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REVIEWED PLN RESUB 1-BLD2024-1018+Geotechnical_Report Iron Vista+9.6.2024_1.40.07_PM+4485342.,.,.,.,.,.REVIEWED ,.,.,.,.,., BY CITY OF EDMONDS BUILDING DEPARTMENT! BLD2024-1018 RESUB Sep 06 2024 CITY OF EDMONDS DEVELOPMENT SERVICES DEPARTMENT COBALT G E 0 S C I E N C E S Geotechnical Investigation Proposed Residential Development 191xx 941h Avenue West Edmonds, Washington September 17, 2020 RECEIVED Aug 02 2022 CITV OF EDMONDS DEVELOPMENT SERVICES DEPARTMENT 11.11ByzI) 041Z116.1 04: , Reviewed by ; , City of Edmonds ; Planning Division '--------------- COBALT G E 0 S C I E N C E S July 6, 2022 Landsverk Quality Homes C/O Brian Kalab, IECO RE: Plan Review Proposed Development 950019oth Street SW Parcel No.'s 004346000006io2,104, io5, & 1o6 Edmonds, Washington Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, Washington 98028 In accordance with your authorization, Cobalt Geosciences, LLC has prepared a plan review letter for the project. We have reviewed the civil plans by IECO dated May 3, 2022 which show the location of a detention pipe below a portion of the site roadway and east of building lots. The detention system will be set about io feet below grade in soils that are anticipated to be dense to very dense glacial till. The vault location and elevations appear suitable from a geotechnical standpoint. We should be on site to verify aspects of the construction. These include but are not limited to soil bearing for footings, vault excavation and backfill placement, footing drainage, fill compaction, slab on grade and road preparation, temporary excavations, erosion control, and drainage systems. Sincerely, Cobalt Geosciences, LLC U,- Phil Haberman, PE, LG, LEG Principal www.cobaltgeo.com (2o6) 331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON Table of Contents 1.o INTRODUCTION............................................................................................................. 1 2.0 PROJECT DESCRIPTION.............................................................................................. 1 3.0 SITE DESCRIPTION....................................................................................................... 1 4.o FIELD INVESTIGATION............................................................................................... 1 4.1.1 Site Investigation Program................................................................................... 1 5.0 SOIL AND GROUNDWATER CONDITIONS.............................................................. 1 5.1.1 Area Geology........................................................................................................ 2 5.1.2 Groundwater........................................................................................................ 2 6.o GEOLOGIC HAZARDS................................................................................................... 3 6.1 Erosion Hazard.................................................................................................... 3 6.2 Seismic Hazard.................................................................................................... 3 7.o DISCUSSION................................................................................................................... 4 7.1.1 General.................................................................................................................4 8.o RECOMMENDATIONS.................................................................................................. 4 8.1.1 Site Preparation................................................................................................... 4 8.1.2 Temporary Excavations........................................................................................ 4 8.1.3 Erosion and Sediment Control.............................................................................. 5 8.1.4 Foundation Design............................................................................................... 6 8.1.5 Reinforced Concrete Retaining Walls................................................................... 7 8.1.6 Slab-on-Grade...................................................................................................... 7 8.1.E Groundwater Influence on Construction.............................................................. 8 8.1.8 Utilities................................................................................................................ 8 8.1.9 Pavements............................................................................................................8 9.o CONSTRUCTION FIELD REVIEWS...........................................................................10 io.o CLOSURE...................................................................................................................10 LIST OF APPENDICES Appendix A — Statement of General Conditions Appendix B — Figures Appendix C — Test Pit Logs GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON September 17, 2020 1.o Introduction COBALT GEOSCIENCES In accordance with your authorization, Cobalt Geosciences, LLC (Cobalt) has completed a geotechnical investigation for the proposed residential development located at 191xx 94th Avenue West in Edmonds, Washington (Figure 1). The purpose of the geotechnical investigation was to identify subsurface conditions and to provide geotechnical recommendations for foundation design, stormwater management, earthwork, soil compaction, and suitability of the on -site soils for use as fill. The scope of work for the geotechnical evaluation consisted of a site investigation followed by engineering analyses to prepare this report. Recommendations presented herein pertain to various geotechnical aspects of the proposed development, including foundation support of the new buildings and pavement design. 2.0 Project Description The project includes construction of multiple single-family residences, driveways, and utility infrastructure. Stormwater will be infiltrated if determined to be feasible. Anticipated building loads are expected to be light to moderate and site grading will include cuts and fills on the order of 4 feet or less. We should be provided with the final plans when they become available. 