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HomeMy WebLinkAboutExhibit_8_Geo_Reportassociated earth sciences incorporated Associated Earth Sciences, Inc. 911 5th Avenue Kirkland, WA 98033 P (425) 827 7701 Subsurface Exploration, Geologic Hazard, and Geotechnical Engineering Report WILLIAMS AVENUE TOWNHOMES Renton, Washington Prepared For: ROGOLO APARTMENTS LLC Project No. 20190451E001 April 8, 2020 Exhibit 8 RECEIVED 07/29/2020 amorganroth PLANNING DIVISION DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E Kirkland | Tacoma | Mount Vernon 425-827-7701 | www.aesgeo.com April 8, 2020 Project No. 20190451E001 Rogolo Apartments LLC P.O. Box 1343 Renton, Washington 98057 Attention: Mr. Joe Bernasconi Subject: Subsurface Exploration, Geologic Hazard, and Geotechnical Engineering Report Williams Avenue Townhomes 82 Williams Avenue South Renton, Washington Dear Mr. Bernasconi: We are pleased to present the enclosed copy of our geotechnical report. This report summarizes the results of our subsurface exploration, geologic hazard, geotechnical engineering studies, and offers geotechnical recommendations for the design and development of the proposed project. We have enjoyed working with you on this study and are confident that the 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 ______________________________ Matthew A. Miller, P.E. Principal Engineer MM/ld 20190451E001-2 DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E SUBSURFACE EXPLORATION, GEOLOGIC HAZARD, AND GEOTECHNICAL ENGINEERING REPORT WILLIAMS AVENUE TOWNHOMES Renton, Washington Prepared for: Rogolo Apartments LLC P.O. Box 1343 Renton, Washington 98057 Prepared by: Associated Earth Sciences, Inc. 911 5th Avenue Kirkland, Washington 98033 425-827-7701 April 8, 2020 Project No. 20190451E001 DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E Subsurface Exploration, Geologic Hazard, Williams Avenue Townhomes and Geotechnical Engineering Report Renton, Washington Project and Site Conditions April 8, 2020 ASSOCIATED EARTH SCIENCES, INC. FSM/ld - 20190451E001-2 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, and geotechnical engineering study for the subject project. Our understanding of the project is based on communications with Mr. Joe Bernasconi and review of a “Boundary and Topographic Survey” prepared by DR Strong Consulting Engineers dated August 27, 2019, “Preliminary Floor Plans” prepared by CW Design Inc. dated December 11, 2019, and architectural elevation renderings. The site location is shown on the “Vicinity Map,” Figure 1. The approximate location of the exploration completed for this study is shown on the “Site and Exploration Plan,” Figure 2. A boring log of the subsurface exploration completed for this study is included in the Appendix. 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. This study included reviewing selected available geologic literature, advancing one exploration boring, and performing geologic studies to assess the type, thickness, distribution, and physical properties of the subsurface sediments and shallow groundwater. Geotechnical engineering studies were completed to establish recommendations for the type of suitable foundations, floor support, anticipated liquefaction-induced settlement, lateral earth pressures, and drainage considerations. This report summarizes our fieldwork and offers geotechnical engineering recommendations based on our present understanding of the property and potential future development. We recommend that we be allowed to review the recommendations presented in this report and revise them, if needed, as the project develops and a design is finalized. 1.2 Authorization Written authorization to proceed with this study was granted by Mr. Joe Bernasconi on December 6, 2019. Our study was accomplished in general accordance with our proposal dated November 8, 2019. This report has been prepared for the exclusive use of Rogolo Apartments 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. DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E Subsurface Exploration, Geologic Hazard, Williams Avenue Townhomes and Geotechnical Engineering Report Renton, Washington Project and Site Conditions April 8, 2020 ASSOCIATED EARTH SCIENCES, INC. FSM/ld - 20190451E001-2 Page 2 2.0 PROJECT AND SITE DESCRIPTION The subject site consists of a rectangular-shaped 0.14-acre parcel located at 82 Williams Avenue South in Renton, Washington (King County Parcel No. 0007200147) (Figure 1, “Vicinity Map”). The parcel is bordered on the west by Williams Avenue South, on the north and south by developed single-family residential parcels, and on the east by a church. The central portion of the parcel is occupied