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HomeMy WebLinkAboutRS_Geotechnical_Engineering_Study_AESI_211001_v1.pdfassociated earth sciences incorporated Associated Earth Sciences, Inc. 911 5th Avenue Kirkland, WA 98033 P (425) 827 7701 Subsurface Exploration, Geologic Hazard, and Preliminary Geotechnical Engineering Report LINDBERGH HIGH SCHOOL ADDITIONS Renton, Washington Prepared For: RENTON SCHOOL DISTRICT NO. 403 Project No. 20000669E005 October 1, 2021 Kirkland | Tacoma | Mount Vernon 425-827-7701 | www.aesgeo.com October 1, 2021 Project No. 20000669E005 Renton School District No. 403 7812 South 124th Street Seattle, Washington 98178 Attention: Mr. Stewart Shusterman Subject: Subsurface Exploration, Geologic Hazard, and Preliminary Geotechnical Engineering Report Lindbergh High School Additions 16426 128th Avenue SE Renton, Washington Dear Mr. Shusterman: We are pleased to present the enclosed copy of the referenced report. This report summarizes the results of tasks including subsurface exploration, geologic hazard analysis, and preliminary geotechnical engineering and offers preliminary recommendations for design of the project. We have enjoyed working with you on this study and are confident that the preliminary recommendations presented in this report will aid in the successful completion of your project. Please contact me if you have any questions or if we can be of additional help to you. Sincerely, ASSOCIATED EARTH SCIENCES, INC. Kirkland, Washington ______________________________ Bruce W. Guenzler, L.E.G. Senior Associate Geologist BWG/ld - 20000669E005-003 SUBSURFACE EXPLORATION, GEOLOGIC HAZARD, AND PRELIMINARY GEOTECHNICAL ENGINEERING REPORT LINDBERGH HIGH SCHOOL ADDITIONS Renton, Washington Prepared for: Renton School District No. 403 7812 South 124th Street Seattle, Washington 98178 Prepared by: Associated Earth Sciences, Inc. 911 5th Avenue Kirkland, Washington 98033 425-827-7701 October 1, 2021 Project No. 20000669E005 Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Project and Site Conditions October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 1 I.PROJECT AND SITE CONDITIONS 1.0 INTRODUCTION This report presents the results of Associated Earth Sciences, Inc.’s (AESI’s) subsurface exploration, geologic hazard analysis, and preliminary geotechnical engineering study for the proposed building additions to Lindbergh High School in Renton, Washington. Our recommendations are preliminary in that the project is in the early design phase. The site location is shown on the “Vicinity Map,” Figure 1. The approximate locations of explorations completed for this study are shown on the “Existing Site and Exploration Plan,” Figure 2. Interpretive exploration logs of subsurface explorations completed for this study are included in Appendix A. 1.1 Purpose and Scope The purpose of this study is to provide subsurface soil and groundwater data to be utilized in the preliminary design of the Lindbergh High School additions project. Our study included reviewing selected available geologic literature, advancing three exploration borings (EB-1 through EB-3), advancing two hand-auger borings (HB-1 and HB-2), reviewing three previous AESI geotechnical engineering studies (2003, 2004, and 2010) done in support of earlier projects on campus, and performing a geologic study of subsurface sediment and groundwater conditions. Geotechnical engineering studies were completed to formulate our recommendations for site preparation, earthwork, the type of suitable foundations and floor slabs, allowable foundation soil bearing pressures, anticipated foundation settlements, erosion considerations, and drainage considerations. This report summarizes our current fieldwork and offers preliminary design recommendations based on our present understanding of the project. 1.2 Authorization Authorization to proceed with this study was given to AESI by means of District Purchase Order 2012000181 dated August 18, 2021. Our study was accomplished in general accordance with our proposal, dated August 10, 2021. This report has been prepared for the exclusive use of Renton School District and its 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. Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Project and Site Conditions October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 2 2.0 PROJECT AND SITE DESCRIPTION The project site is that of the existing Lindbergh High School. The proposed project areas are relatively flat and were graded to the existing configuration during previous earthwork completed to construct the existing campus. Topography of the project vicinity is characterized by an upland plateau with gentle to moderate slopes. A review of mapped critical areas on King County iMAP shows that an undeveloped area along the southeast edge of the project site is flagged as a potential seismic hazard area. No other geotechnical critical areas are flagged on or immediately adjacent to the site. It appears unlikely that building additions constructed near the newly completed explorations will need to address geotechnical critical areas requirements. The proposed project will include substantial renovations and a building addition to the existing Lincoln House in the northeast part of the site. An addition is proposed to be completed southwest of the existing commons. Lastly, an interior courtyard in the science wing will be infilled. 