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1141 Viewland Geotech Report 20190294E001-4-Ra s s o c i a t e d e a r t h s c i e n c e s incorporated Subsurface Exploration, Geologic Hazard, and Geotechnical Engineering Report MAUPIN—CROUCH RESIDENCE Edmonds, Washington Prepared For: TERRI MAUPIN AND PETE CROUCH Project No. 20190294E001 January 10, 2020 a s s o c i a t e d earth sciences i n c o r p o r a t e d January 10, 2020 Project No. 20190294EO01 Terri Maupin and Pete Crouch 1141 Viewland Way Edmonds, Washington 98020 Subject: Subsurface Exploration, Geologic Hazard, and Geotechnical Engineering Report Maupin—Crouch Residence 1141 Viewland Way Edmonds, Washington 98020 Dear Terri Maupin and Pete Crouch: We are pleased to present the enclosed copy of our geotechnical report. This report summarizes the results of our subsurface exploration, geologic hazard, and preliminary geotechnical engineering studies, and offers preliminary 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 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 Anthony'W. Roma' nick, P. E. Senior Project Engineer AWR/ms/Id - 20190294EO01-4 Kirkland Office 1911 Fifth Avenue i Kirkland, WA 98033 P 1425.827.7701 Mount Vernon Office 1508 S. Second Street, Suite 101 i Mount Vernon, WA 98273 P 1425.827.7701 Tacoma Office i 1552 Commerce Street, Suite 102 i Tacoma, WA 98402 P i 253.722.2992 www.aesgeo.com SUBSURFACE EXPLORATION, GEOLOGIC HAZARD, AND GEOTECHNICAL ENGINEERING REPORT MAUPIN—CROUCH RESIDENCE Edmonds, Washington Prepared for: Terri Maupin and Pete Crouch 1141 Viewland Way Edmonds, Washington 98020 Prepared by: Associated Earth Sciences, Inc. 911 51h Avenue Kirkland, Washington 98033 425-827-7701 January 10, 2020 Project No. 20190294EO01 Subsurface Exploration, Geologic Hazard, and Maupin—Crouch Residence Geotechnical Engineering Report Edmonds, Washington Project and Site Conditions I. PROJECT AND SITE CONDITIONS 1.0 INTRODUCTION This report presents the results of Associated Earth Sciences, Inc.'s (AESI) subsurface exploration, geologic hazard, and geotechnical engineering study for a new single-family residence located at 1141 Viewland Way in Edmonds, Washington. Our understanding of the project is based on discussions and email exchanges with you; a topographic survey titled "Boundary and Topographic Survey, Maupin / Crouch Residence," prepared by Terrane, dated July 9, 2019; and our experience working in the project area. The site location is shown on the "Vicinity Map," Figure 1. The approximate locations of explorations completed for this study, along with existing site features, are shown on the "Existing Site and Exploration Plan," Figure 2. The conclusions and recommendations contained in this report should be reviewed and modified, or verified, once project plans are finalized. 1.1 Purpose and Scope The purpose of this study was to provide subsurface data to be utilized in the design and development of the referenced project. Our study included reviewing available geologic literature, completing two exploration borings with a mini -track drill rig, and performing geologic studies to assess the type, thickness, distribution, and physical properties of the subsurface sediments and groundwater. Geotechnical engineering studies were completed to determine site preparation recommendations, structural fill recommendations, the type of suitable foundations, retaining wall/lateral earth pressures, erosion considerations, geologic hazard assessments, and drainage considerations. This report summarizes our fieldwork and offers preliminary recommendations based on our present understanding of the project. We recommend that we be allowed to review the recommendations presented in this report and revise them, if needed, when the project design has been finalized. 1.2 Authorization This report has been prepared for the exclusive use of Terri Maupin and Pete Crouch, and their agents, for specific application to this project. Our work was performed in accordance with our scope of work and cost proposal, dated August 9, 2019. 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. January 10, 2020 ASSOCIATED EARTH SCIENCES, INC. A WRIMS11d - 20190294EO01 -4 Page 1 Subsurface Exploration, Geologic Hazard, and Maupin—Crouch Residence Geotechnical Engineering Report Edmonds, Washington Project and Site Conditions 2.0 PROJECT AND SITE DESCRIPTION The project site is that of the existing single-family residential property located at 1141 Viewland Way in Edmonds, Washington (Snohomish County Parcel No. 00606600001800), as shown on Figure 1 "Vicinity Map." The parcel is rectangular in plan view and is approximately 0.26 acres in size. The parcel is currently occupied by a single-family residence centrally located on the parcel. The site is bounded on the north and west by single-family residences, to the east by 121h Avenue North, and to the south by Viewland Way. Site topography is gently to steeply sloping downward toward the west with a relatively flat area at the center of the parcel which contains the existing residence. The topography of the neighborhood surrounding the subject site is similar, suggesting that the area of the site had been graded to create building lots in a "terraced" configuration many years ago. Slopes near the eastern and western parcel boundaries range in height from 10 feet to 16 feet and have inclinations as steep as 62 percent. We understand that the current project plan includes demolition of the existing residence with the construction of a new single-family residence. The subject site includes Landslide Hazard Areas and Erosion Hazard Areas, as delineated in the City of Edmonds' maps. 3.0 SUBSURFACE EXPLORATION Our field study included advancing two exploration borings at the site (EB-1 and EB-2). The approximate locations of the borings are shown on Figure 2. The conclusions and recommendations presented in this report are based on the conditions encountered in these explorations. The number, locations, and depths of our explorations were completed within site and budgetary constraints. Copies of the exploration logs are included in Appendix A. 