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HomeMy WebLinkAbout998200318 Geotech AssessmentPh 253-896-1011 Fx 253-896-2633 GeoResources, LLC 5007 Pacific 1-Iwy. E., Ste. 20 Fife, Washington 98424-2649 October 13, 2004 Mr. Richard Stmad 150 South 363`d PI Federal Way, Washington 98003 Geotechnical Report -Engineering Services Lot 19 - Talla Shores # 3 - .. Single-Family Residence _ _ ~ Jefferson County, Washington -- PN:998200318 Job No:StmadR.TallaSh.RG INTRODUCTION & SCOPE This report summarizes our site observations and explorations, and provides our conclusions, geotechnical design criteria and engineering recommendations for the proposed residential structure to be constructed on Lot 19 of the Talla Shores residential plat. The subject site is located on east shoreline and bluff of Hood Canal southeast of Port Ludlow in Jefferson County, Washington. The location of the site relative to surrounding features is shown on the Vicinity Map, Figure i. The steep bluff slopes in the east portion of the site require a geotechnical report be prepared in accordance with the Jefferson County Critical Area regulations. Our understanding of the project is based on our discussions with you, our site reconnaissance and explorations, our review of the plans provided and our experience in the area. We understand that the proposed new residential structure will consist of a conventional raised floor structure. The septic system for the residence will be located in the east portion of the site. Stormwater runoff from the structure will be directed to the west portion of the site or tightlined to the shoreline/beach area. We also understand that grading at the site will be minimal, generally related to excavation of the #ounda#ion and utilities. The proposed layout of the site is shown on the Site Plan, Figure 2. The purpose of our services is to evaluate the surface and subsurface conditions at the site as a basis for developing geotechnical recommendations and design criteria for the project, as well as addressing the Jefferson County Critical Area regulations. Specifically, our scope of services for this project includes the following: 1. Review the available geologic, hydrogeologic and geotechnical data~r 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 backhoe test pits. 4. Based on our site observations, subsurface conditions and experience, provide geotechnical recommendations for site grading including site preparation, subgrade 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. 5. Address the Jefferson County Critical Area regulations and provide a recommend setback from the steep slopes at the site. 6. Provide recommendations and design criteria for foundation and floor stab support, including allowable bearing capacity, subgrade modulus, lateral resistance values Strnad -Talla Shores October 13, 2004 Page 2 and estimates of settlement. 7. Provide recommendations and design criteria for design of conventional subgrade/retaining walls, including backfill and drainage requirements, lateral design loads, and lateral resistance values. 8. Provide recommendations for driveway subgrade preparation. SITE CONDITIONS SURFACE CONDITIONS Lot 19 of the Talia Shores # 3 residential plat is located in the east portion of the Port Ludlow upland area and shoreline. Access to the site is via a gravel driveway that extends east from Talla Shores roadway. The site for the well will be located in the northwest portion of the site. The ground surface in the upland portion of the site is generally #lat to gently sloping (5 percent} to the east. Ground surface elevations at the site range from approximately 115 feet in the west to 105 #eet in the east upland area. A natural drainage swale traverses the property to the north. We understand that this property is to remain undeveloped and be a park. The extreme east portion of the site is a steep shoreline bluff slope that extends down to the beach area approximately 70 feet. Site topography is shown on the Site Plan, Figure 2. Areas of 15 to 40 percent slope and greater than 40 percent slope, as well as the top of the bluff, are iliustra#ed on the Site Plan, Figure 2. - We observed no evidence of mass soil movement or deep-seated slope instability in the site area at the time of our site visit. No evidence of significant erosion was observed in the upland portion of the site. Localized areas of erosion and surficial sloughing were observed on the steeper portions of the bluff slope. The upland portion of the site is vegetated with