by a one-story home with a basement reportedly built in 1910 according to King County records. A detached garage is also situated in the southeastern corner of the parcel. A concrete driveway extends from the street along the southern side of the property to the detached garage. Site topography across the parcel is relatively flat with overall vertical relief estimated at less than 5 feet. Current project plans propose demolishing the existing home and detached garage and constructing a new three-story, six-unit townhome structure. We understand that six separate garages will be located on the lowest level of the building with residential living space for each of the townhomes in the upper two levels. We understand no below-grade levels are planned. Other project features will include a new concrete or asphalt driveway along the south side of the new building and utilities. We understand the preliminary project plans are to direct roof water to existing City of Renton-owned storm drainage located in the Williams Avenue South right-of-way. 3.0 SUBSURFACE EXPLORATION Our field study included advancing one exploration boring located in the western end of the property as shown on Figure 2. The conclusions and recommendations presented in this report are based on the exploration completed for this study. The number, location, and depth of the exploration were completed within site and budgetary constraints. 3.1 Exploration Boring The exploration boring was completed by advancing hollow-stem auger tools with a track-mounted drill rig. During the drilling process, samples were obtained at generally 2.5- to 5-foot-depth intervals. The exploration boring was continuously observed and logged by a representative from our firm. The exploration log presented in the Appendix is based on the field log, drilling action, and observation of the samples secured. 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 DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E Subsurface Exploration, Geologic Hazard, Williams Avenue Townhomes and Geotechnical Engineering Report Renton, Washington Project and Site Conditions April 8, 2020 ASSOCIATED EARTH SCIENCES, INC. FSM/ld - 20190451E001-2 Page 3 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 log. 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, as summarized in this report. 4.0 SUBSURFACE CONDITIONS Subsurface conditions at the project site were inferred from the field exploration accomplished for this study and review of applicable geologic literature. As shown on the boring log, the exploration boring generally encountered approximately 8 feet of loose to very loose, fine-grained overbank alluvium underlain by medium dense to dense, coarse-grained channel alluvium to the maximum depth explored of 31.5 feet. The boring log is included in the Appendix. 4.1 Stratigraphy Holocene Alluvium - Overbank Facies Exploration boring EB-1 encountered these fine-grained soils to a depth of approximately 8 feet. The overbank alluvium is comprised of loose to very loose, light brownish orange to light brownish gray, iron-oxide stained, interbedded, silt and fine sand with trace amounts of organics (root hairs). Overbank alluvium is not considered suitable for foundation support and may require mitigation for slab-on-grade floor support. The observed overbank alluvium is susceptible to static settlement caused by the application of footing loads. Foundations loads will need to extend through the overlying loose to very loose overbank alluvium as described in the “Design Recommendations” section of this report. Excavated overbank alluvium material may be suitable for reuse in structural fill applications if such reuse is specifically allowed by project plans and specifications. Reuse would include removing excessively organic and any other deleterious materials, and adjusting moisture content to allow compaction to the specified level, and to a firm and unyielding condition. Based on the exploration completed for this study, we estimate the overbank alluvium was above optimum moisture content for compaction purposes, and therefore may require drying during favorable weather prior to compaction in structural fill applications. DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E Subsurface Exploration, Geologic Hazard, Williams Avenue Townhomes and Geotechnical Engineering Report Renton, Washington Project and Site Conditions April 8, 2020 ASSOCIATED EARTH SCIENCES, INC. FSM/ld - 20190451E001-2 Page 4 Holocene Alluvium - Channel Facies Underlying the overbank alluvium described above, exploration boring EB-1 encountered channel alluvial soils to the maximum depth drilled of 31.5 feet. The channel alluvium generally consisted of brownish orange to brownish olive and gray, medium dense to dense, gravelly sand grading downward to sandy gravel with trace to some amounts of silt throughout. The channel alluvium is suitable for support of foundation loads. Excavated channel alluvial material may be suitable for reuse in structural fill applications but is not anticipated to be encountered in significant quantities during construction of the structure. 