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Home My WebLink About953700404 Geotech AssessmentDecember 7, 1995 Ms. Dee Couto 1707 Water StreeT, #1 Port Townsend, WA 98368 Job 273 RE: Geo~echnical evaluation of lots 953700404 and 95370040~, located in Sec. 29, T. 30 N., R.1 E.W.M., Marrowstone Islgnd~, Jefferson County, WA. Dear Ms. Couto: With your authorization and at the request of Ms. Barbara Blowers, a geotechnical evaluation was made of the subject lots. As I was advised, you propose to file for a variance for both -f'~ts in regard go -Im~orsoD Cn,,n*y setback rmquirement~. Since both lots are the same in regard to height of bluff, formation material, and other factors, this geotechnical report will cover both lots. The field evaluation was made on December 6, 1995. The geotechnical evaluation consisted of ' (1) review of Coastal Zone Atlas of Washings.ton, volume 11 , Jefferson County, Washington Department of Ecology, 1978, (2) review of Soil Survey of Jefferson County Area, Washington, Soil Conservation Service, USDA, 1975, and (3) a field inspection over the lots and adjacent property of tree attitudes, slope conditions, and formation outcrops. Pho~ographs were taken to document existing conditions. In summary, even though the Coastal Atlas classes the bluff portion of the lots as "recen. t unstable", no recent slope instability conditions were noted on either lots. However, slaking and ravel from wetting and drying, is quite obvious in the near vertical portion of the slightly cemented sand member of Vashon formation bluff slope. The. bluff, which, is reported ~o be 82 feet in height, is comprised of two portions, a lower portion (high tide line to about 35 feet up slope) and the upper steep portion (35 feet to the top of the bluff). The lower ~ortion consists of sand slope wash material derived from ~e slaking and covers the same sandy material as exposed in the upper portion of the bluff. The slope wash is standing stable (angle of repose) at an gyerage slope angle of 47 degrees.. Toe material is removed, from t'ime to time, by wave action. However, since ~his portion of MarroWstone Ms. Dee Couto December 7, 1995 -2- Island is well protected in relation to both wind and wave action, the erosion rate is quite iow. Also, logs have been anchored along a portion of the toe area to disapate wave energy. The upper portion of the slope stands at an average slope angle of 80 degrees. Were it not for the sand being slightly cemented, widespread slope failures would be present. The irregular surface of the slope face suggests, that from time to time, that slabs or wedges fail out of the slope. Such failures would be sudden and any facilities located below, would be in jeopardy. However', the existing trees (both on the subject lots and adjacent property) indicate that the bluff hgs been relatively stable in recent years. A section was prepared, using existing slope angles, of the bluff. The section indicates that the top of the bluff i's located about 30 feet east of the high tide line. Allowing for the established stable slope of 47 degrees, with static conditions, the setback needed to achieve that slope angle would be 50 feet. However, allowance has to be made for a dynamic event such as a major earthquake. Therefore, I recommenO that a ~e~ba~k of~6O feet be used. That would result in a projected slope angle of about 40 degrees from the high tide line. Even allowing for a major seismic event, which could result in the loss of 10 to 15 feet (horizontal) out of the bluff, a project life for a structure having a 60 foot setback should EXCEED seventy years with the understanding that recommendations are followed in regard to other factors such as surface drainage, disposal of organic waste material, and location of the septic drainfield. After an extended period of rain, when the ground is fully saturated, any addition rain will result in surface sheet flow. Any concentration of such a flow, especially down the face of the bluff, will result in erosion. For this reason, a "tightline" system must be placed to