Geotechnical Engineering Services and Geologic Hazards Assessment

Transcription

1 Geotechnical Engineering Services and Geologic Hazards Assessment Proposed Segale North Bend Development North Bend, Washington for Segale Properties LLC August 9, 03 Earth Science + Technology

2 Geotechnical Engineering Services and Geologic Hazards Assessment Proposed Segale North Bend Development North Bend, Washington for Segale Properties LLC August 9, 03 0 South Fawcett Avenue, Suite 00 Tacoma, Washington

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4 Table of Contents INTRODUCTION AND PROJECT UNDERSTANDING... GEOLOGY REVIEW... SITE CONDITIONS... Surface Conditions... Subsurface Explorations... Soil Conditions... Groundwater Conditions... GEOLOGIC HAZARDS ASSESSMENT... General... Erosion Hazard Areas... 3 Landslide Hazard Areas... 3 Seismic Hazards... 4 General... 4 Seismic Setting... 4 Seismic Design Criteria... 4 Liquefaction... Lateral Spreading... Ground Surface Rupture... Modifications to Erosion and Landslide Hazard Areas... 6 CONCLUSIONS AND RECOMMENDATIONS... 6 General... 6 Site Preparation and Earthwork... 6 General... 6 Stripping and Clearing... 6 Subgrade Preparation... 6 Erosion and Sedimentation Control... 7 Temporary Excavation Support... 8 Permanent Cut and Fill Slopes and Recommended Buffers... 8 Wet Weather Earthwork... 8 Structural Fill Materials... 9 General... 9 Select Granular Fill... 9 Pipe Bedding... 9 On-Site Soil... 9 Fill Beneath Infiltration Facilities... 9 Structural Fill Placement and Compaction... 0 General... 0 Area Fills and Bases... 0 Trench Backfill... 0 Foundation Design... 0 General... 0 Spread Footings... August 9, 03 Page i File No

5 Lateral Load Resistance... Settlement... Low Retaining Walls... General... Drainage... Design Parameters... Slabs-on-Grade... Site Drainage... 3 Stormwater Considerations... 3 Soil Infiltration Rates... 3 LIMITATIONS... LIST OF FIGURES Figure. Vicinity Map Figure. Site Plan APPENDICES Appendix A. Field Explorations and Laboratory Testing Figure A-. Key to Exploration Logs Figures A- through A-. Log of Test Pits Figures A-6 through A-9. Log of Borings Figures A-0 through A-. Sieve Analysis Results Appendix B. Report Limitations and Guidelines for Use Page ii August 9, 03 GeoEngineers, Inc. File No

6 PROPOSED SEGALE NORTH BEND DEVELOPMENT North Bend, Washington INTRODUCTION AND PROJECT UNDERSTANDING This report presents the results of our geotechnical engineering services and geologic hazards assessment for the proposed residential development on SE Tanner Road in the urban growth area of North Bend, Washington. Our project understanding is based on discussions with Segale Properties LLC and review of a site grading plan by CP H Consultants (CPH), dated August, 03. The project site is located approximately as shown on the Vicinity Map, Figure. The roughly square-shaped property is about 39 acres. The majority of the property is located southeast of Tanner Road and a small portion of the site is northwest of Tanner Road. The middle fork of the Snoqualmie River is adjacent to the northwest corner of the site. We understand the site was previously mined for sand and gravel, and the majority of the existing slopes on site were created during mining operations. The proposed development will include new streets and utilities for single family residential lots. We understand that the majority of site grading will be accomplished using permanent soil slopes; however, retaining walls less than 6 feet high may be utilized where slopes are not feasible. GEOLOGY REVIEW Based on review of the Digital Geology of Washington State at :00,000 scale and :0,000 scale maps, alluvial (Map unit Qa) soil deposits underlie the project site and surrounding area. Alluvial soil is described as sorted combinations of silt sand and gravel deposited in streambeds and alluvial fans. SITE CONDITIONS Surface Conditions The project site is located in North Bend, Washington near the middle fork of the Snoqualmie River. The site is bordered by single family residential tract home developments to the north, east and west. A tree farm borders the site to the south. The northwest corner of the site is adjacent to the middle fork of the Snoqualmie River. The site is presently undeveloped with un-vegetated areas near the center of the site and moderately to heavily wooded areas with evergreen trees and a variable growth of underbrush. An existing sanitary sewer crosses the site from SE Tanner Road to 46 th Place SE. The site was previously mined for sand and gravel. The center of the site is generally flat with a few stockpiles of aggregate products remaining on the site. Cut slopes from the mining operations are present near the east and south site boundaries. The slopes range in height from to 0 feet. In the areas of cuts greater than 40 feet the slopes are about 3H:V (horizontal:vertical). Slopes are generally flatter than about -/H:V in the areas of cuts less than 40 feet. August 9, 03 Page File No

