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Home My WebLink AboutGeoTech Review for MEP2005-00051 - MEP Reports - 8/26/2005 °�' STAr ° MASON COUNTY fl' s o PUBLIC WORKS DIRECTOR/COUNTY ROAD ENGINEER N Shelton,Washington 98584 2�aa N Y a DATE: Aug. 26th, 2005 rasa INTER-DEPARTMENTAL COMMUNICATIONS TO: Chuck McCoy, DCD—Planner FROM: Alan A. Tahja, P/W - Co. Hydr. Engr. WO#PLG-05 SUBJ: Geotechnical Report Review NAME: Wurm/Dimarco SFR g Bear Crk-Dewatto MEP2005-00051 The geotechnical report prepared for the proposed Connie Wurm/Carol Dimarco Single Family Residence (SFR)to be constructed on Lot 3 of Mason County Short Plat No. 2436 has been received and reviewed by Public Works. The report appears to satisfactorily address County requirements for geotechnical reporting. The report's author indicates that, in his opinion, the proposed development of the property will not cause stability problems for either the subject property or neighboring properties. From the contents of the report, I recommend accepting the report as satisfying the County's requirement(s) for stability investigation and geotechnical reporting. The report indicates that"Based on our geotechnical evaluation of the site and our experience in the area, a building setback is not recommended." The report's site plan presents the proposed building location as being on the southwesterly portion of the property where the slopes are fairly gentle. Approval of construction on the site should require that the building's location should roughly comply with the site plan. If the building site is relocated appreciably from the location shown on the site plan, the engineering geologist should be required to revisit the building location issue and provide an addendum to his report to indicate his acceptance of the proposed building's location. Other recommendations and conclusions contained in the report should also be made conditions for project approval. Erosion and sediment control measures recommended in the report should be made conditions for permit issuance. The migration or release of silty water or mud from the applicant's property will be considered a violation of County and State water quality protection regulations. In summary, the geotechnical report appears acceptable, and the development of the lot for residential development should be allowed to proceed, subject to the engineer's recommendations. Please feel free to contact me at County extension 461 if you have any questions regarding these comments, or if you feel any features need further discussion or attention. S .t-�/ ky,7 G. A an A. Tahja File: H: \WP\GEO\Reviews\Wwurm-Dimarco.doc I M,�/� WORK ORDER -PUBLIC WORKS DEPT. W. VMS: (� fir^ 'bs" PERMITS; 2vvs C-' x-> fq Work0, R RmMed bY' . A �by., • • 7�'�uciL vt1�C,,rx;� :,,, . NumbierT cs Type of Worts; v f� v�F Grate: ' CHARGE TO: NAME �+.� AGENCY/COMPANY 7�>i,f-AP,[z-co GlUJUG ADDRESS PHONE ' Pub.workm e. , r ylu!_?=4e165_ MY• E3"MTO Tarty.t+'a: �3 zz, t �� tnTitts � az can • EQMUENrvsEoc MATEMLUSM tF1 Idol cost ewes: MT>E � ENO= rartM - tBett Houis . —_-- EQUIPMENT USED: 81lt! �� • MAI XFJAL WED: 3) BlUE0 DATE PAID.DATE TOTAL ALL --- � was �uRwt -.�,Aiec� • -- Geotechnical Report Lot 3 of SP 2436 Bear Creek-Dewatto Road Mason County, WA Prepared for Carol Dimarca by Geotecbnieal Testing Lab Olympia, WA February 19, 2003 ZT/90 39VJ 00�jvWIQ TSZZG9Z098 TT :VT S00Z/bZ/80 08/24/2005 14: 11 3607692251 DIMAkCll PAGE 02/1 / Geotechnlcai Testing Laboratory A G � O Geotechnica! Services > QA/QC Services Testing Services 10011 slomber8 st,SW vlypvls,WA 9"12 Phone:(3W 754-4812 Fax:(360)754-4W Date: 02121/2003 by. lL Dmvm by: LL R� Checked by, LL /O !� ,02-21-03-01 2 30/ LOG IN O RDA ASEMI PROJECT NAME: DIMARCO MASON COUNTY fi61�i0f�s: 0 10 20 SCALE Z " = 2 0 ' � sue:,t,�, zo,...t C, I, FIGURE 2 ll7 rn)nnc TT-FT r-nn7 )+,7 10M 08/24/2005 14: 11 3607692251 DIMARCO PAGE 16/17 GEOTECHNICA.L TESTING LABORATORY CAROL DIMAiRCo 10701 SCOLA BEACH DR. S W SEATTLE,WA 99146 Re: Geotechnical Report Lot 3,Short plat 2436 Bear Creek-Dewatto Road Mason County,WA INTRODUCTION This report summarizes the results of our geotechnical services for the proposed hone to be located at the above referenced site, in Mason County,WA. The location of the site is sho-ft relative to the surrounding area on the Vicinity Plan,Figure 1. Our understanding of the project is based on our discussions and review of the site and our explorations. We understand that the irregular shaped site is to be developed as a homesite with associated outbuildings,pavment areas and utilities, accessed by