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Home My WebLink About901034003 Geotech Assessment(~~~' ~bI ~~~ v~ ~~ GEOLOGICAL CONSULTING SERVICES CONSULTANTS IN GROUND WATER GEOLOGY ENGINEERING GEOLOGY ~. GEOLOGICAL/GEOTECHNICAL INVESTIGATION Spigarelli Large Lot Subdivision Hadlock, Jefferson County Washington For Don & Mary Spigarelli April 18, 1997 O LAG ITEM ..o,,.:~...~~.~ P~ ~B~C~f_.~. ~1~140 AGATE PASS ROAD, BAINBRIDGE ISLAND, WASHINGTON 98110 LOG '1~ p~~~OdC~~n' ~'~ (~ ~.~AY 2 i 19~ ~'~' '~:,_ ~EFfERSON COUNTY ~ERMiT CENTER- (206) 8425858 GEOLOGICAL CONSI)LTING SERVICES ADril I8. 1997 Mr...Tim Lindsay 14QQ2 35th Ave. S.E. Bothel, WA 98012 Dear Mr. Lindsay: Subject: Geological /Geotechnical Investigation Spigarelli Large Lot Subdivision Hadlocl~, Jefferson County Washington The enclosed report, requested by you, presents the findings of our geological investigation within and adjacent to the above proposed subdivision. Our investigation proceeded on the basis of the requirements set forth by the Permit Center of Jefferson County, in their memorandum of March 27, 1997 that was faxed to you. and subsequently sent to our firm. The data presented in our report were developed from field observations and mapping of the observed geologic units and active streams and springs, review of existing topographic and geologic maps, soil logs developed by Northwestern Territories and A.D.A. Engineering and other pertinent data. Our conclusions and recommendations were created by the analyses and correlation of the information collected and is based on our interpretation of the data and extensive experience with the types of soil and existing conditions. Proposed subdivision maps were supplied by A.D.A. Engineering and any modification of these maps for the purpose of this report were performed by Geological Consulting Services. LOG ITEf~ ~~ 16140 AGATE PASS ROAD, SAINBRIDGE ISLAND, WASHINGTON 98110 (206) 842-5858 We trust this report fulfills your requirements at this time. In the event you have any questions or wish additional information or, if .the present plans are modified, please contact us. Sincerely, Geological Consulting Services l~~t~ Will A. Thomas Certified Engineering Geologist Certified Professional Geologist .ri..:.~•S ~i~ 7•..: ~ . ~ ~ . 4 ~'"~ 1 Si ~i~ r copies to: A.D.A. Engineering Don & Mary Spigarelli ~,~~ 1T~ M #,g v~T_ ~~ g~~c~fe~ `~ GEOLOGICAL/GEOTECHNICAL REPORT Spigarelli Large Lot Subdivision Hadlock, Jefferson County Washington The subject site is located within Section 3, Township 29 North, Range 1 West, W.M.. Approximate location is indicated on Fi re 1, Location Mai,. TopogL hically, the property encompasses both relatively flat lands and variable moderate slopes of a north-south trending ridge. The flat lands consist of relatively smooth surfaces with a gentle down-gradient slope to the north. The easterly facing slopes express a hummocky surface with some small closed depressions and irregular small ridglets. These surface expressions are typical of collapse of the original surface by the melting of buried glacial Cice. Fig r~ e 2, To~grantu_ 'c Man, presents the general topography of the area and Figures 3 thror.~h 5 are sections through Lots 1 and 16, 2 and 15, and Lot 3, exhibiting typical slopes. Location of the sections are indicated on Figure 2. Geologically, surface soils within the entire project site consist of recessional glacial outwash sand, gravel and silt. This formation has been classified, by the U.S. Geological Survey, as that deposited by the receding Vashon-age ice sheet. Within the valley (flat-land) area, the soil types are quite variable. Predominately, the upper two (2} to three (3) feet consist of sandy loam, loam and loamy sand. For clarification, the definition of loam is: a soil composed of a mixture of clay, silt, sand and organic matter. Below three feet, to depths on the order of eight (8) feet, the soils are generally medium to coarse sand and gravel with local lenses of loam. These data are based on soil logs where the deepest boring was to a depth of 105 inches. Data is not available as to the total depth of these deposits. However, it is postulated that they could be as much as 20 feet or more in thickness. The depth to water in the valley portion of the site seemingly is on the order of 50 to 54 inches, with a very slight gradient to the north. Without deeper drilling it cannot be determined whether this is a perched water table, which is very common in these geologic settings, or an unconfined