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HomeMy WebLinkAboutBLD2008-00451 Geotechnical Report • STRATUM GROUP PO Box 2546,Bellingham,Washington 98227 Phone(360)714-9409 October 1,2007 1?-8 Leslie Toussaint V:8 130 NW 56th Street OCT ' Seattle, Washington 98107 S(gO8, iffCp Re: Geology Hazard Assessment fkO� Tax Parcel 701344034 �IpirBeg 1281 Rhododendron Road Toandos Peninsula, Jefferson County, Washington Dear Ms. Toussaint: • Stratum Group is pleased to present the results of our geology hazard evaluation of the above referenced property. The purpose of this geology evaluation was to evaluate the risk of slope failures impacting the proposed building site on the subject property and provide general site development and maintenance recommendations. This evaluation included a visual inspection of the property and vicinity,a visual inspection of the slopes on the property and vicinity, review of available geologic mapping in the area, and inspection of the shoreline at the base of the bluff. The subject property is located along the west side of the Toandos Peninsula in Jefferson County, Washington. The site extends approximately 1,087 feet from the eastern shore of Dabob Bay and is approximately 200 to 230 feet wide. The site location map is provided on Figure 1. SCOPE OF SERVICES The scope of our services included the following: 1) Conducted a site visit to visually inspect the subject property including the slope conditions, shoreline conditions, and relevant conditions in the vicinity of the property. 2) Observed surface soil conditions on the slope and on the uplands above the slope by excavating shallow hand dug test pits. 3) Prepared this report summarizing our findings, including an evaluation of the feasibility of building a yurt and septic system on the subject property, a qualitative evaluation of the steep slope stability,recommendations for site development, and recommendations for further investigation, if necessary. October 1,2007 Parcel 701344034,Toandos Peninsula,Jefferson County,Washington Geology Hazard 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 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 that the site is underlain pre-Fraser undifferentiated deposits(named pre-Vashon stratified sediments in the Birdseye map)with beach deposits along the shoreline. Vashon advance outwash deposits and glacial till are mapped at or near the eastern property boundary. The pre-Fraser sediments are described as consisting of interbedded oxidized brown, red-brown, and gray gravel, sand, silt, and clay. Generally the unit is nonglacial and has abundant peat and woody debris. These sediments include deposits of the Whidbey and Olympia nonglacial periods and have been overridden and consolidated by glacial ice. Site observations are consistent with the above described mapping. Pre-Fraser deposits on the site include fairly uniform grey clay with minor pebbles that blankets the upper portions of the site. The clay is underlain by a thick, brown, stratified sand unit. A pre-Fraser glacial drift with an unsorted mix of clay, sand, and gravel is located beneath the sand unit. The lower slopes of the property along the shoreline are underlain by a mix of soils from the slopes above. The site is mapped as a landslide hazard in the Eastern Jefferson County—Critical Areas Landslide Hazards Map(1999). The map indicates that the subject property and adjacent properties to the north had landslide activities in 1999 and that a high risk area is located further to the north. Observations confirm that there are unstable slopes on the subject property and in the vicinity. The property is located along the south side of a large landslide complex. The steep north facing slope on the site is the southern portion of an old landslide scarp. SPECIFIC SITE OBSERVATIONS A site sketch map of the property with pertinent features indicated is provided in Figure 2. The subject property is accessed via Rhododendron Road. The access enters the south-central portion of the property onto a gently sloping grass-covered area. The grass area is approximately 225 long and 90 feet wide and slopes at approximately six degrees toward the west-northwest. This area is located on a west plunging ridge line between a steep slope on the north and a valley ,t. E CEI VE D Stratum Group File:8.30.07 0 5 2007 EEUN COUNTY UGLI • • • October I,2007 Parcel 701344034,Toandos Peninsula,Jefferson County,Washington Geology Hazard Assessment towards the south. A building pad has been constructed in the eastern end of the grass area. The pad area has been graded to a level surface such that it is approximately five feet high on the west end and has an approximately seven foot high angled slope the east side. The exposed face of the cut slope on the east is underlain by clay. The slope to the north of the building area slopes downward to the north down to Beach Road. The overall angle of the slope is approximately 40 degrees. The slope is forested throughout with a mix of alder,maple, cedar and firs. This slope is the south scarp slope of a large landslide complex located to the north of the subject property. The upper part of the slope is underlain by silts and clays with the bulk of the slope being underlain by sands and gravels. All of the soils on this slope are very compact and are consistent with the description of preglacial deposits. It appears that this old land slide scarp is a very old feature as the slope has been eroded by a number of small slides and the