Countryside Court. Stormwater runoff from the site, roof and hard surfaces,will be collected and directed to on-site retention facilities. The general layout of the site is shown on the Site Map, Figure 2. We further understand that very little grading is required at the site to reach design,grade, b general, grading will consist of the excavation of the foundation in the upper portion of the site;no structural fill is anticipated. We understand that the site will be served by local utility companies. The eastern portion of the site has slopes that are greater than 40 percent. Mason County therefore requires that a geotechnical report be prepared in accordance with the Critical Areas Ordinance. The purpose of our services is to evaluate the surface and subsurface conditions at the site as a basis for providing geotachnical recommendations and design criteria for the project and to satisfy the requirements of the Mason County Critical Areas Ordinance, Oeotechnical Testing Lab is therefore providing geologic and hydrogeologic services for the project. Specifically,our scope of services for this project will include tho following: I- Review the available geologic,hydrogeol4c and geotechnical data for the site area. 2- Conduct a geologic reconnaissance of the site area. 3. Explore the shallow subsurface conditions at the site by monitoring the excavation of boring test pits. 4 Ev aluate the landslide an ion ids at the site r tlu d giros barn per a Mason County Critical,Areas Ordinance regulations. 5. Provide geotechnical recommendations for site grading, including site preparation, sub-grade preparation,fill placement criteria(including hillside grading),suitability of on-site soils for use as structural fill,temporary and permanent cut and fill slopes, drainage and erosion control measures. 6. Provide recommendations and design criteria for the structural foundation and floor slab support, including allowable bearing capacity, sub-grade modulus, lateral resistance values and estimmeg of settlement- 10011 Blomberg Street SW, Olympia,WA 98512 Phone#: (360) 7544612 Fax#: (360) 754-4848 08/24/2005 14: 11 3607692251 DIMARCO PAGE 17/17 GEOTECHNICAL TESTING LABORATORY 7, Provide recommendations and design criteria for design,of conveutioiW retaining walls, including backfill and dminage requirements,lateral design loads,and lateral resistance values. 8. Provide our opinion regarding the feasibility of the location and construction of the stormwater outfall across a portion of the site. 9. Provide preliminary geotecluucal recommendations regarding the construction of the stormwater outfall. SITE CONDITIONS SURFACE CONDITIONS The proposed building site is located in the upland portion of the lower Union River Valley area. The site is situated in an area of moderate developn=t, The site is bounded by a home on two sides of the property. The proposed layout of the site is shown on the Site Plan, Figure 2_ We conducted a reconnaissance of the site area and monitored the exposed excavations at the site. Elevations in the site area range from approximately 150 feet in the east to about 190 feet in the western portion of the site. The building site is located in the level portion of the upper area,where the slopes are between 0%and 13%. The steeper slopes are confined to the northern and eastern,portion of the lot where they steep to between 30 and 45 percent. This area is outside of the proposed building area. We observed no evidence of erosion. Surficial sloughing and soil movement was not observed on the steeper portions of the site and adjacent areas. No evide=of deep-seated slope instability was observed in the site area at the time of our site visit. The site is vegetated with fir,hemlock,maple and cedar trees. A moderate to dense understory consists of grass, brush,young alder trees,salal and Oregon grape with local areas of blackberry. Evidence of a small surface water flow was observed in the eastern portion of the site area at the time of our reconnaissance. The general topography of the site area indicates that the site drains towards the east. SITE GroLoGY T'he site is generally situated within the Hood Canal Drainage Area. The existing topography,as well as the surficial and shallow subsurface soils in the area,are the result of the most recent Vachon stade of the Fraser glaciation that occurred between about 10,000 and 12,000 years ago, and weathering and erosion that has occurred since. A description of the surficial soils is included in the"Site Soils"section of this report, In general, Vashon glacial till underlay the upper portions of the site. SrrE SOILS The Mason County Soil Conservation Survey(1960)has mapped the site soils as Everett gravelly sandy loam in the building area of the site. The Everett soils typically form in sandy glacial till areas and are described as having a rapid permeability and a low erosion hazard. We observed no active erosion or slope disturbance,in the site area during our reconnaissance. 