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Home My WebLink About601343022 Geotech Assessment.~. STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 March 6, 2006 Tim Smit P.O. Box 1365 Grahrn, WA 98338 Re: Geology Assessment Tax Parcels 60134021 and 641343022 Harrington Drive Jefferson County, Washington Dear Mr. Smit: Stratum Group is pleased to present the results of our geology assessment of the above referenced property adjacent to a shoreline bluff. The purpose of this geology assessment was to 1) determine the suitability of the property for the siting of a residence, 2) qualitatively evaluate the risk of slope failures, and 3) provide general site development and maintenance recommendations for development of the property adjacent to a potentially unstable bluff and other steep slopes. Based on my investigation of the site and vicinity, it is my opinion that the northeast portion of the site is suitable for development as a residential property provided the recommendations presented in this report are followed. This assessment included. a visual inspection of the property and slopes on and in the vicinity of the property, review of available geologic mapping in the area,. and a visual assessment of the shoreline at the base of the shoreline bluff. SCOPE OF SERVICES The -scope of our services included the following: 1) Conducted a site visit to visually inspect the subject property and vicinity including the bluff slope conditions,-slope conditions on and in the vicinity of the property and shoreline conditions. 2) Observed surface soil conditions on the shoreline slope and the slopes on the .west side of the property and at the top of the bluff by excavating shallow hand dug test pits. 3) Prepared this report summarizing our findings, including an evaluation of the feasibility of building a residence on the subject property, a qualitative evaluation of the shoreline bluff stability, recommendations. for site development, and .recommendations for further investigation, if necessary. ~ _.;'~ March 6, 2006 Parcels 60134021 and 60134022, Jefferson County, WA Geology Assessment 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 property. The Geologic Man of Surficial Deposits in the Seattle 30' x 60' Quadrangle Washington (Yount, Minard and Dembrof, 1993) and the Geologic Man of West-Central Jefferson County, Washin on (Birdseye, 1976) indicate the subject property and the bluff exposed on the subject property are underlain undifferentiated pre-Fraser glaciation deposits. These sediments are described as consisting of interbedded oxidized brown, red-brown, and gray gravel, sand, silt, and clay. The layers are moderately to well bedded and the unit contains minor amounts of ice- contact deposits and outwash gravel and sand.. Generally the unit is nonglacial and has abundant peat and woody debris. These sediments were deposited sometime prior to the last glacial event and include deposits of the Whidbey and Olympia nonglacial periods. The sediments have been overridden and consolidated by glacial ice. My observations on the bluff face and within road cuts and test pits on the upper portion of the property and on the bluff face in the vicinity of the properly is consistent with the mapping described above. Most of the property and bluff face is underlain by coarse, well graded, rounded gravel with sand and silt. Several discontinuous one- to three-foot thick sand layers and silt layers on the bluff face. All of the silts and gravels were very compact and dense. No woody layers were present on the bluff face on the subject properly; however, woody layers are present at nearby areas on the bluff face. I interpret the deposits to have been deposited in a fluvial environment prior to the last glacial advance as they are in a highly compact and dense condition and some organic material is present. The upland area immediately to the southwest of the subject property and on the uppermost areas of the west end of the site is underlain by glacial till. The till consists of a poorly sorted mix of clay, silt, sand and gravel deposited by glacial ice. The till is very compact and has low permeability. -. ,, ~~ ~ ft ~~~° ~ '~ ~ .q `~ ,,~ n °."i }y ~ 5~~9~~ ,g Stiatum Group "~c~ File:1.19.06 2 t~ March 6, 2006 Parcels 60 t 34021 and 60134022, Jefferson County, WA Geology Assessment SPECIFIC SITE OBSERVATIONS The location of the two lots are shown on the attached Figure i site vicinity map and pertinent information is provided on the attached Figure 2 site-plan sketch. The subject property consists of two parcels that are each 75 feet wide and approximately 1,000 feet long. The two lots extend from near Harrington Drive located near the southwest end of the parcels to the shoreline located along the northeast end of the parcels. The subject property is located in an area with a gentle sloping upland area bounded. on the northeast by a steep shoreline bluff. Several deep steep sided ravines cut across the upland area. The upland area is underlain primarily by glacial till. The ravines appear to be the result of periodic surface water flow across the upland areas to the southwest having cut down through the till to the underlying sand and gravel deposits. Surface water flow within these ravines apparently takes place only during periods of very high intensity rainfall or rain on snow events. Approximately two-thirds of the property is located on the side slopes of a valley that is incised down into the upland area. The northeast end of the property.consists of a gentle sloping upland area bounded on the northeast by a very steep shoreline slope and on the northwest by the steep slope down to a valley located to the northwest.-This gentle sloping area is accessed from Harrington Drive via a gravel road that crosses the properties to the southeast of the subject property. After traversing this upland area,, the gravel road angles down the steep slope of the valley side to the valley bottom to an existing residence located on the property northwest of the subject property. The steep shoreline bluff slope has an average slope angle of approximately 55 degrees. However portions of the slope are vertical particularly near the base of the slope and on the upper parts of the slope. The shoreline slope is underlain by very compact layers of sand and gravel and a few discontinuous silt layers. The very step sections of the slope have exposed soils, but mature trees are present on portions of the bluff slope with younger trees located on areas that have failed within the past number of years. No seeps of water were observed anywhere on the steep shoreline bluff slope despite that the fact that my site visit followed a period of very wet weather during December 2005 and January 2006. The bluff is eroded by a combination of wave action that