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REVIEWED RESUB1 BLD2022-0439+Geotechnical_Report+11.8.2022_10.16.34_AM+3208826RESUB O E O TE C H Nov CITY F E 2ONDS BLD2022-0439 CONSULTANTS, INC_ DEVELOPMENT SERVICES DEPARTMENT •-------------- Reviewed by City of Edmonds ; Nathan and Callie Angle Planning Division --------------- 5194 South Spencer Street ....... ....... Seattle, Washington 98118 = "'REviEwEo" BY via email: nxangle email. com; calllehllpert(a)gmall. com CITY OF EDMONDS BUILDING DEPARTMENT! Subject: Geotechnical Engineering Study Proposed Residential Project 15724 — 72nd Avenue West Edmonds, Washington Greetings: 2401 1 Oth Ave E Seattle, Washington 98102 (425)747-5618 October 3, 2022 JN 21303 Attached to this transmittal letter is our geotechnical engineering report for the residential project to be constructed in Edmonds, Washington. The scope of our services consisted of exploring site surface and subsurface conditions, and then developing this report to provide recommendations for general earthwork and design considerations for foundations, retaining walls, subsurface drainage, and temporary excavations. This work was authorized by your acceptance of our proposal dated August 22, 2022. The first portion of this study is more of a standard geotechnical engineering report that discussed numerous geotechnical engineering issues, conclusions, and recommendations. The latter portion of this report provides direct responses to needed criteria in Edmonds Code (ECDC) that are contained in Chapter 19.10 and 23.08. This report also includes a discussion regarding Critical Areas as noted in Chapter 23. The attached report contains a discussion of the study and our recommendations, as well as comments to the geotechnical engineering information and Critical Area information as noted above. Please contact us if there are any questions regarding this report, or for further assistance during the design and construction phases of this project. cc: Cast Architecture — Forrest Murphy via email: forrest(a)-castarchitecture.com TAJ/DRW:kg Respectfully submitted, GEOTECH CONSULTANTS, INC. D. Robert Ward, P.E. Principal GEOTECH CONSULTANTS, INC. Angle October 3, 2022 GEOTECHNICAL ENGINEERING STUDY Proposed Residential Project 15724 — 72nd Avenue West Edmonds, Washington JN 21303 Page 3 This report presents the findings and recommendations of our geotechnical engineering study for the site of the proposed residential project to be located in Edmonds. This residential lot is located within a 5-lot short plat named Meadowdale Estates, which was established around 2015. This residential lot is located on the southern end of Meadowdale Estates. Based on Sheets A0.1 through A.16, which were prepared by Cast Architecture dated March 25, 2022, we understand that the project consists of a residence and detached garage. A driveway will extend off the northeastern corner of the site through an easement and extend west/southwesterly down to the detached garage, which will be located on east -central portion of the site. The grade of the garage floor will be elevation 346 feet, although the garage will have basement below it whose grade will be at elevation 235 feet. The maximum excavation of approximately 11 feet is needed at the eastern corner of the garage; the eastern property line is approximately 50 feet from this maximum excavation location. The separate residence will be located to the west/northwest of the garage; these two structures will be connected by a covered walkway. The residence will have a main floor of elevation 235 feet, and most of the it will have a lower storage space at elevation 230 feet. With a proposed excavation level of approximately elevation 228 feet, excavations of about 3 to 6 feet are needed. The southern side of the residence is well away from the southern property line, while the northern side of approximately 12 feet from the northern property line. The western side of the residence will be located 65 feet from a steep western slope, as per the original design of the One residence that is under construction is located adjacent to the north of the subject site. It has a footing foundation. Where the adjacent residence is located nearest the proposed site residence, excavations are about 3 to 4 feet below the ground surface in order to reach competent bearing soil for its footings; this correlates to approximately elevations ranging from about elevation 225 feet to the west and elevation 231 to the east. The foundations of the two residences will be at least 20 feet from each other, and the excavation grades are within about 3 feet of each other as the excavation/footing level for the proposed residence that is near the adjacent residence is mostly about elevation 228 feet, but steps up to elevation 231 feet on the northeastern corner of the residence. This information is shown on Sheet A.1 of the architectural Ip ans. The project civil engineering/drainage plans indicate that stormwater from the project will connect to an existing stormwater system that connects to several residences to the north/northwest. The civil engineer/sewer plans for the project indicate that sewage from this project will be pumped up to the east where a sewer system is located in 72nd Avenue West. If the scope of the project changes from what we have described above, we should be provided with revised plans in order to determine if modifications to the recommendations and conclusions of this report are warranted. GEOTECH CONSULTANTS, INC. Angle October 3, 2022 SITE CONDITIONS SURFACE JN 21303 Page 4 The Vicinity Map, Plate 1, illustrates the general location of the site in Edmonds. It is located on the downslope, western side of 72nd Avenue. It is the southern lot of a 5-lot short -plat that was approved in about 2016, although we began our work on the short -plat in 2013. A significant feature west of the short -plat property, as well as a subject site, is a very steep slope (approximately 75 percent inclination) that is about 80 to 100 feet tall. This slope is about 100 feet tall on the western side of the subject site. Residential properties that decline gently to moderately to the west are located at the base of this western slope. The City of Edmonds has mapped the western side of the short -plat property, which includes the subject site, as being within an area known as the North Edmonds Earth Subsidence and Landslide Hazard Area (NEESLH). A map of this hazard area is attached in Appendix A of this report as is required per Edmonds Code 19.10. The steep western slope is also considered a Landslide Hazard for Edmonds code (ECDC) 23.80.020 No other landslide hazard exist on the site in our opinion based on ECDC 23.80.020. The four-sided, somewhat rectangular project site declines westward from 72nd Avenue West. It has approximately 120 feet of frontage along the street right-of-way and an average length of about 360 feet. The steep western slope noted above occupies about one-third of the property. The middle third of the property is somewhat flat to gently inclined up the east above the steep slope, while the eastern, approximate third of the property declines moderately downward from the street right-of- way, mostly in the range of about 25 to 35 percent, to the flat middle third. All portions of the site that have an inclination of 15 percent or higher is considered an Erosion Hazard Area per the ECDC 23.80.020 because of the soil conditions. The property is undeveloped, although it appears that a soil driveway may have extended from the street into the flatter portion of the property in the past. The site is mostly forested with numerous evergreen and deciduous trees, as well as a native underbrush. Several large evergreen trees are located in the flatter area near the top of the steep western slope. The steep western slope is also forested with evergreen and deciduous trees, although the trees appear to be more moderately sized. We have included a site plan of the property as Plate 2, and also a cross-section of site as Plate 3. As noted in the previous section of this report, a residence is being constructed on the adjacent northern property. Excavation depths for that adjacent residence were noted. The property adjacent to the south of the eastern portion of the subject site is undeveloped and forested. These properties have similar Hazard Areas on them similar to the subject site, with the steep slope being a Landslide Hazard Area on their western end and Erosion Hazard Areas where the slope of the lots is greater than 15 percent. Developed residential properties are located east of 72nd Avenue West directly upslope of the subject site. Based on information in Edmond's GIS system, the slope of the properties located with about 300 feet of the subject site have a slope in the range of 15 to 20 percent. Thus, it appears that the properties are considered an Erosion Hazard Area per the ECDC. Landslide information An approximately 40-foot-wide, mostly 3- to 4-foot-deep, surficial landslide occurred on the very steep western slope about 125 feet north of the project site on or just prior to January 1, 1997. The landslide followed a short period of extremely high precipitation when a large amount of snow fell and stuck on the ground in late December, then a large, warm rainstorm fell on or just prior to January 1. The rainstorm event may not have been a 100-year storm, but with the GEOTECH CONSULTANTS, INC. Angle October 3, 2022 JN 21303 Page 5 accumulation of the snow that was on the ground before the warm rain occurred, we believe that the cumulation of precipitation that reached the ground at that time exceeded a 100-year storm. A very significant number of landslides occurred on slopes in the Puget Sound around January 1, 1997. At the time of this landslide, no formal drainage system existed on 72nd Avenue, and it appears that mostly surface water from the street and the area "funneled" down what appeared to be a soil driveway that ended at the top of the slope where the landslide occurred. The landslide area was repaired by some grading at the top of the slope and installing a subsurface drainage system upslope and south to control potential surface and subsurface water in the area upgradient of the presumed water path (from the southeast soil along the previous soil driveway area). In addition, a surface -mounted stormwater pipe now extends over the slope and through a neighboring property to the west to discharge this surface and subsurface water, well west of the slope and subject property. The drainage system installed in 1997 is now a significant part of the control of water from the subdivision, including the subject site. In addition to these drainage improvements done for the short -plat, including the subject site, a formal stormwater system has been constructed in the 72nd Avenue West since 1997. Apparently, the water from this system also discharges water away from the steep western slope south of the subject site. We have made many visits to the site in the last 10 years, and we have not observed indications of instability in the previous slide area, nor the western slope that exists along the entire short -plat, including the subject site. As noted above, the City of Edmonds has mapped the western side of the subject property as being within an area known as the North Edmonds Earth Subsidence and Landslide Hazard Area (NEESLH). As noted earlier, a map of this Area has been attached in the Appendix of this report. This area has been extensively studied, and multiple geotechnical reports have been published, with the most recent being the Area Summary Report published by Landau Associates, dated March 14, 2007. This report describes the overall area as being a large historic/prehistoric landslide that includes a massive downset block of land. Large-scale landsliding of the area has been recorded as occurring in the 1940s and 1950s that reportedly destroyed approximately 6 homes and damaged many others. The historical Dames and Moore (1968) report indicates that the areas of large scale sliding occurred on the lower (western) portion of the overall slide mass to the north and south of the termination of North Meadowdale Drive. As such, the Landau report describes the zone just beneath the steep western slope as being susceptible to the following risks: 1) reactivation of landslide debris (the slide complex) causing ground failure and movement, 2) encroaching landslide debris originating from shallow failures