aquifer system. A perched water table is generally of limited extent with an underlying impervious formation. In this case, water can be forced nearer ®~ ~~~~ ~_ or to the surface creating potential flooding conditions. The ordinary water table {aquifer) ~' - - would be separated some distance below a perched water table -dependent on the >° ~ thickness of the impervious formation. Soils on the hillside slopes of the project are similar to those found in the valley portion but are sandier with more variable fine gravel and lenses of silt within the depths probed. The hillsides are moderately to heavily vegetated. Evidence of soil creep, landsliding and erosion was not noted. Vertical cuts for roads that have been established through these soils indicate only minor sloughing -even in older minimal maintained roads. Drainage of the slopes is to the east and northeast into the valley discussed above. With the exception of a small stream that headwaters to the north and west of the project area and flows onto the property within Lot 16, no stream or spring was located on the property. Soil borings on the slopes did not contact water or seepage. An over-lay indicating our interpretation of soil variables between the slopes and the flat- lands is presented as •aure 6. C'onclusion~ And Recommendations Based on the existing data that have been developed, our site investigation and experience with similar geologic settings, we have concluded that the project can be developed as planned. Actual residence siting and foundation construction can be quite distinct between the sloped lots and flat-land/valley areas. Within the slope areas, site selection for the residences will be controlled primarily on the basis of topography. Within the flat-land site selection would not necessarily be controlled by topography. However, it is our opinion that special consideration will have to given as to the type of foundation design and residence pad preparation. Our evaluations are also pertinent to the construction of access roads when related to cuts, fills and road base preparation. Proposed Lots 1, 2, 3, 15 and 16 will be the most susceptible to erosion and, to a much lesser extent, slope movement. We use the term "slope movement" since we would not anticipate large scale "block" movement of the slopes as long as proper construction and drainage practices are observed. "Slope movement", in our use of the term, would include soil creep and mud-flows which are primarily surficial and not deep-seated landslides. . Granular soils are sensitive to the potential of erosion. This is particularly true when slopes are devoid of vegetation and the rate of water application {by rain-fall, etc.) exceeds the absorption capability of the soil. In general, the soils on the slopes appear to be well-drained, with no ponding of water noted at the time of our site investigation. However, we anticipate the actual permeabiiiry of the soils to be quite variable, from moderate to very good. %~~ ~~~~ ~~~~~~~ Selective logging only should be permitted with as little disturbance of the natural understory as possible. It is our iudgment that the natural angle of repose for these soils is on the order of 45 degrees. Cut slopes should not exceed 1 foot horizontal to 1 foot vertical (l:l) and where feasible, be on the order of 1-I/2 : 1. This recommendation should apply to cut slopes for both road access and pad construction for residences. Hydro-seeding of all barren slopes,.or establishing other vegetal covering should be done as soon as possible to mi#igate erosion potential. Prior .to placement of any fill, including over-cast from excavating, all organic material should be removed from the area to be covered. In ,the case of road construction, placement of .over-cast soils .should be into notches .cut into the hillside and the soil compacted in 6 to 8 inch lifts to 90 % of relative density. For fill around the residence area.the same procedure should be followed. Establishing vegetal cover for fill slopes is also very important for the same reasons as stated above. Selection of sites for residences on the slopes .should . be at locations where the least amount of cut is necessary. During our field investigation we noted topographic areas where certain areas were better than others for residence sites. The foundation for the residences should not be placed on fill but on dense, natural, inorganic soil. The type of foundation design can only. be recommended after sites are selected for the structure and site