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HomeMy WebLinkAboutBLD2004-00407 Geotechnical Report I I .�G ''�F• �av� ,/. %//IF41i,� 3`� III 7lU\ 'IU�II.YI I I I LIMITED SLOPE ASSESSMENT 141 BLUE RIDGE ROAD PORT TOWNSEND,WASHINGTON 1 000 306-624 Project No. 102-04057 July 20,2004 I6094-(6-7 ' Prepared for: I Ms.Susan Fleming I 454 Reinier Rd. Nordland,WA 98358 I Prepared b : P Y Krazan&Associates,Inc. I 4 20714 State Hwy. 305 N.E., Suite 3C PUulsbo, WA 98370 I JUL 21.10 200� (360) 598-2126 JEFFERSON COUNTY DC'I = IKra'zari & ASSOCIATES, INC. ISITE DEVELOPMENT ENGINEERS ISLOOLI - fl � � �I 3b I I ratan & ASSOCIATES, INC. ' GEOTECHNICAL ENGINEERING • ENVIRONMENTAL ENGINEERING CONSTRUCTION TESTING & INSPECTION July 20, 2004 KA Project No. 102-04057 Ms. Susan Fleming ' 454 Renier Rd. Nordland, WA 98358 RE: Limited Slope Assessment Proposed Residential Structure 141 Blue Ridge Road Port Townsend, Washington As requested, we are pleased to present this letter summarizing our limited slope reconnaissance for the ' subject property. The purpose of this report is to provide a professional opinion as to the feasibility of constructing the proposed residence and its potential impact on a nearby slope. In addition, seismic concerns and a detailed slope stability analysis were not performed as part of this assessment. In preparing this letter report, we visited the subject site to observe general site conditions Scope of Services ' Our scope of services are based on conversations with yourself on June 16, 2004 regarding the proposed development are in general conformance with Section 3.6.7 of the Jefferson County Unified ' Development Code effective July 2, 2001 (Amended June 9, 2003). Our scope of services for the project included the following: • A site reconnaissance by a member of our staff to evaluate the surface conditions at the project site. • Preparation of this report presenting the results of our field observations and providing our professional opinion regarding the impact of the proposed development on the subject slopes. SITE LOCATION AND SITE DESCRIPTION ' The property is bound to the north and south by undeveloped lots, to the west by Blue Ridge Road, and to the east by a greenbelt. Access to the site is provided by a driveway off of Blue Ridge Road. The ' subject parcel is part of the Discovery Bay Ridge Division, which for the most part is a relatively level plateau. From our discussions with you on June 16, 2004, we understand that the proposed project will consist of a single-family residence approximately 20 feet from a descending 20 to 30 percent slope which is approximately 30-feet high adjacent to the proposed building area and continues at 20 percent or less ' Ten Offices Serving The Western United States 20714 State Highway 305 NE,Suite 3C• Poulsbo, Washington 98370•(360)598-2126•Fax: (360)598-2127 102-04057 Blue Ridge t ' KA No. 102-04057 July 20, 2004 Page No. 2 ' thereafter. The property is located northwest of Cape George Road in Jefferson County, Washington (see Figure 1, Vicinity Map). The approximate 5-acre site in general primarily consists of a slope, which descends toward the east. The western portion of the parcel is relatively level area that is part of the plateau area of Discovery Bay ' Ridge. This portion of the property was partially cleared at the time of the site visit. The top of the slope nearest the plateau had been logged and was vegetated with low growth native flora. A swale was observed on the northern portion of the site. There was no visible evidence of drainage or of wet soils at the time of the site visit. PROPOSED CONSTRUCTION ' From our discussions with you on April 26, 2004, we understand that the proposed project will consist of a single-family residence on an approximate 5-acre lot. The proposed single-family residence will be ' of light wood frame construction, two stories in height. It is our understanding that the subject residence will be a circular home kit. The proposed location of the home is shown on Figure 2. ' Site Reconnaissance During our reconnaissance a limited site survey was performed. There were no visible indications of ' slope failure and/or instability, which include hummocky terrain, inconsistent patterns of vegetation, tension cracks, seepage zones and course grain material overlaying silt and clay soils, at the time of the site visit. However