occurring upslope, and 3) landsliding occurring in ground that has not previously failed. The latter risk is the one that pertains to the subject site because the area upslope and east of the steep western slope does not appear to have failed or had slope movement. The 2007 Landau report indicated that the subject site would be considered a "Zone D" area. In 1979, Roger Lowe and Associates report for the overall area indicates that the subject site would have a varying probability of being affected by the various types of landsliding described above. A follow up report by GeoEngineers (1985) detailed the reduction slide probability associated with lowering of groundwater levels in the slide complex following installation of subsurface drainage facilities (LID circa 1984) in the eastern portion of the slide mass. Based on Figure 1 of the 1985 report, the western edge of the property (the steep slope) has a 2 percent probability of sliding (due to "debris slides") in a 25-year period. However, as we have noted above, several surface and subsurface drainage features have been added to the site and the adjacent street since 1997; these features reduce the probability of sliding. Therefore, we strongly believe that, based on the analysis done for the 1985 report, the probability of sliding in a 25-year period is no more than 2 percent. GEOTECH CONSULTANTS, INC. Angle October 3, 2022 SUBSURFACE JN 21303 Page 6 Subsurface conditions directly on the site were explored with two test pits excavated in 2013 and we recently explored the site by drilling a test boring in September 2022, at the approximate locations shown on the Site Exploration Plan, Plate 2. However, other test pits and test borings were performed in the Meadowdale Estates, including two that were excavated on the site in 2013. Our exploration program was based on the proposed construction, anticipated subsurface conditions and those encountered during exploration, and the scope of work outlined in our recent proposal. The test boring was drilled on September 20, 2022 using a track -mounted, hollow -stem auger drill. Samples were taken at approximate 2.5- to 5-foot intervals with a standard penetration sampler. This split -spoon sampler, which has a 2-inch outside diameter, is driven into the soil with a 140- pound hammer falling 30 inches. The number of blows required to advance the sampler a given distance is an indication of the soil density or consistency. A geotechnical engineer from our staff observed the drilling process, logged the test borings, and obtained representative samples of the soil encountered. The September 2022 Test Boring Log is attached as Plates 4 and 5. The previous test pits are attached as Plate 6. Soil Conditions The soils revealed in the test pits and down to a depth of approximately 52 feet in the test boring consists mostly of sand with varying amounts of silt. One layer of sandy silt was revealed at a depth of approximately 20 feet in the test boring. The upper soil was loose, becoming medium -dense to dense in the range of 3 to 5 feet. At a depth of approximately 7 feet, the soil became dense and continued down to approximately 24 feet. The average blow counts from the test boring between 7 and 24 feet is approximately 36. Below 24 feet, the soil became dense to very dense, with the average blow counts of approximately 60 down to the maximum explored depth of approximately 52 feet. Groundwater Conditions Groundwater seepage was observed in the test pits at depths of approximately 14 to 18 feet and 50 feet. It should be noted that groundwater levels vary seasonally with rainfall and other factors. The stratification lines on the logs represent the approximate boundaries between soil types at the exploration locations. The actual transition between soil types may be gradual, and subsurface conditions can vary between exploration locations. The logs provide specific subsurface information only at the locations tested. Where a transition in soil type occurred between samples in the test borings, the depth of the transition was interpreted. The relative densities and moisture descriptions indicated on the test pit and test boring logs are interpretive descriptions based on the conditions observed during excavation and drilling. SEISMIC CONSIDERATIONS In accordance with the International Building Code (IBC), the site class within 100 feet of the ground surface is best represented by Site Class Type D (Stiff Soil). As noted in the USGS website, the mapped spectral acceleration value for a 0.2 second (Ss) and 1.0 second period (Si) equals 1.3g and 0.47g, respectively. GEOTECH CONSULTANTS, INC. Angle J N 21303 October 3, 2022 Page 7 The IBC and ASCE 7 require that the potential for liquefaction (soil strength loss) during an earthquake be evaluated for the peak ground acceleration of the Maximum Considered Earthquake (MCE), which has a probability of occurring once in 2,475 years (2 percent probability of occurring in a 50-year period). The MCE peak ground acceleration adjusted for site class effects (FPGA) equals 0.63g. The soils beneath the site are not susceptible seismic liquefaction under the ground motions of the MCE because of their dense nature (and/or) the absence of near -surface groundwater. SLOPE STABILITY ANALYSIS We conducted slope stability analyses on the western portion of the site, including existing steep slope, with the assistance of a computer program Slope/W, which is developed by GeoStudio. Slope stability is essentially governed by the topography of the area being analyzed, the delineation of each soil layer, and the geotechnical parameters of each soil layer. The main geotechnical parameters of each soil are the internal angle of friction and the cohesion. Two analyses have been done that include these items, static and dynamic analyses, and a seismic coefficient has been included in the dynamic analyses as noted further in this section. Based mostly on the onsite test boring, we have used three soil layers in the upper 55 feet; the upper soil is loose to medium -dense sand to 7 feet, then from 7 to 24 feet is dense sandy soil, and then very dense sand soil below 24 feet. Although we did not extend the test boring below a depth of 52 feet, we believe that a very stiff/hard silt soil is situated below the sand soils. This is based on information provided in the 2007 Landau report, and our understanding of soils in the vicinity of the Puget Sound (which is located about 500 feet west of the subject site), and because wet soil was revealed in the test boring at approximately 51 feet. We have included the cross-section used in the analyses, which essentially extends east -west through the center of the property, in Appendix B of this report. We have included all of the soil parameters (density, internal angle of friction, cohesion) for each of the four soil layers in the cross-section. Typically for slope stability, the peak ground acceleration is equal to 2/3rds of the MCE; as noted above, the MCE is a very low probability earthquake event with a theoretical 2% potential of occurring in 50 years, or a one-in-2,475-years event. However, there is an alternate methodology for determining the peak ground acceleration as presented in the LRFD Seismic Analysis and Design of Transportation Geotechnical Features and Structural Foundations Reference Manual (Aug. 2011) by the Federal Highway Administration in conjunction with the National Highway Institute (NHI). This methodology indicates that peak ground acceleration used in a slope stability analysis can be reduced due to the effects of wave scatter at and near steep slopes. Based on the analysis using this methodology, the resulting reduced horizontal force due to peak seismic conditions is 0.24g; the reference and calculation for peak ground acceleration is attached to this report Appendix A. Therefore, the coefficient used in the seismic slope analysis is 0.12g, which is one-half the peak seismic force (as it typical for seismic slope stability analyses). Safety factors of 1.5 and 1.2 are required for slope stability under static and dynamic conditions, respectively. The results of our slope stability analysis are attached to this report as Appendix B. We performed the slope stability analyses for safety factors that meet or exceed 1.5 and 1.2. The analyses indicated that a setback of about 90 feet from the top of the steep western slope would be needed for the western side of the residence to be placed on a shallow footing. Because the previously establish building setback from the steep western slope for the Meadowdale Estates subdivision is 65 feet, the analysis indicates that a retaining wall whose design height is 15 feet is needed to stabilize the western side of the residence at the 65-foot setback. GEOTECH CONSULTANTS, INC. Angle October 3, 2022 CONCLUSIONS AND RECOMMENDATIONS GENERAL JN 21303 Page 8 THIS SECTION CONTAINS A SUMMARY OF OUR STUDY AND FINDINGS FOR THE PURPOSES OF A GENERAL OVERVIEW ONLY. MORE SPECIFIC RECOMMENDATIONS AND CONCLUSIONS ARE CONTAINED IN THE REMAINDER OF THIS REPORT. ANY PARTY RELYING ON THIS REPORT SHOULD READ THE ENTIRE DOCUMENT. The previous test pits and recent test boring conducted on this site encountered competent, medium -dense to dense sand soil at depths of approximately 3 to 5 feet. The soil became denser with depth. It is our opinion that the proposed residence can be supported on the conventional footing foundation provided it bears on the competent soil. Some minor overexcavation may be needed to reach the bearing soil. As noted earlier, the steep slope on the western side of the subject site is considered a Landslide Hazard Area per the ECDC, as well as North Edmonds Earth Subsidence and Landslide Hazard Area (NEESLH). This steep slope extends along the western side of many adjacent properties to the north and south. No other Landslide Hazard Areas exist on or near the site with the exception of this steep western slope. We believe that the 25-foot-buffer adjacent to the western slope and a 65- foot building setback are suitable for this project (as has been used for the remaining Meadowdale Estates lots that contain the steep western slope). However, as noted above, a retaining wall, which we term a stabilization wall, is now needed on the western side of the residence at the 65-foot setback to provide adequate stability for the residence. The plans indicate that no vegetation will be removed in the buffer where most of the large trees on the site are located. and only a few trees will be removed in the 40 feet between the buffer and the building setback. This is very suitable in our opinion. The default buffer is up to 100 feet for this site, but we believe that, because large trees located in the buffer will not be used, a stabilization wall is proposed on the western edge of the residence, and because all stormwater from the project will be placed into an existing stormwater system (so as not to be directed to the steep western slope), it is our opinion that an alteration for the 25-foot buffer and 65-foot building setback in lieu of the default buffer of up to 100 feet is very suitable for this project. Where the subject site is steeper than 15 percent, it is considered an Erosion Hazard Area per the ECDC because of soil conditions. This also applies to the properties adjacent to the north, south and east. We point out that, where there is not disturbance to these properties, and/or they are landscaped or hardscaped, erosion is not an issue. Erosion only occurs where vegetation has removed and/or disturbance/excavation is done to the existing ground; it is a temporary issue. Once a project is completed and landscaping/hardscaping is installed, then the site will not have an erosion issue. The only site nearby that is disturbed currently is adjacent to the north, and it has extensive erosion control measures that appear to be adequately controlling erosion. All sites, including the subject site, will have erosion control measures during construction to ensure erosion doesn't occur and/or affect any neighboring properties. It is our opinion that potential erosion issues for this project can readily be mitigated by the erosion control features noted in the civil plans of the project. No setbacks or buffers are needed where the site is an Erosion Hazard Area. One erosion control feature is covered stockpiles. We have recently corresponded with the project structural engineer and indicated that the side slopes of the stockpiles should