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HomeMy WebLinkAbout901114027 Geotech Assessment NORTHWESTERN TERRITORIES, INC. 717 SOUTH PEABODY STREET, PORT ANGELES, WA 98362 Engineers · Land SurveYors · Geologists Construction Inspection · Materials Testing (360) 452-8491 1-800-654-5545 FAX 452-8498 E-Mail: info@nti4u.com www. nti4u, com GEOTECHNICAL REPORT For 204 Totem Ridge Road, Tax Cf: 901114027 Located in Section 11, Township 29 North, Range 1 West, W.M. Jefferson County, Washington Prepared for HARVEY WINDLE o i- oo--( 6o Prepared by NORTHWESTERN TERRITORIES, INC. 717 S. Peabody Street Port Angeles, Washington 98362 December 19, 2001 December 19, 2001 Harvey Windle Forest Gems 807 Washington St. Port Townsend, WA 98368 Subject: Geotechnical report for 204 Totem Ridge Rd., Tax Cf: 901114027 located in Section 11, Township 29 North, Range 1 West, W.M., Jefferson County, WA Dear Mr. Windle: Introduction' At your request, Bill Payton of Northwestern Territories, Inc. (NTI) conducted a geotechnical inspection of the above referenced property on December 18, 2001. The purpose of the inspection was to observe the slopes in the area of the proposed homesite by visual means and to prepare a report of our findings with regards to the relative stability of these slopes in accordance with the Jefferson County Unified Development Code (UDC) for Geotechnical Reports. It is our understanding that you plan to construct a three bedroom house with a daylight basement occupying a footprint of about 48, by 25'. Plans also call for a shop to be located east and south of the prOposed house. Site Description: The property is located off of Redeemer Way just north of Chimacum in the W.E. Seton Short Plat. The property is hilly and wOoded with a large ravine~to the south. Driveways are in and the areas for the house and shop have been cleared. The proposed homesite is situated on a knoll on the western slope of a ridge that overlooks Chimacum Valley. The property is mapped as a Iow level landslide hazard area by the Jefferson County Department of CommunitY Development. The knoll where the house will be located has slopes that range from about 6 to 20 degrees (11%-37%).' The steepest slope, which is on the east side of the proposed house, is only about 17 feet high and terminates in a .small gully. This gully drains into the large ravine but has been blocked by the construction of the driveway leading to the shop area. The proposed house will sit about six feet from this slope and about ten feet from the 12 degree slope on the west side. The large east-west trending ravine south of the' homesite is about 140 feet away and has a slope of around 25 degrees. The slopes around the homesite are well vegetated with trees and brush and most of the trees had straight trunks. The area where the gully used to drain into the ravine has very little vegetation. This could be because of erosion and/or disturbance by off-road vehicles. The proposed shop will be located on a terrace east of the homesite and 30 or more feet back from the large ravine (the' standard setback distance per Jefferson County UDC). The terrain east and north of the shop location slopes up at angles ranging from 10 to 14 degrees (18%-25%). These slopes were also well vegetated with trees and brush. Periods of heavy rain preceded the site visit and the only water that was noticed, aside from puddles and surface runoff, was ponding in the small ravine east of the homesite. No culvert was noticed under the driveway in the ravine. You expressed an interest in possibly developing this area for use as a storm water detention pond, however, the structural integrity of the fill for use as a dam is unknown. Site Geology: The Soil Survey of Jefferson County maps the s°il in the area of the subject property as the Clallam gravelly sandy loam (CmD). This soil formed in glacial till on terraces on the breaks of canyons and steep drainageways. Runoff of this soil is medium, and the hazard of water erosion is moderate. - The State of Washington Department of EcOlogY, in the "Geologic Map of Eastern Jefferson County Washington" maps the soil in the area of the subject property as Lodgment till of Quaternary age (Qvt). This soil is described as boulders, cobbles and pebbles in a matrix of sand, silt and Clay; a compact and unsorted mixture. Visual observations made of test pits at the homesite revealed silty sand with gravel-and cobbles. These observations generally concur with the above soil descriptions. Conclusions and Recommendations: Based on visual observations made during the site visit, it appears that the slopes around the proposed homesite are grossly stable. The following recommendations should be considered with regards to development of the subject property: 1. Bare soils are subject to erosion, thus ground cover should be encouraged and maintained. Native vegetation that requires little or no irrigation would be most beneficial. Also, the existing establiShed trees and brush provide stabilization to the slopes and should be left in a relatively natural state. 