10011 Blomberg Street SW, Olympia,WA 98512 Phone#: (360) 754-4612 Fax#: (360) 754-4848 08/24/2005 14: 11 3607692251 DIMARCO PAGE 15/17 GEOTECHNICAL TESTING LABORATORY SussuIZFACE]EXPLORATioNs Subsurface conditions at the site were evaluated by observing the excavated power ditch being dug on the steep access road on site. The road cut exposed another eight feet of the subsurface strata, The soils encountered were visually classified in accordance with ASTM D-2487_ SUBSURFACE CONDITIONS In general, undisturbed dense till was observed in the building portion of the site. The undisturbed till was observed under the free canopy across the sur£vca of the entire lot. Some groundwater seepage was observed from the slope area at the site. Based on the mature of the near surface soils, (intermittent outwash and weathered sand over undisturbed soils in,the building area), seasonally perched groundwater conditions should be expected even during periods of extended dry weather. Slope Stability Slopes as steep as 80 percent were observed in the sloping portion of the site. Since slopes of 40 percent or greater with 10 Beet or more of vertical relief occur on portions of the site, Mason County requires that a geologic hazards report be completed per the Critical Areas Ordinance, In general,the undisturbed native soils in the steep slope portions of the site consist of sand with variable amounts of gravel. These sandy soil materials are;in a dense to very dense condition except where they have been disturbed by weathering activity. The near-surface recessional,weathered advance outwash is in a medium dense to dense condition except at the ground surface. The surficial soils are generally in a loose to medium dense condition. These soils_are generally stable relative to deep-seated failure. No evidence of deep-seated landslide activity or signific"erosion was obscured at the site at the time of our site visit. Weathering, erosion, and the resulting sloughing and shallow landsliding are natural processes ftt can affect steep slope areas_ Instability of this nature is typically confined to the upper weathered or disturbed zone, which bas been disturbed and has a lower strength. No evidence of local surficial erosion, raveling,and sloughing was observed in the site area at the time of our site visit. Significant weathering typically occurs in the upper 2 to 3 feet and is the result of oxidation, root penetration,weudry cycles,and freeze/ftw cycles. Erosion in steep slope areas such as this can be reduced through proper design and construction of the storm water systems. Erosion control recommendations for the sloping areas are provided in the"Building Setback"and"Erosion Control" sections of this report_ Proper planning, design and construction techniques will reduce the risk of surficial erosion or movement in these areas. 10011 Blomberg Street SW, Olympia, WA 98512 Phone#: (360) 754-4612 Fax#; (360)7544848 _.J f 08/24/2005 14: 11 3607692251 DIMARCO PAGE 07/17 GEOTECHNICAL TESTING LABORATORY As previously discussed,weathering,erosion, and the resulting surficial sloughing and landsliding are natural processes that affect slope areas. Significant weathering typically occurs in the upper 2 to 3 feet and is the result of _ oxidation, root penetration,wet and dry cycles and Freeze and thaw cycles. These processes can be managed and the risk reduced through proper construction of the residence, including the stormwater system. Erosion control recocnmendati.ons in the slope and buffer areas are provided in the"Building Setback"and"Erosion Control" sections of this report. Building Setback A building: setback from landslide hazard areas is required unless evaluated and reduced by an engineering geologist or a licensed professional engineer. Cleaning,grading and filling within the setback area may not be allowed because the slope stability or erosion hazard will be adversely impacted. Based on our geotechnical evaluation ofthe site acid our experience in the area,a building setback is not recommenW. Building setbacks are usuaUy measured from the bottom of the footing to the face of the slope, in accordance with the Unified Building Code. As previously discussed, weathering,erosion and the resulting surficial, sloughing and shallow landsliding are natural processes that affect slope areas. As noted,no evidence of surficial ravel' or sloughing was observed in the sloping � g portions of the site. To manage and reduce the potential for these natural processes,we recommend the following. 