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HomeMy WebLinkAboutRS_Geotechnical_Report_180308_v1associated earth sciences incorporated Associated Earth Sciences, Inc. 911 5th Avenue Kirkland, WA 98033 P (425) 827 7701 F (425) 827 5424 Subsurface Exploration, Geologic Hazard, and Geotechnical Engineering Report DUVALL AVENUE NE DENTAL BUILDING Renton, Washington Prepared For: Duvall and 17th, LLC Project No. 170437E001 March 8, 2018 SUBSURFACE EXPLORATION, GEOLOGIC HAZARD, AND GEOTECHNICAL ENGINEERING REPORT DUVALL AVENUE NE DENTAL BUILDING Renton, Washington Prepared for: Duvall and 17th, LLC 4111 East Madison Street, #41111 Seattle, Washington 98112 Prepared by: Associated Earth Sciences, Inc. 911 5th Avenue Kirkland, Washington 98033 425-827-7701 Fax: 425-827-5424 March 8, 2018 Project No. 170437E001 Subsurface Exploration, Geologic Hazard, Duvall Avenue NE Dental Building and Geotechnical Engineering Report Renton, Washington Project and Site Conditions March 8, 2018 ASSOCIATED EARTH SCIENCES, INC. TJP/ms - 170437E001-3 - Projects\20170437\KE\WP Page 1 I. PROJECT AND SITE CONDITIONS 1.0 INTRODUCTION This report presents the results of our subsurface exploration, geologic hazard, and geotechnical engineering study for the subject project. Our recommendations are preliminary in that construction details have not been finalized at the time of this report. The location of the subject site is shown on the “Vicinity Map,” Figure 1. The approximate locations of the explorations accomplished for this study are presented on the “Site and Exploration Plan,” Figure 2. In the event that any changes in the nature or design of the proposed layout is planned, the conclusions and recommendations contained in this report should be reviewed and modified, or verified, as necessary. 1.1 Purpose and Scope The purpose of this study was to provide subsurface data to be used in the design and development of the subject project. Our study included reviewing available geologic literature, drilling three exploration borings, and performing geologic studies to assess the type, thickness, distribution, and physical properties of the subsurface sediments and shallow groundwater conditions. Geotechnical engineering studies were also conducted to assess the type of suitable foundation, allowable foundation soil bearing pressures, anticipated settlements, retaining wall lateral pressures, floor support recommendations, and drainage considerations. This report summarizes our current fieldwork and offers preliminary development recommendations based on our present understanding of the project. 1.2 Authorization Authorization to proceed with this study was granted by Mr. Brian Bruce of Duvall and 17th, LLC. This report has been prepared for the exclusive use of Duvall and 17th, LLC, 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. Subsurface Exploration, Geologic Hazard, Duvall Avenue NE Dental Building and Geotechnical Engineering Report Renton, Washington Project and Site Conditions March 8, 2018 ASSOCIATED EARTH SCIENCES, INC. TJP/ms - 170437E001-3 - Projects\20170437\KE\WP Page 2 2.0 SITE AND PROJECT DESCRIPTION The subject site consists of an irregularly shaped parcel of approximately 8,100 square feet located at 1625 Duvall Avenue NE in Renton, Washington. The site is currently occupied by a residential structure reported to have been built in 1943. The residential structure has been converted to an office and is currently occupied by a painting contractor. The existing home/office building includes a basement that daylight to the east. Review of King County records indicates that the daylight basement level extends under most of the structure, but does not extend below the former front porch, located on the west side. The former front porch has been enclosed and now consists of living space. The parcel is bordered to the north by NE 17th Street, to the east by Duvall Avenue NE, to the south by an oil-change-and-lube business, and to the west by a single-family residence. The topography of the site is relatively flat except along its north and east margins where it slopes down toward the adjacent streets. A modular block retaining wall with a maximum height of approximately 6 feet is located along the east property margin. The modular block wall extends around the northeast corner of the property where it abuts an ivy-covered rockery that extends along most of the north side of the site. The rockery has a maximum height of approximately 6 feet. Above the tops of the modular block walls and rockery, the topography slopes up toward the interior of the parcel approximately 3 to 5 feet at a visually estimated inclination of approximately 2H:1V (Horizontal:Vertical). Retaining walls are also located along the south property margin, which