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Home My WebLink AboutBLD2009-00004 GEOTECHNICAL REPORT i • �i -r:y"4� s 00 '' k_ • `,, ua� r j iJ . c '.sift I , . l,. V s I Z�-'� Jr t _�,,,.-��_ _ 'T.kt 17�:: _ K c,,,r_ .r.i, 4 ct j ._� l7-- ' t � „i"-Till_ "lam'-` , ' + ••": �- ` i la r >P '!� ti •r,r, • t f tsift 4� ,1.•'�}� ;- iii r;� � 1 i i vw � tip.' •' r ( ' 'r 41 1, 1��?p0[jI.N r ,� r r _ t — r It : :.- 7:-" Subject '_ I + i', , " _ r Property r .0 �,1, 1 i -- -_,_ '- . 4 Ilk 2 .- { 3 I _ � � i� i I t �, t 4 :e rp - I"ems -.4,- -7 -••:-.'AL .-,=— = i. *`.. F f ti t t � Max- am bi 6e:�vm c.r.,Co*y sc,.,,, .$. �� _ ri G..>..v...�... _.>-_,:�<.: ,.��_�._� k LANDSLIDE EVALUATION FOR EMERGENCY REPAIRS Prepared For the Schindler Residence April 16, 2008 For the Property Located at 270 Rhododendron Lane Brinnon, WA Section 15, Township 25 North, Range 2 West, W.M. Jefferson County, Washington Prepared by NTI Engineering and Surveying 717 S. Peabody Street Port Angeles, Washington 98362 Phone 360-452-8491 F 360-452-8498 Web Site www.riti4u.com=ax E-mail info@nl.i4u.com • s . - . Landslide Evaluation for Emergency Repairs 270 Rhododendron Lane, Brinnon, WA April 16, 2008 Bud & Val Schindler 270 Rhododendron Lane Brinnon, WA 98320-9737 Subject: Landslide Evaluation for Emergency Repairs at 270 Rhododendron Lane, Brinnon, WA Dear Mr. & Mrs. Schindler: Background At your request, NTI Engineering and Surveying (NTI) conducted an evaluation of a landslide that occurred on the high marine bluff at your property during the winter of 2007-08. The purpose of this evaluation was to determine the cause of the landslide and to recommend emergency repair measures to improve the stability of the bluff and mitigate future landslides. The evaluation consisted of research of available literature and geologic maps of the area, a review of previous geotechnical reports for nearby properties completed by NTI and a site visit for visual observations made on April 2, 2008. Site Description The subject "high bank,1 waterfront property is located at 270 Rhododendron Lane off of Highway 101 south of Brinnon. The property overlooks Hood Canal to the east and is bounded on the north and south by residential property and on the west by Rhododendron Lane (Figure 1). 7: � 1F 1 t �+ 1 � aY.. 4 1. 4,ti 1 3F �� ti7 L ,t 4 r ICI Ii y.',. Subject f ', ? 3._ '', a Property r t . E. a; f 1 ^ tL is � F are, - � V I li4 < :2,. � 4� to Ei. rr-'T eily.s ,e,.. , _ ., ,i,„".;PI/ , .:te4 1-..;-''..,, tr*f, ,*,.,t d b ra i'. 4 T 1 ;ESN#' -'s ^... t._„fit-�___, {} r .tip L J -` % po!'t i~ • - Figure 1 M J]S VC.1Qy'bj:..L .Lb»:ytcta!Schms 615 - -• •.., .. _ . ,• 1 • • The house was built in 1983 and is located approximately 24' to 30' back from the edge of the bluff; however, the bluff is undermined by approximately a couple of feet or so. The bluff is approximately 50' high and has average slope angles ranging from 40 to 65 degrees from the horizontal (Figure 2). ,-.;.i,4,.,7.--' V' 42' ' ; -- , -=4,-44:-..1' ',.''1::''', 1:1- '''''''' ' it / -,,,-1-.-- - ,.-.:1,, . , ,,, 7,,,,if,,,z4,:#,,,,- ,--a't7 ..', ,'....4"C. -, ' i ..A1*-4::It* 47;-47 4'-— '--* ''';''''F's:'''' S,,,7 -'' '''.6:.$41Vir' '..- '' - ' ' '.. Ic.:44:14"e•,; Al P�441 , -7 ilf,*-14'''' ''' '' , , N''' . ,t' , ;',k' ii / " -0*,,skt4 4 i O , C N a 'T4,� Li i 8 f ., 1 fig -co, r3 r " .\ , . , ;:,,:--2,1sci, #610-, ,, Ar-1,-,fi*"74(.1,/ ,if-i': - ___I-----i'-'--, „,,--— ;1 U i ";f-f,ifg. ,";.:,1 ''''''\ le y y .'itt .�� t- ' Figure 2 ti —T-- TR ..' ' 4 , = liM�iadolll Je'e�n Co,•rlCcraf a'c Gl, .._ .__.JAY`' This slope angle is steeper than the typical "angle of repose". The typical "angle of repose" is defined as the maximum slope or angle at which loose, cohesionless material remains stable and commonly ranges between 33 and 37 degrees on natural slopes. The landslide involved almost the entire width of the property and was approximately three to five feet thick. It stripped almost all of the vegetation off of the bluff face from near the top of the bluff, to the beach (Photo 1). J� r ,1tG r, i f- 1 0' 4 '�, f cur 1 03 1 tiliir"VI V fly ‘,441\. .4''''''',1' • .1/,',$,A\. .:::lat,-N' 'V.;L.,,5-- 'it.'-'4!"-).,,,, - 4 , 1V-,',14'',,,'1G9991F At., ' tr:.,Vit./ ' , ::',.