1. DraimVe from the roof and drivewayarea should be collected and ' tliaed to the south of the home and � discharged onto the ground, 2. No filling on the slopes unless retained by retaining walls or constructed as an engineered fill. 3. No percolation of surface water within 10 feet of the building. SEISMIC—LIQuvrAc'rlm HAzARn I According to the Seismic Zone Map of the United States contained in Figure 16-2 of the 1997 Uniform Building Code,the project site is located within Seismic Risk Zone 3. :Based on the subsurface conditions observed at the site,we interpret the site conditions to correspond to a seismic Soil Profile Type Sc, for Very Dense Soil,as defined by Table 16-7(UBC)_ This is based on the range of SPT (Standard Penetration Test)blow counts and/or probing with a%-inch diameter steel probe rod. The shallow soil amditioos were assumed to be representative for the site conditions beyond the depths explored. Based on our review of the subsurface conditions,we conclude rbat the site soils.are not susceptible to liquefaction. The near-surface soils are generally in a dense condition and the static water table is Iocated below the surface. Shaking of the already dense soil is not apt to produce a denser configuration and subsequently excess pore water pressures are not likely to be produced. 10011 Blomberg Street SW, Olympia,WA 98512 Phone#: (360)754-4612 Fax#: (360) 7544848 08/24/2005 14: 11 3607692251 DIMARCO PAGE 08/17 GEOTECHNICAL TESTING LABORATORY CONCLUSIONS AND RECOMMENDATIONS GENERAL Based on the results of our site recomiaissance and our experience in the area, it is our opinion that the site is suitable for the const awtioFn of the proposed home. The slopes located in the building portion of the site area and the adjacent property are stable relative to deep-seated instability and will not be affected by the construction. Proper drainage control measures will reduce or eiimii=the potential for erosion inl.this area,and improve slope stability. In general,the sand and gravel soils observed at the site are suitable for use as structural fill material. Material with less than 1.0 percent fines should be considered equivalent to gravel base material. The sandy soils that occur on the site are not moisture-sensitive and not susceptible to disturbance when wet. Conventional constriction equipment may be utilized for work at the site. Conventional spread footings may be utilized at the site for support of the structure_ We do recommend that roof drains be installed for the home. If a daylight basement is planned, a footing drain should be used. A vapor barrier is recommended for all slab-on- grades. Infiltration of the storrnwater is possible at the site- Pertinent conclusions and gcotechnical recommendations regarding the design and construction of the proposed development are presented below. LANDSLIDE--EROSION HAZARD AREAS Classification The Mason County Critical Areas Ordinance defines a landslide hazard area as 1) one containing slopes equal to or greater than 40 percent with more-than a to-fDot vertical relief,aid 2)one containing soils described by the Soil Conservation Service(SCS) Soil Survey as having a"severe"limitation for building site development due to slope- The slopes tasted in the area of the site are in excess of 40-pint aid the vertical relief is in,c s 0f-1'0fc0t` Based on this, this site does meet the technical criteria of a landslide hazard- The soils at the site are mapped as Everett gravelly sand in the building portions of the site. The Ev=ft soils are described as having a low erosion hazard,based on the present slope mi clination- These soils do not nwet the technical criteria of an erosion hazard. Slope Stability Based on our field observations, explorations and our experience with the soil types encountered on the property, we conclude that although portions of tho alapcs exceed 40 percent,they are generally stable relative to deep-seated -seated failure in their present configuration" No changes in slope stability are expected as a result of the proposed home on the site. Development will occur in the more or less level portions of the site based on appropriate engineering and building setback recommendations. Grading in the building portion of the site should be conducted in