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 between the explorations may differ from those inferred by the boring data due to the random nature of deposition and the alteration of topography by past grading and/or filling. The nature and extent of any variations between the field explorations may not become fully evident until construction. 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 drilled using a mini -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 January 10, 2020 ASSOCIATED EARTH SCIENCES, INC. A WRIMS11d - 20190294EO01 -4 Page 2 Subsurface Exploration, Geologic Hazard, and Maupin—Crouch Residence Geotechnical Engineering Report Edmonds, Washington Project and Site Conditions from our firm. The exploration logs presented in the Appendix are based on the field logs, 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 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 boring logs in the Appendix. 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. 4.0 SUBSURFACE CONDITIONS Subsurface conditions at the project site were inferred from the field explorations accomplished for this study, our visual reconnaissance of the site, and review of selected geologic literature. As shown on the field logs, the exploration borings generally encountered fill overlying old landslide debris and older in -place sediments. The following section presents more detailed subsurface information organized from the shallowest (youngest) to the deepest (oldest) sediment types. 4.1 Stratigraphy Grass/Topsoil A surficial layer of grass/topsoil was encountered at the location of our explorations. These organic layers were approximately 4 inches thick. Due to their high organic content, these materials are not considered suitable for foundation support, slab -on -grade floor support, or for use in a structural fill. January 10, 2020 ASSOCIATED EARTH SCIENCES, INC. A WRIMS11d - 20190294EO01 -4 Page 3 Subsurface Exploration, Geologic Hazard, and Maupin—Crouch Residence Geotechnical Engineering Report Edmonds, Washington Project and Site Conditions Existing Fill We encountered fill soils (soils not naturally placed) consisting of loose to dense, moist, light brown to brownish gray, very silty fine to medium sand with variable gravel content to approximately 4 feet below the surface in both explorations. The observed fill included minor amounts of organic material and contained small rootlets. 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 the moisture content is adjusted to allow compaction to the specified level and to a firm and unyielding condition. Existing fill is also expected in unexplored areas of the site, such as the area surrounding and under the existing structure foundations, in existing utility trenches, and at previously graded landscaped areas. Due to their variable density and organic debris content, the existing fill soils are not suitable for structural support of foundations. Landslide Deposits Underlying the fill, exploration borings EB-1 and EB-2 encountered loose to medium dense, moist to wet, very silty, fine to medium sand, with variable amounts of gravel, organics, and wood fragments, to approximate depths below the surface of 13 and 29 feet, respectively. The loose condition of the material encountered, and the presence of organics and wood debris are suggestive of deposits derived from past earth movement (landslides). Due to their variable density and variable content, including organic material, the existing landslide debris is not recommended for foundation support. Pre -Fraser Nonglacial Deposits Underlying the landslide deposits in our exploration borings and extending to the depth explored, we encountered sediments generally consisting of medium dense to very dense or hard, fine to coarse sand with variable silt and gravel ranging to fine sandy silt with trace fine organics. We interpret these sediments to be representative of pre -Fraser nonglacial sediments. The high relative density of the sediments indicates that they have been overrun and consolidated by glacial ice. These sediments were deposited in a nonglacial environment prior to the Fraser glaciation. We anticipate these sediments are present at depths too deep to be considered for excavation and reuse as structural fill. The pre -Fraser nonglacial sediments are generally considered suitable for support of light to moderately loaded foundations. January 10, 2020 ASSOCIATED EARTH SCIENCES, INC. AWRIMS11d-20190294E001-4 Page 4 Subsurface Exploration, Geologic Hazard, and Maupin—Crouch Residence Geotechnical Engineering Report Edmonds, Washington Project and Site Conditions 4.2 Geologic Map Review Review of the regional geologic maps titled Geologic Map of the Edmonds East and Part of the Edmonds West Quadrangles, by J.P. Minard (1983) and Composite Geologic Map of Sno-King Area, by D.B. Booth, B.F. Cox, K.G. Troost, and S.A. Shimel (2004) indicate that the area of the subject site is underlain by Vashon advance outwash deposits (Qva), with pre -Fraser -age deposits mapped downslope of the subject site. Our interpretation of the sediments encountered below the landslide debris is in partial agreement with the geologic units mapped in the area as our borings did not encounter Vashon advance outwash underlying the surficial fill soils but did encounter pre -Fraser age deposits at depth beneath the landslide deposits. 4.3 Hydrology Shallow groundwater at the project site is expected to be present in two zones: a perched groundwater interval in the existing fill soils or landslide deposits, and the unconfined aquifer present in the pre -Fraser sediments. It should be noted that the depth to groundwater and duration or quantity of seepage can vary in response to changes in weather, season, changes in land use, and other factors. • Perched Groundwater: Although not encountered at the time of drilling, perched groundwater within the fill or landslide deposits may be present during the wet season. Perched groundwater occurs when rain or surface water infiltrates through upper, looser, and more permeable soils, such as existing fill and landslide deposits, and becomes trapped on top of siltier intervals located within these deposits. • Pre -Fraser Aquifer: Groundwater was observed near the top of the pre -Fraser nonglacial deposits. We interpret that this groundwater is representative of an unconfined aquifer located within the pre -Fraser sediments. January 10, 2020 ASSOCIATED EARTH SCIENCES, INC. A WRIMS11d - 20190294EO01 -4 Page 5 Subsurface Exploration, Geologic Hazard, and Maupin—Crouch Residence Geotechnical Engineering Report Edmonds, Washington Geologic Hazards and Mitigations 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. 