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HomeMy WebLinkAbout002133005 Geotech AssessmentM ~ ®- -NORTHWESTERN TERRITORIES,~INC. - •. ~' ~ A JLS GROUP COMPANX 717 SOUTH PEA80DY STREET, PORT ANGELES, WA 98362 kJ~ O ~ ~ '' •~' engineers .Land Surveyors Geologists ~ - 3 u~oo ~ : ~:~ ~( . NTI •ConsWcGon Inspection. ' ~Materia/s Testing `• ~ ' (360) 4528491 FAX 452-8498 :www.nti4u.com E-Mall: info~nti4u.com .,-r-Q ~ ~ 3~'~T ~'~ ~~ JLS GROUP, 1') ~14~~, 1:.~u :-. INC. - ~~~' ~ ~ ~~•k~ ' June 27, 2006 - '~ ~ •'~ Mel & Helen Williams. ~ t,w~,-~p~~1i1 ~^t~~t'~ ,, ;. 74'hWest Sequim-Bay.Rd. ~ _ f3 !~ .. Sequim, WA 98382 ~ - Subject; Building:Setbackfrnm. Marine'Bluff at the Property Described as;Tax.Parcel Number 002133005, Goss Road,-Section 13, Township.30;North, Range 2,West, W.M. Jefferson County, Washington. _ _ ?^~*~ ~R~, , ; - . ~.. ,~ Dear Mr. & Mrs.~Williams: - - - , At your request, Bill Payton of Northwestern Territories, Inc. (NTI) met with you and your builder ,.'. .' at the above referenced property for the purpose.of marking the building setback from the : _ marine bluff. • NTI conducted a geotechnical inspection .of the above-mesa °ne bluff was n'ot ecommended, In that report, a s,pe.cific building setback distance from.the - except that the. preliminary .proposed: location of the house 120 feet.back from the bluff was deemed sufficient. Hoy°rever, after.developing specific hoWasPlo tacoted agairi.Pn June of{ h sq apparent that more room was needed,. which is why NTI. • year. - - - Based upon our report, we recommend that the building tenb W,th the Jefferson Couunty Critical established at a minimum of .100'. This distance is consis - Areas Code. The setback was measured with a.tape measure and marked with two:. easeen posts. If you have any questions regarding this matter, or need further assistance; p contact this office. - Sincerely, NORTHWESTERN TERRITORIES, INC. ~ -° Robert A. Leach; P.E., MBA Principal Engineer ~ ~~~ Bill Payton, L.E.G. Engineering Geologist • G:\GenlBill\Reports\WLMM0601.blulf setback Ietter.OLSH0201.doc -" NORTHWESTERN TERRITORIES,. INC. t' • I l 717 SOUTH PEABODY STREET, PORT ANGELES, WA 95362 ~ L p ~ " ~ '"13 Engineers ~ Land Surveyors ~ Geologists 2 ~p ~ ~ _ y~/(J ~ , Construction Inspection ~ Materials Testing 'J 7 J (360) 452-8491 1-800-654-5545 FAX 452-8496 E-Mail: info@nti4u.com ~"'~. tT-a ~-q ~-~z ~~ ~.~q ,~,., ;; ±~ ~ ~ ~- /i NTI ._.. ~s~~l~Gflsol~ "'"~1fi"~ ;, GEOTECHNICAL REPORT for Tax Parcel Number: 002133005, Goss Road in Section 13, Township 30 North, Range 2 West, W.Ni• Jefferson County, Washington Prepared for HELEN OLSON Prepared by NORTHWESTERN TERRIS O ROES, INC. 717 S. Peabody Port Angeles, Washington 98362 March 15, 2002 March 15, 2002 Helen Olson 500 Goss Road Port Townsend, WA 98368 Subject: Geotechnical Report for the Property Described as Tax parcel number 002133005, Goss Road, Section 13, Township 30 North, Range 2 West, W.M. Jefferson County, Washington. Dear Mrs. Olson: Introduction At your request, Bitl Payton of Northwestern Territories, Inc. conducted a geotechnical inspection of the above-mentioned property on March 14, 2002. The purpose of this inspection was to examine the marine bluff at the subject site by visual means and make recommendations regarding the development of the property in accordance with the Jefferson County Unified Development Code. It is our understanding that you own the subject property and plan to build a single family residence on the property. Site Conditions The subject property is located at the end of Goss Road and is reported to be 7.58 acres in size. The property is bounded on the north and south by undeveloped property, on the east by Goss Rd. and on the west by Discovery Bay. The upland is hilly with a few drainage corridors that cross the property. The drainage corridors were mostly dry except for a few small puddles due to recent rains. There appeared to be little to no active erosion in the drainage corridors. The drainage corridors drain towards a ravine north of the proposed house site that leads to the beach. The property is undeveloped, except for an old logging road that crosses the property and leads to the proposed house site which is about 120 feet back from the top of the bluff. The property are several very large t ees ong he propertyative trees, brush and grasses. There The marine bluff at the property is roughly 117 feet high. The angle of the bluff varies from about 40 degrees to near vertical, with an average slope angle of about 46 to 48 degrees. The upper portion of the bluff is mostly well vegetated with trees, brush and grass. The trees are predominantly fir with some madrona. Trunk diameters are around 12 inches. Portions of the mid bluff are near vertical and bare. Portions of the lower bluff are vegetated as the upper bluff and portions are bare, revealing recent shallow slide scars. No springs or seeps were notics~e~d on the bluff face. ~ ,~, ~ ~~~=-~ _ ;, i - !tq ^~ ~ ~, 2 ~~ u 3„. L? ~' r ~.RS~P~ _~'!