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HomeMy WebLinkAbout950100324 Geotech Assessment- ~~ ~ ~ SHORELINE. BLUFF & SLOPE SPECIALISTS Craig ~. ~ hate, ~~Ci • GEOLOGICAL SffE EVALUATIONS GEOTECHNICAL STUDIES, REPORTS & ASSESSMENTS GEOSClENCES CONSULTING LICENSED PROFESSIONAL GEOLOGIST, WASHINGTON 8c ALASKA February 19, 2004 GEOLOGICAL SITE EVALUATION Lots 25 & 26, Trails End Subdivision) NW'/4, Section 16, T27N, R1E W.M. Jefferson County, Washington Property Location and Description: J ' ~ LJ ~ it l'I ~~~ ~ ~ 1EFFERSON COUNTY ENVIRGN~A~NTAL HEr~TH The subject property is located at 1441 Thorndyke Rd., near the community of South Point in Jefferson County, Washington, where it occupies portions of a south- plunging ridge above the east-facing coastal bluffs overlooking the Hood Canal. The property is bounded on the north, south and east by other private lands and on the west by Thorndyke Rd. Property size is approximately 14,400 s.f. or about 1/3 acre. One structure, a sma11, wood-frame cabin is located on the property along the top of the bluff• near the south property line. A Location Map, Slope Stability Map, Seismic Hazards Map and Detail Plat showing drainage and slope features within the Area of Study are also included with this report as Appendices # 1, 2, 3 and 4, respectively. Table of Contents Property Location and Description 1 Topography and Drainage 2 Vegeta#ion 3 Geology and Ground Materials 4 Slope Stability 5 Potential Seismic Hazards 6 Conclusions and Recommendations 7 Summary and Limitations 10 appendixes: Location Map Appendix 1 Slope Stability Appendix 2 Potential 5eisnuc Hazards Appendix 3 Detail: Area of Study Appendix 4 Geological Site Evaluation Mr. Jack Anderson February 19, 2004 Access to the subject property is directly offThorndyke Rd. approximately 1.4 miles south of its intersection with South Point Rd. Access to the biuffface was achieved on foot from the top of the ridge as well as along a narrow pathway at the base of the bluff that leads west from South Point Rd. Topography ansi IDraiaage: Drainage on the upland portion of the subject property is generally to the west and southwest, away from the top of the bluff toward the Thorndyke Rd. with a minor component of drainage toward the adjacent property to the south (see Appendix 4). On the bluff face, drainage is to the east toward the base of the slope and the Hood Canal. Slope angles were measured at several locations on the subject property using a Brunton hand clinometer and laser-sighting device. Slope angles along the bluff face range from 40 to 45 degrees along the north end of the property to as much as 48 degrees along the steeper part of the slope near the south property line. Measured angles behind the top of the bluff varied from less than 3 degrees to as much as 5 degrees. Overall relief on the bluffface is approximately 120 to 130 feet to the top of a narrow bench that occurs along the lower slope face. An additional 20-30 foot drop occurs from the top of the bench to South Point Rd. at the base of the slope. Geological Site Evaluation Mr. lack Anderson February 19, 2004 This site evaluation was conducted in mid-winter, during a period of moderate rainfall, however no visible springs or seeps were identified along the upper bluff face at the property. Evidence of pronounced surface erosion and sloughing of ground materials was identified. m several places along the bhp suggesting that water runofi'onto the bluff face from upland areas is quite heavy during the wetter months of the year and underscoring the need for good water management. The presence of an old cistern situated on the narrow bench near the base of the slope suggests that water which may percolate through the sandy ground materials in.the upper bluffdaylights at some point near the bench where clays or impermeable layers probably occur. Vegetation: Large-growth vegetation in the vicinity of the subject property consists of conifers (Douglas fir, Grand fir, Western hemlock and 'Western red cedar) as well as several varieties of broadleaf trees (Red Alder, Pacific madrone, willow and Bigleaf maple}. Lower-growth plants include Swordfern, Vine maple, Black huckleberry, Salal, Himalayan blackberry and Scotsbroom along with assorted weeds and grasses in cleared areas of the property. Vegetation on the upland portion ofthe property appears to have been cleared Geological Site Evaluation Mr. Jack Anderson February 19, 2004 some years ago. In this areas, vegetation consists mostly of grasses and weeds with one or two large, well-established trees that generally reflect the presence ofwell-drained soils. Vegetation along the bluff face ranges from dense to patchy; a condition which is likely the result of heavy erosion and periodic, localized slides that may occur during the months of heavy rainfall and remove sails and vegetation on the face of the bluff, Geology and Ground Iaiaterials: A review of existing geologic maps of the area indicates that the geologic units