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Home My WebLink AboutBLD2010-00342 Geotechnical Report STRATUM GROUP P.O.Box 2546,Bellingham,WA 98225 Phone(360)714-9409 June 23, 2010 Jon Adkins Exeltech 8729 Commerce Place Drive NE, Suite A Lacey, WA 98516 Re: Geotechnical Analysis Saia Residence Quilcene, Washington Dear Mr.Adkins: I investigated soil conditions at the existing retaining wall/cut slope to the east of the home. The purpose of this investigation was to provide soil parameters and recommendations for the construction of a retaining structure at the site.I also inspected the existing shoreline rock and log bulkhead to assess conditions at the bulk head. Retaining Wall and Cut Slope The existing retaining structure consists of two logs stacked on top of each other. The logs form an approximately 3-foot retaining structure as each log is approximately 1.5 feet in diameter. The logs are supported by steel pilings driven in front of the logs. The slope behind the logs slopes up at an angle of approximately 35 degrees and is grass and brush covered and is underlain by a mix of fill soils derived from the cut slope above and native undisturbed soils. The top edge of the cut slope into the previous is approximately 8 to 10 feet horizontally from the logs. The height of the cut slope is approximately 8 feet. The slope above the cut slope slopes up at an angle of 35 degrees and is tree covered. This slope is underlain by very compact silt and fine sand that I interpret to be preglacial alluvial deposits that have been highly compacted by the overriding glacial ice. Site photos of the existing conditions are provided in Photos 1 and 2 and a cross section sketch of the existing conditions is provided in Figure 1. The existing conditions are such that only very minor raveling of loose material should be expected. Eventually the logs will need to be replaced in order to maintain the current level area between the home and the slope. The existing level area could be expanded by approximately 4 feet by using a 4-foot retaining wall with a 4-foot wide terrace and then another 4-foot retaining wall.This approach will require minimal design strength as long as the wall or rockery is properly supported or constructed. If a wider area is desired a higher wall would be required. The site is underlain by very compact preglacial deposits of silt, sand and gravel with silt being the predominant material. The preglacial soils on the site are capable of providing support for bearing pressures of 3,000 psf for foundation loads. • June 23,2010 Saia Property,Quilcene,WA Geotechnical Analysis Lateral earth pressures which develop against retaining walls will depend on the method of backfilI placement, degree of compaction,backfill slope, type of backfill material,drainage provisions,and the degree to which the wall can yield laterally during and after placement of backfill. When a wall is restrained against lateral movement or tilting(rigid wall), the soil pressure exerted is at-rest soil pressure. Wall restraint may develop if a rigid structural network is constructed prior to backfilling or if the wall is inherently stiff or restrained from rotation. We recommend that rigid walls with level backfill and no surcharge loads be designed for an equivalent fluid weight of 49 pounds per cubic foot(pcf). Lateral loads may be resisted by passive earth pressures developed against the side of the retaining wall. For design purposes,a passive resistance of well-compacted fill placed against the sides of the foundation or wall may be considered equivalent to a fluid with a density of 225 pcf. These values assume drained conditions that will prevent the buildup of hydrostatic pressure in the compacted fill. A safety factor of 1.5 is included in the passive resistance. A coefficient of base friction of 0.35 may be used between the base of footings and the underlying soils. If passive pressure is used in conjunction with frictional resistance to determine lateral resistance to sliding, 1/2 of the passive resistance value presented above should be used, since larger strains are required to mobilize the passive soil resistance,as compared to frictional resistance. Because of the slope above the retaining wall,the design of the retaining walls should include appropriate lateral pressures caused by any adjacent surcharge loads. For uniformly distributed surcharge pressures,a lateral pressure of 0.4 times and 0.65 times the calculated additional surcharge pressure should be added, for yielding and non-yielding walls,respectively. A maximum pseudo-static earthquake load of 0.3 gravity is recommended for soil seismic load. The backfill placed directly behind retaining structures should be free-draining sand and gravel with less than 3%passing the US No. 200 sieve,based on a wet sieve analysis of that portion passing the US No. 4 sieve. The lateral earth pressures recommended above assume drained conditions behind the wall and therefore do not include hydrostatic water pressure. We therefore recommend that a drain be constructed at the base of retaining walls with a minimum 4-inch perforated pipe surrounded by drain rock. We also recommend that the upper 1 to 2 feet of fill behind the wall consist of relatively impervious material to minimize infiltration of surface water. Shoreline Structure The existing shoreline structure consists primarily or rocks. The rock portion is in generally good condition;however,logs along the upper portion of the rockery are in poor condition. It appears Stratum Group File:5.30.10 2 June 23,2010 Saia Property,Quilcene,WA Geotechnical Analysis that waves periodically over top the rockery and the logs and have caused minor soil erosion just above the shoreline protection. Water also is getting behind the rockery such that over time particularly as the logs become more degraded fine grained soils will be eroded out from behind the upper shoreline protection and further damage could occur. Replacing the rotting wood with rocks along the top of the rockery will reduce erosion of soil at the top of the rockery. Raising the rockery an additional 2 feet above the current level should greatly reduce the amount of erosion at the top of the rockery. However, if the rockery level is raised the angle with the existing rockery front should be maintained versus simply placing the upper rocks as a vertical lift.I recommend a slope on 1:1 for the rockery lift. This will reduce the wave energy against the face of the rockery and within the space between rocks in and behind the rockery. The existing rock size on the rockery are suitable for the rockery lift at the top.These rocks should be placed over quarry spalls and separation filter cloth consisting of Layfield LP 400E or substantially equivalent material. The layer of rocks should be placed in as tight as a manner as practical such that spacing between rocks is minimal and with less than 15%total void spaces between the rocks. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions regarding our assessment please contact our office at(360) 714-9409. Sincerely yours, Stratum Group l,tom,� 04-- /174„...., Dan McShane,L.E.G.,M.Sc. C r�: _.i/ Licensed Engineering Geologist ., ;' ' W stiff Stratum Group File:5.30.10 3 .1, ... \ . i...) •S ......) i .„..t. 1) I . PN lid ..L C...:•'S.1 -) 1/40 I Co' ----..t • ...1,. \ -\ , \' % ql..I t t i .......S'''''S W ... \ .1,:s ' ...V- I 1/4.• , -3 I ' - •1 , .1 • 1 . •---k• r' '7 0 ,,..\ ,,.,.,, cs • -. --- •— --.-.C) - ..,. Z \r, .1,, ....... a -.• Cl... . - -- _ •••\.1 I ,.... ••••. ......., ?...) , .„,... l —1-- . .. • .... \ I ••-....., 0 v\ \ / C e--.) ._ I . 1/4-.) • ---:* sl• i '‘....} 1 I '41 k . . . C.. \... ._ sNic —. . .• , . ....) - ' : . 4) ,.. c) , 1,-...„ ..-- fr.,1( June 23,2010 Saia Property,Quilcene,WA Geotechnical Analysis t^ .S• a �1.. tool a" is 1 "1...4:,.0..1 Photo 1. Existing cut slope and log retaining Photo 2. Native silt at cut slope. Sand and structure. gravel is present as well. e A , i r r , r dt 4t:t 4;, `� , ,. 1 • ' , i,- . a, 1,-44„'' 4..`7,) 1.2.. -I 4, , A*: s‘ , Photos 3 and 4.Upper shoreline structure showing rotted logs and erosion. Stratum Group File:5.30.10