61111 WI-110113311la:r_r�_1:].7_V1.3*:%I ia►:: The following paragraphs discuss the stability of the slopes and recommendations to mitigate risks to the public health, safety, or welfare. It must be understood that no recommendations or engineering design can yield a guarantee of stable slopes. Our observations, findings, and opinions are a means to identify and reduce the inherent risks to the owner. The slopes along the eastern and western portions of this property are in excess of 40 percent with greater than 10 feet of vertical relief and are located in areas of historic failures as determined by the landslide deposits underlying the site. As such, they are considered Landslide Hazard Areas by the City of Edmond's City Code (ECC). The site has been subject to past grading, including the abovementioned "terracing' of the surrounding neighborhood and the placement of fill over the parcel during the original construction of the residence. 5.1 LIDAR Mapping As stated above, we encountered landslide debris in our explorations. Based on our review of the Light Detection and Ranging (LIDAR) image encompassing the subject site, the slopes leading upward from the area of downtown Edmonds to the upland encompassing the subject site include several bowl -shaped slide features, including to the immediate east and south of the subject site. However, based on our review of the conditions observed during our site reconnaissance, it is our opinion that the landslide scarps suggested by the LIDAR image near to the subject site likely originated in the ancient past, possibly during or subsequent to ice retreat at the conclusion of the Vashon Stade of the Fraser Glaciation. 5.2 Slope Reconnaissance During our site reconnaissance and our subsurface explorations, we found no visual evidence of tension cracks, emergent seepage, hummocky topography, or other indications of recent slope instability at the subject site. The eastern slope is vegetated with grass, shrubs, and small trees, and the western slope has been altered recently in association with construction of the residence next door. Alteration to the western slope includes the construction of a relatively January 10, 2020 ASSOCIATED EARTH SCIENCES, INC. A WRIMS11d - 20190294EO01 -4 Page 6 Subsurface Exploration, Geologic Hazard, and Maupin—Crouch Residence Geotechnical Engineering Report Edmonds, Washington Geologic Hazards and Mitigations short rockery approximately 3 to 4 feet in height, near the base, removal of vegetation over the bottom two-thirds of the slope, and the installation of a jute net or similar erosion control product and new landscape plants. 5.3 Slope Stability Analysis An analysis of the stability of the western and eastern slopes under existing conditions was completed using the computer program SLOPE/W, version 8.16.2.14053 by GeoSlope International. The program used the Morgenstern —Price method for evaluating a rotational failure. Input parameters for the analysis included slope geometry, geology, and soil -strength parameters. The slope geometry was based on contours from the provided survey. The profile used for our analysis was located along section line A -A' as depicted on Figure 2. AESI modeled the slopes under existing conditions as project plans have not been completed. The geology of the slopes was based on the subsurface conditions encountered in our explorations and our experience. Our slope profile shows a thicker section of fill along the west property line when compared to the fill thickness encountered in EB-2. This is due to the difficulty of distinguishing the fill from landslide deposits in hollow -stem auger samples and the likely grading practice implemented during original site grading. Because the strength parameters between the fill and landslide debris are similar, defining the depth of the contact between these two soils has little effect on the results of our modeling. Soil strength parameters used for our analysis were assumed based on typical published values for similar materials and our prior experience. The values assumed for our analysis are shown on the attached SLOPE/W profiles. For evaluation of slope stability under seismic conditions, a horizontal ground acceleration of 0.27g was used for our analysis. This value is equivalent to % of the peak horizontal ground acceleration associated with a return period of 2,475 years, in accordance with the 2015 International Building Code (IBC). The factor of safety of a slope is the ratio between the forces that resist sliding to the forces that drive sliding. For example, a factor of safety of 1.0 would indicate a slope where the driving forces and the resisting forces are exactly equal. Increasing factor of safety values greater than 1.0 indicate increased stability. An acceptable factor of safety would depend on the level of risk deemed acceptable by the owner and City of Edmonds. The ECC requires the proposed development does not decrease the factor of safety below 1.5 for static conditions and 1.2 for short-term dynamic conditions. Minimum factors of safety of failure extending down to the existing residence from the east slope or starting at the existing residence for slides occurring on the west are provided in Table 1. The results of our SLOPE/W analysis are provided in Appendix B. January 10, 2020 ASSOCIATED EARTH SCIENCES, INC. A WRIMS11d - 20190294EO01 -4 Page 7 Subsurface Exploration, Geologic Hazard, and Maupin—Crouch Residence Geotechnical Engineering Report Edmonds, Washington Geologic Hazards and Mitigations Table 1 Slope Stability Results Slope Minimum Factor of Safety Static Minimum Factor of Safety Seismic East Slope >1.5 >1.2 West Slope >1.5 1.0 Our slope stability analysis indicates that the western slope exhibits factors