be inclined no steeper than approximately 1.5:1 (H:V). The drainage and/or waterproofing recommendations presented in this report are intended only to prevent active seepage from flowing through concrete walls or slabs. Even in the absence of active GEOTECH CONSULTANTS, INC. Angle J N 21303 October 3, 2022 Page 9 seepage into and beneath structures, water vapor can migrate through walls, slabs, and floors from the surrounding soil, and can even be transmitted from slabs and foundation walls due to the concrete curing process. Water vapor also results from occupant uses, such as cooking, cleaning, and bathing. Excessive water vapor trapped within structures can result in a variety of undesirable conditions, including, but not limited to, moisture problems with flooring systems, excessively moist air within occupied areas, and the growth of molds, fungi, and other biological organisms that may be harmful to the health of the occupants. The designer or architect must consider the potential vapor sources and likely occupant uses, and provide sufficient ventilation, either passive or mechanical, to prevent a build up of excessive water vapor within the planned structure. Geotech Consultants, Inc. should be allowed to review the final development plans to verify that the recommendations presented in this report are adequately addressed in the design. Such a plan review would be additional work beyond the current scope of work for this study, and it may include revisions to our recommendations to accommodate site, development, and geotechnical constraints that become more evident during the review process. We recommend including this report, in its entirety, in the project contract documents. This report should also be provided to any future property owners so they will be aware of our findings and recommendations. STABILIZATION WALL As noted earlier, a 15-foot-design-height wall is needed at the western edge of the residence. It should consist of cantilevered, closely -spaced, drilled -concrete piles. Recommendations for the design and construction of this wall are as follows: The concrete piles should be spaced no more than 3 feet edge -to -edge, so that the soil above the theoretical failure plane can arch between the piles if slope instability west of the residence were to occur. The pile wall should be designed for an active soil pressure equal to that pressure exerted by an equivalent fluid with a unit weight of 30 pcf for a total depth of 15 feet beginning at the top of existing ground. This active soil pressure should be assumed to act over the center -to -center pile spacing. An ultimate passive soil pressure equal to that pressure exerted by a fluid with a density of 400 pcf will resist the lateral movement of the soldier piles below the 15-foot depth. For long-term conditions, a safety factor of 1.5 should be applied to the passive soil pressure for the design. The minimum pile length should be 30 feet. The piles would be constructed by setting steel H-beams or rebar cages in drilled holes and grouting the spaces between the steel reinforcements and the soil with concrete for the entire height of each drilled hole. We anticipate that the holes could be drilled without casing, but the contractor should be prepared to case the holes or use the slurry method if caving soil is encountered. Excessive ground loss in the drilled holes must be avoided to reduce the potential for settlement of adjacent structures. If water is present in a hole at the time a soldier pile is poured, concrete must be tremied to the bottom of the hole. CONVENTIONAL FOOTING FOUNDATIONS The proposed structure can be supported on conventional continuous and spread footings bearing on undisturbed, medium -dense to dense, native sand soil. We recommend that continuous and individual spread footings have minimum widths of 16 and 24 inches, respectively. Exterior footings GEOTECH CONSULTANTS, INC. Angle J N 21303 October 3, 2022 Page 10 should also be bottomed at least 18 inches below the lowest adjacent finish ground surface for protection against frost and erosion. The local building codes should be reviewed to determine if different footing widths or embedment depths are required. Footing subgrades must be cleaned of loose or disturbed soil prior to pouring concrete. Depending upon site and equipment constraints, this may require removing the disturbed soil by hand. An allowable bearing pressure of 3,000 pounds per square foot (psf) is appropriate for footings supported on competent native sand soil. A one-third increase in this design bearing pressure may be used when considering short-term wind or seismic loads. For the above design criteria, it is anticipated that the total post -construction settlement of footings founded on competent native soil will be about one-half inch, with differential settlements on the order of one-half inch in a distance of 50 feet along a continuous footing with a uniform load. Lateral loads due to wind or seismic forces may be resisted by friction between the foundation and the bearing soil, or by passive earth pressure acting on the vertical, embedded portions of the foundation. For the latter condition, the foundation must be either poured directly against relatively level, undisturbed soil or be surrounded by. level, well -compacted fill. We recommend using the following ultimate values for the foundation's resistance to lateral loading: PARAMETER ULTIMATE VALUE Coefficient of Friction 0.50 Passive Earth Pressure 300 pcf Where: pcf is Pounds per Cubic Foot, and Passive Earth Pressure is computed using the Equivalent Fluid Density. If the ground in front of a foundation is loose or sloping, the passive earth pressure given above will not be appropriate. We recommend maintaining a safety factor of at least 1.5 for the foundation's resistance to lateral loading, when using the above ultimate values. CONVENTIONAL FOUNDATION AND RETAINING WALLS Retaining walls backfilled on only one side should be designed to resist the lateral earth pressures imposed by the soil they retain. The following recommended parameters are for walls that restrain level backfill: PARAMETER Active Earth Pressure * 35 pcf Passive Earth Pressure 300 pcf Coefficient of Friction 0.50 Soil Unit Weight 130 pcf Where: pcf is Pounds per Cubic Foot, and Active and Passive Earth Pressures are computed using the Equivalent Fluid Pressures. * For a restrained wall that cannot deflect at least 0.002 times its height, a uniform lateral pressure equal to 10 psf times the height of the wall should be added to the above active equivalent fluid pressure. GEOTECH CONSULTANTS, INC. Angle J N 21303 October 3, 2022 Page 11 The design values given above do not include the effects of any hydrostatic pressures behind the walls and assume that no surcharges, such as those caused by slopes, vehicles, or adjacent foundations will be exerted on the walls. If these conditions exist, those pressures should be added to the above lateral soil pressures. Where sloping backfill is desired behind the walls, we will need to be given the wall dimensions and the slope of the backfill in order to provide the appropriate design earth pressures. The surcharge due to traffic loads behind a wall can typically be accounted for by adding a uniform pressure equal to 2 feet multiplied by the above active fluid density. Heavy construction equipment should not be operated behind retaining and foundation walls within a distance equal to the height of a wall, unless the walls are designed for the additional lateral pressures resulting from the equipment. The values given above are to be used to design only permanent foundation and retaining walls that are to be backfilled, such as conventional walls constructed of reinforced concrete or masonry. It is not appropriate to use the above earth pressures and soil unit weight to back -calculate soil strength parameters for design of other types of retaining walls, such as soldier pile, reinforced earth, modular or soil nail walls. We can assist with design of these types of walls, if desired. The passive pressure given is appropriate only for a shear key poured directly against undisturbed native soil, or for the depth of level, well -compacted fill placed in front of a retaining or foundation wall. The values for friction and passive resistance are ultimate values and do not include a safety Restrained wall soil parameters should be utilized for a distance of 1.5 times the wall height from corners or bends in the walls. This is intended to reduce the amount of cracking that can occur where a wall is restrained by a corner. Wall Pressures Due to Seismic Forces The surcharge wall loads that could be imposed by the design earthquake can be modeled by adding a uniform lateral pressure to the above -recommended active pressure. The recommended surcharge pressure is 8H pounds per square foot (psf), where H is the design retention height of the wall. Using this increased pressure, the safety factor against sliding and overturning can be reduced to 1.2 for the seismic analysis. Retaining Wall Back/ill and Waterproofing Backfill placed behind retaining or foundation walls should be coarse, free -draining structural fill containing no organics. This backfill should contain no more than 5 percent silt or clay particles and have no gravel greater than 4 inches in diameter. The percentage of particles passing the No. 4 sieve should be between 25 and 70 percent. If the native sand is used as backfill, a drainage composite similar to Miradrain 6000 should be placed against the backfilled retaining walls. The drainage composites should be hydraulically connected to the foundation drain system. The later section entitled Drainage Considerations should also be reviewed for recommendations related to subsurface drainage behind foundation and retaining walls. The purpose of these backfill requirements is to ensure that the design criteria for a retaining wall are not exceeded because of a build-up of hydrostatic pressure behind the wall. Also, subsurface drainage systems are not intended to handle large volumes of water from surface runoff. The top 12 to 18 inches of the backfill should consist of a compacted, relatively impermeable soil or topsoil, or the surface should be paved. The ground surface must also slope away from backfilled walls to reduce the potential for surface water to percolate into the backfill. Water percolating through pervious surfaces (pavers, gravel, permeable pavement, etc.) must also be prevented from flowing toward walls or into the backfill zone. The compacted subgrade below pervious surfaces and any associated drainage layer should therefore be sloped away. Alternatively, a membrane and subsurface GEOTECH CONSULTANTS, INC. Angle October 3, 2022 JN 21303 Page 12 collection system could be provided below a pervious surface. It is critical that the wall backfill be placed in lifts and be properly compacted, in order for the above -recommended design earth pressures to be appropriate. The wall design criteria assume that the backfill will be well -compacted in lifts no thicker than 12 inches. The compaction of backfill near the walls should be accomplished with hand -operated equipment to prevent the walls from being overloaded by the higher soil forces that occur during compaction. The section entitled General Earthwork and Structural Fill contains additional recommendations regarding the placement and compaction of structural fill behind retaining and foundation walls. The above recommendations are not intended to waterproof below -grade walls, or to prevent the formation of mold, mildew or fungi in interior spaces. Over time, the performance of subsurface drainage systems can degrade, subsurface groundwater flow patterns can change, and utilities can break or develop leaks. Therefore, waterproofing should be provided where future seepage through the walls is not acceptable. This typically includes limiting cold -joints and wall penetrations, and using bentonite panels or membranes on the outside of the walls. There are a variety of different waterproofing materials and systems, which should be installed by an experienced contractor familiar with the anticipated construction and subsurface conditions. Applying a thin coat of asphalt emulsion to the outside face of a wall is not considered waterproofing, and will only help to reduce moisture generated from water vapor or capillary action from seeping through the concrete. As with any project, adequate