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HomeMy WebLinkAbout937800116 Geotech Assessment (2002) NORTHWESTERN TERRITORIES, INC. 717 SOUTH PEABODY STREET, PORT ANGELES, WA 98362 Engineers · Land Surveyors · Geologists Construction Inspection · Materials Testing NTI E-Mail:info@nti4u.com C~'~,,' D www. nti4u.com ' i UPDATE GEOTECHNICAL REPORT for Lot 16, Victoria Loop Cape George Colony in Section 12, Township 30 North, Range 2 West Jefferson County, Washington Prepared for Lynne Sterling Prepared by NORTHWESTERN TERRITORIES, INC. 717 S. Peabody Street port Angeles, Washington 98362 July 15, 2OO2 UPDATE GEOTECHNICAL REPORT for Lot 16, Victoria Loop July 15, 2002 Lynne Sterling PO Box 2050 Port Townsend, WA 98368 Subject: Geotechnical report for the property described as Lot 16, Victoria'LoOp, Cape George Colony in Section 12, Township 30N, Range 2W, Jefferson County, Washington. Dear Mrs. Sterling: Introduction At your request, Bill Payton of Northwestern Territories, Inc; (NTI) conducted an update geotechnical inspection of the above mentioned property on July9, 2002. The purpose of this inspection was to examine the marine bluff at the subject property by visual means and make recommendations regarding the development of the property in accordance with the Jefferson County Unified Development Code. Site Conditions The subject pr0. perty consists of a cleared vacant lot overlooking the Strait of Juan de Fuca (Photo 1 ). The property is bounded on the north by a vacant lot, on the south by an existing home, on the east byVictoria Loop and on the west by a high marine bluff. The upland portion of the propertY is relatively fiat and slopes gently towards the south, and is covered in grass. The bluff is about 9Ofeet high with a mid bluff-bench part way down the bluff. The upper and' lower, portions of the bluff are .very steep and mostly devoid of vegetation. Young alders and brush are growing on the mid bluff bench (Photos 2 and 3). The base of the blUff is subject to active marine erosion fr°m high tides and storm waves. Recent slide debris was noticed at the toe of the bluff. At the time of the site visit, the lower portion of the bluff was moist. It appears that .groundWater percolates through the sandy.upper bluff soils and exitS the bluff at bench level where silty soil precludes furtherinfiltration of the groundwater. The presence of this groundwater contributes to the erosion and sliding of the upper bluff material helping to form the mid bluff bench. Site Geology A visual inspection of the bluff face revealed the upper portion of the bluff to consist of stratified sandy and gravelly deposits. The lower portion of.the bluff consisted of laminated silty deposits. The Coastal Zone Atlas of Washington classifies the bluff soils in the area of the subject property as "Undifferentiated stratified sediments older than Vashon lodgement till" (Qpf). The Atlas describes these soils as consisting mainly of sand and gravel, but in some areas contain Silt, Clay, peat, and possiblYtill. The Atlas also labels the slope stability at the property as "Unstable-Old slide" (Uos). The Atlas was printed in 1978. The marine bluff at the subject property is very typical of the bluffs found throughout northwestern Washington, where a layer of pervious sandy material overlies an impervious silty layer. Groundwater percolates through the sandy strata and then is restricted by the impervious strata. This causes the overlying material to become saturated and increases its pore water pressure. The groundwater also migrates towards the bluff face and exits the bluff above the confining layer. These processes weaken the bluff and contribute to landslide activity. These processes also contribute to the formation of a mid-bluff bench such asis found at the subject property. Another mechanism for erosion of the bluff is the relentless attack on the toe of,the bluff by wave action. This process oversteepens and undercuts the lower bluff, eventually resulting in sliding of the overlying material. When slides occur, the slide debris that forms at the base of the bluff acts as a temporary stabilizing faCtor. The debris temporarily protects the toe of the bluff from wave action but is eventually washed away and the process starts over again. Please review the enclosed Figure 21, which describes this cyclical process of bluff recession in more detail. Historical Data The 1994 geotechnical report for this property calculated an average rate of bluff recession of about 5 to 12 inches per year over the previous 33 years using information from old surveys that date back to 1961. The property was surveyed by NTI in 1994. At that time, markers were placed on the north and south property line, back from the edge of the bluff. The marker on the south property line was placed about 21.3 feet from the bluff, and the marker on the north property line was placed about 14.7 feet from the bluff. The south, marker was found during the course of this inspeCtion and was measured to be about 19.2 feet 2 from the bluff. Thus, it appears that the bluff has receded about 2 feet in the last 8 years at this location. A marker was also found on the north property line at about 12 feet back from the bluff. This marker did not have an NTI cap on it and may not be the marker that was set by NTI in 1994. But if it is, then it appears that the bluff has receded about 2.7 feet at this location. Thus from this information, it appears that the bluff at the subject property'has been receding on average about 3 to4 inches per year, in the last 8 years. It should be noted that the above figures are averages and that a slide could remove several feet of bluff in one event and then become .relatively inactive until the next event. Conclusions and Recommendations It is quite evident that the bluff at the subject property is undergoing bluff recession. There is no way to accurately predict rates of bluff recession or when a landslide will occur. There is also no way to know how