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20-102998-Geotechnical Engineering Report-08-10-2020-V1 SUBSURFACE EXPLORATION, GEOLOGIC HAZARDS, AND GEOTECHNICAL ENGINEERING REPORT FEDERAL WAY SENIOR LIVING Federal Way, Washington Prepared for: Pacific Northern Construction Company 201 27th Avenue SE, Building A, Suite 300 Puyallup, Washington Prepared by: Associated Earth Sciences, Inc. 911 5th Avenue Kirkland, Washington 98033 425-827-7701 Fax: 425-827-5424 September 9, 2014 Project No. KE140420A Subsurface Exploration, Geologic Hazards, Federal Way Senior Living and Geotechnical Engineering Report Federal Way, Washington Project and Site Conditions I. PROJECT AND SITE CONDITIONS 1.0 INTRODUCTION This report presents the results of our subsurface exploration, geologic hazards, and geotechnical engineering studies for the proposed Federal Way Senior Living project. The site location is shown on the “Vicinity Map,” Figure 1. The approximate locations of the exploration pits completed for this study are shown on the “Site and Exploration Plan,” Figure 2. Logs of the subsurface explorations completed for this study are included in the Appendix. 1.1 Purpose and Scope The purpose of this study was to provide geotechnical engineering design recommendations to be utilized in the design of the project. This study included a review of selected available geologic literature, completing seven exploration pits, and performing geologic studies to assess the type, thickness, distribution, and physical properties of the subsurface sediments and shallow ground water. Geotechnical engineering studies were completed to establish recommendations for the type of suitable foundations, allowable foundation soil bearing pressure, anticipated foundation settlement, erosion control, infiltration considerations, and drainage considerations. This report summarizes our fieldwork and offers recommendations based on our present understanding of the project. We recommend that we be allowed to review the recommendations presented in this report and revise them, if needed, when a project design has been finalized. 1.2 Authorization Our study was accomplished in general accordance with our initial scope of work letter dated July 17, 2014. This report has been prepared for the exclusive use of Pacific Northern Construction Company, and their agents, for specific application to this project. Within the limitations of scope, schedule, and budget, our services have been performed in accordance with generally accepted geotechnical engineering and engineering geology practices in effect in this area at the time our report was prepared. No other warranty, express or implied, is made. Our observations, findings, and opinions are a means to identify and reduce the inherent risks to the owner. September 9, 2014 ASSOCIATED EARTH SCIENCES, INC. LDM/ld – KE140420A2 – Projects\20140420\KE\WP Page 1 Subsurface Exploration, Geologic Hazards, Federal Way Senior Living and Geotechnical Engineering Report Federal Way, Washington Project and Site Conditions 2.0 PROJECT AND SITE DESCRIPTION The site is an irregularly shaped parcel (Parcel #872992-0040) located to the east of the intersection of South 272nd Street and Pacific Highway South in Federal Way, Washington. It is currently undeveloped and comprises about 2.5 acres. The parcel is bounded to the north by South 272nd Street, to the south by a King County Park and Ride lot, to the west by existing retail development including a gas station, and to the east by existing residential development. The site is moderately vegetated with sparse trees and brush. An existing L-shaped detention pond is located at the south end of the property. The topography is relatively level over the western portion of the site and gently slopes down to the east in the northeastern area of the site. A 4- to 5-foot-high slope, trending north-south through the north-central portion of the parcel, appears to be the result of previous grading activities which leveled out the western portion of the property. We understand that a new senior living facility will be constructed on the site. The planned development will include a six-story building on the northern side of the parcel. The northern half of the building will contain a below-grade parking level, which will be close to existing grade along the eastern side of building and will require cuts up to 10 feet at the northwestern corner of the proposed building. 3.0 SUBSURFACE EXPLORATION Our subsurface exploration completed for this project included completing seven exploration pits. The conclusions and recommendations presented in this report are based on the explorations completed for this study. The locations and depths of the explorations were completed within site and budget constraints. 3.1 Exploration Pits Associated Earth Sciences, Inc. (AESI) observed excavation of seven exploration pits at the site on August 20, 2014. The approximate locations of the pits are shown on the “Site and Exploration Plan,” Figure 2. Exploration pits were excavated with a track-mounted mini excavator. The pits permitted direct, visual observation of subsurface conditions. Materials encountered in the exploration pits were studied and classified in the field by an engineering geologist from our firm. Disturbed soil samples were selected from the pits, placed in moisture- tight containers, and transported to our laboratory for further visual classification and testing, as necessary. Testing was limited to visual-manual classification of the collected samples and grain-size analyses in general accordance with American Society for Testing and Materials (ASTM) standard practices. After logging the exposed soils all exploration pits were backfilled with the excavated soil and lightly tamped with the excavator bucket. Detailed descriptions of September 9, 2014 ASSOCIATED EARTH SCIENCES, INC. LDM/ld – KE140420A2 – Projects\20140420\KE\WP Page 2 Subsurface Exploration, Geologic Hazards, Federal Way Senior Living and Geotechnical Engineering Report Federal Way, Washington Project and Site Conditions the sediments encountered in each exploration pit are provided on the exploration logs included in the Appendix. Results of grain-size and moisture content testing are also included in the Appendix. 