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Home My WebLink AboutGEO2016-00076 BLD2016-01077 - BLD Engineering / Geo-tech Reports - 10/31/2016 PLANNI Bradley-No le deotechni,,M ervices A Division of The Bradley Group, Inc. 2 Novemb<�) �Jhone O Box 12267; Olympia WA 98508-2267 360-357.7883 • FAX 360-867-9307 RECEIVED Mr, Dennis Eller ' OCT 3 12016 2420 South Taylor Roa Decatur, Illinois 62525 615 W. Alder Street Subject: Prellmi evaluation of the stability of the marine bluff and sew ck from top of slope for a new residence proposed to be constructed at 1114 East Pirates Cove Road, Grapeview area of Mason County, Washington. _ Dear Mr. Eller: As requested, we have Nisited the proposed site of your new residence at I I'l4 East Pirates Cove Road, Lot f of SP 571, Tax Parcel Number 121 187590203 in the Grapeview area of Mas n County, Washington. The purpose of this inspection was to evaluate the stability an erosion potential of the marine bluff and to evaluate setback from the top of the blufl for your new residence. This letter will present our opinions based on our site visit, research, and understanding of your proposed construction. The property is located on a south-facing marine bluff in the Stadium Beach area of Puget Sound. The marl bluff in this area is steep to nearly vertical. Estimated . maximum relief on this property is estimated in the field to be about 50 to 60 feet The height of the bluff decreases to the west and stays at about the same vertical relief to the east. Two soil units)crop out in the bluff face. At the top of the bluff we find a thin toyer of Vashon recessional outwash sands and gravels (Qvr). These soils were fluvially deposited during the retrIeat of the Vashon lobe of the Fraser Glaciation of the Cordilleran glacier in la; Wisconsinan time. In this area these soils mantle the Vashon subglacial till (Qvt). Vachon subglacial till is a mixture of silt, sand, and small gravels which was deposited at t6 base of the advancing ice sheet and was overridden. Loading by•the ice sheet,Ithought to have been about one-half to one mile thick in this area, and the grain size distribution have created a soil unit that has many engineering properties similar to tho a of lean concrete. Subglacial till can stand steeply with apparent stability and only slow erosion occurring. Active erosional processes working on the bluff are freeze-thaw cycles resulting in ice plucking and ice wedging of material from the bluff face. Wind driven rains cause sheet washing of loosened soils from the bluff face, These two processes slowly erode the bluff face. As a result of these processes, we find overhanging soil masses supporting 04100801 Page I of 3 "C7 Y:. 04100801 Page 2 of 3 vegetation at the top of slope. When sufficient material has been eroded,these unsupported soil mass fail under gravitational influence and carry both soil and vegetation to the beach. These events are small, typically a few cubic meters in volume. They are random in oc urrence, and between events apparent stability and slow erosion returns. You must ex vise care in approaching the top of the bluff, as your weight can be sufficient to cause a failure of these overhanging soil masses. These erosion processes are slow with the marine bluff only retreating at a slow rate. In this area we are unaWare of rotational failure developing in the subglacial till unit. This is also reflected in the geologic mapping in this area. In Relative Slope Stability of the Southern Hood Canal Ama, Washington by Mackey Smith and R.J. Carson, Washington Department of Natural Resources, Division of Geology and Earth Resources Map 1-853- F 1977, indicates this area as a Class 2 area, areas believed to be stable under normal conditions. We concur with this opinion. At the toe of bluff, erosion by marine processes does occur during high tides and storms when water elevation can reach the bluff. The highly overconsolidated soils forming the toe of the bluff are naturally resistant to marine erosional processes. Erosion in this area is also slow. Construction of a bulkhead to prevent marine erosion is not recommend at this site. In the current regulatory environment, obtaining permits to construct a bulkhead!is at best difficult. At this site, It is our opinion that a bulkhead would be of limited value as only the toe area is protected. Erosion does continue on the exposed face of the bluff above the bulkhead. Development of a recommend setback from the present top of slope is a compromise between the owner wanting to construct close to the top for the view this property offers against long-term safety of the structure from erosional processes. At this site, we recommend that thej new residence be constructed behind the 1:1 line as measured up from the:toe of bluff. Construction behind the 1:1 line is considered to be a safe setback that will ensure that the residence will not be affected by erosion of the bluff for at least the next 50 years. Construction of decks or other low value structures in the setback zone is permissiIie as long as they do not extend more than twenty feet from the south wall line of the new residence. Based on our field observations, this appears to be the same setback at was used for the residence that was built on Lot 4 of Short Plat S72 to the east of your lot. The other method used for determining setback is the common setback used for adjacent structures. To ensure continued.slow erosion of the bluff, site drainage and clearing of vegetation must be addressed. We recommend a buffer of natural vegetation of at least twenty five feet along the top of the marine bluff. This will protect the vegetation that aids in holding the outwash soils at the top of the bluff. We have no objection to 0.4100801 Page 3 of 3 pruning of trees and shrubs for view enhancement if this pruning is done in a manner that does not Will the vegetation. Control of storm water from impervious surfaces must be addressed in y ur site development plans. No uncontrolled storm water is to be directed over the sl pe. These concentrated flows can create areas of localized saturation and induce sgope instability and accelerated erosion. You must also ensure that uncontrolled stor7 water is not directed onto adjacent properties as a concentrated flow. Based on our site inspection and research, it is our opinion that a residence can be constructed on this property without risk of loss due to soil movements or erosion. Using the recommended setback from the top of the marine bluff will ensure that you. can enjoy this property nd the views it offers with safety for at least the next 50 years. Good vegetation and storm water management will aid in ensuring that this estimated useful life for the structure Is obtained. If you have any questions, or if we may be of additional service to you, please contact us at our Olympia office. Cordially, BRADLEY-NOBLE GEOTECHNICAL SERVICES David C. Strong, L.E.G. y 1� C. '9 C s►ate, y 1129 a� ASP a a\a (fllVtp'C. STRONG F cT 97 5- 81d. Za 1 lP --o l o-7-7 ENVIRONMENTAL HEALTH �F RECEIVED OCT 3 1 2016 615 W. Alder Street d 9 fiCk � X! 4 CeD 7 C- -71 1 4 Age 8 A _ AF)k 77,775, Mc P�PI Fi ;p r No p 7 ALT'H CEW