2.1 Historical Geotechnical Work AESI had previously provided geotechnical engineering for twelve projects onsite. Three of the previous studies included subsurface explorations near the currently proposed building improvement areas. Previous subsurface explorations onsite by AESI generally encountered surficial loose existing fill typically up to about 9 feet thick, underlain by very dense lodgement till. In general, loose existing fill is not suitable for structural support, and dense lodgement till is suitable for structural support with proper preparation. With respect to stormwater infiltration potential, existing fill is not permitted to serve as an infiltration receptor, and lodgement till is not a suitable infiltration receptor due to low permeability. This report relies, in part, on selected subsurface data from previous AESI geotechnical engineering studies onsite. Where we used existing data exploration locations are shown on the “Existing Site and Exploration Plan, Figure 2,” and copies of exploration logs are included in Appendix A. Subsurface data from previous AESI studies onsite included many additional explorations outside of the current work areas, and those more distant explorations are not shown on Figure 2 or included in Appendix A. 3.0 SITE EXPLORATION Our most recent field investigation for the current study was conducted in September 2021 and included advancing three exploration borings and two hand-auger borings. This study is supplemented with subsurface information gathered from our previous reports dated 2003, 2004, and 2010. The existing site conditions, and the approximate locations of subsurface explorations referenced in this study are presented on the “Existing Site and Exploration Plan” (Figure 2). The various types of sediments, as well as the depths where the characteristics of the Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Project and Site Conditions October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 3 sediments changed, are indicated on the exploration logs presented in Appendix A. The depths indicated on the logs where conditions changed may represent gradational variations between sediment types. If changes occurred between sample intervals in our exploration borings, they were interpreted. Our explorations were approximately located in the field by measuring from known site features depicted on the aerial photograph used as a basis for Figure 2. The conclusions and recommendations presented in this report are based, in part, on the explorations completed for this study and previous on-site studies completed by AESI. The number, locations, and depths of the explorations were completed within site and budgetary constraints. Because of the nature of exploratory work below ground, extrapolation of subsurface conditions between field explorations is necessary. It should be noted that differing subsurface conditions may 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 variations between the field explorations may not become fully evident until construction. If variations are observed at that time, it may be necessary to re-evaluate specific recommendations in this report and make appropriate changes. 3.1 Exploration Borings For this study, the three exploration borings were completed by advancing a 3.25-inch, inside-diameter, hollow-stem auger using a track-mounted drill. During the drilling process, samples were generally obtained at 2½- to 5-foot-depth intervals. The borings were continuously observed and logged by a geologist from our firm. The exploration logs presented in Appendix A are based on the field logs, drilling action, and visual observation of the samples collected. Disturbed, but representative samples were obtained by using the Standard Penetration Test (SPT) procedure in accordance with ASTM International (ASTM) D-1586. This test and sampling method consists of driving a standard 2-inch, outside-diameter, split-barrel sampler a distance of 18 inches into the soil with a 140-pound hammer free-falling a distance of 30 inches. The number of blows for each 6-inch interval is recorded, and the number of blows required to drive the sampler the final 12 inches is known as the Standard Penetration Resistance (“N”) or blow count. If a total of 50 is recorded within one 6-inch interval, the blow count is recorded as the number of blows for the corresponding number of inches of penetration. The resistance, or N-value, provides a measure of the relative density of granular soils or the relative consistency of cohesive soils; these values are plotted on the attached exploration boring logs. The samples obtained from the split-barrel sampler were classified in the field and representative portions placed in watertight containers. The samples were then transported to our laboratory for further visual classification. Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Project and Site Conditions October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 4 4.0 SUBSURFACE CONDITIONS 4.1 Regional Geology and Soils Mapping Published geologic mapping for the site and immediate vicinity were reviewed on the United States Geological Survey (USGS) National Geologic Map Database1 , and on the Washington State Department of Natural Resources (DNR) Geologic Information Portal 2. These published regional geologic maps indicate that the site is underlain at shallow depths by Vashon lodgement till. Lodgement till sediments are suitable for support of moderate to heavily loaded structures with normal preparation but are not suitable for use as an infiltration receptor for collected stormwater. Subsurface conditions observed in explorations for this study were generally consistent with the referenced published geologic mapping. Review of the Natural Resources Conservation Service (NRCS) Web Soils Survey shows that the site is mapped as Alderwood gravelly sandy loam (AgC). The survey describes the soils being formed from the weathering of glacial sediments which is also generally consistent with our on-site exploration observations. 4.2 Site Stratigraphy 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. As shown on the exploration logs, soils encountered at the site consisted of fill of variable thickness overlying native sediments interpreted as Vashon lodgement till. The following sections presents more detailed subsurface information on the sediment types encountered at the site. Fill Fill soils (those not naturally placed) were encountered in all of our recent and previous explorations in the current project area with interpreted fill thicknesses ranging between 4 to 9 feet below the existing ground surface. Fill thicknesses at the two hand-augered boring locations exceeded the depth drilled of 3 feet at each location. Figure 2 of this report includes the observed fill thickness at each of the exploration locations at the time of drilling. The fill generally consisted of loose to medium dense, moist, gray to brown, fine to medium sand with variable silt content and variable gravel content. Looser fill with organic content was encountered in exploration boring EB-6 (2003) at depths ranging between 0 