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HomeMy WebLinkAbout950101209 Geotech Assessment13LP~-oo~Z8 ~.,`~ t~ t_~ o(r,- 00117 ~`raic SHORELINE, BLi1~F 8r C(.n('E SYECtAL1STS `` y~ ~, {,,.~~ ~~(~, GEOLOGICAL SITE GVALUA'T1ON5 IAEVSGIENI.EJ 4V NSU~TING GL~NSED PROF 5 ONA t_ GEU1.OGt5T, WASHCpTON & AI.ASIU' January 31, 2006 Ms. Zoe Lamp Department of Community Development Jefferson County 621 Sheridan St. Port Townsend, WA 98368 Re: Revised Setback Provision Geological Site Evaluation2 Report Mr. Chris &erg Tax Parcel # 950-101-209 Dear Ms. Lamp: T ~' `.~"~ ~~ ~~~~ ~ ~~ This letter will serve as a follow-up of my voicemail to you, last Thursday, January 26, 200b. At the request ofMr. Chris Berg and his builder (Lumbermen's Homes), I am willing to reduce the recommended minimum setback distance for his proposed structure from the top of the slope from 30 feet to 28 feet. My understanding is that this reduced setback distance will only be required in one area of the property where a corner of the structure encroaches on the setback buffer. All other provisions and recommendations set forth in my Geological Site Evaluation Repc-rt (dated February 10, 2005} remain in effect and should be observed. i I will follow up this communication to you with a hard copy bearing my license stamp as soon as I return to my office, the end of February. Thank you for accommodating my request to implement this change. Please don't heskt~te to contact me in the event you have any questions or require additional information ../''~~-..~ Sincerely, Vhite, Engineering Geologist Principal Consulta>irt P.O. Pox 3398, ~.verdal.e, i1/asF>inytan 98383 (364) 830-0718 R ~~ 1 ~ ~ ~~~ p .. ~. t._. $,... GEOLQGICAL SITE EVALIFATIUN Prepared fon Mr. Chris Berg . Prepared hy: Craig K. White, Ina Geoscieaces Consulting P.(:~. Bax 3398 Silverdale, V4'ashingtoa 98383 February Ia, 2aa5 Table of Contents Property Location and Description Topography and Drainage Vegetation Geology and Ground Materials Soil Characteristics Slope Stability Seismic Hazards Gonclnsions and Recommendations Summary and Limitations Appendices: Location Map Slope Stability Potential Seismic Hazards Uetait: Area of Study 1 2 3 4 5 b '7 10 Appendix I Appendix 2 Appendix 3 Appendix 4 ~~ (,/t~y~ ~ \\ ~ Q n/+ SHORELINE. BLUFF 8c SLOPE SPECIALISTS ~,..s) ~~~ i ~I~G, ~1 1(,~, GEOLOGICAL SITE CVALUAT10N5 GEOSCIENCES CONSULTfNG uCe sEDPROF' SONALGEOLOCaIST.WAE5HNGTON&ALasFSA February 10, 2005 GEOLOGICAL SITE EVALUATION Tax Prc. # 950-101-209 Lot 13 & 14, Block 12 Trails End Subdivision, 2`~ Ed. Section 16, T27N, R1E W.M. Jefferson County, Washington Property Location and Description: The subject property is located off Dickey St., in the community of South Paint, near Port Ludlow, Washington, where it occupies a portion of the east-facing hillside slopes overlooking the Hood Canal. The property is bounded on the north, south and east by other private lands, and an the west by Dickey St. Property size is approximately 18,300 s.f. or about 0.42 acres. A Location Map, Slope Stability Map, Seismic Hazards Map and Detail Plat showing drainage and slope features within the Area of Study are included with this report as Appendixes 1, 2, 3 and 4, respectively. Access to the subject property is directly from Dickey St., 0.1 miles northwest of its intersection with South Point Rd. No structures are presently located an the subject P.O. Psox 3398, ~11.verd~.e. \VasF>~ta, 98383 (360) 830-0"JI8 Geological Site Evaluation Mr. Chris Berg February 10, 2005 property with the- exception of a small wooden deck and storage shed. A drainfield was installed by the previous owner who had a mobile home on the property, the building site for which was buttressed by a rock rf~taining wall. At the time of this site visit; a proposed budding site for a new structure, approximately where the mobile home was located, was identified with surveyors flagging. Topography and Drainage: Drainage on the subject properly is generally to the east toward the Hood Canal, with a minor component of drainage to the south. A shallow drainage swale extends across the property north and east of the proposed building site from where it extends onto adjacent lots to the south and east. (see Appendix 4, Detail: Area of Study}. Slope angles were rryeasured at several locations on the subject property using a Brunton hand clinometer and laser-sighting device. In the western portion of the property immediately below the proposed building site, measured slope angles ranged from IO -15 degrees to ss much as 20 degrees. In the eastern wind central parts of the property, measured angles were considerably bwer, ranging from 5 - 7 degrees in the area of tl~e drainfield. Overall height of the upper slope face, below the proposed binding site, ranges 2 Geological Site Evaluation Mr. Chris Berg Febniary 10, 2005 from 4 to 6 feet to as much as 25 feet at its highest point. This site evaluation was conducted in mid-winter during periods of light to moderate rainfall, however no springs or seeps were identified along any of the slopes and standing water was generally absent on the property. Evidence of minor erosion was present in some areas along the shallow swale, immediately north of the proposed building site, indicating that significant runoff may occur here during periods of heavy winter runfall and underscoring the need for good water management in the development of the PmP~y Vegetation: Lazge-growth, natural vegetation on the subject property consists of conifers (Douglas fir and Western red cedar} as well as broadleaf trees (most notably, Red alder). Lower-growth plants include Himalayan blackberry, along