provide grade separation between the subject site and the lower lying property to the south. The retaining walls along the south property margin consist of two ecology block walls with maximum heights of approximately 8 feet that are located along the eastern and western ends of the south property line. In between the ecology block walls is a concrete cast-in-place wall with a maximum height of approximately 11 to 12 feet. Along most of the south property line, the topography adjacent to the tops of the walls is relatively flat, but the topography does slope down several feet toward the ecology block wall near the southwest corner of the property. The total relief across the subject site is approximately 7 feet. Access into the property is provided by a gravel driveway that enters the north side of the parcel off of NE 17th Street near its northwest corner. Our understanding of the development plans for the project are based on discussions with Mr. Jerrold Bailet and review of architectural plans prepared by Broderick Architects. It is our understanding that current plans include demolition of the existing home/office building and construction of a two-story, 3,000-square-foot dental building with a below-grade parking garage. The finished floor elevation of the parking garage will be located up to approximately 7 feet below the existing site grades. Maximum excavation depth for construction of the parking garage will be approximately 8 feet. Subsurface Exploration, Geologic Hazard, Duvall Avenue NE Dental Building and Geotechnical Engineering Report Renton, Washington Project and Site Conditions March 8, 2018 ASSOCIATED EARTH SCIENCES, INC. TJP/ms - 170437E001-3 - Projects\20170437\KE\WP Page 3 3.0 SUBSURFACE EXPLORATION Our field study included advancing three exploration borings to gain subsurface information about the site. The various types of sediments, as well as the depths where characteristics of the 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 in the field. Our explorations were approximately located in the field relative to known site features shown on the site plan. The approximate locations of the exploration borings are shown on the “Site and Exploration Plan,” Figure 2. The conclusions and recommendations presented in this report are based, in part, on the exploration borings completed for this study. The number, locations, and depths of the explorations were completed within site and budgetary constraints. Because of the nature of exploratory work below ground, interpolation of subsurface conditions between field explorations is necessary. It should be noted that subsurface conditions differing from those depicted on the logs may be present at the site 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 The exploration borings were completed using a small, track-mounted, hollow-stem auger drill rig. During the drilling process, samples were generally obtained at 2.5- to 5-foot-depth intervals. The exploration borings were continuously observed and logged by an engineering geologist from our firm. The exploration logs presented in Appendix A are based on the field logs, drilling action, and review of the samples collected. Disturbed but representative samples were obtained by using the Standard Penetration Test (SPT) procedure in accordance with American Society for Testing and Materials (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 exploration log in Appendix A. Subsurface Exploration, Geologic Hazard, Duvall Avenue NE Dental Building and Geotechnical Engineering Report Renton, Washington Project and Site Conditions March 8, 2018 ASSOCIATED EARTH SCIENCES, INC. TJP/ms - 170437E001-3 - Projects\20170437\KE\WP Page 4 The samples obtained from the split-barrel sampler were classified in the field and placed in watertight containers. The samples were then transported to our laboratory for further visual classification. 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 applicable geologic literature. The sediments encountered in our explorations generally consisted of medium dense to very dense, granular glacial sediments. The natural sediments were overlain in places by fill. The following section presents more detailed subsurface information organized from the shallowest (youngest) to the deepest (oldest) sediment types. 