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Home My WebLink About BLD2000-00077 Geotechnical Report Zipper Zeman Associates, Inc. Consulting Geotechnical Engineers Ate - 7 ;998 __. J-1 04 July 14, 1998 Kathryn Boyker Charles Thrasher 3049 E Laurelhurst Drive NE Seattle, WA 98105 Re: Results of Limited Geotechnical Study Tract 18 of Unrecorded Plat of Dabob Cove Tracts Jefferson County,Washington At your request, we have performed a limited geotechnical study for your proposed residence on a portion of Tract 18 of the unrecorded plat of Dabob Cove Tracts. A copy of a topographic map, prepared on 14 November 1997 by Wood Surveying, Inc., indicates the approximate location of the proposed residence. The roads indicated on the topographic map are existing and serve Tracts 18, 19 and 20, which we understand are under your common ownership. Site Description As noted, the proposed residence will be located on a portion of Tract 18 in the approximate area where the three existing roads are shown to intersect on the attached topographic map. The area surrounding the residence is primarily vegetated with second growth for trees and native undergrowth. The slope below the residence has been more recently logged of larger timber and consists primarily of native ground cover with scattered conifers. The natural slopes above and below the proposed residence location are generally on the order of 2H:1V, or flatter, with some localized areas approaching 1.5H:1V. Subsurface Conditions Soil exposures along the road cuts in the proposed building area indicate the upland part of the site is underlain at shallow depth by very dense glacial till. This material was deposited and overridden by several thousand feet of ice during the last P. O. Box 5420 Lynnwood, Washington 98046 (425) 6?3-4625 Kathryn Boyker, Charles Thrasher July 14, 1998 Page Two glacial advance and as such, provides excellent foundation support with little or no settlement. Glacial till is also relatively resistant to significant or deep-seated slope stability concerns. As is typical of the Toandos Peninsula, the upland glacial till is underlain by deep stratified deposits of sand and gravel. These soils have also been glacially consolidated and provide excellent foundation support with a minimum of slope stability problems. Reportedly, the on-site well encountered silts and clays at a depth in excess of 125 vertical feet below the building area. Thus, the contact between the sands and gravels, and the underlying fine grained soils would be at least 200 to 250 feet horizontally from the building site. The water bearing contact between the gravels and the underlying fine grained soils is often the site of soil stability problems, but the contact in this case is a significant distance from the building area. Minor amounts of loose granular fill appear to have been placed on the outer shoulders of the road in the building area. While this material has been in place for a number of years and displayed no signs of instability, it will not be suitable for building or slab support. A reconnaissance was made of the slope below the planned residence to check for signs of major slope stability. A few minor surficial slumps were noted, but no significant areas of instability were noted that would affect the proposed residence. Typically, the glacial till and underlying gravel are very resistant to any type of instability. However, during unusual periods of excessively wet weather, such as the snow and rain of January, 1997, the surface soils can become completely saturated and be randomly subject to shallow movement. This type of surficial movement was very common during that particular storm event throughout the Puget Sound area, but it is not indicative of any deeper seated instability. Conclusions and Recommendations Based on the results of our site evaluation, the following conclusions and recommendations are presented. 1. Soils exposed in the building area consist of a glacial till cap underlain by very dense sands and gravels. These soils have been compacted by several thousand feet of ice and will provide excellent foundation support with little or no settlement. P. O. Box 5420 Lynnwood, Washington 98046 (425) 6?3 -4625 Kathryn Boyker, Charles Thrasher July 14, 1998 Page Three 2. Since foundations may bear in the medium dense silty sands above the till, the glacial till or the underlying sands and gravels, we recommend utilizing an allowable bearing pressure of 2500 psf, including both dead and live loads. A higher bearing pressure can certainly be justified in the till or gravels, but this conservative value will allow foundations to be founded in any of the three predominant soil types. No foundation should be founded in or above topsoil or any of the existing fill located on the downslope shoulders of the roadway. 3. Ideally, all existing fill materials and topsoil in the building areas should be removed and replaced with compacted structural fill. On slopes steeper than 5H:1 V, the compacted fill should be keyed into firm native soil and compacted in 8-inch lifts to at least 90% of ASTM:D-1557. It should be noted that although the site soils are granular, they may be difficult or impossible to compact when more than a few percent above their "optimum moisture content". For this reason, earthwork should be scheduled for the dry summer months, if possible. An allowable bearing pressure of 2500/psf may also be used for compacted fill. 4. As an alternative to removing all existing fill, it would be possible to extend all foundations through the fill and any loose surficial soils beneath the fill and topsoil horizon. Footings should bear on medium dense to dense native soils. On sloping ground, footings should be buried a sufficient depth so that they are at least 3 feet horizontally from the slope face. 5. The proposed building area and the natural hillside immediately downslope from the building site are not in a landslide hazard area. The dense to very dense granular soils comprising the site are resistant to deep seated sliding. Isolated surficial sloughing can occur in years of extreme rainfall. This type of movement typically takes place in the colluvium or weathered soil horizon which overlies the underlying glacially compacted soil. 6. It is our professional opinion that the proposed building site has not been subject to landslide activity in the past nor will it be after construction of the residence. Similarly, the proposed building will not decrease slope stability on this or adjacent properties. 7. Due to the granular nature of the soils beneath the cap of glacial till, surface water runoff due to construction of the residence will not be a problem. We would recommend that roof drains be tight-lined well downslope into a natural drainage corridor to avoid saturating the surficial soils in the vicinity of the residence. P. O. Box 5420 Lynnwood, Washington 98046 (425) 673-4625 Iiirrl<athryn Boyker, Charles Thrasher July 14, 1998 Page Four We appreciate this opportunity to be of service. Should you have further questions or require any additional information,please do not hesitate to call. Respectfully submitted, ZIPPER-ZEMAN & ASSOCIATES,IN • Alvin R. Zem ,P.E. szi P. O. Box 5420 Lynnwood, Washington 98046 (425) 6?3-4625 o • Al V A m ''<a m w o -1 m - m < m y. sn1 a o 3 ;a cn o c 5'-'4 o c 73 _ co of rnn o r m v 'E V. m on > -n •n o " '' a O •ti. mx y 0 7) $ g m -a m m co .. Z Y 'n y fA P M o w A C / ...,•, to C) ."'b > m • v z o m gi 1-!: n 01 ";'. o ^1 p [n (..1 C > v Ct '. 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