3.0 Site Description The site is located at 191xx 94th Avenue West in Edmonds, Washington (Figure 1). The property consists of four adjoining parcels with a total area of about 1.11 acres. The property is undeveloped and vegetated with grasses, bushes/shrubs, blackberry vines, along with variable diameter evergreen and deciduous trees. The site slopes gently to moderately downward from east to west at magnitudes of 5 to about 35 percent and relief of about 75 feet. The site is bordered on all sides by residential properties. Access is near the southeast corner and 94tn Avenue West. 4.o Field Investigation 4.1.1 Site Investigation Program The geotechnical field investigation program was completed on August 27, 202o and included excavating and sampling six test pits within the property for subsurface analysis. The soils encountered were logged in the field and are described in accordance with the Unified Soil Classification System (USCS). PO Box 82243 Kenmore, WA 98028 cobaltgeoOgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON September 17, 2020 COBALT GEOSCIENCES A Cobalt Geosciences field representative conducted the explorations, collected disturbed soil samples, classified the encountered soils, kept a detailed log of the explorations, and observed and recorded pertinent site features. The results of the sampling are presented on the exploration logs enclosed in Appendix C. 5.0 Soil and Groundwater Conditions 5.1.1 Area Geology The site lies within the Puget Lowland. The lowland is part of a regional north -south trending trough that extends from southwestern British Columbia to near Eugene, Oregon. North of Olympia, Washington, this lowland is glacially carved, with a depositional and erosional history including at least four separate glacial advances/retreats. The Puget Lowland is bounded to the west by the Olympic Mountains and to the east by the Cascade Range. The lowland is filled with glacial and non -glacial sediments consisting of interbedded gravel, sand, silt, till, and peat lenses. The Geologic Map of the Edmonds East and West Quadrangles, indicates that the site is underlain by Vashon Glacial Till. Vashon Glacial Till is typically characterized by an unsorted, non -stratified mixture of clay, silt, sand, gravel, cobbles and boulders in variable quantities. These materials are typically dense and relatively impermeable. The poor sorting reflects the mixing of the materials as these sediments were overridden and incorporated by the glacial ice. Explorations All of the test pits encountered 6 to 12 inches of topsoil and vegetation underlain by approximately 2.5 to 4.5 feet of loose to medium dense, silty -fine to fine grained sand with gravel (Weathered Glacial Till). These materials were underlain by dense to very dense, silty -fine to fine grained sand with gravel (Glacial Till), which continued to the termination depths of the test pits. 5.1.2 Groundwater Groundwater was not encountered during our exploration work. Mottled and cemented soils were encountered at shallow depths. It is likely that perched groundwater may be present at shallow depths during the wet season. Water table elevations often fluctuate over time. The groundwater level will depend on a variety of factors that may include seasonal precipitation, irrigation, land use, climatic conditions and soil permeability. Water levels at the time of the field investigation may be different from those encountered during the construction phase of the project. 2 PO Box 82243 Kenmore, WA 98028 cobaltgeoOgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON September 17, 2020 6.o Geologic Hazards 6.1 Erosion Hazard COBALT GEOSCIENCES The Natural Resources Conservation Services (NRCS) maps for Snohomish County indicate that the site is underlain by Alderwood—Urban Land Complex (8 to 15 percent slopes) and Alderwood-Everett gravelly sandy loams (25 to 70 percent slopes). These soils would have a slight to severe erosion potential in a disturbed state, depending on the slope magnitude. It is our opinion that soil erosion potential at this project site can be reduced through landscaping and surface water runoff control. Typically erosion of exposed soils will be most noticeable during periods of rainfall and may be controlled by the use of normal temporary erosion control measures, such as silt fences, hay bales, mulching, control ditches and diversion trenches. The typical wet weather season, with regard to site grading, is from October 31st to April ist. Erosion control measures should be in place before the onset of wet weather. 6.2 Seismic Hazard The overall subsurface profile corresponds to a Site Class D as defined by Table 1613.5.2 of the 2015 International Building Code (2015 IBC). A Site Class D applies to an overall profile consisting of dense to very dense soils within the upper too feet. We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website to obtain values for Ss, S,, FQ, and F,,. The USGS website includes the most updated published data on seismic conditions. The site specific seismic design parameters and adjusted maximum spectral response acceleration parameters are as follows: PGA (Peak Ground Acceleration, in percent of g) Ss 129.6o% of g S, 45.8o% of g FA 1.2 Fv Null Additional seismic considerations include liquefaction potential and amplification of ground motions by soft/loose soil deposits. The liquefaction potential is highest for loose sand with a high groundwater table. The relatively dense soil deposits that underlie the site have a low liquefaction potential. 3 PO Box 82243 Kenmore, WA 98028 cobaltgeoOgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON September 17, 2020 17.o DISCUSSION 17-m General COBALT GEOSCIENCES APPROVED BY PLANNING The site is underlain by weathered and unweathered glacial till. The proposed residential buildings may be supported on shallow foundation systems bearing on medium dense or firmer native soils and structural fill placed on suitable native soils. Local overexcavation of fill and/or loose soils may be necessary below proposed foundation elements. Fill depths are expected to vary with location. We recommend detention with overflow of stormwater devices into City stormwater infrastructure. Infiltration of runoff is not feasible in glacial till soils at this site. 8.o Recommendations 8.1.1 Site Preparation Trees, shrubs and other vegetation should be removed prior to stripping of surficial organic -rich soil and fill. Based on observations from the site investigation program, it is anticipated that the stripping depth will be 6 to 18 inches. Deeper excavations will be necessary below large trees and in any areas underlain by undocumented fill materials. The native soils consist of silty -sand with gravel and sandy silt with gravel. These soils may be used as structural fill provided they achieve compaction requirements and are within 3 percent of the optimum moisture. These soils may only be suitable for use as fill during the summer months, as they will be above the optimum moisture levels in their current state. These soils are variably moisture sensitive and may degrade during periods of wet weather and under equipment traffic. Imported structural fill should consist of a sand and gravel mixture with a maximum grain size of 3 inches and less than 5 percent fines (material passing the U.S. Standard No. 200 Sieve). Structural fill should be placed in maximum lift thicknesses of 12 inches and should be compacted to a minimum of 95 percent of the modified proctor maximum dry density, as determined by the ASTM D 1557 test method. 8.1.2 Temporary Excavations Based on our understanding of the project, we anticipate that the grading could include local cuts on the order of approximately 4 feet or less for foundation and utility placement. Any deeper excavations should be sloped no steeper than 1.5H:1V (Horizontal:Vertical) in loose native soils and 1H:1V in medium dense to dense native soils. Steeper excavations (3/4H:1V) are suitable in soils that are very dense. If an excavation is subject to heavy vibration or surcharge loads, we recommend that the excavations be sloped no steeper than 2H:1V, where room permits. Temporary cuts should be in accordance with the Washington Administrative Code (WAC) Part N, Excavation, Trenching, and Shoring. Temporary slopes should be visually inspected daily by a qualified person during construction activities and the inspections should be documented in daily reports. The contractor is responsible for maintaining the stability of the temporary cut slopes and reducing slope erosion during construction. 