4.1 Stratigraphy Sod/Topsoil A surficial, sod/organic topsoil horizon was encountered at the location of exploration boring EB-1, located north of the existing building. The sod/topsoil horizon was approximately 3 to 6 inches thick and is not considered suitable for foundation support or for use in a structural fill. Fill Sediments encountered below the surficial sod/topsoil layer at the location of boring EB-1 and directly below the ground surface at the location of boring EB-2 were interpreted to consist of fill. The fill generally consisted of brown, very silty, gravelly sand. At the location of boring EB-2, the fill also contained some washed gravel from approximately 1.5 to 2 feet. At the locations of borings EB-1 and EB-2, the fill extended to a depth of approximately 2 feet. Greater thicknesses of fill are anticipated to be present adjacent to the basement walls of the existing structure. The existing fill is not considered suitable for foundation support. Vashon Lodgement Till Natural sediments encountered below the fill in exploration borings EB-1 and EB-2, and directly below the crushed gravel surfacing at the location of boring EB-3, generally consisted of dense to very dense, non-stratified, grayish tan to tannish gray, very silty, gravelly sand. One exception was the location of boring EB-1 where these sediments were medium dense above a depth of approximately 5 feet. We interpret these sediments to be representative of Vashon lodgement till. The Vashon lodgement till was deposited directly from basal, debris-laden glacial ice during the Vashon Stade of the Fraser Glaciation, Subsurface Exploration, Geologic Hazard, Duvall Avenue NE Dental Building and Geotechnical Engineering Report Renton, Washington Project and Site Conditions March 8, 2018 ASSOCIATED EARTH SCIENCES, INC. TJP/ms - 170437E001-3 - Projects\20170437\KE\WP Page 5 approximately 12,500 to 15,000 years ago. The high relative density characteristic of the Vashon lodgement till is due to its consolidation by the massive weight of the glacial ice that overran these sediments subsequent to their deposition. The reduced density of the upper portion of the till observed in boring EB-1 is interpreted to be due to weathering. The Vashon lodgement till extended to a depth of approximately 19.5 feet at the location of boring EB-3, and below the maximum depths explored of approximately 10 feet and 21.5 feet at the locations of borings EB-1 and EB-2, respectively. Vashon Advance Outwash Natural sediments encountered below the till in boring EB-3 generally consisted of very dense, stratified fine to medium sand with some gravel, trace silt, and lenses of silty fine sand. We interpret these sediments to be representative of Vashon advance outwash. The Vashon advance outwash was deposited by meltwater streams that emanated from the advancing glacial ice during Vashon time. The high relative density characteristic of the advance outwash is due to its consolidation by the massive weight of the glacial ice that overran these sediments subsequent to their deposition. At the location of boring EB-3, the Vashon advance outwash extended beyond the maximum depths explored of approximately 21 feet. 4.2 Geologic Map Review Review of the regional geologic map titled Geologic Map of Surficial Deposits in the Seattle 30’ by 60’ Quadrangle, Washington by Yount, Minard, and Dembroff (U.S. Geological Survey [USGS] OF-93-233, dated 1993) indicates that the area of the subject site is located near the contact between the Vashon lodgement till and the underlying Vashon advance outwash. Our interpretation of the sediments encountered in our explorations is in general agreement with the geologic map. 4.3 Hydrology Groundwater seepage was encountered in boring EB-3 below the contact between the lodgement till and the underlying advance outwash below a depth of approximately 19.5 feet. Because the advance outwash appeared completely saturated, it is possible that the groundwater within the outwash may be confined by the overlying, low-permeability lodgement till. Consequently, the static head within the advance outwash may be above the elevation of the lodgement till/advance outwash contact. It should be noted that the presence or level of groundwater seepage below the site may vary in response to such factors as changes in season, precipitation, and site use. Subsurface Exploration, Geologic Hazard, Duvall Avenue NE Dental Building and Geotechnical Engineering Report Renton, Washington Geologic Hazards and Mitigations March 8, 2018 ASSOCIATED EARTH SCIENCES, INC. TJP/ms - 170437E001-3 - Projects\20170437\KE\WP Page 6 II. GEOLOGIC HAZARDS AND MITIGATIONS The following discussion of potential geologic hazards is based on the geologic, slope, and shallow groundwater conditions, as observed and discussed herein. 