collect and discharge downspout flow from any structure roof. In addition, while not required but it is recommended, a shallow curtain drain be placed to collect surface and near surface sheet flow before it ca~ flow down the bluff. I am enclosing a standard design that I prepared several years ago for a tightline system. The septic drainfield should be placed as far back from the bluff slope as possible. One reason for this, is that septic effluent is normally slightly acid (pH 5.5 to 6.0). If any carbonates are present in the sand, then the acid will attack them and could result in the sand becoming "loose" and subject to excessive erosion. Also in the same regard, organic waste (grass cuttings, branches, wood chips) should never be stored along the bluff slope or thrown the bluff slope. In addition to adding weight, Ms. Dee Couto December 7, 1995 -3- the oxidation of the organic material results in tannic acid being formed. That will also a~tack any carbonates, and could result in lowering the factor of safety for the slope. Bearing capacity values, after stripping the loose top soil zone, are estimated to be about 1 TSF. This value will be more than ample for any structure having a continuous footing foundation. Sincerely, James B. Scott, P.E. cc: BBlowers Attachment (08/11/92 INSTALLATION OF TIGHT LINE SYSTEM This general and standard design statement, regarding the placement of a "tightline" system, is to be considered as a guide only. Bluff or slope conditions along with the volume of water to discharge, will dictate the actual design of the system. Figure 1 shows a typical section view of such a system. It should be pointed out that the system shown, could be considered to be "over designed" However, it is known that many "tightline" systems have failed because of poor anchorage or the use of under strength pipe. Therefore, it is better to err in favor of over design than to have a system fail. Failure of a tightline system can and does result in very rapid erosion with associated unbalancing of the slope, which then leads to slope instability and landslides. The typical system, as shown, can have a project life of up to 15 years for the exposed portion of the pipe which will ultimately deteriorate. Polyethylene pipe becomes brittle after extended exposure to solar UV. However, buried pipe is reported to have a project life estimated to exceed 100 years. This system design was prepared with the understanding that the system will be placed, using your own labor. This data and the description of how the system will be placed, was not prepared as "plans and specifications" for a bid (contract) item. The weak po'ints of any "tight line" system, depending on the steepness of the slope, are (1) points where couplings are located, (2) effect of UV on the exposed pipe, (3) strength of the pipe (the weight of the pipe tending to pull the pipe apart), and (4) vibration and movement caused by turbulent water flow. Most couplings have been eliminated by using only flexible pipe. However, couplings will be required at discharge points from roof downspouts. Based on data from a manufacturer (see figure 2), it appears that the best pipe in regard to both resistance to UV and strength for a very steep slope, allowing for the estimated flow (25 year storm) generated from a 3000 square foot roof, will be 4 inch Heavy Duty-AASHTO or equivalent, flexible pipe as produced by Hancor, Inc. The product code is HY PL5 (Specification AASHTO M252). The Washington State distributor for Hancor pipe is H. D. Fowler Company of Bellevue. Placement of anchors will be required to reduce or dampen resulting from turbulent flow. Two types of anchoring (concrete blocks and trenching) will be discussed. In the case of a high or steep slope of a bluff, during a very heavy storm, collected water as it discharges at the high tide line, could be like a high pressure water fire hose. To keep the pipe from moving from the vibration and surges INSTALLATION OF TIGHT LINE SYSTEM 08/11/92 -2- of water, a trench and then a series of anchor points will be located