4 PO Box 82243 Kenmore, WA 98028 cobaltgeoOgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON September 17, 2020 COBALT GEOSCIENCES Temporary cut slopes should be covered with visqueen to help reduce erosion during wet weather, and the slopes should be closely monitored until the permanent retaining systems or slope configurations are complete. Materials should not be stored or equipment operated within io feet of the top of any temporary cut slope. Soil conditions may not be completely known from the geotechnical investigation. In the case of temporary cuts, the existing soil conditions may not be completely revealed until the excavation work exposes the soil. Typically, as excavation work progresses the maximum inclination of temporary slopes will need to be re-evaluated by the geotechnical engineer so that supplemental recommendations can be made. Soil and groundwater conditions can be highly variable. Scheduling for soil work will need to be adjustable, to deal with unanticipated conditions, so that the project can proceed and required deadlines can be met. If any variations or undesirable conditions are encountered during construction, we should be notified so that supplemental recommendations can be made. If room constraints or groundwater conditions do not permit temporary slopes to be cut to the maximum angles allowed by the WAC, temporary shoring systems may be required. The contractor should be responsible for developing temporary shoring systems, if needed. We recommend that Cobalt Geosciences and the project structural engineer review temporary shoring designs prior to installation, to verify the suitability of the proposed systems. 8.1.3 Erosion and Sediment Control Erosion and sediment control (ESC) is used to reduce the transportation of eroded sediment to wetlands, streams, lakes, drainage systems, and adjacent properties. Erosion and sediment control measures should be implemented and these measures should be in general accordance with local regulations. At a minimum, the following basic recommendations should be incorporated into the design of the erosion and sediment control features for the site: • Schedule the soil, foundation, utility, and other work requiring excavation or the disturbance of the site soils, to take place during the dry season (generally May through September). However, provided precautions are taken using Best Management Practices (BMP's), grading activities can be completed during the wet season (generally October through April). All site work should be completed and stabilized as quickly as possible. • Additional perimeter erosion and sediment control features may be required to reduce the possibility of sediment entering the surface water. This may include additional silt fences, silt fences with a higher Apparent Opening Size (AOS), construction of a berm, or other filtration systems. • Any runoff generated by dewatering discharge should be treated through construction of a sediment trap if there is sufficient space. If space is limited other filtration methods will need to be incorporated. 5 PO Box 82243 Kenmore, WA 98028 cobaltgeoOgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON September 17, 2020 8.1.4 Foundation Design COBALT GEOSCIENCES The proposed residential buildings and garages may be supported on shallow spread footing foundation systems bearing on undisturbed medium dense or firmer native soils or on properly compacted structural fill placed on the suitable native soils. If structural fill is used to support foundations, then the zone of structural fill should extend beyond the faces of the footing a lateral distance at least equal to the thickness of the structural fill. Depending on the location and finish floor elevations of new buildings, some overexcavation may be required. Fill is likely present near existing buildings and possibly in yard areas. Any fill will need to be removed below new footings and replaced with compacted structural fill as discussed above. For shallow foundation support, we recommend widths of at least 16 and 24 inches, respectively, for continuous wall and isolated column footings supporting the proposed structure. Provided that the footings are supported as recommended above, a net allowable bearing pressure of 2,000 pounds per square foot (psf) may be used for design. A 1/3 increase in the above value may be used for short duration loads, such as those imposed by wind and seismic events. Structural fill placed on bearing, native subgrade should be compacted to at least 95 percent of the maximum dry density based on ASTM Test Method D1557. Footing excavations should be inspected to verify that the foundations will bear on suitable material. Exterior footings should have a minimum depth of 18 inches below pad subgrade (soil grade) or adjacent exterior grade, whichever is lower. Interior footings should have a minimum depth of 12 inches below pad subgrade (soil grade) or adjacent exterior grade, whichever is lower. If constructed as recommended, the total foundation settlement is not expected to exceed 1 inch. Differential settlement, along a 25-foot exterior wall footing, or between adjoining column footings, should be less than 1/2 inch. This translates to an angular distortion of 0.002. Most settlement is expected to occur during construction, as the loads are applied. However, additional post -construction settlement may occur if the foundation soils are flooded or saturated. All footing excavations should be observed by a qualified geotechnical consultant. Resistance to lateral footing displacement can be determined using an allowable friction factor of 0.40 acting between the base of foundations and the supporting subgrades. Lateral resistance for footings can also be developed using an allowable equivalent fluid passive pressure of 225 pounds per cubic foot (pcf) acting against the appropriate vertical footing faces (neglect the upper 12 inches below grade in exterior areas). The allowable friction factor and allowable equivalent fluid passive pressure values include a factor of safety of 1.5. The frictional and passive resistance of the soil may be combined without reduction in determining the total lateral resistance. A 1/3 increase in the above values may be used for short duration transient loads. Care should be taken to prevent wetting or drying of the bearing materials during construction. Any extremely wet or dry materials, or any loose or disturbed materials at the bottom of the footing excavations, should be removed prior to placing concrete. The potential for wetting or drying of the bearing materials can be reduced by pouring concrete as soon as possible after completing the footing excavation and evaluating the bearing surface by the geotechnical engineer or his representative. 