ventilation of basement and crawl space areas is important to prevent a buildup of water vapor that is commonly transmitted through concrete walls from the surrounding soil, even when seepage is not present. This is appropriate even when waterproofing is applied to the outside of foundation and retaining walls. We recommend that you contact an experienced envelope consultant if detailed recommendations or specifications related to waterproofing design, or minimizing the potential for infestations of mold and mildew are desired. The General, Slabs -On -Grade, and Drainage Considerations sections should be reviewed for additional recommendations related to the control of groundwater and excess water vapor for the anticipated construction. SLABS -ON -GRADE The building floors can be constructed as slabs -on -grade atop firm native sand or on structural fill. The subgrade soil must be in a firm, non -yielding condition at the time of slab construction or underslab fill placement. Any soft areas encountered should be excavated and replaced with select, imported structural fill. Even where the exposed soils appear dry, water vapor will tend to naturally migrate upward through the soil to the new constructed space above it. This can affect moisture -sensitive flooring, cause imperfections or damage to the slab, or simply allow excessive water vapor into the space above the slab. All interior slabs -on -grade should be underlain by a capillary break drainage layer consisting of a minimum 4-inch thickness of clean gravel or crushed rock that has a fines content (percent passing the No. 200 sieve) of less than 3 percent and a sand content (percent passing the No.4 sieve) of no more than 10 percent. Pea gravel or crushed rock are typically used for this layer. As noted by the American Concrete Institute (ACI) in the Guides for Concrete Floor and Slab Structures, proper moisture protection is desirable immediately below anyon-grade slab that will be covered by tile, wood, carpet, impermeable floor coverings, or any moisture -sensitive equipment or products. ACI also notes that vapor retarders such as 6-mil plastic sheeting have been used in the GEOTECH CONSULTANTS, INC. Angle J N 21303 October 3, 2022 Page 13 past, but are now recommending a minimum 10-mil thickness for better durability and long term performance. A vapor retarder is defined as a material with a permeance of less than 0.3 perms, as determined by ASTM E 96. It is possible that concrete admixtures may meet this specification, although the manufacturers of the admixtures should be consulted. Where vapor retarders are used under slabs, their edges should overlap by at least 6 inches and be sealed with adhesive tape. The sheeting should extend to the foundation walls for maximum vapor protection. If no potential for vapor passage through the slab is desired, a vapor barrier should be used. A vapor barrier, as defined by ACI, is a product with a water transmission rate of 0.01 perms when tested in accordance with ASTM E 96. Reinforced membranes having sealed overlaps can meet this requirement. In the recent past, ACI (Section 4.1.5) recommended that a minimum of 4 inches of well -graded compactable granular material, such as a 5/8-inch-minus crushed rock pavement base, be placed over the vapor retarder or barrier for their protection, and as a "blotter" to aid in the curing of the concrete slab. Sand was not recommended by ACI for this purpose. However, the use of material over the vapor retarder is controversial as noted in current ACI literature because of the potential that the protection/blotter material can become wet between the time of its placement and the installation of the slab. If the material is wet prior to slab placement, which is always possible in the Puget Sound area, it could cause vapor transmission to occur up through the slab in the future, essentially destroying the purpose of the vapor barrierlretarder. Therefore, if there is a potential that the protection/blotter material will become wet before the slab is installed, ACI now recommends that no protection/blotter material be used. However, ACI then recommends that, because there is a potential for slab curl due to the loss of the blotter material, joint spacing in the slab be reduced, a low shrinkage concrete mixture be used, and 'other measures" (steel reinforcing, etc.) be used. ASTM E-1643-98 "Standard Practice for Installation of Water Vapor Retarders Used in Contact with Earth or Granular Fill Under Concrete Slabs" generally agrees with the recent ACI literature. We recommend that the contractor, the project materials engineer, and the owner discuss these issues and review recent ACI literature and ASTM E-1643 for installation guidelines and guidance on the use of the protection/blotter material. The General, Permanent Foundation and Retaining Walls, and Drainage Considerations sections should be reviewed for additional recommendations related to the control of groundwater and excess water vapor for the anticipated construction. EXCAVATIONS AND SLOPES Excavation slopes should not exceed the limits specified in local, state, and national government safety regulations. Temporary cuts to a depth of about 4 feet may be attempted vertically in unsaturated soil, if there are no indications of slope instability. However, vertical cuts should not be made near property boundaries, or existing utilities and structures. Based upon Washington Administrative Code (WAC) 296, Part N, the soil at the subject site would generally be classified as Type B. Therefore, temporary cut slopes greater than 4 feet in height should not be excavated at an inclination steeper than 1: 1 (Horizontal:Vertical), extending continuously between the top and the bottom of a cut. The above -recommended temporary slope inclination is based on the conditions exposed in our explorations, and on what has been successful at other sites with similar soil conditions. It is possible that variations in soil and groundwater conditions will require modifications to the inclination at which temporary slopes can stand. Temporary cuts are those that will remain unsupported for a relatively short duration to allow for the construction of foundations, retaining GEOTECH CONSULTANTS, INC. Angle J N 21303 October 3, 2022 Page 14 walls, or utilities. Temporary cut slopes should be protected with plastic sheeting during wet weather. It is also important that surface runoff be directed away from the top of temporary slope cuts. Cut slopes should also be backfilled or retained as soon as possible to reduce the potential for instability. Please note that sand can cave suddenly and without warning. Excavation, foundation, and utility contractors should be made especially aware of this potential danger. These recommendations may need to be modified if the area near the potential cuts has been disturbed in the past by utility installation, or if settlement -sensitive utilities are located nearby. All permanent cuts into native soil should be inclined no steeper than 2: 1 (H:V). Water should not be allowed to flow uncontrolled over the top of any temporary or permanent slope. exposed slopes should be seeded with an appropriate species of vegetation to reduce erosion and improve the stability of the surficial layer of soil. DRAINAGE CONSIDERATIONS Footing drains should be used for this residence where: (1) Crawl spaces or basements will be below a structure; (2) A slab is below the outside grade; or, (3) The outside grade does not slope downward from a building. Drains should also be placed at the base of all earth -retaining walls. These drains should be surrounded by at least 6 inches of 1-inch-minus, washed rock that is encircled with non -woven, geotextile filter fabric (Mirafi 140N, Supac 4NP, or similar material). At its highest point, a perforated pipe invert should be at least 6 inches below the bottom of a slab floor or the level of a crawl space. The discharge pipe for subsurface drains should be sloped for flow to the outlet point. Roof and surface water drains must not discharge into the foundation drain system. As a minimum, a vapor retarder, as defined in the Slabs -On -Grade section, should be provided in any crawl space area to limit the transmission of water vapor from the underlying soils. Crawl space grades are sometimes left near the elevation of the bottom of the footings. As a result, an outlet drain is recommended for all crawl spaces to prevent an accumulation of any water that may bypass the footing drains. Providing even a few inches of free draining gravel underneath the vapor retarder limits the potential for seepage to build up on top of the vapor retarder. The excavation and site should be graded so that surface water is directed off the site and away from the tops of slopes. Water should not be allowed to stand in any area where foundations, slabs, or pavements are to be constructed. Final site grading in areas adjacent to the residence should slope away at least 2 percent, except where the area is paved. Surface drains should be provided where necessary to prevent ponding of water behind foundation or retaining walls. A discussion of grading and drainage related to pervious surfaces near walls and structures is contained in the Foundation and Retaining Walls section. Water from roof, storm water, and foundation drains should not be discharged onto slopes; it should be tight lined to a suitable outfall located away from any slopes. GENERAL EARTHWORK AND STRUCTURAL FILL All building and pavement areas should be stripped of surface vegetation, topsoil, organic soil, and other deleterious material. The stripped or removed materials should not be mixed with any materials to be used as structural fill, but they could be used in non-structural areas, such as landscape beds. Structural fill is defined as any fill, including utility backfill, placed under, or close to, a building, behind permanent retaining or foundation walls, or in other areas where the underlying soil needs to support loads. All structural fill should be placed in horizontal lifts with a GEOTECH CONSULTANTS, INC. Angle J N 21303 October 3, 2022 Page 15 moisture content at, or near, the optimum moisture content. The optimum moisture content is that moisture content that results in the greatest compacted dry density. The moisture content of fill is very important and must be closely controlled during the filling and compaction process. The allowable thickness of the fill lift will depend on the material type selected, the compaction equipment used, and the number of passes made to compact the lift. The loose lift thickness should not exceed 12 inches. We recommend testing the fill as it is placed. If the fill is not sufficiently compacted, it can be recompacted before another lift is placed. This eliminates the need to remove the fill to achieve the required compaction. The following table presents recommended relative compactions for structural fill: or Filled slopes and behind 90% retaining walls 95% for upper 12 inches of Beneath pavements subgrade; 90% below that level Where: Minimum Relative Compaction is the ratio, expressed in percentages, of the compacted dry density to the maximum dry density, as determined in accordance with ASTM Test Designation D 1557-91 (Modified Proctor). INFORMATION WITH REGARDS TO ECDC 19.10 and 23.08. The following information is with regards to ECDC 19.10 and 23.08. We have listed the specific requested information noted in these chapters of the ECDC, followed by our responses to the requested information in italics. Per ECDC 19.10.030H, the following seven provisions need to be included in the permit submittal because the site is located in the North Edmonds Earth Subsidence and Landslide Hazard Area (NEESLH): 1. The geotechnical report shall be prepared in accordance with ECDC 23.80.050 and generally accepted geotechnical engineering practices, under the supervision of, and signed and stamped by, the geotechnical engineer. A geologist may be required to be part of the geotechnical consulting staff. The report shall reference the Landau Associates Summary Report (2007) as a technical document reviewed as part of the geologic analysis for the project and discuss all items listed in the permit submittal checklist and shall make specific recommendations concerning development of the site. A geologist from our firm has stamped this report. The report does reference the 2007 Landau report on Page 3. 