5.0 SEISMIC HAZARDS AND MITIGATION Earthquakes occur in the Puget Lowland relatively frequently. The vast 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 1965, 6.5-magnitude event; and the 2001, 6.8-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- to 40-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 The nearest known fault trace to the project site is the Seattle Fault Zone (SFZ) located approximately 1 to 2 miles to the north. Studies of the SFZ by the USGS have provided evidence of surficial ground rupture along a northern splay of the Seattle Fault. According to the USGS studies, the latest movement of this fault was about 1,100 years ago when about 20 feet of surficial displacement took place. This displacement can presently be seen in the form of raised, wave-cut beach terraces along Alki Point in West Seattle and Restoration Point at the south end of Bainbridge Island. The recurrence interval of movement along this fault system is still unknown, although it is hypothesized to be in excess of several thousand years. Based on the distance between the site and the SFZ, and the suspected long recurrence interval, the risk of damage to the subject project by surficial ground rupture along the SFZ is considered to be low during the expected life of the proposed structure and no mitigations are recommended. Subsurface Exploration, Geologic Hazard, Duvall Avenue NE Dental Building and Geotechnical Engineering Report Renton, Washington Geologic Hazards and Mitigations March 8, 2018 ASSOCIATED EARTH SCIENCES, INC. TJP/ms - 170437E001-3 - Projects\20170437\KE\WP Page 7 5.2 Seismically Induced Landslides Given the subsurface conditions present, it is our opinion that the risk of landsliding on the site under either static or seismic conditions is low due to the relatively low height and moderate inclination of the post-development site topography. 5.3 Liquefaction The encountered stratigraphy has a low potential for liquefaction due to its dense state and the general lack of adverse groundwater conditions. No mitigation of liquefaction hazards is warranted. 5.4 Ground Motion It is our opinion that any earthquake damage to the proposed structure, when founded on suitable bearing strata in accordance with the recommendations contained herein, will likely be caused by the intensity and acceleration associated with the event. Structural design of the building should follow 2015 International Building Code (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 MITIGATION The natural sediments underlying the subject site contain substantial quantities of silt and fine sand and will be highly sensitive to disturbance when wet. We recommend the following best management practices (BMPs) to mitigate erosion hazards and potential for off-site sediment transport: 1. Construction activity should be scheduled or phased as much as possible to avoid earthwork activity during the wet season. 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 and staging areas. The contractor should be prepared to implement and maintain the required measures to reduce the amount of exposed ground. 3. Temporary erosion and sedimentation control (TESC) elements and perimeter flow control should be established prior to the start of grading. Subsurface Exploration, Geologic Hazard, Duvall Avenue NE Dental Building and Geotechnical Engineering Report Renton, Washington Geologic Hazards and Mitigations March 8, 2018 ASSOCIATED EARTH SCIENCES, INC. TJP/ms - 170437E001-3 - Projects\20170437\KE\WP Page 8 4. During the wetter months of the year, or when significant storm events are predicted during the summer months, the work area should be stabilized so that if showers occur, it can receive the rainfall without excessive erosion or sediment transport. The required measures for an area to be suitably protected will depend on the time of year and the duration that 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. 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. 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 stockpiles with plastic sheeting, or the use of silt fences around stockpile perimeters. It is our opinion that with the proper implementation of the TESC plans and by field-adjusting appropriate erosion mitigation (BMPs) throughout construction, the potential adverse impacts from erosion hazards on the project may be mitigated. Subsurface Exploration, Geologic Hazard, Duvall Avenue NE Dental Building and Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations March 8, 2018 ASSOCIATED EARTH SCIENCES, INC. TJP/ms - 170437E001-3 - Projects\20170437\KE\WP Page 9 III. PRELIMINARY DESIGN RECOMMENDATIONS 7.0 INTRODUCTION Our explorations indicate that, from a geotechnical standpoint, the parcel is suitable for the proposed development provided the recommendations contained herein are properly followed. The foundation bearing stratum is relatively shallow, and conventional spread footing foundations may be utilized. 