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Home My WebLink About950100324 Geotech Assessment Addendum~prJ ~i5o ~~3~ ~ G~~tf-a°~31 Prepared for: " 1VIr. Jack Anderson Prepared by: Craig K. White, Inc. Geosciences Consulting P.O. Boz 3398 Silverdale, Washington 98383 February 19, 2004 _v ~~ Crain I~.. \~/hite, lnc. GEOLOGICA 5~ E~A ~ ~0 5 ~~A~~ GEOSCIENCES CONSULTING ~ E SEOPRO ~,oN LGEOL ~~ WASHINGTON&ALASKA February 19, 2004 GEOLOGICAL SITE EVALUATION Lots 25 & 26, Trails End Subdivision) NW'/, Section 16, T27N, R1E W.M. Jefferson County, Washington Property Location and Description: The subject property is Iocated 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 bhiffs 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 small, wood-frame cabin is located on the property along the top of the bluff near the south property Iine. 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. .: ,-:. ' 6Vir i P.O.6ox 3398. ~lverdale. \Vashinyton 98383 (360) 830-0]18 Table of Contents Property Location and Description Topography and Drainage Vegetation Geology and Ground Materials Slope Stability Potential Seismic Hazards Conclusions and Recommendations Summaay and Limitations Appendixes: Location Map Slope Stability Potential Seismic Hazards Detail: Area of Study 1 2 3 4 5 6 10 Appendix 1 Appendix 2 Appendix 3 Appendix 4 Geological Site Evaluation Mr. ,Tackflnderson 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 bhtffface was achieved on foot prom 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 and Drainage: r: 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 Thoxndyke Rd. with a Honor component of drainage toward the adjacent property to the south (see Appendix 4). On the bhiffface, 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 bluffface 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 bh~ffvaried from less than 3 degrees to as much as 5 degrees. OveraII relief on the . bhiff face 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. . f~(j~:~ 'i ~ ~vU~. 2 Geological Site Evaluation Mr. JackAnderson 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 in several places along the bluffly suggesting that water runoffonto 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 bIuffdayfights 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, Hunalayan blackberry and Scotsbroom along with assorted weeds and grasses in cleared azeas of the property. Vegetation on the upland portion of the property appears to have been cle e = , ._ 3 Geological Site Evaluation Mr. lack Anderson February 19, 2004 some years ago. In this areas, vegetation consists mostly of grasses and weeds with one or two large, weIl-established trees that generally reflect the presence ofwell-drained sods. Vegetation along the bluffface ranges from dense to patchy; a condition which is h7cely . the result of heavy erosion and periodic, localized slides that may occur during the months of heavy rainfall acid remove sods and vegetation on the face ~of the blull•; Geology and Ground Materials: 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. Reconn?;.~ance mapping shows the upland portion of the study area is capped by Everett, graveIly sandy loaffi 0 to 15 percent slopes (EvC). This nearly Ievel to rolling soil is on glacial outwash terraces where slopes usually range from 4 to 10 percent. The sod is somewhat excessively drained. Permeabdity is rapid and roots may penetrate to depths of 60 inches or more. Runoff is slow and the hazard of water erosion is slight to moderate. The principal soft. along the face of the bhiffwithin the study area is Cassolary sandy loam, 30 to 50 percent slopes (CfE). This steep sod is on canyon slopes and ocean bluffs. The dark gray surface Iayer is absent in many places and the silty clay loam layer in EF: icy - n.;%: 4 Geological Site Evaluation Mr. JackAnderson February 19, 2004 the underlying material is nearer the surface on the upper part of slopes than it is on the lower part. Runoff is 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 7d inches. Along the face of the bluffnear the southern property boundary, the ground materials are exposed in several small, patchy outcrops and consist ofpoorIy-indurated, sandy loam with minor pebbles. Slope Stability: 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 "High Erosion Hazard "due to the presence of steep slopes and poorly-consolidated soils. On Washington State Department of Ecology Coastal Zone Management maps, the bluffface immediately-east of the subject property has been classified as "Unstable ("U`), while the upland portion of the property is identified as "Stable „ ("S') 5 ~:; ; _,, Geological Site Evaluation Mr. Jack Anderson February 19, 2004 . Direct examination of the bluffeast 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 bluffface, 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. ]:n other areas along the bluffface, a number oftdted trees~indica#e the slow, downhill "creep" of surface sods maybe occurring, which may ultimately culminate in slides. Along the base-of the bhp slide debris consisting of trees, stumps and mounds of sod is stdl visible 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 duffing periods ofheavy winter rainfall when water runoffis greatest and the ground materials are most saturated. Potential Seismic Hazards: The Puget Lowland is classified as a Seismic Zone III by the Uniform Budding Code (LTBC). Crustal earthquakes, which occur along surface faults, have been documented in several areas of the Puget Sound region (see Appen~c 3: Seismic Hazards 6 Geological Site Evaluation Mr. Jack Anderson February 19, 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 adjacent 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: I . It is recommended that a minimum setback distance of 35 feet from the top of the bluffbe established for ali structures. Additionally, we recommend that a 15-foot-wide 7 Geological Site Evaluation Mr..7ack Anderson February 14, 2004 natural vegetation buffer be established along the upper bluff edge within which the re- growth of native (lower-growth) plants should be encouraged. Large trees along the bluff face that maybe unstable or impede a "limited view comdor" 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 minimises disturbance to the lower-growth vegetation. 2. Septic tanks should be placed as far back from the top of the bluffas possible and drainfields that maybe constructed on the property should be at least 50 feet from the top of the bhiff to prevent daylighting of effluent along the bluff face and saturation of the sods. 3. Gradmg and preparation of a proposed homesite should be conducted in a manner that wd1 allow runoffwater from the site to drain to the west, towazd natural drainage depressions along the shoulder of Thorndyke Rd., rather than east, towazd the bluffface. Our observations suggest that such re-gradmg 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 towardThorndyke Rd., away from the bluffface. Buried portions of tightlines should utilize 4- ar.6-~ch, , ..;,<,;, 8 - .. Geological Site Evaluation Mr..Tack 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 possble, 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 be selected to minimise the need for heavy watering. The planting of lawn grasses should be limited to relatively Ievel ground. A list of recommended, deep-rooting plants is available, 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 bIu~ 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 folYgviri~ "~ 9 Geological Site Evaluation Mr. Jack flnderson February 19, 200 periods of heavy rainfall, to determine the location of any new springs, seeps or areas of excessive erosion, and to identify any unstable conditions. Recordmg the appearance of these areas in photographs, taken each yeaz, and comparing them with current conditions mayassist in this evaluation. Summary and Limitations: Although the development ofproperties 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, satura#e ground materials and destabilize slopes, resulting in ground movements that often occur without warning. Criven 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 exchssive use of Mr. Jack Anderson for ' r. 10 Geological Site Evaluation Mr. Jack.4nderson 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. No 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 usefulness of this report. %~ wash~~ ~ gro _ . ~ t ~ 0 525 ~ Qa`~ _ used Geo~o :, C~!G K. V~H!TE BOG ~ ~ ~ ~r I1 ~dVhite, Engineering Geologist 19, 2004 APPENDIXES Appendix l: Location Map Appendix 2: Slope Stability Appendix 3: Potential Seismic Hazards Appendix 4: Detail: Area of Study r;,~.3 . , t.~,; 3 ~ ...x34'4 - - c~ _. _ ,c, ~:''. a ~. cv _: r.,, =- . 0 .~ .~ W as iU'] .~ .~ 0 a~ ~_ •"~ ~~ Sam 4 -~._ .~ O 0 0 a-. .~ ,.. p. [~ or Q4 H .~ ~~f ~} ++ s. ~~_~ ~~ ~~ ~ ~i 1 s uvu~ r Slope Stability D Stable Urs Unstable Recent Slide ~I Intermediate Uos Unstable Old Slide Unstable ~ Modified Slope - Unstable Bluff C.v,.,+a•IA/n../.:..s....cs..a.. n_~y__a rar r__~_~_..___. _.-._-__ .u-_ .,, T~ ..~~ ~~ ter' -~~ i. ~ ' _ %~ . 'I)ev9is :~i~d. 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A A -~.. ~ ly O z 0 e . .. d ~- ~~ ~~ ~ ~ ~ ~ ~ ~. ~'h ~ -P ~~2.. m d d a~ c7damh ~. ~ -- b .. b » , e _~-___T s CHARLES H. POLLMAR & ASSOCIATES J ~, ~~ l Dear Client: ` As the Bremertort-ICitsap County Health Dept (BKCHD) ha$ raised tfieir fees in recent years, it has cost us an increasing percentage. of gross revenues when we .write the checks for the county fees out of our twsiness account As such, we actually are .taxed on the county fees when we file our quarterly B & O reports with the State Departrnent of Revenue. Therefore, we can no longer afford to do this. !n an effort to keep our expenses and the cost of our services to you down, we request that you write a separate check to ~e )for short, in the amount indicated on your invoice for County Fees. ~.. j ~~=~~~-~~ ~v~~,~ ~G~1, _~ ~'~~t~c jy~ If for some reason, you cannot write two checks (For accounts with limitations on b~l~~Le~rrt~i them), then please add ~ 10.00 to the amount of the invoice to cover our. increased costs and handling. We apologize.for whatever inconvenience that this causes. Thank you! Sincerely, x. Charles H. Pollmar Licensed Designer NOTE: Payment ItJ-FULL is due within lU Days of the Date of the Invoice, unless arrangements are made to the contrary. Payment later than this w~71 be subject to a X50.00 Surcharge to cover die costs of cleaning out the slot~ghage in the Soil Logs that occurs over time 10081 ARROW POINT DRIVE NE • BAINBRIDGE ISLAND, WA 98110 PNONE: {206}-542-7927