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HomeMy WebLinkAbout976200102 Geotech Assessment (2002)NTi www. nti4u, com 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 . February 21,2002 Robert E. Brasier 1821 E. Marrowstone Rd: Nordland, WA 98358 'RECEIVED FEB 2 6 2002 SUbject: ReviSion to our geotech'nical report dated November 22, 2000; for the tw© adjacent properties with tax #'s 976200102 and .976200103, located on Marrowstone Island in Section 33, Township 30 NOrth, Range 1 East in. Jefferson County. Dear Mr. Brasier:' Due to uneven terrain and building area constraints at the proposed homesite location 100' back from the edge of the bluff in the above referenced report, you have requested' a reduction in the setback distance to a more favorable location.' We hereby revise our report and recommend a setback distance of not less than 70' from the top edge of the bluff to the proposed home. Although the bluff is actively receding, this distance should still provide an adequate buffer for an economic life of roughly 60 years or more, rather than the 75 years stated in the report. if you have any questions or need further assistance, please do not hesitate to contact this office. Sincerely, NORTHWESTERN TERRITORIES, INC. __ 17 / J3~. Jerry Newlin, P.E. Principal Engineer Bill Payton, L.E.G. Engineering Geologist G:\Gen\Bill\Reports\BRAS0001 .report revision.2-21-02.doc ir'it'll'. ;l/Idll.¢'r]l~[ 3604528495 NTI JLS GROUP NORTHWESTERN TERRITORIES, INC. 717 SOUTH PEABODY STREET, PORT ANGELES, WA 083112, Edgidncrc, ! Land StJ/Voyor.~ · Pl~)t,~7or$ ¢.r~$lruction Coordin,.3lior) · M,3lor/ol.~ To~li~ (3~0) 452..840 ! 1 .,~0~-fl54-5545 FAX 452.-8.498 E-Mall. i,'~[.(.,.]"),1( 4,,. c. om PAGE 02 GEOTECHNICAL REPORT FOR THE PROPERTIES DESCRIBED AS TAX PARCEL NUMBERS 976200102 and 976200103 LOCATED IN SECTION 33, TOWNSHIP 30 NORTH, RANGE 1 EAST JEFFERSON COUNTY, WASHINGTON Prepared for MR. ROBERT E. BRASIER Prepared by NORTHWESTERN TERRITORIES, INC 717 South Peabody Stree[ Port Angeles, Washington November, 2000 NORTHWESTERN TERRITORIES, INC. 717 SOUTH PEABODY STREET, PORT ANGELES, wA [18362 Engllleor~ · I :w~ld S~lrveyors · Plonr~cr3 Con.¢t)~clion CoordinnliO~t ~ Meier/als Te.~llr3cj (3~)) 452-8491 1-800.-654.554.~ FAX 4§2-8498 E-Mail: intrc~(f.~'tli4u.com November 22, 2000 Robert E. Brasier 1770 Kirk Place Cartsbad, CA 92008 Subject: Geotechnical Report for the two adjacent properties located on Marrowstone Island in Section 33, Township 30 North, Range 1 East in Jefferson County. Dear Mr. Brasier: General This report is an update of our previous report entitled "Property and Bluff Evaluation on Marrowstone Island" dated May, 1990. On November 15, 2000, a second geotechnical evaluation was made of two adjacent waterfront properties located on the northeastern portion of Marrowstone Island in Jefferson County to verify the current conditions at the subject properties. The two lots, which each measure approximately 90 feet wide by 270 feet deep, are located just north of 2500 East Marrowstone Road in Section 33, Township 30 North, Range 1 East, W.M. ._P..r;operty Site Description The subject adjacent properties are undeveloped and densely wooded. The properties are bounded on the north and south by developed residential property. The properties are bounded on the west by East Marrowstone Road (Photo I). The eastern portion of these two adjacent properties consists of a steep bluff approximately 70 feet in height, which faces Admiralty Inlet (Photo 2). Active marine erosion is currently taking place along this bluff and coastline. The bluff has more than likely receded some in the ten years since the last report was done, however there was no evidence of any large scale landslides having occurred in that time. The subsoils existing beneath the subject properties and bluff areas consist almost entirely of medium grained stratified sands of glacial origin. This type of soil, while providing reasonably good foundations for homes, lacks cohesion and is, therefore, very vulnerable to water erosion. This is evidenced along the bluff portion of the properties where accelerated marine erosion results in what is believed to be somewhat faster than the normal rates of escarpment retreat. Mechanics of Bluff Erosion Like many other areas around Puget Sound, the erosion cycle begins here by undermining of the bluff toe during high tides and accompanying storms. This removal of the lower supporting material results in an oversteepening and eventual collapse of the upper portions of the bluff. The resulting slide debris acts as a toe buttress which temporarily restores the bluff stability and also provides an erosion buffer at the critical lower toe portions of the cliff. However, once this loose pile of sand is removed by subsequent high tides and wave/current