of safety lower than 1.2 under seismic conditions for failures that extend under the residence. As discussed later in our "Foundations" section, we propose new foundations are supported by pipe piles that extend into the pre -Fraser sediments underlying the site. 5.4 Landslide Hazard Mitigation As previously mentioned, our slope stability analysis shows that during strong seismic events, slope failure surfaces from the western slope extend under the residence with factors of safety less than 1.2. To mitigate for these low factors of safety, we recommend that new foundations be supported by a pipe pile foundation system. We have provided pipe pile recommendations in our "Foundations," section. By supporting the new foundations on pipe piles, the project will avoid surcharging the west slope with new loads, will not decrease the factors of safety for either the east or west slope under both static and seismic conditions, and will help mitigate the risk to the residence associated with the low factor of safety for the west slope under seismic conditions. Additional recommendations for landslide hazard mitigation are discussed below. Provided the recommendations in this report are followed, it is our opinion that no adverse impact on the steep slope area or the proposed development will result through the elimination of the steep slope buffer for the western or eastern slope. We recommend a building setback of 10 feet measured from the top of the west steep slope. To the extent possible, we recommend that native vegetation be left on the slopes to provide erosion control. At no time should loose fill be pushed over the top of the slopes or soil excavated from the toe area without support by an engineered retaining structure. Uncontrolled fill on slopes or toe excavation may promote landslides or debris flow activity. AESI should review grading plans if grading is desired at the top of, on, or near the toe of the steep slopes. If the project is designed with the recommendations provided in this report, it is our opinion that the proposed project will not a) not increase surface water discharge or sedimentation to January 10, 2020 ASSOCIATED EARTH SCIENCES, INC. A WRIMS11d - 20190294EO01 -4 Page 8 Subsurface Exploration, Geologic Hazard, and Maupin—Crouch Residence Geotechnical Engineering Report Edmonds, Washington Geologic Hazards and Mitigations adjacent properties beyond predevelopment conditions, b) not decrease slope stability on adjacent properties, and c) not adversely impact other critical areas. 6.0 SEISMIC HAZARDS AND MITIGATION Earthquakes occur in the Puget Lowland with great regularity. 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 2001, 6.8-magnitude event; and the 1965, 6.5- magnitude event. The 1949 earthquake appears to have been the largest in this region during recorded history and was centered in the Olympia area. Evaluation of earthquake return rates indicates that an earthquake of the magnitude between 5.5 and 6.0 is likely within a given 20- 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. 6.1 Surficial Ground Rupture The nearest known fault trace to the project site is the South Whidbey Island Fault Zone (SWIFZ) located approximately 4 miles to the northeast. A recent study by the U.S. Geological Survey (USGS) (Sherrod, et al., 2005, Holocene Fault Scarps and Shallow Magnetic Anomalies Along the Southern Whidbey Island Fault Zone Near Woodinville, Washington, Open -File Report 2005-1136, March 2005) indicates that "strong" evidence of prehistoric earthquake activity has been observed along associated fault strands thought to be part of the SWIFZ. The study suggests as many as nine earthquake events along the SWIFZ may have occurred within the last 16,400 years. The recognition of this fault splay is relatively new, and data pertaining to it are limited, with the studies still ongoing. The recurrence interval of movement along this fault system is still unknown, although it is hypothesized to be in excess of 1,000 years. Due to the suspected long recurrence interval, it is our opinion that the potential for damage to the proposed structure by surficial ground rupture is considered to be low. No mitigations other than complying with 2015 IBC seismic design recommendations are recommended. 6.2 Seismically Induced Landslides Our slope stability modeling indicates that slope failure surfaces that extend from the west slope to under the residence are present with factors of safety under 1.2 for seismic conditions. See Section 5.0 for further discussion and mitigation recommendations. January 10, 2020 ASSOCIATED EARTH SCIENCES, INC. A WRIMS11d - 20190294EO01 -4 Page 9 Subsurface Exploration, Geologic Hazard, and Maupin—Crouch Residence Geotechnical Engineering Report Edmonds, Washington Geologic Hazards and Mitigations 6.3 Liquefaction Liquefaction is a condition where loose, saturated, typically sandy soils lose shear strength when subjected to high -intensity cyclic loads, such as those that occur during earthquakes. The resulting reduction in strength can cause differential foundation settlements and slope failures. Loose, saturated sands that cannot dissipate the buildup of pore water pressure are the predominant type of sediments subject to liquefaction. The landslide deposits encountered in our explorations had zones that are considered loose but were unsaturated, as were the fill soils. The pre -Fraser nonglacial deposits were saturated but were encountered as medium dense to dense and are not likely to liquefy as a result. In our opinion, the subsurface conditions observed in our exploration borings represent a low risk of liquefaction during a design -level seismic event. A detailed liquefaction hazard analysis was not performed as part of this study, and none is warranted in our opinion. 6.4 Ground Motion Structural design of the building should follow 2015 IBC standards using Site Class "D" as defined in Table 20.3-1 of American Society of Civil Engineers (ASCE) 7 - Minimum Design Loads for Buildings and Other Structures. 