second growth timber with scattered old growth trees along the north margin. The east sloping area is vegetated with heavy brush and small to moderate trees. Avery dense understory occurs throughout the site. No evidence of surface water flow or groundwater was observed at the site at the time of our reconnaissance. The general topography of the site area indicates that the site drains generally towards the northeast, towards the drainage swale. SITE GEOLOGY Our interpretation of the geologic conditions at the site area based on our data review, site observations, subsurface explorations and experience in the area. The site is generally situated within the Port Ludlow glacial upland area. The existing topography, as well as the surficial and shallow subsurface soils in the area, is the result of glacial ice during the Vashon stade of the Fraser glaciation that occurred between about 12,000 and 15,00,0 years ago, and weathering and erosion that has occurred since. A description of the surficial soils is included in the "Site Solis" section of this report. In general, Vashon glacial till underlies the area east of the site, and extends into the west portion of the subject site. The glacial till consists of silty sand with variable gravel, cobbles and boulders that was deposited at the base of the advancing glacial ice. The till is commonly refereed to as hardpan and is in a very dense condition where undisturbed. In the west portion of the site, the till is a thinning remnant that has been weathered to a medium dense condition. The east and central portions of the site are underlain by advance outwash sand with minor gravel. The advance outwash material generally consists of sand with minor gravel and variable silt, cobble and boulder con#ent. The outwash material was deposited by melt waters of the advancing glacial ice and is typically in a dense to very Stmad - Talla Shores October 13, 2004 Page 3 dense condition where undisturbed. The soils exposed in the bluff area at the site consists of older glacial and inter-glacial soils. All of these soils have been overridden by the glacial ice and are therefore in a dense to very dense condition where undisturbed. SITE SOILS The SCS (Jefferson County Soii Conservation Survey) has mapped the site soils as Sinclair gravelly sandy loam (SnC or D). These soils have a slight to moderate erosion hazard based on an increasing slope inclination. A copy of the SCS map for the site area is included as Figure 3. We observed very little or no active erosion in the site area during our reconnaissance. Minor erosion in the form of raveling was observed in the exposed loose surficial weathered soils in the disturbed bluff slope areas. Based on our observations, the site soils appear to have a low susceptibility to erosion, particularly where vegetation is established. SUBSURFACE EXPLORATIONS Subsurface conditions at the site were evaluated by monitoring the excavation of 4 rubber-tired backhoe test pits (and perc holes) to depths of 4.5 to 14.5 feet below the existing ground surface. The test pits were located in the field by our representative by pacing from existing site features such as property comers. The approximate locations of the test pits are indicated on the attached Si#e Plan Map, Figure 2. Our representative continuously monitored the excavation of the test pits, maintained logs of the subsurface conditions, and obtained representative samples, as needed. The soils encountered were visually classified in accordance with the system described in Figure 4, ASTM D-2488. The logs of the test pits are included as Figure 5. SUBSURFACE CONDITIONS In general, undisturbed dense glacial outwash was encountered at depth in the test pits excavated in the upland portion of the site. The undisturbed outwash was overlain by 3 to 5 feet of weathered outwash sand with minor gravel and cobbles. The undisturbed advance outwash is typically in a dense to very dense condition. The overlying weathered outwash is in a medium dense condition, except for the near surface soils (approximately upper 0.5 to 2-feet), which are in a loose condition. No groundwater seepage was observed in the test pits excavated a# the site, although we expect that seasonal perched groundwater occurs above the till in the west portion of the site and the adjacent west areas. Localized areas of damp soil and minor seepage were observed west of Talla Road, likely above the contact of the underlying undisturbed glacial till and the overlying less dense soils. The seasonal_~erched