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HomeMy WebLinkAboutBLD2000-00672 Geotechnical Report U7/L7/UU 111U U6:16 NAA OU:S b6L 4040 V151UN 1t1At,H1NhKY lgf UU S APR— 7-98 TUE 15:06 John Lott FAX NO. 360'17943 F. 03 • • GEOTECHNICAL RECONNAISSANCE 1.0 Introduction On March 31, 1998, Steve S. .Luxton, MSc., P.E, completed a geotechnical reconnaissance of a 5-acre parcel owned by Mr. Bill Clevenger of Port Angeles, Washington. The parcel lies on the west side of the Boiton Peninsula:at the edge of Quiicene Bay, The site offers a splendid wilderness environment and secular ( views over Quilcene Bay and the Olympic Mountains beyond. At the time of the reconnaissance inspection, the parcel was offered for sale by Mr. Clevenger. As a condition of the sale, the prospective buyer asked that the parcel and mudslide'at the site be inspected by a geologic engineer. The work was intended to define the suitability of the site for permanent construction and to suggest actions to stabilize the site against further slides and to obliterate the slide scarp. An inspection of the site indicated that, although the slide scarp can be effectively backfilled, effort may be well spent in completing drainage improvements other ents that setback will reduce the chances of other slides taking place and provide an adequate for the proposed house at the site. 2.0 Geologic Setting The Bolton Peninsula is a uplifted and folded portion of the former sea floor that was formed into a ridge by the compression of the advancing crescent basalt ridges that may be seen on the west. The marine environment in the vicinity of the Clevenger parcel is a typical offshore shelf consisting of mud and silt interbedded with silty • sands, Over time the shelf consolidated under the weight of deep sediments, and eventually it was folded and uplifted into the present shape. Much later, after a long period of erosion, the Vashon glaciers advanced into the region. Massive lobes of glacial ice gouged the landscape in the vicinity forming the bays on either side of the siltstone ridge. At the close of the Vashon glaciation, the ice began to retreat from the flanks of the original siltstone ridge forming the Bolton Peninsula. As this occurred, rivers of glacial meltwater flowed along the flank of the ridge captured between the ice and the older terrain. The meltwater left deposits of gravely sand and silty gravelly sand that were observed in the upper part of the site. At intervals, the drainage ways were blocked by Ice and landslides, so the runoff created large lakes of gray silt-laden water that left layers of silt in the subsurface as it settled to the bottom of local lakes, Within the past 100 years or so, a relatively large-scale slide occurred at the site. The slide apparently formed the blocky prominence along the beach within the parcel Evidence at the marine outcrop shows jumbled deposits of siltstone. probably • 04/07/98 TUE 15:03 [TX/RX Na 89321 Q 003 07/Z7/U0 1HU U2S:lb IAA 5U3 b5Z 4640 VISION PIM111N1KY tWUU4 APR-- 7-98 TUE 16:07 John .olt FAX NO, 3607163 P, 04 • of the same age and composition as the Twin River Formation, that have been heavily sheared and fractured by the sliding action. The debris is now weathered and they form a low point that gives a nice aspect to the parcel, 3.0 Geotechnical Indicators and Observations at the Site Attention was drawn to a 40-foot by 25 foot wide mudslide scarp that remains visible on the slope'directly west of the proposed house, site at the parcel. The owner reported that the slide_ occurred during the New Year. storm of 1996-1997, a rare rainfall and ruhoff event of exceptional magnitude: Examination of the lower third of the slide showed that the slide had occurred across the' Twin River silstones as illustrated on the attached figure. • Small springs of water and seepage was observed along the base of the slide area where water was noted running on top of the gray weathered siltstone. Substantial flows of Storm runoff flowed down and across the road approaching the building site at the time of the mudslide. The slope of the earth above the slide indicates that at least some of the road runoff cascaded over the building site saturating the soil and helping to trigger the mudslide. Observations suggested that shallow groundwater is unable to drain vertically at the site. Thus, water seeks lateral routes and groundwater fevels rise, providing floatation and hydraulic pressure to the silty sands and gravels overlying the siltstone. Under exceptional circumstances, when rainfall is abundant, mudslides may be triggered such as the one which occurred below the house site. Mr. Clevenger indicated that the earthwork to form the prospective house site had been completed about 7 % years ago. During that'work, earth materials were dozed down the slope and used to fill the slope as shown on the attached drawings. Examination of the existing slide scarp indicated that most of the slide occurred in the fill material. Much of the fill remains in place. The over-steepening of the toe of the slope above the road may have also been a factor in destabilizing the earth above. Careful observation of the site suggested that the fill slopes that lie on the north and south of the mudslide scarp may fail in a manner similar to the previous mudslide. if such slides do occur, they will be unlikely to broach the weathered siltstone deposits, Rather, they are likely to cross over the siltstone as shown in the attached Drawing 1. The "most probable slide" envelope that traces the plane that future slides may take Is shown on the drawing, Examination of the geometry-of the slope and the location of the siltstone suggests that future slides of this type would be unlikely to remove more than 8 to 10 feet from the rim of the proposed building site in a single event. The occurrence of larger scale historic sliding within the mass of the siltstone suggests that future slides of this proportion are possible. Nevertheless, large scale sliding seems improbable due to the buttressing effect of the older slide debris that 2 04/07/98 TUE 15:03 [TX/RX NO 8932] Q 004 Ui/Z!