7.0 EROSION HAZARDS AND MITIGATION The sloping portions of the parcel are considered Erosion Hazard Areas based on the definition provided in Section 23.80.020 of the ECC. We anticipate that site construction will be limited to the flat areas of the parcel and the existing slopes will not be altered. However, a properly developed, constructed, and maintained erosion control plan consistent with local standards and best management erosion control practices will be required for this project. It will be necessary to make adjustments and provide additional measures to the Temporary Erosion and Sedimentation Control (TESC) plan in order to improve its effectiveness. Ultimately, the success of the TESC plan depends on a proactive approach to project planning and contractor implementation and maintenance. The erosion hazard of the site soils is low to high, depending primarily on slope and runoff velocity. Maintaining cover measures atop disturbed ground provides the greatest reduction to the potential generation of turbid runoff and sediment transport. During the local wet season (October 15t through March 315t), exposed soil should not remain uncovered for more than 2 days, unless it is actively being worked. Ground -cover measures can include erosion control matting, plastic sheeting, straw mulch, crushed rock or recycled concrete, or mature hydroseed. January 10, 2020 ASSOCIATED EARTH SCIENCES, INC. A WRIMS11d - 20190294EO01 -4 Page 10 Subsurface Exploration, Geologic Hazard, and Maupin—Crouch Residence Geotechnical Engineering Report Edmonds, Washington Geologic Hazards and Mitigations 7.1 Erosion Hazard Mitigation To mitigate the erosion hazards and potential for off -site sediment transport, we recommend the following: 1. All TESC measures for the work area should be installed prior to any activity. 2. During the wetter months of the year (typically October through April), or when large storm events are predicted during the summer months, the work area should be stabilized so that if showers occur, the work area can receive the rainfall without excessive erosion or sediment transport. 3. 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. 4. Under no circumstances should concentrated discharges be allowed to flow over the top of steep slopes. 5. Disturbance to vegetation on steep slope areas should be avoided, if possible. 6. 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 straw bales/silt fences around pile perimeters. We anticipate that if our recommendations for erosion mitigation are followed, the development of the project will not significantly increase the site's risk of erosion. January 10, 2020 ASSOCIATED EARTH SCIENCES, INC. A WRIMS11d - 20190294EO01 -4 Page 11 Subsurface Exploration, Geologic Hazard, and Maupin—Crouch Residence Geotechnical Engineering Report Edmonds, Washington Design Recommendations III. DESIGN RECOMMENDATIONS 8.0 INTRODUCTION Our explorations indicate that, from a geotechnical standpoint, the parcel is suitable for the proposed project provided the risks discussed are accepted and the recommendations contained herein are properly followed. The foundation -bearing stratum is moderately deep, ranging from roughly 13 to 29 feet or more below present surface grade. Due to the depth of the bearing soils, a driven pipe pile -supported foundation is recommended for the residence to mitigate the risk of post -construction settlements. 9.0 SITE PREPARATION Site preparation for the proposed residence should include removal of all trees, brush, debris, and any other deleterious material from the area of the planned building footprint. We recommend that any organic topsoil should be stripped from the entire planned building footprint. The sediments encountered in the exploration borings contained a variable percentage of fine-grained material, which makes them 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. If construction will occur during the wet season, we recommend that the building footprint area be graded smooth and sloped to drain. The building footprint should then be blanketed with a minimum of 6 inches of clean, crushed, 2-inch rock (railroad ballast). Following placement of the crushed rock, a pipe pile foundation may be installed. AESI can provide field design recommendations for these areas, if needed. 9.1 Temporary 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 anticipate that temporary, unsupported cut slopes in unsaturated existing fill or landslide deposits, can be planned at a maximum slope of 1.5H:1V (Horizontal:Vertical). Flatter, temporary cut slopes will be needed if drainage is not installed prior to excavation and groundwater seepage is encountered. As is typical with earthwork operations, some sloughing and raveling may occur, January 10, 2020 ASSOCIATED EARTH SCIENCES, INC. A WRIMS11d - 20190294EO01 -4 Page 12 Subsurface Exploration, Geologic Hazard, and Maupin—Crouch Residence Geotechnical Engineering Report Edmonds, Washington Design Recommendations and cut slopes may have to be adjusted in the field. In addition, WISHA/OSHA regulations should be followed at all times. Permanent, unsupported cut or structural fill slopes should not exceed a gradient of 2H:1V. 10.0 STRUCTURAL FILL Structural fill may be necessary to establish desired grades or to backfill around foundations and utilities. 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. 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, adequate recompaction may be difficult or impossible to obtain and should probably not be attempted. In lieu of recompaction, the area to receive fill should be blanketed with washed rock or quarry spalls to act as a capillary break between the new fill and the wet subgrade. Where the exposed ground remains soft and further overexcavation is impractical, placement of an engineering stabilization fabric may be necessary to prevent contamination of the free -draining layer by silt migration from below. After stripping and subgrade preparation of the exposed ground is approved, or a free -draining rock course is laid, structural fill may be placed to attain desired grades. Structural fill is defined as non -organic soil, acceptable to the geotechnical engineer, placed in maximum 8-inch loose lifts, with each lift being compacted to 95 percent of the modified Proctor maximum density using ASTM D-1557 as the standard. 