2. The opinions and recommendations contained in the geotechnical report shall be supported by field observations and, where appropriate or applicable, by literature review, conducted by the geotechnical engineer. The report shall be based on best available science. Test pits and a test boring were done on the subject site, and our firm has significant experience since 2013 with regards to the Meadowdale Estates subdivision that this project site is part of. We believe that best available science has been used to prepare this report. 3. The report shall include an analysis of material gathered through appropriate explorations, such as borings or test pits to a minimum depth of six feet below the proposed lowest footing or pile, an analysis of soil characteristics conducted by or under the supervision of the engineer in accordance with the standards adopted by the American Society of Testing and Materials (ASTM) or other applicable standards. The report must provide subsurface data to support the engineer's conclusions regarding slope stability. These provisions have been followed in our report. 4. If the evaluation involves geologic evaluations or interpretations, the report shall be reviewed and approved by a geologist. It shall be the responsibility of the geotechnical engineer to assure that the geologist meets the qualifications listed in ECDC 19.10.020. A letter of concurrence from the geologist shall be included in the report. A licensed geologist from our firm with well over 20 years of experience has stamped this report. GEOTECH CONSULTANTS, INC. Angle October 3, 2022 JN 21303 Page 16 5. Based upon the North Edmonds landslide area geology and slide mechanisms map and table found in the Landau Associates Summary Report (2007), any lot which contains any portion of any hazard zone or is adjacent thereto (regardless of whether the proposed building pad is located within any hazard area) shall specifically consider within the geotechnical report the following types of typical hazard zones and shall specifically note if the hazard is, or is not, present on the site. The report shall address hazards from encroaching landslide materials, hazards from ground failure in material that has not previously failed, and hazards from ground failure in previously failed material. For each landslide hazard identified on a property, the geotechnical engineer shall identify the types of specific processes associated with the hazard and include design features to reduce such hazards and mitigate impacts. The site is an Erosion Hazard Area where slopes are steeper than 15 percent, and the steep slope on the western portion of the site is a Landslide Hazard Area due to its steep inclination. As noted earlier in this report, erosion only potentially occurs where vegetation has removed and/or disturbance%xcavation is done to the existing ground, this is therefore a temporary issue. Once a project is completed and landscaping/hardscaping is installed, then the site will not have an erosion issue. The only site nearby that is disturbed currently is adjacent to the north, and it has extensive erosion control measures that appear to be adequately controlling erosion. All sites, including the subject site, will have erosion control measures during construction to ensure erosion doesn't occur and/or affect any neighboring properties. It is our opinion that potential erosion issues for this project can readily be mitigated by the erosion control features noted in the civil plans of the project. As for the western Landslide Hazard Area, based on the stability analysis provided earlier that includes the use of the western stabilization wall, the project has suitable safety factors against any instability of the western slope. 6. For properties containing or adjacent to bluffs, the geotechnical engineer shall, as a part of the building permit process, provide analysis of the rate of retreat of the bluff prepared by a geologist and estimate the bluff retreat amount and regression rate for periods of 25 and 125 years. There is no bluff on or adjacent to this site. 7. Geotechnical letter addressing the provisions of Chapter 23.80 ECDC. Information regarding this Chapter 23.80 are addressed below. Per ECDC 23.80.050.A-C A. Preparation by a Qualified Professional. A critical areas report for assessing a potential geologically hazardous area shall be prepared by a geologist licensed in the state of Washington, with experience analyzing geologic, hydrologic, and ground water flow systems, and who has experience preparing reports for the relevant type of hazard. If mitigation measures are necessary, the report detailing the mitigation measures and design of the mitigation shall be prepared by an engineer licensed in the state of Washington, with experience stabilizing slopes with similar geotechnical properties. Critical areas studies and reports on geologically hazardous areas shall be subject to independent review pursuant to ECDC 23.40.090(B). This report is stamped by both a geotechnical engineer and a hydrogeologist. B. Area Addressed in Critical Areas Report. The following areas shall be addressed in a critical areas report for geologically hazardous areas: 1. The project area of the proposed activity; and 2. All geologically hazardous areas within 200 feet of the project area or that have the potential to be affected by the proposal. As noted in this report, the very steep western slope on the site is a Landslide Hazard Area. This steep western slope (Landslide Hazard Area) continues north and south of the site. Also as noted in this report, all slopes on the site that are steeper than 15 percent, as well as the properties to the east are an Erosion Hazard Area. GEOTECH CONSULTANTS, INC. Angle October 3, 2022 JN 21303 Page 17 C. Geological Hazards Assessment. A geology hazard assessment report for a geologically hazardous area shall include a field investigation and contain an assessment of whether or not each type of geologic hazard identified in ECDC 20.80.020 is present or not present and if development of the site will increase the risk of landslides or erosion on or off the site. Geotechnical reports shall be prepared, stamped and signed by a qualified professional. These reports must: 1. Be appropriate for the scale and scope of the project. We believe this report is appropriate for the scale and scope of this project. 2. Include a discussion of all geologically hazardous areas on the site and any geologically hazardous areas off site potentially impacted by the proposed project. If the affected area extends beyond the subject property, the geology hazard assessment may utilize existing data sources pertaining to that area. As noted in this report, the only Landslide Hazard for this report is the steep western slope. as discussed as part of the slope stability analysis, the onsite project will be stable if a potential landslide were to occur on the western slope because of the inclusion of the stabilization wall on the western side of the residence. The stability of the steep western slope and properties west of it will not be decreased due to this project because stormwater from the project will be connected to an installed stormwater system. As discussed earlier, all portions of the site that are steeper than 15 percent are considered an Erosion Hazard Area of the ECDC. Currently, in its forested condition, there is no erosion potential in the proposed development area of the site because of the existing vegetation. However, once the project area is excavated and surface disturbance occurs (which will be 65 feet east of the steep western slope), there could be an erosion potential if suitable erosion control measures are not installed. However, the project plans include suitable erosion control measures, thus we believe potential erosion can suitably be controlled during construction. Following construction, landscaping/hardscaping will be completed and the runoff from the proposed structures and driveway will be controlled with a stormwater system. This stormwater system will also be used for any potential subsurface drainage (footing drains). Therefore, the potential for erosion issues on the site negligible if the project plans are followed. 3. Clearly state that the proposed project will not decrease slope stability or pose an unreasonable threat to persons or property either on or off site and provide a rationale as to those conclusions based on geologic conditions and interpretations specific to the project. Because of the use of the stormwater system for this project, and because a stabilization wall is proposed on the western side of the residence, this project will not decrease the slope stability. This is based on subsurface information obtained on the site, the site topography, and the slope stability analysis given in this report. 4. Provide adequate information to determine compliance with the requirements of this chapter. We believe this report provides this information. 5. Generally follow the guidelines set forth in the Washington State Department of Licensing Guidelines for Preparing Engineering Geology Reports in Washington (2006). In some cases, such as when it is determined that no landslide or erosion risk is present, a full report may not be necessary to determine compliance with this chapter, and in those cases a letter or abbreviated report may be provided. This report is stamped by both a licensed geotechnical engineer and hydrogeologist, thus we believe the guidelines are followed. 6. If a landslide or erosion hazard is identified, provide minimum setback recommendations for avoiding the landslide or erosion hazard, other recommendations for site development so that the frequency or magnitude of landsliding or erosion on or off the site is not altered, and recommendations consistent with ECDC 23.80.060 and 23.80.070. Minimum setbacks are noted in this report to avoid landslide hazard issues with regards to the steep western GEOTECH CONSULTANTS, INC. Angle October 3, 2022 JN 21303 Page 18 slope. No setbacks are needed for Erosion Hazard Areas because erosion control measures are suitably included in the project plans, and also because stormwater from impervious surfaces will be controlled and landscape plans indicate that permanent erosion control measures (vegetation) will be installed. Information regarding ECDC 23.80.060 and 070 are included below. Per ECDC 23.80.050.F, additional Technical Information Requirements for Projects within Erosion and Landslide Hazard Areas. In addition to the basic critical areas report requirements for geologically hazardous areas provided in subsections (A) through (E) of this section, technical information for any development within earth subsidence and landslide hazard areas shall meet the requirements of Chapter 19.10 ECDC and include the following information at a minimum: 1. Site Plan. The critical areas report shall include a copy of the site plan for the proposal showing: a. The height of slope, slope gradient, and cross-section of the project area. The height of the slope and slope gradients are shown on Plate 2 of this report. A cross section is included as Plate 3. b. The location of springs, seeps, or other surface expressions of ground water on or within 200 feet of the project area or that have the potential to be affected by the proposal. We did not observe any locations of seeps or springs. c. The location and description of surface water runoff features. The subdivision has an existing stormwater system which the proposed residence will connect to as noted in the project civil engineering/drainage plans. This system discharges water west of the site via a surface -mounted pipe that extends down the steep western slope north of the subject site. 72"d Avenue West has its own stormwater system, installed sometime after 1997, that also discharges water west of the steep slope just south of the subject site. 2. Hazards Analysis. The hazards analysis component of the critical areas report shall specifically include: a. A description of the extent and type of vegetative cover. It is described in the Surface section of this report. b. A description of subsurface conditions based on data from site -specific explorations. Described in the Subsurface section of this report. c. Descriptions of surface and ground water conditions, public and private sewage disposal systems, fills and excavations, and all structural improvements. These are described in the initial portions of this report. d. An estimate of slope stability and the effect