/UU 1t1U U0:11 rAA JUJ DOG J040 v1JIUIN 1?1AUHhiNr:K1 L JUUJ APR- 7-98 TUE 15:08 John likott. FAX NO, 36077S3 P. 05 remains at the base of the slope. In addition, the redistribution of weight and mass that occurred during the slide has probably reduced the potential for further large- scale sliding. • 4.0 Site Engineering Recommendations Sitework may include backfilling the•mudslide sdarp and/or completing work at the site that would reduce the possibility of additional smaller scale mudslides or shallow block slides that might endanger the proposed building site. } 4.1 Backfilling the Mudslide Scarp ' . The soil above and behind the mudslide is unlikely to be the subject of further sliding but the scarp is unsightly at present. The mudslide scarp may be filled by excavating an 8-foot wide flat base area and loading the toe of the scarp with large blast rock as illustrated on Drawing #2. The blast rock toe-revetment should extend to the top of the road below. Above the rock, a free-draining ballast of sandy gravel may be placed in layers of about 2 feet. Each layer should be compacted and flattened. Place a road fabric over each 2-foot thick layer of gravel and fill another two feet. If the scarp is backfilled in this manner it will be substantially stronger than the adjacent fill materials, This work will require about 100 tons of revetment stone and about 100 yards of imported gravel backfill and about 2 'h days of heavy equipment time and some hand labor for an approximate cost of about $4500. 4.2 Drainage Improvements at the Road Leading to the House Site The chances of additional sliding at the west side of the proposed building site may be reduced by completing certain improvements to the drainage at the site. Runoff that follows the'road must be diverted away from the building site. Runoff could be • better controlled by the following actions; • • Pitch the road cross-slope to the north so that water flows off of the road to the • ditch on the north side • Improve the ditch,on the north side of the road so that it may carry all of the runoff collected in that area, • . • • Construct of a crossing culvert that would collect water captured on the south side of the road and transfer it across to the main ditch on the north, • Place a rock revetment in the ditch so that the velocity of runoff is slowed and further erosion is prevented, Pitching the road cross-slope to the north side ditch can be accomplished by importing some sandy gravel ballast, grading and compacting it. The ditch itself • S • 04/07/98 TUE 15:03 [TX/RX NO 8932] Lj 005 UY/l f/UU 111U U0:10 PICA ovo UO6 , O40 V 131HIV ITSHL111IV Hit 1 IQJ UUb ' APR- 7-98 TUE 15:09 John •ot t FAX CIO, 360779 93 P. 06 , • should be excavated.to a depth of 2.5 feet and an overall width of 7 feet. The,ditch should•be completely lined with-6 to 8-inch hand-placed rock blast spells. A•124rich concrete crossing culvert about 25,feet in. length should'be adequate to divert, the ' ' water that is captured on the south side of the road. • ,, • . • if it isperformed with attention to detail so,'tl the • _ - "This'work should:cost about ; �; - • . completed,job.is.both functional and pleasing to the eye. • •: . -4.3 .Dralnage.improvements at the House Site c: . - ,.,,,• • Trench Dra4n,.' • • '. . - ,. •- ;�. • , . • = ; Additional protection against slope instability may be obtained,by constructionof an interceptor trench drain along the'axis of'the.toe Of the.slope at the east side',of the • - prospective house site. The trench drain will-collect surface•water leaving the slope above the house site and capture some of the subsurface water that would''tend to destabilize the slopes west of the house site, The trench drain is illustrated on the ' • attached Drawing#2. .' . . . " The completed .trench drain would be about 100 feet in length and •it .can be constructed.far about $28QJ if included with other work that requires the use of a backhoe et the site and provided that the road Is repaired so that trucks may'access the site. Slope Toe Revetment and or Retaining Wall - o A 6 foot high retaining wall or rock revetment'will be needed at the toe of the:slope • •}sele-;x7. _.above the house site. The retaining wall 'will provide additional width to the.-pad so • 'that the house may be built with a prudent setback. The most width would be gained by construction of an engineered concrete retaining wall'about 6.feet in height leaving about 2 feet of freeboard behind the wall so that any small slips of earth are arrested. Such a retaining wall would allow a house width of not more than 30 feet with.a • prudent setback of about 35 to 40 feet. r ' A concrete retaining wall of this type would cost about.$550Q to.build.. Alternately, a large.rock'revetment could be placed against the toe of the slope as"illustrated on. . . Drawing r#2.- Such.'a revetment.