4.0 SUBSURFACE CONDITIONS Subsurface conditions at the project site were inferred from the field explorations accomplished for this study, visual reconnaissance of the site, and review of selected applicable geologic literature. Because of the nature of exploratory work below ground, interpolation of subsurface conditions between field explorations is necessary. It should be noted that differing subsurface conditions may sometimes be present due to the random nature of deposition and the alteration of topography by past grading and/or filling. The nature and extent of any variations between the field explorations may not become fully evident until construction. 4.1 Stratigraphy Fill Fill soils (soils not naturally placed) were encountered in all exploration pits up to 7 feet in depth except at the locations of pits EP-3 and EP-5. This fill generally consisted of loose to medium dense silty sand with gravel with occasional organics and was generally similar in gradation to the underlying native lodgement till. The encountered fill likely resulted from past grading activities, including the leveling of grades along the western side of the site and construction of the existing pond. Fill thicknesses can vary over short distances and may be deeper than observed in our explorations. The fill was thickest near the top of the slope that separates the northwestern and northeastern portions of the site, at the location of EP-4. This slope appears to be the result of previous grading activities. Due to their variable density and content, the existing fill soils are not suitable for foundation support. Excavated fill material may suitable for use in structural fill applications if suitable moisture conditions are achieved and all deleterious materials are removed. At the time of exploration, we estimate that most of the fill soils that we observed were near or above optimum moisture content for compaction purposes, and therefore may require drying during favorable weather prior to compaction in structural fill applications. September 9, 2014 ASSOCIATED EARTH SCIENCES, INC. LDM/ld – KE140420A2 – Projects\20140420\KE\WP Page 3 Subsurface Exploration, Geologic Hazards, Federal Way Senior Living and Geotechnical Engineering Report Federal Way, Washington Project and Site Conditions Vashon Lodgement Till Sediments encountered below the existing fill consisted of a medium dense to very dense, poorly sorted mixture of clay, silt, sand, and gravel. We interpret these sediments to be representative of Vashon lodgement till. The till was deposited directly from basal, debris- laden glacial ice during the Vashon Stade of the Fraser Glaciation approximately 12,500 to 15,000 years ago. The high relative density of the unweathered till is due to its consolidation by the massive weight of the glacial ice from which it was deposited. The till extended to the bottom of all completed pits. Undisturbed medium dense to very dense lodgement till sediments are suitable for foundation and paving support. Due to the high percentage of fine-grained material present in these sediments, they are susceptible to disturbance when wet. Careful management of moisture- sensitive soils will be needed to reduce the potential for disturbance of wet till and costs associated with repairing disturbed sediments. Excavated lodgement till material is suitable for use in structural fill applications if suitable moisture conditions are achieved. At the time of exploration, we estimate that most of the lodgement till soils that we observed were near or above optimum moisture content for compaction purposes, and therefore may require drying during favorable weather prior to compaction in structural fill applications. Lodgement till generally has a low potential for infiltration due to its high density and high content of fine-grained sediments. 4.2 Geologic Mapping Review of the regional geologic map (Booth, D.B., Waldron, H.H., and Troost, K.G., 2004, Geologic Map of the Poverty Bay 7.5' Quadrangle, King and Pierce Counties, Washington: U.S. Geological Survey (USGS), Scientific Investigations Map SIM-2854, scale 1:24,000) indicates that the area of the subject site is underlain by Vashon lodgement till. Our interpretation of the sediments encountered in our explorations is in general agreement with the regional geologic map. 4.3 Hydrology No seepages were observed in any of the pits at the time of exploration. A mottled coloration was observed at the location of EP-6 at a depth of 9 feet, which may be indicative of seasonal or intermittent ground water at this depth. We anticipate that perched ground water may occur seasonally at the site. Perched water occurs when surface water infiltrates down through relatively permeable soils (such as the existing fill or weathered till) and becomes trapped or “perched” atop a comparatively impermeable barrier, such as unweathered lodgement till. The perched water will generally flow “downhill” over the top of the unweathered till surface at September 9, 2014 ASSOCIATED EARTH SCIENCES, INC. LDM/ld – KE140420A2 – Projects\20140420\KE\WP Page 4 Subsurface Exploration, Geologic Hazards, Federal Way Senior Living and Geotechnical Engineering Report Federal Way, Washington Project and Site Conditions depth. The duration and quantity of seepage will largely depend on the soil grain-size distribution, topography, seasonal precipitation, on- and off-site land usage, and other factors. 