construction and placement of structures will have on the slope over the estimated life of the structure. Based on the slope stability analysis noted earlier in this report, which includes the stabilization wall, suitable safety factors are adhered to for this project. This includes the affect of a large-scale seismic event. Because the residence will be founded on competent soils, and the foundations will be at or behind the stabilization wall, and because stormwater from the project will be controlled and not allowed to flow toward the steep slope, the construction and placement of structures on the site will not have any affect on the stability of the slope. e. An estimate of the bluff retreat rate or an estimate of the percent risk of landslide area expansion that recognizes and reflects potential catastrophic events such as seismic activity or a 100-year storm event. As noted earlier, an extreme precipitation event occurred in and around January 1, 1997 (as noted earlier, we believe that this precipitation exceeded a 100-year storm) that resulted in a slide with a depth in the range of 3 to 4 feet. It would take centuries in our opinion for precipitation events to reach the residence area. As noted in the in the slope stability section of this report, the static and dynamic (seismic) analysis of the steep western slope (which includes the stabilization wall at the western edge of the GEOTECH CONSULTANTS, INC. Angle October 3, 2022 JN 21303 Page 19 residence, indicate that a static and dynamic safety factors for the residence exceeds the required safety factors. f. Consideration of the run -out hazard of landslide debris and/or the impacts of landslide run - out on downslope properties. Residential sites are located at the base of the site's steep western slope. As with any steep slope in the Puget Sound Area, landslides can occur during times of significant precipitation and potentially during large seismic events. The runout could potentially reach neighboring residences, although we have not done an analysis of this because it is not part of this project. However, because the stormwater from this residence project and other neighboring residences are connected to a system, less water will reach the steep western slope after the residence is constructed as compared to the current undeveloped condition. Thus, this project not increasing the potential for a landslide on the steep western slope (it actually is slightly lessening it). g. A study of slope stability including an analysis of proposed cuts, fills, and other site grading. A slope stability analysis in given in this report. h. Recommendations for building siting limitations. The initial design of the subdivision had a building setback of 65 feet. As noted earlier in this report, in order to keep this setback, a stabilization wall is needed on the western edge of the residence as it is discussed earlier in this report. There are no other siting limitations from a geotechnical engineering standpoint in our opinion. i. An analysis of proposed surface and subsurface drainage, and the vulnerability of the site to erosion. Currently, in its forested condition, there is no erosion potential at the site because of the existing vegetation. However, once the site is excavated and surface disturbance occurs, there could be erosion potential if suitable erosion control measures are not installed. However, the project plans include suitable erosion control measures, thus we believe potential erosion can be suitably controlled during construction. Following construction, landscaping/hardscaping will be completed and the runoff from the proposed structure and driveway will be controlled with a stormwater system. This stormwater system will also be used for any potential subsurface drainage (footing drains). 3. Geotechnical Engineering Report. The technical information for a project within a landslide hazard area shall include a geotechnical engineering report prepared by a licensed engineer that presents engineering recommendations for the following: a. Parameters for design of site improvements including appropriate foundations and retaining structures. These should include allowable load and resistance capacities for bearing and lateral loads, installation considerations, and estimates of settlement performance. These are in this report. b. Recommendations for drainage and subdrainage improvements. These are noted in the report, and are also now contained in project plans. c. Earthwork recommendations including clearing and site preparation criteria, fill placement and compaction criteria, temporary and permanent slope inclinations and protection, and temporary excavation support. These are included in this report. d. Mitigation of adverse site conditions including slope stabilization measures and seismically unstable soils, if appropriate. A stabilization wall will be constructed on the western side of the residence. Per ECDC 23.80.060.A, alterations of geologically hazardous areas or associated buffers may only occur for activities that: 1. Will not increase the threat of the geological hazard to adjacent properties beyond predevelopment conditions. Because of the use of the stabilization wall, the use of the existing stormwater system, and once the site is fully landscape, it is our professional opinion that this project will not increase the threat of the geological hazard area to adjacent properties beyond the predevelopment conditions. GEOTECH CONSULTANTS, INC. Angle October 3, 2022 JN 21303 Page 20 2. Will not adversely impact other critical areas. This project will not impact other critical areas based on our response to 1. 3. Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or less than predevelopment conditions. This project is mitigated to code standards to be equal to greater than predevelopment conditions. 4. Are certified as safe as designed and under anticipated conditions by a qualified engineer or geologist, licensed in the state of Washington. The stamping geotechnical engineer certifies it as safe under the anticipated conditions provided the recommendations in this report and the project plans are followed. Per ECDC 23.80.070, activities on sites containing erosion or landslide hazards shall meet the requirements of ECDC 23.80.060 (as noted in the previous section above) and the specific following requirements: 1. Minimum Building Setback. The minimum setback shall be the distance required to ensure the proposed structure will not be at risk from landslides for the life of the structure, considered to be 120 years, and will not cause an increased risk of landslides taking place on or off the site. A setback shall be established from all edges of landslide hazard areas. The size of the setback shall be determined by the director consistent with recommendations provided in the geotechnical report to eliminate or minimize the risk of property damage, death, or injury resulting from landslides caused in whole or part by the development, based upon review of and concurrence with a critical areas report prepared by a qualified professional. Based on the slope stability analysis, with the use of the stabilization wall on the western side of the residence, a setback of 65 feet is suitable for the life of the structure (120 years). 2. Buffer Requirements. A buffer may be established with specific requirements and limitations, including but not limited to, drainage, grading, irrigation, and vegetation. Buffer requirements shall be determined by the director consistent with recommendations provided in the geotechnical report to eliminate or minimize the risk of property damage, death, or injury resulting from landslides caused in whole or part by activities within the buffer area, based upon review of and concurrence with a critical areas report prepared by a qualified professional. As is consistent with the rest of the Meadowdale Estates short plat, we believe a 25-foot, no -disturbance buffer is suitable for this project. This is because most of the large trees on the site are contained within this buffer. 3. Alterations of an erosion or landslide hazard area, minimum building setback and/or buffer may only occur for activities for which a hazards analysis is submitted and certifies that: a. The alteration will not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions. The proposed development will direct stormwater to an established stormwater system, and thus it will not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions. b. The alteration will not decrease slope stability on adjacent properties. The only properties that would be affected by the development are downslope of it. This development will be located well away from the top of the steep slope and surface water will be controlled so that it will not reach the steep slope. Therefore, this alteration will not affect the downslope properties. c. Such alterations will not adversely impact other critical areas. It will not. 4. Design Standards within Erosion and Landslide Hazard Areas. Development within an erosion or landslide hazard area and/or buffer shall be designed to meet the following basic requirements unless it can be demonstrated that an alternative design that deviates from one or more of these standards provides greater long-term slope stability while meeting all other provisions of this title. The requirement for long-term slope stability shall exclude designs that require regular and periodic maintenance to maintain their level of function. The basic development design standards are: GEOTECH CONSULTANTS, INC. Angle October 3, 2022 JN 21303 Page 21 a. The proposed development shall not decrease the factor of safety for landslide occurrences below the limits of 1.5 for static conditions and 1.2 for dynamic conditions. If stability at the proposed development site is below these limits, the proposed development shall provide practicable approaches to reduce risk to human safety and improve the factor of safety for landsliding. In no case shall the existing factor of safety be reduced for the subject property or adjacent properties. Per the slope stability analyses noted in this report, with the inclusion of the stabilization wall, these limits are met. b. Structures and improvements shall be clustered to avoid geologically hazardous areas and other critical areas. All structures and improvement will be located upslope and east of the stabilization wall. c. Structures and improvements shall minimize alterations to the natural contour of the slope, and foundations shall be tiered where possible to conform to existing topography. The plans indicate that the development will essentially follow the natural contours of the site in as much as the slope of the driveway will allow access from 72" d Avenue West (from the east) on the upslope side of the site. The floor level of the western side of the residence will be located near the existing ground surface. d. Structures and improvements shall be located to preserve the most critical portion of the site and its natural landforms and vegetation. The structures and improvements will be located on the eastern side of the site where the slope is gently to moderately slope, and at least 65 feet from the steep slope. e. The proposed development shall not result in greater risk or a need for increased buffers on neighboring properties. It will not. f. The use of retaining walls that allow the maintenance of existing natural slope area is preferred over graded artificial slopes. That is being done for this project. g. Development shall be designed to minimize impervious lot coverage. We understand that the development has been designed per Edmonds lot coverage standards. Water from impervious surfaces will be controlled into an existing stormwater system. LIMITATIONS The conclusions and recommendations contained in this report are based on site conditions as they existed at the time of our exploration and assume that the soil and groundwater conditions encountered in the test borings and test pits are representative of subsurface conditions on the site. If the subsurface conditions encountered during construction are significantly different from those observed in our explorations, we should be advised at once so that we can review these conditions and reconsider our recommendations where necessary. Unanticipated conditions are commonly encountered on construction sites and cannot be fully anticipated by merely taking samples in explorations. Subsurface