7 PROPOSED SEGALE NORTH BEND DEVELOPMENT North Bend, Washington Subsurface Explorations We explored soil and groundwater conditions at the site by excavating 4 test pits on November 0, 0 and advancing 4 borings on December 8, 0 at the approximate locations shown on the Site Plan, Figure. The test pits ranged in depth from 8 feet below ground surface (bgs) to 8 feet bgs. The borings were advanced to depths between 6. feet bgs and 7. feet bgs. Falling Head Percolation tests in accordance with the Environmental Protection Agency s (EPA s) Onsite Wastewater Treatment and Disposal Systems (980), as described in the 009 King County Surface Water Design Manual were conducted at four locations (TP-I-, I-, B-I-3, and B-I-4). The Falling Head Percolation tests were conducted within feet of the ground surface. A description of our site exploration and laboratory testing programs along with exploration logs and laboratory test results are presented in Appendix A. Soil Conditions At the exploration locations where soil had not been disturbed by the previous mining activity, we observed silty sand with organic matter at the surface to a depth of about 6 inches to 3 feet. Below the silty sand with organic matter, we observed gravel deposits with varying amounts of silt and sand to the full depths explored. In areas that had been disturbed during mining activities, the silty sand with organic matter was not present and we generally observed the gravel deposits with varying amounts of silt and sand from the ground surface to the full depths explored. We interpret gravel deposits with varying amounts of silt and sand this soil unit to be alluvium. Within the alluvium deposits, we also encountered cobbles, boulders, occasional sand lenses and localized zones with increased silt content. Groundwater Conditions We encountered groundwater seepage in test pits TP-, TP-, and TP- at depths of to feet bgs. We interpret this groundwater to be surficial perched groundwater. Heavy rains in the week leading up to the exploration likely resulted in surficial perched groundwater. We encountered groundwater seepage in test pits TP-, TP-, TP-3, TP-4 and TP-9 at depths between 4 and feet bgs which, we interpret to be perched groundwater on localized zones of silty soil. Seasonal perched groundwater will likely occur in various locations throughout the project site depending on time of year and silt content of soils. We did not observe perched groundwater seepage on the existing slopes at the site. Based on the time of year the explorations were completed, observed groundwater levels are probably at or near seasonal high levels. GEOLOGIC HAZARDS ASSESSMENT General The North Bend Municipal Code (NBMC), Chapter 4. Geologically Hazardous Areas lists three potential geologic hazards: ) Erosion; ) Landslide; and 3) Seismic. Our assessment of each hazard related to the project site is discussed below. Page August 9, 03 GeoEngineers, Inc. File No

8 PROPOSED SEGALE NORTH BEND DEVELOPMENT North Bend, Washington Erosion Hazard Areas Erosion hazard areas are defined in Section (B)() of the NBMC as areas having severe or very severe rill and inter-rill erosion hazard according to the U.S. Department of Agriculture s (USDA) Natural Resources Conservation Services (NRCS). Concentrated stormwater runoff is a major cause of erosion and soil loss. The NRCS classifies the existing surficial soil as EvB and EvC (Everett Gravely Sandy Loam 0 to percent slopes and to percent slopes, respectively). Everett Series soils are described as consisting of somewhat excessively drained soils underlain by very gravely sand at a depth of 8 to 36 inches. Erosion hazard is described as slight to moderate for EvB and EvC soils. Based on our review of topography, portions of the existing slopes are greater than percent. It is our opinion the exposed alluvium soil on the existing slopes greater than percent has a moderate erosion hazard. We provide specific recommendations for temporary and permanent site erosion control measures in the Erosion and Sedimentation Control section below. Landslide Hazard Areas Landslide hazard areas are defined in Section (B)() of the NBMC as areas potentially subject to landslides based on a combination of geologic, topographic and hydrologic factors. Landslide Hazard areas include: Areas of historic failure identified on published maps. Areas with all three of the following characteristics: Slopes steeper than percent Hillsides intersecting geologic contacts that contain impermeable soils frequently interbedded with permeable granular soils Impermeable soils overlain with permeable soils; and areas with springs or groundwater seepage. Areas with slope movement within the last 0,000 years. Slopes that are parallel or sub-parallel to planes of weakness in subsurface materials. Slopes having a gradient steeper than 80 percent and subject to rock fall during seismic shaking. Areas potentially unstable due to rapid stream incision, stream bank erosion or undercutting by wave action. Areas located in a canyon or an active alluvium fan, potentially subject to inundation by debris flow or catastrophic flooding. Areas with slopes steeper than 40 percent with a vertical relief of 0 or more feet Based on our assessment, portions of the existing slopes near the east and south portions of the site meet the criteria for landslide hazard areas because they are steeper than 40 percent, with a vertical relief of 0 or more feet. Landslide hazard areas are indicated on the Site Plan (Figure ). We did not observe evidence of global slope instability during our site visits; however, it is likely that surficial sloughing could occur on these slopes. Stockpiles of aggregate products from the August 9, 03 Page 3 File No

9 PROPOSED SEGALE NORTH BEND DEVELOPMENT North Bend, Washington mining operations meet the criteria for landslide hazard areas; however, we understand these stockpiles will be removed from the site during site grading activities. We provide recommendations for appropriate slope setbacks and permanent slope construction in the construction recommendations section below. Seismic Hazards General Seismic hazard areas are defined in Section (B)(3) of the NBMC as areas that are subject to severe risk of damage as a result of earthquake-induced ground shaking, slope failure, settlement, soil liquefaction, lateral spreading, or surface failure. The following sections discuss the seismic hazards present at the site and provide seismic design criteria. Seismic Setting The site is located in western Washington, which is seismically active. Seismicity in this region is attributed primarily to the interaction between the Pacific, Juan de Fuca and North American plates. The Juan de Fuca plate is subducting beneath the North American plate at the Cascadia Subduction Zone (CSZ). This produces both intercrustal (between plates) and intracrustal (within a plate) earthquakes. Research is ongoing regarding large magnitude CSZ-related intercrustal earthquake activity along the Washington and Oregon coasts. Geologists are reporting evidence that suggests several large magnitude earthquakes (magnitude 8 to 9) have occurred along the CSZ in the last,00 years, the most recent of which occurred about 300 years ago. Five large subduction zone earthquakes have been observed globally since 960: ) in 960, a magnitude 9. earthquake occurred in Chile; ) in 964, a magnitude 9. earthquake occurred in Alaska; 3) in 006, a magnitude 9. earthquake occurred in Indonesia; 4) in 00, a magnitude 8.8 occurred of the coast of Chile; and ) in 0 a magnitude 9.0 occurred in Japan. No earthquakes of this magnitude have occurred along the CSZ during the recorded history of the Pacific Northwest. Hundreds of smaller intracrustal earthquakes have been recorded in western Washington. Four of the most recent earthquakes were: ) in 946, a magnitude 7. earthquake occurred in the Vancouver Island, British Columbia area; ) in 949, a magnitude 7. earthquake occurred in the Olympia area; 3) in 96, a magnitude 6. earthquake occurred between Seattle and Tacoma; and 4) on February 8, 00, a magnitude 6.8 occurred at Nisqually near Olympia. Seismic Design Criteria We understand seismic design of structures will be performed using the procedure outlined in the 0 International Building Code (IBC). The parameters provided below may be used for design. Page 4 August 9, 03 GeoEngineers, Inc. File No

