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HomeMy WebLinkAboutBLD2010-0309 Geotechnical Report NORTHWESTERN TERRITOIES, INC. 717 SOUTH PEABODY STREET,PORT ANGELES,WA 98362 Engineers■Land Surveyors I•Geologists -71'- Construction Inspection I Materials Testing ljr PHONE (360) 452-8491 1-800-654-5545 FAX (360) 452-8498 NTIE-Mail:infoAnti4u.com Mr.John Kueber 2616—25th Ave. West Seattle,Washington 98199 June 23,2010 Subject Geotechnical Review and Prudent Setback Analysis for Lot 3 of the Dabob Seclusion Plat - Jefferson County Parcel 701324006 Dear Mr.Kueber: During May and June of 2010, NTI Engineering and Land Surveying completed a geotechnical study of your parcel at McDonald Road near Quilcene, Washington. The parcel is listed as Jefferson County Parcel #701324006 and it lies within the South Half of Section 32, Township 27 N, Range 1 W. WM. We understand that you intend to construct a thoughtfully designed"Greenpod"house on the parcel. The parcel, which is shown as Lot 3 on the attached survey, is set out by rebar stakes at the top of the marine bluff about 200 feet high. The study was ordered to assess the geologic hazard and the prudent setbacks for the proposed structures at the site and to provide other recommendations and assistance relating to the site development. 1.0 Background Information The parcel is located on a 200 foot high marine bluff. Dabob Bay lies to the southeast below the steep bluff that ends at the high tide line. The bluff rises along a line steeper than 50 degrees from the horizontal. A shallow ravine carrying a creek lies a few hundred feet to the west. The vicinity of the proposed building site is shown in the aerial photograph below. Aerial View of Subject Property • • The surface soils of the area include Alderwood Gravelly Sand Loam terrace deposits that were mapped by the Soil Conservation Service. Significantly, outcrops of Quilcene Silt Loam were mapped by the Soil Conservation Service along the lowest third of the marine bluff and over a large area further to the west. The upper terrace sand and gravel was deposited over the pre-existing terrain of soft weathered shale rock which is described in more below. The two-hundred foot-high bluff stands at a steep angle of more than 50 degrees from the horizontal below this building site. During site study, significant seepage of shallow groundwater was noted on adjoining parcels on the west. 2.0 The Geologic Setting The subject property is a glacial terrace deposit of the Pleistocene glaciation of Puget Sound. The bluff escarpment exposes a sequence of recessional glacial outwash and terrace deposits that were left at the site some 12,000 years to,perhaps, 15,000 years before the present. The sandy terrace soils overlie a pre- glacial siltstone that forms the lower portion of the bluff. This compact clayey silt and siltstone is of probably of Eocene age and may have been part of a freshwater lake environment that pre-dated the seawater flows into Puget Sound. During a long period, the material was folded up by tectonic forces and became an eroded ancient landscape. During the Ice Age permeable gravelly sands and gravels were overlaid on this surface to form the modern landscape. Much of the material forming the upper part of the bluff and site originated in what is now British Columbia. The sands and gravel was carried to the south by glacial ice beyond the ice front within rivers of meltwater that cascaded down the ancestral Hood Canal. 3.0 The Geotechnical Indicators at the Site An important indicator at site was the observation of a larger-scale slump sliding affecting the property on the west of the building site. A rotational slump slide scarp was noted between 400 and 600 feet to the west of Lot 3. That slide probably occurred quite recently. In some sections along the line of the active slide, vertical displacements of up to 15 feet and multiple failure surfaces were noted. Another"step" of the same slide occurred about 25 feet north of the main slip zone although at this time it is only a foot or two in height. The line of recent instability trends to the northeast across the southerly portions of Lot 1 and Lot 2 as shown on the attached plat. In a portion of Lot 2, a benched area was observed that appears to be an older slump block whose width would have been on the order of 40 or more feet. Review of lidar(side looking radar) image below suggests that the slide activity and instability gradually reduces to the east from a maximum in the vicinity of Lot 1 and further to the west. The radar image below shows the slumping that is described above. Unstable slumping areas are shown in darker shades on the west, especially within Lot 1. Slump blocks are also visible within Lot 2. The region to the east appears to be stable but the mechanism of slumping could be present. Local well logs and geologic mapping provide insight into the structure of the subsurface and the erosional processes working there. The general characteristics and scale of the local sliding is illustrated in Figure 1 attached. The drawing shows the rotational slides being driven by high groundwater table due to the impervious and clayey shale materials at depth. During periods of high rainfall,the slumps occur in an unpredictable way due to the floatation provided by the water and the reduction of friction across the • • slippery surface of the wet clayey material that underlies the site. The scale and mechanism of those slides is illustrated in Figure 1 and the image below. ,. 701324008 701 #11 7013244 .