8.0 SITE PREPARATION 8.1 Clearing and Stripping All topsoil, vegetation, and any other deleterious materials should be stripped from the proposed building and pavement areas. Areas where loose surficial soils exist due to grubbing operations should be considered as fill to the depth of disturbance and treated as subsequently recommended for structural fill placement. Any existing fill soils located below the building area should be removed to expose the underlying, medium dense to very dense natural lodgement till sediments. These sediments were encountered in our explorations at depths of approximately 1 to 2.5 feet. 8.2 Temporary and Permanent Cut Slopes In our opinion, stable construction slopes should be the responsibility of the contractor and should be determined during construction based on the local conditions encountered at that time. For planning purposes, we anticipate that temporary, unsupported cut slopes within the medium dense fill or weathered till horizon can be made at a maximum slope of 1.5H:1V. Temporary, unsupported cut slopes within the dense to very dense, unweathered till can be planned at a maximum slope of 1H:1V. Flatter inclinations may be recommended in areas of seepage. As is typical with earthwork operations, some sloughing and raveling may occur, and cut slopes may have to be adjusted in the field. In addition, WISHA/OSHA regulations should be followed at all times. Permanent cut slopes should not exceed an inclination of 2H:1V. Subsurface Exploration, Geologic Hazard, Duvall Avenue NE Dental Building and Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations March 8, 2018 ASSOCIATED EARTH SCIENCES, INC. TJP/ms - 170437E001-3 - Projects\20170437\KE\WP Page 10 8.3 Site Disturbance The lodgement till sediments underlying the site contain a high percentage of fine-grained material. These sediments are considered to be highly moisture-sensitive and subject to disturbance when wet. The contractor must use care during site preparation and excavation operations so that the underlying soils are not softened. If disturbance occurs, the softened soils should be removed and the area brought to grade with structural fill. Consideration should be given to protecting access and staging areas with an appropriate section of crushed rock or asphalt treated base (ATB). If crushed rock is considered for the access and staging areas, it should be underlain by engineering stabilization fabric (such as TenCate Mirafi 500X or approved equivalent) to reduce the potential of fine-grained materials migrating through the rock during wet weather and making the area muddy. The fabric will also aid in supporting construction equipment, thus reducing the amount of crushed rock required. We recommend that at least 10 inches of rock be placed over the fabric. Crushed rock used for access and staging areas should be of at least 2-inch size. 9.0 STRUCTURAL FILL Placement of structural fill may be necessary to establish desired grades in some areas or to backfill utility trenches or around foundations. 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. 9.1 Subgrade Compaction After overexcavation/stripping has been performed to the satisfaction of the geotechnical engineer/engineering geologist, the upper 12 inches of exposed ground should be recompacted to a firm and unyielding condition. If the subgrade contains too much moisture, suitable recompaction may be difficult or impossible to attain 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. Subsurface Exploration, Geologic Hazard, Duvall Avenue NE Dental Building and Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations March 8, 2018 ASSOCIATED EARTH SCIENCES, INC. TJP/ms - 170437E001-3 - Projects\20170437\KE\WP Page 11 9.2 Structural Fill Compaction 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. Utility trench backfill should be placed and compacted in accordance with applicable municipal codes and standards. The top of the compacted fill should extend horizontally a minimum distance of 3 feet beyond footings or pavement edges before sloping down at an angle no steeper than 2H:1V. Fill slopes should either be overbuilt and trimmed back to final grade or surface-compacted to the specified density. 9.3 Moisture-Sensitive Fill Soils in which the amount of fine-grained material (smaller than 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 on-site lodgement till sediments or existing fill are suitable for use as structural fill provided they are free of roots or other deleterious materials and have a moisture content suitable for achieving the specified compaction. These sediments contain a high percentage of fine-grained material and are considered highly moisture-sensitive. If the moisture contents of these sediments at the time of construction are above the optimum for achieving suitable compaction, they should be moisture-conditioned prior to their use as structural fill. Such moisture-conditioning could consist of spreading out and aerating the soil during periods of warm, dry weather. Construction equipment traversing the site when the silty natural sediments are very moist or wet can cause considerable disturbance. If fill is placed during wet weather or if proper compaction cannot be attained, 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. 