6 PO Box 82243 Kenmore, WA 98028 cobaltgeoOgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON September 17, 2020 8.1.5 Stormwater Management COBALT GEOSCIENCES The site is underlain by glacial till which typically has a very low permeability. We encountered mottled soils above the unweathered glacial till. Additionally, the unweathered till was cemented. It is our opinion that infiltration is not feasible due to the soil and anticipated groundwater conditions during the wet season. Infiltrating runoff would migrate laterally along the weathered -unweathered glacial till contact and into adjacent properties. This migrating runoff could flow into any adjacent basements, utilities, or fill zones. We performed a small scale pilot infiltration test in TP-1 at a depth of 3 feet below grade. Following saturation, testing, and application of correction factors for site variability (o.8), influent control (o.9), and testing (0.5), the rate was o.1 inches per hour. We encountered ponded stormwater below the area after completion of the testing. We recommend utilizing dispersion trenches if there is adequate space. Other options include detention vaults with overflow to City infrastructure and possibly rain gardens and permeable pavements, depending on their location and elevations. We can provide additional input upon request and once a civil plan has been prepared. 8.1.6 Slab -on -Grade We recommend that the upper 12 inches of the existing fill and/or native soils within slab areas be re - compacted to at least 95 percent of the modified proctor (ASTM D1557 Test Method). Often, a vapor barrier is considered below concrete slab areas. However, the usage of a vapor barrier could result in curling of the concrete slab at joints. Floor covers sensitive to moisture typically requires the usage of a vapor barrier. A materials or structural engineer should be consulted regarding the detailing of the vapor barrier below concrete slabs. Exterior slabs typically do not utilize vapor barriers. The American Concrete Institutes ACI 36oR-o6 Design of Slabs on Grade and ACI 302.1R-04 Guide for Concrete Floor and Slab Construction are recommended references for vapor barrier selection and floor slab detailing. Slabs on grade may be designed using a coefficient of subgrade reaction of 18o pounds per cubic inch (pci) assuming the slab -on -grade base course is underlain by structural fill placed and compacted as outlined in Section 8.1. A 6 inch thick capillary break should be placed over the prepared subgrade. This should consist of 5/8 inch clean angular rock or pea gravel. A perimeter drainage system is recommended unless interior slab areas are elevated a minimum of 12 inches above adjacent exterior grades. If installed, a perimeter drainage system should consist of a 4 inch diameter perforated drain pipe surrounded by a minimum 6 inches of drain rock wrapped in a non -woven geosynthetic filter fabric to reduce migration of soil particles into the drainage system. The perimeter drainage system should discharge by gravity flow to a suitable stormwater system. Exterior grades surrounding buildings should be sloped at a minimum of one percent to facilitate surface water flow away from the building and preferably with a relatively impermeable surface cover immediately adjacent to the building. 7 PO Box 82243 Kenmore, WA 98028 cobaltgeoOgmail.com 2o6-331-1097 COBALT GEOTECHNICAL INVESTIGATION GEOSCIENCES EDMONDS, WASHINGTON September 17, 2020 8.1.7 Groundwater Influence on Construction Groundwater was not encountered during the test pit exploration work. We anticipate that perched groundwater could be present at shallow depths during the wet season. If groundwater is encountered, we anticipate that sump excavations and small diameter pumps systems will adequately de -water short-term excavations, if required. Any system should be designed by the contractor. We can provide additional recommendations upon request. 8.1.8 Utilities Utility trenches should be excavated according to accepted engineering practices following OSHA (Occupational Safety and Health Administration) standards, by a contractor experienced in such work. The contractor is responsible for the safety of open trenches. Traffic and vibration adjacent to trench walls should be reduced; cyclic wetting and drying of excavation side slopes should be avoided. Depending upon the location and depth of some utility trenches, groundwater flow into open excavations could be experienced, especially during or shortly following periods of precipitation. In general, silty and sandy soils were encountered at shallow depths in the explorations at this site. These soils have low cohesion and density and will have a tendency to cave or slough in excavations. Shoring or sloping back trench sidewalls is required within these soils in excavations greater than 4 feet deep. All utility trench backfill should consist of imported structural fill or suitable on site soils. Utility trench backfill placed in or adjacent to buildings and exterior slabs should be compacted to at least 95 percent of the maximum dry density based on ASTM Test Method D1557. The upper 5 feet of utility trench backfill placed in pavement areas should be compacted to at least 95 percent of the maximum dry density based on ASTM Test Method D1557. Below 5 feet, utility trench backfill in pavement areas should be compacted to at least 90 percent of the maximum dry density based on ASTM Test Method D1557. Pipe bedding should be in accordance with the pipe manufacturer's recommendations. The contractor is responsible for removing all water -sensitive soils from the trenches regardless of the backfill location and compaction requirements. Depending on the depth and location of the proposed utilities, we anticipate the need to re -compact existing fill soils below the utility structures and pipes. The contractor should use appropriate equipment and methods to avoid damage to the utilities and/or structures during fill placement and compaction procedures. 8.1.9 Pavement Recommendations The near surface subgrade soils generally consist of silty sand with gravel. These soils are rated as good for pavement subgrade material (depending on silt content and moisture conditions). We estimate that the subgrade will have a California Bearing Ratio (CBR) value of 10 and a modulus of subgrade reaction value of k = 200 pci, provided the subgrade is prepared in general accordance with our recommendations. We recommend that, at a minimum, 18 inches of the existing subgrade material be moisture conditioned (as necessary) and re -compacted to prepare for the construction of pavement sections. Deeper levels of recompaction or overexcavation and replacement may be necessary in areas where fill and/or very poor (soft/loose) soils are present. Any soils that cannot be compacted to required levels and soils that have more than 40 percent fines by weight should be removed and replaced with imported structural fill. 8 PO Box 82243 Kenmore, WA 98028 cobaltgeoOgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON September 17, 2020 COBALT GEOSCIENCES The subgrade should be compacted to at least 95 percent of the maximum dry density as determined by ASTM Test Method D1557. In place density tests should be performed to verify proper moisture content and adequate compaction. The recommended flexible and rigid pavement sections are based on design CBR and modulus of subgrade reaction (k) values that are achieved, only following proper subgrade preparation. It should be noted that subgrade soils that have relatively high silt contents will likely be highly sensitive to moisture conditions. The subgrade strength and performance characteristics of a