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HomeMy WebLinkAbout701222011 Geotech AssessmentLIMITED SLOPE ASSESSMENT PROPOSED RESIDENTIAL STRUCTURE 1602 TOANDOS LANE QUILICENE, WASHINGTON JOB NUMBER 102-03050 JUNE 22, 2003 'Prepared for: William and Julie Gaz P. O. Box 122 Indianola, WA 98342-0122 Prepared by: KRAZAN & ASSOCIATES, INC. GEOTECHNICAL ENGINEERING DIVISION 20714 State Highway 305 NE, Suite 3C Poulsbo, Washington 98370 (360) 598-2126 ASSOCIATES, INC. GEOTECHNICAL ENGINEERING · ENVIRONMENTAL ENGINEERING CONSTRUCTION TESTING & INSPECTION June 22,2003 William and Julie Gaz P. O. Box 122 Indianola, WA 98342-0122 KAPr~ectNo. 102-03050 Limited Slope Assessment Proposed Residential Structure 1602 Toandos Lane Quilicene, Washington As requested, we are pleased to present this letter summarizing our limited slope assessment for the subject property. The purpose of this letter report is to provide a professional opinion as to the feasibility of constructing the proposed residential structure. Note that a detailed slope stability analysis was not performed as part of this assessment. In preparing this letter, we visited the subject site to observe general site conditions, reviewed a well log for the subject parcel and reviewed slope stability maps for Jefferson County in accordance with the scope of services outlined in our proposal KA No. PG 03-124, Dated June 9, 2003. Our scope of services are based on responding to a June 2, 2003 letter generated by the Jefferson County Department of Community Development regarding the proposed development and 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. · Logging one exploratory test pit within the footprint of the proposed building and review of the well log for the subject parcel. · 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. PROPOSED CONSTRUCTION From our discussions with you on June 6, 2003 we understand that the proposed project will consist of the construction of a two-story residential structure approximately 100 from the top of a steep slope. The structure will be of wood frame design with a daylight basement. The project location is shown on the attached Vicinity Map, Figure 1. Eleven Offices Serving The Western United States 20714 State Highway 305 NE, Suite 3C · Poulsbo, Washington 98370 * (360) 598-2126 · Fax: (360) 598-2127 102-03050 Gaz SFR KANo. 102-03050 June 22,2003 Page No. 2 SITE LOCATION AND SITE DESCRIPTION General The property is located west of Toandos Lane on the Toandos Peninsula in Jeffers0_n County, Washington. The property is bound to the north by a forested parcel with a cabin, to the south by a vacant forested parcel, to the west by Dabob Bay, and to the east by Toandos Lane. The parcel is 5 acres in size and rectangular in shape. A primitive drive provides access to the site off of Toandos Lane. (See Figure 2). The site consists of a relatively level plateau above a relatively steep slope descending to the west. The slope is approximately 300 feet in height. The site was developed with a small cabin and a septic system at the time of the site visit. The septic system was installed in 1995. The cabin was in place when the current property owners purchased the property approximately 13 years ago and is approximately 190 square feet in size. The central portion of the parcel on the plateau was logged approximately 8 years ago. The remainder of the plateau area is forested, as is the majority of the steep slope to the west. In general, the slope areas are vegetated with a mixture of mature cedars and firs. Localized areas of the slope have a moderate under story of brush. There was no visible evidence of ponding water or surface drainage at the time of the site visit. We understand from you that some erosion occurred near the bottom of the slope approximately 5 years ago. The impacted area was relatively small in size with no significant impact on the slope. 