4.2 Geologic Mapping Review of the regional geologic maps titled Geologic Map of King County, compiled by Derek B. Booth et al. (March 2007) and Geologic Map of the Renton Quadrangle, King County, Washington, by D.R. Mullineaux (1965) indicates the subject site is underlain by alluvium. Our interpretation of the sediments encountered at the subject site is in general agreement with the detailed geologic maps. 4.3 Hydrology Groundwater was observed within the alluvium at the time of drilling in boring EB-1 at a depth of approximately 19 feet below the surface. The observed groundwater is interpreted to be representative of the regional unconfined groundwater table. The depth to the groundwater is expected to vary seasonally with the amount and frequency of rainfall, nearby river levels, and on-site and off-site land use. DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E Subsurface Exploration, Geologic Hazard, Williams Avenue Townhomes and Geotechnical Engineering Report Renton, Washington Geologic Hazards and Mitigations April 8, 2020 ASSOCIATED EARTH SCIENCES, INC. FSM/ld - 20190451E001-2 Page 5 II. GEOLOGIC HAZARDS AND MITIGATIONS The following discussion of potential geologic hazards is based on the geologic, slope, and ground and surface water conditions, as observed and discussed herein. 5.0 SEISMIC HAZARDS AND MITIGATION Earthquakes occur in the Puget Lowland with great regularity. The majority of these events are small and are usually not felt by people. However, large earthquakes do occur, as evidenced by the 1949, 7.2-magnitude event; the 1965, 6.5-magnitude event; and the 2001, 6.8-magnitude event. The 1949 earthquake appears to have been the largest in this area during recorded history. Evaluation of return rates indicates that an earthquake of a magnitude between 6.0 and 7.0 is likely within a given 25- to 40-year period. Generally, there are three types of potential geologic hazards associated with large seismic events at this site: 1) surficial ground rupture, 2) liquefaction, and 3) ground motion. The potential for each of these hazards to adversely impact the proposed project is discussed below. 5.1 Surficial Ground Rupture The project site is located approximately 8.5 miles northeast of a suspected trace of the Tacoma Fault Zone and approximately 3 miles south of the southern boundary of the Seattle Fault Zone. Geophysical surveys of the Puget Lowland in 1998 suggested the presence of a fault zone along the north side of the Tacoma basin generally between Tacoma and the south end of Hood Canal. More recent studies by the U.S. Geological Survey (USGS) have provided evidence of surficial ground rupture along the Tacoma Fault near Burley, Washington. The recognition of this fault is relatively new, and data pertaining to it are limited, with studies still ongoing. According to the USGS studies, the latest movement of this fault was about 1,100 years ago when several meters of surficial displacement took place. This displacement can presently be seen in the form of raised tidal flats in the Burley Lagoon, Lynch Bay, and North Cove, resulting in the present-day freshwater marshes and forested lowlands. Further evidence of the faulting activity in this system has been seen at Wollochet Bay, where forested areas were inundated by water. Additionally, linear fault scarp features in the ground surface were exposed in recent Light Detection and Ranging (LIDAR) maps. The scarp features are located near Allyn, Washington, and have been described as trending parallel to the Tacoma Fault system. Studies also point to the fault system continuing across the Puget Sound, and it is inferred as continuing through Commencement Bay and the Puyallup River Delta. The slip rate along the Tacoma Fault DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E Subsurface Exploration, Geologic Hazard, Williams Avenue Townhomes and Geotechnical Engineering Report Renton, Washington Geologic Hazards and Mitigations April 8, 2020 ASSOCIATED EARTH SCIENCES, INC. FSM/ld - 20190451E001-2 Page 6 is thought to be approximately 2 millimeters per year. However, very little is known about the recurrence interval of earthquakes along the Tacoma Fault. Our current understanding of the Seattle Fault suggests that several fault traces are spread across a relatively wide zone. Recent studies by the USGS (e.g., Johnson et al., 1994, Origin and Evolution of the Seattle Fault and Seattle Basin, Washington, Geology, v. 22, p. 71-74 and Johnson et al., 1999, Active Tectonics of the Seattle Fault and Central Puget Sound Washington - Implications for Earthquake Hazards, Geological Society of America Bulletin, July 1999, v. 111, n. 7, p. 1042-1053) have provided evidence of surficial ground rupture along a northern splay of the Seattle Fault. The recognition of this fault is relatively new, and data pertaining to it are limited, with the studies still ongoing. 