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HomeMy WebLinkAboutBLD2000-00778 Geotechnical Report 11/27/2000 22: 42 206354007 COYLEDRUG • PAGE 01 STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phore (360) 714-9409 September 4, 1998 Gary Elmer 5111 NE 201st Pi Lake Forest Park. WA 98155-1817 Re: Engineering Geology Reconnaissance,Shoreline Bluff Lot 2, of Government Lot 3, Sec 3, TWP 25N, R1W Coyle. Washington Dear Mr. Elmer: We are pleased to present the results of our engineering geology reconnaissance of the 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 of the property adjacent to a potentially unstable bluff. This evaluation was limited to a visual inspection of the property, bluff face, available geologic mapping in the area, and 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 and shoreline conditions. 2) Observed surface soil conditions on the bluff face 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. 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 11/27/2000 22: 42 2063640107 C:OOYLEDRUG PAGE 02 • September 4, 1998 Lot 2, Government Lot 3, Sec 3, Twp. 25N R 1W, Coyle, WA EngineeringGeologyRcconnaissance,Shoreline Bluff layer of glacial related and interglacial fluvial sediments were deposited over the underlying bedrock in the vicinity of the subject property. The _.ologic Map of Surficial Tye ai n the Seattle 30' x 60" Quadrgn , Was in ton (Yount, Minard and Dembrof, 1993) and the Rlggic Map of We T-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. Our observations on the bluff face and within road cuts and a test pit on the upper portion of the property and on the bluff face in the vicinity of the property 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. We observed several discontinuous one- to three- foot thick sand layers and silt layers on the bluff face. We observed very hard, varved silts near the base of the bluff at several locations. All of the silts and gravels were very compact and dense. We did not observe any woody layers on the bluff face on the subject property; however, we have observed woody layers at nearby areas on the bluff face. We 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 The subject property consists of an upland area bounded on the south by a steep shoreline bluff and on the west by a ravine. The upland portion slopes very 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 overall slope of the steep shoreline bluff averages approximately 70 degrees; however, portions of the bluff are vertical. The total height of the steep portion of the bluff is approximately 90 feet. Except for an approximately 30 foot high by 60 foot wide recent slide scar, most of the bluff is well vegetated with a mix of brush and trees. The brush consists of a mix of evergreen huckleberry, salal, ferns, salmon berry, and poison oak. A number of mature madrone and douglas fir trees are growing on the steep bluff face. Stratum Group 2 File:9.2.98 11/27/2000 22: 42 20E3640107 Car''LEDRUG PAGE 03 • • September 4, 1998 .......... ... t..i n, A.:� 1, T.Lik. n Cnvlo. WA Bnginaering GoologyRoemansiss fusee,Sbmrell nr Bluff The steep bluff slope above the shoreline on the subject property consists primarily of very compact sand and gravel with silt and clay that we interpret to be pre-Fraser fluvial deposits. A few discontinuous silt/clay layers less than one-foot thick are present on the bluff face. A silt/clay layer extends across a portion of the bottom two to six feet of the bluff. The silt/clay layers are very hard and compact. Small seeps of water were observed within a thin sand layer approximately 10 feet above the beach level. The sand layer from which water was seeping appeared to contain less silt and clay than the sand and gravel layers above and below, and hence is more permeable. Except for this area of seepage the remaining portions of the bluff face and the upland portion of the subject property were dry at the time of our site visit on September 2, 1998. 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 east by wave action. Very little landslide debris has accumulated at the base of the bluff. We 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. The areas of seeping water did not appear to be eroding at a faster rate than the layers above or below the seeps. The westernmost portion of the property slopes down towards a ravine at a maximum angle of approximately 45 degrees. A rough road follows the side of ravine down to the beach at the southwest corner of the subject property. The bottom of the ravine near the beach was wet. The ravine bottom upstream from the beach was dry. The bottom of the ravine was well vegetated and appeared to rarely have much water flow. Except for shallow minor soil creep, there was no evidence of slope instability in the ravine. We interpret the ravine to be a fairly old land form that may have formed shortly after the retreat of the last glaciers in the area. CONCLUSIONS AND RECOMMENDATIONS Based on our visual inspection of the subject property and vicinity, we conclude that a residence 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. We recommend a set back distance of 50 feet from the top of the steep portion of the bluff. We recommend a set back distance Stratum Group 3 ri e:9,2,98 11/27/2000 22:42 2063640107 COYLEDRUG I-'Hat qµ • • September 4. 1998 Lot 2, Government Lot 3, Sec 3, Twp. 25N R 1W, Coyle, WA EngineeringceologyRecoufaisusnce,Shareline Bluff from the ravine of 25 feet. It is our 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. We do not anticipate that the development of the subject property will cause any negative impacts on the stability of the slope to the east of the subject property as long as our 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 east 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. 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. Alternatively this water could be discharged via tightline to the base of the ravine west of the building site. 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. 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 rough road to the bottom of the ravine on the western portion of the property should be used only as a trail. Stratum Group 4 Fiic:9.2.98 11/27/2000 22:42 20636447 COYLEDRUG • PAGE 0E September 4, 1998 Lot 2, Government Lot 3, Sec 3, Twp. 25N R 1W, Coyle, WA Buslncez ina Geology R ,Shoreline Bluff The proposed septic drain field is located approximately 200 feet away from the top of the bluff. We do not anticipate that the septic drainfield will have any adverse affects on the stability of the bluff. 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. 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, Stratum Group i L. Dan McShane, M.S, Senior Geologist Stratum Group 5Pile;9,2.98