4.4 Infiltration Considerations We understand that infiltration into the on-site soils is being considered for the management of storm water runoff generated on-site. As discussed above, near-surface site soils consist of existing fill over glacial till. We do not recommend attempting to infiltrate into exiting fill due to its highly variable condition. The on-site native sediments consist of lodgement till which is generally composed of a very dense, unsorted matrix of silty fine to medium sand with variable percentages of clay, gravel, cobbles, and boulders. Due to its very high relative density and high content of fine-grained material, till typically has a very low permeability and is not considered a suitable receptor for infiltrating storm water runoff. September 9, 2014 ASSOCIATED EARTH SCIENCES, INC. LDM/ld – KE140420A2 – Projects\20140420\KE\WP Page 5 Subsurface Exploration, Geologic Hazards, Federal Way Senior Living and Geotechnical Engineering Report Federal Way, Washington Geologic Hazards and Mitigations II. GEOLOGIC HAZARDS AND MITIGATIONS The following discussion of potential geologic hazards is based on the geologic, slope, and ground and surface water conditions, as observed and discussed herein. 5.0 SEISMIC HAZARDS AND MITIGATIONS Earthquakes occur regularly in the Puget Lowland. The majority of these events are small and are usually not felt by people. However, large earthquakes do occur, as evidenced by the 1949, 7.2-magnitude event; the 2001, 6.8-magnitude event; and the 1965, 6.5-magnitude event. The 1949 earthquake appears to have been the largest in this region during recorded history and was centered in the Olympia area. Evaluation of earthquake return rates indicates that an earthquake of the magnitude between 5.5 and 6.0 is likely within a given 20-year period. Generally, there are four types of potential geologic hazards associated with large seismic events: 1) surficial ground rupture, 2) seismically induced landslides, 3) liquefaction, and 4) ground motion. The potential for each of these hazards to adversely impact the proposed project is discussed below. 5.1 Surficial Ground Rupture Generally, the largest earthquakes that have occurred in the Puget Sound area are sub-crustal events with epicenters ranging from 50 to 70 kilometers in depth. Earthquakes that are generated at such depths usually do not result in fault rupture at the ground surface. Limited and sporadic surficial faulting or earth rupture as a result of deep seismic activity has been documented to date in the region. Current research indicates that surficial ground rupture is possible in the Seattle and Tacoma Fault Zones. The Seattle and Tacoma Fault Zones are areas of active research. Our current understanding of these fault zones is poor, and actively evolving. The site is located south of the currently mapped limits of the Seattle Fault Zone. Preliminary maps of the Tacoma Fault Zone depict possible fault splays mapped west of the site and projecting generally toward the site area, but mapped fault splays fall short of the project site. If the currently mapped fault splays are projected, the nearest fault splay projects within 1 mile north of the subject site. The recurrence interval of movement along these fault systems is still unknown, although it is hypothesized to be in excess of several thousand years. Due to the suspected long recurrence interval, the potential for surficial ground rupture is considered to be low during the expected September 9, 2014 ASSOCIATED EARTH SCIENCES, INC. LDM/ld – KE140420A2 – Projects\20140420\KE\WP Page 6 Subsurface Exploration, Geologic Hazards, Federal Way Senior Living and Geotechnical Engineering Report Federal Way, Washington Geologic Hazards and Mitigations life of the structure, and no mitigation efforts beyond complying with the current (2012) International Building Code (IBC) are recommended. 5.2 Seismically Induced Landslides It is our opinion that the potential risk of damage to the proposed improvements by seismically induced slope failures is low due to the lack of steep slopes in the project area. 5.3 Liquefaction Liquefaction is a process through which unconsolidated soil loses strength as a result of vibrations, such as those which occur during a seismic event. During normal conditions, the weight of the soil is supported by both grain-to-grain contacts and by the fluid pressure within the pore spaces of the soil below the water table. Extreme vibratory shaking can disrupt the grain-to-grain contact, increase the pore pressure, and result in a temporary decrease in soil shear strength. The soil is said to be liquefied when nearly all of the weight of the soil is supported by pore pressure alone. Liquefaction can result in deformation of the sediment and settlement of overlying structures. Areas most susceptible to liquefaction include those areas underlain by non-cohesive silt and sand with low relative densities, accompanied by a shallow water table. The subsurface conditions encountered at the site pose little risk of liquefaction due to relatively high density of glacial soils encountered. No detailed liquefaction analysis was completed as part of this study, and none is warranted, in our opinion. 5.4 Ground Motion Structural design of the building should follow 2012 IBC standards using Site Class “C” as defined in Table 20.3-1 of American Society of Civil Engineers (ASCE) 7 – Minimum Design Loads for Buildings and Other Structures. 