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HomeMy WebLinkAbout701311020 Geotech Assessment NORTHWESTERN TERRITORIES, INC. .. 717 SOUTH PEABODY STREET, PORT ANGELES, WA 98362 Engineers · Land Surveyors · Geologists Construction Inspection · Materials Testing '~ i i ' ' ~ (360) 452-8491 1-800-654-5545 FAX 452-8498 E-Mail: info@nli4u.com NTI www. n ti4u. corn · · JAN 2 5 2002 UPDATE GEOTECHNICAL REPORT For GLENVIEW SHORT PLAT Located in Lot A, in Section 31, Township 27 North, Range 1 West, W.M. Jefferson County, Washington Prepared for GLENN O. GUSTAVSON Prepared by NORTHWESTERN TERRITORIES, INC. 717 S. Peabody Street Port Angeles, Washington 98362 Januaw 24,2002 Januaw 24,2002 Glenn Gustavson PO Box 609 Quilcene, WA 98376 Subject: Update Geotechnical Report for Lot A, Glenview Short Plat in Section 31, T 27 N, R 1 W, W.M., Jefferson County, WA Dear Mr. Gustavson: At your request, Bill Payton of Northwestern Territories, Inc. (NTI) met with you at the above location on January 15, 2002 to conduct an update geotechnical inspection of Lot A. It is understood that you plan to sell Lot A, and that this report will be used to satisfy Jefferson County's requirement for a geotechnical report. The original report, completed in August, 1993 by NTI for Lots A, B and C, is included with, and made a part of, this update. The update geotechnical inspection consisted of a visual inspection of the lot including a traverse of the bluff and beach via a pathway down the face of the bluff to determine if there had been any new slide activity since the original report. No evidence of further instability was noticed and thus, the conclusions and recommendations of the original report are still valid. The following recommendations should also be considered with regards to development of the subject property: 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 the bluff. o Runoff from roofs and other hard surfaces such as driveways and decks should be controlled and routed to a detention basin, dry well, other drainage control device or to the existing natural ravines such that surface water discharge or sedimentation to adjacent properties will not be significantly higher than pre-development conditions. The water should not be infiltrated between the house and the bluff. An engineered drainage control plan should be developed for this propertY to address these issues. 2 Septic systems and/or detention basins should not be constructed between the house and the bluff. All drainage control devices should be maintained in good working order and inspected at least once a year. Silt fences or other sediment control devices may need to be utilized in order to prevent sedimentation to adjacent properties during construction. Based on the findings, recommendations and limitations of this report: There has been landslide activity on the bluff face in the past, however, the report recommendations should provide for minimal landslide hazard to the proposed home. An analysis of slope stability indicates that the proposed home would not be subject to risk of landslide under normal conditions. 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 home site would be stable under normal geologic conditions. 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 an 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. 3 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. Jerry Newlin, P.E. Principal Engineer Bill Payton, L.E.G. Engineering Geologist' G:\Gen\Bill\Repods\GUSG0201.bluff stability.31(27-1).Bolton Peninsula.doc 4 NORTHWESTERN TERRITORIES, INC. Engineers · Land Su~eyors · Planners Construction Coordination · Materials Testing SLOPE STABILITY EVALUATION GUSTAVSON SHORT PLAT Lot 3, Pope Resource Subdivision of Government Lots 1 and 2 Section 31 North, Township 27 North, Range 1 West, W.M. Jefferson County, Washington Prepared for GLENN 0. GUSTAVSON Prepared by NORTHWESTERN TERRITORIES, INC. August, 1993 717 SOUTH PEABODY, PORT ANGELES, WA 98362 (206) 452-8491 1-800-654-5545 FAX 452-8498 SLOPE STABILITY EVALUATION SCOPE AND PURPOSE Our evaluation of bluff stability was reconnaissance in nature and consisted of visual observations by Mr. Blase Cylwik, Engineering Geologist. Observation efforts included traverse of the bluff slope, shoreline and upper areas of the property designated for residential construction. No subsurface exploration was performed. The purpose of our study was to provide information concerning the proposed division of the property into three residential lots--Lots A and B west of Quilcene Road, and Lot C east of the road. Por- tions of Lots A and B are also present east of the road. Our study concerns that portion of the property lying west of the road. PROPERTY DESCRIPTION The property proposed to be divided is near the south terminus of the Bolton Peninsula and consists of Lots A and B that extend about 350 feet west of Quilcene Road down to Quilcene Bay. Each of the lots are 150 feet wide except that an area of about 200 feet by 200 feet and another of irregular shape about 600 feet long east of the road are portions of Lots A and B, respectively. These areas are to be used for septic drainfields. Lot C lies east of these latter areas (Figure 1). REGIONAL GEOLOGY The bluff and property to the east are underlain by massive but highly jointed grey to brown, well consolidated siltstone of the