accordance with geoteehnical recommendations provided herein. Site drainage will be collected and directed to the site's stormwater system. loot Blomberg Street Sw,Olympia,WA 98512 Phone 4. (360) 7544612 Fax 4: (360)754-4848 08/24/2005 14: 11 3607692251 DIMARC0 PAGE 09:'17 GEOTECHNICAL TESTING LABORATORY EwsioN CONTROL It is our opinion that the potential erosion hazard of the site is not a limiting factor for the proposed development. -Removal of natural vegetation on fc steep slopes shoald 1be-munmized and limited to the active construction areas. Temporary and permanent erosion control measures should be installed and maintained during construction or as soon as practical thereafter to limit the additional influx of water to exposed areas. Erosion control measures should include,but not be limited to,bemas and swales with ground cover/protection in exposed areas. Graded areas should be shaped to avoid concentrations of rurwff onto cut or fill slopes, natural slopes or other erosion-sensitive areas. Temporary ground cover such as jute matting,excelsior matting,wood chips, straw or clear plastic sheeting should be used until permanent erosion protection is established. No earthwork is anticipated on the steep portions of the site. EARTHWORK Site Preparation All areas to be excavated should be cleared of deleterious matter including any existing structures,foundations, abandoned utility lines,debris and vegetation. Based on.our exploration,-we-estisnatethat stripping on the order of 6 to 8 inches will be necessary to remove the root zone and surfieial soils containing ergwlics. Areas with deeper, unsuitable orpnics should be expected in the vicinity of depressions or heavy vegetation. Stripping depths of up to 1 foot may occur in these areas. These materials.may be stockpiled and later used for erosion control and landscaping. Materials that cannot be used for landscaping or erosion control should be removed from the project site. Where placement of fill material is required,the exposed subgrade areas should be proof-rolled to a firm and unyielding surface prior to placement of any fill. We recommend that trees be removed in such a manner that a majority of the roots are removed unless located on a slope. Excavations for tree stump removal in the building area should be backfilled with structural fill compacted to the density requirements described in the"Structural Fill" section of this report. If fill is used for the building pad,we recommend that a member of our staff evaluate the exposed sub-grade conditions after removal of vegetation and topsoil stripping is completed and prior to placement of fill. Any soft,loose or otherwise unsuitable areas observed during construction should be compacted, if practical, or over- excavated and replaced with structural fill,based on the recornmendations of this report. Structural Fill All fill material should be placed as structural fill. 10011 Blomberg Street SW, Olympia, WA 98512 Phone#: (360)754-4612 Fax#: (360)754-4848 08/24/2005 14: 11 3607692251 DIMARCO PAGE 10/11 GEOTECHNICAL TESTING L"ORATORY The structural fill should be placed in horizontal lifts of appropriate thickness to allow adequate and uniform compaction of each lift. Fill should be compacted to at least 90 percent of MOD(maximum dry density as determined in accordance with ASTM D-1557/698)to within 2 feet of the sub-grade and 95 percent MDD in the upper 2 feet. The appropriate lift thickness will depend on the fill characteristics and compaction equipment used. We recommend that the appropriate lift thickness be evaluated by our field representative during construction. We recommend that our represeutative be present during site grading activities to observe the work and perform field density tests. The suitability of material for use as structural fill will depend on the gradation and moisture content of the soil. As the amount of fines (material passing No. 200 sieve) increases, soil becomes increasingly sensitive to small changes in moisture content and adequate compaction becomes more difficult to achieve. During wet weather,we rccomitend use of well-graded sand and gravel with less than 10 perrettt(by weight)passing the No. 200 sieve based on that fraction passing the 3/a-inch sieve. If prolonged dry weather prevails during the earthwork and foundation installation phase of construction,a somewhat Higher(up to 12 percent)fires content will be acceptable. Material placed for structural fill should be free of debris,organic matter,trash and cobbles greater than 6 inches in diameter. The moisture content of the fill