6.0 EROSION HAZARDS AND MITIGATIONS The following discussion addresses Washington State Department of Ecology (Ecology) erosion control regulations that will be applicable to the project. The on-site conditions do not meet the City of Federal Way definition for Erosion Hazard Areas. In addition, there are no erosion hazards shown in the immediate project vicinity on the Critical Areas map produced by the City. In our opinion, implementation of the following recommendations should be adequate to address the Ecology and City of Federal Way requirements for management of erosion hazards. September 9, 2014 ASSOCIATED EARTH SCIENCES, INC. LDM/ld – KE140420A2 – Projects\20140420\KE\WP Page 7 Subsurface Exploration, Geologic Hazards, Federal Way Senior Living and Geotechnical Engineering Report Federal Way, Washington Geologic Hazards and Mitigations The Ecology Construction Storm Water General Permit (also known as the National Pollutant Discharge Elimination System [NPDES] permit) requires weekly Temporary Erosion and Sedimentation Control (TESC) inspections, turbidity monitoring and pH monitoring for all sites 1 or more acres in size that discharge storm water to surface waters of the state. Because we anticipate that the proposed project will require disturbance of more than 1 acre, we anticipate that these inspection and reporting requirements will be triggered. The following recommendations are related to general erosion potential and mitigation. Project planning and construction should follow local standards of practice with respect to temporary erosion and sedimentation control. Best management practices (BMPs) should include but not be limited to: 1. Construction activity should be scheduled or phased as much as possible to reduce the amount of earthwork activity that is performed during the winter months. 2. The winter performance of a site is dependent on a well-conceived plan for control of site erosion and storm water runoff. The site plan should include ground-cover measures, access roads, and staging areas. The contractor should be prepared to implement and maintain the required measures to reduce the amount of exposed ground. 3. TESC measures for a given area to be graded or otherwise worked should be installed soon after ground clearing. The recommended sequence of construction within a given area after clearing would be to install TESC elements and perimeter flow control prior to starting grading. 4. During the wetter months of the year, or when large storm events are predicted during the summer months, each work area should be stabilized so that if showers occur, the work area can receive the rainfall without excessive erosion or sediment transport. The required measures for an area to be “buttoned-up” will depend on the time of year and the duration the area will be left un-worked. During the winter months, areas that are to be left un-worked for more than 2 days should be mulched or covered with plastic. During the summer months, stabilization will usually consist of seal-rolling the subgrade. Such measures will aid in the contractor’s ability to get back into a work area after a storm event. The stabilization process also includes establishing temporary storm water conveyance channels through work areas to route runoff to the approved treatment/discharge facilities. September 9, 2014 ASSOCIATED EARTH SCIENCES, INC. LDM/ld – KE140420A2 – Projects\20140420\KE\WP Page 8 Subsurface Exploration, Geologic Hazards, Federal Way Senior Living and Geotechnical Engineering Report Federal Way, Washington Geologic Hazards and Mitigations 5. All disturbed areas should be revegetated as soon as possible. If it is outside of the growing season, the disturbed areas should be covered with mulch, as recommended in the erosion control plan. Straw mulch provides a cost-effective cover measure and can be made wind-resistant with the application of a tackifier after it is placed. 6. Surface runoff and discharge should be controlled during and following development. Uncontrolled discharge may promote erosion and sediment transport. Under no circumstances should concentrated discharges be allowed to flow over the top of steep slopes. 7. Soils that are to be reused around the site should be stored in such a manner as to reduce erosion from the stockpile. Protective measures may include, but are not limited to, covering with plastic sheeting, the use of low stockpiles in flat areas, or the use of silt fences around pile perimeters. It is our opinion that with the proper implementation of the TESC plans and by field-adjusting appropriate mitigation elements (best management practices) during construction, the potential adverse impacts from erosion hazards on the project may be mitigated. September 9, 2014 ASSOCIATED EARTH SCIENCES, INC. LDM/ld – KE140420A2 – Projects\20140420\KE\WP Page 9 Subsurface Exploration, Geologic Hazards, Federal Way Senior Living and Geotechnical Engineering Report Federal Way, Washington Preliminary Design Recommendations III. PRELIMINARY DESIGN RECOMMENDATIONS 7.0 INTRODUCTION Our exploration indicates that, from a geotechnical standpoint, the property is suitable for the proposed improvements provided the recommendations contained herein are properly followed. The subject site is underlain in places by a layer of existing fill that is variable in thickness and density. Existing fill or loose soils are not suitable for support of new foundations, and warrant remedial preparation where occurring below paving. Fill soils should be removed from below foundation areas and replaced with structural fill. Medium dense to very dense native deposits or structural fill placed over medium dense to very dense native deposits are suitable for support of shallow foundations with proper preparation. 