Twin River formation that is of lower Miocene age. This formation is overlain in the upper bluff and portions of the property by poorly sorted silt, sand and gravel sediments. These sediments are of glacial outwash origin and were deposited over 10,000 years ago. They are generally dense but are not well consolidated and rela- tively permeable. The bluff has been formed by marine erosion and undercutting along the shoreline, resulting in oversteepening and subsequent periodic downward movement of bluff materials by erosion, slow creep of surface soils and slumping. In general, the rate of average annual bluff retreat can vary from a few inches to several feet. In this area, the bluff does not appear to be aggressive marine undercut- ting, perhaps because of the relatively firm siltstone and protec- tion afforded by Quilcene Bay. LOCAL CONDITIONS The bluff area along Lots A and B is 75 to 100 feet in height and has an overall slope of about 37 degrees. The slope is densely tree and brush covered. Most of the trees appear to be second to third growth. Over most of the bluff the trees stand nearby vertically indicating surficial and grass slope stability during their growth. A landslide is present on the bluff between Lots A and B (Figure 1). The base of the landslide is about 100 feet wide along the shoreline. It narrows and extends to near the top of the bluff. The slide probably occurred over 50 years ago and appears to have been subject to slow downward creep since that time. The landslide probably was caused by a combination of adversely oriented jointing in the siltstone, erosion along the ravine and over-steepening of the bluff by marine erosion. The base of the landslide is seen in Photos 1 and 2. A shallow ravine extends northwest from Quilcene Road across the northeast corner of Lot B and western slope of Lot A. The sides of the ravine are brush and tree-covered (Photo 3). Another similar ravine extends across the northeast corner of Lot A (Photo 4). Erosion or downcutting in the ravines appears to have been negligi- ble over the past at least several years. A shallow bowl-shaped depression along the south side of the ravine' in Lot B may represent pg.st shallow slumping of soils exposed by ravine erosion. Slumping does not appear to be presently active (Photo 3). The large relatively level areas adjacent to and west of Quilcene Road appear to be largely natural and appear permanent as a result of having been recently cleared. CONCLUS IONS (i) The relatively level portions of Lots A and B west of Quilcene Road appear to be adequately stable to support residential construction. It is understood that septic drainfields for each lot will be east of Quilcene Road. (2) (3) (4) Runoff carried by the ravine down the bluff on Lot A may have been a contributing factor to formations of the landslide on the bluff. However, the ravine does not appear to be aggres- sively eroding the slope or adjacent landslide. The bluff appears to be grossly stable. The upper part of the bluff above the landslide is relatively steep and may be subject to accelerated sloughing or erosion because of this. The possible shallow slump area along the ravine on Lot B appears to be stable and should not present a hazard if structural footings are founded in the underlying firm or dense earth materials. RECOMMENDAT IONS (i) Permanent structures should be located a minimum of 50 feet back (east) of the existing top-of-bluff edge and 25 feet back of the top of ravine slopes. (2) Structural foundations should be founded at least 18 inches into firm or dense in-plate natural earth materials. Addi- tional considerations may be necessary if any fill or loose soils are encountered adjacent to the top-of-ravine banks. (3) Septic system drain fields should be located east of Quilcene Road as proposed. (4) (5) Sloping areas along the ravines and top of bluff should be planted with deep-rooted ground cover to minimize erosion and surface sloughing. Runoff from roof d6Qnspouts and paved areas should be con- ducted via lined downdrains to velocity dissipators in the flowline of existing natural ravines. STANDARD DISCLOSURE STATEMENT This report is based on a visual inspection of the existing site 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 an investigation without benefit of scientific data and calculations. Unless construction plans are reviewed and the construction work is inspected by the Engineer, no warranty is made by the Engineer concerning conformity of the construction to the conclusions and recommendations of this report. The sole objective of this investigation has been to diagnose the geotechnical conditions at the site. The report has been prepared for the exclusive use of Glenn 0. Gustavson and may be used by others only with expressed written permission of the Engineer. The report has not been prepared for use by others or 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. Blase Cylwik ing Geo Engineer lo Professiona fc: Gustvsn. Rpt Dir: Eng/Rpt. S T R J DATE: /7' DESIGNED BY: ~'~./I :~' ~'yz&J//<' PROJECT : ~TAU~ ~ ~ FOR : ~ ~. SHT. / OF / NORTHWESTERN TERRITORIES, INC. Engineers · Land Sm'veyors · FYanners Construction Coordination · Materials Testing 717 SOUTH PEABODY. PORT .ANGELES. WASHINGTON 98362 - (206) 452-8491 - SOUTH - RAVINE BASE OF LANDSL LOT A PHOTO 1 - NORTH - BASE OF LANDSLIDE LOT B PHOTO 2 H