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HomeMy WebLinkAbout950200203 Geotech Assessment( NORTHWESTERN TERRITORIES, INC. 717 SOUTH PEABODY STREET, PORT ANGELES, WA 98362 Engineers I~ Land Surveyors I Geologists Construction Inspection ~11 Materials Testing NT! (360) 452-849, 1-800~654-5545 FAX 452-8498 E-Mail: ini'o~)nti4u.com 'j~:~",, JUL -3 2002 GEOTECHNlCAL REPORT For Lots 3 and 4, Block 2, Trails End 4th Addition Located in Section 16, Township 27 North, Range 1 East, W.M. Jefferson CountY,'Washington Prepared for DIANE HALVERSON WILLIAMS Prepared by NORTHWESTERN TERRITORIES, INC. 717 S. Peabody Street Port Angeles, Washington 98362 June 13,2002 GEOTECHNICAL REPORT For Lots 3 and 4, Block 2, Trails End 4th Addition June 2002 Diane Halverson Williams 130 Madrona Vista Port Ludlow, WA 98365 Subject: Geotechnical report for Lots 3 and 4, Trails End 4th Addition, Tax #'s 950200202 and 203, Section 16, T27N, R1E, Jefferson County, WA Dear Ms. Williams: Introduction At your request, Bill Payton of Northwestern Territories, Inc. conducted a geotechnical inspection of the above-menii~Oned property on June 7, 2002. The purpose of this inspection was to examine 'the bluff at the subject property by visual means and report our findings in regards to the relative stability of the bluff in conjunction with future development of the property. Site Description The subject property consists of two vacant lots located on the east side of Thorndyke Road at the intersection of Seattle Drive. The property is bounded on the north by developed residential property, on the west by Thorndyke Rd., on the south by wooded land and on the east by a high bluff. The majority of the property has been cleared and is overgrown with grass and brush. There is a wooded strip along the south side of the property. The bluff is well vegetated with trees, brush and ferns. The bluff has been logged in the past and most of the existing trees are between six and twelve inches in diameter. Most of the existing trees and the old stumps have straight trunks which suggests that minimal or no slope creep is occurring. Some minor surficial slumping was noticed in areas of thin vegetative cover. No springs or seeps were noticed on the bluff face, however, the area below the base of the bluff (off of Edna Place) was wet. The bluff is about 100 feet high and has a slope of about 30 to 34 degrees from horizontal, which is at or below the natural angle of repose for loose cohesionless material. 2 Geoloqic Conditions The Soil Survey of Jefferson County maps the soils in the area of the subject property as the Alderwood gravelly sandy loam (AIC). This unit is described as having formed in glacial till with a slowly permeable cemented layer at 20 to 40 inches. There is a perched water table above this cemented layer during the winter. Permeability above the cemented layer is moderately rapid, runoff is slow to medium and the hazard of water erosion is slight to moderate. The Survey maps the bluff face soils as the Cassolary sandy loam (CfE). This soil formed in reworked glacial and marine sediments. Runoff of this soil is rapid and the hazard of water erosion is severe. The Coastal Zone Atlas of Washington maps the following soils in the area of the subject property. The Vashon lodgement till (Qvtl) is mapped on lot 3 and the west side of lot 4. The Vashon advance outwash (Qva) is mapped on the east side of lot 4 and the bluff is mapped as Undifferentiated stratified sediments older than Vashon lodgement till (Qpf). The Vashon lodgement till (Qvtl) is described as a compact mixture of boulders, cobbles, pebbles, sand, silt and clay, generally overlain by 1 to 5 feet of ablation till. The Atlas also describes this soil as' being excellent for foundation stability and good for seismic stability. The Atlas maps the slope stability of this soil as Stable (S). The Vashon advance outwash (Qva) is described as a member of the Vashon Drift. This soil is composed of well sorted, well stratified sandy gravel. The Atlas describes the foundation stability of this soil as good to excellent but may be poor on slopes that approach the angle of repose (30 to 39 degrees). The slope stability of this soil is described as generally stable in slopes up to the angle of repose and may stand in steeper slopes for short periods. Seismic stability is described as good. The Atlas maps the slope stability of this soil and the Undifferentiated soils found on the bluff as Unstable (U). Visual observations of four test pits located on the property (presumably for septic design) revealed sand with a trace of gravel. Likewise, surficial soils exposed on the bluff face were sand. Conclusions and Recommendations The subject property appears grossly stable at present. The sandy soils on the property and bluff have the potential to be very erosive if sufficient ground cover is not maintained. Tree removal or other disturbance of the vegetation on the bluff face could affect the subject property. 3 JUL -' 3 2002 C, EP Or. gE',, The following recommendations should be considered with regard development of the prope~y: 1. Maintaining ground cover will help reduce eros'ion from surface runoff. Any -bare areas that develop should be revegetated. Native vegetation that requires little or no irrigation would be the most beneficial. 2. Surface water should not be allowed to flow over the face of the bluff and cause erosion of the bluff face. This can be controlled with vegetation and using berms or swales to direct runoff to a drainage system. 3. Surface runoff from hard surfaces such as roofs, driveways, walkways and patios should be controlled and routed to a community storm sewer or to a drainage system if a community storm sewer is not available. Surface water discharge to adjacent properties should not exceed predevelopment conditions. 4. Silt fences or other sediment control devices may be needed during construction so that sedimentation o:~to adjacent properties does not exceed predevelopment conditions.. 5. 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. 6. All drainage control devices should be maintained in good working order and inspected at least once a year. 7. The Soil Survey mentions the possibility of a shallow perched water table during the wet season. Thus it would be beneficial to control wetness under the home in the building design. One measure would be the use of footing drains. 8. A building setback of at least 35 feet from the edge of the bluff should be established. 9. The septic drainfield should not be constructed between the house and bluff. 10.An engineered drainage control plan should be developed for this property to address items. 3, 4, 5 and 6 above. A contractor knowledgeable in the construction of these types of features should be retained for this work. For f~rther 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". Based on the findings, recommendations and limitations of this report, the .,. proposal should pose no unreasonable threat to persons or property nor decrease slope stability. ' ~::; 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 other development regulations 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, anrJ 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\Reports\WIDI0201.slope stability. 16(26-1E).Thorndyke Rd.doc