%,. V 'I,4,:t, *k C , 0(0 ri,","•,,,e4f-, , , " ' ,,-4 / ,4 ,psy ft,1.71:- ' ii-:c ..,,..;:i ,i ri: ,t, ,,,,il , , -Aiiiili:,t,' - 4 J' J d� ray J -, \r' • 4" i k—/ '•;�+Ih4 4. ,' Y ' ; is ( y J nQ Photo 1 2 • • The vegetation along the top of the bluff was spared but was undercut, and some of the trees near the toe of the bluff were spared. However, some of the trees that are still standing at the toe may have been carried down the bluff in the slide debris. Frequently, trees that have been disturbed like this will die due to root damage. The existing beach access stairs and landing at the beach were destroyed by the slide. No seeps or springs were observed on the bluff; however, erosional rills and gullies were present indicating that water has flowed over the top of the bluff. The exposed soil on the bluff face consisted of predominantly clean sand with occasional layers of sandy gravel, with a layer of till at the top of the bluff. The sandy soil raveled readily as the bluff face was traversed. Site Geology The Washington State Department of Ecology's Coastal Zone Atlas, 1978 maps the soils in the area of the subject property as Vashon till (Qvt) at the top of the slope and underlain by Vashon advance outwash (Qva). The till is described as a very compact mix of poorly sorted, nonstratified gravel, sand, silt and clay with occasional boulders. This soil is excellent for foundation stability, good for seismic stability, and stands in steep natural and cut slopes for long periods. It may ravel and spall by wetting and drying, and freezing and thawing. The advance outwash is described as well stratified, well sorted sandy pebble to cobble sized gravel. In some places, sand predominates. This soil is good for seismic stability and good to excellent for foundation stability, but may be poor on slopes that approach the angle of repose of the material. This soil is generally stable in slopes up to the angle of repose, and may stand in steeper slopes for short periods. The "angle of repose" is defined as the maximum slope or angle at which loose, cohesionless material remains stable, and commonly ranges between 33 and 37 degrees on natural slopes. The Department of Ecology's "Geology and Ground-Water Resources of Eastern Jefferson County, Washington" map of the soils in the area of the subject property is generally consistent with the Coastal Zone Atlas descriptions. According to the Soil Survey of Jefferson County Area, Washington (United States Department of Agriculture, 1975), the site surface soils are mapped as HoC — Hoodsport very gravelly sandy loam. This soil formed in glacial till, becomes very hard at a depth of 20 to 36 inches and has a perched water table above the hard pan during the rainy season. Visual observations made at the site are consistent with the above soil descriptions. Mechanics of Bluff Recession There are many forms of bluff recession that occur in the coastal regions of northwest Washington. Common processes include the erosion of the toe of the bluff by wave 3 • • action, and the resultant sloughing of upper bluff soils due to being undermined and over steepened. Landslides also frequently occur during the winter due to over saturation of the bluff soil. These processes seem to be occurring at the subject property and triggered the landslide. When waves attack the toe of an unprotected bluff, the lower bluff soils are eroded away. Eventually, this erosion will undermine and/or oversteepen the bluff to a point where the soil can no longer support itself at such a steep angle. Then the overlying bluff soils will slough off, depositing material at the toe of the bluff. This will have the effect of temporarily reducing the angle of the toe of the bluff to a more stable angle. The upper bluff may slide to a less steep and more stable angle, or it may stay at a relatively steep angle. The slide debris that gets deposited at the toe of the bluff will protect the toe from wave erosion until it is eventually eroded away by waves, and the bluff undermining process will start over again. Accelerated toe erosion occurs more often in the winter when the tides are generally higher, and the storm waves larger. Likewise, many of the upper bluff landslides that occur in our region happen in the winter or spring when the ground is saturated with water, and especially after heavy rainfall events. When the soil becomes saturated, there is a decrease in the cohesion between the