material should be adjusted as necessary for proper compaction. Suitability of On--Site Soils as Fill During dry weather construction,any non-organic on-site soil may be considered for use as structural fill,provided it meets the criteria described above in the structural fill section and can be compacted as recommended. If the material is over-optimum ittoisture content when excavated,it will be necessary to aerate or dry the soli prior to placement as structural fill. The workability of a material for use as structural fill will depend on the gradation and moisture content of the soil. As the amount of fines increases, soil becomes more sensitive to small changes in moisture content and adequate compaction becomes more difficult or impossible to achieve. In general,the native soils(sand and gravel)observed at the site with less than 10 percent fines(material passing the US No. 200 Sieve)are suitable for use as structural fill. This material is comparable to WSDOT"select borrow". Material with less than 5 pent fines will be suitable as structural fill during wet weather conditions, if fill material is imported to the site for wet weather construction,we recommend that it be a sand and gravel mixture such as}ugb duality pit run with less than 10 percent fines. CUT AND FELL SLOPES All job site safety issues and precautions are the responsibility of the contractor providing services and or work. The following cut/fill slope guidelines are provided for planning purposes. Temporary cut slopes will likely be necessary during grading operations. As a general guide, temporary slopes of 1.5 to I (horizontal to vertical)or flatter may be used for temporary cuts in the upper 3 to 4€eet of the glacially consolidated soils that are weathered to a loosMnedium dense condition. 10011 Blomberg Street SW, Olympia,WA 98512 Phone#: (360)754-4612 Fax#: (360) 754-4848 08/24/2005 14: 11 3607692251 DIMARCO PAGE 11/17 G EOTECHNICAL TESTING LABORATORY Tenxporary slopes of 1 to 1 or flatter may be used in the uu-weathered dense to very dense sands and gravels or till. These guidelines assume that all surface loads are kept at a minimum distance of at least one half the depth of the cut away frOR,the top of the slope and that significant seepage is not present on the slope face- Flatter cut slopes will be necessary where significant raveling or seepage occurs. Subsurface drainage may be required if seepage areas are discovered in the building area. Surface drainage should be directed away from the area of the home and driveway. Some nunor raveling may occur with time. All eNposed slopes should be seeded as soon as practical to facilitate the development of a protective vegetative cover or otherwise protected. FOUNDATION SUPPORT All exterior footing elements should be embedded as least 18 inches below grade for frost protection. Where foundation elements are located near slopes of 5 percent or rnorv,the footings should be located a minirmrrn of 2 times the footing width from the slope face(horizontally),and founded in medium dense or denser native soils or properly prepared structural fill- We reommwad a n ofZ£net€o+�asc�latodkotinp and at kast+6,v*chm for-continuous wall f'oofmSs- Footings founded as described above can be designed using an allowable soil bearing capacity of 2,000 psf(pounds per square foot)for combined dead and long-term live Ioads in areas of medium dense to dense soils. The weight of the footing and any overlying backfill may be neglected. The allowable bearing value may be increased by one-third for transient toads such as those induced by Seismic events or wind loads. Lateral loads may be resisted by friction on the base of footings and floor slabs and as passive pressure oil the sides of footings. We recommend that an allowable coefficient of friction of 0.60 be used to calculate friction between the concrete and the underlying soil- Passive pressure may be determined using an allowable equivalent fluid density of 35 pcf(pounds per cubic fax), Factors of safety have been applied to these values. We estimate that settlements of footings designed and constructed as recommended will be less than 1 inch,for the anticipated load conditions, with differential settlements between comparably loaded footings of inch or less. Most of the settlements should occur essentially as loads are being applied. However, disturbance of the foundation sub- grade during construction could result in larger settlements than predicted. FLOOR SLAB SUPPORT Slabs-on-grade should be supported on medium dense or dense native soils or on structural fill prepared as