10 PROPOSED SEGALE NORTH BEND DEVELOPMENT North Bend, Washington TABLE. SEISMIC DESIGN CRITERIA 0 IBC Seismic Design Parameters Spectral Response Acceleration at Short Periods (SS).08g Spectral Response Acceleration at -Second Periods (S) 0.40g Site Class C Design Peak Ground Acceleration (PGA) 0.44g Design Spectral Response Acceleration at Short Periods (SDS) 0.7g Design Spectral Response Acceleration at -Second Periods (SD) 0.38g Liquefaction Soil liquefaction refers to the condition by which vibration or shaking of the ground, usually from earthquake forces, results in the development of excess pore pressures in saturated soils with subsequent reduction in soil shear strength. In general, soils that are susceptible to liquefaction include very loose to medium dense, clean to silty sands and very soft to stiff non-plastic silts that are below the water table. The site is mapped as having a moderate to high susceptibility to liquefaction according to the Liquefaction Susceptibility Map of King County, Washington (Tetra Tech, 00). County-wide liquefaction susceptibility maps are typically based on regional geologic mapping and are not detailed enough to consider site-specific soil conditions. These maps are typically best suited for use in the absence of site-specific information. Using soil information from our explorations, we evaluated the risk of liquefaction at the project site. Based on the observed soil densities and relatively high gravel content at the site, it is our opinion that the potential for liquefaction at this site is low. Lateral Spreading Lateral spreading related to seismic activity generally involves lateral displacement of large, surficial blocks of non-liquefied soil when a layer of underlying soil loses strength during seismic shaking. Lateral spreading usually develops in areas where sloping ground or large grade changes (including retaining walls) and loose or liquefiable soils are present. Based on our understanding of the subsurface conditions, it is our opinion that the risk of lateral spreading is low. Ground Surface Rupture We reviewed two maps to identify potential fault related ground surface rupture at or near the project site; Digital Geology of Washington State at :00,000 scale and :0,000 scale. Based on our review, there are no mapped faults within 00 feet of the project site. Because of the anticipated site location with respect to the nearest known active crustal faults and the presence of relatively thick alluvium deposits overlying bedrock, it is our opinion that the risk of ground rupture at the site due to crustal faulting is low. August 9, 03 Page File No

11 PROPOSED SEGALE NORTH BEND DEVELOPMENT North Bend, Washington Modifications to Erosion and Landslide Hazard Areas Based on our review of the CPH site grading plan and our discussions with Segale Properties, we understand that portions of the site, which are currently classified as erosion hazard areas and/or landslide hazard areas, are proposed to be graded as part of the development. Provided that earthwork and site grading is performed in accordance with the recommendations provided in this report, it is our opinion that the proposed development will meet the requirements in Sections and (E) of the NBMC. Recommended buffers from erosion and landslide hazard areas are discussed in the Permanent Cut and Fill Slopes and Recommended Buffers section below. CONCLUSIONS AND RECOMMENDATIONS General Based on our subsurface explorations, site observations, laboratory testing and engineering analyses, it is our opinion that the site is suitable for the proposed development from a geotechnical standpoint. The following sections present our specific conclusions and recommendations for site earthwork, foundation support and other geotechnical aspects of the project. Site Preparation and Earthwork General We anticipate that site development work will include stripping and clearing, excavating for utilities and grading, preparing foundation bearing surfaces and pavement subgrades and placing and compacting fill and backfill materials. We expect that the majority of site grading can be accomplished with conventional earthmoving equipment in proper working order. However, cobbles and boulders were encountered in our explorations. The contractor should be prepared to deal with cobbles and boulders. The following sections provide recommendations for earthwork, site development and fill materials. Stripping and Clearing The existing trees, brush, forest duff, and organic-rich soil should be removed from the building footprints, driveways and any other flat work areas. Stripping should extend at least feet beyond all structural areas for buildings and pavements. Trees, stumps and roots greater than about /-inch-diameter should be removed as part of the stripping and clearing process. Based on our explorations we anticipate a stripping depth of about 6 to inches may be required in vegetated areas. Greater stripping depths may be required to remove fill and/or localized zones of loose or organic-rich soil, or if stripping operations cause excessive disturbance to subgrade soil. The actual stripping depth should be determined during excavation and construction. Subgrade Preparation Subgrades for roadways and foundations should be thoroughly compacted to a uniformly firm and unyielding condition on completion of stripping, and before placing structural fill to establish grades. We recommend that prepared subgrades be proofrolled to identify areas of yielding prior Page 6 August 9, 03 GeoEngineers, Inc. File No