silty subgrade material may be dramatically reduced if this material becomes wet. Based on our knowledge of the proposed project, we expect the traffic to range from light duty (passenger automobiles) to heavy duty (delivery trucks). The following tables show the recommended pavement sections for light duty and heavy duty use. ASPHALTIC CONCRETE (FLEXIBLE) PAVEMENT LIGHT DUTY Asphaltic Concrete Aggregate Base* Compacted Subgrade* ** 2.5 in. 6.o in. 12.0 in. HEAVY DUTY Asphaltic Concrete Aggregate Base* Compacted Subgrade* ** 3.5 in. 6.o in. 12.0 in. PORTLAND CEMENT CONCRETE (RIGID) PAVEMENT Min. PCC Depth Aggregate Base* Compacted Subgrade* ** 6.o in. 6.o in. 12.0 in. * 95% compaction based on ASTM Test Method D1557 **A proof roll may be performed in lieu of in place density tests The asphaltic concrete depth in the flexible pavement tables should be a surface course type asphalt, such as Washington Department of Transportation (WSDOT) 1/2 inch HMA. The rigid pavement design is based on a Portland Cement Concrete (PCC) mix that has a 28 day compressive strength of 4,000 pounds per square inch (psi). The design is also based on a concrete flexural strength or modulus of rupture of 550 psi. 9 PO Box 82243 Kenmore, WA 98028 cobaltgeoogmail.com 2o6-331-1097 COBALT GEOTECHNICAL INVESTIGATION GEOSCIENCES EDMONDS, WASHINGTON September 17, 2020 9.o Construction Field Reviews Cobalt Geosciences should be retained to provide part time field review during construction in order to verify that the soil conditions encountered are consistent with our design assumptions and that the intent of our recommendations is being met. This will require field and engineering review to: ■ Monitor and test structural fill placement and soil compaction ■ Observe bearing capacity at foundation locations ■ Observe slab -on -grade preparation ■ Observe excavation stability Geotechnical design services should also be anticipated during the subsequent final design phase to support the structural design and address specific issues arising during this phase. Field and engineering review services will also be required during the construction phase in order to provide a Final Letter for the project. io.o Closure This report was prepared for the exclusive use of Landsverk Quality Homes, Inc. appointed consultants. Any use of this report or the material contained herein by third parties, or for other than the intended purpose, should first be approved in writing by Cobalt Geosciences, LLC. The recommendations contained in this report are based on assumed continuity of soils with those of our test holes, and assumed structural loads. Cobalt Geosciences should be provided with final architectural and civil drawings when they become available in order that we may review our design recommendations and advise of any revisions, if necessary. Use of this report is subject to the Statement of General Conditions provided in Appendix A. It is the responsibility of Landsverk Quality Homes, Inc. who is identified as "the Client" within the Statement of General Conditions, and its agents to review the conditions and to notify Cobalt Geosciences should any of these not be satisfied. 10 PO Box 82243 Kenmore, WA 98028 cobaltgeoOgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON September 17, 2020 Respectfully submitted, Cobalt Geosciences, LLC Original signed by: HONrWA y9 a ✓ 'i yam. 54896 s CIST AONA1.�- 9/17/2020 Phil Haberman, PE, LG, LEG Principal PH/sc PO Box 82243 Kenmore, WA 98028 cobaltgeoogmail.com 2o6-331-1097 179 2513 ��• K d G e COBALT GEOSCIENCES 11 APPENDIX A Statement of General Conditions Statement of General Conditions USE OF THIS REPORT: This report has been prepared for the sole benefit of the Client or its agent and may not be used by any third party without the express written consent of Cobalt Geosciences and the Client. Any use which a third party makes of this report is the responsibility of such third party. BASIS OF THE REPORT: The information, opinions, and/or recommendations made in this report are in accordance with Cobalt Geosciences present understanding of the site specific project as described by the Client. The applicability of these is restricted to the site conditions encountered at the time of the investigation or study. If the proposed site specific project differs or is modified from what is described in this report or if the site conditions are altered, this report is no longer valid unless Cobalt Geosciences is requested by the Client to review and revise the report to reflect the differing or modified project specifics and/or the altered site conditions. STANDARD OF CARE: Preparation of this report, and all associated work, was carried out in accordance with the normally accepted standard of care in the state of execution for the specific professional service provided to the Client. No other warranty is made. INTERPRETATION OF SITE CONDITIONS: Soil, rock, or other material descriptions, and statements regarding their condition, made in this report are based on site conditions encountered by Cobalt Geosciences at the time of the work and at the specific testing and/or sampling locations. Classifications and statements of condition have been made in accordance with normally accepted practices which are judgmental in nature; no specific description should be considered exact, but rather reflective of the anticipated material behavior. Extrapolation of in situ conditions can only be made to some limited extent beyond the sampling or test points. The extent depends on variability of the soil, rock and groundwater conditions as influenced by geological processes, construction activity, and site use. VARYING OR UNEXPECTED CONDITIONS: Should any site or subsurface conditions be encountered that are different from those described in this report or encountered at the test locations, Cobalt Geosciences must be notified immediately to assess if the varying or unexpected conditions are substantial and if reassessments of the report conclusions or recommendations are required. Cobalt Geosciences will not be responsible to any parry for damages incurred as a result of failing to notify Cobalt Geosciences that differing site or sub -surface conditions are present upon becoming aware of such conditions. PLANNING, DESIGN, OR CONSTRUCTION: Development or design plans and specifications should be reviewed by Cobalt Geosciences, sufficiently ahead of initiating the next project stage (property acquisition, tender, construction, etc), to confirm that this report completely addresses the elaborated project specifics and that the contents of this report have been properly interpreted. Specialty quality assurance services (field observations and testing) during construction are a necessary part of the evaluation of sub -subsurface conditions and site preparation works. Site work relating to the recommendations included in this report should only be carried out in the presence of a qualified geotechnical engineer; Cobalt Geosciences cannot be responsible for site work carried out without being present. APPENDIX B Figures: Vicinity Map, Site Plan 10.2 PO Box 82243 Kenmore, WA 98028 cobaltgeoogmail.com 2o6-331-1097 1Z2"Yl.UUIJ' W 1L1 1U.DUU- W WGS84 122'19.000' W a ,1 WASHINGTON j ! ^ �• ii r r ° Edmonds r a /" ` - - .