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. Related fault systems in the Puget Sound region have been hypothesized to have reoccurrence intervals in excess of several thousand years. Due to the suspected long recurrence interval, and the distance of this fault system from the site, the potential for surficial ground rupture is considered to be low during the expected life of the project, and no mitigation efforts beyond complying with the requirements of the local jurisdictions and the International Building Code (IBC) in use by the local jurisdiction are recommended. 5.2 Liquefaction Liquefaction is a process through which unconsolidated soil loses strength as a result of vibratory shaking, such as that which occurs during a seismic event. During normal conditions, the weight of the soil is supported by both grain-to-grain contacts and by the 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 decrease in soil shear strength. The soil is said to be liquefied when nearly all 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 clean sand or silt with low relative densities accompanied by a shallow water table. To assess the risk, we performed a liquefaction hazard analysis for this site in accordance with guidelines published in Seed & Idriss, 1982; Seed et al., 1985; and Kramer, 1996. Our liquefaction analysis was completed with the aid of LiquefyPro computer software Version 5.8 (2009) by CivilTech Corporation. The soil and groundwater conditions observed in EB-1 were used to define subsurface conditions at depth for the analysis. DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E Subsurface Exploration, Geologic Hazard, Williams Avenue Townhomes and Geotechnical Engineering Report Renton, Washington Geologic Hazards and Mitigations April 8, 2020 ASSOCIATED EARTH SCIENCES, INC. FSM/ld - 20190451E001-2 Page 7 The liquefaction potential is dependent on several site-specific factors such as soil grain size, density, site geometry, static stresses, level of ground acceleration considered, and duration of the event. The 2015 IBC seismic design parameter for peak ground acceleration (PGA) was determined by the latitude and longitude of the project site using the USGS U.S. Seismic Design Maps website1. The USGS website interpolated PGA at the project site to be 0.592g, with a 2 percent chance of exceedance in 50 years. The earthquake parameters used in our liquefaction analysis were a magnitude 7 earthquake occurring directly beneath the site with a peak horizontal ground acceleration of 0.592g per the USGS U.S. Seismic Design Maps website. Results of our liquefaction analysis are provided in the Appendix. Based on our liquefaction analysis under the current standard, the subsurface conditions encountered below the site pose a moderate risk of seismic liquefaction and associated liquefaction-induced differential settlement. The liquefaction-induced overall settlement for the site with the above-mentioned parameters is estimated to range from approximately 2 to 4 inches. However, the settlement is predicted to occur below a depth of 20 feet. We estimate that the differential settlement over a distance of 100 feet would be on the order of one-half of the total seismic settlement. As discussed in the “Foundations” section of this report, the proposed structure could be supported on rock-filled trenches to partially mitigate the effects of the liquefaction, such as differential settlement. 5.3 Ground Motion Seismic design of the project should follow the 2015 IBC guidelines. Seismic site class selection is outlined in American Society of Civil Engineers (ASCE) 7-16. Based on our subsurface explorations, the site is considered to be in Seismic Site Class E. 6.0 EROSION HAZARDS AND MITIGATION In order to control erosion and reduce the amount of sediment transport off the site during construction, the following recommendations should be followed: 1. Properly embedded silt fencing should be placed around the construction area. The fencing should be periodically inspected and maintained, as necessary, to ensure proper function. 