groundwater will likely be absent or at its lowest point during late summer and early fall. SLOPE STABILITY In general, the undisturbed native soils at the site consist of dense to very dense sand with variable amounts of silt, gravel, cobbles and boulders, glacial outwash in the upland area and dense to very dense older glacial and inter-glacial soils in the lower portions of the site. These soil materials are in a dense to very dense condition except where they have been disturbed by weathering. Near surface and shallow weathered sand and silty sand soil deposits are in a medium dense to dense condition except at the ground surface. The surficial-soils are generally in a loose to medium dense condition. No evidence of significant landslide activity or erosion was observed a# the site or immediate adjacent areas at the time of our. site visit. Localized areas of surficial sloughing and erosion were observed on portions of the shoreline bluff. Proper Strnad - Talla Shores October 13, 2004 Page 4 planning, design and construction techniques (bulkhead) will reduce the risk of potential development related increases surficial erosion or shallow soil movement in these areas. Erosion control recommendations are provided in the "Erosion and Sediment Control" section of this report. CONCLUSIONS AND RECOMMENDATIONS GENERAL Based on the results of our site reconnaissance, subsurface exploration programs and data review, and our experience in the area, it is our opinion that the site is suitable for the proposed residential development, provided the recommendations in this report are incorporated into the design. The steep buff slopes located in the east portion of the site are generally stable relative to deep-seated failure and will not be adversely affected by properly designed and constructed development. Grading at the site will likely be minimal, generally related to construction of the proposed residence and utilities. Proper temporary and permanent erosion/sediment and drainage control measures will minimize the potential for erosion and the transport of sediment to the shoreline area. The sandy soils that occur in the upland portion of the site are suitable for use as structural fill during moderate wet to dry weather. Perched ground water conditions may be associated with the soils in the extreme west portion of the site during or following extended or heavy periods of precipitation. Portions of the shallow soils at the site were observed to be at or above the optimum moisture content during our site evaluation. It will likely be necessary to dry (moisture condition) over optimum soils prior to use as structural fill. ' it is our opinion that the west portion of the site is suitable for stormwater infiltration - provided the system is properly designed and constructed. No stormwater infiltration should occur in the east portion of the site. Alternatively, the collected stormwater may be tightlined to the beach area. Pertinent conclusions and geotechnical recommendations regarding the design and construction of the proposed development are presented below. LANDSLIDE HAZARD AREAS The slopes located in the extreme east portion ofi the site slope at between 40 and 160 percent. The steeper bluff slopes at the site and adjacent areas meet the technical criteria of a Landslide Hazard area per the Jefferson County Critical Area Ordinance. Localized areas of steeper slope occur in the lower portion of the shoreline bluff and adjacent areas. The underlying soils at the site consist of glacially consolidated advance sand and older soils that are in a very dense condition where undisturbed. A thin veneer of weathered soil occurs at the ground surface and on the bluff surface. These soils are in a medium dense to dense condition, except at the ground surface where it is weathered. Based on our site observations and experience, it is our opinion that the soils at the site are stable relative to deep-seated slope movement. No evidence of landslide activity was observed at the site or the adjacent areas. Provided the site is developed in accordance with the recommendations provided in this report, no change in slope stability at the site is expected. The soils at the site are mapped as Sinclair soils by the SCS. Building site limitations listed in the SCS for these soils are based on steep slope inclinations. The residential structures at the site will be located in the flatter upland areas at the site. The driveway will extend east form Talla Road. The risk of shallow or surficial