formation of a few stream channels associated with convergent wet areas. The steep slope has evidence of soil creep and periodic shallow soil slope failures. The sloping ridge line slopes gently towards the west towards the shoreline but is abruptly truncated by the steep shoreline bluff. The uppermost portion of the shoreline bluff is very steep with portions of the slope being nearly vertical. The slope then lessens to approximately 45 degrees down to the shoreline itself. This slope is mostly tree covered with a few small bare areas where the slope is very steep. The uppermost part of the slope is underlain by silt and clay with compact sand and gravel a short distance down the slope. The lower portions of the slope are covered with mixed material from old slope failures. The height of the shoreline bluff is approximately 300 feet. A seasonal stream valley is located on the south side of the property and makes up the south side of the ridge. The forested slope down to the bottom of the valley are gentle slopes on the order of 25 degrees on the upper part of the valley and steepens to 40 degrees further down the valley until the valley is truncated by the shoreline bluff. As noted above indications of slope movement and past shallow slides are present on the steep north slope and on the steep shoreline slope. The movement includes soil creep, small mud flows triggered by tree falls, and small failures within the sand units. In addition,three areas of settlement and cracking are present along the outer edge of the grass-covered area at the top of the steep north slope and the top of the steep shoreline slope. The cracking and settlement of the edge areas extent into the gentle sloping areas approximately 5 feet. . E) Stratum Group File:8.30.07 • • October 1,2007 Parcel 701344034,Toandos Peninsula,Jefferson County,Washington Geology Hazard Assessment ASSESSMENT OF GEOLOGIC RISK Three potential slide hazards were identified on the subject property: 1)steep forested slopes that extend toward north toward Beach Road from the grass-covered building area,2)the steep shoreline bluff, and 3)the deep-seated landslide failure area to the north. These three areas are each discussed below. Steep North Slope A forested slope extends from the grass-covered building area toward the north from the grass- covered areas toward Beach Road. The slope is generally on the order of 45 degrees on the upper part with less steep areas as the bottom of the slope is reached. Indications of shallow movement were observed on these slopes through soil creep, small shallow failures,and soil movement triggered by tree falls. Areas of convergent topography and seeps of water are present on the slope as well as failures associated with these areas are evident as well. It is our opinion that shallow failures will continue to occur along the over steepened portions of the slopes and especially during the wet winter months Two areas settlement and cracking are present along the edge of the grass-covered area adjacent to the steep slope. The ground in these areas has settled approximately one to three feet in arcuate scarps approximately 15 feet wide. It is our understanding that the landslides occurred on the steep slope in the late 1990s. The recent shallow settlement areas on the edge of the steep slope are located within the upper clay unit on the property. Native plants have been recently planted within the failure areas. The largest shallow failure is located north-northwest of the building area and has extended approximately 12 feet from the forested slope. The proposed building area is located approximately 27 feet south of the failure at the closest point. These shallow failures are the result of the over steep slope immediately down slope. More failures will likely occur in the shallow clay layers during wet times of the year and/or seismic activity. The setback for the yurt structure is 25 feet from the edge of the closest failure. This will allow for two similar sized failures before it would impact the structure. Shoreline Bluff The shoreline bluff is the result of wave erosion at the base of the bluff. Wave erosion is limited in size due to the confinement within the Dabob Bay. Some landslide debris was noted along the shoreline of the subject property and adjacent properties. The debris consisted of clay, sand, and downed trees. This debris acts as a natural armor to the base of the bluff. Over time the erosion at the base of the bluff creates steep slopes and under the right weather conditions the steep shoreline slope should be expected to have shallow rapid slope failures that will eventually I'\) ECEIVED Stratum Group File:8.30.07 OCT 0 5 2007 • JEFFERSON COUNTY D,CD • • October 1,2007 Parcel 701344034,Toandos Peninsula,Jefferson County,Washington Geology Hazard Assessment impact the west end of the gentle sloping ridge line. However,the erosion rate at the base of the bluff appears to be minor at the present time and the beach conditions are such that wave action at the base of the failed soils is only during times of very high tides. Failures similar in scale and mechanism to the steep north facing slope on the property are present on the shoreline bluff slope as well. Settlement has taken place within the clay soils on the upper edge of the slope. Future failures along the edge of the slope and ion the slope itself should be expected. Deep Seated Landslide The steep north facing slope on the subject property is the southern side of a large deep seated