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Home My WebLink About501043004 Geotech Assessment January 3, 2008 Wendy Stafford 6007 Chennault Beach Drive Mukilteo, WA 98275 STRATUM GROUP P.O. Box 2546, Bellingham, WA 98227 Phone (360) 714-9409 Re: Geology Hazard Assessment MLA#: MLAA07-00534 681 Donald Drive Quilcene, Washington Dear Ms Stafford: Stratum Group is pleased to present the results of our geology assessment of the above referenced property adjacent to a shoreline bluff. A home is already located on the property. A new septic system is proposed on the site. The purpose of this geology assessment was to 1) evaluate the potential risk of slope failures impacting the existing home site and the proposed septic system, 2) evaluate. the risk posed by the construction of the new septic system to the stability of the slope, and 3) provide general site development and maintenance recommendations for the property adjacent to a potentially unstable bluff. Based on my investigation of the site and vicinity, it is my opinion that the home site is not at risk from slope failures over the expected life of the structure (100 years) and that construction of the proposed septic system will not be at risk nor with construction of the system increase the risk of slope failures at the site or off site provided the recommendations presented in this report are followed. This assessment included a visual inspection of the property, slopes on and in the vicinity of the property, available geologic mapping in the area, and investigation of the shoreline at the base of the 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 upland areas by excavating shallow hand dug test pits. 3) Prepared this report summarizing our findings,. including an evaluation of the risk of slope failures impacting the residence on the subject property and the proposed January 3, 2008 681 Donald Road, Jefferson County, WA Geology Assessment septic system, a qualitative evaluation of the shoreline bluff stability, recommendations for site maintenance, and recommendations for further investigation, if necessary. GENERAL GEOLOGY Northwestern Washington has been occupied by continental glaciers at least four times during the Pleistocene Epoch (L6 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 Geolo i_g c Map of Surficial Deposits in the Seattle 30' x 60' Quadrangle, Washington (Yount, Minard and Dembrof, 1993) and the Geolo ig c Map of West-Central Jefferson County, Washington (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 observationson the bluff face and uplands of the subject property and vicinity are consistent with the mapping described above, The property and the shoreline bluff slope is underlain by coarse, well graded, rounded gravel with sand and silt. Several discontinuous one- to three-foot thick sand layers and .silt layers area present on the bluff slope. All of the gravel, sand and silt layers are very compact and dense. No woody layers or clay layers are present on the bluff face on the subject property. 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. SPECIFIC SITE OBSERVATIONS A site plan sketch showing pertinent features is provided in Figure 2. The- subject property consists of a sloping upland area bounded on the south by a steep shoreline bluff. The upland portion slopes gently towards the south, and is covered primarily with second growth Douglas fir and madrone trees with an understory of salal, evergreen huckleberry, and ferns. The property is accessed via Donald Road. An access drive off of Donald Road accesses an existing .home on the propem'• Stratum Grbup File:12.15.07C ~ r~ 2 January 3, 2008 681 Donald Road, Jefferson County, WA Geology Assessment The southeast corner of the home is located approximately 35 feet. from. the top edge of the shoreline bluff. The home is angled such that the rest of the home is further from the top edge of the bluff slope. The proposed septic drain field and reserve drain field are located upslope and well back from the top of the steep shoreline slope.. The overall slope of the steep shoreline bluff averages approximately 45 degrees; however, portions of the uppermost part of the bluff and the base of the bluff area steeper. The total height of the steep portion of the bluff on the subject properly is approximately 90 feet. Except for the nearly vertical to vertical sections of the steep shoreline bluff, the steep shoreline slope is well vegetated with a mix of brush and trees. The brush consists of a mix of salal, snow berry, trailing blackberry, and poison oak. A number of mature madrone