� LOT 3 f 701324009 ; ° \ LOT- 2 „ E 70132501 ; P' %4 ara..ded by,b e 1 Carr,Calks Set oos Gts d 0 t -• ,3 of i Lidar Image of the Vicinity Showing Lot 3(Parcel 701324006) 4.0 Minimum Marine Bluff Setback Recommended In terrain affected by this kind of step-sliding, it is difficult to predict the next occurrence of the sliding and exactly how far back is truly safe. The type of sliding occurring here has been seen elsewhere to result in a series of "steps" each with a flattened top that develops successively larger steps on the seaward side. The whole area of instability may be a relatively wide area, although the safety increases as you move away from the bluff because the displacements are likely to be less on the upslope side of the unstable area. For site planning purposes on this parcel, it should be assumed that a characteristic slump slide could occur sometime in the foreseeable future and 30 to 40 feet of the bluff could suddenly drop down as shown in Figure 1. The general geometry of nearby slide areas and the scale and steepness of the bluff suggest that secondary slump steps could extend back 65 feet or more from the existing rim of the bluff. The value of the house on a bluff of this height would be substantially diminished by the perception of slide hazard once an escarpment moves to within about 50 feet. An additional setback of 50 feet also provides a logical measure of extra protection should an unexpectedly large and improbable earthquake occur. Thus, this distance was added to the characteristic slide width to obtain a setback recommendation of 115 feet. In view of these observations, we suggest that you obtain as much setback as is possible. In this situation, the more setback, the greater the safety of life and investment. At a minimum, we recommend that all new residential structures be located as far as possible behind a line that varies from 115 feet north of the surveyor's top of bluff monument on the east line of the property, to a point that is 130 or more feet back from the rim of the bluff measured along the west line of the property in the northerly direction. • • These are the minimum setbacks. If possible, provide more setback for improved safety and peace of mind. (See Figure 2,the setback diagram.) An undisturbed vegetated buffer of 30 feet should be observed as measured from the edge of the bluff to the north. The continued growth of native plants and trees should be observed in this area and no trees should be removed from any portion of the bluff or the buffer. 5.0 Measures to Observe During Construction The following should be observed in site design and during construction: 1. Do not dump spoils from excavation or re-grading of this site over the bluff of slope on the west. Instead,move the excess soil off the site. 2. Avoid drainage schemes or grade revisions which would route any water over the bluff on the east. Subsurface disposal of drainage water is not recommended at this site due to the destabilizing effect it may have on the nearby slopes. Instead, route drainage water to the low area beyond the slope on the west if possible. 3. Install silt fencing or silt traps to reduce the fine sand and silt in the runoff from the site at the end of drainage pipes and along all open drainage routes. Remove a year after construction completion. 6.0 Other Recommendations The following generalized recommendations apply to this site: 1. Avoid underground lawn irrigation systems and unnecessary below-ground water pipes that may break and lead to the uncontrolled flow of water to the soil. Keep irrigation to a minimum. The sand soil of this site is subject to slumping and rapid erosion under water flow. 2. Trees and other vegetation stabilize the bluff soil and they help remove water from the soil. Established slope vegetative cover should be left in a natural state to insure the continued health and growth of strong roots. 3. Surface water should never be allowed to flow over the face of the bluff as a concentrated flow and cause erosion of the bluff face. Erosion can be controlled by developing a well-planned drainage system. Please see the referenced Department of Ecology Publications for more information on this subject. 4. Tree limbs, lawn clippings, and excess soil from construction excavations should not be thrown over the bluff. These materials can damage or destroy the underlying vegetation, which in turn increases the erosion potential of the bluff. Earth and construction spoils add unnecessary weight to the bluff, and when saturated with water,they may trigger land sliding. 5. Surface runoff from hard surfaces such as roofs, driveways, walkways and patios should be controlled and routed to a drainage flow control system so that the rate of water discharge is not increased by the development action. Drainage control devices should be maintained in good working order and inspected regularly. • • For further information please review the three publications (included with the original of this report) published by the Washington State Department of Ecology (DOE) entitled: "Slope Stabilization and Erosion Control Using Vegetation," "Vegetation Management: A Guide for Puget Sound Bluff Property Owners" and "Surface Water and Groundwater on Coastal Bluffs." These publications can also be viewed on the DOE website at http://www.ecy.wa gov/biblio/sea.html under the 1993 and 1994 year heading. The DOE website also contains additional useful information regarding slope stability and site development, and this reference is highly recommended. 7.0 The Limitations of This Investigation and Report This report applies only to the described property and it is for the exclusive use of Mr. John Kueber for prudent setback determination relating to the residential structures under discussion at the time of the report. The conclusions and recommendations of this report are not transferable to the adjoining or nearby properties. This report is based upon surface observations of the site and the adjoining bluff areas. The . limited study was done without the benefit of borings,test pits or careful measurements taken over time. The setback recommendations of this report are based upon the fundamental assumption that the future of this bluff will resemble its past as discerned from the geomorphology and the erosional mechanism(s) of the bluff. The recommended setback may prove insufficient in the very large but also quite improbable earthquakes that have been suggested by B. Atwater and others. Users of this report are encouraged to provide as much setback as possible for improved peace of mind and safety. This report, the field work and the recommendations herein were conscientiously completed in accordance with the principles of geotechnical science and in conformance with the professional practice of engineering. NTI provides no other warranty,neither express nor written, herewith. Users who require a level of reliance beyond the observations and recommendations of this limited study are encouraged to obtain more detailed geotechnical and geologic studies. Sincerely, NTI ENGINEERING AND LAND SURVEYING i -;x �'?r 3 6 tci. `.? . yr"R Steve S. Luxton MSc. 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