2. Construction access should be stabilized with gravel to minimize tracking sediment offsite. 3. If possible, construction should proceed during the drier periods of the year. 1 http://earthquake.usgs.gov/designmaps/ DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E Subsurface Exploration, Geologic Hazard, Williams Avenue Townhomes and Geotechnical Engineering Report Renton, Washington Geologic Hazards and Mitigations April 8, 2020 ASSOCIATED EARTH SCIENCES, INC. FSM/ld - 20190451E001-2 Page 8 4. Surface runoff and discharge should be controlled during and following construction. Uncontrolled discharge may promote erosion and sediment transport. 5. If excavated soils are to be stockpiled on the site for reuse, measures should be taken to reduce the potential for erosion from the stockpile. These could include, but are not limited to, covering the pile with plastic sheeting, and the use of straw bales/silt fences around stockpile perimeters. DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E Subsurface Exploration, Geologic Hazard, Williams Avenue Townhomes and Geotechnical Engineering Report Renton, Washington Design Recommendations April 8, 2020 ASSOCIATED EARTH SCIENCES, INC. FSM/ld - 20190451E001-2 Page 9 III. DESIGN RECOMMENDATIONS 7.0 INTRODUCTION It is our opinion that, from a geotechnical standpoint, the site is suitable for the proposed building provided the recommendations contained herein are properly followed. Our explorations generally encountered approximately 8 feet of loose to very loose overbank alluvium underlain by medium dense to dense channel alluvial sediments extending beyond the depth of our exploration. The overbank alluvium sediments are considered normally consolidated and are conducive to post-construction consolidation when subject to new loads such as those from the placement of fill or from new foundations. Based on the exploration and analyses completed to date, we recommend foundations be constructed on rock-filled trenches to mitigate post-construction static consolidation settlement beneath the proposed building. The rock-filled trenches would extend through the loose to very loose overbank alluvium into the medium dense to dense overbank alluvium. Based on our exploration, we estimate that the depth of rock-filled trenches would be on the order of 8 to 9 feet. It is important to note that raising final grades will induce consolidation of the native alluvial sediments but also could partially mitigate liquefaction-induced settlement. We recommend that once final grading plans are developed, we are allowed to review the plans and update our recommendations to account for new loads from proposed fills, or foundation loads. 8.0 SITE PREPARATION Existing buried utilities, vegetation, topsoil, asphalt, and any other deleterious materials should be removed where they are located below planned construction areas. All disturbed soils resulting from demolition activities should be removed to expose underlying undisturbed native sediments and replaced with structural fill, as needed. Erosion and surface water control should be established around the clearing limits to satisfy local requirements. Below planned on-site paving, the soft, compressible native soils should be exposed, proof-rolled if possible, and compacted to 95 percent of the modified Proctor maximum dry density. If a firm and unyielding condition is achieved, no further remedial preparation would be needed. If excessively yielding conditions are encountered, the unsuitable native soils would be partially removed and replaced with imported structural fill. The depth of replacement of the existing fill below paving should be determined at the time of construction when field DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E Subsurface Exploration, Geologic Hazard, Williams Avenue Townhomes and Geotechnical Engineering Report Renton, Washington Design Recommendations April 8, 2020 ASSOCIATED EARTH SCIENCES, INC. FSM/ld - 20190451E001-2 Page 10 conditions are known. Alternatively, soil cement treatment could be used to complete remedial preparation of existing fill below planned new paving areas. 8.1 Temporary and Permanent Cut Slopes In our opinion, stable, temporary construction slopes should be the responsibility of the contractor and should be determined during construction. For planning purposes, we recommend temporary cut slopes of 1.5H:1V (Horizontal:Vertical) within the overbank alluvium and existing fill (if present). Flatter, temporary cut slopes are recommended in areas of groundwater 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 always be followed . 