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HomeMy WebLinkAboutGeoTech Report - BLD Engineering / Geo-tech Reports - 3/21/2003 -52. ems TO BE KEPT IN THE PA(ULL r -ILE Geotechnical Report 1 Hawk View Place Union, WA Prepared for William & Ruth Tremblay by Geotechnical Testing Lab Olympia, WA March 21 , 2003 GEOTECHNICAL TESTING LABORATORY WILmAM& RUTH TREMBLAY 207 TILLEY AYE. CENTRALIA,WA 98531 Re: Geotechnical Report Parcel#322-35-75-00010 1 Hawk View Phice Union,WA INTRODUCTION This report summarizes the results of our geotechnical consulting services for the proposed home to be located on the vacant lot at 1 Hawk View Place in Mason County, WA. The location of the site is shown 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 a new home is to be located on the above referenced lot. Stormwater runoff from the site,roof and hard surfaces, will be collected and directed to the east of the proposed home. 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. In general,grading will consist of the excavation of the foundation in the upper portion of the site;no structural fill is anticipated. The northern 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 geotechnical recommendations and design criteria for the project and to satisfy the requirements of the Mason County Critical Areas Ordinance. Geotechnical Testing Lab is therefore providing geologic and hydrogeologic services for the project. Specifically, our scope of services for this project will include the following: 1. Review the available geologic,hydrogeologic 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 observing a test pit. 4. Evaluate the landslide and erosion hazards at the site per the 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), 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 estimates of settlement. 7. Provide preliminary geotechnical recommendations regarding the construction of the stormwater outfall. 10011 Blomberg Street SW, Olympia, WA 98512 Phone#:(360)7544612 Fax#:(360)7544848 GEOTECHNICAL TESTING LABORATORY SITE CONDITIONS SURFACE CONDITIONS The proposed building site is located in the upland area of the south shore of the Hood Canal. The site is situated in an area of modest development along Hawks Drive. The site is bounded by forested vacant lots on each side 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 bluff at the site. Elevations in the site area range from approximately 250 feet in the north to about 500 feet in the southern portion of the site. The building site is to be located in the level portion of the upper area,where the slopes are between 0%and 5%. The steepest slope is confined to the north portion of the lot where the slope exceeds 40 percent. This area is outside of the proposed building area. Site topography is shown on the Site Plan, Figure 2. We observed no evidence of major erosion. No surficial sloughing or soil movement was observed on the steeper portion of the site or adjacent areas. No evidence 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,cedar and madrona trees. A moderate to dense understory consists of grass, brush,young alder trees, salal and Oregon grape with local areas of blueberry. No evidence of surface water flow was observed in the site area at the time of our reconnaissance. The general topography of the site area indicates that the site drains towards the south. SITE GEOLOGv The site is generally situated within the Lower Puget Sound highlands. The existing topography,as well as the surficial and shallow subsurface soils in the area, are the result of the most recent Vashon 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 site. SITE SOILS The Mason County Soil Conservation Survey(1960)has mapped the site soils as Alderwood Gravelly Sandy Loam in the building area of the site. The Alderwood soils typically form from mixed gravelly sandy till and have a low erosion hazard. We observed very little active erosion or slope disturbance in the site area during our reconnaissance. SUBSURFACE EXPLORATIONS Subsurface conditions at the sites were evaluated by observing a boring log dug to a depth of twelve ft. at the site. SUBSURFACE CONDITIONS In general, undisturbed dense sand and gravel till was observed in the building portion of the site. The undisturbed till was observed across the building portion of the lot. 10011 Blomberg Street SW,Olympia, WA 98512 Phone#:(360)754-4612 Fax#:(360)754-4848 GEOTECHNICAL TESTING LABORATORY No groundwater seepage was observed from the slope areas at the site. Based on the nature of the near surface soils, (intermittent outwash and weathered gravels and sands over undisturbed soils in the building area), seasonally perched groundwater conditions should be expected