•could be adequately constructed.for• 3 50o`'.'bul}:..the lock .will require more Width, _forcing tile• house footprint' a little closer:;. o•.,;the' escarpment. Alternatively; a retaining wall/daylight basement may be engineered -to' fit the site and to have sufficient strength to resist small slides from the sldpe�above the house pad, - • • 04/07/98 TUE 15:03 [TX/RX NO 88321 [j006 U//L1/UU LL1U UO:1a CAA UUJ OOL JOY ) V1J1V1V 1Y1HLri11VLKY WJUU, APR- 7-98 TUE 15:10 John •o t t FAX NO, 360779 93 r. t1• • 4.4 Drainage Improvements Below the House Site Water must be transported to the beach without causing erosion or further instability. Water from the ditch on the north side of the approach road may be routed to the beach in two ways described below: Restrained Drainage Tube • • Drainage from the ditch north .of the,road may be collected into a fabric hose or , "drainage tube that resembles a fire hose. A 12-inch diameter hose of this type anchored by a concrete wall embedded in the ground would.allow drainage to be directed over the slope along the northerly line of the parcel.. Such a device should be designed by an engineer and construction costs would be about $3800 for a properly detailed job that will give enduring service. Rock Revetment of Existing Ditch Along the Road to the Beach Drainage could continue to flow along the road to the beach although it is desirable to divert water away from the slopes that He below the house site, If drainage is to remain in its current alignment, the ditch should be deepened and rock rip raps should be placed along the ditch all the way to the beach to reduce future erosion. It would be best to avoid any widening of the road to the beach. Thus, if it is absolutely necessary to maintain the road for narrow rough terrain vehicles, then, drainage must be routed through a separate tube system. Completion of hand placed rip rap and improved ditching to the beach along the existing road should cost about $2200 for a carefully completed job that is attractive and functional_ 5.0 Summary and Conclusions • Gross Slope Stability Generally the property appears to have good prospects for continued gross stability. The buttressing effect of the existing landslide debris,below the proposed..house site is helpful in stabilizing the deeper siltstone against deep-seated slides.• The.historic slide has also redistributed the mass and weight of the slope so.that large,scale slides are less likely. Local Stability of the Proposed Building Site The proposed building site has been eroded by mudsiiding due to the presence of subsurface water and flooding with surface water during storms. The existing conditions, geometry of the building site and the presence of fills on the slope beyond the building site suggests that similar slides in adjacent areas could occur in the . • • 04/07/98 TUE 15:03 [TX/RX NO 8932] Ij007 U(/L(/UU inu vo.1.7 rnA UV) uoL O04O vialuj' atikmii'tcm L�UVC1 APR— 7-98 TUE 15: 10 John Elko" FAX NO, 36077943 1". Uti • future. Site planning should be carried out to provide at least 35 feet and preferably 40 feet of setback from the tine where the building site descends rapidly to the beach below. This can be achieved by an appropriate earth retaining structure behind the prospective house and by selecting a house plan that is relatively.narrow. The house may also be built with an integral daylight basement that is appropriately engineered. Site improvements Can Reduce the Risk of Sliding ' Several site improvements may be considered to reduce'the risk'of future•sliding.by controlling and directing'drainage. These' improvements include better,drainage along the 'approach .road, .ditch improvements and rip rap, - drainage ;tube construction, a trench drain along the east side of the building site and the addition . of a culvert to direct water away from the house site. These improvements are expected to cost a total of about$12,000. Retaining Structure Behind the Prospective House Suggested • In order to achieve the desirable setback it Will be necessary to situate the house well back on the lot. This is achievable at the site if some retaining of earth is done on the east side of the building site. Costs of this may vary from $3500 to $5500. Mudslide Scarp Can be Repaired Although the existing mudslide scarp is unlikely to be the site of additional sliding, the scarp is unsightly at present. The mudslide scarp may be filled beginning with a large rock toe revetment and finishing with free-draining sandy gravel fills with fabric reinforcements. Access to the slide is difficult and the work is expected to cost about $4500. This work enhances the appearance of the site but it does nothing to enhance the future stability of the home site. • . 6.0 Limitations • This report and the conclusions herein were based upon a reconnaissance.inspection . • of the subject parcel. Although much can be inferred from inspection of surface • conditions, the subsurface cannot be completely• understood without borings at frequent intervals, more detailed mapping of the surface and laboratory testing of soil samples. Users who require a higher degree of reliance on the investigations and conclusions of the report are encouraged to obtain such studies. Unless construction plans are reviewed and the construction work is inspected by the Engineer, no warranty is made by the Engineer concerning conformity of the construction to the conclusions and recommendations of this report. 6 04/07/98 TUE 15:03 [TX/RX NO 8932] C{+~j008 0i/Lr/UV inu Uo.Lv FAA )U) ooa oo4a) V1J1U1N ❑Iaunl1Nnxcx IZJOU ' APR- 7-98 TUE 15:11 John •o k t. FAX NO, 360779 93 i'• 09 - ' - This report;and its conclusions were prepared in 'accordance with the prirfaip►es of ,geologic science and 'standards, of professional erigineerin {-practice. No other • werranty►, either express or implied,-it provided herewith" 1 " :'' Steve.S."Luicton;'.MSc. P.E. • ...