and 5 feet below existing ground surface. Deleterious materials such as plywood, plastic, and metal fragments were observed in 1 https://ngmdb.usgs.gov/ngmdb/ngmdb_home.html 2 https://www.dnr.wa.gov/geologyportal Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Project and Site Conditions October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 5 explorations EB-1 (2010), EB-1 (2004), EB-6 (2003) and HB-2 (2021). Existing fill is not recommended for foundation support and may require remedial preparation below new paving. Excavated existing fill material is suitable for reuse in structural fill applications if such reuse is specifically allowed by project plans and specifications, if excessively organic and any other deleterious materials are removed, and if moisture content is adjusted to allow compaction to the specified level and to a firm and unyielding condition. Existing fill is not suitable for infiltration of stormwater. Vashon Lodgement Till In all of our exploration borings with the exception of the hand-augered borings, we observed dense to very dense, unsorted, silty fine sand with varying amounts of gravels interpreted to represent lodgement till sediments. The observed depth to the top of lodgement till sediments ranged between 4 and 9 feet (EB-2, 2004 and EB-1, 2004, respectively) below existing ground surface elevation. In each boring that lodgement till was observed it extended beyond the depths of the explorations, with the deepest observations at 35 feet (EB-1, 2010). The upper 4 to 6 feet of the lodgement till in EB-1 (2010) and EB-6 (2003) was generally weathered and less dense, oxidized, and siltier than the lower, unweathered portions of the unit seen in other explorations. 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. Consequently, these materials are dense to very dense, possess high-strength, low-compressibility characteristics, and are relatively impermeable. The lodgement till is suitable for foundation support with proper preparation. Excavated lodgement till is suitable for use in structural fill applications if allowed by project specifications and provided that the moisture content is adjusted to allow compaction to a firm and unyielding condition at the specified level. The lodgement till has a large proportion of fine-grained material making it susceptible to disturbance when wet. Lodgement till is not a suitable infiltration receptor. 4.3 Hydrology Groundwater seepage was encountered in exploration borings EB-1 (July 2010) at 30 feet, EB-1 (March 2004) at 4 feet, EB-2 (March 2004) at 1 foot, and EB-3 (March 2004) at 5 to 7 feet. It is our opinion that the groundwater in each of these borings except EB-1 (July 2010) was perched near the interface between overlying existing fill and underlying lodgement till. Perched water occurs when surface water infiltrates down through relatively permeable soils, such as existing fill or weathered lodgement till and becomes trapped or “perched” atop a comparatively low-permeability barrier, such as the unweathered lodgement till. When water becomes perched within fill or above the unweathered till, it may travel laterally and may follow flow paths related to permeable zones that may not correspond to ground surface topography. Groundwater Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Project and Site Conditions October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 6 deeper within the till such as at the location of EB-1 (July 2010) can accumulate in coarser-grained pockets or lenses and is typically discontinuous. The presence and quantity of groundwater will largely depend on the soil grain-size distribution, topography, seasonal precipitation, site use, on- and off-site land usage, and other factors. Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Geologic Hazards and Mitigations October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 7 II.GEOLOGIC HAZARDS AND MITIGATIONS We reviewed mapped geologic hazards on the King County iMap, the previously referenced DNR map, and the City of Renton GIS (https://maps.rentonwa.gov/). The reviewed maps do not identify the presence of regulated critical slopes or erosion hazard areas on or immediately adjacent to the project areas. However, the DNR map and King County iMap show that there is a section near the southeastern edge of the project site that is mapped as a potential liquefaction hazard, which is discussed further below. It appears unlikely, in our opinion, that the current project will need to address requirements related to geotechnical critical areas. 5.0 LANDSLIDE HAZARDS AND MITIGATIONS The topography of the current project areas is relatively flat to gently sloping. We reviewed topographic contours presented on Figure 2 and did not identify any slopes greater than 40 percent within or near the project areas. Based on visual reconnaissance of the site existing slopes appear to have performed well with no visual indications of unusual erosion, slope instability, or emergent groundwater seepage. Given the subsurface conditions on the site and the inclination and height of the slopes, it is our opinion that the risk of damage to the proposed improvements by landslide activity on these slopes under both static and seismic conditions is low. No detailed quantitative assessment of slope stability was completed as part of this study, and none is warranted to support the project as currently proposed, in our opinion. 6.0 SEISMIC HAZARDS AND MITIGATIONS All of Western Washington is at risk of strong seismic events resulting from movement of the tectonic plates associated with the Cascadia Subduction Zone (CSZ), where the offshore Juan de Fuca plate subducts beneath the continental North American plate. The site lies within a zone of strong potential shaking from subduction zone earthquakes associated with the CSZ. The CSZ can produce earthquakes up to magnitude 9.0, and the recurrence interval is estimated to be on the order of 500 years. Geologists infer the most recent subduction zone earthquake occurred in 1700 (Goldfinger et al., 20121). Three main types of earthquakes are typically associated with subduction zone environments: crustal, intraplate, and interplate earthquakes. Seismic records in the Puget Sound region document a distinct zone of shallow crustal seismicity (e.g., the Seattle Fault Zone). These shallow fault zones may include surficial expressions of previous seismic 1 Goldfinger, C., Nelson, C.H., Morey, A.E., Johnson, J.E., Patton, J.R., Karabanov, E., Gutierrez-Pastor, J., Eriksson, A.T., Gracia, E., Dunhill, G., Enkin, R.J, Dallimore, A., and Vallier, T., 2012, Turbidite Event History—Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone: U.S. Geological Survey Professional Paper 1661–F, 170 . Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Geologic Hazards and Mitigations October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 8 events, such as fault scarps, displaced shorelines, and shallow bedrock exposures. The shallow fault zones typically extend from the surface to depths ranging from 16 to 19 miles. A deeper zone of seismicity is associated with the subducting Juan de Fuca plate. Subduction zone seismic events produce intraplate earthquakes at depths ranging from 25 to 45 miles beneath the Puget Lowland including the 1949, 7.2-magnitude event; the 1965, 6.5-magnitude event; and the 2001, 6.8-magnitude event and interplate earthquakes at shallow depths near the Washington coast including the 1700 earthquake, which had a magnitude of approximately 9.0. 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 or lateral spreading, 3) liquefaction, 4)ground motion. The potential for each of these hazards to adversely impact the proposed project is discussed below. 6.1 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. Surficial ground rupture is possible during shallower crustal events in areas close to the Seattle Fault Zone, which is located approximately 4.5 miles to the north and is the closest mapped fault zone to the project. Due to the suspected long recurrence interval, and the distance of the site to known fault traces, the potential for surficial ground rupture to occur at the project site is considered to be low during the expected life of the proposed structures. 6.2 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 very soft to stiff, non-cohesive silt and very loose to medium dense, non-silty to silty sands with low relative densities, accompanied by a shallow water table. Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Geologic Hazards and Mitigations October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 9 There is a portion of the southeastern edge of the site that is mapped as a liquefaction hazard. The mapped hazard is outside of the proposed improvement areas. The proposed project is not expected to have high risk of damage due to liquefaction because substantial deposits of saturated loose granular sediments were not observed. A detailed liquefaction hazard analysis was not performed as part of this study, and none is warranted based on existing subsurface data, in our opinion. 6.3 Ground Motion/Seismic Site Class (2018 International Building Code) Structural design of the new building additions should follow 2018 International Building Code (IBC) standards. We recommend that the project be designed in accordance with Site Class “C” in accordance with the 2018 IBC, and the publication American Society of Civil Engineers (ASCE) 7 referenced therein, the most recent version of which is ASCE 7-16. 7.0 EROSION CONTROL Project plans should include implementation of temporary erosion controls in accordance with local standards of practice. Control methods should include limiting earthwork to seasonally drier periods, if possible, use of perimeter silt fences, stabilized construction entrances, and straw mulch in exposed areas. Removal of existing vegetation should be limited to those areas that are required to construct the project, and new landscaping and vegetation with equivalent erosion mitigation potential should be established as soon as practical after grading is complete. During construction, surface water should be collected as close as possible to the source to minimize silt entrainment that could require treatment or detention prior to discharge. Timely implementation of permanent drainage control measures should also be a part of the project plans, and will help reduce erosion and generation of silty surface water onsite. The Ecology Construction Storm Water General Permit requires weekly Temporary Erosion and Sedimentation Control (TESC) inspections, turbidity monitoring and pH monitoring for all sites 1 or more acre in size that discharge stormwater to surface waters of the state. Because we anticipate that the proposed project will not require disturbance of more than 1 acre, we anticipate that these inspection and reporting requirements will not be triggered. The following recommendations are related to general erosion potential and mitigation. 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 stormwater runoff. The site plan should include ground-cover measures, Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Geologic Hazards and Mitigations October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 10 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 unworked. During the winter months, areas that are to be left unworked 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 stormwater conveyance channels through work areas to route runoff to the approved treatment/discharge facilities. 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 (BMPs) during construction, the potential adverse impacts from erosion hazards on the project may be mitigated. Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 11 III. PRELIMINARY DESIGN RECOMMENDATIONS 8.0 INTRODUCTION Our explorations indicate that, from a geotechnical engineering standpoint, the proposed building additions are feasible provided the recommendations contained herein are properly followed. The project area is underlain 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. South Addition - the southmost addition area, near hand-auger exploration HB-2, might benefit from the use of pin piles or aggregate pier ground improvement due to the depth of existing fill and constraints imposed by adjacent existing structures. Pin piles are feasible and are discussed in general terms later in this report. We should be allowed to offer situation-specific geotechnical engineering recommendations if pin piles are selected for foundation support. Removal and replacement of existing fill is also a feasible way to support the planned south addition if the overexcavation program can be completed without encroaching on support soils for existing adjacent structures. Support soils for existing structures should be considered to be defined by a line projected down and away from all foundation elements at an angle of 1H:1V (Horizontal: Vertical). North Additions and Courtyard Infill - The north addition areas and courtyard infill appear likely to have suitable bearing soils at a depth similar to existing adjacent structures, and therefore removal and replacement of existing fill soils below the planned building additions is thought to be feasible. This report provides geotechnical engineering recommendations focused on the remove/replace approach for dealing with existing fill below planned new buildings. We are available on request to discuss other possible foundation support approaches. It is worth noting that the hand-auger boring in the courtyard infill area did not penetrate deep enough to confirm the depth to native soils. At the time this report is written the nature of the courtyard infill project is not known. If the courtyard infill will be a building addition or other substantial structure, we recommend completion of a deeper exploration in the courtyard that was beyond the scope of the current study. This report is preliminary because a project concept has not yet been prepared at the time this report is written. AESI should be allowed to review the final project plans once they have been developed to update our recommendations, as necessary. Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 12 9.0 SITE PREPARATION Erosion and surface water control should be established around the perimeter of the excavation to satisfy City of Renton requirements. Building Pad Areas - Site preparation should include removal of all existing pavement, structures, buried utilities, and any other deleterious material from below the new building additions. Existing fill should be removed to expose suitable native materials suitable for structural support. Structural fill may then be placed as needed to reach building pad grade. At the time this report is written a site development plan has not been selected. We should be allowed to review the site development plan when one is selected and discuss possible site preparation and structural support plans that are appropriate to the project plan. Paving Areas - Areas of planned paving should be prepared by stripping existing vegetation and topsoil, removing structures and utilities to be demolished, and excavating to planned paving subgrade elevation. The resulting subgrade should then be evaluated visually, compacted, and proof-rolled. Exposed soils are expected to consist of existing fill and lodgement till depending on the location and finished subgrade elevation. Areas with organic or deleterious material, or areas that yield during proof-rolling should receive additional preparation tailored to proof- rolling results and field conditions at the time of construction. 9.1 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 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. We do not anticipate the need for extensive dewatering in advance of excavations. However, the contractor should be prepared to intercept any groundwater seepage entering the excavations and route it to a suitable discharge location. Final exterior grades should promote free and positive drainage away from buildings at all times. Water must not be allowed to pond or to collect adjacent to foundations or within immediate building areas. 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, Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 13 a minimum gradient of 1 percent should be provided, unless provisions are included for collection and disposal of surface water adjacent to the structure. 9.2 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. 9.3 Proof-Rolling and Subgrade Compaction Following the recommended clearing, site stripping, planned excavation, and any overexcavation required to remove existing fill, 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. Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 14 9.4 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. 9.5 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 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. 9.6 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 can be made at a maximum slope of 1.5H:1V 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 groundwater seepage is encountered in cut slopes, or if surface Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 15 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. 9.7 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. 10.0 STRUCTURAL FILL Structural fill should be placed and compacted according to the recommendations presented in this section and requirements included in project specifications. All references to structural fill in this report refer to subgrade preparation, fill type, 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. 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 at least 95 percent of the modified Proctor maximum dry density using ASTM D-1557 as the standard. In the case of roadway and utility trench filling, the backfill should be placed and compacted in accordance with City of Renton standards. For planning purposes, we recommend the use of a well-graded sand and gravel for road and utility trench backfill. At this time we are not aware of any planned right-of-way work associated with the project. The contractor should note that AESI should evaluate any proposed fill soils prior to their use in fills. This would require that we have a sample of the material at least 3 business days in advance of filling activities 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 is not recommended during the winter months or under wet site and weather conditions. Most of the on-site soils are Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 16 moisture-sensitive and have natural moisture contents over optimum for compaction and will likely require moisture-conditioning before use as structural fill. In addition, construction equipment traversing the site when the soils are wet can cause considerable disturbance. If import soil is required, 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 and at least 30 percent retained on the No. 4 sieve. A representative from our firm should observe the subgrades 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 the owner in developing a suitable monitoring and testing frequency. 