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HomeMy WebLinkAboutBLD2004-00343 Geotechnical Report IKrazari & ASSOCIATES, INC. GEOTECHNICAL ENGINEERING • ENVIRONMENTAL ENGINEERING CONSTRUCTION TESTING & INSPECTION June 8,2004 KA Pro'• ► e. ! -I'009 4"Y1LA OU - '30Z 1 Lc OL4 — 3Le3 If .T N.,.--„�. Mr. Cary Casal :, :! �`' '1. V Casal Homes 260 Mumby Road Nordland, WA 98358 JUN 10 RE: Limited Slope Assessment JEFFERSON COUNTY DCD Proposed Residential Structure 1076 Schwartz Road Nordland,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 4, 2004 and also Mr. Greg Ballard of the Jefferson County Department of Community Development regarding the proposed development are in general conformance with Section 3.6.7 of the Jefferson County Unified Development Code amended July 2, 2001. 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 letter 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 Description From our discussions with you on June 4, 2004, we understand that the proposed project will consist of a single family residence approximately 45 feet from an approximately 40-foot high slope. The property is located east of Schwartz Road on Marrowstone Island in Jefferson County, Washington. The property is bound to the north and south by single-family residential structures, to the east by Admiralty Inlet, and to the west by Schwartz Road. The 5-acre site consists of a forested plateau that gradually descends to the east. The subject parcel is terminated to the east by an approximately 40-foot high steep slope with Admiralty Inlet beyond. The Ten Offices Serving The Western United States 20714 State Highway 305 NE,Suite 3C•Poulsbo,Washington 98370•(360)598-2126•Fax:(360)598-2127 102-04009 Casal Homes �" KA No. 102-04009 June 8, 2004 Page No. 2 eastern portion of the site had been cleared of trees and was grass covered at the time of the site visit. The steep slope had young alder trees and secondary plant growth. Access to the beach was provided by stairs on the adjacent property to the south. 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. A visual inspection of the slope face from the beach indicated primarily glacial till. There was some insignificant sloughing noted but overall the slope appeared to be stable in its current repose. There was visible evidence of water seepage at the time of the site visit. A review of the water well log for the subject property indicated that brown cemented subsoil and rocks exist from 2 to 21 feet below the well house elevation which in turn is underlain by brown cemented gravel and rocks to a depth of 80 feet. 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. The United States Department of the Interior United States Geological Survey(USGS), Geologic Map of The 30' by 60' Seattle Quadrangle, Washington dated 1991, indicates that the property and surrounding area is located within Vashon advanced outwash deposits. The Vashon sediments were deposited during the Early to Late Pleistocene and consist of oxidized red-brown gravel and cobbles with zones of iron oxide cementation, and nonglacial deposits of red-brown, oxidized sand and gravel and brown to gray fine-grained sand, silt and clay. The Washington Division of Geology and Earth Resources Geologic Map GM-50 dated 2002 gives the sediments a designation of Glacial Till. 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 stable(1) or normally stable unless modified by man. The "Coastal Zone Atlas of Jefferson County, Washington" Department of Ecology, 1979, indicates that the site vicinity is in an area mapped as stable(S). Krazan &Associates,Inc. Ten Offices Serving The Western United States 102-04009 Casal Homes KA No. 102-04009 June 8, 2004 Page No. 3 Subsurface Soil Conditions and Water Well Log Information The site's subsurface soil conditions were evaluated by observing exposed soil outcroppings located along the shoreline, exposed slopes and road cut areas. Soil The observed soils consisted of medium dense to very dense glacial till consisting of cemented sandy silt with trace gravel and cobble. Groundwater and Apparent Hydrology The static groundwater table as indicated by the water well drilling log was at roughly elevation 124. The well was drilled 118 feet above mean sea level January 15, 1993. 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 Whidbey gravelly sandy loam, which is listed as having slight to moderate erosion potential. CONCLUSIONS AND RECOMMENDATIONS It is our opinion the site does not meet erosion hazard, 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. 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 and the proposed setback of 45 feet exceeds current Jefferson County criteria. Krazan &Associates,Inc, Ten Offices Serving The Western United States 102-04009 Casa!Homes KA No. 102-04009 June 8, 2004 Page No. 4 It is our understanding that ground surface water flow will slope away from the proposed residential structure toward a dry hole situated upland on the property. All water from pavement areas, roof drains, and footing drains should be tight-lined to an appropriate outlet. 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. 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. Based on phone conversation with Jefferson County,we understand Jefferson County will address seismic concerns for the property. 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 (ESC) 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. Krazan & Associates, Inc. Ten Offices Serving The Western United States 102-04009 Casal Homes KA No. 102-04009 June 8, 2004 Page No. 5 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 without 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. Foundation and earthwork construction is characterized by the presence of a calculated risk that soil and groundwater conditions have been fully revealed by the original foundation investigation. This risk is derived from the practical necessity of basing interpretations and design conclusions on limited sampling of the earth. The recommendations made in this report are based on the assumption that soil conditions do not vary significantly from those disclosed during our field investigation. If any variations or undesirable conditions are encountered during construction, the Geotechnical engineer should be notified so that supplemental recommendations can be made. The 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 not be valid. The Geotechnical engineer should be notified of any changes so the recommendations can be reviewed and reevaluated. This report is a geotechnical engineering investigation with the purpose of evaluating the soil conditions in terms of foundation design and slope stability. The scope of our services did not include any environmental site assessment for the presence or absence of hazardous and/or toxic materials in the soil, groundwater or atmosphere, or the presence of wetlands. Any statements, or absence of statements, in this report or on any boring log regarding odors, unusual or suspicious items, or conditions observed are strictly for descriptive purposed and are not intended to convey engineering judgment regarding potential hazardous and/or toxic assessment. The 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 developments. We emphasize that this report is valid for this project as outlined above, and should not be used for any other site. Krazan &Associates,Inc. Ten Offices Serving The Western United States 102-04009 Casal Homes KA No. 102-04009 June 8, 2004 Page No. 6 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. Respectfully submitted, / KRAZAN& ASSOCIATES INC. Nip 0 .,, •t-c e------->"--------- t i ialotigitib.n. / 4,1tV • Shawn E. Williams, L. G. / r� ` Senior Environmental Geologist // `' _ '� 0 2208 • SEW/wrj ��sed Gee Krazan &Associates,Inc. Ten Offices Serving The Western United States 102-04009 Casal Homes