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HomeMy WebLinkAbout601211001 Geotech Assessment STRATUM GROUP 1451 Grant Street, B¢llingham, WA 98225 . December 21, 1999 AU$ - 5 ~002 Michael Anderson JEFFERSON COUNTY P.O. Box 2305 DEPT. OF COMMUNITY DEVELOPMENT Kirkland, WA 98083 Re: Engineering Geology Reconnaissance, Shoreline Bluff Tax Parcel 601211001 and 601211002 NE 1/4 of Section 21, Township 26 N, Range 1 W Jefferson County, Washington Dear Mr. Anderson: 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 residence, 2) qualitatively evaluate the risk of slope failures, and 3) provide general site development and maintenance recommendations for development of a building site adjacent to a potentially unstable bluff. This evaluation was limited to a visual inspection of the property and vicinity, a visual inspection of the bluff face, review of available geologic mapping in the area, and inspection of the shoreline at the base of the bluff. The property is located on an approximately 400-foot high bluff on the west side of the Toandos Peninsula. The location of the property is indicated on Figure 1. The property is approximately 62 acres in size and is 'located in the northeast 1/4 of Section 21, Township 26N, Range 1 W. 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. 3) Prepared this report summarizing our findings, including an evaluation of the feasibility of building a residence on the upland portion of the subject property, a qualitative evaluation of the shoreline bluff stability in regards to December 21, 1999 Anderson Property, NE 1/4 Section 21 T26N, R1W Geologic Evaluation of Shoreline Bluff building a residence on the property, 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 layer of glacial related and interglacial fluvial sediments were deposited over the underlying bedrock in the vicinity of the 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 shoreline and bluff exposed on the subject property is underlain glacial till and undifferentiated pre-Fraser glaciation deposits. The glacial till is described as consisting of a mixture of clay, silt, sand and gravel and is in a very compact condition. 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. Our observations of the bluff face on 'the subject property and upland areas on the subject property and vicinity of the property are consistent with the mapping described above. We observed that the glacial till at the top of the shoreline bluff varied from approximately 20 feet thick to 40 feet thick. The pre-Fraser deposits consisted primarily of layered sands with silt, sandy silts, and occasional gravel layers. SPECIFIC SITE OBSERVATIONS The subject property is approximately 62 acres in size. No buildings are presently located on the property. The upland portion of the property slopes gently to the west and south and is tree-covered primarily with Douglas fir and a wide variety of brush. Based on the size of the trees, it appears that the upland portion of the property was logged within the past 10 years. The shoreline bluff is approximately 360 feet high. The overall average slope from the top Stratum Group 2 File: 11.30.99 December 21, 1999 Anderson Property, NE 1/4 Section 21 T26N, R1W Geologic Evaluation of Shoreline Bluff of the steep bluff to the beach is approximately 45 degrees. However, portions of the bluff are much steeper and other portions are less steep. Near the central portion of the property, the upper portion of the bluff is nearly vertical for approximately 30 feet, but the lower slopes of the bluff slope on the order of 30 degrees or less. Both north and south of this area the lower portion of the bluff is much steeper, but the upper areas of the buff are not as steep. Vegetation varies on the bluff slope from areas of no vegetation to stands of mature Douglas fir and madrone. Vegetation cover is dependent on the steepness of the slope and the time since previous slope failures. Several recent slope failures on various portions of the bluff are covered only with brush, and other areas are covered with stands of red alder. Several shallow surface soil failures have occurred within the past few years on the bluff face. Landslide debris consisting of failed soil and trees has been deposited at the base of the buff and forms several gentle sloping areas approximately 5 feet above the beach. Soils exposed on the steep portions of the bluff slope consist primarily of layered very compact silty sand and sandy silt with occasional lenses of gravel. The uppermost portion of the bluff near the center of the property .is underlain by very compact massive silty sand with gravel, cobbles and boulders. We interpret the layered sands and silts to be pre-Fraser fluvial deposits and the massive silty sand with gravel, cobbles and boulders to be glacial till. Small seeps of water were observed within one thin sand layer approximately 200 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 November 30, 1999. Very heavy rainfall had taken place the day before our site visit, and the previous month had been wet. 