8.2 Subgrade Protection If building construction will proceed during the winter, we recommend the use of a working surface of the existing concrete, crushed rock, or quarry spalls to protect exposed soils, particularly in areas supporting concentrated equipment 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 suitable subgrade protection. Foundation concrete should be placed and excavations backfilled as soon as possible to protect the bearing surfaces. 8.3 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. 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 subgrade conditions. Groundwater was observed relatively deep in our boring at the time of drilling. However, depending on the depth of excavation required to construct the proposed improvements, and the time of year construction is performed, the construction contractor should be prepared to perform temporary dewatering of excavations which may include the use of sump pumps. DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E Subsurface Exploration, Geologic Hazard, Williams Avenue Townhomes and Geotechnical Engineering Report Renton, Washington Design Recommendations April 8, 2020 ASSOCIATED EARTH SCIENCES, INC. FSM/ld - 20190451E001-2 Page 11 9.0 STRUCTURAL FILL 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 95 percent of the modified Proctor maximum density using ASTM D-1557 as the standard. Soils excavated onsite are acceptable for use in structural fills if they can be moisture-conditioned and compacted to project specifications for the intended use. In the case of roadway and utility trench filling, the backfill should be placed and compacted in accordance with current City of Renton codes and standards. Where adjacent to slopes, the top of the compacted fill should extend horizontally outward a minimum distance of 3 feet beyond the locations of the perimeter footings or pavement edges before sloping down at an angle of 2H:1V. The contractor should note that any proposed fill soils must be evaluated by AESI prior to their use in fills. This would require that we have a sample of the material 72 hours in advance to perform a Proctor test and determine its field compaction standard. Soils in which the amount of fine-grained material (smaller than the No. 200 sieve) is greater than approximately 5 percent (measured on the minus No. 4 sieve size) should be considered moisture sensitive. Use of moisture-sensitive soil in structural fills should be limited to favorable dry weather conditions. Some of the native soils present onsite contained significant amounts of silt and are considered highly moisture sensitive. In addition, construction equipment traversing the site when the soils are wet can cause considerable disturbance. If fill is placed during wet weather or if proper compaction cannot be obtained, a select import material consisting of a clean, free-draining gravel and/or sand should be used. Free-draining fill consists of non-organic soil with the amount of fine-grained material limited to 5 percent by weight when measured on the minus No. 4 sieve fraction with at least 25 percent retained on the No. 4 sieve. 10.0 FOUNDATIONS (ROCK-FILLED TRENCHES) As previously stated, the shallow subsurface sediments (overbank alluvium) encountered at the project site are not suitable for support of foundation loads. Because the overbank alluvium is considered too deep to economically extend the footings down to suitable bearing soils, rock-filled trenches extended down to the medium dense to dense channel alluvium are recommended for foundation support. DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E Subsurface Exploration, Geologic Hazard, Williams Avenue Townhomes and Geotechnical Engineering Report Renton, Washington Design Recommendations April 8, 2020 ASSOCIATED EARTH SCIENCES, INC. FSM/ld - 20190451E001-2 Page 12 The trenches should have a minimum width of 4 feet (or as designated by the field engineer/engineering geologist) and be excavated down to the medium dense to dense overbank alluvium. Because of the potential for caving, the actual trench width may be greater than specified. It would be appropriate to backfill the trenches as the excavation proceeds to reduce caving. The use of a larger, track-mounted backhoe will greatly speed trench excavation over the use of a conventional, rubber-tired backhoe. In order to reduce disturbance of the bearing soils exposed in the trench, it is strongly recommended that the teeth of the backhoe bucket be covered with a smooth cutting edge. To determine when suitable bearing has been achieved and to verify proper rock placement, the geotechnical engineer/engineering geologist must be present on a full-time basis during footing trench