present at the subject property are glacial and interglacial deposits of Quaternary age, most of which are less than 20,000 years old. Reconnaissance mapping shows the upland portion of the study area is capped by Everett, gravelly sandy loam 0 to 15 percent slopes (EvC). This nearly level to rolling soil is on glacial outwash terraces where slopes usually range from 4 to 10 percent. The soil is somewhat excessively drained. Permeability is rapid and roots may penetrate to depths of 60 inches or more. Runoff is slow and the hazard ofwater erosion is slight to moderate. The principal soil along the face of the bluff within the study area is Cassolary sandy loam, 34 to ~0 percent slopes (CfE}. This steep soil is on canyon slopes and ocean bluffs. The dark gray surface layer is absent in many places and the silty clay loam layer in Geological Site Evaluation Mr. Jack Anderson February 19, 2004 the underlying material is nearer the surface on the upper part of slopes than it is on the lower part. Runoffis rapid in this unit and the hazard of water erosion is severe. Examination of the ground materials in the vicinity of the subject property generally supports the above geologic interpretation. In the upland portion of the adjacent lot to the north, fine- to medium grained, sandy Loam was identified in three soil log test pits excavated to a depth of roughly 74 inches. Along the face of the bluff near the southern property boundary, the ground materials are exposed in several small, patchy outcrops and consist ofpoorly-indurated, sandy loam with minor pebbles. Slope Stabilaty: On maps showing Geologically-Critical Areas, prepared by the Jefferson County Department of Community Development, the coastal bluffs at the subject property have been identified as having a "Nigh Erosion Hazard "due to the presence of steep slopes and poorly-consolidated soils. On Washington State Department of Ecology Coastal Zone Ivlanagement maps, the bluff face imYnediately east of the subject property has been classified as "£Instable ("U"}, while the upland portion of the property is identified as "Stable" ("S'). Geological Site Evaluation Mr. Jack.4nderson February 19, 200 Direct examination of the bluff east of the subject property, shows several areas where ground movements have occurred, some probably within fairly recent years. Several small, circ-shaped areas along the bluff face, immediately south of the subject property, were probably the result of localized slides. 'This conclusion is supported by the presence of stands of "same-age" alders that re-sprouted in these areas after the slide events occurred. In other areas along the bluff face, a number of tilted trees indicate the slow, downhill "creep" of surface soils maybe occurring, which may ultimately culminate in slides. Along the base of the bluff, slide debris consisting of trees, stumps and mounds of soil is still visble from earlier ground movements that occurred along the bluff face . Based on historical information and our. general knowledge of this area, these slide incidents probably occurred during periods of heavy winter rainfall when water runoff is greatest and the ground materials are most saturated. Potential Seismic Hazards: The Puget Lowland is classified as a Seismic Zone III by the Uniform Building Code {UBC). Crustal earthquakes, which occur along surface faults, have been documented in several areas of the Puget Sound region (see Appendix 3: Seismic Hazards Geological Site Evaluation tLir. Jack ~inderson February i9, 2004 Map}. However, the largest earthquakes in this region have been subcrustal events, ranging in depth from 30 to 60 miles or more. These deeper-focus earthquakes are not related to surface faults but are usually caused by movements along plate boundaries or within deeper sub-crustal slabs. Although the mapping of faults 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 classifications of the coastal bluffs adjacerrt to the subject property as "Unstable "with "High Erosion Hazard ". However, it is also our opinion that,-with adequate setback from the top of the bluff for structures, good water management, re-vegetation in the area of the upper bluff edge, and implementation of the slope stability measures outlined, below, there is no reason that the development of this property should not occur. Specific recommendations are as follows: 1. It is recommended that a minimum setback distance of 3S feet from the top of the bluff be established for all strictures. Additionally, we recommend that a 1 S-foot-wide 7 Geological Site Evaluation Mr..lack Anderson February 19, 2009 natural vegetation buffer be established along the upper bluff edge within which the re- growth ofnative (lower-growth} plants shoukt be encauraged. Large trees along the bluff face that may be unstable or impede a "limited view corridor" may be cut, as necessary, however the stumps and root systems of such trees should not be disturbed as they will resist erosion for a period of time. Removal of trees should also be performed in a manner that minimi~.es disturbance to the lower-growth vegetation. 