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HomeMy WebLinkAbout821265005 Geotech AssessmentPh 253-279-1023 Fx 253-638-8992 DEPT. OF COMMUNITY DEVELOPMENT GeoResources 5113 Paeific Hwy E., Ste. 1-I Fife, Washington 98424-2642 March 7, 2002 Olympic Ridge 15940 Lindsay Lane Paulsbo, WA 98370 Attn: Mr. Joe Klinker Geotechnical Consultation Single-Family Residential Lot Lot 5, White Rock Cove Tracts Jefferson County, Washington Job No. OlympicRidge.04 INTRODUCTION This report summarizes our site observations and conclusions, and provides geotechnical design criteria and recommendations for the support of the proposed residential structure at Lot 5 within the White Rock Cove residential tracts, east of White Cove and Paradise Bay Roads. The location of the site is shown on the Vicinity Map, Figure 1. We understand that you propose to construct a conventional residential structure and driveway at the site. In general, grading for the proposed site development will be minimal. The purpose of our services is to evaluate the soil conditions at the site and provide foundation support recommendations and design criteda for the proposed structure. Subsurface conditions were evaluated using the existing test pits at the site, available geologic data and our site reconnaissance. Our services are provided at your request. SITE CONDITIONS SURFACE CONDITIONS The project site is situated on the east shoreline bluff of the Hood Canal-Port Ludlow Upland area. The proposed residential structure will be located in an area of established and newer residential construction. The ground surface in the upland portion of the site is fiat to gently sloping to the east. A steep shoreline bluff occurs in the extreme east portion of the site, reaching approximately 80 feet. The shoreline bluff slope ranges from approximately 100 percent to 200 percent with near vertical areas. Vegetation at the site generally consists of grasses with scattered brush and trees in the upland area. Vegetation on the bluff face was intermittent, generally absent from areas of recent erosion and surficial sloughing. Based on the SCS and Coastal Zone Atlas maps, the soils inthe site area generally consist of sand and gravel with variable silt, cobbles and boulders in the lower and middle portions of the bluff, with silty sand and fine sandy silt in the upper bluff and upland areas. The soils are mapped by the SCS as KtC, KtD (Kitsap silt loam) and Bm (Belfast silty clay loam). The erosion hazard for these soils ranges from slight to severe, based on slope steepness. Portions of the shoreline bluff in the site area are mapped as "U" or unstable in the Coastal Zone Atlas. Olympic Ridge M~irch 7, 2002 i JEi~-~_no..~h ~do,~ iY Page 2 [ O~PT. OF CO~.~MUNITY DEVELOPMENT No evidence of erosion was observed in the upland area. Localized areas of erosion and sloughing were observed on the bluff face. The sloughing was surficial in nature, typical of a steep shoreline bluff. SUBSURFAC CONDITIONS Six existing test pits were observed at the site. The test pits extended to depths of 5 to 6.5 feet below the existing site grades. The soils generally consisted of silty fine sand and fine sandy silt with occasional organic material and fine gravel content to the full depth explored. The native undisturbed soils were in a stiff to very stiff condition (generally medium dense to dense at depth). A layer surficial weathered soil materials, silty sand with abundant organic debris (topsoil) was encountered to a depth of approximately 1/2-foot. The surficial soils were in a loose condition. Based on our site observations and experience in the area, the site is underlain by glacial till as a relatively shallow depth, 10 to 20 feet below the ground surface. The glacial till is typically in a very dense condition. The till soils have a high strength value and are generally stable relative to deep-seated slope movement. Seasonal perched groundwater occurs at a shallow depth at the site. Test pits open at the time of our site visit indicate groundwater is approximately 1 to 2 feet below the existing ground surface during wet winter conditions. CONSLUSIONS AND RECOMMENDATIONS GENERAL Based on the results of our subsurface exploration program, it is our opinion that the site is suitable for the proposed residential development. No surficial erosion was observed in the flatter portion of the site at the time of our site visit. Proposed grading at the site will be minimal. Proper erosion/sediment control measures will mitigate the potential for erosion in the disturbed areas. The silty soils that occur at the site are moisture-sensitive and susceptible to disturbance when wet. Perched ground water conditions will likely be associated with these soils during or following extended periods of precipitation. To reduce grading and construction costs, we recommend that earthwork be undertaken during dry weather conditions. The proposed residential structure can be supported on conventional foundation elements with appropriate drainage. To provide a dry working surface and usable crawl space, and reduce the risk of seismic damage, we recommend that the building area be supported on a minimum of 12-inches of structural fill placed as structural fill. Specific foundation support recommendations are provided in the "BUILDING FOUNDATIONS" section of this report. LANDSLIDE HAZARD AREAS Classification Jefferson County Critical Areas defines a landslide hazard area as 1) one containing slopes equal to or greater than 40 percent, and 2) one containing soils described by the Soil Conservation Service (SCS) Soil Survey as having a "severe" limitation for building site development. Slopes of 40 percent or greater were observed in the extreme east portion of the site. The soils at the site are mapped by the SCS as KtC, KtD (Kitsap silt loam) and Bm (Belfast silty clay loam). The erosion hazard for these soils ranges from slight to severe, based on slope steepness. Building site limitations listed in the SCS for these soils are related to steep unstable slopes and wet ground conditions, or high water table. Based on the above, we conclude that the site does meet the technical criteria for a landslide hazard