�,' = r� �f r., � ls. • .'Prtifessiciriai Engineer - . • i�� ,•'eta ;~ r*gyp : y: • :t.R 1: Newlin, P.E.' T + rs '• :I • • Principal Engineer ' SSUJRN::eas • • • • 04/07/98 TUE 15:03 [TX/EX NO 8932] iJ009 07/Z//M.' inu uo:zu riu. au., JJ.J <5040 VISIUN MAUHINEMY cult; 7-98 TUE 15: 12 John IOW. 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AclArOk/ 7E Sire Af sor-t5NT -` Engineers.Land Surveyors.Planners • ECT: - �1 _ - 0/LA C her 1.4tiVerSir ConstrucUOA coordination.AMRrerla/s Teeing j��� 711SOUTHPEASODY•PORTANGELES,WASHINGTON MIS■(7SOI 4$24411 r OF 7- 04/07/98 TUE 15:03 (TX/RX NO 89321 [7J011 leIVVG OVI v. Vv_ ":" iv __ "" r.;,-.... o.ia ,.... r lulu -giWt uasi la001 trit bI' *.lit �URYEY { • , -,—.444, •e , NORTHWESTERN TERRITORIES; INC, . • TIT SOUTH PEaBODY Si Heel,PORTAK6tlLts,WA .. . - Engineers■Land Surveyors I Plerrnas �� . Donstructkan Coo!dlnatiar R Mrten'sis'Tes4,9 (300)452- 491 1800-654-5545 FM 462-0s88 EPAra n 4u.00m . . . lYwwAWA.CeAt . . . • • • . • . . .. ...: • . . . , .. .. .. .. - . . . . . . .• . . . .. . . ... • • . : .. . .. . . . . ... . .• .. • . .. . . • • • .. . : • September 5,•2000 . .. . . •, • . . . .. . . ©ouglas Harbord. • 13150 South Macksburg Road Canby, OR 97013 • . • RE: •Stte'Improvements•at 4264 E.Quilcetre Rd. in Section 31, Township 27'North, - ' -Range 1 West, WM, Jefferson County,WA. • • Dear Mr. Harbard; • - This letter is in response to your request for a revision of our letter dated'August 18, -• 2000 and addressed to Mr. Tollefson, your representative. In a phone conversation .. • with you on September 1, 2000, you requested that we revise•the above mentioned • ' • • letter such that you could occupy the proposed home without completing the . . . • - improvements mentioned in sections 4.1 and 4.4 of the original report dated April, '1998. . • .The original report states in section 4.1 that the soil above and behind the mudslide • located on the slope west of the proposed house site is unlikely to be the subject'of : - ' • , .• '' further sliding. The site visit conducted on August 16, 2000 confirmed that no new • •- sliding had taken, place. The original report also states in section 1 that•aithough the ...' ••.- . •-, • slide scarp can be effectively backfiiled, effort may be well spent in completing drainage• •• • . i1s Ili'uvc!►ints and other cite enhancements that will reduce the chances of ether slides • • taking'place and provide an adequate setback for the proposed house at'the site: - • '•-• Improvements to the drainage above the proposed homesite have already been'made• ° *and the trench drain and retaining wall mentioned in section 4.3 of the original report will be completed prior to placement of the proposed manufactured home on the - ; subject site. • •'In regards to the drainage ditch mentioned in section 4.4 of the original report;'it . . ' ' - appears from the site visit conducted.on August 16,.2000 that the improvements-made.- • • thus far have stabilized the erosion of this area and that further improvements may not • • 'be necessary•unless further erosion takes place_ •. . 09/06/00 WED 07:53 [TX/RX NO 9186] 4D 001 V.7/V V/V V +e siar iv.v a�.a v� •a.aavi, larIvaasi LIAL ___ vvv •••�••' 1�11 t11�I lt,B .`iUl[Yr.,Y t002 • • • • ... Douglas Harbord ' ' -Site Improvement• • September 5, 2000 Page 2 of 2 • • ' Based on the above discussion. we therefore•deem it acxevtable to.occueV the . ' •• • •proposed dwelling-without completing the modifications mentioned in sections 4.1'and , 4:4 of the original report unless future erosion of these areas warrants such ••• • improvomont.I. Limitations: This letter is based-on a visual irisOeclien of the erxisltriy site conditions, barring major • • - catastrophic events such'as earthquakes•or abnormal weather conditions_ No scientific .' •. . -measurements, tests or calculations were performed. The findings and letter are limited: . ' ' ' to the normal standard of care of the industry for an investigation without benefit of ' scientific data and calculations. Northwestern Territories; in and/or its raproventatives' . • • .assume no responsibility for loss or damage. • • The sole objective of.this Investigation has been to diagnose the geotechriical' •' • • • -• 'ruridiGuus at the site. The letter lies been piepaie d WI Mitt ttxolusiver•use UI Douglas -• 1 1orbzrd (ownar)was/or Tollallai-ic 0vtletrYc4lvn (L-uildaa.r). It rno7• 4.. aa.suJ kr,-o111uro c+r+i;y• • ' With expressed written permission of the Engineer. The report has not been prepared for use by others or other uses. The observations, interpretations, and conclusions herein are based on generally accepted professional engineering and geologic • principles and practice. This warranty is in lieu of all others, either expressed or implied. Sincerely, NORTHWESTERN TER RITORIE S,..INC, •... .•• . i.. 1ofwist84i'; • -;-. •• y-• s '1 . S•yc. i r.a' -)4- - 1 f 4 a • ' J. R. Jerry Newlin,.P.E. �ars�B�v1 '. Principal Engineer sic , • »aarss'• ���mz _ 7 .