8.0 SITE PREPARATION Erosion and surface water control should be established around the clearing limits to satisfy local requirements. Existing buried utilities, vegetation, topsoil, and any other deleterious materials should be removed where they are located below planned construction areas. All disturbed soils should be removed to expose underlying, undisturbed, native sediments and replaced with structural fill, as needed. All excavations below final grade made for clearing and grubbing activities should be backfilled, as needed, with structural fill, as discussed under the “Structural Fill” section of this report. Once clearing and grubbing activities have been completed, existing fill, where encountered, should be addressed. We recommend that existing fill be removed from below areas of planned foundations to expose underlying, undisturbed native sediments, followed by restoration of the planned foundation grade with structural fill. Removal of existing fill should extend laterally beyond the building footprint by a distance equal to the depth of overexcavation. For example, if existing fill is removed to a depth of 2 feet below a planned footing area, the excavation should also extend laterally 2 feet beyond the building footprint in that area. Where existing fill is removed and replaced with structural fill, conventional shallow foundations may be used for building support. The required depth of removal should be determined in the field based on actual conditions encountered during excavation. 8.1 Site Drainage and Surface Water Control The site should be graded to prevent water from ponding in construction areas and/or flowing into excavations. Exposed grades should be crowned, sloped, and smooth drum-rolled at the end of each day to facilitate drainage. Accumulated water must be removed from subgrades and work areas immediately prior to performing further work in the area. Equipment access September 9, 2014 ASSOCIATED EARTH SCIENCES, INC. LDM/ld – KE140420A2 – Projects\20140420\KE\WP Page 10 Subsurface Exploration, Geologic Hazards, Federal Way Senior Living and Geotechnical Engineering Report Federal Way, Washington Preliminary Design Recommendations may be limited, and the amount of soil rendered unfit for use as structural fill may be greatly increased if drainage efforts are not accomplished in a timely sequence. If an effective drainage system is not utilized, project delays and increased costs could be incurred due to the greater quantities of wet and unsuitable fill, or poor access and unstable conditions. Although we did not encounter ground water at the time of our explorations (late summer), we anticipate that perched ground water may be encountered over the unweathered till during the wet season at the site. We do not anticipate the need for extensive dewatering in advance of excavations. However, the contractor should be prepared to intercept any ground water seepage entering the excavations and route it to a suitable discharge location. Final exterior grades should promote free and positive drainage away from the building at all times. Water must not be allowed to pond or to collect adjacent to foundations or within the immediate building area. We recommend that a gradient of at least 3 percent for a minimum distance of 10 feet from the building perimeters be provided, except in paved locations. In paved locations, a minimum gradient of 1 percent should be provided, unless provisions are included for collection and disposal of surface water adjacent to the structure. 8.2 Subgrade Protection If building construction will proceed during the winter, we recommend the use of a working surface of sand and gravel, crushed rock, or quarry spalls to protect exposed soils, particularly in areas supporting concentrated equipment traffic. In winter construction staging areas and areas that will be subjected to repeated heavy loads, such as those that occur during construction of masonry walls, a minimum thickness of 12 inches of quarry spalls or 18 inches of pit run sand and gravel is recommended. If subgrade conditions are soft and silty, a geotextile separation fabric, such as Mirafi 500X or approved equivalent, should be used between the subgrade and the new fill. For building pads where floor slabs and foundation construction will be completed in the winter, a similar working surface should be used, composed of at least 6 inches of pit run sand and gravel or crushed rock. Construction of working surfaces from advancing fill pads could be used to avoid directly exposing the subgrade soils to vehicular traffic. Foundation subgrades may require protection from foot and equipment traffic and ponding of runoff during wet weather conditions. Typically, compacted crushed rock or a lean-mix concrete mat placed over a properly prepared subgrade provides adequate subgrade protection. Foundation concrete should be placed and excavations backfilled as soon as possible to protect the bearing surface. September 9, 2014 ASSOCIATED EARTH SCIENCES, INC. LDM/ld – KE140420A2 – Projects\20140420\KE\WP Page 11 Subsurface Exploration, Geologic Hazards, Federal Way Senior Living and Geotechnical Engineering Report Federal Way, Washington Preliminary Design Recommendations 8.3 Proof-Rolling and Subgrade Compaction Following the recommended clearing, site stripping, and planned excavation, the stripped subgrade within the building areas should be proof-rolled with heavy, rubber-tired construction equipment, such as a fully loaded, tandem-axle dump truck. Proof-rolling should be performed prior to structural fill placement or foundation excavation. The proof-roll should be monitored by the geotechnical engineer so that any soft or yielding subgrade soils can be identified. Any soft/loose, yielding soils should be removed to a stable subgrade. The subgrade should then be scarified, adjusted in moisture content, and recompacted to the required density. Proof-rolling should only be attempted if soil moisture contents are at or near optimum moisture content. Proof-rolling of wet subgrades could result in further degradation. Low areas and excavations may then be raised to the planned finished grade with compacted structural fill. Subgrade preparation and selection, placement, and compaction of structural fill should be performed under engineering-controlled conditions in accordance with the project specifications. 