2. The area where the gully used to drain into the ravine should be 3 revegetated to reduce erosion. If erosion persists, it may need to be further protected from erosion (i.e. lined with rock, etc.). 3. The pond created in the gully shOuld have a way of releasing excess water to reduce the possibly of overtopping or breaking through the driveway leading to the shop. During the site visit, there appeared to be only a couple feet or so of water in the pond. It may be that the soil can infiltrate the water sufficiently enough that the pond would not fill up and that a release mechanism is not needed. It may be possible to monitor the situation to determine if improvements are necessary. A portable sump pump and hoses could be kept on hand if needed. 4. Heavy irrigation or other activities that would contribute large quantities of water to the soil should be avoided. One cause of landslides is the presence of excessive groundwater in the soil near slopes. 5. Runoff from roofs and other hard surfaces such as driveways and patios should be controlled and routed to a detention basin, dry well, other drainage control device such that surface water discharge or sedimentation to adjacent properties will not be higher than pre- development conditions. Further, footing drains should be incorporated into the design of the home. 6. All drainage control devices should be maintained in good working order and inspected at least once a year. 7. Silt fences or other sediment Control devices may need to be utilized in order to prevent sedimentation to adjacent properties during construction. An engineered drainage and erosion control plan should be developed for this property to address items 2, 3, 5, 6, and 7 above. A contractor knowledgeable in the construction of these types of features should be retained for this work. Based on the findings, recommendations and limitations of this report: 1. There is minimal landslide hazard as proven by a lack of evidence of landslide activity in the vicinity in the recent past. 2. An analysis of slope stability indicates that the proposed home would not be subject to risk of landslide under normal conditiOns. 3. The proposal would not increase surface water discharge or Sedimentation to adjacent properties. 4. The proposal would not decrease slope stability on adjacent properties. 5. The proposed homesite would be stable under normal geologic conditions. For further information please review the three pamphlets, which are enclosed with the original of this report, published by the Washington .State Department of Ecology entitled: "Slope Stabilization and Erosion Control .Using Vegetation", "SurfaCe Water and Groundwater on Coastal Bluffs" and "Vegetation Management: A Guide for Puget Sound Bluff Property Owners". Limitations This report is based on a visual inspection of the existing site conditions, barring major catastrophic events such as earthquakes or abnormal weather conditions. No scientific measurements, tests or calculations were performed. The findings and report are limited to the normal standard of care of the industry for a'n -investigation without benefit of scientific data .and calculations. The sole objective of this investigation has been to diagnose the geotechnical conditions at the subject property. The property may be affected by regulations pertaining to issues such as wetlands or wildlife habitat that are not addresSed in this report. The report has been prepared for the exclusive use of our client in conjunction with the aboVe referenced project. It may be used by others only with expresSed written permission of the Engineer. The report has not been prepared for use by others or for other uses. The observations, interpretations, and conclusions herein are based on generally accepted professional engineering and geologic principles and Practice. This warranty is in lieu of all others, either expressed or imPlied. Sincerely, NORTHWESTERN TERRITORIES, INC. .~ '~' J.R. JerryNewlin, P.E. '~, ' ~ Principal Engineer Bill Payton, Engineering Geologist G:\Gen\Bill\Reports\WI NH0101 .slope stability. 11 (29-1).Chimacum. J CUDC.doc 5 j~ I.!i~m;C....,payto r!