- . ' . ... . . . P• • . Bill Payton, Geologist ' • • • • . • 4+0.4.vanwirnrou000,o.,,c2,,,pe • • • 09/06/00 WED 07:53 (TX/Ra NO 91861 4b002 • • GEOTECHNICAL RECONNAISSANCE 1.0 introduction On March 31, 1998, Steve S. Luxton, MSc., P.E. completed a geoterhnical reconnaissance of a 5-acre parcel owned by Mr. Bill Clevenger of Port Angeles, Washington. The parcel lies on the west side of the Bolton Peninsula,at the edge of Quilcene Bay, The site offers a splendid wilderness environment and spectacular views over Ouilcene Bay and the Olympic Mountains beyond. At the time of the reconnaissance inspection, the parcel was offered for sale by Mr. Clevenger. As a condition of the sale, the prospective buyer asked that the parcel and mudslide at the site be inspected by a geologic engineer. The work was intended to define the suitability of the site for permanent construction and to suggest actions to stabilize the site against further slides and to obliterate the slide scarp. An inspection of the site indicated that, although the slide scarp can be effectively backfilled, effort may be well spent in completing drainage improvements and other site enhancements that will reduce the chances of other slides taking place and provide an adequate setback for the proposed house at the site. 2.0 Geologic Setting The Bolton Peninsula is a uplifted and folded portion of the former sea floor that was formed into a ridge by the compression of the advancing crescent basalt ridges that may be seen on the west. The marine environment in the vicinity of the Clevenger parcel is a typical offshore shelf consisting of mud and silt interbedded with silty sands, Over time the shelf consolidated under the weight of deep sediments, and eventually it was folded and uplifted into the present shape. Much later, after a long period of erosion, the Vashon glaciers advanced into the region. Massive lobes of glacial ice gouged the landscape in the.vicinity forming the bays on either side of the siltstone ridge. At the close of the Vashon glaciation, the ice began to retreat from the flanks of the original siltstone ridge forming the Bolton Peninsula. As this occurred, rivers of glacial meltwater flowed along the flank of the ridge captured between the ice and the older terrain. The meltwater left deposits of gravely sand and silty gravelly sand that were observed in the upper part of the site. At intervals, the drainage ways were blocked by ice and landslides, so the runoff created large lakes of gray silt-laden water that left layers of silt in the subsurface as it settled to the bottom of local lakes. Within the past 100 years or so, a relatively large-scale slide occurred at the site. The slide apparently formed the blocky prominence along the beach within the parcel. Evidence at the marine outcrop shows jumbled deposits of siltstone, probably 1 04/07/98 TUE 15:03 [TX/RX NO 89321 4P003 10/29/99 OL(O ions L ScuLt FP1X NU �buir�oc�� e same age and composition as the Twin River Formation, that have been vily sheared and fractured by the sliding action. The debris is now weathered and y form a low point that gives a nice aspect to the parcel, 3.0 Geotechnical Indicators and Observations at the Site Attention was drawn to a 40-foot by 25 foot wide mudslide scarp that remains visible on the slope'directly west of the proposed house.-site at the parcel. The owner reported that the slide occurred during the New Year, storm of 1996-1997, a rare rainfall and runoff event of exceptional magnitude' roExa is inate o nn ot the herl s ie r third as of the slide showed that the slide had occurred illustrated on the attached figure. SmallV . springs of water and seepage was observed along the base of the slide area tial where water was noted running on top of �9he weathered asiltstone.he building nsite fiows of storm runoff flowed down a slide at the time of the mudslide. The slope o the e wid above teesaturat saturating indicates hte soil that a d least some of the road runoff cascade helping to trigger the mudslide. Observations suggested that shallow groundwater is unable to drain vertically at the site. Thus, water seeks lateral routes ands9and gravtelser loveriymg rise, the si�st°n$ing OeUnder tation and hydraulic pressure to the silty exceptional circumstances, when rainfall is house absitundant, mudslides may be triggered such as the one which occurred below the Mr. Clevenger indicated that the earthwork to form the rk prospective materials house us e site te eohad ad been completed about 7 % years ago. During that wo down the slope and used to fill slope as shown most of the sade occurred in the Examination of the existing slide scarp indicated that fill material. Much of the fill remains in lace.a factoren destabilizeing ning the earth of the tabovethe slope above the road may have also he Careful observation of the site suggested hat the fmlila�Po the previouses that lie on tmudslideand south of the mudslide scarp may fail in aann r s . If such slides do occur, they will be unlikely to broach the weathered siltstone deposits, Rather, they are likely to cross over the siltstone as shown in the attached Drawing 1. The "most probable slide" envelope that traces the plane that future slides may take is shown on the drawing. Examination of the geometry .ofotthee slope ope nand thy e lorecation of the siitstone suggests that future slides of this type more than 8 to 10 feet from the rim of the proposed building site in a single event. The occurrence of larger scale historic sliding within the mass of the siltstone suggests that future slides of this proportion are possible. Nevertheless, large scale sliding seems improbable due to the buttressing effect of the older slide debris that 2 04/07/98 TUE 15:03 [TX/RX NO 8932] 2004 503 OO .,ten., . 1Jt ) .nt' ni.vnni John FAX FAX NO, 3607796293 P. 05 • > r n, the on of ht mass !ns at the base of a slideo has lrrobabt o reduceddihar'pfential forigurtherand occurred during th p Y large- ale sliding. 