8.4 Overexcavation/Stabilization Construction during extended wet weather periods could create the need to overexcavate exposed soils if they become disturbed and cannot be recompacted due to elevated moisture content and/or weather conditions. Even during dry weather periods, soft/wet soils, which may need to be overexcavated, may be encountered in some portions of the site. If overexcavation is necessary, it should be confirmed through continuous observation and testing by AESI. Soils that have become unstable may require remedial measures in the form of one or more of the following: 1. Drying and recompaction. Selective drying may be accomplished by scarifying or windrowing surficial material during extended periods of dry and warm weather. 2. Removal of affected soils to expose a suitable bearing subgrade and replacement with compacted structural fill. 3. Mechanical stabilization with a coarse crushed aggregate compacted into the subgrade, possibly in conjunction with a geotextile. 4. Soil/cement admixture stabilization. 8.5 Wet Weather Conditions If construction proceeds during an extended wet weather construction period and the moisture-sensitive site soils become wet, they will become unstable. Therefore, the bids for site grading operations should be based upon the time of year that construction will proceed. It is expected that in wet conditions additional soils may need to be removed and/or other September 9, 2014 ASSOCIATED EARTH SCIENCES, INC. LDM/ld – KE140420A2 – Projects\20140420\KE\WP Page 12 Subsurface Exploration, Geologic Hazards, Federal Way Senior Living and Geotechnical Engineering Report Federal Way, Washington Preliminary Design Recommendations stabilization methods used, such as a coarse crushed rock working mat to develop a stable condition if silty subgrade soils are disturbed in the presence of excess moisture. The severity of construction disturbance will be dependent, in part, on the precautions that are taken by the contractor to protect the moisture- and disturbance-sensitive site soils. If overexcavation is necessary, it should be confirmed through continuous observation and testing by a representative of our firm. 8.6 Temporary and Permanent Cut Slopes In our opinion, stable construction slopes should be the responsibility of the contractor and should be determined during construction. For estimating purposes, however, we anticipate that temporary, unsupported cut slopes in the existing fill or loose to medium dense native deposits can be made at a maximum slope of 1.5H:1V (Horizontal:Vertical) or flatter. Temporary slopes in dense to very dense till sediments may be planned at 1H:1V. As is typical with earthwork operations, some sloughing and raveling may occur, and cut slopes may have to be adjusted in the field. If ground water seepage is encountered in cut slopes, or if surface water is not routed away from temporary cut slope faces, flatter slopes will be required. In addition, WISHA/OSHA regulations should be followed at all times. Permanent cut and structural fill slopes that are not intended to be exposed to surface water should be designed at inclinations of 2H:1V or flatter. All permanent cut or fill slopes should be compacted to at least 95 percent of the modified Proctor maximum dry density, as determined by ASTM:D 1557, and the slopes should be protected from erosion by sheet plastic until vegetation cover can be established during favorable weather. 8.7 Frozen Subgrades If earthwork takes place during freezing conditions, all exposed subgrades should be allowed to thaw and then be recompacted prior to placing subsequent lifts of structural fill or foundation components. Alternatively, the frozen material could be stripped from the subgrade to reveal unfrozen soil prior to placing subsequent lifts of fill or foundation components. The frozen soil should not be reused as structural fill until allowed to thaw and adjusted to the proper moisture content, which may not be possible during winter months. 9.0 STRUCTURAL FILL All references to structural fill in this report refer to subgrade preparation, fill type and placement, and compaction of materials, as discussed in this section. If a percentage of compaction is specified under another section of this report, the value given in that section should be used. September 9, 2014 ASSOCIATED EARTH SCIENCES, INC. LDM/ld – KE140420A2 – Projects\20140420\KE\WP Page 13 Subsurface Exploration, Geologic Hazards, Federal Way Senior Living and Geotechnical Engineering Report Federal Way, Washington Preliminary Design Recommendations After stripping, planned excavation, and any required overexcavation have been performed to the satisfaction of the geotechnical engineer, the upper 12 inches of exposed ground in areas to receive fill should be recompacted to a firm and unyielding condition as determined by the geotechnical engineer. If the subgrade contains silty soils and too much moisture, adequate recompaction may be difficult or impossible to obtain and should probably not be attempted. In lieu of recompaction, the area to receive fill should be blanketed with washed rock or quarry spalls to act as a capillary break between the new fill and the wet subgrade. Where the exposed ground remains soft and further overexcavation is impractical, placement of an engineering stabilization fabric may be necessary to prevent contamination of the free-draining layer by silt migration from below. After recompaction of the exposed ground is tested and approved, or a free-draining rock course is laid, structural fill may be placed to attain desired grades. Structural fill is defined as non-organic soil, acceptable to the geotechnical engineer, placed in maximum 8-inch loose lifts, with each lift being compacted to 95 percent of the modified Proctor maximum density using ASTM:D 1557 as the standard. For on-site utility trench backfill, including the backfill resulting from the removal of existing utility lines below the planned structure, we recommend the structural fill standard described above. In the case of roadway and utility trench filling