action, the erosion cycle again repeats itself. This process results in a landward retreat of the escarpment face over a period of time. Various estimates have been made of this average rate of bank retreat where active marine erosion is at work along the bluff toe. These vary from an average of six inches per year to as much as two feet per year, depending upon the resistant properties of the soil, such as cohesion, and the intensity of the erosive attack by the sea (e.g., fetch distance, longshore current activity, prevailing wind direction, etc.) Considering the soil conditions and marine environment at the subject properties, it is estimated that an average rate of bluff erosion at the two subject properties may be on the order of between one and one-and-one-half feet per year. However, this should not be a problem considering the substantial depth of these lots and the consequent room to provide an adequate set back distance from the bluff edge. Conclusions and Recommendations (1) For the construction of a permanent type home, a minimum set back distance of 100 feet from the bluff edge is recommended at each of these properties. (2) If a more movable type structure were to be considered, such as a mobile home or a specially constructed conventional home that could be relatively easily moved, a set back distance of around 50 feet might be expedient. Ir the bluff edge ever reached within 30 feet of such a home, a move further landward on the lot would then be necessary. (3) Ali runoff surface water should either be directed away from the bluff or tightlined down the face of the bluff to prevent accelerated erosion from this source. Surface water discharge to adjacent properties should not exceed predevelopment conditions. (4) All drainage control devices should be maintained in good working order and inspected at least once a year. (5) It will be necessary to maintain ground cover to reduce erosion from surface runoff. Native vegetation that requires little or no irrigation would be the most beneficial. Also, vegetation on the bluff face should be incouraged. (6) Silt fences or other sediment contr°i devices may need to be utilized in order to prevent sedimentation to adjacent properties during construction. (7) Septic systems should not be located between the home and the bluff. (8) A safety fence erected ten or fifteen feet landward from the precipitous bluff edge should be considered for personal protection, (9) The undisturbed sandy subsoil at these properties is capable of supporting footing loads of up to 3,000 .pounds per square foot, once the one foot thick topsoil has been removed. Based on the findings, recommendations and limitations of this report: A. There has been landslide activity in the vicinity in the past, however, the recommended setback distance should provide for minimal landslide hazard to the proposed home for an economic life of about 75 years or more. B. An analysis of slope stability indicates that the proposed home is not subject to risk of landslide under normal conditions. CT The proposal would not increase surface water discharge or sedimentation to adjacent properties. ...... ' NOV 1 7 2003 ! :::-::::- ;:i!:'~; '~i.!~ "7 11¢17/2003 10:22 3604528498 NTI JLS GROUP PAGE 06 Certification of Item C is contingent upon proper construction and maintenance of an engineered, drainage, erosion and sediment control plan per standard Jefferson County requirements, 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 site. The report has been prepared for the exclusive use of Mr. Robert E. Braiser. 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 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. ewli~ P.E. Principal Engineer Bill Payton Geologist G'~Gen~8~IILRnport~BRAS05o 1.bluff r,l~h~llty.33(~0,1E).Mam:w.~one Isb~d.wpd Photo 1' View of subject properly looking northeast from East Marrowstone Road. 11V17~2003 10~22 3604528498 NTI JLS GROUP (NO'RTHW~ERN TERRITORIES, INC. Construction ~~ ~ Mat~d~ Teeing Ni'J PAGE 08 FILE PROPEETYAND BLUFF EVALUATION ON t~%~ROWSTONE ISLAND Jefferson County, Washington Prepared For JO,ANNWHIPPLE Prepared By NORTHWESTERN TERRITORIES, INC. May, 1990 717 SOUTH PEABODY, PORT ANGELES, WA 98362 (,'Z06) 452-8491 1.-8~,654-,(i545 11/177200~ 10:22 3604528498 NTI JLS GROUP PAGE 09 ,, ~ NOV ]7 2003 PROPERTY AND BLIIFF EVALUATION ON ~BU~WSTONE rSLAND On May 22, 1990, a geotechnical evaluation was made of two adja- cent waterfront properties located on the northeastern portion of Marrowstone Island in Jefferson County. The two lot~, which each measure approximateiy 90 feet wide by 270 feet deep, are located Just north of 2500 East Marrowstone Drive in Section 33, Township 30 NOrth, Range i East, W. M. JO Ann Whipple, who is a prospective purchaser of these proper- ties, has requested that a geotechnical examination be made with particular emphasis placed upon safe set back distances from the bluff edge as a guide to future home construction. PROPERTY SITE D.]