landslide. The slide is approximately 1,200 feet wide and extends inland from the shoreline approximately 1,400 feet. There is no evidence of recent large scale movement or expansion of the slide area as the headwall scarps are tree covered and no tension cracks or recent large scarps are present on the upper slopes of the landslide complex. Recent movement of the landslides within the landslide complex occurred in 1997. The central portion of the landslide, located two properties north of the subject property,was reactivated during extremely heavy rains through the winter and spring of 1996/1997. Landslide debris covered Beach Road and filled a residential home located on the lower central portion of the slide with mud. The landslide surface is generally hummocky and evidence of soil movement both old and within the past ten years is apparent. The central lower portion of the slide area has numerous wet areas and seeps. The large slide complex is likely the result of an area of perched water that periodically forms during very wet weather periods causing soil saturation and high pore water pressure within the sand unit. Rapid large scale expansion of the slide area appears to have ended a very long time ago based on the eroded condition of the headwall scarp. Expansion is likely to result as the lower portion of the slide continues to slide on a periodic basis towards the bay causing the steep upper slopes to loose support and eventually fail. Because the subject property is on the limb of the slide pore pressure under the steep north slope note.likely build up in a manner that would lead to sudden large scale failures. The close proximity of the subject property to the old landslide feature does pose a potential risk to the subject property. RECEIVE ) Stratum Group OCT 0 5 2007 File:8.30.07 EFFF SON CUONI' OCR • • October 1,2007 Parcel 701344034,Toandos Peninsula,Jefferson County,Washington Geology Hazard Assessment CONCLUSIONS AND RECOMMENDATIONS I conclude that the construction of a yurt and septic system to be placed on the property in the proposed locations will not be at risk from slope failures over the expected life of the structure (25 years). A yurt also will have the advantage that if slope failures within the clay soils begin to expand towards the yurt,the structure can be readily moved even if settlement takes place within the shallow clay soils. However, it is my opinion that the yurt site and the septic drain field site will not be at risk of slope failures over the next 25 years as long as yurt and drain field are located at least 25 feet from the top edge of the steep slopes on the subject property. A more permanent standard home construction would require a greater setback than 25 feet to ensure the home would not be at risk from slope failures over the expected life of a standard foundation home (100 years). A larger setback is not practical on this site due to the property boundaries and other types of engineering to mitigate slope movement are not feasible at this site. While the size of the potential building pad area would be highly limiting for a standard construction home, a yurt is a prudent choice for this site. A yurt structure allows for reasonable use of the property.. The additional water input from the septic system will not increase the risk of landslide movement on the property. Because the slope is towards the west the modest drainage increase will be dispersed over a fairly large area. We recommend that the reserve septic drainage field be located at least 20 feet from the shallow failures. The following recommendations should also be followed in developing the site: 1. No occupancy structures should be built within 25 feet of the steep slopes on the north and west sides of the gentle sloping ridge around the perimeter of the upland grassy area of the subject property. This distance will provide adequate space for two additional shallow failures (based on the largest shallow failure)before the structure would be compromised. 2. The occupancy structure should be limited to a yurt or mobile type home with independent pile supports. 3. Native vegetation on the slope should be maintained. Removal of potential hazard trees or limbing and topping of trees for view purposes is acceptable. 4. No soil, debris or landscaping debris should be placed on the slopes or within the areas of shallow failures. Additional material on or along the edges of the slopes can cause additional failures. 5. Storm water should be dispersed and infiltrated into the ground and not concentrated near the top of the slope. Dispersion and infiltration areas should be located towards the south side of the ridge area. Dispersion/infiltration can be accomplished by a level infiltration/dispersion trench located along the southern boundary of the property. 4 el Stratum Group File:8.30.07 II • October 1,2007 Parcel 701344034,Toandos Peninsula,Jefferson County,Washington Geology Hazard Assessment We do not anticipate that the development of the subject property as proposed with a yurt or mobile type home or trailer will cause any negative impacts on the stability of other slopes or the shoreline bluff as long as our recommendations are followed. 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 000AL Ref .4,, /a Dan McShane, L.E.G., M.Sc. Kim Ninnemann, B.S. Licensed Engineering Geologist Geologist . ">i r ilia;4_441K4HilAlli ; T°:71 ' ' ;. )4' - Daniel Mc$h.A•!, RECEIVED Stratum Group O C T 0 5 2007 File:8.30.07 JEFFERSON COUNTY OCR •