9.4 Structural Fill Testing The contractor should note that any proposed fill soils must be evaluated by Associated Earth Sciences, Inc. (AESI) prior to their use in fills. This would require that we have a sample of the material at least 3 business days in advance to perform a Proctor test and determine its field compaction standard. Subsurface Exploration, Geologic Hazard, Duvall Avenue NE Dental Building and Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations March 8, 2018 ASSOCIATED EARTH SCIENCES, INC. TJP/ms - 170437E001-3 - Projects\20170437\KE\WP Page 12 A representative from our firm should observe the stripped subgrade and be present during placement of structural fill to document 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 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. 10.0 FOUNDATIONS 10.1 Allowable Soil Bearing Pressure Spread footings may be used for building support when founded either directly on the medium dense to very dense natural sediments, or on structural fill placed over these materials. We recommend that an allowable foundation soil bearing pressure of 2,500 pounds per square foot (psf) be used for design purposes, including both dead and live loads. These sediments were encountered in our explorations at depths of approximately 1 to 2.5 feet. If higher foundation loads are anticipated, an allowable foundation bearing pressure of 4,000 psf may be used, provided that all footings bear directly on the dense to very dense, unweathered till. The unweathered till was encountered in our explorations at depths ranging from approximately 2 to 5 feet, but is likely deeper adjacent to the basement walls of the existing building. An increase in the allowable bearing pressure of one-third may be used for short-term wind or seismic loading. If structural fill is placed below footing areas, the structural fill should extend horizontally beyond the footing edges a distance equal to or greater than the thickness of the fill. 10.2 Footing Depths Perimeter footings for the proposed structure should be buried a minimum of 18 inches into the surrounding soil for frost protection. No minimum burial depth is required for interior footings; however, all footings must penetrate to the prescribed stratum, and no footings should be founded in or above loose, organic, or existing fill soils. 10.3 Footings Adjacent to Cuts The area bounded 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 edges of steps or cuts in the bearing soils. Subsurface Exploration, Geologic Hazard, Duvall Avenue NE Dental Building and Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations March 8, 2018 ASSOCIATED EARTH SCIENCES, INC. TJP/ms - 170437E001-3 - Projects\20170437\KE\WP Page 13 10.4 Footing Settlement Anticipated settlement of footings founded as described above should be on the order of 1 inch or less. However, disturbed soil not removed from footing excavations prior to footing placement could result in increased settlements. 10.5 Footing Subgrade Bearing Verification All footing areas should be observed by AESI prior to placing concrete to verify that the exposed soils can support the design foundation bearing pressure and that construction conforms with the recommendations in this report. Foundation bearing verification may also be required by the City of Renton. 10.6 Foundation Drainage Perimeter footing drains should be provided as discussed under the “Drainage Considerations” section of this report. 11.0 LATERAL WALL PRESSURES All backfill behind walls or around foundations should be placed following our recommendations for structural fill and as described in this section of the report. Horizontally backfilled walls that 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 55 pcf. Walls that retain sloping backfill at a maximum angle of 50 percent should be designed for 45 pcf for yielding conditions and 65 pcf for restrained conditions. If areas to receive vehicle traffic (e.g. parking areas or driveways) are located adjacent to walls, a surcharge equivalent to 2 feet of retained soil should be added to the wall height in determining lateral design forces. 