11.0 FOUNDATIONS Preliminary recommendations are presented for conventional shallow foundations supported directly by dense native soils, or by new structural fill overlying suitable native soils. Preliminary pin pile recommendations are provided for areas where existing fill soils cannot be removed and replaced due to the presence of existing adjacent structures. 11.1 Shallow Foundations We expect the depth from existing grade to bearing soil in building addition areas will vary, however we anticipate that existing fill does not extend deeper than foundation level for existing buildings adjacent to the additions. The existing on-site fill was thickest (about 9 feet in depth) in the southwestern portion of the site, in the vicinity of EB-1 (2004). Where present, existing fill should be removed below the building pad, exposing medium dense to very dense native 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. Higher foundation soil bearing pressures may be suitable if new footings will be supported entirely on dense to very dense native soils. An increase of one-third may be used for short-term wind or seismic loading. Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 17 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 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 14.0 of this report. 11.2 Pin Piles Pin piles should be installed by a local contractor with demonstrated expertise in pin pile installations. We recommend preliminary planning assuming pin pile axial compressive capacity of 12,000 pounds on a 3-inch-diameter pile. Piles with smaller and larger capacities are possible, if needed. We should be allowed to offer situation-specific pin pile recommendations if project plans are prepared that include the use of pin piles. In general, pin piles are installed with an air or hydraulic impact hammer until the specified refusal criteria are met. If multiple pipe sections are required, the pipes should be joined with an extension pin inside the pipe, and/or a sleeve on the outside. If uplift loads are expected to be placed on the piles at any time, the connections should also be securely welded to prevent pipe separation at joints. Although vertical pin piles can provide small uplift and lateral capacities, we recommend that these contributions be neglected in designing the new foundation system. The structural engineer should provide pile spacing, locations, splicing details, foundation connection details, and any other structural design recommendations that are needed. Pin piles are driven until specific refusal criteria are achieved. Pile lengths are difficult to estimate in advance. At this site in addition to achieving the required driving resistance, piles are required to reach a minimum depth. We recommend that piles fully penetrate existing fill soils to bear on Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 18 lodgement till, and that all piles achieve a minimum penetration depth of 5 feet below the level of any adjacent temporary or permanent excavations. It should be noted that the subsurface conditions that will be encountered during pile driving could include construction waste and compacted fill soils. We recommend that we be allowed to observe the installation of pin piles. We would observe materials, equipment, and procedures, and confirm refusal for each pile. The purpose of our observations is to confirm that the conditions observed in our explorations and assumed in preparation of our recommendations are consistent with those encountered at the time of construction, and to confirm that the materials, procedures, and refusal criteria are consistent with those we assumed while formulating our recommendations contained in this report. We recommend that we be allowed to agree on mutually acceptable driving resistance criteria with the pile contractor selected for the project, and that the agreed-on driving criteria be verified by at least one load test for each pile type. 12.0 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. If any of the building addition areas will use pin pile support for foundations, floors could be supported on pin piles. Alternatively, floor slabs could be supported by a new layer of compacted structural fill at least 2 feet thick, underlain by existing fill soils. If this alternative is selected, the existing fill soil should be prepared in accordance with the “Site Preparation” section of this report, and new fill should be placed as recommended in the “Structural Fill” section. If existing fill is left in place below new floors and pin piles are not used for floor support, there is some potential for larger than normal post-construction settlement. The settlement risk is offset by substantial cost savings at the time of construction. We are available on request to discuss floor support over existing fill. Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 19 13.0 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 to resist active earth pressure represented by 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 2018 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 midpoint 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 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. 13.1 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: Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 20 •Passive equivalent fluid = 250 pcf •Coefficient of friction = 0.35 14.0 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 drain rock. 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. 