slope movement at the site can be reduced through appropriate design and earthwork and drainage/erosion controls. It should be noted however, that there is an inherent risk associated with steep shoreline bluff slopes. This risk can be reduced or partially mitigated, but cannot be eliminated. Geotechnical Stmad - Talla Shores October 13, 2004 Page 5 recommendations for the project are discussed in detail in the following sections of this report. Building Setback Based on our understanding of the proposed minimal grading at the site and the location of the driveway and residential structure the steeper bluff slopes in the east portion of the site will remain undisturbed. Where slopes are steeper than 40 percent, effectively the shoreline bluff slope, we recommend a Structural Setback of 50 feet. This building setback includes the setback and buffer areas and is measured horizontally from the #ace of the slope to the base of the footing. As noted, no evidence of significant surficial erosion, raveling, sloughing or landslide activity was observed in the upland area during our site visits. Localized areas of surficial erosion and sloughing were observed on the steeper bluff slope. To manage and reduce the potential risk for these natural processes, we recommend the following: • No drainage of concentrated surface water or significant sheet flow onto or near the slope area, unless as a slope seep such as a septic system. Drainage from the roof and driveway areas should be infiltrated in the west portion of the site or should be collected and tightlined to the toe of the slope/shoreline. An appropriate energy dissipa#ion system should be installed to spread the flow and prevent erosion at the discharge point, as appropriate. • No filling on or near slopes greater than 15 percent unless constructed as an engineered slope fill or retained by engineered retaining walls. SEISMIC -LIQUEFACTION According to the Seismic Zone Map of the United S#ates contained in Figure 16-2 of the 1997 UBC (Uniform Building Code} or IBC (International Building Code), the project site is located within Seismic Risk Zone 3. Based on the subsurface conditions observed at the site, the site condi#ions correspond to a seismic Soil Profile type Sp, for Dense Soil, as defined by Table 16-J (UBC). This is based on the blow counts encountered in the boring, the range of SPT (Standard Penetration Test) blow-counts and probing with a'/2-inch diameter steel probe rod in our test pits a# the site. The recent 6.8 magnitude earthquake in the Puget Sound area provided an in-situ design- - level test at the site. No ground disturbance was observed at the site following the earthquake. Based on our review of the subsurface conditions, we conclude that the site soils are not susceptible to liquefaction. The site soils consist of silty sand with gravel and fine to medium sand that are generally in a dense to very dense condition. The static water table is located well below the site. Shaking of the already dense soil is not apt produce a denser configuration and subsequently excess pore water pressures are not likely to be produced. EROSION AND SEDIMENTATION CONTROL Erosion hazard areas are typically defined as "those areas that are classified as having moderate to severe, severe or very severe erosion potential by the Soil Conservation Service, United States Department of Agriculture (USDA}." The subject property is located in an area mapped by the Soil Conservation Service as Sinclair gravely sandy loam. The erosion hazard for this soil ranges from little to moderate, based on slope inclination. 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 should be minimized and limited to the active construction areas. Temporary and permanent erosion control Stmad - Talla Shores October 13, 2004 Page 6 measures should be installed and maintained during construction or as soon as practical thereafter to-limit the additional influx of water to exposed areas and protect potential receiving waters. Erosion control measures should include, but not be limited to, berms and swales with check dams to channel surface water runoff, ground cover/protection in exposed areas and silt fences. Graded areas should be shaped to avoid concentrations of runoff onto cut or fill slopes, natural slopes or other erosion-sensitive areas. Temporary ground cover/protection such as jute matting, excelsior matting, wood chips or clear plastic sheeting should be used until permanent erosion protection is established. EARTHWORK Site Preparation