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 at least 3 business days 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 soils in structural fills should be limited to favorable dry weather conditions. The on -site soils contained variable amounts of silt and are considered moisture -sensitive, and we expect that this material may be difficult to compact to structural fill specifications, particularly during and following wet weather. If proper compaction of the on -site soils cannot be achieved, we recommend that a select, import January 10, 2020 ASSOCIATED EARTH SCIENCES, INC. A WRIMS11d - 20190294EO01 -4 Page 13 Subsurface Exploration, Geologic Hazard, and Maupin—Crouch Residence Geotechnical Engineering Report Edmonds, Washington Design Recommendations material consisting of a clean, free -draining gravel and/or sand 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. A representative from our firm should inspect the stripped subgrade and be present during placement of structural fill to observe the work and perform a representative number of in -place density tests. In this way, the adequacy of the earthwork may be evaluated as filling progresses and any problem areas may be corrected at that time. It is important to understand that taking random compaction tests on a part-time basis will not assure uniformity or acceptable performance of a fill. As such, we are available to aid the owner in developing a suitable monitoring and testing frequency. 11.0 FOUNDATIONS We recommend the use of steel pipe piles to support new foundations. Recommendations for pipe pile foundations are included in this section. For preliminary estimating purposes, pile lengths in the 20- to 40-foot range may be assumed. Actual pile lengths may differ significantly from the estimated range depending on local variations in soil conditions, pile size, and driving equipment used. Pile lengths can best be determined by driving a series of test piles. If the new home is located closer to the western steep slope, then the existing home additional horizontal pile surcharges may be necessary to mitigate seismic slope movement. AESI should be allowed to review the recommendations in this section once project plans have been finalized. 11.1 Pipe Pile Foundations Pipe piles should consist of 3-, 4-, or 6-inch-diameter pipe, depending on the required structural loads. The piles should be galvanized steel pipe, driven with a suitable hydraulic hammer to the refusal criteria shown in Table 2. The following table provides required minimum hammer weights, refusal criteria, and allowable loads for pipe piles. January 10, 2020 ASSOCIATED EARTH SCIENCES, INC. A WRIMS11d - 20190294EO01 -4 Page 14 Subsurface Exploration, Geologic Hazard, and Maupin—Crouch Residence Geotechnical Engineering Report Edmonds, Washington Design Recommendations Table 2 Pipe Pile Design Parameters Allowable Axial Pipe Minimum Hammer Refusal Compressive Diameter Wall Size Criterion* Load** (inches) Thickness (pounds) (seconds) (kips) 3 Schedule 40 400 25 10 4 Schedule 40 650 20 20 6 Schedule 40 1,500 15 30 * Refusal is defined as less than 1 inch of penetration in "X" seconds under constant driving. ** Allowable load to be verified by load tests in accordance with ASTM International (ASTM) D-1143 "quick load test." Anticipated settlement of pile -supported foundations should be less than % inch. Pile installation must be observed by AESI to verify that the design bearing capacity of the piles has been attained and that construction conforms to the recommendations contained herein. The City of Edmonds may also require such inspections. Lateral resistance can be derived from passive soil resistance against the buried portion of the foundation (i.e., the grade beam) or from the installation of batter piles. A passive equivalent fluid of 200 pounds per cubic foot (pcf) can be used to account for lateral resistance. Lateral resistance for batter piles should be taken as the horizontal component of the axial pile load. Batter piles are typically installed at 1H:4V inclination. Pile Inspections The actual total length of each pile may be adjusted in the field based on required capacity and conditions encountered during driving. Since completion of the pile takes place below ground, the judgment and experience of the geotechnical engineer or their field representative must be used as a basis for determining the required penetration and acceptability of each pile. Consequently, use of the presented pile capacities in the design requires that the installation of all piles be observed by a qualified geotechnical engineer or engineering geologist from our firm, who can interpret and collect the installation data and examine the contractor's operations. AESI, acting as the owner's field representative, would determine the required lengths of the piles and keep records of pertinent installation data. A final summary report would then be distributed following completion of pile installation. Load testing should be performed to verify that the design bearing capacity of the piles has been attained. Because of the variation in the soil types and their densities, we recommend that AESI monitor the load testing program. A common pile load testing program would consist of one or more 200-percent verification tests of the design bearing capacity of the pile in the January 10, 2020 ASSOCIATED EARTH SCIENCES, INC. A WRIMS11d - 20190294EO01 -4 Page 15 Subsurface Exploration, Geologic Hazard, and Maupin—Crouch Residence Geotechnical Engineering Report Edmonds, Washington Design Recommendations soil. Verification test piles are usually loaded in 25-percent increments that are held for 2 minutes up to the final load of 200-percent design load. The 200-percent load is commonly held for 20 minutes and creep -measured. The load is then reduced by 25-percent increments to evaluate the effect of elasticity in the pile to overall displacement. 