•" t5W Project 73 Location - - 1 j . lalb°tRdr t 1:74th. 476tii St SV11 -_ - - --- r �� 't°i • � 'a I� �' '° 178thStSW 418 fSW rNvrlN •180 z \ 81'st PI z o : • . ,-2f - _ - - �� ' '82nd St -SW P sw o o —> �, n /'^ _r 1. '_ a '. �_ � . � � • e al sw � � _ V n 106th it 514 3 � ash d •-`�� ro,Ql � �`� •s,.= '" •61 �`,� 1 1m • 1v n• d 1 ' t I • Oth 3 SW f7 i� °---I ,balk 92nd .192nd PI SVIf� • . •� pd� / , q _`\.,Q t^6..• O Ifl�� •Q :i m •II _ 3 '193r'a Way— e' .Y yiewla-n Wa' l� Ir • > a c ,I c3 T� > y--• i Z d.• a- 194th 1N t g -� z 200th St SW1 00 0 o o c. , z _6 z , ?ul n '.3ti: 3 § •r ::M 3a I. 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'r i' 122'23.000' W 122'22.000' W 122'21.000' W 122,20.000' W WG584 122'19.000' W 0 5 I TN MN NATIONAL 1000 0 1000 2000 30M 4000 M"`� 16° N GEOGRAPHIC 5 0 KItOMETfRS "`t IA 1000 0 METERS 1000 11/26/14 Cobalt Geosciences, LLC Proposed Res. Development VICINITY P.O. Box 82243 1 th Avenue West �P Kenmore, WA g8o28 COBALT 91XX 94 (206) 331-1097 Edmonds, Washington FIGURE i www.cobaltgeo.com cobaltgeo(&gmail.com TP-1 Approximate 9 Test Pit Location Proposed Res. Development 191xx 94th Avenue West Edmonds, Washington SITE PLAN FIGURE 2 N A Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 www.cobaltgeo.com cobaltgeopgmail.com APPENDIX C Exploration Logs Unified Soil Classification System (USCS) MAJOR DIVISIONS SYMBOL TYPICAL DESCRIPTION Clean Gravels GW Well -graded gravels, gravels, gravel -sand mixtures, little or no fines Gravels (more than 50% (less than 5% fines) GP Poorly graded gravels, gravel -sand mixtures, little or no fines COARSE GRAINED SOILS of coarse fraction retained on No. 4 sieve) Gravels with Fines (more than 12% fines) GM Silty gravels, gravel -sand -silt mixtures GC Clayey gravels, gravel -sand -clay mixtures (more than 50% retained on Clean Sands ;•; sw Well -graded sands, gravelly sands, little or no fines No. 200 sieve) Sands (50% or more of coarse fraction (less than 5% fines) sP Poorly graded sand, gravelly sands, little or no fines passes the No. 4 sieve) Sands with Fines sM Silty sands, sand -silt mixtures (more than 12% fines) sc Clayey sands, sand -clay mixtures ML Inorganic silts of low to medium plasticity, sandy silts, gravelly silts, FINE GRAINED (50% or more Silts and Clays (liquid limit less than 50) Inorganic cL or clayey silts with slight plasticity Inorganic clays of low to medium plasticity, gravelly clays, sandy clays silty clays, lean clays Organic rganic oL Organic silts and organic silty clays of low plasticity passes the MH Inorganic silts, micaceous or diatomaceous fine sands or silty soils, No. 200 sieve) Silts and Clays (liquid limit 50 or more) Inorganic elastic silt CH Inorganic clays of medium to high plasticity, sandy fat clay, or gravelly fat clay Organic OHOrganic clays of medium to high plasticity, organic silts HIGHLY ORGANIC SOILS Primarily organic matter, dark in color, and organic odor PT Peat, humus, swamp soils with high organic content (ASTM D4427) Classification of Soil Constituents MAJOR constituents compose more than 50 percent, by weight, of the soil. Major constituents are capitalized (i.e., SAND). Minor constituents compose 12 to 50 percent of the soil and precede the major constituents (i.e., silty SAND). Minor constituents preceded by "slightly" compose 5 to 12 percent of the soil (i.e., slightly silty SAND). Trace constituents compose o to 5 percent of the soil (i.e., slightly silty SAND, trace gravel). Relative Density (Coarse Grained Soils) Consistency (Fine Grained Soils) N, SPT, Relative N, SPT, Relative Blows/FT Density Blows/FT Consistency 0-4 Very loose Under 2 Very soft 4 -10 Loose 2-4 Soft 10 - 30 Medium dense 4-8 Medium stiff 30 - 50 Dense 8 -15 Stiff Over 50 Very dense 15 - 30 Very stiff Over 3o Hard Grain Size Definitions Description Sieve Number and/or Size Fines <#200 (o.o8 mm) Sand -Fine #200 to #40 (o.o8 to 0.4 mm) -Medium #40 to #10 (0.4 to 2 mm) -Coarse #10 to #4 (2 to 5 mm) Gravel -Fine #4 to 3/4 inch (5 to 19 mm) -Coarse 3/4 to 3 inches (19 to 76 mm) Cobbles 3 to 12 inches (75 to 305 mm) Boulders >12 inches (305 mm) 1 Moisture Content Definitions 1 Dry Absence of moisture, dusty, dry to the touch Moist Damp but no visible water Wet Visible free water, from below water table Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 Soil Classification Chart Figure Ci (206) 331-1097 _ www.cobaltgeo.com cobaltgeo(&gmail.com Test Pit TP-1 Date: August 27, 2020 Depth: 7' Groundwater: None Contractor: Client Provided Elevation: Logged By: PH Checked By: SC 0 o Moisture Content (%) N Q Plastic I Liquid U N Limit Limit N Material Description o c ? o DCP Equivalent N-Value G 0 10 20 30 40 50 ------ ---- -- ToosoMYe__ Catn io-------------------------------- 1 SM/ Loose to medium dense, silty -fine to fine grained sand with gravel, 'I ML mottled yellowish brown to grayish brown, dry to moist. 2 (Weathered Glacial Till) ---- ---- -- --------------------------------------------- SM Dense to very dense, silty -fine to fine grained sand with gravel, 4 ML grayish brown, moist. (Glacial Till) 5 -Till is cemented 6 YZ End of Test Pit 7' 8 9 10 Date: August 27, 2020 Depth: 8' Groundwater: None Contractor: Client Provided Elevation: Logged By: PH Checked By: SC o Moisture Content (�) NPlastic 1 Liquid U E 3 Limit Limit N Material Description o ? o DCP Equivalent N-Value C 0 10 20 30 40 50 IIII Topsoil/Vegetation SM/ Loose to medium dense, silty -fine to fine grained sand with gravel, 2 ML mottled yellowish brown to grayish brown, dry to moist. (Weathered Glacial Till) 3 4 5 ----- ---- -- ------------------------------------------- 6 SM Dense to very dense, silty -fine to fine grained sand with gravel, ML grayish brown, moist. (Glacial Till) 7 Till is cemented End of Test Pit 8' 9 10 Cobalt Geosciences, LLC Proposed Development P.O. Box 82243 lxx th Avenue West Test Pit Kenmore, WA 98028 COBALT1 9 94 (2o6) 331-1097 • Edmonds, WA Logs www.cobaltgeo.com cobaltgeo(Rigmail.com Test Pit TP-3 Date: August 27, 2020 Depth: 7' Groundwater: None Contractor: Client Provided Elevation: Logged By: PH Checked By: SC 0 o Moisture Content (%) N Q Plastic I Liquid U N Limit Limit N Material Description o c ? o DCP Equivalent N-Value G 0 10 20 30 40 50 ------ ---- -- ToosoMYe__ Catn io-------------------------------- 1 SM/ Loose to medium dense, silty -fine to fine grained sand with gravel, 'I ML mottled yellowish brown to grayish brown, dry to moist. 2 (Weathered Glacial Till) ----- ---- -- --------------------------------------------- SM Dense to very dense, silty -fine to fine grained sand with gravel, 4 ML grayish brown, moist. (Glacial Till) 5 -Till is cemented 6 YZ End of Test Pit 7' 8 9 10 Date: August 27, 2020 Depth: 8' Groundwater: None Contractor: Client Provided Elevation: Logged By: PH Checked By: SC 0) o Moisture Content (�) N JO -0 Plastic Liquid U E 3 Limit Limit N Material Description o ? o DCP Equivalent N-Value C 0 10 20 30 40 50 IIII Topsoil/Vegetation SM/ Loose to medium dense, silty -fine to fine grained sand with gravel, 2 ML mottled yellowish brown to grayish brown, dry to moist. (Weathered Glacial Till) 3 �— --- —-------- SM/ Dense to very dense, silty -fine to fine grained sand with gravel, 5--- ML grayish brown, moist. (Glacial Till) i 7 1 14.1.111 1 1 Till is cemented End of Test Pit 8' 9 10 Cobalt Geosciences, LLC Proposed Development P.O. Box 82243 lxx th Avenue West Test Pit Kenmore, WA 98028 COBALT1 9 94 (2o6) 331-1097 • Edmonds, WA Logs www.cobaltgeo.com cobaltgeo(Rigmail.com Test Pit TP-5 Date: August 27, 2020 Depth: 7' Groundwater: None Contractor: Client Provided Elevation: Logged By: PH Checked By: SC 0 o Moisture Content (%) N Q Plastic I Liquid U N Limit Limit N Material Description o c ? o DCP Equivalent N-Value G 0 10 20 30 40 50 ------ ---- — -- ToosoMYe__ Cation -------------------------------- 1 SM/ Loose to medium dense, silty -fine to fine grained sand with gravel, ML mottled yellowish brown to grayish brown, dry to moist. 