11.1 Wall Backfill The lateral pressures presented above are based on the conditions of a uniform backfill consisting of either the on-site glacial sediments, or imported sand and gravel compacted to 90 to 95 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 unacceptable settlement behind the walls. Thus, the compaction level is critical and must be tested by our firm during placement. Subsurface Exploration, Geologic Hazard, Duvall Avenue NE Dental Building and Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations March 8, 2018 ASSOCIATED EARTH SCIENCES, INC. TJP/ms - 170437E001-3 - Projects\20170437\KE\WP Page 14 11.2 Wall Drainage 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 for the full wall height using imported, washed gravel against the walls. 11.3 Passive Resistance and Friction Factor Lateral loads can be resisted by friction between the foundation and the supporting natural sediments or structural fill soils, or by passive earth pressure acting on the buried portions of the foundations. The foundations must be backfilled with compacted structural fill to achieve the passive resistance provided below. We recommend the following design parameters: • Passive equivalent fluid = 300 pcf • Coefficient of friction = 0.35 The above values are allowable. 11.4 Seismic Surcharge As required by the 2015 IBC, retaining wall design should include a seismic surcharge pressure in addition to the equivalent fluid pressures presented above. We recommend a seismic surcharge pressure of 12H and 15H 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 wall. 12.0 FLOOR SUPPORT Slab-on-grade floors may be constructed either directly on the medium dense to very dense, natural lodgement till sediments, or on structural fill placed over these materials. Areas of the slab subgrade that are disturbed (loosened) during construction should be recompacted to an unyielding condition prior to placing the capillary break. If moisture intrusion through floor slabs is to be limited, the slabs should be constructed atop a capillary break consisting of a minimum thickness of 4 inches of washed pea gravel or washed, crushed rock. The pea gravel or washed, crushed rock should be overlain by a 10-mil (minimum thickness) plastic vapor retarder. Subsurface Exploration, Geologic Hazard, Duvall Avenue NE Dental Building and Geotechnical Engineering Report Renton, Washington Preliminary Design Recommendations March 8, 2018 ASSOCIATED EARTH SCIENCES, INC. TJP/ms - 170437E001-3 - Projects\20170437\KE\WP Page 15 13.0 DRAINAGE CONSIDERATIONS The natural glacial sediments generally contain a high percentage of silt and are considered to be highly moisture-sensitive. Traffic from vehicles and construction equipment across these sediments when they are very moist or wet will result in disturbance of the otherwise firm stratum. Therefore, prior to site work and construction, the contractor should be prepared to provide drainage and subgrade protection, as necessary. 13.1 Wall/Foundation Drains All retaining and perimeter foundation walls should be provided with a drain at the footing elevation. The drains should consist of rigid, perforated, polyvinyl chloride (PVC) pipe surrounded by washed gravel. The level of the perforations in the pipe should be set approximately 2 inches below the bottom of the footing, and the drains should be constructed with sufficient gradient to allow gravity discharge away from the building. All retaining walls should be lined with a minimum, 12-inch-thick, washed gravel blanket provided to within 1 foot of finish grade, and which ties into the footing drain. Roof and surface runoff should not discharge into the footing drain system, but should be handled by a separate, rigid, tightline drain. Exterior grades adjacent to walls should be sloped downward away from the structure to achieve surface drainage. 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 the foundation or within the immediate building area. It is recommended that a gradient of at least 3 percent for a minimum distance of 10 feet from the building perimeter 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. Additionally, pavement subgrades should be crowned to provide drainage toward catch basins and pavement edges. 14.0 STORMWATER INFILTRATION The site is underlain by Vashon lodgement till. Because of its high silt content and high relative density, the permeability of the unweathered till is low and it is not considered a suitable receptor soil for stormwater infiltration. Although the permeability of the weathered till horizon is greater than that of the unweathered till, the weathered till horizon at the site is thin and discontinuous. Consequently, water infiltrated into the weathered till horizon would tend to migrate laterally at a shallow depth atop the underlying unweathered till. As a result, stormwater infiltrated into the weathered till horizon can daylight in downslope areas, potentially adversely impacting on-site structures and adjacent properties. For this reason, stormwater infiltration at the subject site is not recommended. KINGCOUNTY KING COUNTY KING COUNTY Copyright:© 2013 National Geographic Society, i-cubed 0 20001000 FEET ± NOTE: BLACK AND WHITEREPRODUCTION OF THIS COLORORIGINAL MAY REDUCE ITSEFFECTIVENESS AND LEAD TOINCORRECT INTERPRETATION VICINITY MAP PROJ NO. DATE: FIGURE:170437E001 2/18 1Document Path: G:\GIS_Projects\aTemplates\aVM_Template\VM_MXD\170437E001 F1 VM_DuvallAveCommercial.mxdDATA SOURCES / REFERENCES:USGS: 7.5' SERIES TOPOGRAPHIC MAPS, ESRI/I-CUBED/NGS 2013KING CO: STREETS, PARCELS, CITY LIMITS 1/18 LOCATIONS AND DISTANCES SHOWN ARE APPROXIMATE KitsapCounty Snohomish County Pierce County King County Duvall Ave NES R 9 0 0 NE 17th St !