12.0 LATERAL WALL PRESSURES All backfill behind retaining 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 retaining 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 pcf. Fully restrained, horizontally backfilled, rigid walls that cannot yield should be designed for an equivalent fluid of 50 pcf. If roadways, parking areas, or other areas subject to vehicular traffic are adjacent to retaining walls, a surcharge equivalent to 2 feet of soil should be added to the wall height in determining lateral design forces. Retaining walls that retain sloping backfill at a maximum angle of 2H:1V should be designed using an equivalent fluid pressure of 55 pcf for yielding conditions or 75 pcf for fully restrained conditions. In accordance with the 2015 IBC, retaining wall design should include seismic design parameters. Based on the site soils and assumed wall backfill materials, we recommend a seismic surcharge pressure in addition to the equivalent fluid pressures presented above. A rectangular pressure distribution of 5H and 10H pounds per square foot (psf) (where H is the height of the wall in feet) should be included in design for "active" and "at -rest" loading conditions, respectively. The resultant of the rectangular seismic surcharge should be applied at the midpoint of the walls. The lateral pressures presented above are based on the conditions of a uniform horizontal backfill consisting of the on -site, natural, glacial sediments or imported sand and gravel 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 wall. Footing drains must 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 the ground surface using imported, washed gravel against the walls placed to be continuous with the footing drain. January 10, 2020 ASSOCIATED EARTH SCIENCES, INC. A WRIMS11d - 20190294EO01 -4 Page 16 Subsurface Exploration, Geologic Hazard, and Maupin—Crouch Residence Geotechnical Engineering Report Edmonds, Washington Design Recommendations 12.1 Passive Resistance and Friction Factors Retaining wall grade beams/keyways cast directly against undisturbed dense soils in a trench may be designed for passive resistance against lateral translation using an allowable equivalent fluid equal to 200 pcf. The passive equivalent fluid pressure diagram begins at the top of the buried portion of grade beam. Since the structure will be pile -supported, we do not recommend using base friction for resistance to lateral loads. 13.0 FLOOR SUPPORT Due to the loose nature of the subgrade soils, we recommend that structural support be provided for settlement -sensitive, slab -on -grade floors. The floors should be cast atop a minimum of 4 inches of washed pea gravel or washed crushed rock to act as a capillary break where moisture migration through the slabs is to be controlled. The capillary break material should be overlain by a 10-mil-thick vapor retarder material prior to concrete placement. American Concrete Institute (ACI) recommendations should be followed for all concrete placement. 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, polyvinyl chloride (PVC) pipe surrounded by washed pea gravel. The level of the perforations in the pipe should be set at or slightly below the bottom of the footing grade beam, and the drains should be constructed with sufficient gradient to allow gravity discharge away from the building. In addition, all retaining walls should be lined with a minimum, 12-inch-thick, washed gravel blanket that extends to within 1 foot of the surface and is continuous with the foundation drain. Roof and surface runoff should not discharge into the foundation drain system, but should be handled by a separate, rigid, tightline drain. In planning, exterior grades adjacent to walls should be sloped downward away from the structure to achieve surface drainage. All collected runoff must be tightlined to a City -approved location. 15.0 PROJECT DESIGN AND CONSTRUCTION MONITORING Our recommendations are preliminary in that definite building locations and construction details have not been finalized at the time of this report. We are available to provide additional geotechnical consultation as the project design develops and possibly changes from that upon January 10, 2020 ASSOCIATED EARTH SCIENCES, INC. A WRIMS11d - 20190294EO01 -4 Page 17 Subsurface Exploration, Geologic Hazard, and Maupin—Crouch Residence Geotechnical Engineering Report Edmonds, Washington Design Recommendations which this report is based. If significant changes in grading are made, we recommend that AESI perform a geotechnical review of the plans prior to final design completion. In this way, our earthwork and foundation recommendations may be properly interpreted and implemented in the design. We are also available to provide geotechnical engineering and monitoring services during construction. The integrity of the 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. If these services are desired, please let us know, and we will prepare a proposal. 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 Bruce L. Blyton, P.E / Senior Principal Engineer Anthony W. Romanick Senior Project Engineer Attachments: Figure 1: Vicinity Map Figure 2: Existing Site and Exploration Plan Appendix A: Exploration Logs Appendix B: SLOPE/W Results January 10, 2020 ASSOCIATED EARTH SCIENCES, INC. A WRIMS11d - 20190294EO01-4 Page 18 A A 1 J � ■�Y�J� --r\I I . MM VIEWLAND WAY% nt Snohomish County — 13 R • �' eagal'i r '•f ! �� I _ 9l Iark �rf'�`✓��`�` II a .� ®-- Jr El lw 161 17 .17 99 I. ' UNINCOR-RORATE'D. J •-� Copyright:© 2013 Natidnal Geogra;phlc So�iety, i-cubed a s s o c i a t e d N earth sciences A e incorporated 0 1000 2000 VICINITY MAP FEET DATA SOURCES/REFERENCES: MAUPIN-GROUCH RESIDENCE USGS: 7.5' SERIES TOPOGRAPHIC MAPS, ESRI/1-CUBED/NATIONAL NOTE: BLACK AND WHITE GEOGRAPHIC SOCIETY2013 REPRODUCTION OF THIS COLOR EDMONDS, WASHINGTON SNOHOMISH CO: STREETS, CITY LIMITS, PARCELS, 2/19 ORIGINAL MAY REDUCE ITS EFFECTIVENESS AND LEAD TO PROD NO. DATE: FIGURE: LOCATIONS AND DISTANCES SHOWN ARE APPROXIMATE INCORRECT INTERPRETATION 190294EO01 9/19 1 is a 10 SITE O EXPLORATION BORING CROSS SECTION CONTOUR 10 FT CONTOUR 2 FT DATA SOURCES/REFERENCES: WA STATE LIDAR PORTAL: NORTH PUGET SOUND 2016 CONTOURS FROM LIDAR SNOHOMISH CO: STREETS, PARCELS 1/18 AERIAL: KING CO., PICTOMETRY INT. 2017 LOCATIONS AND DISTANCES SHOWN ARE APPROXIMATE KAI -- r >, ; i a s s o c i a t e d N earth sciences i n c o r p o r a t e d 50 EXISTING SITE AND Feet EXPLORATION PLAN MAUPIN-CROUCH RESIDENCE NOTE: BLACK AND WHITE REPRODUCTION OF THIS COLOR EVERETT, WASHINGTON ORIGINAL MAY REDUCE ITS EFF ECTIVENESS AND LEAD TO PROJ NO. DATE: FIGURE: INCORRECT INTERPRETATION 190294EO01 10/19 2 APPENDIX A Exploration Logs associated Exploration Borin earth sciences Project Number Exploration Number Sheet Incorporated 190294EO01 EB-1 1of1 Project Name Maupin-Crouch Residence Ground Surface Elevation (ft) —238 Location Edmonds, WA Datum NAVD 88 Driller/Equipment Geologic Drill / Mini -Track Date Start/Finish 9/19/1Q 9/1 /19 Hammer Weight/Drop 140# / 30 Hole Diameter (in) F E cL c a) n � J U) 3 Blows/Foot w d) a S 12 �, 0 u) o o T in DESCRIPTION " m m r ° 10 20 30 40 Grass / Topsoil - 4 inches S-1 2 2 $ Fill Slightly moist, light brown to gray with minor oxidation in upper 6 inches, 21 very silty, fine SAND, some medium sand, some gravel; minor organics; no S-2 :. apparent structure (SM). 