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. Because of the recent slides on the bluff, soil and logs have accumulated at the base of the bluff. The soil and logs are acting as a natural shoreline armoring for the base of the bluff. However, over time erosion and undermining of this material will occur, and the bluff will again become undermined by wave action. The material eroded from the bluff face is removed from the base of the bluff and transported along the shore towards the north by wave action. Stratum Group 3 File: 11.30.99 December 21, 1999 Anderson Property, NE 1/4 Section 21 T26N, RIW Geologic Evaluation of Shoreline Bluff We did not observe any evidence (tension cracks or trees rotated inward away from the top of the bluff or indications of past uplift of the beach area) 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. A stream with head waters on the east and northeastern areas of the property drains surface water from these areas and flows within a deep ravine to the shore on the west side of the property (see Figure 1). The side slopes of the stream ravine are on the order of 45 degrees and are covered primarily with mature western red cedar and big leaf maple with an understory of ferns. Soil creep and shallow topsoil failures are evident on most of the slopes within the ravine. Alluvial material from the creek has created a wider beach at the mouth of the creek including a small bar with a backwater area behind it. 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 (as indicated in Figure 2) 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 that any buildings built on the subject property be set back from the top edge of the steep shoreline bluff slope at least 100 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 pr°i3erty will cause any negative impacts on the stability of the slope or shoreline bluff 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 north 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 Stratum Group ~' File: 11.30.99 December 21, 1999 Anderson Property, NE 1/4 Section 21 T26N, R1W Geologic Evaluation of Shoreline Bluff 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, and should be located at least 100 feet from the steep shoreline bluff slope and at least 50 feet from the steep ravine slope. 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 steep bluff. Native vegetation, particularly tress and low native brush on the slope between the building site and the shoreline bluff should be disturbed as little as possible. No trails or grading should be performed on the bluff face. If trees are thinned or limbed for view purposes, no debris should be placed within 30 feet oi' the bluff face. Some thinning of 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 septic drain field should be located at least 100 feet from the top of the steep shoreline bluff slope and at least 50 feet from the slope of the ravine. As long as the septic drainfield is used by a single family residence, it should not have any negative impacts on slope stability as long as the above recommended setback distances are followed. Stratum Group 5 File: 11.30.99 December 21, 1999 Anderson Property, NE 1/4 Section 21 T26N, RIW Geologic Evaluation of Shoreline Bluff Please note that there are inherem 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 Dan McShane, M.S. Senior Geologist McShane [ Stratum Group 6 File: 11.30.99 122°50.111" NAD2? 47°4Z.f33', 1~46.2z I000 0 1000 2000 $000 4000 F~ET Pzi.~t~d £mra TOPO! ©199~ Wildflower Productions (ww'~.tol~o,com) ! Potentia Building e Potential Area a_ -~ Building o ~ ~ -~ Area ~ Set] ~ Creek Steep Slope 1~1 inch = 400 feet ""~ "~'~i.':'~..:-'.'~'~5fiii~iii~i:'~i~ S t r a t u m F i g u r e 2 ~.?.:.~-,~ Group Site Sketch STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 July 31, 2002 AUG ' § ~ Michael Anderson , JEFFERSON COUNTY P.O. Box 789 DEPT. OF COMMUNITY DEVELOPMENT Issaquah, WA 98027 Re: Septic Drain Field Location Tax Parcel 601211001 and 601211002 NE 1/4 of Section 21, Township 26 N, Range 1 W Jefferson County, Washington Dear Mr. Anderson: As per our conversation on July 30, 2002 this letter is to provide guidance on the location of the septic drain field location on the above described property. The drain field can be located anywhere on the upland portion of the property as long as it is located 100 feet back from the steep shoreline slope or 50 feet from the steep ravine slope. These distances are included in our December 21, 1995 Geologic Evaluation of the property. The 100-foot and 50-foot setback distances are from the top of the steep slope. For purposes of our report, we cOnsidered the top of the steep shoreline slope to be the sharp slope break where the slope to the shore becomes greater than 45 degrees (1:1). A similar slope break occurs above the ravine as well. The setback from the steep ravine should be measured from' the sharp slope break above the ravine. Should you have any further questions regarding recommended setback distances please contact our office at (360) 714-9409. Sincerely yours, Stratum Group Dan McShane, M.S., P.G. Professional Geologist STRATUM GROUP 1451 Grant Strc~ l~llingham, WA 98225 Phone 060) 714~9409 September 26, 2002 Michael Anderson P.O. Box 789 Issaquah, WA 98027 100-foot Buffer Flagging Tax Parcel 601211001 and 601211002 NE 1/4 of Section 21, Township 26 N, Range 1 W Jefferson County, Washington Dear Mr_ Anderson: I visited the above descnbe, d parcel on September 8, 2002. At that time the 100-foot bluff setback was gagged by tying ta~ to trees located apl~roximately 100 feet from the top of the stee~ shoreline bluff slope. The septic drain field test pits are located well back from the setback. Should you have any further questions regarding recommended setback distances please contact our office at 060) 714-9409. S~acerely yours, Stratum Group Dan McShan~, M.S., P.O. Profe~,ional C.~ologi~ Denlel Mc,~hane