All areas to be graded/excavated should be cleared of deleterious matter including any existing structures, foundations, abandoned utility lines, debris and vegetation. Graded areas should be stripped of any forest duff and organic-laden soils. Stripping should be limited to the organic rich material and not include soil material with occasional root fibers. Based on our explorations, we estimate that stripping on the order of 4 to 6-inches will be necessary to remove the forest duff, root-mat zone and surficial soils containing significant organics. Areas with deeper, unsuitable organics should be expected in the vicinity of depressions, steeper slopes or dense vegetation. Stripping depths of up to 2 feet may occur in these areas. These materials may be stockpiled and later used for erosion control and landscaping/revegetation. Materials that cannot be used for landscaping or erosion control should be removed from the project site. Any existing areas of fill material if encountered and present below proposed final grades of future home sites or project improvement areas, should be removed and recompacted in accordance with the recommendations provided in this report. Where placement of fill material is required, the exposed subgrade soil areas should be compacted to a firm and unyielding surface prior to placement of any fill. We recommend that trees be removed by overturning in fill areas so that a majority of the roots are removed. Excavations for tree stump removal should be backfilled with structural fill compacted to the densities described in the "Structural Fill" section of this report. We recommend that a member of our staff evaluate the exposed subgrade conditions after removal of vegetation and topsoil stripping is completed and prior to placement of structural fill. The exposed subgrade soil should be proofrolled with heavy rubber-tired equipment during dry weather or probed with a 1/2-inch-diameter steel rod during wet weather conditions. Any soft, loose or otherwise unsuitable areas delineated during prQOfrolling or probing should be recompacted, if practical, or overexcavated and replaced with structural fill, based on the recommendations of our site representative. Structural Fill All fill material/trench backfill should be placed as structural fill unless in landscape or open space areas. 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) to within 2 feet of subgrade and 95 percent MDD in the upper 2 feet. Structural fill placed on slopes of so percent or steeper should be keyed and benched into the underlying medium dense soils. 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 Stmad - Talla Shores October 13, 2004 Page 7 .~- s ,_.E~ field representative during construction. We recommend that our representative 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 US 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 5 percent {by weight) passing the US No. 200 sieve based on that fraction passing the 3/4-inch sieve. If prolonged dry weather prevails during the earthwork and foundation installation phase of construction, a somewhat higher (up to 10 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 Materials as Fill During dry weather construction, any nonorganic on-site soil may be considered for use as structural fiill, 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. Many ofi the soils encountered in our test pits appeared above optimum moisture conten#. The workability of material fior use as structural fill will depend on the gradation and moisture content of the soil. As the amount of fines increases, soil becomes increasingly more sensitive to small changes in moisture content and adequate compaction becomes more difficult or impossible to achieve. The areas of sand with occasional gravel encountered in a majority of the site are suitable for use as structural fill under generally any weather condition. We recommend that completed graded-areas be restricted from traffic or protected prior to wet weather conditions. The graded areas may be protected by paving, placing asphalt-treated base, a layer of free-draining material such as pit run sand and gravel or crushed rock (2-inch minus) material containing less than 5 percent fines, or some combination of the above. If fill material is imported to the site for wet weather construction, we recommend that it be a sand and gravel mixture such as high quality pit run with less than 5 percent #ines. All fill materials should be placed as structural fill and compacted to at least 95 percent of the MDD. During wet weather conditions, traffic should be con#ined to protected areas. CUT AND FILL SLOPES _ All job site safety issues and precautions are the responsibility of the contractor providing services/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 1 (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 loose/medium dense condition. Temporary slopes of 1 to 1 or flatter may be used in the unweathered dense to very dense sands and gravels or till. Where ground water seepage is encountered, flatter temporary slopes may be required. 