12 PROPOSED SEGALE NORTH BEND DEVELOPMENT North Bend, Washington to placement of fill or other structural elements. Proofrolling should be accomplished with a heavy piece of construction equipment such as a loaded dump truck or front-end wheel loader. Where proofrolling is not feasible due to space constraints, the exposed subgrade soil should be probed by an experienced person, using a steel probe rod. If soft or otherwise unsuitable areas are revealed during proofrolling or probing that cannot be compacted to a stable and uniformly firm condition, we recommend that: ) the subgrade soils be scarified (e.g., with a ripper or farmer s disc), aerated and recompacted; or ) the unsuitable soils be removed and replaced with structural fill, as needed. Erosion and Sedimentation Control Portions of the site slopes are identified as erosion hazard areas. To minimize the risk of erosion on site slopes during construction, it will be important to use erosion and sedimentation controls. Typical erosion and sedimentation control measures are described below. Potential sources or causes of erosion and sedimentation can be influenced by construction methods, slope length and gradient, amount of soil exposed and/or disturbed, soil type, construction sequencing and weather. Implementing an erosion and sedimentation control plan will reduce the project impact on erosion-prone areas. The plan should be designed in accordance with applicable city, county and/or state standards. The plan should incorporate basic planning principles, including: Scheduling grading and construction to reduce soil exposure. Re-vegetating or mulching denuded areas. Directing runoff away from denuded areas. Reducing the length and steepness of slopes with exposed soils. Decreasing runoff velocities. Preparing drainage ways and outlets to handle concentrated or increased runoff. Confining sediment to the project site. Inspecting and maintaining control measures frequently. Some sloughing and raveling of exposed or disturbed soil on slopes should be expected. We recommend that disturbed soil be restored promptly so that surface runoff does not become channeled. Temporary erosion protection should be used and maintained in areas with exposed or disturbed soils to help reduce erosion and reduce transport of sediment to adjacent areas and receiving waters. Permanent erosion protection should be provided by paving, structure construction or landscape planting. Until the permanent erosion protection is established and the site is stabilized, site monitoring may be required by qualified personnel to evaluate the effectiveness of the erosion control measures and to repair and/or modify them as appropriate. Provision for modifications to the erosion control August 9, 03 Page 7 File No

13 PROPOSED SEGALE NORTH BEND DEVELOPMENT North Bend, Washington system based on monitoring observations should be included in the project erosion and sedimentation control plan. Temporary Excavation Support Excavations deeper than 4 feet should be shored or laid back at a stable slope if workers are required to enter. Shoring and temporary slope inclinations must conform to the provisions of Title 96 Washington Administrative Code (WAC), Part N, Excavation, Trenching and Shoring. Regardless of the soil type encountered in the excavation, shoring, trench boxes or sloped sidewalls will be required under Washington Industrial Safety and Health Act (WISHA). The contract documents should specify that the contractor is responsible for selecting excavation and dewatering methods, monitoring the excavations for safety and providing shoring, as required, to protect personnel and structures. In general, based on our observations and explorations, temporary cut slopes in on-site soils should be inclined no steeper than about -/H:V. This guideline assumes that all surface loads are kept at a minimum distance of at least one-half the slope height away from the top of the slope and that significant seepage is not present on the slope face. Flatter slopes will be necessary where significant seepage occurs, soils are disturbed or if voids are created during excavation. Based on our explorations, we recommend the contractor be prepared to deal with caving soils during excavation of utility trenches. Sloughing and raveling of temporary cut slopes should also be expected. Temporary covering with heavy plastic sheeting should be used to protect slopes during periods of wet weather. Permanent Cut and Fill Slopes and Recommended Buffers Based on our explorations and understanding of site soil conditions, we recommend permanent cut and fill slopes be constructed at a maximum inclination of -/H:V. For slopes inclined between -/H:V and H:V, we recommend a minimum 0-foot buffer from top and toe of slope. For slopes inclined at H:V or flatter, it is our opinion a buffer is not necessary for property lines and fences, but we recommend that occupied and storage structures be set back at least 0 feet from top and toe of slope. We do not recommend construction of permanent slopes steeper than -/H:V. These inclination and buffer recommendations are intended to address the requirements of NBMC Section , and are appropriate for proposed new slopes and existing slopes. Permanent slopes should be re-vegetated as soon as practical to reduce the risk of surface erosion and sloughing. Temporary protection should be used until permanent protection is established. In order to achieve uniform compaction, we recommend that fill slopes be overbuilt and subsequently cut back to expose well-compacted fill. Wet Weather Earthwork In our explorations, we observed localized zones of soil with fines content (material passing the U.S. Standard No. 00 sieve) greater than to 0 percent. These zones of soil are moisture sensitive and may be difficult to work with and compact when wet. Disturbance of near-surface soil should be expected if earthwork is completed during periods of wet weather. The NBMC specifies Page 8 August 9, 03 GeoEngineers, Inc. File No