there were observed loosed soils on portions of the top of the slope, which ' apparently had been pushed over the top of the slope during clearing activities for the proposed building location. ' A septic test pit was observed near the top of the slope on the southern portion of the plateau area. A second septic test pit was observed on the slope face itself. Dense to very dense cemented glacial till was observed in both of the test pit excavations. The approximate locations of the observed test pits are ' shown on Figure 2. ' Geologic Setting and Slope Stability 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 that includes 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. Krazan &Associates,Inc. ' Ten Offices Serving The Western United States 102-04057 Blue Ridge ' KA No. 102-04057 July 20, 2004 Page No. 3 The 1989 US Depai fluent of the Interior, US Geological Survey map "Surficial Geologic Map of the Port Townsend 30-by 60-minute Quadrangle,Puget Sound Region, Washington"indicates the site is in an area of Advance Outwash Deposits (map symbol Qva) which consists of sand, gravel, silt, and clay deposited by meltwater flowing from an advancing glacier ice margin of the Vashon-aged Puget lobe. The Washington Division of Geology and Earth Resources Geologic Map of Washington —Northwest Quadrant, dated 2002, also indicates that the property is in an area mapped as glacial advance outwash (map symbol Qga). The texture and structure of this material vary from place to place, but is generally composed of moderate to well-sorted, distinctly stratified, light gray to tan, medium to coarse sand and pebbly sand containing minor amounts of fine sandy silt, and scattered lenses and layers of gravel. A review of the State of Washington, Depat tment of Ecology Geologic Map of Eastern Jefferson ' County also indicates this site is within an area of glacial advance outwash (Qva, Quaternay Vashon Advance Outwash). The Soil Survey of Jefferson County Area, Washington, indicate the soils comprising the site are Clallam gravelly loamy sand (map symbol CmD), which are well-drained soils that formed on glacial till, that have a very slowly permeable cemented layer at a depth of 20 to 40 inches. The two observed test pits confirmed this description. ' Slope Stability A review of the "Relative Slope Stability in Northeastern Jefferson County, Washington" map by ' Martin J. Gayer, 1976, was performed in conjunction with preparing this study. The mapping by Martin Gayer indicates that the subject property is located in an area mapped as Class (3), areas of poor natural stability. ' Subsurface Soil Conditions and Water Well Log Information The site's subsurface soil conditions were evaluated by observing previously excavated septic test pits, exposed soil outcroppings along slopes and road cut areas. I Soil The observed soils consisted of medium dense to very dense glacial till consisting of cemented sandy ' silt with trace gravel and cobble. While the geologic maps show the site on glacial advance outwash, the soil survey map indicates an area of transition between soils formed on glacial till and those formed on advance outwash. Groundwater and Apparent Hydrology ' A drilling log of a nearby water well indicated that the static groundwater table was at an elevation of roughly 233 feet above mean sea level. The well was drilled 350 feet above mean sea level July 14, 1992. t Krazan&Associates,Inc. ' Ten Offices Serving The Western United States IO2-04057 Blue Ridge KA No. 102-04057 July 20, 2004 Page No.4 ' Perched ground water may exist above the level of the near surface dense underlying stratum, during or just after wet weather conditions. Perched groundwater develops where vertical infiltration of surface ' precipitation is impeded by a relatively impermeable soil layer, resulting in horizontal migration of the groundwater within overlying more permeable soils. ' Soil Erosion Hazards Erosion hazard areas are defined as those areas that because of natural characteristics, including ' vegetative cover, soil texture, slope, gradient, and rainfall patterns, or human-induced changes to such characteristics, are vulnerable to erosion. The Jefferson County guidelines classifies all areas that are identified in the United States Department of Agriculture, Soil Conservation Service, Soil Survey as having a moderate to very severe