within City rights-of-way, the backfill should be placed and compacted in accordance with current City of Federal Way codes and standards. The top of the compacted fill should extend horizontally outward a minimum distance of 3 feet beyond the locations of the roadway edges before sloping down at an angle of 2H:1V. The contractor should note that any proposed fill soils must be evaluated by AESI prior to their use in fills. This would require that we have a sample of the material 72 hours in advance to perform a Proctor test and determine its field compaction standard. Soils in which the amount of fine-grained material (smaller than the No. 200 sieve) is greater than approximately 5 percent (measured on the minus No. 4 sieve size) should be considered moisture-sensitive. Use of moisture-sensitive soil in structural fills should be limited to favorable dry weather conditions. The native and existing fill soils present on-site contained variably high amounts of silt and are considered moisture-sensitive. Therefore, we anticipate that the use of on-site soils as structural fill may require moisture-conditioning to achieve proper compaction. For non- structural applications, the on-site material is generally considered suitable, as long as it is free of vegetation, topsoil, and any other deleterious materials. In addition, construction equipment traversing the site when the soils are wet can cause considerable disturbance. If fill is placed during wet weather or if proper compaction cannot be obtained, a select import material consisting of a clean, free-draining gravel and/or sand should be used. Free-draining fill consists of non-organic soil with the amount of fine-grained material limited to 5 percent by weight when measured on the minus No. 4 sieve fraction with at least 25 percent retained on the No. 4 sieve. September 9, 2014 ASSOCIATED EARTH SCIENCES, INC. LDM/ld – KE140420A2 – Projects\20140420\KE\WP Page 14 Subsurface Exploration, Geologic Hazards, Federal Way Senior Living and Geotechnical Engineering Report Federal Way, Washington Preliminary Design Recommendations A representative from our firm should inspect the stripped subgrade and be present during placement of structural fill to observe the work and perform a representative number of in-place density tests. In this way, the adequacy of the earthwork may be evaluated as filling progresses, and any problem areas may be corrected at that time. It is important to understand that taking random compaction tests on a part-time basis will not assure uniformity or acceptable performance of a fill. As such, we are available to aid in developing a suitable monitoring and testing program. 10.0 FOUNDATIONS We expect the depth to bearing soil to vary across the building footprint relative to the foundation subgrade elevation of the planned building. The existing fill was thickest (about 7 feet in depth) in the northwestern portion of the site, in the vicinity of EP-4, near the top of a north-south trending slope. This slope appears to have been constructed during previous grading activities. Based on the provided site plans, a significant portion of this fill will be removed as part of the cuts necessary for the below-level parking. Cuts up to 10 feet are anticipated at the northwestern corner of the building. Where present, existing fill should be removed below all foundations, exposing medium dense to very dense native till sediments. Spread footings may be used for building support when founded directly on undisturbed native sediments, on structural fill placed over suitable native sediments. If foundations will be supported by a combination of very dense native sediments and new structural fill, we recommend that an allowable bearing pressure of 3,000 pounds per square foot (psf) be used for design purposes, including both dead and live loads. If foundations will be supported entirely by very dense native sediments, an allowable foundation soil bearing pressure of 5,000 psf may be used for design. An increase of one-third may be used for short-term wind or seismic loading. Perimeter footings should be buried at least 18 inches into the surrounding soil for frost protection. However, all footings must penetrate to the prescribed bearing stratum, and no footing should be founded in or above organic or loose soils. It should be noted that the area bound by lines extending downward at 1H:1V from any footing must not intersect another footing or intersect a filled area that has not been compacted to at least 95 percent of ASTM:D 1557. In addition, a 1.5H:1V line extending down from any footing must not daylight because sloughing or raveling may eventually undermine the footing. Thus, footings should not be placed near the edge of steps or cuts in the bearing soils. Anticipated settlement of footings founded as described above should be on the order of ¾ inch or less. However, disturbed soil not removed from footing excavations prior to footing September 9, 2014 ASSOCIATED EARTH SCIENCES, INC. LDM/ld – KE140420A2 – Projects\20140420\KE\WP Page 15 Subsurface Exploration, Geologic Hazards, Federal Way Senior Living and Geotechnical Engineering Report Federal Way, Washington Preliminary Design Recommendations placement and footings placed above loose soils could result in increased settlements. All footing areas should be inspected by AESI prior to placing concrete to verify that the design bearing capacity of the soils has been attained and that construction conforms to the recommendations contained in this report. Such inspections may be required by the governing municipality. Perimeter footing drains should be provided, as discussed under the “Drainage Considerations” Section 13.0 of this report. 