14 PROPOSED SEGALE NORTH BEND DEVELOPMENT North Bend, Washington that earthwork activities in landslide hazard areas may only be conducted between May st and October st, or as designated by the City of North Bend. Structural Fill Materials General Structural fill must be free of debris, organic material and rock fragments larger than 6 inches. The workability of material used as structural fill will depend on the gradation and moisture content of the soil. As the amount of fines increases, soil becomes increasingly sensitive to small changes in moisture content and adequate compaction may become difficult or impossible to achieve. If construction is performed during wet weather conditions, we recommend using select granular fill as described below. If prolonged dry weather prevails during the earthwork phase of construction, a somewhat higher fines content may be acceptable. Select Granular Fill Select granular fill should consist of well-graded sand and gravel or crushed rock with a maximum particle size of 3 inches and less than percent fines by weight based on the minus 3/4-inch fraction. Organic matter, debris or other deleterious material should not be present. In our opinion, material conforming to Washington State Department of Transportation (WSDOT) Specification (Aggregates for Ballast and Crushed Surfacing), (Aggregate for Gravel Base), or (Borrow) is suitable for use as select granular fill, provided that the fines content is less than percent (based on the minus 3/4-inch fraction) and the maximum particle size is 3 inches. Pipe Bedding Trench backfill for the bedding and pipe zone should consist of well-graded granular material with a maximum particle size of 3/4 inch and less than percent passing the U.S. Standard No. 00 sieve. The material should be free of roots, debris, organic matter and other deleterious material. On-Site Soil Based on our subsurface explorations, it is our opinion that the inorganic soil present on site may be considered for use as structural fill, provided it can be placed and compacted as recommended. The sand and gravel soil generally has a low fines content, and may be suitable for use as select granular fill during periods of wet weather. We observed localized zones of higher silt content in the sand and gravel soils; these zones may be more difficult to work with during wet weather. Fill Beneath Infiltration Facilities We understand site grading plans may include localized zones of fill beneath infiltration facilities. In order to maintain infiltration rates consistent with the design recommendations presented herein, we recommend fill beneath infiltration facilities be granular in nature and have less than percent fines. In our opinion, either on-site soil or imported material meeting these recommendations may be considered for use beneath infiltration facilities. August 9, 03 Page 9 File No

15 PROPOSED SEGALE NORTH BEND DEVELOPMENT North Bend, Washington Structural Fill Placement and Compaction General Structural fill should be compacted at a moisture content near optimum. The optimum moisture content varies with the soil gradation and should be evaluated during construction. Silty soil and other fine-grained soil can be difficult or impossible to compact during wet conditions. Fill and backfill material should be placed in uniform, horizontal lifts and uniformly compacted with vibratory compaction equipment. The maximum lift thickness will vary depending on the material and compaction equipment used, but generally should not exceed inches in loose thickness. Fill placement on slopes steeper than H:V should be benched into the slope face and include keyways. The configuration of the benches and keyways depends on the equipment being used and the slope geometry. Area Fills and Bases Structural fill placed to raise site grades and aggregate base materials under foundations, slabs and pavements should be placed on a prepared subgrade composed of uniformly firm and unyielding inorganic naturally occurring soils or compacted fill. Structural fill placed in structure areas should be compacted to at least 9 percent of the maximum dry density (MDD) determined by ASTM International (ASTM) Test Method D 7 (modified Proctor). Structural fill placed more than feet below subgrade in pavement areas should be compacted to at least 90 percent of the MDD. We recommend at least 9 percent of the MDD for structural fill placed within feet of subgrade. Trench Backfill For utility excavations, we recommend that the initial lift of fill over the pipe be thick enough to reduce the potential for damage during compaction but generally should not be greater than about 8 inches. In addition, rock fragments greater than about inch in maximum dimension should be excluded from this lift. In building areas, trench backfill should be uniformly compacted in horizontal lifts to at least 9 percent of the MDD based on ASTM D 7. Fill placed more than feet below subgrade in pavement areas may be compacted to at least 90 percent of the MDD (ASTM D 7). Fill placed within feet of subgrade in pavement areas should be compacted to at least 9 percent of the MDD (ASTM D 7). In nonstructural areas, trench backfill should be compacted to a firm and unyielding condition. Foundation Design General Residential structures at the site may be founded on continuous wall or isolated column footings established on undisturbed medium dense or denser sand and gravel soil or on compacted structural fill that extends to such soil. Loose or organic-rich soils should be overexcavated and replaced with structural fill where present at or below foundation grades. The overexcavation should extend laterally beyond the footing perimeter a distance equal to one-half the depth of Page 0 August 9, 03 GeoEngineers, Inc. File No

16 PROPOSED SEGALE NORTH BEND DEVELOPMENT North Bend, Washington overexcavation. We recommend a minimum width of inches for continuous wall footings and 8 inches for isolated column footings. For frost protection, we recommend footing elements be embedded at least 8 inches below the lowest adjacent external grade. Spread Footings Footings founded on medium dense or denser sand and gravel soil or compacted structural fill extending to such soil may be designed using an allowable soil bearing pressure of 3,000 pounds per square foot (psf). This value applies to long-term dead and live loads exclusive of the weight of the footing and any overlying backfill and may be increased by one-third when considering total loads, including transient loads such as those induced by wind and seismic forces. Lateral Load Resistance Lateral loads on foundation elements may be resisted by passive soil pressure on the sides of footings and other below-grade structural elements and by friction on the base of footings. Passive soil pressure may be estimated using an equivalent fluid density of 300 pounds per cubic foot (pcf), assuming that the footings and below-grade elements are backfilled with structural fill placed and compacted as recommended. The top foot of soil should be neglected when calculating passive resistance unless the area is covered by pavement or a slab-on-grade. Frictional resistance may be estimated using 0.3 for the coefficient of base friction. The above values include a factor of safety of about.. The passive earth pressure and friction components may be combined, provided that the passive pressure component does not exceed two-thirds of the total. Settlement Based on typical residential and structure loads, we estimate that settlement of footings designed and constructed as recommended should be less than inch, with differential settlements of / inch or less between comparably loaded isolated footings or along 0 feet of continuous footing. Most of the settlement should occur as loads are being applied. Loose or soft soil below footings or disturbance of foundation subgrades during construction could result in larger settlements than estimated. Low Retaining Walls General The recommendations provided below are intended for design of walls less than 6 feet tall and are not appropriate for walls with over 6 feet of grade separation. We are available to provide recommendations for walls over 6 feet tall upon request. Drainage We recommend that positive drainage be designed behind all retaining structures. This can be accomplished by providing a zone of free-draining material behind the wall with perforated pipes to collect seepage water. The drainage material should consist of coarse sand and gravel containing less than percent fines based on the fraction of material passing the 3/4-inch sieve. The wall drainage zone should extend horizontally at least 8 inches from the back of the wall. August 9, 03 Page File No