conditions can also vary between exploration locations. Such unexpected conditions frequently require making additional expenditures to attain a properly constructed project. It is recommended that the owner consider providing a contingency fund to accommodate such potential extra costs and risks. This is a standard recommendation for all projects. The recommendations presented in this report are directed toward the protection of only the proposed structure from damage due to slope movement. Predicting the future behavior of steep slopes and the potential effects of development on their stability is an inexact and imperfect science that is currently based mostly on the past behavior of slopes with similar characteristics. Landslides and soil movement can occur on steep slopes before, during, or after the development of property. The property owner must ultimately accept the possibility that some slope movement could occur at or near the steep western slope. However, the development will not be affected as noted in this study. GEOTECH CONSULTANTS, INC. Angle J N 21303 October 3, 2022 Page 22 This report has been prepared for the exclusive use of Nathan and Callie Angle, and their representatives, for specific application to this project and site. Our conclusions and recommendations are professional opinions derived in accordance with our understanding of current local standards of practice, and within the scope of our services. No warranty is expressed or implied. The scope of our services does not include services related to construction safety precautions, and our recommendations are not intended to direct the contractor's methods, techniques, sequences, or procedures, except as specifically described in our report for consideration in design. Our services also do not include assessing or minimizing the potential for biological hazards, such as mold, bacteria, mildew and fungi in either the existing or proposed site development. ADDITIONAL SERVICES Geotech Consultants, Inc. should be retained to provide geotechnical consultation, testing, and observation services during construction. This is to confirm that subsurface conditions are consistent with those indicated by our exploration, to evaluate whether earthwork and foundation construction activities comply with the general intent of the recommendations presented in this report, and to provide suggestions for design changes in the event subsurface conditions differ from those anticipated prior to the start of construction. However, our work would not include the supervision or direction of the actual work of the contractor and its employees or agents. Also, job and site safety, and dimensional measurements, will be the responsibility of the contractor. During the construction phase, we will provide geotechnical observation and testing services when requested by you or your representatives. Please be aware that we can only document site work we actually observe. It is still the responsibility of your contractor or on -site construction team to verify that our recommendations are being followed, whether we are present at the site or not. The following plates are attached to complete this report: Plate 1 Vicinity Map Plate 2 Site Exploration Plan Plate 3 Site Cross -Section Plates 4 - 5 Test Boring Log Plate 6 1997 Test Pits Plate 7 Typical Footing Drain Detail Appendix A NEESLH Map, Peak Ground Acceleration Calculation Appendix B Slope Stability Analysis GEOTECH CONSULTANTS, INC. Angle October 3, 2022 JN 21303 Page 23 We appreciate the opportunity to be of service on this project. Please contact us if you have any questions, or if we can be of further assistance. TAJ/DRW:kg Respectfully submitted, GEOTECH CONSULTANTS, INC. oc Wash � 1 voaoaeasT 2210 used Geo�o DVnothy Alan Johnson J a 10/3/2022 Timothy A. Johnson Licensed Hydrogeologist D. Robert Ward, P.E. Principal GEOTECH CONSULTANTS, INC. GEOTECH CONSULTANTS, INC. 1 Meadov:dale Beach Park I I ------------- r I (Source: Snohomish County GIS) VICINITY MAP 15724 - 72nd Avenue West Edmonds, Washington Job No: Date: Plate: 21303 1 Sept.2022 1 1 n►nQTN z;,s—ENT 0:) cANUW ELLWlxlx-)5131' -..q5 • '- i \ \` ! t.dx- \ �PgISTHG m 1 -- CNI°vt REWMiNeNe.°t.)---:,""'----'gMo . -d' aCE55 Q UTT��tt EA�xENi . 0. TM°IYQ9500t) A M.xd Y � f _ i Ex s SN du' J t1 \ � \ � \ / se. adgRfiarE 3°x 95 � t3:a DEL � XY / t'III I� nt t \ � t t64— �vuaN' 'nSDS �d11, fYx 1)� II)I1111 ��1 �111 11 ))11 \P \�\ \`\\\\ \ ` \� - \ $ \ s�T-T LOT4 ' <' �\ 29, 673 SF —GROSS 15724 721V0 AVE W Legend: Test Boring Location 0 Test Pit Location Slope Stability Cross Section Location GEOTECH CONSULTANTS, INC. ppRCC 0002 ao t-t/r a°. iaw nvE, gr ry mratE �5513j0o (rvv, stu�na t.r s e za w or cuco Imr mta) SITE EXPLORATION PLAN 15724 - 72nd Avenue West Edmonds, Washington Job No: Date: Plate: 21303 1 Sept. 2022 No Scale 1 2 A' 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 Distance (Feet) GEOTECH CONSULTANTS, INC. H 280 270 260 250 240 230 ._ O 220 41 210 200 O 180 j 170 N 160 W 150 140 130 120 460 480 500 SITE CROSS SECTION 15724 - 72nd Avenue West Edmonds, Washington Job No: Date: Plate: 21303 1 Sept.2022 3 �'1 e, ti BORING 1 5 10 5 19 33 35 0 15 47 20 28 25 51 30 Description 1SM Gray silty SAND, fine-grained, mist to very moist, loose 2' -becomes rust -brown, very moist, medium -dense -with lenses of gray silt and trace roots 3' sP Gray and rust -brown, gravelly, slightly silty to silty SAND, fine-grained, very sM moist to wet, dense 4' Gray very gravelly SAND, fine-grained, wet, dense SP -wet sampling rods at 14' 5' 6 Gray -brown sandy SILT with rusting and lenses of gray and rust -brown, wet ML silty sand, low plasticity, moist, angular bedding, very stiff to hard Gray with trace rusting, slightly silty SAND, very fine-grained, moist, very dense V - i - - - - - - - - - - - - - - - - - - - - - - - * Test Boring 1 is continued on Plate 4 GEOTECH CONSULTANTS, INC. TEST BORING LOG 15724 - 72nd Avenue West Edmonds, Washington Job Date: Logged by: Plate: 21303 Sept.2022 MKM 4 30 42 18 11 35 40 45 50 55 72 76 70 47 BORING 1 (Continued) G� �5 Description Gray SAND to slightly sitly SAND, very fine-grained, moist, dense rsm P 9 -with a thin seam of rusted silt, becomes very dense � 10� Gray SAND trace silt, fine-grained, moist, very dense SP 11 -becomes moist to very moist SP 12' SM Gray with rusting, slightly silty SAND with silty sand and silt seams, very fine- raned wet dense to very dense * Test boring was terminated at 51.5 feet on September 20, 2022. * Perched groundwater was encountered at 11 feet, from 14 to 18, and at 50 feet during drilling. GEOTECH CONSULTANTS, INC. TEST BORING LOG 15724 - 72nd Avenue West Edmonds, Washington Job Date: Logged by: Plate: 21303 1 Sept.2022 1 MKM 1 5 0 10 0 10 n" OeQ�r` �o`°��o�a��\e G5 G U TEST PIT I Description Topsoil over; Brown, mottled, silty SAND with gravel and some organics, very dense, loose to medium dense -becomes cemented, no organics, medium dense to dense -becomes gray, no gravel, more silty, wet * Test Pit terminated at 7.0 feet on July 3, 2013. * Slight groundwater seepage was observed at 5.0 feet during excavation. * No caving observed during excavation. 5 J5G d Topsoil over, - Description Brown, slightly silty, gravelly SAND with organics, slightly moist, loose 5P" si -becomes mostly gray, no organics, less silty, moist, dense SM * Test Pit terminated at 6.0 feet on July 3, 2013. * No groundwater was observed during excavation. * No caving observed during excavation. GEOTECH CONSUINANTF, ;P,C. TEST PIT LOG_ 15620 72nd Avenue West Edmonds, Washington Job 3245 jDate, 2013 ILoogedbDRWy, I plate: 6 Slope backfill away from foundation. Provide surface drains where necessary. Backfill (See text for requirements) e c 0 M M Nonwoven Geotextile Filter Fabric o Washed Rock LL (7/8" min. size) 000 0 0 0 0 0 0 Oo O�O�OoO Oo oO o0 o 0 oo 0 n 4 min. Tightline Roof Drain (Do not connect to footing drain) Possible Slab .o. oP•o. o.o•o. 0.°•0�000.°•0�000.° a.opQ o°Oo op� o°O vo.op� o�O °a.op� o�O °o.opQ o�O.Q o e°o. .o e°o. .o °o. .o •o '.o a°o. .o 4" Perforated Hard PVC Pipe (Invert at least 6 inches below slab or crawl space. Slope to drain to appropriate outfall. Place holes downward.) Vapor Retarder/Barrier and Capillary Break/Drainage Layer (Refer to Report text) NOTES: (1) In crawl spaces, provide an outlet drain to prevent buildup of water that bypasses the perimeter footing drains. (2) Refer to report text for additional drainage, waterproofing, and slab considerations. GEOTECH CONSULTANTS, INC. FOOTING DRAIN DETAIL 15724 - 72nd Avenue West Edmonds, Washington Job No: I Date: Plate: :7] 21303 1 Sept.2022 Appendix A NEESLH Map & Peak Ground Acceleration Calculation 21303 - Angle OV All North Edmonds Earth Subsidence and 33 in Landslide Hazard r ►- J i Areas Ma i '' r Legend Ii North Edmonds Earth Subsidence and J Landslide Hazard Area f "'f„ (See ECDC 23.80.020 I3.1 and ECDC 19.10) T (Note: Boundaries are the approximate extent previous landslidin ' hazards are resent of pre g, p adjacent to the landslide boundaries) IFSeep Slope Areas: ':' f CSlope of 40% or steeper and — with a vertical relief of ten (10) ft or more (See ECDC 23.80.020 B.2) �, v I Minimum buffer equal to the height J rj of the steep slope or 50 feet, whichever f - is greater (see ECDC 23.80.070 A.1) (The buffer shown is the minimum buffer J / - L adjacent to the North Edmonds Subsidence,,--,r i4 and Landslide Hazard Area; a similar buffer would apply to steep slope areas, -_ I but is not shown on this map for clarity) � -J- _ I 2 ft Topographic Contour Vertical datum: NAVD88 �� •7 77 F-1 Parcel /—; 400 Scale in Feet 4 Map created by Landss ,auAociates, Inc. (May 2006) Aerial photo: City of Edmonds / ----. Topographic contours and sleep slopes daia derived from LiDAR data: City of Edmonds (February 27, 2005) / �I ` �F___T7�7 Peak Acceleration Reduction Due to Wave Scattering Near Slope Face Source: LRFD Seismic Analysis and Design of Transportation Geotechnical Features and Structural Foundations, NHI Course No. 130094; Federal Highway Administration; August 2011 6-2 where a = a slope height reduction factor and k ,, is the average peak acceleration in the potential faihue mass. taking into account spatial incoherence (or wave scattering). The following relationship is presented in NCHRP Report 611 for the value of a for slopes and embankments of up to 100 ft in height founded upon Site Class C. D. and E soil conditions: a=1-0.01•H•(0.5-P-0 6-3 where H = slope height (feet) and a is a function of the shape of the acceleration response spectrtmi and is given by: R =F, 'St / kuku 6-4 where F, = AASHTO site factor for the spectral acceleration at 1 second and S, = the spectral acceleration at 1 second for Site Class B. CALCULATION: For Site Class D, kmax (MCEg) = 0.629g under ASCE 7-16 (2018 IBC) H= 100 feet Fv for Site Class B = 1.0 (ASCE 7-16) S1 = 0.473 (ASCE 7-16) R = 1.0 * 0.473 / 0.629 = 0.752 (ASCE 7-16) a = 1+0.01 * 100' * (0.5*0.752-1) = 0.376 Kav = a * Kmax = 0.376 * 0.629 = 0.24 Kh (for input into slope stability analysis = 0.5 * 0.24 = 0.12g (ASCE 7-16) Appendix B Slope Stability Analysis 21303 - Angle 21303 - Angle Static A' 0 20 40 60 80 100 120 140 A 280 270 260 250 240 230 220 210 200 O 190 180 N 170 uJ 160 150 140 130 120 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500 Distance (Feet) 21303 - Angle Static A' Medium -Dense 120 pcf c=0 psf phi=32 degrees Dense 130 pcf c=0 psf phi=38 degrees / 0 20 40 60 80 100 120 140 A 280 270 260 65' __ 250 �__--- 240 230 220 --- ---- - - - 210 .