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. Stmad - Talla Shores October 13, 2004 Page 8 We recommend a maximum slope of 2 to 1 for permanent cut and fill slopes. Where 2 to 1 slopes are not feasible, retaining structures should be considered. Fill placed on slopes that are steeper than 5 to 1 should be "keyed" into the undisturbed native soils by cutting a series of horizontal benches. The benches should be 1'h times the width of equipment used for grading and a maximum of 3 feet in height. Subsurface drainage may be required in seepage areas. 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 Provided the building setback can be achieved from the top of the 40 percent slope, conventional spread/strip footings may be used for the structure. We recommend that spread/strip footings be founded on medium dense to dense native soils or on structural fill that extends to suitable native soils. The soil at the base of the excavations should be disturbed as little as possible. All loose, soft or unsuitable material should be removed or recompacted, as appropriate. A representative from our firm should observe the foundation excavations to determine if suitable bearing surfaces have been prepared. Ail footing elements should be embedded at least 18 inches below grade for frost protection. 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. Where footings extend to or are founded in the undisturbed very dense glacial till, the allowable bearing capacity may be increased to 3,000 psf. The vertical and lateral bearing pressures for footings will be reduced in proximity to slopes. The vertical bearing pressure and the lateral design pressure are based on a horizontal ground surface being present for at least twice the width of the footing. 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 base of footings and floor slabs and as passive pressure on the sides of footings. We recommend that an allowable coefficient of friction of 0.35 be used to calculate friction between the concrete and the underlying soil. Passive pressure may be determined using an allowable equivalent fluid density of 300 pcf (pounds per cubic foot). 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 1/2 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. In the event that the building setback cannot be met, we recommend that the foundation elements be extended vertically to meet the setback criteria (measured from the face of the slope). This may include vertical extension of the footing/stemwall, needle piles, or piers. Site specific recommendations can be provided at your request. FLOOR SLAB SUPPORT Slabs-on-grade should be supported on medium dense or denser native sails 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 gravel containing less than 3 percent fines. Where the native soils at the site Stmad - Talla Shores October 13, 2004 Page 8 meet this criteria, they may be substituted. 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 file 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. A subgrade modulus of 400 kcf (kips per cubic foot) maybe used for floor slab design. We estimate that settlement of the floor slabs designed and constructed as recommended, will be 1/2 inch or less over a span of 50 feet. SUBGRADE AND RETAINING WALLS The lateral pressures acting on subgrade and retaining 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 walls are backfilled with granular well-drained soil, the design active pressure may be taken as 35 pcf (equivalent fluid density). This design value assumes a level backslope and drained conditions as described below. 