during periods of extended wet weather. Wpe Stability Slopes steeper than 40 percent were observed in the sloping portion of the site. Since slopes of 40 percent or greater with 10 feet 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 soil components 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. Evidence of deep-seated landslide activity was not observed at the site at the time of our site visit. Weathering, erosion, and the resulting sloughing and shallow landsliding are natural processes that can affect steep slope areas. Instability of this nature is typically confined to the upper weathered or disturbed zone, which has 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, wet/dry cycles, and freeze/thaw 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 movenwA in these areas. CONCLUSIONS AND RECONIlAENDATIONS GENERAL Based on the results of our site reconnaissance and our experience in the area, it is our opinion that the site is suitable for the proposed home. The slopes located north of 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 proposed home and drainfield. Proper drainage control measures will reduce or eliminate the potential for erosion in this area, and improve slope stability. 10011 Blomberg Street SW, Olympia, WA 98512 Phone#:(360)754-4612 Fax#:(360)754-4848 GEOTECHNICAL TESTING LABORATORY Conventional construction 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 and footing drains be installed for the home. A vapor barrier is recommended for all slab-on-grades. Infiltration of the stormwater is not possible at the site. Pertinent conclusions and geotechnical recommendations regarding the design and construction of the proposed home 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 10-foot vertical relief; and 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 located in the area of the site are in excess of 40 percent and the vertical relief is in excess of 10 feet. Based on this,the site does meet the technical criteria of a landslide hazard. The soils at the site are mapped as Alderwood gravelly sandy loam in the building portions of the site. The Alderwood soils are described as having a low erosion hazard, based on the present slope inclination. These soils do not meet 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 the slopes exceed 40 percent,they are generally stable relative to deep-seated failure in their present configuration. Peak Shear Stress vs.Normal Stress Sample#1 2500 F 8] 2000 (qa 1500 l0 L1000 y� -t-1 f 112 con (LD 500 0 0 500 1000 1500 2000 2500 Normal Stress(Psf) No changes in slope stability are expected as a result of the proposed home on the site. Development will occur in the 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 geotechnical recommendations provided herein. 10011 Blomberg Street SW, Olympia, WA 98512 Phone#: (360)754-4612 Fax#:(360)754-4848 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 recommendations in the slope and buffer areas are provided in the"Building Setback" and"Erosion Control" sections of this report. Building Setback A building setback area from landslide hazard areas is required unless evaluated and reduced by an engineering geologist or a licensed professional engineer. Cleaning and grading within the setback area may be allowed because the slope stability or erosion hazard will not be adversely impacted. Slope stability was modeled using the GEO-SLOPE/W program(version 5.13)in both static and dynamic conditions (ca=0.3). Factors of safety were determined using Bishop's,Janbu, and the Morgenstern-Price methods. Previously,the gravelly sandy loam was determined to have a unit weight of 138 pcf, cohesion of 0 psf, and a shear angle(+)of 48". Under static conditions,the slopes remained stable to deep-seated and shallow failure. Under dynamic loading,the 3000 computations demonstrated that the slope may be susceptible the surficial raveling, but large deep-seated failure was not demonstrated by our model See the figure below for most stable solution of a surficial failure with a F.S. equal to 2.0 under dynamic conditions. Based on our geotechnical evaluation of the site and our experience in the area, we recommend a building setback of 5 feet from a slope area of 15 percent or greater. The building setback may be measured from the bottom of the footing to the face of the steep slope, in accordance with the Unified Building Code. may Ma"s Method:Morgenste—Pnce Direction of Sp Movement Left to Pogtt SRp Surface Ophon:Grid and Radus Seismic Coefficient Honz"I and Vertical 571 530 5Ja F314y Gr&,W • 5J0 Pli s,p SOil Model.Mohr-CoubmO . eto Lktit Wert[ 138 500 ohevon' 500 48 •90 �� :e0 QJIO 400 m � ly3 •Se ` 450 0 \ 440 •tp \� t0 400 -`---{-- R _ 400 p m .0 ep 80 Oa ua 1W IN Ise JOo m LO A. JBO oWAM.cro Dynamic Conditions—F.S.