� 200 190 LD - -- -- -- ---- 180 N 170 LLJ 160 Hard Silt 150 125 pcf c=200 psf 140 phi=33 degrees 130 120 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500 Distance (Feet) 9/26/22, 1:19 PM Static Static Report generated using GeoStudio 2012. Copyright © 1991-2016 GEO-SLOPE International Ltd. File Information File Version: 8.15 Title: 21303 Angle Created By: Matt McGinnis Last Edited By: Matt McGinnis Revision Number: 36 Date: 9/26/2022 Time: 1:18:41 PM Tool Version: 8.15.6.13446 File Name: 21303 AA' Complete reduction factor.gsz Directory: C:\Users\MattM\Geotech Consultants\Shared Documents - Documents\2021 Jobs\21303 Angle (DRW)\Slope Stability\Sept 2022 Slope Stability - With Boring\ Last Solved Date: 9/26/2022 Last Solved Time: 1:18:44 PM Project Settings Length(L) Units: Feet Time(t) Units: Seconds Force(F) Units: Pounds Pressure(p) Units: psf Strength Units: psf Unit Weight of Water: 62.4 pcf View: 2D Element Thickness: 1 Analysis Settings Static Kind: SLOPE/W Method: Morgenstern -Price Settings Side Function Interslice force function option: Half -Sine PWP Conditions Source: Piezometric Line Apply Phreatic Correction: Yes Use Staged Rapid Drawdown: No Slip Surface Direction of movement: Right to Left Use Passive Mode: No Slip Surface Option: Entry and Exit Critical slip surfaces saved: 1 Resisting Side Maximum Convex Angle: 1 ° Driving Side Maximum Convex Angle: 5 ° Optimize Critical Slip Surface Location: No Tension Crack file:///C:/Users/MattM/geotech consultants/shared documents - documents/2021 jobs/21303 angle (drw)/slope stability/sept 2022 slope stability- with b... 1/5 9/26/22, 1:19 PM Static Tension Crack Option: (none) F of S Distribution F of S Calculation Option: Constant Advanced Number of Slices: 30 F of S Tolerance: 0.001 Minimum Slip Surface Depth: 0.1 ft Search Method: Root Finder Tolerable difference between starting and converged F of S: 3 Maximum iterations to calculate converged lambda: 20 Max Absolute Lambda: 2 Materials Medium -Dense Model: Mohr -Coulomb Unit Weight: 120 pcf Cohesion': 0 psf Phi': 32 ° Phi-B: 0 ° Dense Model: Mohr -Coulomb Unit Weight: 130 pcf Cohesion': 0 psf Phi': 38 ° Phi-B: 0 ° Pore Water Pressure Piezometric Line: 1 Very Dense Model: Mohr -Coulomb Unit Weight: 135 pcf Cohesion': 100 psf Phi': 42 ° Phi-B: 0 ° Pore Water Pressure Piezometric Line: 2 Hard Silt Model: Mohr -Coulomb Unit Weight: 125 pcf Cohesion': 200 psf Phi': 33 ° Phi-B: 0 ° Slip Surface Entry and Exit Left Projection: Range Left -Zone Left Coordinate: (8, 125.6) ft Left -Zone Right Coordinate: (135, 197.90977) ft Left -Zone Increment: 10 file:///C:/Users/MattM/geotech consultants/shared documents - documents/2021 jobs/21303 angle (drw)/slope stability/sept 2022 slope stability- with b... 2/5 9/26/22, 1:19 PM Static Right Projection: Range Right -Zone Left Coordinate: (265, 236.47619) ft Right -Zone Right Coordinate: (430, 260) ft Right -Zone Increment: 10 Radius Increments: 10 Slip Surface Limits Left Coordinate: (0, 124) ft Right Coordinate: (487, 272) ft Piezometric Lines Piezometric Line 1 Coordinates X (ft) Y (ft) Coordinate 1 151.08 210 Coordinate 2 313 223 Coordinate 3 487 253 Piezometric Line 2 Coordinates X (ft) Y (ft) Coordinate 1 106 176 Coordinate 2 313 187 Coordinate 3 487 216 Points X (ft) Y (ft) Point 1 0 124 Point 2 50 134 Point 3 183 234 Point 4 313 238 Point 5 407 260 Point 6 437 260 Point 7 487 272 Point 8 487 124 Point 9 250 236 Point 10 487 267 Point 11 487 250 Point 12 487 212 Point 13 313 231 Point 14 313 221 Point 15 313 185 Point 16 103.2 174 file:///C:/Users/MattM/geotech consultants/shared documents - documents/2021 jobs/21303 angle (drw)/slope stability/sept 2022 slope stability- with b... 3/5 9/26/22, 1:19 PM Static Point 17 1 173.69 22 Point 18 151.08 �2log Regions Material Points Area (ft') Region 1 Hard Silt 1,8,12,15,16,2 26,453 Region 2 Very Dense 16,18,14,11,12,15 12,866 Region 3 Dense 18,17,13,10,11,14 4,297.5 Region 4 Medium -Dense 17,3,9,4,5,6,7,10,13 2,100 Current Slip Surface Slip Surface: 4 F of S: 1.61 Volume: 8,222.7739 ft' Weight: 1,063,321 Ibs Resisting Moment: 2.5424699e+008 Ibs-ft Activating Moment: 1.5749703e+008 Ibs-ft Resisting Force: 652,038.02 Ibs Activating Force: 403,754.4 Ibs F of S Rank (Analysis): 1 of 1,331 slip surfaces F of S Rank (Query): 1 of 1,331 slip surfaces Exit: (8, 125.6) ft Entry: (265, 236.47619) ft Radius: 347.21814 ft Center: (10.623107, 472.80823) ft Slip Slices X (ft) Y (ft) PWP (psf) Base Normal Frictional Strength Cohesive Strength Stress (psf) (psf) (psf) Slice 1 12.2 125.61908 0 110.78578 71.945128 200 Slice 2 20.6 125.75889 0 322.57429 209.48219 200 Slice 3 29 126.10225 0 513.75868 333.63879 200 Slice 4 37.4 126.64976 0 680.37183 441.83863 200 Slice 5 45.8 127.40238 0 818.66746 531.64886 200 Slice 6 54.433333 128.39405 0 11243.4736 807.52119 200 Slice 7 63.3 129.6385 0 11946.7365 1,264.2254 200 Slice 8 72.166667 131.11753 0 2,600.7199 1,688.9273 200 Slice 9 81.033333 132.8342 0 3,196.0749 2,075.5553 200 Slice 89.9 134.79214 0 3,726.3125 2,419.8957 200 10 Slice gg 766667 136.99557 0 4,188.0834 2,719.7731 200 11 Slice 104.6 138.55284 0 4,467.6809 2,901.3459 200 12 Slice 110.508 140.30065 0 4,741.7396 3,079.3217 200 13 Slice 119.524 143.14402 0 5,108.9302 3,317.7781 200 14 Slice 128.54 146.26104 0 5,411.4218 3,514.2184 200 file:///C:/Users/MattM/geotech consultants/shared documents - documents/2021 jobs/21303 angle (drw)/slope stability/sept 2022 slope stability- with b... 4/5 9/26/22, 1:19 PM Static 15 Slice 137.556 149.65963 0 5,656.0804 3,673.1016 200 16 Slice 146.572 153.34879 0 5,850.6876 3,799.481 200 17 Slice 154.84833 156.98809 0 5,982.7003 3,885.211 200 18 Slice 162.385 160.54038 0 6,065.2706 3,938.8328 200 19 Slice 169.92167 164.31768 0 6,128.7135 3,980.0331 200 20 Slice 178.345 168.83202 0 6,154.2915 3,996.6436 200 21 Slice 189.19581 175.11708 0 5,732.9027 3,722.9905 200 22 Slice 197.0347 179.8793 59.629253 5,079.1909 4,519.6336 100 23 Slice 203.28065 183.99532 -175.83469 4,602.9247 4,144.492 100 24 Slice 212.48637 190.35531 -541.14073 3,922.6792 3,531.9962 100 25 Slice 221.6921 197.1695 -934.70914 3,245.803 2,922.5341 100 26 Slice 230.89782 204.47258 -1,358.698 2,564.7656 2,309.3253 100 27 Slice 240.10355 212.30576 -1,815.6727 1,870.225 1,683.9581 100 28 Slice 245.39686 216.99415 35.850236 1,569.8964 1,198.5283 0 29 Slice 248.04365 219.46117 -103.93078 1,356.1995 1,059.5792 0 30 Slice 254.13269 225.36147 -439.4414 869.0028 678.93939 0 31 32Ce 261.63269 232.9523 0 286.13008 178.79392 0 file:///C:/Users/MattM/geotech consultants/shared documents - documents/2021 jobs/21303 angle (drw)/slope stability/sept 2022 slope stability - with b... 5/5 21303 - Angle Seismic A' 1.24 i Medium -Dense 120 pcf c=0 psf phi=32 degrees Dense 130 pcf c=0 psf phi=38 degrees / 65' Hard Silt 125 pcf c=200 psf phi=33 degree 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 48 Distance (Feet) A 280 270 260 250 240 230 220 210 200 O — 190 M 180 N 170 LU 160 150 140 s 130 120 0 500 9/26/22. 1:20 PM Seismic Seismic Report generated using GeoStudio 2012. Copyright © 1991-2016 GEO-SLOPE International Ltd. File Information File Version: 8.15 Title: 21303 Angle Created By: Matt McGinnis Last Edited By: Matt McGinnis Revision Number: 36 Date: 9/26/2022 Time: 1:18:41 PM Tool Version: 8.15.6.13446 File Name: 21303 AA' Complete reduction factor.gsz Directory: C:\Users\MattM\Geotech Consultants\Shared Documents - Documents\2021 Jobs\21303 Angle (DRW)\Slope Stability\Sept 2022 Slope Stability - With Boring\ Last Solved Date: 9/26/2022 Last Solved Time: 1:18:45 PM Project Settings Length(L) Units: Feet Time(t) Units: Seconds Force(F) Units: Pounds Pressure(p) Units: psf Strength Units: psf Unit Weight of Water: 62.4 pcf View: 2D Element Thickness: 1 Analysis Settings Seismic Kind: SLOPE/W Method: Morgenstern -Price Settings Side Function Interslice force function option: Half -Sine PWP Conditions Source: Piezometric Line Apply Phreatic Correction: Yes Use Staged Rapid Drawdown: No Slip Surface Direction of movement: Right to Left Use Passive Mode: No Slip Surface Option: Entry and Exit Critical slip surfaces saved: 1 Resisting Side Maximum Convex Angle: 1 ° Driving Side Maximum Convex Angle: 5 ° Optimize Critical Slip Surface Location: No Tension Crack file:///C:/Users/MattM/geotech consultants/shared documents - documents/2021 jobs/21303 angle (drw)/slope stability/sept 2022 slope stability- with b... 1/5 9/26/22. 1:20 PM Seismic Tension Crack Option: (none) F of S Distribution F of S Calculation Option: Constant Advanced Number of Slices: 30 F of S Tolerance: 0.001 Minimum Slip Surface Depth: 0.1 ft Search Method: Root Finder Tolerable difference between starting and converged F of S: 3 Maximum iterations to calculate converged lambda: 20 Max Absolute Lambda: 2 Materials Medium -Dense Model: Mohr -Coulomb Unit Weight: 120 pcf Cohesion': 0 psf Phi': 32 ° Phi-B: 0 ° Dense Model: Mohr -Coulomb Unit Weight: 130 pcf Cohesion': 0 psf Phi': 38 ° Phi-B: 0 ° Pore Water Pressure Piezometric Line: 1 Very Dense Model: Mohr -Coulomb Unit Weight: 135 pcf Cohesion': 100 psf Phi': 42 ° Phi-B: 0 ° Pore Water Pressure Piezometric Line: 2 Hard Silt Model: Mohr -Coulomb Unit Weight: 125 pcf Cohesion': 200 psf Phi': 33 ° Phi-B: 0 ° Slip Surface Entry and Exit Left Projection: Range Left -Zone Left Coordinate: (10, 126) ft Left -Zone Right Coordinate: (129, 193.3985) ft Left -Zone Increment: 10 file:///C:/Users/MattM/geotech consultants/shared documents - documents/2021 jobs/21303 angle (drw)/slope stability/sept 2022 slope stability- with b... 2/5 9/26/22. 1:20 PM Seismic Right Projection: Range Right -Zone Left Coordinate: (265, 236.47619) ft Right -Zone Right Coordinate: (430, 260) ft Right -Zone Increment: 10 Radius Increments: 10 Slip Surface Limits Left Coordinate: (0, 124) ft Right Coordinate: (487, 272) ft Piezometric Lines Piezometric Line 1 Coordinates X (ft) Y (ft) Coordinate 1 151.08 210 Coordinate 2 313 223 Coordinate 3 487 253 Piezometric Line 2 Coordinates X (ft) Y (ft) Coordinate 1 106 176 Coordinate 2 313 187.00292 Coordinate 3 487 214 Seismic Coefficients Horz Seismic Coef.: 0.12 Points X (ft) Y (ft) Point 1 0 124 Point 2 50 134 Point 3 183 234 Point 4 313 238 Point 5 407 260 Point 6 437 260 Point 7 487 272 Point 8 487 124 Point 9 250 236 Point 10 487 267 Point 11 487 250 Point 12 487 212 file:///C:/Users/MattM/geotech consultants/shared documents - documents/2021 jobs/21303 angle (drw)/slope stability/sept 2022 slope stability- with b... 3/5 9/26/22. 1:20 PM Seismic Point 13 1 313 231 Point 14 313 221 Point 15 313 185 Point 16 103.2 174 Point 17 173.69 227 Point 18 151.08 210 Regions Material Points Area (ft') Region 1 Hard Silt 1,8,12,15,16,2 26,453 Region 2 Very Dense 16,18,14,11,12,15 12,866 Region 3 Dense 18,17,13,10,11,14 4,297.5 Region 4 Medium -Dense 17,3,9,4,5,6,7,10,13 2,100 Current Slip Surface Slip Surface: 4 F of S: 1.24 Volume: 8,194.3213 ft3 Weight: 1,059,777.5 Ibs Resisting Moment: 2.3914498e+008 Ibs-ft Activating Moment: 1.9227692e+008 Ibs-ft Resisting Force: 620,046.98 Ibs Activating Force: 498,786.58 Ibs F of S Rank (Analysis): 1 of 1,331 slip surfaces F of S Rank (Query): 1 of 1,331 slip surfaces Exit: (10, 126) ft Entry: (265, 236.47619) ft Radius: 345.27003 ft Center: (11.848719, 471.26508) ft Slip Slices X (ft) Y (ft) PWP (psf) Base Normal Stress (psf) Frictional Strength (psf) Cohesive Strength (psf) Slice 1 14 126.02493 0 113.11704 73.459062 200 Slice 2 22 126.16751 0 336.08967 218.25919 200 Slice 3 30 126.49577 0 547.72751 355.6984 200 Slice 4 38 127.01022 0 741.95533 481.83142 200 Slice 5 46 127.7117 0 911.99612 592.25721 200 Slice 6 54.433333 128.66037 0 11387.4411 901.01479 200 Slice 7 63.3 129.87958 0 21156.7602 1,400.6165 200 Slice 8 72.166667 131.3344 0 2,863.7563 1,859.7451 200 Slice 9 81.033333 133.02788 0 3,487.0092 2,264.4902 200 Slice 10 89.9 134.96361 0 4,011.9925 2,605.4184 200 Slice 11 gg 766667 137.14582 0 4,432.492 2,878.4939 200 Slice 12 104.6 138.68955 0 4,667.1691 0 1 4,880.1936 3,030.8951 200 Slice 110.508 140.42432 3,169.2348 200 file:///C:/Users/MattM/geotech consultants/shared documents - documents/2021 jobs/21303 angle (drw)/slope stability/sept 2022 slope stability- with b... 4/5 9/26/22. 1:20 PM Seismic 13 Slice 119.524 143.24844 0 5,134.0936 3,334.1194 200 14 Slice 128.54 146.34725 0 5,306.1168 3,445.8325 200 15 Slice 137.556 149.7287 0 5,413.7471 3,515.7285 200 16 Slice 146.572 153.40181 0 5,474.6377 3,555.2713 200 17 Slice 154.84833 157.02733 0 5,494.5156 3,568.1802 200 18 Slice 162.385 160.56798 0 5,488.8525 3,564.5025 200 19 Slice 169.92167 164.33452 0 5,479.0543 3,558.1395 200 20 Slice 178.345 168.83799 0 5,445.589 3,536.4068 200 21 Slice 189.20663 175.11777 0 5,027.3056 3,264.7704 200 22 Slice 197.05957 179.88127 59.668882 4,551.1791 4,044.174 100 23 Slice 203.30881 183.99547 -175.66487 4,096.2541 3,688.2837 100 24 Slice 25 212.51464 190.35124 -540.69957 3,459.1521 3,114.6346 100 Slice 221.72046 197.16357 -934.14335 2,845.6523 2,562.2369 100 26 Slice 230.92629 204.46752 -1,358.1775 2,241.9831 2,018.6906 100 27 Slice 240.13212 212.30479 -1,815.3973 1,632.9801 1,470.3419 100 28 Slice 245.42498 216.99623 35.861195 1,382.0932 1,051.7917 0 29 Slice 30 248.05746 219.45219-103.30534 1,196.9463 935.15694 0 Slice 254.13817 225.35154-438.79845 772.75847 603.74508 0 31 32Ce 261.63817 232.95246 0 259.7481 162.30863 0 file:///C:/Users/MattM/geotech consultants/shared documents - documents/2021 jobs/21303 angle (drw)/slope stability/sept 2022 slope stability - with b... 5/5