2. Septic tanks should be placed as far back from the top of the bluff as possible and drainfields that may be constructed on the property should be at least SO feet from the top of the bluff to prevent daylighting of effluent along the bluff face and saturation of the soils. 3. Grading and preparation of a proposed homesite should be conducted in a manner that will allow runoffwater from the site to drain to the west, toward natural drainage depressions along the shoulder of Thorndyke Rd., rather than east, toward the bluff face. Our observations suggest that such re-grading can be achieved with a minimum. of ground disturbance and earth moving activities. 4. An effective water collection system (gutters and downspouts) should be installed on any new structures. Collected water should be tightlined toward Thorndyke Rd., away, from the bluff face. Buried portions of tightlines should utilize 4- or 6-inch, Geological Sife Evaluation Mr. Jack Anderson February 19, 2004 smoothwall, sewer/drain pipe, tightly cemented at the joints. 5. In no case should water from any source be allowed to discharge onto the bluff face, and areas where excessive water flow occurs should be protected by catchment boxes or curtain drains that collect and shunt the water away from these areas. 6. Efforts should be made to promote and maintain a healthy growth of deep- rooting vegetation on all slopes, whenever possible, to aid in retaining surface soils and to reduce the effects of erosion. Shrubs and plants used on slopes should be deep-rooting varieties and should also ~ selected to minimise the need for heavy watering. The planting of lawn grasses should be limited to relatively level ground. A list of recommended, deep-rooting plants is ava~7able, on request, or you may wish to contact a local landscape architect or Certified Washington State Nurseryman. 7. The accumulation of construction materials, stumps, felled trees, branches, cuttings or other yard debris on the bluff should also be discouraged as these materials may inhibit the growth of healthy vegetation. Walking or climbing on any portions of the blur which may loosen or disturb ground materials, should also be avoided in favor of established pathways. 8. Periodic inspections of the bluff face should be conducted, especially following Geological Site Evaluation Mr. Jack A"nderson February 19, 20D4 periods of heavy rainfall, to determine the location of any new springs, seeps or areas of excessive erosion, and to identify any unstable conditions. Recording the appearance of these areas 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 coastal bluffs in the Puget Sound region is common, it should be acknowledged by property owners that these areas may be inherently unstable and involve higher risks than other areas due to the steepness of slope faces and the unconsolidated nature of the ground materials. Frequent and severe winter storms commonly produce high volumes of water runoffthat may cause extensive surface erosion, saturate ground materials and destabilize slopes, resulting in ground movements that often occur without warning. Given these conditions, it should be expected that erosion and periodic, minor sloughing of ground materials on or adjacent to the subject property may be a continuing problem. While the potential for larger-scale movements, such as may be precipitated by a seismic event, are-not well-understood 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 exchisive use of IVIr. Sack Anderson for 10 Geological Site Evaluation Mr. Jack Anderson February 19, 2004 specific application to the referenced site. Within the limits of scope and budget, this study was conducted in accordance with generally-accepted practices employed at the time this work was done. Nv other warranty of conditions is expressed or implied. You should also be aware 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 different 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 critical review. Although this time limit is somewhat arbitrary, it is suggested that 2 years be considered a reasonable limit for the usefuness of this report. ~~~ of w a s hi,~9 ~, ~a .. f . ~,j~~:ri:,; :~eo~ogat . y ~ 525 ~, ns ed Ge°~~ ~' Cry' fG K. WHfTE ig K, te, Engineering Geologist 19, 2004 11 APPENDICES Appendix 1: Location Map Appendix 2: Slope Stability Appendix 3: Patential Seismic Hazards Appendix 4: DetaiL• Area of Study :_~ ~'{~~ f ~ 1 ~ a tv ~' ~ ~ f t'"'1 t cat O =- _ ~ ti _- '- -'t _ . - - - -:. , '- - - - ' z_ ._ _____ -t_-_ . ... ,, .,, ._ _ :• '- - ... Ghf - - - ~ ~ 1 _ . __-. _ _ _~ _ ~ _ e,,l~ 4i+1 i-~ a ~i .~ $Y N ~- Q ,.,~_~ ~_ ~, -, ~, ~~ !y~ ~" ~~ ~. v e ~..~ ~°'~ ~i e ~~ ~4 'yet 0 cv cv ,-. N ,fit ~ W " - ~'ti %~ ,`', 4 ~ ~ °. ~ ~ ~; ~`~ ' _~ r~~ y• I •I ~ - ~~ .. ~~: 1,4 ~'. ._ `~ M1. . ~~~` .: '~ e ~.. ~`-=~ ~~ 4 Tj. 1- W W La. 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