2 (Weathered Glacial Till) 3 ------- 4 -- SM/ --------------------------------------------- Dense to very dense, silty -fine to fine grained sand with gravel, ML grayish brown, moist. (Glacial Till) 5 -Till is cemented 6 YZ End of Test Pit 7' 8 9 10 Date: August 27, 2020 Depth: 8' Groundwater: None Contractor: Client Provided Elevation: Logged By: PH Checked By: SC 0) o Moisture Content (�) N JO -0 Plastic Liquid U E 3 Limit Limit N Material Description o ? o DCP Equivalent N-Value C 0 10 20 30 40 50 IIII Topsoil/Vegetation SM/ Loose to medium dense, silty -fine to fine grained sand with gravel, 2 ML mottled yellowish brown to grayish brown, dry to moist. (Weathered Glacial Till) 3 —A--- ---- - -- -------------------------------------------- SM/ Dense to very dense, silty -fine to fine grained sand with gravel, 5 ML grayish brown, moist. (Glacial Till) b 7 Till is cemented 9 10 End of Test Pit 8' Cobalt Geosciences, LLC Proposed Development P.O. Box 82243 • 1 lxx th Avenue West Test Pit Kenmore, WA 98028 • B9 94 (2o6) 331-1097 • Edmonds, WA Logs www.cobaltgeo.com cobaltgeo(Rigmail.com Test Pit TP-7 Date: August 27, 2020 Depth: 7' Groundwater: None Contractor: Client Provided Elevation: Logged By: PH Checked By: SC 0 o Moisture Content (%) N Q Plastic I Liquid u N Limit Limit N Material Description o c ? o DCP Equivalent N-Value G 0 10 20 30 40 50 ------ ---- — -- ToosoMYe__ Cation -------------------------------- 1 SM/ Loose to medium dense, silty -fine to fine grained sand with gravel, ML mottled yellowish brown to grayish brown, dry to moist. 2 (Weathered Glacial Till) 3 —4---------- -- --------------------------------------------- SM/ Dense to very dense, silty -fine to fine grained sand with gravel, 5 ML grayish brown, moist. (Glacial Till) b YZ -Till is cemented End of Test Pit 7' 8 9 10 Cobalt Geosciences, LLC Proposed Development P.O. Box 82243 \ • 1 lxx th Avenue West Test Pit Kenmore, WA 98028 • 9 94 (2o6) 331-1097 —•- • Edmonds WA Logs www.cobaltgeo.com cobaltgeo(Rigmail.com REVIEWED BY CITY OF EDMONDS COBALT G E 0 S C I E N C E S July 26, 2023 Updated December 18, 2023 Landsverk Quality Homes Attn: Duane Landsverk & Joseph Rowett 24113 56th Avenue West Mountlake Terrace, Washington RE: Plan Review & Addendum Proposed Development 19128 94th Ave West Edmonds, Washington RESUB Jan 16 2024 DEVELOPMENT SERVICES DEPARTMENT BLD2022-0926 Cobalt Geosciences, LLC P.O. Box 1792 North Bend, WA 98045 In accordance with your authorization, Cobalt Geosciences, LLC has prepared a plan review and geotechnical addendum letter for the proposed development. We have reviewed the provided civil plans by IECO dated May 3, 2022 with updates July 2023. We have no specific comments at this time. The proposed pipe anchors (concrete blocks keyed into native soils) appear to be suitable from a geotechnical standpoint. We should verify final locations and subgrade soil conditions for these features. We should be on site during construction to verify soil bearing of foundation areas, stormwater facility placement, temporary excavations, building locations, erosion control, any subsurface drainage placement, and soil compaction. We previously emailed the structural engineer regarding soil parameters for use in the concrete detention vault design. These included an at -rest earth pressure of 55 pcf for walls, a 14H psf seismic surcharge for at -rest conditions, and a soil bearing pressure of 5,000 psf for vaults set at least 5 feet below grade. These values may be used for design. We note that the site is underlain by dense to very dense glacially consolidated soil deposits. These materials have a low risk of global instability; however, there can be a slight chance of sloughing within the upper, weathered soil zones, particularly if stripped of topsoils and vegetation through a wet season without ongoing grading taking place. The risk of landslide activity is very low for the planned development and adjacent slope areas provided proper TESC is in place during construction and grading activities are performed in accordance with the approved plans, permit requirements, and under supervision by the geotechnical engineer. The rate of bluff retreat is estimated to be 0 to 2 centimeters annually. This is most likely to occur where slope magnitudes exceed 70 percent. This would be more consistent with soil creep and not actual bluff retreat since there is no bluff on site. The risk of landslide runout is essentially zero since the risk of landslide activity is very low to low. If local sloughing occurred due to improper clearing, etc., we would conservatively estimate small sloughs to have 3 to 7 feet of runout. www.cobaltgeo.com M (2o6) 331-1097 December 18, 2023 Page 2 of 2 Geotechnical Addendum No specific mitigation or mitigation sequencing appears to be warranted based on the type of construction in conjunction with the geologic conditions. We recommend that work progresses continuously until completed once the site is prepared with erosion control measures and is stripped of vegetation. Site grading should include benches along with temporary excavations as needed and vegetation removal should occur in areas only where work will occur. Our geotechnical report and recommendations are suitable and reliable for the currently known soil conditions at the site and our knowledge of the project to date. We should be on site during construction to verify aspects of construction and to provide any additional input as grading takes place. Please contact us if you have any questions. Sincerely, Cobalt Geosciences, LLC "0NY vti��WAsy�ti9�� 06 "O1054896 <� FS, 'ONAL 12/18/2023 Phil Haberman, PE, LG, LEG Principal vvash�/� qr��EfVinwnng,'WCOMIyceo www.cobaltgeo.com (2o6) 331-1097 COBALT G E 0 S C I E N C E S November 28, 2022 Landsverk Quality Homes Attn: Mr. Joseph Rowett josephPlgh-inc.com RE: Geotechnical Addendum Proposed Development 950019oth Street SW Parcel No.'s 004346000006io2,104, 105, & 1o6 Edmonds, Washington RESUB Mar07 2023 CITV OF EDMONDS DEVELOPMENTSERVICES DEPARTMENT BLD2022-0926 Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, Washington 98028 In accordance with your authorization, Cobalt Geosciences, LLC has prepared an addendum letter for the project. Steep Slope/Landslide Hazards There are local slopes near the property that have magnitudes of at least 40 percent and relief of more than 10 feet. Per the City of Edmonds GIS maps, areas that have magnitudes of 40 percent or more and relief of at least 10 feet are located near the north property line and elsewhere on properties to the northwest and northeast of the subject property (within 50 feet of the subject property). Areas within the property do not meet the criteria of steep slope or landslide hazards since their vertical relief is less than 10 feet. These areas can be safely benched and graded as part of site development. Overall, the steep slope areas on adjacent parcels appear stable at this time with no evidence of instability or erosion. Note that we observed these areas from the subject site and aerial photographs. The total relief is about 15 feet for slopes to the northwest of the site and relief of about 40 feet for slopes to the north and northeast of the site. While these areas consist of steep slope hazards, they do not exhibit evidence or characteristics of landslide hazard areas. Code Information 23.80.o6o Development standards — General requirements. 