erosion potential as erosion hazard areas. The soils comprising the site are mapped in the Soil Survey as Clallam gravelly loamy sand, with slopes of 15 to 30 percent, which is listed as having moderate erosion potential. ' CONCLUSIONS AND RECOMMENDATIONS It is our opinion the site does meet erosion hazard classification, and landslide hazard requirements presented in the Jefferson County Code (Article VI-G. Geologically Hazardous Areas). ' In our opinion, based on our slope reconnaissance, and historical landslide research for this area, the slopes are relatively stable against surficial slope movement and the potential for deep-seated rotational failures of the underlying dense material is low. In our opinion, the risk of damage to the proposed ' building, or to adjacent properties, from soil instability is minimal and the proposed construction will not significantly increase the potential for soil movement under both static and seismic loading conditions. The proposed setback of 20 feet for the residential structure will be sufficient provided other recommendations in this report are followed. Note that we have not analyzed the overall stability of the slope. It is our opinion that such an analysis is not necessary for the proposed construction as the impact of the structure on the slope is minimal at worst. The loose soil noted on portions of the top of the slope are not significant and do not pose a threat to the overall stability of the slope. It is our understanding that ground surface water flow will slope away from the proposed residential ' structure. All water from pavement areas, roof drains, and footing drains should be tight-lined to an appropriate outlet and not directed to flow over the top of the slope. Groundwater Concerns Perched ground water may exist above the level of the dense underlying stratum, during or just after wet ' weather conditions. Perched groundwater develops where vertical infiltration of surface precipitation is impeded by a relatively impermeable soil layer, resulting in horizontal migration of the groundwater within overlying more permeable soils. Krazan & Associates, Inc. Ten Offices Serving The Western United States 102-04057 Blue Ridge KA No. 102-04057 111 July 20, 2004 Page No. 5 ' Landslide Hazards and Seismic Hazard ' Based on historical landslide research for this area, the slopes have been identified as stable (S). However, based on our slope reconnaissance of the area, in our opinion, the slopes are relatively stable against surficial slope movement and the potential for deep-seated rotational failures of the underlying ' dense material is low. In our opinion, the risk of damage to the proposed buildings, or to adjacent properties, from soil instability ' is minimal and the proposed construction will not significantly increase the potential for soil movement. Note that we have not performed a slope stability analysis at this site and have not performed a seismic hazard analysis. ' Note that the site should be graded in such a way that surface water runoff is not directed over the top of slopes, either during construction or when construction is complete. This is especially important for a slope that is bare of vegetation. EROSION AND SEDIMENT CONTROL Erosion and sediment control should be used to minimize the transportation of sediment to wetlands, streams, lakes, drainage systems, and adjacent properties. The site soils may be eroded by channelized ' water or sheet flow storm runoff. Therefore, it is recommended that all site preparation and excavation work be completed during the normally drier portion of the year. During periods of heavy rainfall, ditching should be used to divert water away from stripped areas and visqueen should be used to cover the slopes and soil stockpiles to aid in preventing excessive surface erosion. This covering also aids in preventing infiltration of water into the unprotected soils. All disturbed soil areas and slopes should be ' replanted with fast-growing, deep-rooted grass, shrubs and other ground cover as soon after final grading as practical. If the vegetation is not fully established prior to the onset of wet weather, the slopes should be covered with visqueen to aid in preventing excessive erosion and water infiltration. ' We also recommend that erosion control measures be implemented in graded areas. The site should be graded to reduce