described in the"Structural Fill"section of this report. We recommend that floor$labs be directly underlain by a rninintum 6- inch thickness of coarse sand and/or gravel containing less than 5 percent fines (by weight). The drainage material should be placed in one lift and compacted to an unyielding condition. A synthetic vapor barrier should be used for the control of moisture migration through the slab, in particular where adhesives are used to anchor carpet or tile to the slab. A thin layer of sand may be placed over the vapor barrier and immediately below the slab to protect the liner during steel and/or concrete placement_ The lack of a vapor barrier could result in wet spats on the slab,particularly in storage areas. 1001 l Blomberg Street SW, Olympia, WA 98512 Phone#: (360) 754-46I2 Fax#: (360)7544848 g 08/24/2005 14: 11 3607692251 DIMARCO PAGE 12/17 _ GEOTECHNICAL TESTING LABORATORY A subgrade modulus of 400 kcf(lips per cubic foot)may be used for floor slab design. We estimate that settlement of the floor slabs designed and constructed as recommended, will be ''/,inch or less over a span of 50 feet. Retaining Wall Retaining walls may be utilized on the sloping portion of the site to retain fill material, or for below grade parking garage or basements. The lateral pressures acting on the subgrade and mtainiug walls will depend upon the nature and density of the soil behind the wall. It is also dependent upon the presence or absence of hydrostatic pressure. If the adjacent exterior wall space is backfilled with clean granular,well-drained soil (washed rock),the design active pressure may be taken as 35 pcf(equivalent fluid density). This design value assinnes a level backslopc and drained conditions as described below_ Retaining walls located on or near the toe of a slope that extends up behind the wall should be designed for a lateral pressure, which includes the surcha.rgc effects of the steep slope in proxinxnity to the wall, Although not expected at this site,the following data is provided for planning purposes. For an irregular or composite slope,the equivalent slope angle may be detem-fined by extending a line upward from the toe of the wall at an angle of 1 to l (Horizontal to Vertical)to a point where the line intersects the ground surface. The surcharge effects may be modeled by increasing the equivalent fluid pressure for flat ground by the percentage given in the following table: Slope Inclination: Equivalent Fluid Pressure Slope Angle Percent Increase Equivalent Fluid Pressure Horizontal 0% 35 pcf 3H:1V 25% 44 pcf 2H:1V 50% 53 pcf lH.lV 75% 61 pcf If the walls are greater than 4 feet in height,exclusive of the footing,additional design considerations should be applied. Positive drainage,which controls the development of hydrostatic pressure,can be accomplished by placing a zone of coarse sand and gravel behind the walls. The granular drainage material should contain less than 5 percent fines. The drainage zone should extend horizontally at least 18 inches from the back of the wall. The drainage zone should also extend from the base of the wall to within 1 foot of the top of the wall_ The drainage zone should be compacted to approximately 90 percent of the MDD_ Over compaction should be avoided as this can lead to excessive lateral pressures. A perforated PVC pipe with a minimum diameter of 4 inches should be placed in the draimge zone aloe th the wall to direct accumulated water to an appropriate discharge location- We e base of We recommend that a non-woven geotextile filter fabric be placed between the drainage material and the remaitnjng wall bac"ll to reduce silt migration into the drainage zone. The infiltration of silt.into the drainage zone can,with time, reduce the permeability of the granular material. 10011 Blomberg Street SW, Olywpia,WA 98512 Phone#: (360) 754-4612 Fax#: (360) 754-4848 08/24/2005 14: 11 3607692251 DIMARCO PAGE 13/17 G EOTECHNICA.L TESTING LABORATORY The filter fabric should be placed in such a way that it fully separates the drainage material and the back-fill,and should be extended over the top of the drainage zone. Lateral loads may be resisted by friction on the base of footings and as passive pressure on the sides of footings and the buried portion of the wall. We recommend that an allowable coefficicnt of friction of O,50 be used to calculate friction between the concrete and the underlying soil. Passive pressure may be determined using an allowable equivalent fluid density of 35 pcf(pounds per cubic foot). Factors of safety have been applied to these values_ Rctainin al&A ernaC