~,CRrl~?!.O.N' The eastern portion of these two adjacent properties consists of a steep bluff approximately 60 feet in height, which faces Admi- ralty Inlet (see photographs). Active marine erosion is current- ly taking place along this bluff and coastline. · The subsoils existing beneath the subject propertie~ and bluff areas consists almost entirely of medium grained stratified sands of glacial origin. This type of Soil, while providing reasonably good foundations for homes, lacks cohesion and lB, therefore, very vulnerable to water erosion. This is evidenced along the bluff portion of the properties where accelerated marine erosion results in what is believed to be ~omewhat faster than the normal rates of escarpment retreat. MECHANICS OF BLUFF £RO$ION Like many other areas around Puget Sound, the erosion cycle begins here by undermining of the bluff toe during high tides and accompanying storms. This removal of lower support results in an oversteepening and eventual collapse of the upper portions of the bluff. The resulting slide debris acts as a toe buttress which temporarily restores the bluff stability and also provides an erosion buffer at the critical lower toe portions of the cliff. However, once this loose pile of sand is removed by subsequent high tides and wave/current action, the erosion cycle again repeats itself. This process results in an inexorable retreat of the escarpment face over a period of time. PA~£ 10 Various estimates have been made of this average rate of bank retreat where active marine erosion is at work along the bluff toe. These vary from an average of six inches per year to as much as two feet per year, depending upon the resistant proper- ties of the sot1, such as cohesion, and the intensity of the erosive attack by the sea (e.g., fetch distance, longshore cur- rent activity, prevailing wind direction, etc.) Considering the soil conditions and marine environment at the subject properties, it is estimated that an average rate of bluff erosion at the two sub3ect properties may be on the order of between one and one-and-one-half feet per year. However, this should not be a problem considering the substantial depth of these lots and the consequent room to provide an adequate set back distance from the bluff edge. ,CONCLUSIONS_AND RECOMMENDATIONS (1) For the construction of a permanent type home, a minimum set back distance of 100 feet from the bluff edge is recommended at each of these properties. (2) If a more movable type structure were to be considered, such as a mobile home or a specially constructed conventional home that could be relatively easily moved, a Set back distance of around 50 feet might be expedient. If the bluff edge ever re~¢hed within 30 feet of such a home, a move further landward on the lot would then be necessary. (3) Bluff erosion and associated escarpment retreat could be substantially reduced if a low (five to six foot high) sea wall were to be constructed along the shoreline near the base of the bluff. However, sOme of the effectiveness of such a sea wall would be reduced if the neighboring proper- ties were not similarly protected. (4) Ail runoff surface water shoUld either be directed away from the bluff or tightlined down the face of the bluff to pre- vent accelerated erosion from this source. (5) A safety fence erected ten or fifteen feet landward from the precipitous bluff edge should be considered for personal protection (see photographs). (6) The undisturbed sandy subsoil at these properties is capable of supporting footing loads of up to 3,000 pounds per square foot, once the one foot thick topsoil layer has been re- moved. PAGE 11 STANDARD DISCLO$_~E STATEMENT This report is based on a visual inspection of the existing site conditions and/or facility. 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 invest/ga- tion without benefit of scientific data and calculations. Unless foundation plans are reviewed and the Construction work is in- spected by the Engineer., no warranty is made by "the Engineer concerning conformity of the con~truction to the conclusions and recommendations of this report. :, · · - : .. Norman A. Dixon, PEG Engineering Geologist J. R. Jerry'Newlin, PE Principal Engineer lc: Whipple.May Dir: NAD/Rpt NOV 17 2003 i:;i '! Photograph 1 Looking north to subject bluff area beyond house. Note steep upper por- tion and lower sand slide debris which acts as temporary erosion buffer. NTI JLS GROUP Photograph 2 South to subject bluff from shoreline. Note overhanging bluff top on subject property. PAGE 12 Photograph 3 Showing bluff encroachment on mobile home located ~everal hundred feet north of subject property.