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Home My WebLink About501025008 Geotech Assessment STRATUM GROUP 1451 Grant Street, Bellingham, W A 98225 Phone (360) 714-9409 RECE~'TF.n JUt 0 2 211m JEffERSON COUNIY DeD C MaY2~ Terry McHugh John L. Scott Real Estate 2219 West Simms Way Port Townsend, W A 98368 Re: Engineering Geology Reconnaissance, Shoreline Bluff Lot 8, HazelPoint Large Lot Subdivision Government Lot 1, Section 2, Township 25 N, Range 1 W Jefferson County, Washington Dear Mr. McHugh: We are pleased to present the results of our engineering geology reconnaissance ofthe above referenced property adjacent to a shoreline bluff. The purpose of this geology evaluation 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 ofthe shoreline property. This evaluation was limited to a visual inspection ofthe property and vicinity, a visual inspection of the slopes on the property, review of available geologic mapping in the area, and inspection 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 including the bluff face slope conditions, shoreline conditions, and relevant conditions in the vicinity of the property . 2) Observed surface soil conditions on the bluff face and on the uplands above the bluff by excavating shallow hand dug testpits. 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. May 28, 2002 Lot 8, Hazel Point Large Lot Subdivision Engineering Geology Reconnaissance, Shoreline Bluff R.ECEIVED .0211I8 ~CHlH~UN CUUNlY fiCO 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 ofthe subject property. The Geologic Map of Surficial Deposits in the Seattle 30' x 60' Quadrangle, Washington (Yount, Minard and Dembrof, 1993) and the Geologic Map of West-Central Jefferson County, Washington (Birdseye, 1976) indicate the property is underlain by undifferentiated pre-Fraser glaciation deposits. The pre-Fraser 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. Yount, Minard, and Dembrof (1993) indicate that recessional outwash deposits are present on the upland areas of the subject property, and Birdseye (1976) indicates that stratified ice-contact deposits are located in this same area. Observations on the subject property and the vicinity of the property are consistent with the mapping described above. Most of the slopes on the property area underlain by coarse, well graded, rounded gravel with sand and silt in a very compact condition. The upper slopes of the property area underlain by soft silt and loose sand that that is consistent appear to be poorly stratified ice margin and ice wasting deposits. SPECIFIC SITE OBSERVATIONS The site location map is provided on Figure 1. The subject property generally slopes towards the east from an elevation of approximately 200 feet down to the essentially sea level at a convergent sand spit. Most of the property is tree-covered with a mix of Douglas fir, western red cedar, alder, madrone and maple. The slope ofthe property is generally approximately 10 degrees with some areas as steep as 20 degrees. No evidence of unstable slopes is evident on the property. The eastern side ofthe property is bounded by.a convergent sand and gravel spit. This shoreline is an accreationary shore feature, that is, the shoreline is growing outward from deposition of . sand and gravel being transported via wave action from the eroding bluffs both west and north of the subject property. The shoreline west of the subject property is in a fairly natural condition Stratum Group File:5.22.02C 2 . , May 28, 2002 Lot 8, Hazel Point Large Lot Subdivision Engineering Geology Reconnaissance, Shoreline Bluff with no bulkheads that would disrupt the sediment needed to maintain the spit. Two bulkheads are present immediately north ofthe spit. However, these bulk heads do not appear to be causing interference of shoreline sediment transport and given the length of the feeder bluff available to the north of the spit are probably having a minimal effect on sediment supply to the spit. Stratum Group File:5.22.02C 3 : May 28, 2002 Lot 8, Hazel Point Large Lot Subdivision Engineering Geology Reconnaissance, Shoreline Bluff ltECEIVED dUl022lD1 ~f~flliU:r~i willi i'w ii lJ;jJ) CONCLUSIONS AND RECOMMENDATIONS Slopes on the subject property are stable and shoreline erosion is not an issue at this site. Therefore the risk of slope failures or erosion impacting a residence on the subject property is minimal. We have reviewed and inspected the proposed building area and septic drain field locations and conclude that these sites are all suitable from a slope stability standpoint. The shoreline spit east of the property is presently stable and in fact is growing. The construction of shoreline armoring on other properties north or west of this spit may overtime starve the spit of the sediment it will need in order to remain stable. The owner of the subject property should contact Jefferson County officials regarding any proposed shoreline protection projects along the bluffs north or west of the spit. With the exception of the two rock bulkheads north of the spit 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 stOml water catch basins must not be introduced into the perimeter footing drain. We recommend that roof drains and any other drainage be discharged into a rigid perforated dispersion pipe. The dispersion pipe should be placed in a level infiltration trench excavated perpendicular to the slope. Native vegetation, particularly tress and low native brush should be disturbed as little as possible. Trees between residence and shoreline bluff may be limbed or thinned for view purposes. We appreciate 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, ]:roup Dan McShane, M. c., L.E.G. Licensed Engineering Geologist Stratum Group File:5.22.02C 4 122048'00" W Figure 1. 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