undermines the base of the bluff, ravelling of material off the exposed soils on the unvegetated portions of the bluff, and periodic shallow topsoil failures, The material eroded from the bluff face is removed from the base of the bluff and transported along the shore towards the northwest by wave action. Wedges of landslide Stratum Group File: L 19:06 3 ,, March 6, 2006 Parcels 60l 34021 and 60134022, Jefferson County, WA Geology Assessment material with trees growing on the wedges are present at numerous locations along the base of the bluff along this stretch of shoreline. I did not observe any evidence (tension cracks or trees rotated inward away from the top of the shoreline bluff] indicating an incipient global-type or deep-seated failure on the subject property. The presence of straight mature Douglas firs on the bluff face indicates that the erosion rate on the steep slope must be relatively slow. An approximately 1,000 foot long ravine is located along the northwest side of the property. The ravine starts near Harrington Drive near the southwest end of the property from an otherwise minor area of convergent sloping ground with little indication of stream flow. The base of the ravine appears to rarely have surface water flow and there was no evidence of surface water flow in the ravine during the past winter despite the very wet weather in December 2005 through January 2006. This observation is consistent with observations I have made in other ravines to the north. Surface water flow likely takes place in these ravines during rain on snow events when the thin glacial till soils have little or no storage capacity due to frozen ground. The side slopes of the ravine on the northwest side of the property are on the order of 40 degrees with a few slightly steeper areas. The road access down the side of the ravine to the property to the north has cut into a portion of the slope. This slope is standing vertical and is underlain by compact sand and gravel layers. Except for shallow minor soil creep, there is no evidence of slope instability on the ravine side slopes. I interpret the ravine to be a fairly old landform that may have formed shortly after the retreat of the last glaciers in the area. Periodic water flow from less permeable soils above the ravine area may periodically send water flowing down the ravine. sUaium Group File:1.19.06 4 ' T. March 6, 2006 Parcels 60134021 and 60134022, Jefferson County, WA Geology Assessment CONCLUSIONS AND RECOMMENDATIONS Based on my visual inspection of the subject property and vicinity, I conclude that a residence(s) can be located on the upland portion of the property such that bluff erosion or landsliding would not threaten the structure within the expected life of the structure (75 years) as long as our recommendations are followed. I recommend a set back distance of 35 feet from the top of the steep portion of the bluff. I recommend a set back distance from the top of the steep slope of the ravine of 20-feet. It is my opinion from an engineering geology standpoint that these distances will be adequate to allow for expected bluff face failures that will result from the continued erasion of the .base of the bluff by wave action and ravelling of the exposed soils on the bluff face: as well as any possible soil failures on the steep side slopes of the ravine. I do not anticipate that the development of the subject property will cause any negative impacts on the stability of the -shoreline slope or ravine slopes as long as the following recommendations are followed. Because the bluff is an eroding bluff, slope failures should be expected. to occur on a periodic basis. The only way to prevent continued erosion of the base of the. bluff is to construct hard armoring at the shoreline at the base of the bluff. However, the eroding bluff acts as a feeder bluff (erosion of the bluff provides sediment) for the beaches northwest and southeast of the subject property, and any shoreline armoring will have a negative impact on properties down drift from the bluff. The construction of shoreline armoring on other properties may cause an increase of erosion of the subject property. The owner of the subject property should contact Jefferson County officials regarding any proposed shoreline protection projects along the bluff or any shoreline protection construction. There are currently no shoreline-armored areas along this stretch of coastline. Soils on the upland portion of the property appear to be relatively well drained, and therefore we do not anticipate discharging of footing drains will be necessary. Roof drainage and any storm water catch basins must not be introduced into the perimeter footing drain. Roof drains and any other drainage should be discharged into a perforated dispersion pipe. The dispersion pipe should be placed in a level dispersion trench. Site grading soils or debris, landscape debris, or any other material should not be disposed of over the bluff face or placed at the top of bluff or placed on the steep ravine slopes. Over time debris will break down and form an unstable mass that will slide damaging the slopes below. Stratum Group File:1.19.06 5 iT March 6, 2006 Parcels 60134021 and 60134022, Jefferson County, WA Geology Assessment Native vegetation, particularly tress and low native brush within 30 feet of the top of the bluff should be disturbed as little as possible, and we recommend that no trails or grading be performed on the bluff face. If trees are thinned or limbed for view purposes on the bluff face, all debris should be removed from the bluff face. Some thinning of small Douglas fir trees growing between the top of the bluff face and the proposed home site should not cause any problems as long as the native brush understory is left in place. The road to the bottom of the ravine on the western portion of the property may continue to be used as it currently is but should not be widened unless road construction plans are reviewed and approved by an engineering geologist or geotechnical engineer. A septic drain field should not hav y av the bluff as long as the drain field is locat t least 40 feet from any slopes steeper than 40 degrees. Please note that there are inherent risks associated with building on 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 asrequired. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions regarding our reconnaissance please contact our office at (360) 714-9409. Sincerely yours, Str tum Group ~-~ ~~~~~ Dan McShane, .Sc., L.E.G. Licensed Engineering Geologist Stratum Group Fi]e:1.19.06 6 Fgure 1. Sit Vidnity 122°48.000' W 122°47.000' MJ WGS84 122°46.400' 1N Ji .. d i ;l ' f`` ~`~~ ~ . 'I? ~~ - r~l j I ~ r - ~ ~ ! - °_ __- - ,.._._y:C _ i _ .>.~ Vic...-.,- r _r ..~ ~ ~.~~ _I ._. _ -. 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