15.0 PAVEMENT AND SIDEWALK RECOMMENDATIONS The pavement sections included in this report section are for driveway and parking areas onsite, and are not applicable to right-of-way improvements. At this time, we are not aware of any planned right-of-way improvements; however, if any new paving of public streets is required, we should be allowed to offer situation-specific recommendations. Pavement and sidewalk areas should be prepared in accordance with the “Site Preparation” section of this report. Soft or yielding areas should be overexcavated to provide a suitable subgrade and backfilled with structural fill. New paving may include areas subject only to light traffic loads from passenger vehicles driving and parking, and may also include areas subject to heavier loading from vehicles that may include buses, fire trucks, food service trucks, and garbage trucks. In light traffic areas, we recommend a pavement section consisting of 3 inches of hot-mix asphalt (HMA) underlain by 4 inches of crushed surfacing base course. In heavy traffic areas, we recommend a minimum pavement section consisting of 4 inches of HMA underlain by 2 inches of crushed surfacing top course and 4 inches of crushed surfacing base course. The crushed rock courses must be compacted to 95 percent of the maximum density, as determined by ASTM D-1557. All paving materials should meet gradation criteria contained in the current Washington State Department of Transportation (WSDOT) Standard Specifications. Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 21 Depending on construction staging and desired performance, the crushed base course material may be substituted with asphalt treated base (ATB) beneath the final asphalt surfacing, if desired. The substitution of ATB should be as follows: 4 inches of crushed rock can be substituted with 3 inches of ATB, and 6 inches of crushed rock may be substituted with 4 inches of ATB. ATB should be placed over a native or structural fill subgrade compacted to a minimum of 95 percent relative density, and a 1½- to 2-inch thickness of crushed rock to act as a working surface. If ATB is used for construction access and staging areas, some rutting and disturbance of the ATB surface should be expected to result from construction traffic. The contractor should remove any ATB areas damaged by construction equipment and replace them with properly compacted ATB prior to final surfacing. 16.0 INFILTRATION FEASIBILITY The on-site soils consist of fill and dense silty glacial till. The existing fill is not permitted to serve as an infiltration receptor, and lodgement till is not a suitable infiltration receptor due to low permeability. Stormwater infiltration is not recommended. 17.0 PROJECT DESIGN AND CONSTRUCTION MONITORING We recommend that AESI perform a geotechnical review of the plans prior to final design completion. In this way, we can confirm that our recommendations have been correctly interpreted and implemented in the design. The City of Renton may require a plan review by the geotechnical engineer as a condition of permitting. We recommend that AESI be retained to provide geotechnical special inspections during construction, and preparation of a final summary letter when construction is complete. The City of Renton may require such geotechnical special inspections. The integrity of the earthwork and foundations depends on proper site preparation and construction procedures. In addition, engineering decisions may have to be made in the field in the event that variations in subsurface conditions become apparent. Subsurface Exploration, Geologic Hazard, Lindbergh High School Additions and Preliminary Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations October 1, 2021 ASSOCIATED EARTH SCIENCES, INC. ART/ld - 20000669E005-003 Page 22 We have enjoyed working with you on this study and are confident these recommendations will aid in the successful completion of your project. If you should have any questions or require further assistance, please do not hesitate to call. Sincerely, ASSOCIATED EARTH SCIENCES, INC. Kirkland, Washington ______________________________ Aaron R. Turnley, G.I.T. Senior Staff Geologist ______________________________ Bruce W. Guenzler, L.E.G. Matthew A. Miller. P.E. Senior Associate Geologist Principal Engineer Attachments: Figure 1. Vicinity Map Figure 2. Existing Site and Exploration Plan Appendix A. Exploration Logs KING COUNTY KING COUNTY RENTON PROJ NO. NOTE: BLACK AND WHITE REPRODUCTION OF THIS COLOR ORIGINAL MAY REDUCE ITS EFFECTIVENESS AND LEAD TO INCORRECT INTERPRETATION DATE:FIGURE: ±\\kirkfile2\GIS\GIS_Projects\aY00post0716\000669 Lindbergh HS\aprx\20000669E005 F1 VM_Lindbergh.aprx | 20000669E005 F1 VM_Lindbergh | 9/13/2021 2:53 PMDATA SOURCES / REFERENCES: USGS: 7.5' SERIES TOPOGRAPHIC MAPS, ESRI/I-CUBED/NGS 2013 KING CO: STREETS, CITY LIMITS, PARCELS, PARKS 3/20 LOCATIONS AND DISTANCES SHOWN ARE APPROXIMATE 0 2000 Feet VICINITY MAP LINDBERGH HS IMPROVEMENTS RENTON, WASHINGTON 20000669E005 9/21 1 King County ¥ ¥ ¥ ¬« ¬«169 !( SITE King County !( !( !( !( !( !( !( !( !( !(4204104304 0 0 410400430 42 0 39042 0400 King CountyRentonHB-1 HB-2 EB-6, 5.5FT EB-1, 8FT EB-1, 4FT EB-2, 7FT EB-3, 6FT EB-1, 9FT EB-2, 4FT EB-3, 5FT EagleView Technologies, Inc. ± BLACK AND WHITE REPRODUCTION OF THIS COLOR ORIGINAL MAY REDUCE ITS EFFECTIVENESS AND LEAD TO INCORRECT INTERPRETATION \\kirkfile2\gis\GIS_Projects\aY00post0716\000669 Lindbergh HS\aprx\20000669E005 F2 ES_Lindbergh.aprx | 20000669E005 F2 ES_Lindbergh | 9/27/2021 9:23 AMPROJ NO.DATE:FIGURE: 0 80 FEET NOTE: HISTORICAL EXPLORATIONS OUTSIDE THE CURRENT WORK AREA ARE NOT SHOWN ON THE FIGURE AND NOT INCLUDED IN THE APPENDIX DATA SOURCES / REFERENCES: PSLC: KING COUNTY 2016, GRID CELL SIZE IS 3'. DELIVERY 1 FLOWN 2/24/16 - 3/28/16 CONTOURS FROM LIDAR KING CO: STREETS, PARCELS, 3/20 AERIAL PICTOMETRY INT. 2019 LOCATIONS AND DISTANCES SHOWN ARE APPROXIMATE 20000669E005 9/21 2 EXISTING SITE AND EXPLORATION PLAN LINDBERGH HS ADDITIONS RENTON, WASHINGTON LEGEND SITE !(EXPLORATION BORING, DEPTH OF FILL (2021) !(HAND BORING (2021) !(EXPLORATION BORING, DEPTH OF FILL (2010) !(EXPLORATION BORING, DEPTH OF FILL (2004) !