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 drainage zone along the base of the wall to direct accumulated water to an appropriate - - discharge location. We recommend that a nonwoven geotextile filter fabric be placed between the drainage material and the remaining wall backfill 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. The filter fabric should be placed .such that it fully separates the drainage material and the backfill, 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 coefficient of friction of 0.35 be used to calculate friction between the concrete and the underlying soil. Passive pressure may be determined using an allowable equivalent fluid density of 300 pcf (pounds per cubic foot). Factors of safety have been applied to these values. PAVEMENT/DRIVEWAY 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). The Driveway will extend across the wes# portion of the site. The sandy soil material should be suitable for use as subbase material in #his area, and should be compacted as described in the STRUCTURAL FILL section of this report, or replaced with a pit run sand and gravel structural fill material. Where the underlying soils are in a marginal condition, we recommend that an appropriate geotextile fabric be placed prior to the replacement/pfacement of the structural fil-. SITE DRAINAGE All ground surfaces, pavements and sidewalks should be sloped away from the Stmad - Talla Shores October 13, 2004 Page 10 residences and associated structures. Surface water runoff should be controlled by a system of curbs, berms, drainage swales, and or catch basins, and conveyed to the west portion of the site for infiltration, or tightline to the shoreline area. We recommend that conventional roof and footing drains be installed for all structures. Drains should be provided behind a!I retaining walls. The roof drain should not be connected to the footing drain unless an adequate gradient or structure will prevent a surcharge of the footing drain. LIMITATIONS We have prepared #his report for use by Mr. Richard Strnad and members of the 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 limited site reconnaissance, and should not be construed as a warranty of the subsurface conditions. Variations in subsurface conditions are possible between the explorations and may also occur with time. A contingency for unanticipated conditions should be included in the budget and schedule. Sufficient monitoring, tes#ing and consultation should be provided by our firm during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations 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 and specifications. 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. ® O Stmad - Talla Shores October 13, 2004 Page 11 Within the limitations of scope, schedule and budget, our services have been executed in accordance with generally accepted practices in this area at the time this report was prepared. No other conditions, express or implied, should be understood. Respectfully Submitted, GeoResources, LLC Bradley P. Biggerstaff, LEG Principal Kurt Groesch, PE Principal ~. -~ ~~ . ~. ~, ~~~_ ~~ Via, ~~~~,~~, i 't ~ r, r, -~ ~r F=~:..:.~x~.~-.~~~,r ~~ ~_.~ ~' li{i I ~ ~~~ BPB:KG:bpb Docl D: StmadR.TallaSh. RG R R't ~ 05 ter, ~ W 3 COUR: UCFR ai _ WAY lftkER ~T rya CT .~. ~~~i ~ ~ :` u y~ ;~, d+._ . ~ ._ . ___ - __ -__.. __. _. y~--. __ '~.~~ ~~ ~ `^' ~f ,. ,_ ~~~ ~' _ ~~: ~ ~.~ © 2002 Thomas Bros. GeoResources, LLC L _ BASr1LT ~ ~~ ~~~~ - a SIM ~ ~O 1 ~ ,. S~:V;r lt~lt.tt ~~YE _ ~ ~` 5007 Pacific Highway East, Suite 20 Fife, Washington 98424 Phone: 253-896-1011 Fax: 253-896-2633 Figure 1: Site vicinity Map Taiia Shores Road D TJ O O ?~ m~ m -~-~ cn m ~r -o O ~n ~_ O z -~ w ~, 0 m o ~ ~ n "l7 rn m z ~ -~ n O m ~ z ~ ~ ~ ~ m ~ ~ ~ m m 7J D ~~ r m O ~' m D m D ~ ~~ m o v ~~~ w 00 ~ n. ~~ rn~~ -` `~ _ o~~• 1 ~^^'' 7~ x ~, N ~ ~ W C 00 ~ ~ O N W W O O n V/ r n ~ n . r ~ 'Tj ° ° ~ m~ ~~ ~ ~~C ~ o ~ ~ m m ~ ~~ ~~ ~a~ o~N ~' Q- ~ o N m ~ o o °P~ ~~~ o Q ~ ~ ~ 0 S -, a n CD I I W O a~ a A..~-:' ~~ rrr Z 0 ,---~ ~. -~ _ i. __ __ ~; _ 3 ;- ~ 3 ~~ __ ~~ t3 1. Not to scale GeoResources, LLC 5007 Pacific Highway East, Suite 20 Fife, Washington 98424 Phone: 253-896-1011 Fax: 253-896-2633 figure 3: USDA SCS Ii~Iap Tana Shores Road SOIL CLASSIFICATION SYSTEM MAJOR DIVISIONS GROUP GROUP NAME SYMBOL GRAVEL CLEAN GW WELL-GRADED GRAVEL, FINE TO COARSE GRAVEL E GRAV L GP POORLY-GRADED GRAVEL COARSE GRAINED More than 50% SOILS Of Coarse Fracton GRAVEL GM SILTY GRAVEL H FINES Retained on WIT No. 4 Sieve GC CLAYEY GRAVEL SAND CLEAN SAND SW WELL-GRADED SAND, FINE TO COARSE SAND More than 50% Retained on SP POORLY-GRADED SAND No. 200 Sieve More than 50% Of Coarse Fraction SAND SM SILTY SAND Passes WITH FINES No. 4 Sieve SC CLAYEY SAND SILT AND CLAY INORGANIC ML SILT FINE CL CLAY GRAINED SOILS Liquid Limit Less than 50 ORGANIC OL ORGANIC SILT, ORGANIC CLAY SILT AND CLAY INORGANIC MH SILT OF HIGH PLASTICITY, ELASTIC SILT More than 50% Passes CH CLAY OF HIGH PLASTICITY, FAT CLAY No. 200 Sieve Liquid Limit 50 or more ORGANIC OH ORGANIC CLAY, ORGANIC SILT HIGHLY ORGANIC SOILS PT PEAT NOTES: 1. Field classification is based on visual examination of soil in general accordance with ASTM D2488-90. 