> 1.0 As previously discussed, weathering,erosion and the resultant surficial sloughing and shallow landsliding are natural processes that affect slope areas. Some surficial raveling or sloughing from the past was observed in the sloping portions of the site. To manage and reduce the potential for these natural processes,we recommend the following: 10011 Blomberg Street SW,Olympia, WA 98512 Phone#:(360)754-4612 Fax#:(360)7544848 GEOTECHNICAL TESTING LABORATORY 1. No drainage of concentrated surface water or significant sheet flow onto or near the slope area. Drainage from the roof and driveway area should be collected and tightlined to the discharge area east of the proposed home with the same setback as the proposed home. 2. No filling within the buffer or setback zone unless retained by retaining walls or constructed as an engineered fill. SEISMIC—LIQUEFAMON HAZARD 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-J(UBC). This is based on the range of SPT (Standard Penetration Test)blow counts and/or probing with a 'h-inch diameter steel probe rod. The shallow soil conditions were assumed to be representative for the site conditions beyond the depths explored. Based on our review of the subsurface conditions,we conclude that the site soils are not susceptible to liquefaction. The near-surface soils are generally in a dense condition and the static water table is located 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. EROSION 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 the steep slopes(400/a,or greater)should be minimized and limited to trimming and landscaping consistent with home gardening. 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, berms and swales with ground cover/protection or silt fencing in exposed areas. Graded areas should be shaped to avoid concentrations of runoff onto cut or fill slopes, natural slopes or other erosion-sensitive areas. No earthwork is anticipated on the steep portions(40%or greater)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 observations,we estimate that stripping on the order of 6 to 8 inches will be necessary to remove the root zone and surficial soils containing organics. Areas with deeper,unsuitable organics 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. 10011 Blomberg Street SW,Olympia, WA 98512 Phone#:(360)754-4612 Fax#:(360)754-4848 GEOTECHNICAL TESTING LABORATORY Where placement of a daylight basement is anticipated, excess material should be broadcast over the lot in such a way as to not adversely affect other building or parking areas. Excavations for 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 structural fill is needed we recommend that a member of our staff evaluate the exposed subgrade conditions after removal of vegetation and topsoil stripping is completed. Any soft, loose or otherwise unsuitable areas delineated during foundation preparation or probing should be compacted, if practical,or over-excavated and replaced with structural fill,based on the recommendations of our report. Structural FUl All fill material should be placed as structural fill. 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 MDD(maximum dry density as determined in accordance with ASTM D-1557/698)to within 2 feet of subgrade 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. 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 recommend use of well-graded sand and gravel with less than 10 percent(by weight)passing the No.200 sieve based on that fraction passing the'/4-inch sieve. If prolonged dry weather prevails during the earthwork and foundation installation phase of construction, a somewhat higher(up to 12 percent)fines 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 FM 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 moisture content when excavated, it will be necessary to aerate or dry the soil prior to placement as structural fill. CUT AND FILL 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 feet of the glacially consolidated soils that are weathered to a looselmedium dense condition. Temporary slopes of I to I or flatter may be used in the unweathered dense to very dense sands and gravels or till. 10011 Blomberg Street SW, Olympia, WA 98512 Phone#:(360)754-4612 Fax#:(360)754-4848 GEOTECHNICAL TESTING LABORATORY These guidelines assume that all surface loads are kept at a minimum distance of at least one half the depth of the cut away from 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. Surface drainage should be directed away from all slope faces. Some minor raveling may occur with time. All slopes should be seeded as soon as practical to facilitate the development of a protective vegetative cover or otherwise protected. FOUNDATION SUPPORT Where foundation elements are located near slopes of 15 percent or more,the footings should be located a minimum 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 recommend a minimum width of 2 feet for isolated footings and at least 16 inches for continuous wall footings. 