and Douglas fir trees are growing on the steep bluff face. Soils on the steep bluff slope consist primarily of very compact sand and gravel that I interpret to be pre-Vashon glaciation fluvial deposits.. Surface soils consist of thin coluvium of loose sand and gravel derived from the underlying fluvial deposits. No springs or seeps or seasonally wet areas are present on the bluff slope. The time of my site visit was a few days after an extreme heavy rain on snow event where as much as 16 inches of rain was recorded over a two day period a few miles south of the subjectproperty. Despite this recent very high rain fall event I observed no indications of surface water flow on the .property. 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 soil failures. The material eroded from the bluff face is removed from the base of the bluff and transported along the shore towards the west and east by wave action. Very little landslide debris has accumulated at the base of the bluff. I did not observe any evidence (tension cracks or trees rotated inward away from the top of the 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. CONCLUSIONS AND RECOMMENDATIONS Based on my visual inspection of the subject property and vicinity, I concl~lde that the existing residence or the proposed septic system are not at risk from potential landslides or erosion induced slope failures within the expected life of the structure (100 years) as long as my recommendations are followed. Stratum Group File:12.15.07C w, E ~`f 3 E ~ i7 ~~°~~t,.._ ~ January 3, 2008 681 Donald Road, Jefferson County, WA Geology Assessment Building Setbacks The present-home is located 35 feet at its closest point to the top of the steep shoreline slope. I recommend that any other future additions or buildings be set back at least 30 feet from the top edge of-the steep shoreline slope. It is my opinion from an engineering geology standpoint that this distance will be adequate to allow for expected bluff face failures that will result from the continued erosion of the base of the bluff by wave action and ravelling of the exposed soils on the bluff face over the expected life of the structure of 100 years Shoreline 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 east and west 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. Storm and Septic Drainage Soils on the upland portion of the property appear to be relatively well drained. The soils appear to consist primarily of coarse grained deposits. The present manner of dispersed drainage should be maintained. Any additional buildings if constructed should have roof drains and any other drainage should be discharged into a level dispersion/infiltrationtrevch excavated perpendicular to the slope. Dispersion trenches should be setback at least 30 feet from the top of the shoreline bluff. The proposed septic drain field. should not cause any problems for slope stability as long as the drain field is located at least 40 feet back from the top of the steep shoreline slope. The proposed drain fields are located at least 115 feet back from the top of the steep. shoreline bluff slope. Vegetation 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. Over time debris will break .down and form an unstable mass t ~ 1 ~~lama m the slopes below. ~ 3~~'~' ~~ ~ (r Stratum Crroup File:12.15.07C ~EEfE~SON C~U~~Y DAD January 3, 2008 681 Donald Road, 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. No trails or grading be performed on the shoreline 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. At least one tree per 30-foot center should be left within the buffer area. 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 as required. 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, Stratum Group ~%~~.~~ Dan McShane, M.Sc., L,E.G. Licensed Engineering Geologist Stratum Group File:12.15.07C 5 ~. ` ~~#F ~~r z -a4°" ~' c~ P;r' '~;~. ~,~ ~ «'_. ;: '~.~ a`e' .~,.... M,,.. s } 0.` ~ 4 _ A - l r.~ p . .. A'p ~: v~ '., x~" ~.,.... ...-.; r~-.. , ., ,.....,...., s.~. ~ f, Fgure 1. Site Location 47°41.057' N, 122°48.852' W W'GS84 122°50.000' W 122°49.000' W WGS84 122°4$,OUO' W 1 c c n r b c c c b ° r d z c a c~ c~ st 0 n v z 0 0 ni ~r n a- 0 0 d~ ° n 0 0 0 ° n ~- 122°50.000' W 122°49.000' lA/ WGS84 122°4$.000' W TN MN O S t 81~E 1 ~ ®~ fEEI p 9l~- a00D todETEE~ 11} _ NIa~ tiseadedwithlY?Pt]!~ ~.~003 tZetio~al ~ {wvo~ ~ ~oo~opo) ~_~ ~' i did9 `~~ dG.dsrl~*~'1!~ ~tl'f.'°'F~~~r4~' .:$ ,P ~f~1'i ~',~~ ~~ s ~°"°° ~:~: Stratu Figure 2. °~ °~ ~, ~ ;': Group °~~~ ~ ,4g, ~~t Strut Site Plan Sketch l{ .xT ~ ~~ d Y #?3 k t,~. ~~. ~ ' ~1, k ~~~~{