11.0 FLOOR SUPPORT If crawl-space floors are used, an impervious moisture barrier should be provided above the soil surface within the crawl space. Slab-on-grade floors may be used over medium dense to very dense native soils, or over structural fill placed as recommended in the “Site Preparation” and “Structural Fill” sections of this report. Slab-on-grade floors should be cast atop a minimum of 4 inches of washed pea gravel or washed crushed “chip” rock with less than 3 percent passing the U.S. No. 200 sieve to act as a capillary break. The floors should also be protected from dampness by covering the capillary break layer with an impervious moisture barrier at least 10 mils in thickness. 12.0 FOUNDATION WALLS All backfill behind foundation walls or around foundation units should be placed as per our recommendations for structural fill and as described in this section of the report. Horizontally backfilled walls, which are free to yield laterally at least 0.1 percent of their height, may be designed using an equivalent fluid equal to 35 pounds per cubic foot (pcf). Fully restrained, horizontally backfilled, rigid walls that cannot yield should be designed for an equivalent fluid of 50 pcf. Walls with sloping backfill up to a maximum gradient of 2H:1V should be designed using an equivalent fluid of 55 pcf for yielding conditions or 75 pcf for fully restrained conditions. If parking areas are adjacent to walls, a surcharge equivalent to 2 feet of soil should be added to the wall height in determining lateral design forces. As required by the 2012 IBC, retaining wall design should include a seismic surcharge pressure in addition to the equivalent fluid pressures presented above. Considering the site soils and the recommended wall backfill materials, we recommend a seismic surcharge pressure of 5H and 10H psf, where H is the wall height in feet for the “active” and “at-rest” loading conditions, respectively. The seismic surcharge should be modeled as a rectangular distribution with the resultant applied at the mid-point of the walls. The lateral pressures presented above are based on the conditions of a uniform backfill consisting of excavated on-site soils, or imported structural fill compacted to 90 percent of September 9, 2014 ASSOCIATED EARTH SCIENCES, INC. LDM/ld – KE140420A2 – Projects\20140420\KE\WP Page 16 Subsurface Exploration, Geologic Hazards, Federal Way Senior Living and Geotechnical Engineering Report Federal Way, Washington Preliminary Design Recommendations ASTM:D 1557. A higher degree of compaction is not recommended, as this will increase the pressure acting on the walls. A lower compaction may result in settlement of the slab-on-grade or other structures supported above the walls. Thus, the compaction level is critical and must be tested by our firm during placement. Surcharges from adjacent footings or heavy construction equipment must be added to the above values. Perimeter footing drains should be provided for all retaining walls, as discussed under the “Drainage Considerations” section of this report. It is imperative that proper drainage be provided so that hydrostatic pressures do not develop against the walls. This would involve installation of a minimum, 1-foot-wide blanket drain to within 1 foot of finish grade for the full wall height using imported, washed gravel against the walls. A prefabricated drainage mat is not a suitable substitute for the gravel blanket drain unless all backfill against the wall is free-draining. 12.1 Passive Resistance and Friction Factors Lateral loads can be resisted by friction between the foundation and the natural glacial soils or supporting structural fill soils, and by passive earth pressure acting on the buried portions of the foundations. The foundations must be backfilled with structural fill and compacted to at least 95 percent of the maximum dry density to achieve the passive resistance provided below. We recommend the following allowable design parameters: • Passive equivalent fluid = 300 pcf • Coefficient of friction = 0.35 13.0 DRAINAGE CONSIDERATIONS All retaining and perimeter foundation walls should be provided with a drain at the base of the footing elevation. Drains should consist of rigid, perforated, PVC pipe surrounded by washed pea gravel. The level of the perforations in the pipe should be set at or slightly below the bottom of the footing grade beam, and the drains should be constructed with sufficient gradient to allow gravity discharge away from the building. In addition, all retaining walls should be lined with a minimum, 12-inch-thick, washed gravel blanket that extends to within 1 foot of the surface and is continuous with the foundation drain. Roof and surface runoff should not discharge into the foundation drain system, but should be handled by a separate, rigid, tightline drain. In planning, exterior grades adjacent to walls should be sloped downward away from the structure to achieve surface drainage. September 9, 2014 ASSOCIATED EARTH SCIENCES, INC. LDM/ld – KE140420A2 – Projects\20140420\KE\WP Page 17 D e s M o i n e s F e d e r a l W a y K e n t Copyright:© 2013 National Geographic Society, i-cubed REFERENCE: USGS, KING CO VICINITY MAPFEDERAL WAY S ENIO R LIVINGFEDERAL WAY, WASHINGTON FIGURE 1 DATE 8/14 PROJ. NO. KE140420A 0 20001000 FEETNOTE: BL ACK AND WHITE REPROD UCTION OF THIS C OLOR ORIGINAL MAYREDUCE IT S EFFECTIVENES S A ND L EAD TO IN CORRECT INTE RPRETATION.Document Path: H:\GIS_Projects\aTemplates\aVM_Template\ProjectVicinity_King.mxd± S 272nd St SR 99S I T E !(9 9 ¥5 S 2 7 2 N D S T NOTE: BLACK AND WHITE REPRODUCTION OF THIS COLOR ORIGINAL MAYREDUCE ITS EFFECTIVENESS AND LEAD TO INCORRECT INTERPRETATION.a s s o c i a t e de a r t h s c i e n c e si n c o r p o r a t e dFEET25 500NREFERENCE: BARGHAUSENFIGURE 2DATE 08/14PROJ. NO. KE140420AProjects 140420 Federal Way Senior Living \ 140420 Site and Exploration Plan.cdrSITE AND EXPLORATION PLANFEDERAL WAY SENIOR LIVINGFEDERAL WAY, WASHINGTONEP-5EP-3EP-2EP-4EP-7EP-1EP-6APPROXIMATE LOCATIONOF EXPLORATION BORINGTYPAPPROXIMATE LOCATIONOF EXPLORATION BORINGTYP