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 loads 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 loads such as those induced by seismic events or wind loads. Lateral loads may be resisted by friction on the bases of footings and floor slabs and as passive pressure on the sides of footings. We recommend that an allowable coefficient of friction of 0.25 be used to calculate friction between the concrete and the underlying soil. Passive pressure may be determined using an allowable equivalent fluid density of 350 pcf(pounds per cubic foot). 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 Y2 inch or less. Most of the settlements should occur essentially as loads are being applied. However,disturbance of the foundation subgrade 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 slabs be directly underlain by a minimum 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 spots on the slab,particularly in storage areas. Retaining Wall Retaining walls are not anticipated on this project. 10011 Blomberg Street SW,Olympia, WA 98512 Phone#:(360)754-4612 Fax#:(360)754-4848 GEOTECHNICAL TESTING LABORATORY 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 swales,and directed to a discharge area at least 20 feet from the proposed home and recognizing the same setback from the bluff. We recommend that conventional roof and footing drains be installed for the home and garage. The roof drain should not be connected to the footing drain. If footing drains are installed,the drain invert should be below the bottom of the footing. We do not expect any adverse affects on the recharge condition of the groundwater system. LEWTATIONS We have prepared this report for use by William and Ruth Tremblay and members of their 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. 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 encountered are consistent with those indicated by the recommendations 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 from 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 methods,techniques,sequences or procedures,except as specifically described in our report for consideration 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 fully applicable. If such changes are made, we should be given the opportunity to review our recommendations and provide written modifications or verifications, as appropriate. t Wesh Respectfully submitted, GEOTECHNICAL TESTING LABORATORY y 7 �i 7 ti� Harold Parks, �q 1I27 �O sd GOO Engineering geologist a HAROLD PARKS 31 d 3 10011 Blomberg Street SW, Olympia, WA 98512 Phone#:(360)754-4612 Fax#:(360)754-4948 GEOTECHNICAL TESTING LABORATORY Geotechnical General notes SOIL PROPERTY SYMBOLS N: Standard"N"penetration: Blows per foot of 140 pound hammer falling 30 inches on a 2 inch O.D. split-spoon. Q,,: Unconfined compressive strength,tons/ft2 Q,: Penetrometer value,unconfined compressive strength, lbs/ft2 V: Vane value,ultimate shearing strength, lbs/ft2 M: Water content,% LL: Liquid limit,% PI: Plasticity index, % D: Natural dry density, lbs/ft3 WT: Apparent groundwater level at time noted after completion. DRILLING AND SAMPLING SYMBOLS SS: Split-Spoon- 1 3/8" I.D., 2"O.D.,except where noted. ST: Shelby Tube-3"O.D., except where noted. AU: Auger Sample. GB: Grab Sample. DB: Diamond Bit. CB: Carbide Bit. WS: Washed Sample. RELATIVE DENSITY AND CONSISTENCY CLASSIFICATION Terms(Non-Cohesive Soils) Standard Penetration Resistance Very Loose 0-2 Loose 2-4 Slightly Compact 4- 8 Medium Dense 8- 16 Dense 16-26 Very Dense Over 26 Terms(Cohesive Soils) Q.-(tons/ftz) Very Soft 0-0.25 Soft 0.25 -0.50 Firm (Medium) 0.50- 1.00 Stiff 1.00-2.00 Very Stiff 2.00-4.00 Hard 4.00+ PARTICLE SIZE Boulders 8 in. + Coarse Sand 5 mm-0.6 mm Silts 0.074 mm -0.005 mm Cobbles 8 in.-3 in. Medium Sand 0.6 mm-0.2 mm Clays 0.005 mm&Smaller Gravel 3 in.-5 mm Fine Sand 0.2 mm -0.074 mm 10011 Blomberg Street SW,Olympia, WA 98512 Phone#: (360)7544612 Fax#: (360)7544848 I 1 �C � —r��.�� ���_•►'_�,�/ �if1,-' ice:s�'l%? • 1 I r ` � GEOTECHNICAL TESTING LAB Date: Mar.7,03 File#: Boring Log#: 1 Client: Tremblay Boring Type: Excavator Depth Drilled: 12 feet Field Change 6n) 1)esLnption in Soils %M N %'%" %44 %4200 Comments 6.0 f opn oil 12.0 gravel\ sand 8.7". 18.0 01 0% 13°,, dense 24.0 30.0 36.0 42.0 Sandi nra%J 48.0 54.0 60.0 66.0 72.0 78.0 84.0 90.0 96.0 102.0 108.0 114.0 120.0 and Boren, 126.0