Loading...
HomeMy WebLinkAbout977700024 Geotech Assessment (2006)Ziaaer Zeman Associates, inc. Geotechnical and metal Conwldng ~- ~td4~ Eotit~ny ' 8065019 May 25, 2006 Allied/Nationwide Insurance Company 8804258"' Street East Graham, Washington 98338 Attention: Mr. Rob Crotl Subject: Geotechnical Engineering Evaluation Tour de Mond Residence - S30 Reimer Road Nordland, Washington (Marrowstone Island) Claim # 46A - 71361 Dear Mr. Croft: This letter summarises our site reconnais~ee and property observations and oonclasians completed by Zipper Zeman Associates, Inc. (ZZA) at the above address. The purpose of the gootechniol engineering evaluation was to assess east faciung slope conditions for reconstmction of the damaged beach access stairway down the face of the steep blu#1 Our geotechnical data includes foundation and anchorage types and opacities, -and preliminary topography based on available LiDAR and Jefferson County data. Our scope of services inch~ded a review of ~ information for the site vicinity, a site recxnmaissance and hand auger boring performed on April 21, 2006, review of pertinent geologic information, limited geotechnical engineering analyses and preparation of this letter. Authorization to proceed with this study was provided by you on April 1 S, 2006. LiDAR topography was submitted previously under separate cover. The LiDAR topography is based on aerial mapping, and is not intended to replace actual measurement of topography at critical locations. This letter is an instrument of service and the observations and conclusions presented herein are in respect to the subject property and have been prepared in accordance with generally accepted geoteehnical engineering pracfiice. This letter has been prepared for the exclusive use of Allied/Nationwide Insurance Company and their agents, for specific application to this project and the stated purpose, SITE DESCRIPTION The subject property is locatiad on Marrowstone Island at 530 Reimer Road in Nordland, Washington. The high~ank waterfront residence has an expansive wood deck directly east of the structure and a stone block patio, south of the wood deck. The stone block patio connects to steps and the remains of damaged wood stairway to the east over the steep slope. We understand the wood stairway was destroyed during a windstorm on February 4, 2006. 18905 33~ Avenue West #117, Lynnwood, WA 48036 425-771-3304 Fax: 425-771-3549 Tour de Mond R~denoe - 530 Reimer Road 81065019 May 25, 2006 Pale 2 A site was performed on Apri121, 2006. The owner, Ms. Dec Tour de Mond showed us the stairway area and explained some of the site history. We understand Ms. Tour de Mond has owned the house approximately 8 years and has extensively remodeled the main house and added a gazebo and improved the septic system and seQffiate garage sinu~irre. Ms. Tour de Mond noted the damaged wood stairway traversed straight down the slope and was supported by steel brackets fouocled in the slope soils. During our site visit we noted scvuai of the brackets exposed on the slope surface. Ms. Tout de Mond discussed her desire to have the stairway rebuilt north of the existing location, nacth of the wood declr, in a lawn area She is concerned with construction equipment damaging her septic system a+od stone patio area if the stairway is rebuilt at the south area. we advanced our hand auger boring HA-1, at the t~ of the east facing slope north of the wood dock in the area Ms. Tour de Maw would like to construct the beach access stairway. SURFACE CONDITIONS The upland area inchxling the residence, attached wood deck, stone patio area and several outbuildings are atop a generally level landscaped area.. Limited lawn areas were noted to the north and west of the residence. Native vegetation including serial and small to large cedar and fir trees were observed acxoss the site. Numervns Douglas fir trees ranging from 18 to 3b inches in diameter were noted on the upland property with several within 15 feet of the break in slope. The septic tank and drainfield, gazebo and garage arc located west of the residence in a lawn .area. The east facing slope is on the order of 60 fee# in height, based oar the topographic map of the area. The slope angle ranges from near vertical to 1 Horizontal : 1 Vertical (1H:1V). The top of slope area is ge~rally thickly covered with serial. Over the slope the near vertical areas are exposed native soils. Limited vegetation including surface grass and brash and small trees were observed on the 1H:1V slope areas. Though we could not access the beach area fiercer the top of slope, we did obscxvc some surficial soil slumps which accumulated at the base of the slope. Ow observations did not observe any deep seated landslide features and the soil movement appears to be limited to surficial sloughing. The surficial soil sloughing is related