( SITE DUVALL AVENUE DENTAL BUILDING RENTON, WASHINGTON ¬«900 RENTON DUVALL AVENUE NENOTES:1. BASE MAP REFERENCE: BRODERICKARCHITECTS; DENTALOFFICE BUILDING FOR JAROLD BAILET; SHEETA1.0;DATED 9/8/17170437 Duvall Dental bldg \ 170437E001 F2 S-E Plan.cdrDUVALLAVENUE DENTAL BUILDINGRENTON, WASHINGTONSITE AND EXPLORATION PLANPROJ NO. DATE: FIGURE:170437E001 2/18 2BLACKAND WHITE REPRODUCTION OF THIS COLOR ORIGINALMAY REDUCE ITSEFFECTIVENESSAND 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 dFEET5100NCONTOUR INTERVAL = 2’LEGEND:EXPLORATION BORINGSITE BOUNDARYEBNOTE: LOCATIONAND DISTANCES SHOWNAREAPPROXIMATE.EB-1EB-2EB-3 APPENDIX A Exploration Logs Sod / Topsoil Fill Washed gravel cuttings return ~1.5 to 2 feet. Weathered Vashon Lodgement Till Very moist, grayish tan, very silty, gravelly, SAND; nonstratified (SM). Vashon Lodgement Till Very moist, tannish gray, very silty, gravelly, SAND; nonstratified (SM). (Refusal on a rock at 7 feet; driller moved over 1 foot and resumed drilling.) 5 6 4 19 27 50 45 50/3" 50/6" S-1 S-2 S-3 S-4 Bottom of exploration boring at 10 feet due to refusal on a rock. No groundwater encountered. 1 of 1 Unknown Sheet Depth (ft)Exploration Number 170437E001 M - Moisture 7 inches 40 Datum S T Graphic10 Other TestsHole Diameter (in) DESCRIPTION Location Water Level ()Approved by: 30 Blows/Foot Driller/Equipment Blows/6"Geologic Drill / Mini-Track Well5 10 15 20 25 30 35 Water LevelProject Name EB-1 SymbolTJP2" OD Split Spoon Sampler (SPT) 3" OD Split Spoon Sampler (D & M)JHSCompletionSamples Ground Surface Elevation (ft) Grab Sample 2/22/18,2/22/18 Logged by: Shelby Tube Sample 140# / 30" Ring Sample No Recovery Water Level at time of drilling (ATD) Duvall Ave NE Dental Building 426 Project Number 20 Renton, WA Date Start/Finish Hammer Weight/Drop Sampler Type (ST): Exploration Log AESIBOR 170437.GPJ February 28, 20181010 77 5050/3" 5050/6" Fill Moist, brown, gravelly, very silty, SAND (SM). Vashon Lodgement Till Very moist, grayish tan, gravelly, very silty, SAND; nonstratified (SM). Becomes moist, tan gray. Becomes very moist. Contains pockets of very silty, fine sand. 14 31 40 31 50/6" 30 50/6" 20 50/6" 19 32 50/4" S-1 S-2 S-3 S-4 S-5 Bottom of exploration boring at 21.5 feet No groundwater encountered. 1 of 1 Unknown Sheet Depth (ft)Exploration Number 170437E001 M - Moisture 7 inches 40 Datum S T Graphic10 Other TestsHole Diameter (in) DESCRIPTION Location Water Level ()Approved by: 30 Blows/Foot Driller/Equipment Blows/6"Geologic Drill / Mini-Track Well5 10 15 20 25 30 35 Water LevelProject Name EB-2 SymbolTJP2" OD Split Spoon Sampler (SPT) 3" OD Split Spoon Sampler (D & M)JHSCompletionSamples Ground Surface Elevation (ft) Grab Sample 2/22/18,2/22/18 Logged by: Shelby Tube Sample 140# / 30" Ring Sample No Recovery Water Level at time of drilling (ATD) Duvall Ave NE Dental Building 426 Project Number 20 Renton, WA Date Start/Finish Hammer Weight/Drop Sampler Type (ST): Exploration Log AESIBOR 170437.GPJ February 28, 201871 5050/6" 5050/6" 5050/6" 5050/4" Surface: Crushed Gravel Vashon Lodgement Till Moist to very moist, grayish tan, gravelly, very silty, SAND; nonstratified (SM). Becomes tan gray. Becomes very moist to wet and grayish tan. Vashon Advance Outwash Wet, grayish tan, fine to medium SAND, some gravel, trace silt; contains lenses (< ~3 inches thick) of silty, fine sand (SP). 26 50/5" 29 18 31 50/6" 26 50/6" 14 50/6" S-1 S-2 S-3 S-4 S-5 Bottom of exploration boring at 21 feet 1 of 1 Unknown Sheet Depth (ft)Exploration Number 170437E001 M - Moisture 7 inches 40 Datum S T Graphic10 Other TestsHole Diameter (in) DESCRIPTION Location Water Level ()Approved by: 30 Blows/Foot Driller/Equipment Blows/6"Geologic Drill / Mini-Track Well5 10 15 20 25 30 35 Water LevelProject Name EB-3 SymbolTJP2" OD Split Spoon Sampler (SPT) 3" OD Split Spoon Sampler (D & M)JHSCompletionSamples Ground Surface Elevation (ft) Grab Sample 2/22/18,2/22/18 Logged by: Shelby Tube Sample 140# / 30" Ring Sample No Recovery Water Level at time of drilling (ATD) Duvall Ave NE Dental Building 426 Project Number 20 Renton, WA Date Start/Finish Hammer Weight/Drop Sampler Type (ST): Exploration Log AESIBOR 170437.GPJ February 28, 20185050/5" 4949 5050/6" 5050/6" 5050/6"