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Home My WebLink About701153014 Geotech Assessment~un 06 02 O~:5~p SubTerra Ino ~25 888 ~7~5 p.1 Geologic Reconnaissance Report Sema Property Vacation Home PN: 2002-19 June 6, 2002. Scott Sema 1213 12~h Avenue Southwest Puyallup, Washington, 98371 Subject: Geologic Reconnaissance for Vacation Home Construction Dabob Bay, Washil~gl. on Dear Mr. Serna: At your request, in June 2001 a geologic reconnaissance was completed for your vacation home construction site, located at the top of a steep bluff in the northeastern part of Dabob Bay. The property is located in the southwest comer of Section 15, T27N, RIW, about 10 miles southwest of Port Ludlow, Washington. At the time of our site visit, yo~ had not yet purchased the property, but you requested verbal recommendations as to whether the site could be suitable for home construction. We indicated verbally to you at that time that the site could be suitable for construction provided that a 75-foot top-of- slope setback could be maintained for the nnsite septic system, and that a 120-foot setback could be maintained for the home construction. We understand that prior to purchasing the property you measured for these recommended setbacks and you determined that the site afforded sufficient space for you to construct your home within these limits. As part of the building permit to construct your home, we understand that JuiTerson County personnel requestcd our verbal recommendations to be placed in writing, along with the observations made during our site reconnaissance. The purpose of this letter is to satisfy these County requirements for a building permit. PROJECT AND SITE CONDITIONS The site lies at the top of a steep bluff in the northwestern portion of Dabob Bay, about one mile nndh of Camp Discovery. The subject site comprises about six acres, and is relatively long and narrow with the property oriented in an east-west direction. The property is 300-feet wide in a north-south direction, and extends from between 700 and '1100 feet east-west from the top of the bluff dr~wn to the beach on Dabob Bay. We understand you wish to construct one single-family vacation home along the upper, eastern portion of the site. 1 PN: 2002-19 · Serna pro?_e,d,y . Dabob Bay Vacation Home 6/6/2002 The western two thirds of the property is composed of a steeply sloping west-facing bluff .r with an overall gradient of nearly 100 percent over a vertical height of about 400 feet. The eastern portion of the site is moderately-to-gently sloping, with a relatively level a aa, ~_;--: near the far eastern end of the site. The northern boundary of the site slopes to the north toward a local drainage swale that drains to the west, perpendicular to the bluff face. Total topographic relief at the site is about 460 feet, wilh eleva{ions ranging from sea level at the western end of the property to about 460 feet at the eastern edge of thej property. - ~i Vegetation at the site consists of relatively large second growth evergreen trees and deciduous trees, with an under story of ferns and brush. At the time of our site visit, thel proposed home site (the relatively level area in the eastern portion of the property) had been cleared of trees and leveled, and an access road to the site was the only development. SOILS The soils underlying the subject site were exposed in cuts along the access road in the eastern portion of the site and in the bluff face to the southwest of the site. The upper eastern portions of the site are underlain by dense lodgement till. This i~ an unsorted mixture of gravel silt and sand deposited at the sole of the Vashon glacial ice sheet about 15,000 years ago. The till was consolidated to a dense state by the weight of the ice that reached an e=timated thicknes~ or 3,500 feet in this area a[ its maximum extent. The process of consolidation gives till relatively high strength characteristics, and the high silt content makes this material relatively impermeable with respect to ground water flow, Underlying the till and exposed in the upper portions of the biuff face is a dense gravelly fine to coarse sand with some silt. This matedal was interpreted as Vashon advance outwash that was deposited from melt water streams emanating from the advancing Vashon glacial ice sheet, The advance outwash was subsequently overridden by the ice and also consolidated to a dense state, Because of its composition, this material is relatively permeable with respect to ground water flow, and has served as a local aquifer in this area. The lower portions of the bluff, below about the 250-foot elevation, were underlain by hard silt with local lenses of sand and gravel. The material was interpreted as pre- Vashon deposits that may be considerably older than the overlying Vashon outwash and till. The hard character of this material may be the result of repeated glacial ice advances that occurred in this area over the past several hundred thousand years. The high silt content of this material makes it relatively impermeable to ground water flow. SLOPE CONDITIONS The steep, west-facing bluff that occupies the western two thirds of the site is considered an active marine bluff that is undergoing periodic slope retreat, Two mechanisms are acting on the bluff to produce the majority of the retreat. One is the wave active that acts on the toe of the slope, taking away material along the beach that is replaced by material further up the slope. The second mechanism is ground water that builds up within the advance outwash, perching on top of the pre-Vashon hard silt at about the 250-foot elevation. This ground water buildup can lead to the sudden periodic failuro of the outer 2 PN: ZUrJZ-15t Jun ub u~ u~:a~p ~uolerra lnc v~a v~ ~a p,~ Serna.,.Property .... . .,. Dabob. Bay,,, Vacation Home 6/6/2002 colluvial layer that naturally accumulates along the outer edge of the slope. A few years prior to our site visit, a rather large, sudden failure of this type occurred along the subject bluff on the adjacent property to the south. We fully expect that a similar such occurrence could occur on your property over the next several years. It is our judgment, based on the site reconnaissance, that the rate o1' slope retreat along this bluff, over a period of several years, is on the order of about one foot per year. This retreat will not be continuous, but rather will be periodic and erratic, and dependent up! the fluctuations in annual rainfall and storm events. , ' OPINIONS AND RECOMMENDATIONS Based on our geologic reconnaissance and slope assessment, it is our opinion that th~ '---' proposed 120-foot top-of-slope setback for your home is suitable to p'roted the propose; ~ home from slope failure during its anticipated lifetime. The proposed 75-foot setback f6¢c the septic drain field should also be acceptable, provided the drain field is sloped to i direct and maintain drainage toward the swale along the northern side of the sitc, and i not toward the bluff to the west. The purpose of this requirement is to keep additional i ..... ground water away from the bluffface. ',~ ', During and following construction we would also recommend the following: · The glacial sediments encountered at the sCte are suitable inundation-bearing soils. Foundation footings should be placed on a non-yielding natural subgrade beneath any organic or loosened soil zones. · All storm water from impermeable surfaces, including roadways and roofs, should be tightlined into a suitable storm water drainage system, and not be allowed to discharge as a point source onto steeply sloping areas. · The vegetation downslope of the building areas should be left in its existing natural condition to protect the slope from erosion. The cleadng of view corridors through selective I~a[¥esting and/or Iimbing of ~rees ~s acceptable, provided that the overall integrity of the underlying soil is maintained, and the underlying root systems are not damaged. · To minimize soil creep and erosion, no unprotected cuts should be made into the steeply sloping areas, and at no time should fill be placed on steep slopes. We have enjoyed working with you and are confident that our recommendations will lead to the successful completion of your project. If you have any questions or,, nocd further information, please do not hesitate to call. Sincerely Yours, George H. Bennett, P.G. Senior Geologist 3 PN: 2002-19