area. Mitigation for the landslide hazard at this site consists of a Building Setback from the steep slope bluff area and drainage controls. Olympic Ridge ~v~x~ 2 7 March 7, 2002 Page 3 L____ Liquefaction 0£PT. OF C0,~viMUN!TY D£VELOPME~'qT Based on our review of the subsurface conditions, we conclude that the site soils are not generally susceptible to liquefaction. The near-surface silty soils are generally in a medium dense/stiff condition, but the static water table is located near the ground surface during typical winter conditions. Shaking of the already medium dense/stiff silty soil will not likely produce a denser configuration and subsequently excess pore water pressures will not likely to be produced. However, the combination-of sandy silt soils with the high groundwater table can result in thixotropic conditions during significant seismic conditions. Thixotropic soil disturbance may result in portions of the soil acting as a plastic medium during periods of significant lateral ground acceleration. This type of soil disturbance can subsequently relate to foundation damage is not mitigated. Mitigation options include the placement of structural fill material, geotextile fabric and/or increased foundation strength. These options are discussed in the "BUILDING FOUNDATION" section of this report. EROSION AND SEDIMENTATION CONTROL Erosion hazard areas are defined by Jefferson County 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 silt loam (B), and Xerochrepts (47F) soils. The erosion hazard for these soils ranges from slight to severe. It is our opinion that the site does meet the technical criteria of an erosion hazard area. The proposed residential structure will be located in the flatter portion of the site, appropriately setback from the steeper shoreline bluff slope. No disturbance of the shoreline bluff area is proposed. Although disturbance at the site will be minimized, removal of the natural vegetation or disturbance of the soil material results in an increased risk of erosion with any soil. It is our opinion that the potential erosion hazard at the site is can be mitigated through proper drainage and erosion control measures. Removal of natural vegetation should be minimized 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 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 sod, forest duff or organic-laden soils. Based on our explorations, we estimate that stripping on the order of 4 or 6 inches will be necessary to remove the sod/root zone and surficial soits containing significant organic material. Areas with deeper, unsuitable organics should be expected in the vicinity of depressions, slopes-or heavy vegetation. In addition, localized areas of old stripping material have been placed in the central portion of the site. Stripping depths of up to 2 feet or more may be required in these areas. 2 7 200 Olympic Ridge March 7, 2002 Page 4 -,-" '~E~i These materials may be stockpiled and later used for erosion control and landscaping and/ or reve§etation. Materials that cannot be used for landscaping or erosion control should be removed from the project site. Where placement of fill material is placed, the exposed subgrade 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 proof-roiled with heavy rubber-tired equipment during dry weather or probed with a lt2-inch-diameter steel rod during wet weather conditions. Any soft, loose or otherwise unsuitable areas delineated during proofrolling or probing should be recompacted, if practical, or over-excavated and replaced With structural fill, based on the recommendations of our site representative. The footings should be founded on a minimum 'of 12 inches of pit run sand and gravel/crushed rock structural fill compacted to at least 95 percent of the appropriate MDD in accordance with ASTM D-1557. The structural ill! should bear on suitable subgrade soil material, medium dense condition with minimal organic or deleterious material. Significant organic debris encountered during foundation preparation should be removed and replaced with structural fill material. Geotextile fabric may be utilized below the structural fill material to further reduce the r Sk of seismic impacts. BUILDING FOUNDATIONS We recommend that the building be supported on a minimum of 12 inches of granular structural fill material. The structural fill should extend a minimum of 3 feet beyond the limits of the foundation elements. This will elevate the building above the shallow pemhed groundwater and reduce the risk of damage during significant seismic conditions. The seismic risk may be further reduced with the use of a geotextile fabric (Mirifi 300X or approved equivalent) below the fill material. In general, footings founded on medium dense or denser compacted structural fill can be designed using an allowable soil bearing pressure of 1,500 psf for combined dead and long-term live loads, exclusive of the weight of the footing and any overlying backfill. These values may be increased by one-third for transient loads such as those induced by seismic events or wind loading. We recommend a minimum width of 30 inches for isolated footings and 24 inches for continuous wall footings. All exterior-footing elements should be embedded at least 18 inches below the lowest adjacent finished grade. We recommend that any disturbed soils in the footing excavations be removed, or if practical, recompacted prior to concrete placement. We estimate that settlements of footings designed and constructed as recommended will be less than 1 inch, with differential settlements between comparably loaded footings of 1/2 inch or less. Settlements will occur essentially as loads are applied. Disturbance of the foundation subgrade during construction could result in larger settlements than predicted. We recommend that all foundations be provided with footing drains. Roof drains should not be connected to the footing drain system unless adequate measures are taken to prevent surcharge of the footing drains. FLOOR SLAB SUPPORT Where slab-on-grade floors (including garage floors) are utilized, they should be supported on medium dense or denser native soil or on structural fill prepared as recommended