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Home My WebLink About601354006 Geotech Assessment John Bethel, LG ' 16313 Kelly Rd, NE Duvall, WA 98019 Friday, May 25, 2007 Don Marshall 14426 320th Ave. NE Duvall WA 98019 Dear Mr. Marshall: At your request I visited your property On Hazel Point in Jefferson County and reviewed published geologic information regarding this area. The reason for my investigation has been to evaluate the site for existing geologic hazards and to provide recommendations that will minimize geologic risks to your proposed construction and minimize the adverse impacts of your construction on nearby ecologically sensitive areas. In addition I am specifically addressing the questions raised in the September 29, 2008 letter from Michelle Farfan of the Jefferson County Department of Community Development. This report is based on visual reconnaissance of the site and review of published maps, Iidar imagery, and geologic information. The conclusions and recommendations in this report are my professional judgments based on this information. I conducted no subsurface exploration for this report. The conclusions and recommendations in this report are based on a geologic assessment of the site and are not engineering recommendations, You should be aware that marine shorelines and the adjacent upland areas are inherently dynamic, Shoreline erosion and landsliding are common, natural, and essential processes for proper function of a shoreline ecosystem. The recommendations in this report are intended to minimize the geologic risk associated with development on this property but that risk cannot be eliminated. As the property owner it will be your responsibility to review and understand the recommendations in this report, to insure that they are implemented, and to monitor conditions on your property during and after development. Based on my investigation I believe that, contingent on the recommendations herein, there is a stable and appropriate building site on this lot for a single-family residence and detached garage supported by a conventional foundation. I have reviewed the location of the proposed septic drainfield and believe that a drainfield at that location poses no significant threat to site stability. Site Description The subject parcel (Parcel Number 601354006) is located on the east side of the Toandos Peninsula in the vicinity of Hazel Point (Figure 1). The property is marine waterfront with approximately 200 feet of frontage on Hood Canal. It is approximately 850 feet deep with an easterly aspect, a total vertical relief of approximately 225 feet, and a steep beach bluff estimated to be 50 feet in height. A gravel County road crosses the property in a generally north-south direction midway through the property. Geologic ri,',:" .'; i i'J !~q ~ 1" (R:)n;, \ tf. LJlt ~ If' I ; A tb 'f I ., Q II '*"'7 t .. '" "t LUtr - 1 - ~ti.' I.u, " ,vYi rl~:;, iOYf",/t>, , , mapping 1 of the site shows that most of the property is underlain by pre-Vashon stratified sediments with a cap of Vashon till at the western end of the property. I have made several visits to the site including visits on 12/2/06, 01/14/07, and 3130107, The attached site map shows the relevant features I observed during my visits. The western portion of the property consists of a rolling area forested with second growth conifers. The middle of the property is also wooded but becomes much steeper. The county road diagonals down through this steeper portion of the property, Below the County road the property levels out into a rolling, east-sloping bench. There is evidence of minor past clearing and grading in this area. The bench is truncated at its easterly edge by a steep bluff 50 to 75 feet in height (estimated) with an average inclination of 1.5:1 (H:V) (estimated). Buildina Area The area you have indicated as your preferred building area is located between the County Road and the top of the beach bluff (Figure 2). There is a subdued east-west trending ridgeline in this area located generally on the southern portion of your property. There are gentle swales that parallel this ridgeline located both to the north and south. There is evidence for landslide movement at the eastern end of each of these swales. You reported that there was landslide movement in the north swale in 2005 that resulted in a debris avalanche over the bluff and onto the beach. Most of this landslide movement was on the adjoining parcel to the north, but a portion of the failure extended onto the north edge of your parcel. A lobe of landslide debris remains on the beach face below this failure. Examination of the ground above the bluff in this swale shows clear evidence of active ground movement extending approximately 100 feet from the top of bluff on the adjoining property. This evidence includes open soils cracks, leaning trees, and racking of the open shelter located on this adjacent property. During my visits in winter and spring there was seepage at the ground surface just above the top of bluff in this area, Local topographic