4.0 Site Engineering Recommendations • Sitework may include backfilling the•mudsllde scarp and/or completing work at the site that would reduce the possibility of additional smaller scale mudslides or shallow block slides that might endanger the proposed building site. • 4.1 Backfilling the Mudslide Scarp The soil above and behind the mudslide is unlikely to be the subject of further sliding but the scarp is unsightly at present. The mudslide scarp may be filled by excavating an 8-foot wide flat base area and loading the toe of the scarp with large blast rock as illustrated on Drawing #2. The blast rock toe.revetment should extend to the top of the road below. Above the rock, a free-draining ballast of sandy gravel may be placed in layers of about 2 feet. Each layer should be compacted and flattened. Place a road fabric over each 2-foot thick layer of gravel and fill another two feet. if the scarp is backfilled in this manner it will be substantially stronger than the adjacent fill materials, This work will require about 100 tons of revetment stone and about 100 yards of imported gravel backfill and about 2 'h days of heavy equipment time and some hand labor for an approximate cost of about $4500. r-- 4.2 Drainage improvements at the Road Leading to the House Site The chances of additional sliding at the west side of the proposed building site may be reduced by completing certain improvements to the drainage at the site. Runoff that follows the road must be diverted away from the building site. Runoff could be better controlled by the following actions; • • Pitch the road cross-slope to the north so that water flows off of the road to the ditch on the north side • Improve the ditch.on the north side of the road so that it may carry all of the runoff collected in that area, • Construct of a crossing culvert that would collect water captured on the south side of the road and transfer it across to the main ditch on the north, • Place a rock revetment in the ditch so that the velocity of runoff is slowed and further erosion is prevented, Pitching the road cross-slope to the north side ditch can be accomplished by importing some sandy gravel ballast, grading and compacting it. The ditch itself 3 04/07/98 TUE 15:03 (TX/RX NO 89321 gl005 `$ I.A, A JUJ UOG J04J V1J1UIv :121l,11l `I1;1•1 ca��v 09 John cotl. FAX NO. 3607796293 P, 06 V ould be,excavated.to a depth of 2.5 feet and an overall width of 7 f e t A e1 dctch ould•be completely lined with-6 to 8-inch'hand-placed rock bt spncrete crossing culvert about 25 feet in. lenth should be adequate to diert,'the ter that is captured on the south side of the road. .z - This'work should cost about $ 0 if It is performed with attention to detail so ttiat'the ' completed job is bath functic totheee_na( and pleasingY . -4.3 .Dirainage Improvements at the House Site ..:,: - •• : ,i,: " ` . ' Trench brain: S . • '. - . Additional protection against slope instability may be obtained,by construction:of:an interceptor trench drain along the axis of the toe of the.slope at the east side,of the prospective house site. The trench drain will colleat surface'water leaving would'tend te above the-house site and capture some of the subsurface water that destabilize the slopes west of the house site. The trench drain is illustrated on the attached Drawing#2. The corhpleted trench drain would be about 100 feet in length and it..can be constructed far about $28 0 if included with other work that requires the use of a backhoe at the site and provided that the road is repaired so that trucks may-access the site. • Slope Toe Revetment and or Retaining Wall A 6 foot high retaining wall or rock revetment`will be needed at the toe of the_slope 6. : __above the house site. The retaining wall will provide additional width to the.pad so that the house may be built with a prudent setback. The most width would be•gained by construction of an engineered concrete retaining wall'about 6 feet in height leaving about 2 feet of freeboard behind the wall so that any small slips of earth are arrested. Such a retaining wall would allow a house width of not more than 30 feet with a • prudent setback of about 35 to 40 feet. A concrete retaining wall of this type would cost about,$550Q to build. Alternately, a large rock revetment could be placed against the toe of'the slope as.illustrated on . Drawing,#2, Such 'a revetment:could be adequately constructed_for•$3 00 `but,..the rock .will require more 'width, .forcing the house footprint a -little closer,;to•.;the' '' escarpment. Alternatively; a retaining wall/daylight basement may.be engineered to fit the site and to have sufficient strength to resist smell slides from the slope above the house pad. • tit, 04/07/98 TUE 15:03 [TX/RX NO 89321 4006 SOS 882 3845 vtStUN MACHINERY UUr bw John 410F LL FAX NO. 3607796293 N. U r • lnage Improvements Below the House Site r must be transported to the beach without causing erosion or further instability. or from the ditch on the north side of the approach road may be routed to the ach in two ways described below: .