excavation and backfill. A pump may be required to control seepage so that the bearing level can be visually determined. Seepage entering the excavation on an overnight basis must be removed prior to commencing trench excavation the following day. After the bearing stratum has been reached, the trench should be immediately backfilled. We recommend the use of 2- to 4-inch-size crushed rock for backfill. The crushed rock must be tamped into place to achieve a tightly-packed mass; this may be done with either a “Hoepac”-type compactor mounted on the backhoe or with the excavator bucket. Staging areas should be maintained so that the rock is not contaminated by mud prior to placement in the trench. Equipment access to trench locations should also be maintained. Once the rock-filled trenches are suitably backfilled and compacted we recommend an allowable bearing capacity of 2,000 psf for a conventional spread footing foundation. 11.0 DRAINAGE CONSIDERATIONS Traffic across the on-site soils when they are damp or wet will result in disturbance of the stratum. Therefore, during site work and construction, the contractor should provide surface drainage and subgrade protection, as necessary. All perimeter foundation walls should be provided with a drain at the footing elevation. 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 at the bottom of the footing, and the drains should be constructed with sufficient gradient to allow gravity discharge away from the building. The perforations should be located on the lower portion of the pipe. In addition, any retaining or subgrade walls should be lined with a minimum, 12-inch-thick, washed gravel blanket, backfilled completely with free-draining material, or lined with a drainage mat, such as Mira-Drain 6000, over the full height of the wall (excluding the first 1 foot below the surface). This drainage material should tie into the footing drains and must be installed and backfilled in DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E Subsurface Exploration, Geologic Hazard, Williams Avenue Townhomes and Geotechnical Engineering Report Renton, Washington Design Recommendations April 8, 2020 ASSOCIATED EARTH SCIENCES, INC. FSM/ld - 20190451E001-2 Page 13 strict accordance with the manufacturer’s specifications. Roof and surface runoff should not discharge into the footing drain system, but should be handled by a separate, rigid, tightline drain. To minimize erosion, stormwater discharge or concentrated runoff should not be allowed to flow down any steep excavation cuts. In planning, exterior grades adjacent to walls should be sloped downward away from the structure to achieve surface drainage. Runoff water from impervious surfaces should be collected by a storm drain system that discharges into the site stormwater system. 12.0 FLOOR SUPPORT A slab-on-grade floor may be used supported by rock-filled trenches or over structural fill as discussed below. Where moisture transmission through the slab must be controlled, the floor should be cast atop a minimum of 4 inches of washed pea gravel or ½- to ¾-inch washed crushed rock material to provide a capillary break. The floor should also be protected by a moisture vapor retarder (minimum 6-mil-thick polyethylene sheeting). The vapor retarder must be protected from punctures. If a sand “blotter layer” is placed over the vapor retarder, it must be protected from excess moisture prior to placing the slab. If some cracking and settlement of the floor is tolerable, an alternative would be to support the slab on a thin structural fill. After overexcavating at least 3 feet below finish floor grade, a structural fill would be placed. After the fill is completed and approved, the moisture barrier and free-draining layer may be placed. The floor slab can then be cast on top of the free-draining layer. The floor slab should not be tied into the building’s foundation but should be free to settle independent of footings. The slab should contain bar reinforcement to reduce differential movement across any cracks that might develop. If the potential for floor cracking and settlement must be minimized, the floor should be cast on structural fill or rock-filled trenches. Another alternative would be to support the slab on rock trenches. The trenches would be constructed as discussed under the “Foundations” section of this report. The spacing of the trenches would be determined by a structural engineer based on such things as the amount of reinforcement included in the floor slab design and the amount of acceptable settlement or deflection of the slab. DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E Subsurface Exploration, Geologic Hazard, Williams Avenue Townhomes and Geotechnical Engineering Report Renton, Washington Design Recommendations April 8, 2020 ASSOCIATED EARTH SCIENCES, INC. FSM/ld - 20190451E001-2 Page 14 13.0 PROJECT DESIGN AND CONSTRUCTION MONITORING We recommend that AESI perform a geotechnical review of the plans prior to final design completion. In this way, our recommendations may be properly interpreted and implemented in the design. This plan review is not included in the current scope of work and budget. We are also available to provide geotechnical engineering and monitoring services during construction. 