17 PROPOSED SEGALE NORTH BEND DEVELOPMENT North Bend, Washington Perforated smooth-walled rigid PVC pipe having a minimum diameter of 4 inches should be placed at the bottom of the drainage zone along the entire length of the wall, with the pipe invert at or below the elevation of the base of the wall footing. The drainpipes should discharge to a tightline leading to an appropriate collection and disposal system. An adequate number of cleanouts should be incorporated into the design of the drains in order to provide access for regular maintenance. In general, roof downspouts, perimeter drains or other types of drainage systems should not be connected to retaining wall drain systems. Design Parameters The pressures presented assume that backfill placed within feet of the wall is compacted by hand-operated equipment to a density of 90 percent of the MDD and that wall drainage measures described above are included. For walls constructed as described above, we recommend using an active lateral earth pressure corresponding to an equivalent fluid density of 3 pcf for the level backfill condition. For walls with backfill sloping upward behind the wall at H:V, an equivalent fluid density of pcf should be used. This assumes that the tops of the walls are not structurally restrained and are free to rotate. For the at-rest condition (walls restrained from movement at the top) and level backfill conditions an equivalent fluid density of pcf should be used for design. For seismic conditions, we recommend a uniform lateral pressure of H (where H is the height of the wall) psf be added to these lateral pressures. Note that if the retaining system is designed as a braced system but is expected to yield a small amount during a seismic event, an active earth pressure condition may be assumed and combined with the uniform seismic surcharge pressure. The recommended pressures do not include the effects of surcharges from surface loads. If vehicles will be operated within one-half the height of the wall, a traffic surcharge should be added to the wall pressure. The traffic surcharge can be approximated by the equivalent weight of an additional feet of backfill behind the wall. Additional surcharge loading conditions should also be considered on a case-by-case basis. Retaining walls founded on native soil or structural fill extending to these materials may be designed using the allowable soil bearing values and lateral resistance values presented above in the Foundation Design and Excavation section of this report. We estimate settlement of retaining structures will be similar to the values previously presented for building foundations. Slabs-on-Grade A modulus of subgrade reaction of 0 pounds per cubic inch (pci) may be used for designing slabs-on-grade provided that the subgrade consists of undisturbed medium dense or denser naturally occurring sand and gravel soil or structural fill extending to such soil. The upper foot of structural fill in slab areas should consist of select granular fill. For slabs-on-grade designed and constructed as recommended, we estimate settlements of less than inch. We estimate that differential settlement of the floor slabs will be / inch or less over a span of 0 feet. We recommend that slabs-on-grade be underlain by a minimum 6-inch-thick capillary break layer to reduce the potential for moisture migration into the slab. The capillary break material should consist of well-graded sand and gravel or crushed rock with a maximum particle size of 3/4 inch and less than 3 percent fines. The capillary break material should be placed in one lift. If dry slabs Page August 9, 03 GeoEngineers, Inc. File No

18 PROPOSED SEGALE NORTH BEND DEVELOPMENT North Bend, Washington are required (for example, where adhesives are used to anchor carpet or tile to the slab), a waterproof liner should be placed as a vapor retarder below the slab. Site Drainage Stormwater Considerations We recommend that all surfaces be sloped to drain away from the proposed building areas. Pavement surfaces and open spaces should be sloped such that surface runoff is collected and routed to suitable discharge points. Roof drains should be tightlined to a suitable stormwater disposal system. Soil Infiltration Rates We evaluated long-term design infiltration rates for the site soils using two methods: ) falling head percolation tests and ) correlation based on grain-size analysis results. We performed falling head percolation tests in accordance with the Environmental Protection Agency s (EPA s) Onsite Wastewater Treatment and Disposal Systems (980), as described in the 009 King County Surface Water Design Manual. To estimate infiltration rates based on grain-size analysis results, we used methodology provided in the 00 Washington State Department of Ecology (Ecology) Stormwater Management Manual for Western Washington (SMMWW). Both methods specifically account for long-term clogging due to siltation and biomass buildup in the infiltration facility. Based on these results, we recommend a long-term design infiltration rate of 9 inches per hour for the native gravelly soils present on site. We observed localized areas of siltier soils surrounding TP-, TP- and I-, and near B-, B- and B-I-3. In these areas, we recommend a long-term design infiltration rate of inch per hour. Although not specifically observed in our explorations, other localized areas of silty soils may be present on the site. August 9, 03 Page 3 File No