soil grains and an increase in the pore-water pressure. This condition can trigger landslides and debris flows on slopes. Often, there will be an impermeable soil layer part way down the bluff, which prevents the downward migration of groundwater and causes the water to migrate laterally, exiting the bluff above the impermeable layer. Surface runoff flowing over the top of the bluff and down the face can also cause erosion and damage vegetation on the bluff. For this reason, it is important to control on-site drainage and runoff in order to minimize negative impacts to the bluff. It is also important to maintain vegetation on the bluff face, where possible, in order to reduce erosion of the bluff soils. Conclusions and Recommendations Based upon our evaluation, it is our conclusion that the slide was triggered by a combination of the processes described in the preceding section. Left unchecked, it is our opinion that periodic slides will continue, and eventually threaten the home. Future landslides may also devalue the property. In the interest of mitigating the effects of future slides and preserving the value of the property, we recommend action be taken to stabilize the bluff as much as possible. While it may be technically possible to stabilize the bluff to such a degree that future slides are all but eliminated, it may not economically feasible. This approach would likely entail a series of several retaining walls from the toe to near the top of the bluff. A more economical, and also correspondingly less effective approach, would be to install a retaining wall at the toe and maybe a second wall above the first (Figure 3, in Appendix). This coupled with drainage improvements at the top of the bluff and re- establishment of vegetation on the bluff would likely mitigate the erosional processes that are contributing to the slides and slow down the bluff recession. This work should 4 • • be considered an emergency repair with the goal of being completed prior to the next rainy season. It is our understanding that the neighboring property to the north experienced a similar landslide in December of 2003 when storm waves and higher than normal tides severely eroded the toe of the bluff. At some point prior to the slide, soft-type armoring consisting of logs anchored to the beach had been installed. This system failed to prevent the erosion and emergency repairs were performed which included the construction of a rock bulkhead to replace the logs. Toe Revetment: Hard-Type Recommended We recommend a hard type bulkhead such as concrete or rock for the bluff toe protection. If a second bulkhead in constructed above the first one, it would probably need to be a pile and lagging type wall with tieback anchors. The retaining walls could also be designed to provide support for the reconstructed beach access stairs. Retaining — Revetment Structure Criteria The actual design of the retaining wall is beyond the scope of this evaluation, but these guidelines should be followed: • The lower retaining wall should extend the entire width of the property or as far as economically possible and should be on the order of four to five feet high. • Ideally, the wall would be of such height as to prevent storm waves from overtopping the wall. • Rock should be of a large enough size to remain in place during the worst storm wave conditions (on the order of 4' diameter). • The ends of the wall should be keyed into the slope to resist wave erosion around the wall. The wall should also be buried deep enough to resist undermining (on the order of 4'-5'). • It would be best to have no gap between the new wall and the existing wall to the north. • The wall should be sufficiently chinked from behind with large granular material so that the backfill soil does not wash out through the wall. Filter fabric could be utilized between the native sandy soil and the coarse backfill to further reduce the potential for material to be washed through the wall. Any imported wall backfill material should be free draining granular material. • For concrete walls, proper wall drainage should be incorporated so that hydrostatic forces behind the wall do not develop or else the wall should be designed to handle such forces. NTI's Geotechnical and Structural Engineering group can design the retaining wall if desired. 