2. Soil classification using laboratory tests is based on ASTM D2487-90. 3. Description of soil density or consistency are based on interpretation of blow count data, visual appearance of soils, and or test data. SOIL MOISTURE MODIFIERS: Dry- Absence of moisture, dry to the touch Moist- Damp, but no visible water Wet- Visible free water or saturated, usually soil is obtained from below water table _ _ _. ~~ ~ ~ GeoResources, LLC SOIL CLASSIFICATION SYSTEM 5007 Pacific Hwy. E, Ste 20 Fife, Washington 98424-2648 Ph. 253-896-101 ~ Fx. 253-896-2633 FIGURE 4 TEST PIT LOGS STRNAD RESIDENTIAL SITE TALLA SHORES ROAD JEFFERSON COUNTY, WASHINGTON TEST PIT 1 -Located in SE portion of site, cleared area Depth (ft) Soil Type Description 0.0 - 0.5 Duff/ Topsoil 0.5 - 2.5 SP Brn SAND w/ silt, occ gravel and organics (roots to 5", tree} (loose, moist) 2.5 - 4.5 SP Brn SAND w/ occ gravel and trace silt and org (roots) (med dense, moist) 4.5 - 7.0 SP Brn SAND w/ gravel and Honor silt {loose to med dense, moist) 7.0 - 12.0 SP Brn SAND w/ occ gravel and trace silt (dense, moist) Minor caving observed. No groundwater seepage observed TEST PIT 2 -Located in central portion of site, south driveway area Depth (ft) Soil Type Description 0.0 - 1.5 Duff/ Topsoil 1.5 - 2.5 SP Brn SAND w/ silt, occ gravel and organics (roots to 6", tree) (loose, moist) 2.5 - 4.5 SP Brn SAND w/ occ gravel and silt and org (roots) (med dense, moist) 4.5 - 7.0 SM Brn si SAND w/ gravel (med dense to dense, moist} (wea till) 7.0 - 11.0 SP Brn SAND w/ occ gravel and trace silt (dense, moist) Minor caving observed No groundwater seepage observed TEST PIT 3 -Located in west portion of site, driveway area (perc hole) Depth (ft) Soil Type Description 0.0 - 1.5 _ Duff/ Topsoil 1.5 - 2.5 SP Brn SAND w/ silt, occ gravel and organics (roots to 6" free) (loose, moist) 2.5 - 4.0 SP Brn SAND w/ occ gravel and silt and org (roots) (med dense, moist) 4.0 - 7.5 SM Brn si SAND w/ gravel (med dense to dense, moist) (wea till) 7.5 - 8.5 SP Brn SAND w/ occ gravel and trace silt (dense, moist) Minor caving observed No groundwater seepage observed Memo To: Linda Adkins, Je From: Brad Biggerstaff CC: File Date: January 3, 2004 Re: Stmad Residential Lot, Based on our discussions with Rick,:=~ undersfaritl that you have questions regarding the septic suitability of the Talla Shores residential site. As indicated in our geotechnical report, the site soils consist of sand with minor gravel to the #ull depth excavated in the test pits excavated in the east and central portion of the site, up to 12 feet A thin layer of silty sand, possibly weathered glacial till, was encountered in the west portion of the lot between 4 and 7 feet below the adjacent ground surface. No groundwater seepage was encountered in the test pits at the site. We are aware of the perched groundwater conditions that occur on the lots to the north and south of the subject site. No groundwater was encountered at the site, likely fuse of the generally clean sand soils. Based on our explorations and experience in the area, the glacial till is absent at the site, likely eroded away over the last several thousand years. Because the sand extends to the ground surface, no perching condition can occur. The generally clean sand with no groundwater provides the perfect conditions for a septic system. Both the main and reserve systems will be situated in these soils. The controlled discharge and low volume of effluent released from the septic sys#em will have not adverse impact on the bluff stability at the site. We do recommend that the septic systems be located greater than 25 feet form the top of the bluff slope. The septic system drainfields were relocated to the east portion of the lot so that the roof water infiltration system could be located in the west portion of the site. The roof system will discharge a greater volume of water than the septic system. Locating the roof system farther form the bluff slope provides the water a greater opportunity to disperse both vertically and laterally, thus ~iucing the potential impact to the bluff slope. We hope this addresses these issues. If you have further questions, please contact us to discuss. • Page 1