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Home My WebLink About950101312 Geotech Assessment---e.r 3t.D~(o- ill ~ STRATUM GROUP - 1451 Grant Street, Bellingham, wA 98225 Phone (360) 714-9409 September 29, 2005 ~~~E~~~~ Cynthia Towner P.O. Box 351 ~3; ``... ~ .~ 4 ~. Kingston, WA 98346 ~~ Re: Geology Ilaz~ra Evalnsfion JEffERSOH COUNn UCO Tax Parce1950101312 and 950101313 Jefferson County, WA Dear Ms Towner: Based on my site visit to the above-described property, a home can be constructed on the site such that potential slope failures on the steep east slope of the property will not impact the home site within the expected life of the structure. Stratum Group herein presents the results of our geology hazard evaluation of the above referenced properly. The purpose of this geology evaluation was to qualitatively evaluate the risk of slope failures impacting the proposed building azea on the subject properly.' This evaluation included a visual inspection of the property and vicinity, a visual inspection of the slopes on the property and vicinity, .and a review of available geologic mapping in the area. The property is located on the east side of Thorndyke Road in the Bridge Haven area of Jefferson County. The property is located on a gentle sloping to level area between Thorndyke Road to the west and a steep east facing slope to the east. The location of the property is indicated on Figure 1. SCOPE OF SERVICES The scope of our services included the following: 1) Conducted a site visit to visually inspect the subject property including the slope conditions and relevant conditions in the vicinity of the property. 2) Observed surface soil conditions on the slopes and on the uplands above the slopes on and in the vicinity of the property. 3) Prepared this report summarizing our findings, including a qualitative evaluation of the slope stability in regards to the proposed residence on the property, recommendations for site development, and recommendations for further investigation, if necessary. r ~~;;3 September 29, 2005 Tax Parcels 950101312 and 950101313, Thorndyke Road Jefferson Cow~ty, WA Geologic Evaluation GENERAL GEOLOGY '~~~~ ~`~~ ~~~~ Northwestern Washington has been occupied by continental glaciers at least four times during the Pleistocene Epoch (1.6 million to 10,000 years ago). During these glacial and accompanying interglacial periods, the underlying bedrock was eroded and a relatively thick layer of glacial related and interglacial fluvial sediments were deposited over the underlying bedrock in the vicinity of the subject properly. The Geolog~Map of Surficial Deposits in the Seattle 30' by 60' Quadrangle, Washingrton (Yount, Minard, and Dembroff, 1993) and the Geolopc Map ofEast-Central Jefferson County, Washington (Birdseye, 1976) indicate the subject property and vicinity is underlain Vashon age glacial advance outwash and Pre-Vashon glaciation stratified deposits. Observations on the subject property, slopes in the vicinity of the property are consistent with the above mapping. The upland gentle sloping to level area of the property and the upper portions of the steep slope on the east side of the property are underlain by sand and gravel outwash deposits: This unit is in a compact condition; however, the lack of silt gives the unit little cohesive ,,,~ strength. The lower portions.of the slope and the slopes below the steep slope are underlain by very compact sand and gravel and silt layers. The Relative Slope Stability in East-Central Jefferson County, Washintgfon (Birdseye, 1976) map indicates the subject property and steep slopes in the vicinity of the subject property are unstable areas. However, Birdseye did not identify any areas of former or recent landslides on or in the vicinity of the subject property at the time the map was produced. Observations confirm that there are potential unstable slopes and unstable slopes on the subject property and the vicinity. Shallow rapid slope failures have taken place on the steep east-facing slope in the vicinity of the property since the time of Birdseye's mapping of slope stability including a slide that took place on the steep slope just south of the subject property. SPECIFIC SITE OBSERVATIONS A sketch map of the property with pertinent features indicated is provided in Figure 2. The subject property is bounded on the west by Thomdyke Road. The upland area of the property consists of a partially cleared level to gentle sloping area. This upland area is approximately 119 feet wide along the south boundary of the property and is approximately 147 feet wide on the north side of the property. The upland portion of the property is located at an elevation of approximately 240 feet. The upland area is grass