runoff control,Ian z ;+ _ IiJller- termed slope vegetation maintenance plans. 1. "Slope Stabilization and Erosion Control Using Vegetation:A Manual of Practice for Coastal Property Owners",May 1993,Publication 93-30. 2. "Vegetation Management:A Guide for Puget Sound Bluff Property Owners",May 1993, Publication 93-31. 3. "Surface Water and ground Water on Coastal Bluffs:A Guide for Puget Sound Property Owners",June 1995,Publication 95-107. These publications can be obtained through the Olympia office of the Department of Ecology. Report Limitations This report has been prepared for the exclusive use of the Rick Davis and his agents for use in developing the referenced project.The conclusions and recommendations in this report are based on my visual observations,interpretation of site conditions as they presently exist,and the expectation that the exploratory efforts adequately define the subsurface conditions throughout 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 sites or properties without additional exploration and review of those sites by this firm.The results of this study are intended for the use of the original client only.Use of this report in whole or part by third parties will require a written agreement be in place between the consultant and the third-party. NOTE: Although I have reviewed subsurface conditions as part of this study,I have not conducted analytical laboratory testing of any samples obtained,have not 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 owner and the contractor should make themselves aware of and become familiar with applicable local,state,and federal safety regulations,including current O.S.H.A. 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,Allen L.Hart Engineering Geologist(including consultants and subcontractors)is providing the preceding information and recommendations solely as a service to Rick Davis.Under no circumstances should the ALLEN L.HART ENGINEERING GEOLOGIST 1720 North Oakes,Tacoma,Washington 98406 Project No.0506042 • Report of Soil Study and Geological Aliment 162 Dusty Road Brinnon,Washington July 7,2005 Page No.6 provision of this information or recommendations be construed to mean that the firm Allen L.Hart Engineering Geologist(including consultants and subcontractors)is 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 if I may be of additional assistance,please call my office at(253)752- 8963. Sincerely sky ., l Allen L.Hart,CPG,RG/CEG a Licensed Engineering Geologist `E"gl"°°r5 0°° ?ALLEN HA T ALLEN L.HART ENGINEERING GEOLOGIST 1720 North Oakes,Tacoma,Washington 98406