v Typically, reinforced-earth block wall systems are more cost effective for long term walls than the other options. Specific design criteria for these options can be provided at your request by the block manufacturers. PAVEMENT SUBGRADE We recommend that pavement subgrades be prepared in accordance with the previously described site preparation and structural fill recommendations. The upper 2 feet of roadway subgrade should have a density of at least 95 percent of the MDD (ASTM D-1577) SITE DRAINAGE All ground surfaces,pavements and sidewalks should be sloped away from the residences and associated structures. Surface water runoff should be controlled by a system of curbs, berms, drainage stivales,and/or catch basins, and conveyed to the site's retention system_ We recommend that,convent onal roof and footing drains be instAled forlhe 4xuuemW 9m*ge. Drains should be provided behind all retaining walls. The roof drain shouldilot be sec-ted''to' the footing drain. Footing drains must be msWlod,the draw invert should be below the bottom of the footing. We recommend thatthe collected stor►mvater ruriol£at the site be directed to the south side of site and discharged onto the ground. We do not ex-pect any adverse affects on the recharge condition of the groundwater system. The infiltration of a portion of the site's stormwater runoff will match the existixtg shallow groundwater recharge conditions. Because the recharge of the local shallow aquifer will not be significantly affected,the net impact to the regional aquifer will be insignificant- The recharge area for the regional aquifer essentially occurs over the entire Union River Valley Watershed area. LIMITATIONS We have prepared this report for use by Carol Dimarco and members of her design team,for use in the design of a portion of this project. The data used in preparing this report,and this report, should be provided to prospective contractors for their bidding or estimating purposes only. Our report,conclusions and interpretations are based on data from others and our site reconnaissance, and should not be construed as a warranty of the subsurface conditions. 10011 Blomberg Street SW, Olympia, WA,985I2 Phone#: (360) 754-4612 Fax#: (360) 754-4848 �o 08/24/2005 14: 11 3607692251 DIMARCO PAGE 14/17 G EOTECHNICAL TESTING LABORATORY The data used in preparing this report,and this report, should be provided to prospective contractors for their bidding or estimating purposes only. Our report, conclusions and interpretations are based oa data from others and our site reconnaissance,and should not be construed as a warranty of the subsurface conditions. Variations in subsurface conditions are possible and may also occur with time. A contingency for unanticipated conditions should be included in the budget and schedule. Sufficient consultation should be made with our firm during construction to confirm that the conditions elncounterW are consistent with those indicated by the rcomi srendations and for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether earthwork and foundation installation activities comply with contract plans, If our analysis and recommendations are followed,we do not anticipate any on site or off site impact from the construction. It is our conclusion that potential landslide hazards f om the landslide area can be overcome so as not to cause,harm to property,public health,and safety,or the environment. The scope of our services does not include services related to environmental remediation and construction safety precautions. Our recommendations are not intended to direct the contractor's metbods, technicpies, sequences or procedures,except as specifically described in our report for considerabcn in design, If there are any changes in the loads,grades, locations, configurations or type of facilities to be constructed, the conclusions and recommendations presented in this report may not be frilly.applicable. If such changes are made,we should be given the opportunity to review our recommendations and provide written modifications or verifications, as appropriate. Respectfully submitted, GEOTECHNICAL TESTING LABORATORY 7 Harold Parks, Engineering geologist sd eo�o HAROLD PARKS 7'3/ 10011 Blomberg Street SW, Olympia, WAS 98512 Phone,4: (360)7544612 Fax#: (360) 7544848 t `y f r • : � • . •^�•• yr�rj o e r%�. r , �P�3 •.:.: Pi�� ;••,: r' Jllt� / D•:S�-wad w yB��^�? /� 1 li �;t a., j! .. .IY/�!1\� IF� •i�..�... ._.A+ �.:..:�1'��1 � ,`..?'.c„U \9�✓J��i< A c N ,p A 30' LOG IN RDA � ASEM C 0 10 20 SCALE 1 " = 20 ' C. 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