and brush covered with a few fir trees. saw, ~ Fib: 9.14.05 .., fi~~ September 29, 2005 X Tax Parcels 950101312 and 950101313, Thorndyke Road Jefferson County, WA Geologic Evalnatioo ~~~rC~SO~ C~Up~ D6U The east side of the upland area is bounded by a steep east-facing slope that slopes down to the east at an overall angle of 40 degrees down to a bench area of gentle slopes approximately 100 feet in elevation below the upland area. However, portions of the upper slope on the south side of the property are steeper with slopes as steep as 60 degrees. The slope on the north side of the property is less steep with sections of the slope being on the order of 35 degrees. With the exception of the area south of the subject property, the slope is plainer. The area: south of the property including the southen~most portion of the steep slope on the subject property consists of an area of convergent topography. The upper part of the steep slope as well as the upland area is underlain by sand and gravel with very little silt or clay. These soils have low cohesion. The lower portions of the steep slope are underlain by layers of very compact sand, gravel and silt. The lower layers are very hard and compact with high cohesion. Numerous seeps and springs are present on the lower portions of the slope as a result of the porous upper slope soils being underlain by the very compact impermeable soils on the lower slopes. The area of convergent topography corresponds to a spring. Water was flowing from this spring at the time of my site visit in May 2005. ~.,~ Because of the low cohesive .strength of the upper slope soils, slope failures can be triggered by soil saturation on the lower slopes. This type of failure is evident on the convergent slopes above the spring area. Vegetation clearing and poor water management from run off appears to have played a role in shallow slides on the slope in the area as well. Most of the mature trees have been cleared off the steep slope to the east of the upland portion of the property. The slope is now covered primarily with alder trees on the lower slopes and maple trees on the upper slopes with a mix of a few evergreens an understory of ferns and salal. A few areas of bare soil are present on the southern most portion of the steep slope in an area of relative recent slide activity. CONCLUSIONS AND RECOMMENDATIONS Based on my site observations and soils investigations, the proposed building area as indicated on the site sketch map will not be at risk of future landsliding. The building should be sited at least 30 feet back from the top of the steep slope except for at the far southern boundary of the property where the setback should be at least 50 feet from the top of the steep slope above the area of convergent topography. In addition to the setback the following recommendations should be followed: Roof run off and any other storm water run off should be directed away from the steep slopes and directed into a dispersion trench on the west side of the property. saaam, ~ Fik: 9.14.05 3 .,~` , .~" ~. p~;" "'" September 29, 2005 Tax Parcels 9501013 ] 2 and 950101313, Thomdyke Road Jefferson County, WA Geologic Evalnstion ii a r., q '~9#yyj~ jj~' The septic drain filed should follow the same setback recommendations as the home. Native trees and plants on the slope and within the .setback buffer should be disturbed as little as possible. Limbing and topping of trees to maintain view corridors is acceptable; however, no trimming debris or any other debris should be placed_on the steep slope or at the top of the steep slope. Because of the past disturbances to the slope on the south portion of the slope below the subject property, these slope areas are susceptible to being overgrown with blackberries. Blackberries should be prevented from taking over the slope as they have poor root strength and poor water reteirtion in the wintertime. CLOSURE Flease note that there are inherent risks associated with lots near or adjacent to steep slopes. These are risks that the building owner should recognize and be willing to accept. If conditions appear different than those described in this report, or other concerns arise, we request that we be notified so we can review those areas and modify our recommendations as required. We appreciate the opportunity to be of service to you. Should you have any questions regarding please contact our office at (360) 714-9409. Sincerely yours, S tam Groap Dan McShane, M.Sc., L.E.G. Licensed Engineering Geologist svannn ~ File: 9.14.05 4 ~. ~~ ~~ ~~ .. v+ tS" c -~',p . S a S ~ ~ v, ''~ ~ . '~ ,~ ~ ~ ~ ~' y~ U ..,~ N ~ ~ ~ ~ ~ ~ ~ ~~ LL a~ ~ ~l 3~ ~~ a -~ ~+ ~~ ~ ~~~ ! !! ~! 1!I iii e.~. s .~ 0 o, .n, c.t' -~ cg ~ ° ,~ ~ ~ v J ~ .J _ ~ ~~~