with a variety of ornamental plants and fruit trees, as well as native grasses ate weeds in cleared areas of the property. Most natural vegetation an the s~ibject property has been removed, probably by the previous owners or subdivision developer. Weeds and grasses have replaced the natural Geological Site Evaluation Mr. Chris Berg February I4, 2045 vegetation, along with isolated thickets of Himalayan blackberry that extend down from the proposed building site to the base of the slope. Near the southeast corner of the Property, a large stand of well established, second-growth conifers and hardwoods is present along with species of native lower-growth plants that generally reflect the presence of moderately well-drained soils. Geology and Grvnnd Materials: A review of existing geologic maps of the area indicates the subsoil ground materials present at the subject property are glacial and interglacial deposits ofQuaternary age, most of which are less than 20,000 years old. Reconnaissance mapping shaves the principal geologic unit at the subject property is Esperance Sand, a sequence of fluvial sands and interbedded gravels with occasional lenses of clay, and a basal "blue clay" (sometimes mapped with the underlying K:itsap formation). The unconsolidated nature of the Esperance sands makes them highly vulnerable to surface erosion where exposed slang slopes and steep coastal bluffs in the Puget Sound region. Esperauce sands commonly achieve thicknesses of up to 30 feet and the unit may serve as a prirnaty aquifer where it occurs in the subsurface. Examination of limited exposures on and in the vicinity of the subject property 4 Geological Site Evaluati~ Mr. Chris Berg February I4, 2445 generally supports the reconnaissance mapping, Several shallow excavations near the roadway an the property reveals the presence of tan, poorly-consolidated, silty to sandy soils, probably sourced by the underlying Esperance Sand. The same ground materials are also exposed in several areas along the steep bhiffface west of Dickey St., indicating the distn~ution of this unit is fairly widespread throughout the area and. that that it probably occurs in the subsurface at the proposed budding site. Soil Characteristics: A review of soil types in the vicinity of the subject properly indicates the predominant soils are Kitsap silt loam, 0 to 15 percent slopes. This nearly-level to rolling.soil occurs on terraces and benches where, in mast places natural slopes range from 4 to 9 percent. Inchided with this soil in mapping are small, areas of Kitsap silt loam 15 to 30 perce~, K:itsap gravelly loam d to 3Q percent slopes, and small areas of Alderwoad, Sinclair, Cassalary, Everett, Haypus and vadianola soils. This soil is only moderately well drained and permeabilifiy is very slow. Roots penetrate to a depth of 30 to 40 inches. Runoff is slow to medium and the hazard of water erosion is slight to moderate. A perched water table may occur at a depth of 1 '/2 to 3 feet. Principal soil use is far the production of trees, recreation areas, rural home sites and wildlife habitat. Geological Site Evaluation Mr. Chris Berg February !0, 20©~ SIape Stability: On Washington State Department of Ecology Coastal Zone Management maps (3efferson County volume), the hillside slopes in the vicinity of the subject property have been classified as Untable (U) due to the presence of steep slopes and poorly-indurated ground materials. Examination of the east-facing slope at the subject property reveals several areas, irnn~diately below Dickey St., where minor erosion from water runoff along the roadway into the shallow drainage swale has occurred. No evidence of any significant erosion was identified along the slope below the proposed building site and water from the upland portion of the property appears to be effectively distributed across the land surface where rt is absorbed by natural percolation into the sandy soils. Some ofthis runoffmay also be collected from curtain drains in the area. of the proposed building site where water appears to be shamed through tightlines to the base of the slope. Of some concern is the integrity of the rock wall against which backfiIl has been emplaced to ~Ip create a level btiildmg site. Although no indications of instability in the wall were identified during our site visit, it is our opinion that the wall should be inspected by a civil or stY-uctural engineer to ensure it was properly constructed. 6 Geological Site Evaluation Mr. Chris Berg February 14, 2045 Seismic Hazards: The Puget Lowland is classified as a Seismic Zone III by the Uniform Building Cade (tTBC}. Crustal earthquakes, which occur along surface faults, have been documented in several areas of the Puget Sound region (see Appendix 3: Seismic Hazards Map). However, the largest earthquakes in this region have been subcrustal events, ranging in depth from 30 to 60 miles ar more. These deeper-focus earthquakes are not related to surface faults but aze usually caused by movements along plate boundaries or within deeper sub-crustal slabs. Although the mapping of fauns and study of earthquakes in the Puget Sound region is a continuing effort, the potential effects of a large seismic event on hillsides and slopes in this area are still not completely understood. Conclusions and Recommendations: The observations and interpretations outlined in this report support the classification ofthe hillaside slopes in the vicinity of the subject property as "Unstable" and, although recent activity along slopes at the subject property appears to be limited to minor surfi{ce erosion, we cannot guarantce that a larger movement will not occur at some