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. The United States Department of Agriculture, Soil Conservation Service, Soil Survey of Jefferson County Area, Washington (SCS) designates the upper portion of the site as Sinclair gravelly sandy loam, 0 to 15 percent slopes. This soil unit is listed as having a moderate limitation for dwellings Krazan & Associates, Inc. Eleven Offices Serving The Western United States 102-03050 Gaz SFR KANo. 102-03050 June 22,2003 Page No. 3 without basements and a slight to moderate hazard of water erosion. The lower portion of the site is designated as Kitsap silt loam, I5 to 30 percent slopes. This soil unit is listed as having a severe limitation for dwellings without basements and a moderate to severe hazard of water erosion. Surface Conditions The Washington State Department of Ecology's Coastal Zone Atlas indicates that the steep slope below the existing cabin is an unstable area with a recent slide on the lower portion of the slope. The portion of the site between the top of the slope and the access road (including the proposed house location) is shown as Intermediate. Intermediate slopes are generally steeper than 15 percent that are not otherwise considered unstable. The eastern portion of the parcel is mapped as a stable area. "Relative Slope Stability In East-Central Jefferson County, Washington" map by R. U. Birdseye indicates similar designations for the area as shown on Figure 3. Jefferson County maps the western portion of the property as being in an area mapped as a seismic hazard. The remainder of the site including the proposed residence is not mapped as a seismic hazard area. Jefferson County also shows the western portion of the parcel as mapped as an area with a high landslide hazard. The central portion of the parcel is mapped as a moderate landslide risk and the eastern portion of the lot including the proposed home site is mapped as a slight landslide risk area. The water well is located in the upper portion of the lot at approximately elevation 320. The well was drilled to a depth of 305 feet with the static water level encountered at approximately 247 below the ground surface (bgs). The depth to water is similar to other wells in the general area. There was no visible water seepage in the exploratory test pit and there was no visible evidence of water seepage or springs within 1000 feet of the proposed single-family residence at the time of our site visit. Site Reconnaissance There were no visible indications of slope failure and/or instability at the time of our site visit. Indications of slope failure or instability include hummocky terrain, inconsistent patterns of vegetation, tension cracks, seepage zones and course grain material overlaying silt and clay soils,. The observed curvature of the trees was slight but consistent. There are slides on properties to the north and south of the subject parcel but not on adjacent lots. However, as discussed previously, there has been minor erosion near the base of the steep slope in the past. During the site visit an exploratory test pit was excavated within the footprint of the proposed residence utilizing a trackhoe. The test pit location is shown as Figure 2 and the test pit log is attached to this letter report. The soils encountered were for the most part consistent with regional mapping of the site. Cemented hardpan (till) was encountered at approximately five feet below the existing grade. No water seepage was noted in the test pit. The small cabin on the property is situated near the top of the steep slope. The cabin has been in place for at least 13 years. The current property owner stated that there had been some sloughing of the slope Krazan & Associates, Inc. Eleven Offices Serving The Western United States 102-03050 Gaz SFR KA No. 102-03050 June 22, 2003 Page No. 4 below the residence approximately 5 years ago. Portions of the parcel were selectively logged approximately 8 years ago. There was no visible indication of structural distress on the cabin at the time of our site visit and no visible indications of recent slide activity or erosion. CONCLUSIONS AND RECOMMENDATIONS Based on our understanding of the Jefferson County Unified Development Code, specifically Section 3.6.7 Geologically Hazardous Areas, the steep western portion of the site is likely an Erosion Hazard Area (due to the severe erosion hazard potential of the Kitsap Silt Loam, 15 to 30 percent slopes listed in the SCS), a Landslide Hazard Area (due to the severe building limitation for dwellings without basements designation of the Kitsap Silt Loam, 15 to 30 percent slopes listed in the SCS and the Unstable and Unstable Recent Slide mapping on the Coastal Zone Atlas), and a Seismic Hazard Area (due to mapping as such by Jefferson County). The less steep eastern portion of the site (including the proposed residence) does not meet any of the hazard criteria. The required buffer under this section from a Landslide Hazard Area is 30 feet from the top of the