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. Construction monitoring services are not part of this current scope of work. 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 ______________________________ Frank S. Mocker, L.G., L.E.G. Matthew A. Miller, P.E. Project Geologist Principal Engineer Attachments: Figure 1: Vicinity Map Figure 2: Site and Exploration Plan Appendix: Exploration Log and Liquefy Pro Results DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E DATA 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 VICINITY MAP WILLIAMS AVENUE APARTMENT SITE RENTON, WASHINGTON 20190451E001 3/20 1 KING COUNTY RENTON Copyright:© 2013 National Geographic Society, i-cubed ± 0 2000 Feet 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\aaY2019\190451 Williams Ave\aprx_mxd\190451E001 F1 F2 Williams.aprx¥ ¥ ¥405 ¬«167 ¬«169 !(WellsAveSWilliams Ave SAccessRd S Tobin St S R i ve r s ide D r S Tillicum St Williams Aly SKing County SITE ¬«900 DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E NOTES: 1. BASE MAP REFERENCE: DR STRONG CONSULTING ENGINEERS, JOE BERNASCONI, BOUNDARY AND TOPOGRAPHIC SURVEY, 8/27/19 190451 82 Williams Ave \ 20190451E001 F2 S-E.cdrWILLIAMS AVENUE APARTMENTS RENTON, WASHINGTON SITE AND EXPLORATION PLAN PROJ NO.DATE:FIGURE: 20190451E001 3/20 2 BLACK AND WHITE REPRODUCTION OF THIS COLOR ORIGINAL MAY REDUCE ITS EFFECTIVENESS AND LEAD TO INCORRECT INTERPRETATION. a s s o c i a t e d e a r t h s c i e n c e s i n c o r p o r a t e d FEET 10 200 N CONTOUR INTERVAL = 2’ LEGEND: EXPLORATION BORING SITE BOUNDARY EB NOTE: LOCATION AND DISTANCES SHOWN ARE APPROXIMATE. EB-1 DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E APPENDIX DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E 1 1 1 1 1 2 2 2 2 12 17 21 13 14 16 3 7 7 9 13 16 5 8 5 Bottom of exploration boring at 31.5 feet Groundwater encountered at 19 feet with BOH at 20 feet. Topsoil - 6 inches Very loose, brown, silty, fine SAND, some organics; nonstratified (SM). Overbank Facies Very loose, moist, light brownish orange with iron oxide, fine SAND, some silt, trace organics; nonstratified (SP-SM). Moist, light brownish gray with iron oxide mottling, SILT, trace fine sand, trace organics (rootlets); massive (ML). As above; increased iron oxide staining. Channel Facies Driller notes gravelly drill action below 8 feet. Moist, brownish orange with iron oxide, very gravelly, fine to coarse SAND, trace silt; crudely bedded (SW). Gravelly drill action 10 to 15 feet. Wet, brownish olive, very gravelly, medium to coarse SAND, trace to some fine sand, trace silt; nonstratified (SP-SM). Sample is wet due to drilling to 20 feet then extracting sample. Gravelly drill action. Adding bentonite mud to reduce heave. Wet, gray, very sandy, fine to coarse GRAVEL, trace silt; sand is predominantly medium to coarse grained; massive (GW). Gravelly drill action. Very moist to wet, brownish orange, sandy, fine to coarse GRAVEL, trace to some silt; crudely bedded (GW-GM). Gravelly drill action. Wet, light brown, fine SAND, trace to some silt, trace medium to coarse sand; massive to stratified; one thin (<1 inch thick) highly oxidized orange, silt interbed at 31.25 feet (SP-SM/ML). S-1 S-2 S-3 S-4 S-5 S-6 S-7 S-8 1 of 1 NAVD 88 FSM2" OD Split Spoon Sampler (SPT) 3" OD Split Spoon Sampler (D & M)Water LevelProject Name JHSWater Level ()Approved by: 30 Blows/Foot Samples Ground Surface Elevation (ft) Grab SampleSymbol 8 40 Datum Hammer Weight/Drop Sampler Type (ST): S T Project Number 20 Renton, WA Date Start/Finish CompletionLocation Sheet Depth (ft)Exploration Number 20190451E001 3/12/20,3/12/20 Logged by: Shelby Tube Sample 140# / 30 Geologic Drill Partners / HSA w/ XL Trailer Drill Well38 5 10 15 20 25 30 EB-1 Ring Sample No RecoveryGraphic 10 Other TestsHole Diameter (in) DESCRIPTION Driller/Equipment Blows/6"Exploration Boring Water Level at time of drilling (ATD) Williams Avenue Apartments M - Moisture AESIBOR 20190451E001.GPJ March 17, 202022 33 44 3838 3030 1414 2929 1313 DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E DocuSign Envelope ID: 7AA8C5B2-956E-4E15-93A2-9CB8F148EB1E