time in the fimme. However, with an adequate setback distance from the top of the slope for proposed structures, control of sua-face water runoff and implementation of the slope 7 Geological Site Evaluation Mr. Chris Berg February 10, 2005 stabilization measures outlined below, we see no reason why the development of this property, as proposed, should not occur. Specific recomn~ndations aze as follows: l . It is recommended that a minimum setback distance of 30 feet from the top of the slope (upper edge of the rock wall) be observed for the bearing walls of the primary residence structure. Porches or decks, which are not an integral part of the main structure may be placed as close as 15 feet from the tap of the sigpe. Within this 15-foot setback distance, we also recommend that a natural vegetation buffer be created within which the growth ofdeep-rooting plants should be encouraged. . 2. An effective water-collection system (gutters and downspouts) should~be installed on all structures with water ti8htlined unto the natural drainage Swale near the base of the slope. Buried portions of all new trghtiines should utr'lize 4 ar 6-inch, srnoothwall, sewerldrain pipe, tightly cemented at the joints. Efforts should also be made to determine the nature of the drain system alreaaiy in place in the area of the proposed building site (the wrinkle-pipe that extends across the face of the rock wall) to preserve its integrity during construction. 3. Ci~adirrg and site preparation should be conducted in a manner that will direct Geological Site Evaluation Mr. Chris Berg February 10, 2005 water from uphill sources and impervious adjacent areas to flaw away from the proposed stnuture, preferably in the direction of the natural drainage Swale below the building site. Consideration should also be given to installing a foundation drain on the uphdl (west) side of the structure to intercept surface and near-surface runoff water from the higher- elevation areas on the properly that, otherwise, might accumulate against the foundation. The outlet ends of the foundation drains maybe connected to the tightline system. 4. Efforts should be made to promote and maintain a healthy growth of deep- rooting vegetation, wherever possible, in the vicinity afthe slopes on the property to aid in retaining surface soils and reduce the effects of erosion. The accumulation of construction materials, stumps, branches, grass cuttings or other yard debris on ail slopes should be discouraged, as these materials may inhibit the growth of healthy vegetation. 5. A earefiil examination of the mck retaining wail below the proposed building site should be conducted by a qualified, civil or structural engineer to ensure the iirtegrity ofthis structure. Niaintenaace and repairs of the rock wall should be performed, as necessary to protect the structures above the wall. 6. Frequent inspec.,tions of the hillside slope should be made, especially after periods of heavy rainfall, to determine the location of any new springs and identify areas of Geological Site Evaluation Mr. Chris Berg February 10, 2005 excessive water runoffor erosion. Recording the appearance ofpotentially-unstable azeas in photographs, taken each year, and comparing them with current conditions may assist in this evaluation. - Summary and Limitations: Although the development of properties along hillside slopes in the Puget Sound region is common, it should be acknowledged by property owners that these areas maybe inherently unstable and involve higher risks than other areas due to the steepness of slope faces anti the unconsolidated nature of the ground materials, Frequent and severe winter storms commonly produce high vohunes of water runoff that may cause extensive slu~ace erosion, saturate ground materials and destabilize slopes, resulting in gmund movements that often occur without warning. Given these conditions, it should be expected that erosion and periodic minor sloughing of ground materials on the subject property may be a continuing problem. V~Jhile the potential for larger-scale movemens, such as may be precipitated by a seismic event, are not well understand in this area, the hazards that may be posed by such an occurrence should also not be ignored. This report has been prepared for the exclusive use of Mr. Chris Berg for specific application to the referenced site, and the co~h~sions and recommendations presented, herein, should be applied in their entirety. Within the limits of scope and budget, this study was corxiucted in accordance with generally-accepted practices employed at the time this 14 Geotogica} Site Evacuation Mr. Chris Berg February 1 Q 2005 work was done. No ©ther warranty of conditions is expressed or implied. you should also. be awaze that these conclusions and recommendations are based on a general knowledge of this area and the interpretations of surface and subsurface conditions as they are believed to exist. These conditions may, in fact, be differezrt than interpreted and events may inevitably occur that were not predicted. Clients should also be aware that there is a practical limit to the usefulness of this report and recommendations without a site re-evaluation and crrttcal review. Although this time limit is somewhat arbitrary, it is suggested that 2 years be considered a reasonable limit for the usefulness of this report. ~# wa, ~`^~r7 G` ,? 4„ 5=~ ~ ~ ~/ ~ S~~ ~.. e, Engineering Geologist dry I0, 2005 it APPENDICES Appendix 1: Location Map Appendix Z: Slope Stability Appendix 3: Potential Seismic Hazards Appendiz 4: Detail: Area of Study z ~`` ' ,~'-~ ~ .t ~ ~ ~t ~ V ~ •. 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