surface run off over slope areas. By reducing surface runoff, the potential for erosion, ' raveling, gully washers, shallow slides and debris flows will be significantly reduced. LIMITATIONS ' Geotechnical engineering is one of the newest divisions of Civil Engineering. This branch of Civil Engineering is constantly improving as new technologies and understanding of earth sciences improves. ' Although your site was analyzed using the most appropriate current techniques and methods, undoubtedly there will be substantial future improvements in this branch of engineering. In addition to improvements in the field of Geotechnical engineering, physical changes in the site either due to excavation or fill placement, new agency regulations or possible changes in the proposed structure after the time of completion of the soils report may require the soils report to be professionally reviewed. In light of this, the Owner should be aware that there is a practical limit to the usefulness of this report Krazan &Associates,Inc. ' Ten Offices Serving The Western United States 102-U4057 Blue Ridge I KA No. 102-04057 July 20,2004 Page No. 6 Iwithout critical review. Although the time limit for this review is strictly ,arbitrary, it is suggested that two years be considered a reasonable time for the usefulness of this report. IThe conclusions of this report are based on the information provided regarding the proposed construction. If the proposed construction is relocated or redesigned, the conclusions in this report may Inot be valid. The Geotechnical engineer should be notified of any changes so the recommendations can be reviewed and reevaluated. IThe geotechnical information presented herein is based upon professional interpretation utilizing standard engineering practices and a degree of conservatism deemed proper for this project. It is not warranted that such information and interpretation cannot be superseded by future geotechnical I developments. We emphasize that this report is valid for this project as outlined above, and should not be used for any other site. If you have any questions, or if we may be of further assistance, please do not hesitate to contact our office at(360) 598-2126. � I I Respectfully submitted, WI*, �g Of W_ , .` KRAZAN& ASSOCIATES,INC. 7 3 t I ' V i��/� Shawn E. Williams, L.E.G. , i ' \ ,,.�E","""i'g Geologist w. : Zr0nmtal Geologist j• 'sed G®°`,77 ' Shawn E. Williams 7 — ' ._o -- I I I I Krazan &Associates,Inc. I Ten Offices Serving The Western United States 102-04057 Blue Ridge ' KA No. 102-02097 July 20, 2004 Page No. 6 REFERENCES 111 U.S. Geological gi Survey, 7.5-minute,minute,Port Townsend Washington topographic quadrangle map, dated 1953. Revised 1994. Soil Survey of Jefferson County Area,Washington,USDA Soil Conservation Service, 1980. Geologic Map of Washington-Northwest Quadrant, Geologic Map GM-50 (Joe Dragovich&others, 2002). Surficial Geologic Map of The Port Townsend 30' by 60' Quadrangle g Puget g Sound Region, Washington, United States Department of Interior United States Geological Survey(USGS), dated 1988. Coastal Zone Atlas of Washington, Volume 9, Jefferson County, Washington Department of Ecology, 1979. ' Jefferson CountyCritical Areas Ordinance, Jefferson efferson County Depai taient of Community Development, Amended June 9, 2003 I i I Krazan &Associates, Inc. Ten Offices Serving The Western United States 102-04057 Blue Ridge I I )../ : _/_,... __1 -----:,,,Ntc.--' ', - ' '1 ,''', s'N,\ ' ',,, . 1/4„.i.k._.,.._„,,,,,,, \\3\ il / ---,...4 , ,,-- ...,„„.,,,,,, ) fit 4 0 if -1")\\s1111\ I ---\ ' SITE , ! N. I CAPE GEORGE ROAD i 1 ' i , �,, k `(� S 1 'r► , s ‘t ,,./ , 1,81 :. >- ..,,,,y,,,\ .. i \ ,,,,, c.. ,9 /. : 11iN( 1 \\ ,. 11 1I `'� dIi , ~tom 'f''' , t ° �, \ t , ._ 1 / ,,,,,),\ ,...,,,,,,/ 1 i /��-0 If RI, �. if f �' !i _Az -- --" ramp Meeting ,,'/ / ---,, I _ , G �...-,•--- --_ 1 - .:.- Ber Printed from TOPO! t'1997 Wildflower Productions(un..n r.topo.com) Note: Map adapted from TOPO!© 1997,Wildflower Productions. I FIGURE 1 — SITE VICINITY MAPAill I KRAZAN & ASSOCIATES, INC.20714 State Route 305-Suite 3C Job name: Blue Ridge Slope Assessment Location:Port Townsend,Washington Poulsbo,WA 98370 Job No. : 102-04057 360-598-2126 Client: Ms.Susan Fleming IDate:6/25/04 I f� r;4 ......_ • . b i, i 1 1 , = i i _ i :. ,1 i ii. 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