10 15 s Moist, brownish gray, very silty, fine SAND, trace gravel; minor mottling; 13 broken gravel in sampler, blowcounts likely overstated; low recovery; no 5 apparent structure (SM). 3 S 3 : .. Gravelly drilling at 4 feet -1 5 9 Landslide Deposits 4 Moist to very moist, gray to dark gray with zones of moderate oxidation, very silty, fine to medium SAND, some gravel; layers of organics and woody debris; disturbed texture (SM). 10 S-4 Very moist to wet, light brownish gray to brown with bands of minor to 3 A13 moderate oxidation throughout, very silty, fine to medium SAND, some 5 gravel; disturbed texture (SM). 8 --———————————————————————— — — — — —— X Pre -Fraser Nonglacial Deposits Water on rods at -13 feet. 15 S-5 Wet, gray to brownish gray, fine SAND, trace to some silt, trace gravel; 5 A massive; minor mica flakes (SP/SP-SM). 8 19 11 20 S 6 Wet, gray, fine SAND, trace to some silt, trace gravel; minor to moderate 11 mica; massive (SP-SM/SP). 18 7 29 25 Wet, gray, fine to medium SAND, trace to some silt, trace to some gravel; S-7 _ minor to moderate mica content; some faint angular stratification 12 (SP-SM/SP). 23 58 35 Bottom of exploration boring at 26.5 feet Groundwater encountered at -13 feet. 30 35 40 Sampler Type (ST): m 2" OD Split Spoon Sampler (SPT) ❑ No Recovery M - Moisture Logged by: TG m 3" OD Split Spoon Sampler (D & M) Ring Sample Q Water Level Q Approved by: JHS ® Grab Sample 0 Shelby Tube Sample 1 Water Level at time of drilling (ATD) ass 0cl a t e d Exploration Boring earth sciences Project Number Exploration Number Sheet incorporated 190294EO01 EB-2 1 of Project Name Maupin-Crouch Residence Ground Surface Elevation (ft) —237 Location Edmonds, WA Datum NAVI71RR Driller/Equipment Geologic Drill / Mini -Track Date Start/Finish 9/19/1 A 9/19/1 A Hammer Weight/Drop 140# / 30 Hole Diameter (in) F m C U O 0 c O � a) > J °' Zo N Blows/Foot Y U a) S m> >N >E�5o p T � (D rn DESCRIPTION o C) @ 3: m ° 10 20 30 40 Topsoil - -4 inches S-1 _ - _- 1 10 20 30 Fill Slightly moist, light brownish gray with minor oxidation, very silty, fine to = medium SAND, some gravel; minor organics and rootlets; no apparent S-2 -=--= = structure (SM). 11 10 A 19 - Slightly moist to moist, light brownish gray with minor oxidation, very silty, 9 9 fine to medium SAND, some gravel; no apparent structure (SM). 5 -- = - - `Gravelly drilling at 4 feet. -J S-3 - - Landslide Deposits q _ Moist, brownish gray to dark gray with minor oxidation, silty, fine to medium 4 _ SAND, trace gravel; minor wood debris; minor organics; disturbed texture (SM). 10 S-4 = _= - Moist to very moist, dark gray to brownish gray with zones of minor to 3 Al _ moderate oxidation, silty, fine to medium SAND, some gravel; disturbed 5 _- -- = texture; minor organics (SM). 10 15 S 5 Very moist, dark gray, very silty, fine to medium SAND, some gravel; s -= - = disturbed texture; minor organics and organic odor (SM). 2 5 20 S 6 As above; contains woody debris. 5 - _ _- 6 11 _- - 5 Driller reports gravelly drilling at 24 feet. 25 S"� = _ Very moist to wet, gray to dark gray, silty, fine to medium SAND, some 10 gravel; minor organics; pockets of sandy silt; disturbed texture (SM). 10 20 10 : Water on rods at 29 feet. T 30 Pre -Fraser Nonglacial Deposits S-g Wet, dark gray, gravelly, fine to coarse SAND, some silt; quartz 4 A44 fragments?; moderately stratified (SP-SM). 11 33 35 S-g Wet, dark gray, sandy, SILT, trace gravel; minor mica content; single 8 A3 interbed (--3 inches thick) of silty, fine to medium sand (SM); minor fine 14 organics within sandy silt; organic odor; faintly laminated (ML). 22 40 S-10 Wet, dark gray, fine sandy, SILT, trace medium sand; minor to moderate 10 mica content; faint laminations (ML). 16 a1 25 Bottom of exploration boring at 41.5 feet Groundwater encountered at -29 feet. Sampler Type (ST): m 2" OD Split Spoon Sampler (SPT) ❑ No Recovery M - Moisture Logged by: TG ® 3" OD Split Spoon Sampler (D & M) Ring Sample �Z Water Level () Approved by: JHS IN Grab Sample Shelby Tube Sample 1 Water Level at time of drilling (ATD) APPENDIX 6 SLOPE/W Results W 61 Color Name Unit Cohesion' Phi' Piezometric Weight (psf) (') Line (Pcf) ■ Fill 120 50 33 1 Landslide 120 50 31 1 Deposits Pre -Fraser 130 100 36 1 Sediments 1.2 Maupin-Crouch Residence 190294EO01 Slope Profile Seismic V� 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200 A Distance (ft.) A' W 61 Color Name Unit Cohesion' Phi' Piezometric Weight (psf) (') Line (Pcf) ■ Fill 120 50 33 1 Landslide 120 50 31 1 Deposits ❑Pre -Fraser 130 100 36 1 Sediments 1.0 Maupin-Crouch Residence 190294EO01 Slope Profile Seismic V� 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200 A Distance (ft.) A' W 61 Color Name Unit Cohesion' Phi' Piezometric Weight (psf) (') Line (Pcf) ■ Fill 120 50 33 1 Landslide 120 50 31 1 Deposits Pre -Fraser 130 100 36 1 Sediments 1.9 Maupin-Crouch Residence 190294EO01 Slope Profile Static V� 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200 A Distance (ft.) A' W 61 Color Name Unit Cohesion' Phi' Piezometric Weight (psf) (') Line (Pcf) ■ Fill 120 50 33 1 Landslide 120 50 31 1 Deposits ❑Pre -Fraser 130 100 36 1 Sediments 1.5 Maupin-Crouch Residence 190294EO01 Slope Profile Static V� 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180 185 190 195 200 A Distance (ft.) A'