along the pipe down the slope. These anchors will be 1/4 or 3/8 inch rebar, driven at least 2.0 feet into the slope, bent over to form a loop and then secured by vinyl coated wire. A concrete anchor block (0.5 to 1.0 cubic foot) is recommended at the point of discharge at the high tide line. The higher the head (vertical height of bluff), the larger the anchor block. The function of the trench at the top of the bluff is to provide a good anchor for the up-hill portion of the pipe. The first anchor point will be located about three feet down slope from the point where the pipe appears out of the trench. Then anchors (driven rebar), will be placed every 20 to 50 feet (depending on ~l'ope angle) along the pipe (see Figure 1). If you do your own labor, the cost of placing the anchor points and pipe, should be quite economical with regard to material costs. However, the labor effort to dig the trench, might be a di££icult task, i£ "hard pan" is encountered .... After the system has been placed into service, an annual inspection of the pipeline is recommended. At the first sign of the pipe becoming brittle, it should be replaced. James B. Scott, P. E. 0 o r~ Fig. 1 HEAVY DUTY-AASHTO pipe is a flexible high density polyethylene pipe, corrugated on the inside and outside. HEAVY DUTY-AASHTO pipe is heavier than HEAVY DUTY pipe. This extra weight provides additional sliffness desired in some installations. It is often used for highway-relaled drainage projecls or for applications where heavy loads will be placed on the pipe. HEAVY DUTY-AASHTO pipe also works well for general drainage applications. . Inside Ou£nide Diameter Diameler S_~.pecificalion 3" 3.6" AASIlTO M252 4" 4.6" AASHTO M252 6" 7.0" AASHTO M252 8" 9.9" AASHTO M252 10" 1 i.9" AASHTO M252 12" 14.1" AASHTO M294 15" 17.7" AASHTO M294 18" 21.5" AASHTO M294 24" 28.4" AASHTO M294 Product Style Code Plain HY PL! 03 Pelf HY PFI 03 Perf/Wrap HY PWI 03 Plain HY PL5 04 Pelf HY PF5 04 Perf/Wrap H~ PW$ 04- Plain HY PLI 06 Pelf HY PFI 06 Pelf/Wrap HY PWI 06 Plain HY PLI Pelf HY PFI t~ Pelf/Wrap HY PWI 08 Plain HY PLI 10 Pelf HY PFI 10 Pelf/Wrap HY PWI 10 Plain HY PLI 12 Pelf HY PFI 12 Pelf (AASHTO HY D34 12 M3~ Pat~m) PelffWrap HY PWI 12 Plain HY PLI I$ Pelf lit PFI I$ Pelf (AASHTO HY D34 I$ ~36 Pattern) Perf/Wrap HY PWl 15 Plain HY PL! 18 Perf HY PFI 18 Pelf (AASHTO HY D36 18 M36 Pattern) Perf/Wrap HY PWI 18 Plain HY PLI 24 Perf HY PFI 24 Perf (AASHTO HY D36 24 M36 Patlern) Perf/Wmp HY PWl 24 SUGGESTED APPUCATIONS Storm drainage systems Culverts and cross drains Roadway edge drains CHECKUST OF ITEMS TO CONSIDER WHEN PURCHASING THIS PRODUCT Fittings (see page 4) Exlra couplers for fittings Geotextile wrap The need for perforations Fig. 2 JBScott & Associates 3601 West 5th Street Anacortes, WA 98221 -INVOICE- PROJECT: Lots located in Sec. 29, T. 30 N., R. 1E.W.M., Jefferson County, WA. JOB NO: 273 DATE: 12/07/95 SERVICES: Geotechnical Evaluation TO: Ms. Dee Couto 1707 Water Street, #1 Port Townsend, WA 98368 DESCRIPTION OF SERVICES: December 06, 1995 - Field Reconnaissance & Evaluation Travel Time - 1.5 hr x $45.00 @ ................. Mileage - 70 mi x $0.30 ......................... Ferry - Lump Sum ................................ Ferry Travel Time - 1.0 hr x $45.00 @ ........... Field Time - 0.7 hr x $90.00 @ .................. Office Time (Draft Report) 1.5 hr x $90.00 @ .... December 07, 1995 - Final Letter-Report Office Time 0.5 hr x $90.00 @ ................... $ 67 50 21 00 3 50 45 00 63 00 135 00 45.00 Credit of $200.00 paid on 12/06/95. DUE THIS INVOICE: DUE FROM PREVIOUS INVOICE: 0 00 TOTAL ' .~0 00 TOTAL AMOUNT DUE' I hereby certify, as principal of the firm of JB SCOTT & ASSOCIATES, that the charges as summarized, are fair and reasonable and in accordance with the terms of the agreement and/or work authorization dated: 12/05/95 mes B. Scott, P.E. Date LIMITATIONS: JBScott & Associates provides services in fields of Geology and Geotechnical Engineering in accordance with curren% professiona~:-standar~s. No warranty, expressed or implied is granted as part of perfQrmance of