specific soil classification and drainage has been determined. However, the load bearing capacity of these soils can be assumed to be on the order of 2,500 pounds per square foot (psf~, unless adverse soil conditions are encountered. As with all construction on or around slopes proper drainage control is essential. Drainage design can only be formulated once final siting is selected. The surface soils within the flat-land area seemingly are much less permeable than those on the slopes. With less permeable soils we would expect some local ponding of water to occur. With a relatively shallow water table (+/- four feet and very gentle valley gradient, there is potential for localized flooding. We have noted that the proposed storm drainage plans, prepared by A.D.A. Engineering, addresses this issue and directs the drainage into three depressed areas which are then set aside as open space, with the mouth of the drainage system exiting to the north out of the property boundaries. In our opinion this design is compatible to existing conditions. For lots 4 through 14, individual soil investigations for each lot should be performed for design of foundations once the structure site is selected. It is our opinion there are possible conditions that could cause differential settlement of the foundations or, in the incident of an intense seismic event, liquefaction of the soils. ~~~ IT~(~ ~;: _~ ~`~"- 3 Preparation of the sub-grade for construction of the roadways within the flat-land areas will require additional work because of variable soil conditions that will be encountered. It is expected that local soft areas will be discovered whereby additional effort for compaction will be required. It may even be necessary to excavate poor material and replace it with good quality soil. Contingency plans should also include the necessity of increasing the thickness of the compacted base coarse for ballast. SUMMARY Based on the data analyses, conclusions and recommendations presented above, it is our opinion the site can be developed as presented. As with any development of this size it is to be anticipated it will be necessary to make field modifications as construction proceeds. Erosion of the granular soils on the slopes is the primary concern. Steepness of cut and fill slopes should not be greater than 1' : 1 -preferably 1 1/2:1. Cut and fill slopes should be re-vegetated as soon as possible. Only selective logging should be allowed with as little disturbance to the understory as possible. . Foundations for residences on the slopes should not be placed on fill. All fill, including roadway fill should be compacted to 90% of relative density. In the flat-land area, extra precautions for roadway and structure foundation construction will be necessary due to the potential for inferior drainage and soil conditions. Site specific soils investigation should be performed for all lots once the construction site is determined, in order to make recommendations for foundation design and drainage. ~, sN ~~w ~SZt 11 ~_ _~ ry ,~ ~`~-. - - (~ 1 I Mill Pt. ~I ! .-. ^~ ~ .,. .I / S• l 1 it fscavery. ~ . ~~ ~1 1 _ ~~ ' ~ FOR MOAE DETAIL SEE !AAP d 13A6 rj /. ~ r. - ~ I I I ~ I 9 ~o ,,, `'City y Jct. ~ /Lake i II C~~ / i ' f I1 M 30 ~ I U ^!!~ Delanty Lakej '_` A ~___ Q ~ 1 `O ~~ I LtnD ~ 1 ' 1r ,~ 1`11 O I \' ..____.._ __ . 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I 1 / i .......... 1 1 ~-- I 1 1 f--''----~ ~ '~ / /~ `~NOI'S4'1~'E ~p,`6 ~ ' ~ ~l'S4'lIG'E - -4.445' I 1 I ~ I I i ~ ~ ~ 1_r ' Vml 1 i I ~ ~ / ~T 1 // 224.84' ~ i i J ~L i ll\:J I , ~ ' i ~- ~ ~ r N~ (] s~ 1, Oy i ~' i '~ N y~l j i/ ~~X / ~, ( /~ ~/ i ~ `ll` ~ I ~ i / ~ ' ~ i ~~1 ! 1 I ~ 111 i J 0 111111 /~ 1~ ~' 1 ---` O i ~• , ``_~" ` \` 1 ~/Q~y'f~f'ti 1 ~ D ~ ~: / 1 1 i' p ~ ~ Q ~ i~ b / \``~ x\111 ~ ~ ; `- ~% 1 {~~ Q (,~} ry ~ rO ~ ZS , 1 C~ (.~ / .. /i' Y' I U~J ~ 2Jc~~(~ @ LC~S 1 i }J I '~• '~1 W ~ ~ 11 1¢nl~ 1 /' '/ O O y~~j ~I ~1 /lri yc O~% /• ~ ' ~ C~ ~J ~ y9 F~Y~ 1 i 1 ~ ~ ,'IQf, ~~ L ~ 11" 1 / 1 In\)I (1~ 1 PU ~Uwi-~ p'D -C~l D I r a O >Y ~ 1 //• I _1 (n 1'~ Iv u u u u u y y u u~ u u ul u V fbf~~P ~ ~ -~~ I I I ~ (")2 ~ i~ uaNU-ss°- °OV$°- '" n 1 C~ u uu w V 1 1 `~ ~' ~ o ~ ~ I 1 i ~ I x1 4 ~~~111 `' ca f~? I ~_ s n ~~ "~ ~~t-~VAT'tan~- ~'FET c~ c:4 G~ n ~ ~ ~ ~ ~ 7 ~ .~~,..~ ~ ~~ .,,~ ~ m S M ~ ~1 o+ `~ 0 a p ~D C 4 c~, ~~ N r- c ~`i ~^~ ~ ~ ~~+ 1~~ C, Q ~' l~ n ~ c~ v1 _ ~ ire ~ ~ 1~ CZ .„ ~•,3 (( II~ .