• Restrained Drainage Tube • Drainage from the ditch north of the,road may be collected into a fabric hose or drainage tube that resembles a fire hose. A 12-inch diameter hose of this type anchored by a concrete wait embedded in the ground would'allow drainage to be directed over the slope along the northerly line of the parcel. Such a device should be designed by an engineer and construction costs would be about $3800 for a properly detailed job that will give enduring service. • Rock Revetment of Existing Ditch Along the Road to the Beach Drainage could continue to flow along the road to the beach although it is desirable to divert water away from the slopes that He below the house site, If drainage is to remain in its current alignment, the ditch should be deepened and rock rip raps should be placed along the ditch all the way to the beach to reduce future erosion. It would be best to avoid any widening of the road to the beach. Thus, if it is absolutely necessary to maintain the road for narrow rough terrain vehicles, then, drainage must be routed through a separate tube system. Completion of hand placed rip rap and improved ditching to the beach along the existing road should cost about $2200 for a carefully completed job that is attractive and functional. 5.0 Summary and Conclusions Gross Slope Stability Generally the property appears to have good prospects for continued gross stability. The buttressing effect of the existing landslide debris below the proposed..house site is helpful in stabilizing the deeper siltstone against deep-seated slides_• The. historic . slide has also redistributed the mass and weight of the slope so'that large scale slides are less likely. • .. Local Stability of the Proposed Building Site The proposed building site has been eroded by mudsiiding due to the presence of subsurface water and flooding with surface water during storms. The existing conditions, geometry of the building site and the presence of fills on the slope beyond the building site suggests that similar slides in adjacent areas could occur in the 5 . 04/07/98 TUE 15:03 [TX/RX NO 8932] Ia007 9 FAX 503 682 3845 VISION MACHINERY 2008 10 Joe Scott FAX NO, 3601/293 r, Uti future. Site planning should be carried out to provide at least 35 feet and preferably 40 feet of setback from the line where the building site descends rapidly to the beach oft below. This can be achieved by an appropriate earth retaining structure behind the prospective house and by selecting a house plan that is relatively, narrow. The house may also be built with an integral daylight basement that is appropriately engineered. Site Improvements Can Reduce the Risk of Sliding Several site improvements may be considered to reduce the risk of future•sliding by controlling and directing.'drainage. These improvements include better, drainage along the approach road, ditch Improvements and rip rap, drainage tube construction, a trench drain along the east side of the building site and the addition of a culvert to direct water away from the house site. These improvements are expected to cost a total of about$12,000. Retaining Structure Behind the Prospective House Suggested In order to achieve the desirable setback it will be necessary to situate the house well back on the lot. This is achievable at the site if some retaining of earth is done on the east side of the building site Costs of this may vary from $3500 to $5500. Mudslide Scarp Can be Repaired Although the existing mudslide scarp is unlikely to be the site of additional sliding, the scarp Is unsightly at present. The mudslide scarp may be filled beginning with a large rock toe revetment and finishing with free-draining sandy gravel fills with fabric reinforcements. Access to the slide is difficult and the work is expected to cost about $4500. This work enhances the appearance of the site but it does nothing to enhance the future.stability of the home site. 6,0 Limitations This report and the conclusions herein were based upon a reconnaissance inspection of the subject parcel. Although much can be inferred from inspection of surface conditions, the subsurface cannot be completely understood without borings at frequent intervals, more detailed mapping of the surface and laboratory testing of soil N. samples. Users who require a higher degree of reliance on the investigations and conclusions of the report are encouraged to obtain such studies. Unless construction plans are reviewed and the construction work is inspected by the Engineer, no warranty is made by the Engineer concerning conformity of the construction to the conclusions and recommendations of this report. 6 04/07/98 TUE 15:03 [TX/RX NO 8932] 2008 .�••,• ,azkW'11VRRY J009 ,, t t FAX NO, 36077 293 t', Os • ,A ,r • ''.'its conclusions were prepared in accordance with the principles of . -- and 'standards of professional engineering `pr'ectice. No other Sher express or implied. is provided herewith. yours,• • ' r.'• • :- Sieve S...C4tan;` MSC. P.E ' '• x?/ =: .PrQfessiOnai Engineer - • • • �� ,/� erg-: -ry��:rc g •' j ' • :il.R: Newlin, P.E. IMPIRitiPrincipal Engineer i � �� . ' SSUJRN:eas •• = • • • • • • 04/07/98 TUE 15:03 [TX/RX NO 8932) Q 009 • NORTHWESTERN TERRITORIES, INC. A JLS GROUP COMPANY JAN 2 4 ' ,.t- 717 SOUTH PEABODY STREET,PORT ANGELES,WA 98362 ❑ Engineers ❑Land Surveyors o Geologists Jt-1=_;J-SON COUNTY N7' ❑ Construction Inspection ❑ Materials Testing �:-r fl LDtV[LOf°.. r j (360)452-8491 FAX 452-8498 www.nti4u.com E-Mail:info©nti4u.com JLS GROUP, INC. January 23, 2003 Douglas Harbord 13150 South Macksburg Road Canby, OR 97013 RE: Deck construction at 4264 E. Quilcene Rd. in Section 31, Township 27 North, Range 1 West, WM, Jefferson County, WA. Dear Mr. Harbord; This letter is in response to your request for a letter from us to the Jefferson County Department of Community Development regarding the reduction of the vegetative buffer at the above referenced property. A deck has been built adjacent to the home, which extends to within 21 feet of the top