to the steep angle of the slope, exposure of the slope face to wind and rain and dense nature of the bank soils which limits protective surficial vegetative growth, in our opinion. SUBSURFACE CONDITIONS During our reconnaissance, we advanced one hand auger boring at the top of the slope, north of the residence and deck. The interpretive log of the hand auger boring is attached to this letter. We encountered loose silty sand with some gravel and roots to a depth of approximately 2.5 feet, which became medium dense from approximately 2S to 3.5 feet in depth. Dense to very dense silty, gravelly sand ink as weathered glacial till was obscrvod firm approximately 3.5 feet bo the boitom at 4.5 feet We could not advance the hand auger boring doeper than 4.5 feet dae to gravelly conditions in the very dense gl~ial till. 5lOSSn)9 =(aurdthia~8 Roidemx .. Aordlan3 sVasfiingccx~ Tour de Mond Residence - 530 Retnier [toad 81065019 May 25, 2006 Pale 3 The regional geology of the site area was reviewed in the Geologic Map of Port Townsend Qreadraxgle, Puget Sound Region, lYashington (I i 19&1~ compiled in 1989 by Pessl and others. The site area is classified as Qvt-Quaternary Vashon faill in the upland area. The till consists of a mixture of sand, silt, gravel and clay which has bcea glacially c~nsolidat~ed. Below the glacial till is nn~apped bedmek Ts-Sodimentary Rocks (Eocene). Tl~e Ts bodioclk oa~sists of sandstone with interbedded siltstone. Based on our hand auger boring and observations of We slope soils we would ooacatt with the mapped geology. We observed upper glacial till which extended dovvnslope amp the sandstone bedrock. Though we could not access the beach area, we did observe the upper glacial till and the sandstone bench at the base of the slope. Based on our observations looking over the slope, a glacially consolidated sand (Qva) may be between the upper till and lower sandstone, but could not be field verified. The lower sandstone beach appears to be on the ordex of 10 feet in height with a near vertical slope facx. Sandstone boulders were observed sc~ered across the beach area. CONCLUSIONS ANA RECOMMENDATIONS Based on our site reconnaissance and hand auger boring, review of tlx iofarmation sources and site history, we offer the following conclusions and recommendations regarding reconstruction of the stairway: ~ T'he slope consists of upper glacially consolidated soils (glacial till) atop sandsmne bedrock, with possibly advance sand between the upper till and lower sandstone. Competent dense and very dense soil and bedrock comprise the bluff area with limited surficial sloughed soils. Slope angles ranging from near vertical to 1H:1V for the approximate 60 foot high bh~ff were observed. The bhrff area appears stable and only limimd surficial soil sloughing should be expected, in our opinion. Jefferson County Sensitive Areas Map uses the Coastal Zone Atlas m classify the bluff area as Unstable and Unstable Recent Landslide, probably due to the slope steepness. • At the top of the slope our hand auger boring HA-1 encountered medium dense conditions at a depth of Z.5 feet, dense conditions at a depth of 3.5 feet and very dense conditions at approximately 4S feet. We would interpret the lower dense to very dense soils in our boring as weathered and unweathered glacial ti~lI, as also observed on the east- . facing slope. Mark Schaefer of Pacific Engineering Technologies is designing a new stairway to replace the stairway that was damaged in the February 4, 2006 windstorm. According m Mark, tentative design may inchrde a cable supported stairway with a top and bottom concrete foundation block. The stairs would be supported by stringers along the cables. Anchors supported in the slope soils would serve as standoffs for the cables and not as bending members. Possibly two or three intermediate anchor locations would be installed ~ the slope as standoffs for the flexiible cable stairway. $1065(! 19 -T~xu de IWtxid R~~;.e 3~'urtiLynd, W'~ltink*ttm Tour de Mond Residence - 530 Retnier Road 131065019 May 25, 2006 Pane 4 • Anchors for the standoffs may inchide helical anchors or pin piles. Two factors which increase the difficulty of installing uud-slope anchors inchide: 1. The steepness of the slope and limited access for construction equipment may require hand labor supported by safety ropes for all slope anchor installations; 2. The density of the slope soils, which includes upper glacial h~l and lower sandstone, may limit adequate penetration depths for helical anchors. In discussions with several contractors, helical anchors world not adequately P~ the glacial till soils. Pin piles installed with a jackhammer would be. the most appropriate anchor type. The power unit would be at the top of the stopc with hydraulic hoses and the jackhammer on the slope. Hand art benches would be nary