breaks and bends in the trunks of some large conifers suggest that there may have been older landslide movement as much as 300 feet back from the top of the bluff along the axis of this northern swale on the adjoining lot. Examination of the bluff face below this area of failure revels very dense laminated sediments ranging from gravelly to silty sand. Most of the bluff height appears to be in- place, undisturbed sediment. Landslide movement appears to be confined to a relatively thin layer of soil. It appears that sliding is localized to this surficial layer because of a shallow perched groundwater table developed on top of a fine-grained lens in the glacial stratified sediments. Recent landsliding is also evident in at the eastern end of the southern swale near your southern lot line. Sliding here is less extensive than that seen to the north. A small area of surface seepage on your south property line shows possible evidence of past movement. Further south another small slide in January of this year took out a short section the old roadway that diagonals down the beach bluff in this area. In contrast to the instability observed to the north and south, the subdued ridgeline on the southern portion of your property (the proposed building site) shows no evidence of instability. A near-vertical beach bluff exposure at the east end of this ridge shows -2- .' ":"': ,1 '{,d?n ,,,,"'i~MJ dense, stratified gravelly sand. The steepness of this exposure clearly indicates the competence of this deposit. There is no evidence of seepage emerging from the bluff face or from the ground surface above the bluff in this area. A short steep slope separates the ridgeline from the swale to the north. This steep slope is in part the cut slope for the old road that runs along the base of this slope. This steep slope may also be a lateral scarp bounding the old landslide area to the north. Orainfield Area The area currently proposed for a septic drainfield is located near the western end of your parcel. This is an area of rolling topography above the steeper shoreline slope. Soils in this area, as exposed in test pits excavated for drainfield design, are dense silty sand. There is no indication of instability in the vicinity of the proposed drainfield. Recommendations For the purpose of the following recommendations, steep slopes are considered slopes in excess of 40% (2.5 H : 1 V), The following recommendations are intended for site planning purposes. I recommend you have a geological review of your building design to verify consistency with these recommendations. Buildina Area - Buildings should be set back a minimum of 140 feet from the top of the bluff on the north side of the ridgeline, and 130 from the top of the bluff along the south property line (Figure 2). Building setbacks should be measured to the outside edge footing. Roof overhangs may extend up to a maximum of 3 feet past the footing on the east side of the proposed structure. An unroofed, permeable deck supported on piers or pier blocks may extend up to 20 feet past the foundation on the east side of the proposed structure, - A minimum of 10 feet building setback is recommended from the steep slope on the north side of the central ridgeline. Roof overhangs may extend up to a maximum of 3 feet past the footing on the north side of the proposed structure. - No fill should be placed in the building setback areas. - The building. setback areas may be landscaped and trees may be removed from this building setback area, but no trees (except hazard trees) should be removed from the steep slopes on site, or from within 25 feet of the top of the beach bluff. Any trees on site may be limbed in accordance with good arboreal practice that maintains the integrity of the tree. Jefferson County may have specific regulations regarding tree removal and you should consult with County staff before cutting any trees. - Roof and footing drains should be collected and routed to the beach via a 4" minimum butt-welded HOPE tightline. You should contact Jefferson County to determine if there are specific engineering or design elements involved with dealing with runoff from the proposed building. ~ I~\a 'IR;R \V lRO - 3 - JUt 2 42f1U7 ,'tf#Hiu" j~'(8UV(II'11 ecosystems. By following the recommendations presented in this report I believe it is possible to safely construct a residence on your property while allowing these natural processes to continue. This report is intended primarily for site planning purposes. I recommend you have a geological review of your proposed site development plan to verify consistency with these recommendations. You should provide a copy of this report to your building designer and contractor. If you have any questions regarding the observations or recommendations in this report please feel free to contact me for clarification, Sincerely, ~~1 John Bethel, LG John Bethel \ ; ",' ''', ./ ,1." ! .., ;rl :\~ 1111 0'\ . " :~~ ~.:L ~J :........;,. l -5- L:'~::~~;I\ .t;:li., 'I' .... ' Figure 1. 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