~~V ~ ~ C:~ ~~ rf r .b '~ b `1 h ~ ,h ( C 'K ~ t W 1 r fl ~~` }J 0 /~ Q ~ ~. ~' v~~ ~- ~ tt~~ - ~- c ~ r v C ~ ~ n n Selective logging only should be permitted with as little disturbance of the natural understory as possible. It is our judgment that the natural angle of repose for these soils is on the order of 45 degrees. Cut slopes should not exceed 1 foot horizontal to 1 foot vertical (1:1) and where feasible, be on the order of 1-1/2:1. This recommendation should apply to cut slopes for both road access and pad construction for residences. Hydro-seeding of all barren slopes, or establishing other vegetal covering should be done as soon as possible to mitigate erosion potential. Prior to placement of any fill, including over-cast from excavating, all organic material should be removed from the area to be covered. In the case of road construction, placement of over-cast soils should be into notches cut into the hillside and the soil compacted in 6 to 8 inch lifts to 90 % of relative density. For fill around the residence area the same procedure should be followed. Establishing vegetal cover for fill slopes is also very important for the same reasons as stated above. Selection of sites for residences ~ on the slopes should be at locations where the least amount of cut is necessary. During our field investigation we noted topographic areas where certain areas were better than others for residence sites. The foundation for the residences should not be placed on fill but on dense, natural, inorganic soil. The type of foundation design can only be recommended after sites are selected for the structure and site specific soil classification and drainage has been determined. However, the load bearing capacity of these soils can be assumed to be on the order of 2,500 pounds per square foot (psf), unless adverse soil conditions are encountered. As with all construction on or around slopes proper drainage control is essential. Drainage design can only be formulated once final siting is selected. The surface soils within the flat-land area seemingly are much less permeable than those on the slopes. With less permeable soils we would expect some local ponding of water to occur. With a relatively shallow water table (+/- four feet) and very gentle valley gradient, there is potential for localized flooding. We have noted that the proposed storm drainage plans, prepared by A.D.A. Engineering, addresses this issue and directs the drainage into three depressed areas which are then set aside as open space, with the mouth of the drainage system exiting to the north out of the property boundaries. In our opinion this design is compatible to existing conditions. For lots 4 through 14, individual soil investigations for each lot should be performed for design of foundations once the structure site is selected. It is our opinion there are possible conditions that could cause differential settlement of the foundations or, in the incident of an intense seismic event, liquefaction of the soils. _~. Preparation of the sub-grade for construction of the roadways within the flat-land areas will require additional work because of variable soil conditions that will be encountered. It is expected that local soft areas will be discovered whereby additional effort for compaction will be required. It may even be necessary to excavate poor material and replace it with good quality soil. Contingency plans should also include the necessity of increasing the thickness of the compacted base coarse for ballast. Based on the data analyses, conclusions and recommendations presented above, it is our opinion the site can be developed as presented. As with any development of this size it is to be anticipated it will be necessary to make field modifications as construction proceeds: Erosion of the granular soils on the slopes is the primary concern. Steepness of cut and fill slopes should not be greater than 1• : 1 -preferably 1 1/2:1. Cut and fill slopes should be re-vegetated as soon as possible. Only selective logging should be allowed with as little disturbance to the understory as possible. Foundations for residences on the slopes should not be placed on fill. A11 fill, including roadway fill should be compacted to 90% of relative density. In the flat-land area, extra precautions for roadway and structure foundation construction will be necessary due to the potential for inferior drainage-and soil conditions. Site specific soils investigation should be performed for all Lots once the construction site is determined, in order to make recommendations for foundation design and drainage. ~t~(~ I`~EIV1 _~~._~~