at the mid-slope anchor locations for the safety roped offhand labor to operate fire jackhammer. A 90 or 140 pound jackhammer would be used to advance the 2-inch diameter (I.D.) Schedule 80 steel pipe on the slope. A steel paint at the tip of dre pin pile would increase pion into the dense slope soils. According t+u a pin pile contractor, the 140 pound jackhammer (Rhino} is easier to Dandle than the 90 pound. jackhammer because it is shorb~r, sets securely over the top of the pile and advances down without r+ecNining the apE:rator to stand on. • The steepness of the slope requires that pin piles be driven at an angle as close to perpendicular to the slope face as possible. However, a maximum batter angle of 4S degroes is reconunended. The 2-inch pin piles should be driven to refusal criteria of less than l-inch of penetration in 60 seconds of continuous driving using a minimum 90- pound jackhammer under the full weight of ono-mazr. Alternatively, for the 140 pound jackhammer, the 2-inch piles should be driven to refusal critieria of less than 1-inch of penetration in 40 seconds of continuous driving. We recommend an allowable vertical axial capacity of 4 kips per pile for 2-inch pin piles installed to refusal criteria. For piles driven at a 4S degree angle, a 1-lap capacity should be assumed. It should be noted that pin piles driven to refusal are adequate to resist axial Gee loads only, and lateral loads can not be resisted by pin piles. Options for stairway reconstruction inchyde limiting impact to the slope and exposed soil conditions. Standard stairway consdudion over the slope would be diftic~ilt due to the slope steepness and the very dense nature of the glacial till and sandstone bedrock. Additionally, a standard stairway would be more disruptive to the bhrff face. We understand the least slope disturbance would involve a cable lift or tram construction, but at a considerable greater cost. We understand foundation ~aremcets me an upper concrete foundation at the top of the skyQe and a lower foundation at the bottom of the slope. Mid-slope foundations into the bluff Bing would be limited for a cable lift or tram construction. The proposed cable supported stairway or lift tram structure may be supported on conventional concrete spread footings at the top and bottom of the slope. These footings should be founded directly on the medium dense to very dense native soils, or on a zone 51065ttI9-T.~urAei~.liwtdRe&i3mte ;dix'dland,lirashin,~ttm Tour de Mond Residence -- 530 Reir~ar Road 81065019 May 25, 2006 Fage 5 of compacted stnyctural fi11 extending down to the native soil. Depending on stairway structure selected the base foundation maybe atop sandstone bedrock. • Spread footings suppori~ed in the manner recommended above may be designed. with a maximum allowable bearing pressure of 2,500 pounds per square foot (psf). This allowable bearing pressure maybe used by up to one-third to accommodate seismic or wind loads. For these portions of foundations embedded more than 12 inches below finish surr+nimding grade, we recommend using an allowable passive earth pressure of 300 pcf in st<uctural fill and medium dense to dense native soils, respo~ively. We recommend using an allowable base friction coefficient of 0.35. Anchorage at the upper and lower cable ends should be provided by mass ooncx~ete embedded sufficiently to provide passive earth pressure resistance to lateral loads. A~ factors. of safety should be applied to load resistance. Any upward or uplift components of loads should be frilly resisted by dead weight of the concrete. Upper anchors should be set back a minimum of 25 feet from the top of slope. CLOSURE We mate the opporhmity to be of service to you, and we hope this letter meets your current needs. If y~ .should have any questions, Please do not hesitate to oontac~ us at your convenience. Respectfully submitted, Zipper Zeman Associates Inc. Cbrt R. Thompson, P.G. Senior Geologist ~, John E. Zipper, P.E Principal Enclosure: Hand Auger Boring Log l3A 1 A 106Si ~ i 9 -Pant do h,Saui R~tidenxr -• Tt~innd. l~.'a~tingta: Hand Anger Boriig HA-I I.ocatioa: At top of East facing slope, 12 feet west of slope treat .. __ ....__._.... w .. ,. Project: Tour de Mond East Sbpe Project No: 81065019 ~-D~cava#od: Apci121, 2006 Depth ~~ Material Descriptiaa ** Nc Sanplc Tesfieg Surtaco grass over Topsoil l Loose, moist. ora~ish-1an~xown, silty SAND with some gravel and moderate m t S-1 o s 2 Loose, moist, brawn, silty SAND with some gravel and scattered r t S-2 oo s 3 Mediumdense, moist, brown, silty SAND with some gravel - _ _ _ S-3 4 Deese tp very clensa, moist, brown-SAY. ~Y, gravelly sAND (weathered glacial till) s~ s Refusal atop gravels at 4.5 feet HA-1 co~nple~d at 4.5 feet on M21/~06. No caving sidewalk were observed. No groundwater seepage observed at the time of excavation.