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HomeMy WebLinkAbout935100008 Geotech Assessment (1997) AddendumAchievement by Design File: FSJ395JRL/kkh March 18, 1998 . . Russell Ferguson Kitsap Tire 20245 Viking Way Poulsbo, WA 98370 Dear Mr. Ferguson; On May 15, 1997, I accompanied you to your property at 851 Thorndyke Road near South Point in Jefferson County 'for the .purpose of inspecting .a debris slide that OcCUrred on _the east side of your property. The top' of the slide came ~within~ .!0 ~._gf...the east Wall6f-tl~"~0use. The'house was not damaged, however. During the inspection I viewed the house, foundation, slope, and Climbed down a portion of the slope. I probed under the footing and into the soil on the slope with a % inch diameter steel rod to estimate soil type, relative density, and relative moisture of the soil. ADA Engineering of Poulsbo performed a topographic survey of the-property and slide area. ! prepared a design for stabilizing the house foundation by underpinning and stabilizing the' upper part of the slope by lowering the 'grade and providing a rock wall keyed into the very dense sand. In their review comments, the County requested more definitive soils information; so Allen Hart, Engineering Geologist, was hired to provide a boring and geotechnical inspection of the site. The county also asked for this geotechnical engineering report containing information on the site geology, landslide mechanisms and potential, proposed repair, effect of earthquakes on house and wall after repair and storm drainage conditions. The site is described in Mr. Hart's report (attached).. Th°rndyke Road slopes down to the north' and your lot Slopes down to .-the east. -The' front (West) yard is nearly flat,' sloping at a-rate of 8 (horizontal): I (vertical). The slope steepens at the back (east) of the house to'a rate of 1.3(H):l(V).- The slope extends eastward another 120 (+/-) feet from your east property line-at an average rate of 1-.8(H):1(V). Then it flattens.out to a rate of 6.1(H):l(V). The USGS quadrangle map indicates that the approximate elevation' of Thorndyke Road'at-the site is 340 ft (mean sea level datum). .. The west yard of your property is vegetated'with lawn, shrubs, and trees. The east yard had almost no vegetation in the slide scarp, .but alongside of the scarp it was vegetated With ivy, blackberry bushes, shrubs, and small trees. 'You had covered the area with straw to control erosion. .. ART ANDERSON ASSOCIATES 148 FIRST STREET, BREMERTON, WA 98337-1899 (360) 479-5600 FAX (360) 479-5605 File: FSJ395JRL/kkh March 18, 1998 Page 2 of 5 At the time of my visit, the soils down the slide scarp were quite visible. I climbed down to the scarp face and probed the soil. It was very dense fine sand and was uniform from near the top of the slide to the property line. The original intent of the drilling program was to advance a hole near the foundation underpinning, and to advance a second hole near the rock wall. The soils appear quite uniform so one hole in each location should be sufficient to properly characterize the soils design purposes. The drilling crew felt it was unsafe to work on the steep slope without the benefit of a benCh cut into it; so they extended the upper hole to the elevation that the base of the second hole was extended to. This procedure revealed that the soil is quite uniform with the depth as well as area. The gray Very dense sand found in the hole is very similar to the gray very dense sand found on the surface of the scarp near the location of the wall. The brown medium dense silty sand near the top of the hole is very similar to the soils exposed along the edges of the slide scarp. This slope is similar to many others that I have seen in eastern Jefferson County and Kitsap County. Over the long term (more than 100 years) the very dense material is stable. The medium dense upper layer on the slope comes from weathering of the very dense layer by the action of wetting, drying, freezing, and thawing. The mediUm dense material is stable over the long term on a 2(H):I(V) slope if it has good drainage and a good vegetative cover, but on a 1.2(H):1(V) slope it is only stable for the short term. Over time the thickness of the medium dense layer builds up and its weight increases. During an exceedingly wet season the friction at the base of the medium dense layer is reduced to the point that the weight of the layer is enough to tear the root system on the perimeter of the slide and the layer flows down the slope. Since these types of slides are fairly likely to occur at least once during the life of the a house; it is prudent to construct the house so that it is protected from damage due to slides that may occur, in the case of your house the primary defense is to extend the footings to dense/very dense material that will not be included in the slide. In your case, the east wall footings appear to be founded on medium dense sand, a portion of which may be fill. To extend the footings to the dense/very dense layers (N >_ 30 blows/ft) the medium dense sand will be removed in blocks and replaced by cast-in-place blocks of non-reinforced concrete. The bearing pressure on the footing is calculated to be approximately 1500 Ib/sq ft, including the weight of the concrete. The allowable bearing value of the very dense gravelly sand is 3,000 Ib/sq.ft. Even if the medium dense material were to slide right up to the edge of the house, the footings would be on cOmpetant material. A retaining wall is proposed to be I°cated approximately 20 ft. east of the house. Its purpose is to reduce the rate of weathering for the dense to very dense Sand which supports the house. The secondary purpose is to provide a durable upper edge for the medium dense soil which did not slide. File: FSJ395JRL/kkh March 18, 1998 Page 3 of 5 The wall is designed of rock using the NCMA procedure for segmented retaining walls. The wall is of constant thickness to