slope. The proposed residence is to be located approximately 100 fee~ from the top of the slope. Therefore, the construction of the proposed residence will not entail the alteration ora landslide hazard area or its buffer. The following recommendations are general for construction near steep slopes. · Due to the potential for erosion, water should not be allowed to flow over the slope. · Trees greater than about 6 inches in diameter within the Landslide Hazard Area or its buffer should not be removed unless diseased or undermined or otherwise in danger of falling. The advice of a licensed arborist should be obtained regarding trees to be removed in this area. Trees less than 6 inches in diameter may be removed if necessary to improve views and larger trees may be selectively limbed. Again, we recommend the advice of a licensed arborist regarding trees to be limbed. Do not place surcharge loads, such as piles of soil, within the Landslide Hazard Area buffer and do not push loose soil onto the slope. 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. Krazan & Associates, Inc. Eleven Offices Serving The Western United States 102-03050 Gaz SFR KA No. 102-03050 June 22, 2003 Page No. 5 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 field 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. 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 logs regarding odors, unusual or suspicious items, or conditions observed are strictly for descriptive purposes and are not intended to convey engineering judgment regarding potential hazardous materials. 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. 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, t KRAZAN & ASSOCIATES, INC. Shawn E. Williams, R.G. d Senior Environmental Geologist SEW/tsp S. Parkington, P.E. Senior Geotechnical Engineer Krazan & Associates, Inc. Eleven Offices Serving The Western United States 102-03050 Gaz SFR KA No. 102-03050 June 22, 2003 Page No. 6 REFERENCES U.S. Geological Survey, 7.5-minute Seabeck, Washington topographic quadrangle map, dated 1953. Photoinspected 1973. Geologic Map of Washington-Northwest Quadrant, Geologic Map GM-50 (Joe Dragovich & others, 2002). Geologic Map of The 30' by 60' Seattle Quadrangle, Washington, United States Department of Interior United States Geological Survey (USGS), dated 1991. Coastal Zone Atlas of Washington, Jefferson County, Washington Department of Ecology, 1979. Relative Slope Stability in East-Central Jefferson County, Washington map by R. U. Birdseye dated 1976. Geology and Groundwater Resources of Eastern Jefferson County, Washington, Water Supply Bulletin 54, Department of Ecology. Dated April 1981. Soil Survey of Jefferson County Area, Washington, United States Department of Agriculture Soil Conservation Service, Dated August 1975. Lidar and aerial photographs. Krazan & Associates, Inc. Eleven Offices Serving The Western United States 102-03050GazSFR Printed from TOPO! © 199? ~i,~w~ r Prod~r~ (w~,~'.t opo.com) Note: Map adapted from TOPO! © 1997, Wildflo~vcr Produ¢~ons. KRAZAN & ASSOCIATES, INC. 20714 State Route 305-Suite 3C Poulsbo, WA 98370 360-598-2126 FIGURE 1 - SITE VICINITY MAP Job name: GAZ SFR Location: Jefferson County, Washington Job No.: 102-03050 Client: Mr. William Gaz Date: 6/11/03 -% Note: Relative Slope Stabilit~ in East-Central Jefferson Count~, Washington by R.U. 1976. KRAZAN & ASSOCIATES, INC. 20714 State Route 305-Suite 3C Poulsbo, WA 98370 360-598-2126 FIGURE 3 - SLOPE STABILITY MAP Job name: GAZ SFR Location: Jefferson County, Washington Job No.: 102-03050 Client: Mr. William Gaz Date: 6/11/03 Project: Gaz SFR Client: Mr. William Gaz Location: Toandos Peninsula Depth to Water: Not Encountered Log of Test Pit TP-1 Project No: 102-03050 Figure No.: A-1 Logged By: S.E.W. Elevation: SUBSURFACE PROFILE SAMPLE DATA om Water Content (%) Description 0~ ~ o~ EE ~ · ~0 -, c ~ '~ 10 20 30 40 50 Ground Surface SANDY SILT (ML) . Soft, reddish tan, moist. Scattered cobbies. SILTY SAND WiTH GRA VEL (SM) Medium dense to dense, fine grained sand, tan, moist. Scattered cobbles. Becomes cemented at 5 feet. (GLACIAL TILL) End of Test Pit No sloughing of test pit sidewalls. Method: Trackhoe Excavator: Operator: Krazan and Asso'ciates 20714 State Highway 305 N.E. Suite 3C Poulsbo, Washington 98370 Excavation Date: 6/6/03 Sheet: I of 1 ,~4AT.F_RtAE2: - --.". ! S'F.(OM := :_if.h-ainee. licep~ed driller's F. em~'r~2n NJ ;iP?'"~':,~TX }h":"~': -'-r- 7' . n:: ~,-~ ','--