far the bluff will recede in any one event. From the above mentioned reports, it appears the bluff has receded about 16 to 35 feet in the last 41 years. The 1994 geotechnical report uses an average recession rate of 0.7 feet per.year, which still seems valid based on the above information. ASsuming a 50-year economic life of a new home, the bluff could be expected to recede about 35 feet during the next 50 years based on the forgoing information. It would be prudent to provide a minimum 40-foot setback at the time the house is considered uninhabitable. Therefore, ~a minimum setback of 75 feetffom the.topof the b'luffto the "foundation footprint" of the house is recommended: Consideration should be given to a type of home that could be relocated when the bluff encroached tOo close to the home. The following recommendations should be considered with regards to development of the property: ,1. Maintaining ground cover will help reduce erosion 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 the beach via tightline. 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 onto 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 basic septic system components and drainfield, as shown on the septic design plans previously approved for this site (case no. SEP94-00070) should be adequate from a geotechnical perspective. 8. An' engineered drainage control plan should be developed-for this property to address items 2, 3, 4, and 6 above. NTI can accomplish this task if requested. A contractor knowledgeable in the construction of these types of features should be retained for this work. 9. The drainage ditch along VictOria Loop should be improved if necessary in order to prevent roadway runoff from flowing across the property. 10, Information from other reports-indicates that residents have noticed ground cracks along the edge of the bluff prior to slide events. It would be prudent to develop.a monitoring program in order to be aware of the possible development of such cracks. It would also be helpful to monitor the rate of bluff recession by establishing a benchmark and taking regular measurements. Based on the findings, recommendationS and limitations of this report: a. The landslide hazard to the proposed home would be reduced due to the mitigation measures proposed in this report. b. An analysis of slope stability indicates that the proposal could be modified such that hazards are reduced. c. .The proposal would not increase surfacewater discharge or sedimentation to adjacent properties beyond predevelopment conditions. d. The proposal would not decrease slope stability on adjacent properties. e. The proposed home would be stable under the normal geologic conditions that have occurred over the last 41 years at the property. 4 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 abnorma 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. O. "~. Jerry Newlin, P.E. Principal Engineer Bill Payton, L.E.G. Engineering Geologist G:\Gen\Bill\Reports\STRL0201 .bluff stability. 12(30-2) Cape George.doc 5 Photo 1' View of subject property looking west. Photo 2: View of bluff at 'property. Photo 3: View of bluff at property. · - A Atthe beginning of an idealized cycle, the bluff ,-",Z / '/ has a uniform slope. Water infiltrates the sur- face soils and perches above the relatively im- ~'"'~ / / / permeable materials at the base of this sandy soil/[ /r -~ i-~ ~ / erosion of ground surface sequence. Saturation creates pore-water till !. -~ ,,- ) '~ J -( during overland flow pressures that reduce the effective strength of ........ these materials. sand ' perched ground-water and{--- ~ ~-. ' ~ring ~ravel I ~ j G~OUNa ..~ wave erosion i i ~ , , removes less ' I slide debris permeable ) /'"... potential failure materials l '-.~ surface potential deep failure surface . /, / / B Runoff and precipitation introduced by the ~. ;. ._~. ~' / / , sources shown in A have infiltrated and weak- ~.~/~J~ irl~ ened the sediments, causing failure of the un- -~ ~ ~ / . ~~-~l~lll~J~-J~-;~,u.. .................... .~ewous slo-e '~rofi" consolidated upper sand unit. Once mobilized, soil/['-'~"-Y~ ..: .,~ ~ r- I' K "-' the sand moves (sometimes episodically, tdl [.._ -- ~ , , :,/~"~.. m~d-slo e sometimes continuously) along the contact [ ~ "..~,~z~' / bench debris from upper with the underlying less permeable unit on the sarld / ,, ,,, ,,.~,~t~ ~ ("-...~,,,,'1,~ slope contributes 'bo mid-slope bench, often cascading as a secon- t , ', ¥' ~,,4~ - . . and J ", ",~"~'.'~ '~; ~,,~,~ seconda~ landshdlng dary landslide off the bluff formed by the lower ~ravel / ',~~.¥_.~ Jr..,,~ ~ / unit. This migration of material across the ~ [ ~~.,.~~o~ ~. ~ ] bench decreases the buttressing of the upper ~~'~., /'7'~'- ! lower bluff. Failure surfaces can be deep (those that I X~. ' I ~ ' ) ~ '" project into the lower, less permeable materi- less I ~,~."~'~"~,,,. j~ bluff als) as well as shallow. permeable [,j,,.~,,, i , ~~ J materials j --... potential failure surfaces if ground ~ potential not stabilized deej~ failure SUrTace £.- . ~ /'~7.~ r. '- , / / /-Z-./--fi-- -.-) _ ,.-~',z ~ '/' %' , d/ <//// ~ Benched bluff retreat continues. Movement of ~ ~'~ ~X,_~__~. ' ~d~,~,.-~ / / , .~ slide debris toward the lower bluff further de- ~_~.~_.~~ ~_..~..%~,,,) / / stabilizes the upper bluff, causing continued sloughing onto the bench. Either failure of the so~ ~c~ /// upper bluff onto the bench or failure of the till_[ .- -- -~ ~t --~, . ~ slope continues to retreat, slide debris off the lower bluff can trigger a cy- I '~',~t ~,,~;,,/, eventually encroaching on the house cie of movement. Movement along a deep- sand J ~,~-~ ~'~'~-,~. ~/,~ seated surface can reset this sequence of and I '~.~,~ ~ .~."~'.~?~,~:, ' events. permeable B'"".. materials / - . "-...-...~ '~~ ,~_.~ potential -'-, .............. deep failure surface Figure Zl. This sequence of sketches shows the idealized, potentially cyclical process by which bluffs in the northern Puget Sound area are forming and retreating.