assure internal stability of wall elements. It is set 4.5 ft. into the dense/very dense sand to assure proper base support. The I(H):4(V) batter of the wall puts the reaction inside of the center of the base, and the base bearing pressure will be approximately 2400~/S.F. The combination of batter and embedment puts the factor of wall safety of base sliding at 2.58 which exceeds the 1.5 minimum. A layer of 10 mil polYethylene film will be placed under the base rocks to separate water that enters the wall drainage system from the very dense gravely sand. This will provide protection against weathering of the material upon which the base rocks rest. Under the polyethylene and bp the back face of the slope will be placed permeable, nonwoven filter fabric so that if there are water Seeps through the very dense layer, they will not be able to wash materials into the voids of the spall backfill. · To' further reduce the pressure on the slope, all medium dense soil eaSt of' tl~e foundation will be removed. This will necessitate extending the deck support posts down to the very dense gravely sand and will require removal of the gardening shed under the north end of the deck. At a later time a deck at the basement level may be constructed to provide basement floor level access on the east side of the house. Design of the deck is not within our scope of'work. A wedge of open graded crushed rock will be placed above the rock wall at a slope of 1.5(H):1(V), and the area above the crushed rock wedge will be sloped to 2(H):I(V), which is stable for the weathered soil. One of the major contributors of the surface slide was the uncontrolled flow of water from the south and west on Thorndyke Road. The CoUnty has since improved the drainage on the west side of Thorndyke Road to keep it from crossing to the east side. A problem that still exists is uncontrolled water from the east side of the road. The high point of Thorndyke Road is located at EagleView Lane to the south. A roadside ditch is provided t° prevent road runoff from runr~ing east across lawns as it naturally did prior to the road. Lot 8 just south of yours has not been developed and no roadside ditch exists there; so all of the east side roadway water crosses that lot and ends up in the slide area. Since a good deal of the water that crosses lot 8 is water that came from south of lot 8 and would have naturally flowed east across lots east of lot 8 prior to conStruction of Thorndyke Road, it would seem appropriate that the roadway runoff be channeled further north along the road right of way until a public outfall to the east is reached, and that this work be done at County expense. The roof water that comes from your lot will be collected in 4 in.'.diameter tight lines an~d routed to the storm drainage outfall pipe at the retaining wall. The roof drainage quantity for the 100 year storm is estimated to be 0.12cfs. Water that flows overland from your lot and from lot 8 will reach the crushed rock fill behind the rock wall and percolate into the wall drainage system and to the storm drainage out"fall. The outfall File: FSJ395JRL/kkh March 18, 1998 Page 4 of 5 will extend along the south edge of the slide and routed to a level spreader where the flow will be converted to sheet flow and allowed to flow over the wooded slope at is naturally would. The outfall flow of 1.15cfs including Thorndyke Road flow would be routed in a 6 in. diameter pipe. Erosion control for the slide area above the wall will be provided by crushed rock. Bel°w the rock wall you already spread straw and vegetation is reestablishing. There presently is erosion damage occurring in the southwest corner of the slide zone due to the water flows coming from lot 8. The proposed construction will stop this source of erosion. The rest of the slide zone appears to be holding up well. If you wish to supplement the down-slope vegetation that is now naturally occurring, there are two Washington State Department of Ecology publications, nos. 93-30 and 93-31 that address the best types of plants for controlling erosion on slopes. These publications are available from the County Cooperative Extension Office. Prior to beginning construction, a 'silt fence should be installed just down slope of the wall construction. The work should be done during dry weather and exposed soil should be covered if rain is expected. Th"e silt fenCe should be removed when the work has been completed. This report has been prepared for the exclusive use of Russell Ferguson and his agents for use in this specific project. The conclusions and recommendations in this report are based on my visual observations of the site as it existed at the time of the field investigation. The data and report may be used for preparing bids or estimates, but the report conclusions and interpretations should not be construed as a warranty of the subsurface conditions between points of probing and at different times of year. There are Possible variations in subsurface conditions. If soil and/or slope conditions other than those in this report are encountered or suspected, we should be contacted to review the encOuntered conditions and provide additional recommendations as may be required. The scope of this report does not include services related to construction safety precautions and these recommendations are not' intended to direct the contract°r's methods, techniques, sequences or procedures except as specifically described in this report for consideration in design. Within the limitations of scope, schedule and budget for this work, it is warranted that the work has been done in accordance with generally accepted practices followed in this area at the time this report was made. No other warranty, expresSed or implied is ~. made. File: FSJ395JRL/kkh March 18,. 1998 Page 5 of 5 o. If you have fUrther questions, .please contact me. J)~mes R: Levey,_ Ph..D.~P.~~~~ ,Senior Civil Engineer .~~~~~.,/~,~ 'EnclOsures . .. . .