(EXPLORATION BORING, DEPTH OF FILL (2003) CITY BOUNDARY PARCEL CONTOUR 10 FT CONTOUR 2 FT APPENDIX A Exploration Logs Topsoil - 4 to 6 inches Fill Moist, grayish brown, silty, fine SAND, some medium to coarse sand, trace gravel; unsorted (SM). Vashon Lodgement Till Moist, grayish brown, silty, fine SAND, some medium to coarse sand, some gravel; unsorted (SM). As above. Driller reports hard drilling. Moist, grayish brown, silty, fine SAND, some medium to coarse sand, some broken gravel; diamict (SM). Moist, grayish brown, silty, fine SAND, some medium to coarse sand, trace gravel; diamict (SM). Driller reports hard drilling. Moist to very moist, grayish brown, silty, fine SAND, some medium to coarse sand; contains broken gravel; occasional sand lens; diamict (SM). S-1 S-2 S-3 S-4 S-5 S-6 20 26 27 12 20 27 20 35 50/5" 26 50/3" 50/6" 35 43 50/5" Bottom of exploration boring at 21.4 feet No groundwater encountered. Ground Surface Elevation (ft) Grab SampleSymbol 3.25 inch ID, +/- 8 inch OD 40 Datum Hammer Weight/Drop Sampler Type (ST): ~425 5 10 15 20 EB-1 Ring Sample No RecoveryGraphic 10 Other TestsHole Diameter (in) DESCRIPTION Driller/Equipment Blows/6"JHS ART2" OD Split Spoon Sampler (SPT) 3" OD Split Spoon Sampler (D & M)Water LevelProject Name Water Level ()Approved by: 30 Blows/Foot SamplesDepth (ft)S T Exploration Number 20000669E005 9/11/21,9/11/21 Logged by: Shelby Tube Sample 140# / 30 Advanced Drill Technologies / D-50 Track Mount Exploration Boring Water Level at time of drilling (ATD) Lindbergh High School Additions M - Moisture Project Number 20 Renton, WA Date Start/Finish CompletionLocation Sheet 1 of 1 NAVD 88 WellAESIBOR 20000669E005.GPJ September 27, 202153 4747 5050/5" 5050/3" 5050/6" 5050/5" Topsoil - 4 to 6 inches Fill Moist, brownish gray with faint iron oxide staining, silty, fine SAND, some medium to coarse sand, some gravel; unsorted (SM) Moist, grayish brown, silty, fine SAND, some medium to coarse sand, trace gravel; unsorted (SM). Drilling action changes at 7 feet. Vashon Lodgement Till Moist, grayish brown, silty, fine SAND, some medium to coarse sand, trace gravel; diamict (SM). Moist, grayish brown, silty, fine SAND, trace medium to coarse sand; diamict (SM). Moist, grayish brown, silty, fine SAND, some medium to coarse sand, trace gravel; diamict (SM). No recovery due to gravel. S-1 S-2 S-3 S-4 S-5 S-6 6 4 3 3 3 8 30 50/6" 20 50/6" 50/6" 50/2" Bottom of exploration boring at 20.2 feet No groundwater encountered. Ground Surface Elevation (ft) Grab SampleSymbol 3.25 inch ID, +/- 8 inch OD 40 Datum Hammer Weight/Drop Sampler Type (ST): ~427.5 5 10 15 20 EB-2 Ring Sample No RecoveryGraphic 10 Other TestsHole Diameter (in) DESCRIPTION Driller/Equipment Blows/6"JHS ART2" OD Split Spoon Sampler (SPT) 3" OD Split Spoon Sampler (D & M)Water LevelProject Name Water Level ()Approved by: 30 Blows/Foot SamplesDepth (ft)S T Exploration Number 20000669E005 9/11/21,9/11/21 Logged by: Shelby Tube Sample 140# / 30 Advanced Drill Technologies / D-50 Track Mount Exploration Boring Water Level at time of drilling (ATD) Lindbergh High School Additions M - Moisture Project Number 20 Renton, WA Date Start/Finish CompletionLocation Sheet 1 of 1 NAVD 88 WellAESIBOR 20000669E005.GPJ September 27, 202177 1111 5050/6" 5050/6" 5050/6" 5050/2" Topsoil - 4 to 6 inches Fill Moist, brownish gray with oxidation, silty, fine SAND, some medium to coarse sand; unsorted (SM). Moist, grayish brown, silty, fine SAND, some medium to coarse sand, trace gravel; fill lifts; unsorted (SM). Vashon Lodgement Till Drill change at 6 feet. Moist, grayish brown, silty, fine SAND, some medium to coarse sand, trace broken gravel; diamict (SM). As above. As above; faint iron oxide staining. As above. S-1 S-2 S-3 S-4 S-5 S-6 3 2 5 15 20 28 14 50/6" 19 50/6" 50/4" 50/5" Bottom of exploration boring at 20.4 feet No groundwater encountered. Ground Surface Elevation (ft) Grab SampleSymbol 3.25 inch ID, +/- 8 inch OD 40 Datum Hammer Weight/Drop Sampler Type (ST): ~422.5 5 10 15 20 EB-3 Ring Sample No RecoveryGraphic 10 Other TestsHole Diameter (in) DESCRIPTION Driller/Equipment Blows/6"JHS ART2" OD Split Spoon Sampler (SPT) 3" OD Split Spoon Sampler (D & M)Water LevelProject Name Water Level ()Approved by: 30 Blows/Foot SamplesDepth (ft)S T Exploration Number 20000669E005 9/11/21,9/11/21 Logged by: Shelby Tube Sample 140# / 30 Advanced Drill Technologies / D-50 Track Mount Exploration Boring Water Level at time of drilling (ATD) Lindbergh High School Additions M - Moisture Project Number 20 Renton, WA Date Start/Finish CompletionLocation Sheet 1 of 1 NAVD 88 WellAESIBOR 20000669E005.GPJ September 27, 202177 4848 5050/6" 5050/6" 5050/4" 5050/5" Landscaping Mulch - 2 to 3 inches Fill Loose, moist, light brown, silty, fine SAND, some gravel, some medium to coarse sand; unsorted (SM). As above. S-1 S-2 Bottom of exploration boring at 3 feet No groundwater encountered. Ground Surface Elevation (ft) Grab SampleSymbol 3.25 inch ID, +/- 8 inch OD 40 Datum Hammer Weight/Drop Sampler Type (ST): ~425 5 HB-1 Ring Sample No RecoveryGraphic 10 Other TestsHole Diameter (in) DESCRIPTION Driller/Equipment Blows/6"JHS ART2" OD Split Spoon Sampler (SPT) 3" OD Split Spoon Sampler (D & M)Water LevelProject Name Water Level ()Approved by: 30 Blows/Foot SamplesDepth (ft)S T Exploration Number 20000669E005 9/11/21,9/11/21 Logged by: Shelby Tube Sample 140# / 30 Hand Auger Exploration Boring Water Level at time of drilling (ATD) Lindbergh High School Additions M - Moisture Project Number 20 Renton, WA Date Start/Finish CompletionLocation Sheet 1 of 1 NAVD 88 WellAESIBOR 20000669E005.GPJ September 27, 2021 Landscaping Mulch - 4 to 6 inches Fill Loose, moist, light brown, fine SAND, some silt, some gravel, some medium to coarse sand; unsorted (SP-SM). Loose, moist, light brown, fine SAND, some silt, some gravel, some medium to coarse sand; metal debris; unsorted (SP-SM). S-1 S-2 Bottom of exploration boring at 2.5 feet No groundwater encountered. Ground Surface Elevation (ft) Grab SampleSymbol 3.25 inch ID, +/- 8 inch OD 40 Datum Hammer Weight/Drop Sampler Type (ST): ~397.5 5 HB-2 Ring Sample No RecoveryGraphic 10 Other TestsHole Diameter (in) DESCRIPTION Driller/Equipment Blows/6"JHS ART2" OD Split Spoon Sampler (SPT) 3" OD Split Spoon Sampler (D & M)Water LevelProject Name Water Level ()Approved by: 30 Blows/Foot SamplesDepth (ft)S T Exploration Number 20000669E005 9/11/21,9/11/21 Logged by: Shelby Tube Sample 140# / 30 Hand Auger Exploration Boring Water Level at time of drilling (ATD) Lindbergh High School Additions M - Moisture Project Number 20 Renton, WA Date Start/Finish CompletionLocation Sheet 1 of 1 NAVD 88 WellAESIBOR 20000669E005.GPJ September 27, 2021