19 PROPOSED SEGALE NORTH BEND DEVELOPMENT North Bend, Washington TABLE. SOIL INFILTRATION RATES Test Pit No. Soil Sample No. Soil Sample Test Depth (feet) Approximate Elevation of Sample Test (feet) Approximate Groundwater Elevation (feet) Percent Fines 3 USCS 4 Soil Classification Method of Estimation of Infiltration Rate Estimated Longterm Design Infiltration Rate (Inches per Hour) TP GP-GM Grain Size Analysis TP GP Grain Size Analysis 9 TP Not Observed GW Grain Size Analysis 9 TP GW Grain Size Analysis 9 TP- 3 0 Not Observed GW Grain Size Analysis 9 TP-I- 49 Not Observed 4 GP Falling Head Percolation 0.0 B- 0 8 Not Observed 8 GP-GM Grain Size Analysis B Not Observed 7 GW-GM Grain Size Analysis B-I Not Observed SM Falling Head Percolation 0. B-I-4 77 Not Observed 7 GW-GM Falling Head Percolation 0.0 I- No Sample Not Observed No Sample No Sample Falling Head Percolation 0.7 Notes: For selected soil samples and test locations Based on survey map provided by CPH Consultants. 3 Fines = Silt and clay-sized particles passing U.S. No. 00 (0.7 mm) sieve. 4 Unified Soil Classification System (USCS). Page 4 August 9, 03 GeoEngineers, Inc. File No

20 PROPOSED SEGALE NORTH BEND DEVELOPMENT North Bend, Washington These rates are an estimate of subsurface infiltration properties. Information on the final location, elevation and size of the infiltration facility was not available at the time of our explorations. Local jurisdictions may require an in-situ infiltration test to check that the preliminary infiltration rate(s) used for design are appropriate for the conditions encountered. We recommend that the project plans include provisions for GeoEngineers to observe the final surface during construction to check the infiltration surface for variances from recommended infiltration rates. FACILITY PROTECTION AND MAINTENANCE We recommend infiltration facilities be protected during construction to help reduce the potential for clogging. Siltation control facilities such as temporary settling basins, silt fences and hay bales should be provided, as appropriate. Suspended solids can clog the soil and reduce the infiltration rate. Overcompaction and excessive subgrade disturbance can also decrease the infiltration rate of the naturally occurring soils. To avoid overcompaction or disturbance of the prepared pond subgrade, construction equipment should be kept out of the infiltration facility after it is excavated to grade. Infiltration facilities should also be periodically maintained. Maintenance will likely include removal of accumulated sediment and debris. TREATMENT CONSIDERATIONS According to the 009 King County Surface Water Design Manual Section.4, the existing site soils are generally not suitable for stormwater treatment, because the infiltration rate for the gravelly soils is too high to achieve the required treatment. If stormwater treatment is required we recommend alternative measures, such as amended treatment layers, be considered. LIMITATIONS We have prepared this report for use by Segale Properties LLC and their authorized agents in support of design of the proposed residential development in North Bend, King County, Washington. Within the limitations of scope, schedule and budget, our services have been executed in accordance with generally accepted practices in the field of geotechnical engineering in this area at the time this report was prepared. No warranty or other conditions, express or implied, should be understood. Please refer to Appendix B titled Report Limitations and Guidelines for Use for additional information pertaining to use of this report. August 9, 03 Page File No

21 464Th Ave SE 434Th Ave SE SE 4Th St Office: TACO Path: P:\0\0904\GIS\090400_F.mxd Map Revised: 0 December 0 syi SE Mt Si Rd 434Th Ave SE 90 SE 49Th St 436Th Ave SE Bpa Access Rd 436Th Ave SE SE Tanner Rd 437Th Pl SE SE 4Nd St SE 44Th Ln SE 36Th St SE 43Rd St 438Th Ave SE SE 30Th St SE 39Th St Olympic NP 446Th Ave SE 444Th Ave SE 446Th Ave SE SE 4Nd St Snoqua l South t Forrk 40 SE 30Th Pl SE 4Nd Pl SE St St SE St Pl 4Nd Ave SE lm i ie Ri iverr W a s h i n g t o n 90 Olympia Mount Rainier NP SE 30Th Pl Notes:. The locations of all features shown are approximate.. This drawing is for information purposes. It is intended to assist in showing features discussed in an attached document. GeoEngineers, Inc. cannot guarantee the accuracy and content of electronic files. The master file is stored by GeoEngineers, Inc. and will serve as the official record of this communication. 3. It is unlawful to copy or reproduce all or any part thereof, whether for personal use or resale, without permission. Data Sources: ESRI Data & Maps, Street Maps 00 Transverse Mercator, State Plane South, North American Datum 983 North arrow oriented to grid north St Ave SE M iidd King County Snoqua l le l Fo rrk 43Rd Pl SE 4Th Pl SE SITE lm ie i R ii vve 47Th Ave SE SE 4Th St 49Th Ave SE SE 0Th St rr 46St Pl SE 46Th Dr SE SE 40Th St SE North Bend Way SE 9Th St SE 34Th St 468Th Ave SE Vicinity Map 468Th Ave SE SE 9Th St 468Th Ave SE 469Th Pl SE 470Th Ave SE SE Middle Fork Rd SE 44Th St Grouse Ridge Access Rd SE 3Rd St,000 0,000 Feet SE 6Th St SE 9Th St SE 37Th St Segale North Bend Development North Bend, Washington SE 3St Pl 474Th Ave SE Figure

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23 APPENDIX A Field Explorations and Laboratory Testing