5 • • Drainage Considerations With regards to drainage from above, there is an existing tightline drain at the north end of the property, but the roof drain at the southeast corner of the house runs to the top of the bluff and is discharged there. It is recommended that this drain and any other drains be additionally tightlined down the bluff to just above the ordinary high water mark, or they should be connected to the existing tightline drain. We also recommend that a shallow swale be constructed across the top of the bluff to intercept any surface runoff and prevent it from flowing over the top of the bluff. This water from this swale could also be routed to the existing tightline drain. If a curtain drain is installed in lieu of a swale, it should not be dug so deep as to penetrate through the till layer and into the underlying sand. If this occurred, the water would likely be transmitted downward through the pervious sand rather than be captured in the drain. The bluff face is currently almost completely devoid of vegetation. The bluff is steep and the soils are sandy. It is anticipated that establishing good vegetative cover on the bluff will be challenging. However, anything that can be done to promote vegetation growth on the bluff face should be attempted. Low growing, deep rooted drought tolerant species would be the best choice. Please see the DOE publications included with the original of this report for further information on slope stabilization using vegetation. Tree Removal Recommended There are two large fir trees at the top edge of the bluff near the north end of the property. Due to bluff erosion, these trees are severely undercut. If they fall over or are blown over in a windstorm, they would wrench the root base out of the soil and leave a large disturbed area removing several feet of bluff. The trees are also tall enough that they would hit the house if they fell towards it. We recommend that these trees be removed, ideally with the trunks left in place so that the roots can continue to provide support to the bluff soils until they rot. It would be best to have the trees removed from above and not let them fall down the bluff, as the damage to the remaining bluff vegetation would likely be considerable. 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; this reference is highly recommended. Limitations This report has been prepared for your exclusive use in conjunction with the above referenced project. The report has not been prepared for use by others or for other 6 • • locations. It may be used for other purposes only with the expressed written permission of the Engineer. Within the limits of scope, schedule and budget, this report was prepared in general accordance with accepted professional engineering and geological principles and practices in this or similar localities at the time the report was prepared. No other warranty, expressed or implied, is made as to the conclusions and professional advice included in this report either express or implied. The observations, conclusions and recommendations presented in this report were based on our visual observations of the subject property at the time of our site visit; no laboratory tests were performed. Soil and geologic conditions can vary significantly between test holes and/or surface outcrops. If there is a substantial lapse of time, conditions at the site have changed or appear different than those described in this report, we should be contacted and retained to evaluate the changed conditions and make modifications to our report if necessary. Sincerely, NTI Engineering and Surveying As,S. Lis..0i Steve S. Luxton, MSc. PE Civil Engineer IOAtAL I EXPIRES:31234!O _ 1 Bill Payton, L.E.G. a$fr;�, Engineering Geologist 4'`.3'ro` 147, ' ingi etrbo • v .v 'Cl 191 ,-• Ga0 ; William C. Payton Jr. B:\Reports\SCHY0801.slide evaluation.15(25-2).Black Point.Brinnon.doc 7 • • C Fb 1st suer SF-Con,6 ' ISAFILL Toe "NiiiitReve-rheArr • RI R 5KCTcH RR 1L-w.1rrzinor' Fk3SSS NoT fbi COh2iTRUCTION SCALE- S ► 5 T 25 R 2 I DATE: g/t s/oe er.>, NTI Engineering Surveying DESIGNED BY: �P PiAft � Northwestern Territories, Inc. PROJECT: 2 70 RA—o&p Q dv✓1 Lc `ra' �� - Engineers•Land Surveyors•Geologists•Materials Testing FOR: S Can‘r•l a er P.es1 denC e. NTI 717 SOUTH PEABODY•PORT ANGELES,WASHINGTON 98362•(360)4524491 1 www.nt14u.com info@ntl4a.com SHT: � OF 9