of the bank. This necessitates a reduction in the vegetative buffer from the standard 30 feet in order to accommodate the deck. On January 14, 2003, Bill Payton, Engineering Geologist with Northwestern Territories, Inc. (NTI) visited the site to observe the deck and the bank in order to determine the feasibility of reducing the buffer and any mitigating measures that may be useful. The deck is a very lightly loaded structure and we anticipate that the effect of its weight upon the soil will not decrease slope stability. Roof gutters and drains were noticed on the home. This water is collected and routed towards a creek south of the home via a plastic pipe. There is a very small ditch across the bench in front of the deck a few feet back from the top of the bank that probably collects surface water from the bench and keeps it from flowing down the face of the bank. Vegetation between the deck and the top of the bank is very sparse. The condition of the slope face appears relatively unchanged from the last site visit made on August 16, 2000. Based upon our site visit, we recommend that the vegetative buffer be reduced to 20 feet with the following recommendations: 1 . The drainage patterns on the bench should be monitored during a rainstorm to determine the effectiveness of the current drainage features at keeping surface water from flowing over the top of the slope. Improvements should be made if necessary. 2. Another option to reduce surface water on the bench if necessary, would be to construct a "false roof" under the deck and collect the runoff into the existing house drainage system. 3. The vegetative buffer should be planted in deep rooted native plants in order to reduce surface erosion. Any bare areas on the slope face should also be revegetated. 4. Heavy irrigation or other activities that would contribute large quantities of water to the soil should be avoided. One cause of landslides is the presence of excessive groundwater in the soil. 5. All drainage control devices should be maintained in good working order and inspected at least once a year. For further information please review the three pamphlets, which are enclosed with the original of this report, published by the Washington State Department of Ecology entitled: "Slope Stabilization and Erosion Control Using Vegetation", "Surface Water and Groundwater on Coastal Bluffs" and "Vegetation Management: A Guide for Puget Sound Bluff Property Owners". Limitations This report is based on a visual inspection of the existing site conditions. No scientific measurements, tests or calculations were performed. The findings and report are limited to the normal standard of care of the industry for an investigation without benefit of scientific data and calculations. The sole objective of this investigation has been to diagnose the geotechnical conditions at the site. The report has been prepared for the exclusive use of our client in conjunction with the above mentioned project. It may be used by others only with expressed written permission of the Engineer. The observations, interpretations, and conclusions herein are based on generally accepted professional engineering and geologic principles and practice. This warranty is in lieu of all others, either expressed or implied. cco A.Sincerely, O� of Nws q0, NORTHWESTERN TERRITORIES, INC. A. �0, r T'F 13772 p , O C/STER,% � L' 1 FSS�ONAL ENG\ A Robert A. Leach, P.E., MBA EXPIRES 12/30/2004 tl N CC Principal Engineer LL, � +mil C2 ,114 Bill Payton, L.E.G. Engineering Geologist *c� 8 191 G:\Gen\Bill\Letter\31(27-1)Harbord deck letter.doc FiV I i l d in C. Payton 2 Expires 11-06-03 • • SON co OG�� 4:$› *95�lING,C° JEFFERSON COUNTY DEPARTMENT OF COMMUNITY DEVELOPMENT 621 Sheridan Street Port Townsend, WA 98368 Al Scalf, Director April 3, 2003 CeNA11474"...0" Douglas Harbord 13150 S. Macksburg Rd. Canby, OR 97013 Dear Mr. Harbord: We are in receipt of the supplemental geo technical information addressing the location and stability of the deck. Taking this report into consideration for the deck structure it is necessary for you to adhere and complete the following recommendations that were called out in this report: o The drainage patterns on the bench should be monitored during a rainstorm to determine the effectiveness of the current drainage features at keeping surface water from flowing over the top of the slope. Improvements should be made if necessary. o Another option to reduce surface water on the bench if necessary would be to construct a "false roof' under the deck and collect the runoff into the existing house drainage system. o The vegetative buffer should be planted in deep rooted native plants in order to reduce surface erosion. Any bare areas on the slope face should also be revegetated. o Heavy irrigation or other activities that would contribute large quantities of water to the soil should be avoided. One cause of landslides is the presence of excessive groundwater in the soil. Building Permits/Inspections Development Review Division Long Range Planning (360) 379-4450 ascalf a(�,co.jefferson.wa.us FAX: (360) 379-4451 • • o All drainage control devices should be maintained in good working order and inspected at least once a year. In our letter dated 1/21/03, we had brought to your attention that we needed to have a framing inspection scheduled for this permit and what was needed for that inspection. To date, the framing inspection has not been called in. If you should have any questions, please feel free to contact our office. Sin.f i 0 s Al Scalf Director/ Building Official