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Home My WebLink About821333016 Geotech AssessmentE3RA December 12, 2006 T06415 Dick and Louise Cole 3466 Teal Lake Road Port Ludlow, Washington 98365 (360) 437-0699 _ a ~ ~•a ~n ~ ~, ~~' U' :~ ~ ~ao~ €;, ~7EV:LOPMENT J PO Box 44890 Tacoma WA 98444 253-537-9400 253-537-9401 fax Subject: Geological Assessment-Landslide Hazard Geotechnical Letter Parcel # 821333016 3466 Teal Lake Road Port Ludlow, Washington Dear Dick and Louise: E3RA is pleased to submit this Geological Assessment-Landslide Hazard Geotechnical Letter for the above-referenced project site. The purpose of our assessment is to address Title 18 of Jefferson County's Critical lreas Qrdinances as they relate to Landslide hazards on the site. Plans call for the construction of an 18 by 14 or 16 foot addition on the north side of your existing modular home. Our scope of work is limited to surface observations, geotechnical research, and letter preparation. This report has been prepared for the exclusive use of Dick and Louise Cole and their consultants, for specific application to this project, in accordance with generally accepted geotechnical practice. PROJECT DESCRIPTION The project site is a roughly square parcel that measures 140 to 145 feet on a side, although acrescent- shaped piece has been removed from the northwest corner, to accommodate the right-of--way of an adjacent cul-de-sac. An existing modular home with a carport and detached storage/shop building occupy the northwest corner of the site. The site is bordered by open land and a cul-de-sac on the north and open land on the west. Residential parcels lie nearby to the east and south. Plans call for building an 18 by 14 or 16 foot addition on the uphill, relatively level, north side of the modular home_ A moderately steep fill embankment is located 6 to 15 feet south of the modular home. SITE CONDITIONS During our reconnaissance of the site and nearby areas on November 27, 2006, we observed that site generally slopes down to the south at grades that average about 20 percent, except where modified by prior grading. The prior grading consists of cutting and filling in the vicinity of the modular home, in order to create a level building area. Generally, fills there are on the south side of the modular home, 6 to 15 feet or more from the south perimeter of the structure; vary from a few feet to about 10 feet in height, and grade at 2H:1V or less (50 percent or less). Cuts are generally on the north side of the modular home .. 3 ~?~R - 9 2J07 ~ i.~.~'' 3 December 12, 2006 ~ ERA, Inc. T06415! Cole Geologic Assessment ~ , ; .,..."7T where the new construction is to occur, and appear to consist of site leveling with no significant resultant cut slopes. Other grading consists of a small cut near the southwest corner of the site, where an access road from Teal Lake Road traverses the south boundary area. This cut is about 10 feet or less in height and grades down to the access road at about 2H:1 V. Slopes nearby offsite are similar or gentler than those onsite. Mapping by Jefferson County indicates that the north four fifths of the site, which includes the area to be developed, presents a slight landslide hazard. The southernmost fringe of the site, where. no development will. occur, presents a moderate landslide hazard. The Geologic Map of Eastern Jefferson County identifies onsite soils as Vashon Ice-Contact Stratified Drift. The Natural Resources Conservation Service maps the site as Carlsborg Gravelly, Loamy Sand, CaD, formed in glacial outwash terrace deposits on 15 to 30 percent slopes. Erosion and Seismic Hazards Maps do not depict the project site and nearby adjacent areas as erosion or seismic hazards. No indications of flowing water or surface hydrology were observed onsite. No seeps, springs, or other surface expressions of groundwater flow, were observed onsite or immediately offsite. Springs or seeps were not observed within 1,000 feet of the site, but could exist without our notice. We would expect that seeps and springs within 1,000 feet of the site would appear on slopes where relatively impermeable, silty lacustrine deposits underlie sandy, gravelly glacial outwash soils similar to those onsite. We did not observe Landslide Hazard Indicators, such as hummocky terrain, scarps, slump blocks, etc., on the site or nearby the site. Part of the site is mapped by the Coastal Zone Atlas and is classified I, Intermediate. The same reference identifies slopes offsite to the south, and closer to State Route 104, as U, Unstable. CONCLUSIONS AND RECOD7~~NDATIONS Slopes on the site are gentle to moderate and, based on our observations and research, stable. Because the planned new addition will be built on level ground and because building loads for residential structures such as the planned addition are relatively light, the proposed new addition should have no significant impact upon site geology and there should be no appreciable increase in the potential, slight, landslide hazard that the area to be developed presents. Although the site is not considered an erosion hazard, we recommend that basic erosion control system be implemented. This system should collect and filter all surface run off through either silt fencing or a series of properly placed and secured straw bales. We anticipate a system of berms and drainage ditches around construction areas will provide an adequate collection system. If silt fencing is selected as a filter, this fencing fabric should meet the requirements of WSDOT Standard Specification 9-33:2 Table 3. In addition, silt fencing should embed a minimum of 6 inches below existing grade. If straw baling is used as a filter, bales should be secured to the ground so that they will not shift under the weight of retained December 12, 2006 E3RA, Inc. T06415/ Cole Geologic Assessment water. Regardless of the silt filter selected, an erosion control system requires occasional observation and maintenance. Specifically, holes in the filter and areas where the filter has shifted above ground surface should be replaced or repaired as soon as they are identified. We also recommend that roof runoff from the new addition not be allowed to flow freely over the fill slope on the south side of the modular home. Collected roof runoff should be directed to the west, away from the fill slope, or tight lined to the south beyond the fill slope. The collected roof runoff should then can dissipate on splash blocks or infiltrated into site soils in accordance with Jefferson County criteria. CLOSURE The conclusions and recommendations presented in this report are based, in part, on our interpretations and assumptions regarding subsurface conditions; therefore, if variations in the site conditions are observed at a later time, we may need to modify this report to reflect those changes. We appreciate the opportunity to be of service on this project. If you have any questions regarding this report or any aspects of the project, please feel free to contact our office. Respectfully submitted, E3RA, Inc. t', 4 f- L{' ~ ~ ~~~ ~' --.,, ,r_ _,.__....~ Y 7 t. `~ MAR - ~ 2007 j _..`1 u . ~r ~.__ J~`~E;.~~,~~NT Fred Rennebaum,L.E.G. Senior Geologist P.O Box 44890 Tacoma, Washington 98444 (253) 537-9400