24 PROPOSED SEGALE NORTH BEND DEVELOPMENT North Bend, Washington APPENDIX A FIELD EXPLORATIONS AND LABORATORY TESTING Explorations Subsurface conditions at the project site were exploredd by excavating 4 test pits on November 0, 0 and advancing 4 borings on December 8, 0. The testt pits were excavated using an excavator provided and operated by Segale Properties. The test pits ranged in depths from 8 feet below ground surface (bgs) to 8 feet bgs. The boringss were advanced using a track-mounted drill rig under subcontract to GeoEngineers. The borings were advanced to depths between 6. feet bgs and 7. feet bgs. The locations of the test pits and borings were determined by hand held GPS. The elevations presented on the test pit and boring logs are based on the topographic survey data provided by CPH Consultants. The locations and elevations of the explorations should be considered approximate. Locations of the explorations are provided on the Site Plan, Figure. Our field representative obtained samples, classified the soils, maintained a detailed log of each exploration and observed groundwater conditions where applicable. The samples were retained in sealed plastic bags. The soils were classified visually in general accordance with the system described in Figure A-, which includes a key to the exploration logs. Summary logs of the explorations are included as Figures A- through A-9. The densities noted on the test pit logs are based on the difficulty of excavation and our experiencee and judgment. Infiltration Testing Long-term design infiltrationn rates for the site soilss were evaluated using the Falling Head Percolation tests at four locations (TP-I-, I-, B-I-3, and B-I-4).. We performed falling head percolation testss in accordance with the EPA s Onsite Wastewater Treatment and Disposal Systems (980), as described in the 009 King County Surface Water Design Manual. Geotechnical Laboratory Test Results Soil samples obtained from the test pits and borings were transported to our laboratory and examined to confirm or modify field classifications, as well as to evaluate engineering properties of the soil. Representative samples were selected for laboratory testing. Gradation tests (ASTM D 4) and moisture content determination ns (ASTM D 6) were performed on representative soil samples. Results of gradation tests are presented in Figures A-0 through A-. August 9, 03 Page A- File No

25 MAJOR DIVISIONS SOIL CLASSIFICATION CHART SYMBOLS GRAPH LETTER TYPICAL DESCRIPTIONS ADDITIONAL MATERIAL SYMBOLS SYMBOLS TYPICAL GRAPH LETTER DESCRIPTIONS GRAVEL AND GRAVELLY SOILS CLEAN GRAVELS (LITTLE OR NO FINES) GW GP WELL-GRADED GRAVELS, GRAVEL - SAND MIXTURES POORLY-GRADED GRAVELS, GRAVEL - SAND MIXTURES AC CC Asphalt Concrete Cement Concrete COARSE GRAINED SOILS MORE THAN 0% RETAINED ON NO. 00 SIEVE FINE GRAINED SOILS MORE THAN 0% PASSING NO. 00 SIEVE MORE THAN 0% OF COARSE FRACTION RETAINED ON NO. 4 SIEVE SAND AND SANDY SOILS MORE THAN 0% OF COARSE FRACTION PASSING NO. 4 SIEVE SILTS AND CLAYS SILTS AND CLAYS GRAVELS WITH FINES (APPRECIABLE AMOUNT OF FINES) CLEAN SANDS (LITTLE OR NO FINES) SANDS WITH FINES (APPRECIABLE AMOUNT OF FINES) LIQUID LIMIT LESS THAN 0 LIQUID LIMIT GREATER THAN 0 GM GC SW SP SM SC ML CL OL MH CH OH SILTY GRAVELS, GRAVEL - SAND - SILT MIXTURES CLAYEY GRAVELS, GRAVEL - SAND - CLAY MIXTURES WELL-GRADED SANDS, GRAVELLY SANDS POORLY-GRADED SANDS, GRAVELLY SAND SILTY SANDS, SAND - SILT MIXTURES CLAYEY SANDS, SAND - CLAY MIXTURES INORGANIC SILTS, ROCK FLOUR, CLAYEY SILTS WITH SLIGHT PLASTICITY INORGANIC CLAYS OF LOW TO MEDIUM PLASTICITY, GRAVELLY CLAYS, SANDY CLAYS, SILTY CLAYS, LEAN CLAYS ORGANIC SILTS AND ORGANIC SILTY CLAYS OF LOW PLASTICITY INORGANIC SILTS, MICACEOUS OR DIATOMACEOUS SILTY SOILS INORGANIC CLAYS OF HIGH PLASTICITY ORGANIC CLAYS AND SILTS OF MEDIUM TO HIGH PLASTICITY CR TS Crushed Rock/ Quarry Spalls Topsoil/ Forest Duff/Sod Groundwater Contact Measured groundwater level in exploration, well, or piezometer Measured free product in well or piezometer Graphic Log Contact Distinct contact between soil strata or geologic units Approximate location of soil strata change within a geologic soil unit Material Description Contact Distinct contact between soil strata or geologic units Approximate location of soil strata change within a geologic soil unit HIGHLY ORGANIC SOILS NOTE: Multiple symbols are used to indicate borderline or dual soil classifications Sampler Symbol Descriptions.4-inch I.D. split barrel Standard Penetration Test (SPT) Shelby tube Piston Direct-Push Bulk or grab Blowcount is recorded for driven samplers as the number of blows required to advance sampler inches (or distance noted). See exploration log for hammer weight and drop. A "P" indicates sampler pushed using the weight of the drill rig. PT PEAT, HUMUS, SWAMP SOILS WITH HIGH ORGANIC CONTENTS %F AL CA CP CS DS HA MC MD OC PM PI PP PPM SA TX UC VS NS SS MS HS NT Laboratory / Field Tests Percent fines Atterberg limits Chemical analysis Laboratory compaction test Consolidation test Direct shear Hydrometer analysis Moisture content Moisture content and dry density Organic content Permeability or hydraulic conductivity Plasticity index Pocket penetrometer Parts per million Sieve analysis Triaxial compression Unconfined compression Vane shear Sheen Classification No Visible Sheen Slight Sheen Moderate Sheen Heavy Sheen Not Tested NOTE: The reader must refer to the discussion in the report text and the logs of explorations for a proper understanding of subsurface conditions. Descriptions on the logs apply only at the specific exploration locations and at the time the explorations were made; they are not warranted to be representative of subsurface conditions at other locations or times. KEY TO EXPLORATION LOGS FIGURE A-