~~'~' The toe of the bluff is slowly being eroded by wave action during high tides and storms. This results in oversteepening and subsequent periodic failure of the bluff face. When slides occur, the slide debris that forms at the base of the bluff temporarily protects the toe of the bluff from wave action. Eventually this debris will be washed away, and the process will start over. Site Geolo The USDA Soil Survey of Jefferson County classifies the soil at the subject property as the Cassoiary sandy loam (CfD). This soil formed in reworked glacial and marine sediments and is found on terraces along the breaks of ravines or marine bluffs. The Survey states that the runoff of this soil is medium and the hazard of water erosion is moderate. The Coastal-Zone Atlas of Washington classifies the soil at the_property as the Vashon advance outwash (Qva). The Atlas describes these soils as being a member of the Vashon Drift that lies stratigraphically between the Esperance Sand and Vashon till. Vashon advance outwash typically consists of well sorted, well stratified sandy pebble-to cobble-sized gravel. In some places, sand predominates. The unit generally contains minor amounts of silt and clay. The Atlas also classifies the bluff in the area of the subject property as being unstable with old slide activity (Uos). Observations made at the property are consistent with the above soil descriptions. Soils exposed on the bluff face revealed compact gravelly silty sand of glacial origin: There were many cobbles and boulders exposed at the surface on the upland portion of the property. Conclusions and Recommendations As with most of the shoreline of western Washington, the bluff at the subject site is slowly undergoing bluff recession. It is very difficult, if not impossible to predict rates of bluff recession or if or when a landslide may occur. Bluff recession can occur in several ways and is generally cyclic in nature. The bluff may weather slowly over time and recede very little or none for several years. Then a portion of the bluff may slough off suddenly. It appears that the main cause of bluff recession at the subject property is triggered by the erosion of the toe by wave action. Judging from the size of the trees on the bluff face, the overall bluff recession is probably. pccurring at a very slow rate. Therefore, the proposed 120 foot setback from the top of the bluff to the proposed house should be sufficient. The following recommendations should also be considered with ~~~ <~•°~, development of the subject property: ~ y , j,a _, 3 Iri C~IIVl7~ ikf4~~ili~. , r~ t . w,f ~_ ` 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 property and the bluff. Therefore, the land between the proposed house and the bluff should be left in a relatively natural state. Selective thinning and pruning can be utilized to improve the view. 2. 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 soi! near the bluff. 3. Runoff from roofs and other hard surfaces such as driveways and decks should be controlled and routed to a detention basin, dry well, natural drainage corridor, other drainage control device or to the beach via tightline such that surface water discharge or sedimentation to adjacent properties will not be higher than pre-development conditions. The water should not be infiltrated between the house and the bluff. Surface runoff should not be allowed to flow over the edge of the bluff. An .engineered drainage .control plan. should be developed for this property to address these issues. 4. Septic systems and/or detention basins should not be constructed between the house and the bluff. 5. All drainage control devices should be maintained in good working order and inspected at least once a year. 6. 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 recommended setback distance should provide for minimal landslide hazard to the proposed home during its economic life. 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 _ ro"perti s. 5. The proposed homesite would be stable under normal geologic conditions. ~~ r a .~ ,~ ~ , ~,< YS . a ~~ 11, .c. ~ ,~<~ 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 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. 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 j ~z ~ ~ ~~c r~ ~^, ~~ ,.. ~ x ~ ~ , . ~u~s 2~/.lG ~c :-