above. March 7, 2002 ,FkkLno~[, .alOOflY - I Page 5 DEPT. OF COMMUNITY 0EVELOPMENT. ~ Typically, 12 inches of pit run sand and gravel over medium dense/very stiff native soils will provide adequate support for slabs-on-grade. We recommend that floor slabs at the site be underlain by a 4-inch thickness of uniformly graded gravel or sand containing no more than 3 percent fines to provide a capillary break. The capillary break material should be placed in one lift and 'compacted to a minimum of 95 percent of the MDD. The capillary break material should be connected to a suitable drain outlet to provide an exit for any accumulated seepage. A vapor barrier, such as a polyethylene liner, is recommended. A thin layer of "clean" sand may be placed over the vapor barrier and immediately below the slab to protect the polyethylene liner during steel and/or concrete-placement. A subgrade modulus of 250 kcf (kips per cubic foot) may be used for 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. LATERAL RESISTANCE Lateral loads may be resisted by friction on the base of footings and floor slab and as passive pressure on the sides of footings. We recommend a coefficient of friction of 0.65 be used to calculate friction between the concrete and soil, Passive pressure may be determined using an equivalent fluid weight of 400 pcf (pounds per cubic foot) above the water table, and 180 pcf for saturated soils. This assumes that structural fill is placed against the sides of the footings and that the top of the fill is confined by either a concrete floor slab or pavement. A safety factor of 1.5 is conventionally applied to these values. RETAINING AND SUBGRADE WALLS Retaining and/or subgrade wa, lis are not expected at this site. Basements or deep crawl spaces are not recommended at this site. However, in the event that shallow subgrade walls are required (stem-type walls), they can be supported on shallow footings founded on dense native soils or structural fill, if properly prepared. Footings bearing on undisturbed native soils or structural fill as described above can be designed using an average allowable bearing value of 1,500 psf with a maximum toe pressure of 2,000 psf, when the adjacent downhill slope is 4 to 1 (horizontal to vertical), or flatter. Lateral loads on conventional retaining structures founded as described above may be resisted by friction on the base of the wall footings and as passive pressure on the sides of footings. We recommend using an ultimate coefficient of friction of 0.65 to calculate friction between the concrete and dense native soils or on structural fill. Passive pressure may be determined using an equivalent fluid weight of 400 pcf. This assumes that structural fill is placed against the sides of the footings. A safety factor of 1.5 should be applied to these values, for sliding and overturning. The lateral active soil pressures acting on reinforced concrete retaining walls depend on the nature, density and configuration of the soil behind the wall. We recommend that walls supporting horizontal backfill be designed using an equivalent fluid density of 35 pcf for a level backslope behind the wall. This pressure is based on backfill placed within 2 feet of the wall being compacted by hand-operated equipment to a density of 90 percent of the MDD and consisting of clean sand or sand and gravel. The recommended pressure does not include the effects of sur- charges from surface loads. 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. Olympic Ridge '"" :'~ , <?,i.: ..... : t March 7, 2002 Page 6 Temporary cut slopes will likely be necessary during grading/utility operations. As a general guide, temporary slopes of t.5 to 1 (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 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. 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. 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. 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. DRIVEWAY SUBGRADE We recommend that driveway subgrades be prepared in accordance with the previously described site preparation and structural fill recommendations. The upper 18 inches of the driveway subgrade should have a density of approximately 95 percent of the MDD (ASTM D-1577). Structural fill may be required to provide adequate support for concrete or paved driveway surfaces. If possible, a crushed rock driveway surface should be utilized until dry weather conditions and sufficient traffic has compacted the subgrade soils, late summer or fall. SITE DRAINAGE All ground surfaces, pavements and sidewalks should be sloped away from the residence and associated structures. Surface water runoff may be controlled by a system of curbs, berms, drainage swales, and or catch basins, and conveyed to an appropriate discharge point. This will likely consist of a tightline over the shoreline bluff to the beach area. The tightline should be appropriately anchored approximately 20 feet back from the top of the bluff and joint connections should include redundant systems. An appropriate energy dissipation system should be used at the discharge point. We recommend that conventional roof and footing drains-be installed for all structures. Drains should be provided behind all retaining walls. The roof drain should not be connected to the footing drain unless an adequate gradient will prevent a surcharge of the footing drain. LIMITATIONS We have prepared this report for use by Mr. Joe Klinker 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, testing 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. 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 Olympic Ridge March 7, 2002 Page 7 contractor's methods, techniques, sequences or procedures, except as specifically described in our report for consideration in design. 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. We hope this satisfies your current needs. If you have any questions regarding this letter or need additional information please call me. Yours Very Truly, Prlncil~ ?, AR 2 / 2002 JEH-Lh;~h .dumlY DEPT. OF COMMUNITY DEVELOPMENT LSI/ADaPT Engineering Kurt Groesch, PE Principal DoclD.OlympicR.04L Two Copies Submitted 1 ~ SEE IViAP 11 T 2