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; 2. Will not adversely impact other critical areas; 3. Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or less than predevelopment conditions; and 4. Are certified as safe as designed and under anticipated conditions by a qualified engineer or geologist, licensed in the state of Washington. APPROVED BY PLANNING www.cobaltgeo.com (2o6) 331-1097 November 28, 2022 Page 2 of 4 Geotechnical Addendum The project will include local grading and lot creation within moderately steep slope areas and for the north lot, near local steep slope hazard areas. It is our opinion that this work will not affect global or local stability provided work is monitored by the geotechnical engineer and erosion control measures are in place during construction. It will be necessary to bench and key anX existing slope areas with new structural fills (where proposed) and construction of properlX designed retaining walls where proposed and necessary. Similarly._ grading can be completed near the mapped steep slope hazard areas near the north propeM line provided any temporary excavations are protected from erosion and are created per our recommendations. These areas, if proposed to be graded, may be faced with retaining walls if necessarylproposed. The steep slope hazard areas appear to terminate at the north property line. The slope areas are cross gradient (do not slope toward the property) and it is our opinion that no buffer or setback is required from this area. However, we must review the final plans to verify that site grading and development in these areas do not require shoring or other design considerations, such as shoring. The proposed construction can be completed with no increase of the threat of geologic hazards on adjacent properties, will not impact other critical areas, and can be considered safe as designed under anticipated conditions. Again, we should be provided with the grading and development plans for this area to confirm suitability. 23.80.07o Development standards — Specific hazards. A. Erosion and Landslide Hazard Areas. Activities on sites containing erosion or landslide hazards shall meet the requirements of ECDC 23.80.o6o, Development standards — General requirements, 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; No specific setback is required from steep slope areas. Properly designed walls or graded areas are generally suitable near the north property line and closest steep slope areas. 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; No buffer is required from the localized steep slope hazard area near the north property line. We anticipate that new homes will be located at least.-, feet from the north property line and hazard areas. Proper erosion control, temporary cuts, and permanent walls or graded areas are anticipated and required. www.cobaltgeo.com (2o6) 331-1097 November 28, 2022 Page 3 of 4 Geotechnical Addendum 3. Alterations. 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; b. The alteration will not decrease slope stability on adjacent properties; and c. Such alterations will not adversely impact other critical areas; Provided earthwork activities are performed in accordance with the approved plans, all runoff is fully controlled, and periodic geotechnical oversight is performed, the development will not decrease slope stability on adjacent properties, will not increase surface water discharge or sedimentation beyond current levels, and will not impact other critical areas. Temporary and permanent erosion and sediment control devices should be in at all times during construction. The steep slope hazard areas themselves will not be disturbed or graded. 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: 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; b. Structures and improvements shall be clustered to avoid geologically hazardous areas and other critical areas; 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; d. Structures and improvements shall be located to preserve the most critical portion of the site and its natural landforms and vegetation; e. The proposed development shall not result in greater risk or a need for increased buffers on neighboring properties; f. The use of retaining walls that allow the maintenance of existing natural slope area is preferred over graded artificial slopes; and g. Development shall be designed to minimize impervious lot coverage; The very limited project scope does not pose a risk to critical areas or the need to increase buffers on adjacent properties. Essentially, the current stability will not be affected by the proposed construction since the proposed grading and slope stability will be equal to what is currently present once completed (or more stable). Factors of safety against landslide movements are well above minimum values based on the soil types, topography, and locations of the development. www.cobaltgeo.com (2o6) 331-1097 November 28, 2022 Page 4 of 4 Geotechnical Addendum 5. Vegetation Retention. Unless otherwise provided or as part of an approved alteration, removal of vegetation from an erosion or landslide hazard area or related buffer shall be prohibited; 6. Seasonal Restriction. Clearing shall be allowed only from May 1st to October 1st of each year; provided, that the director may extend or shorten the dry season on a case -by -case basis depending on actual weather conditions, except that timber harvest, not including brush clearing or stump removal, may be allowed pursuant to an approved forest practice permit issued by the city of Edmonds or the Washington State Department of Natural Resources; 7. Point Discharges. Point discharges from surface water facilities and roof drains onto or upstream from an erosion or landslide hazard area shall be prohibited except as follows: a. Conveyed via continuous storm pipe downslope to a point where there are no erosion hazard areas downstream from the discharge; b. Discharged at flow durations matching predeveloped conditions, with adequate energy dissipation, into existing channels that previously conveyed storm water runoff in the predeveloped state; or c. Dispersed discharge upslope of the steep slope onto a low -gradient, undisturbed buffer demonstrated to be adequate to infiltrate all surface and storm water runoff, and where it can be demonstrated that such discharge will not increase the saturation of the slope; and We concur with the above code items and have no additional comments at this time. We should be on site to verify aspects of the construction. These include but are not limited to soil bearing for footings, vault excavation and backfill placement, footing drainage, fill compaction, slab on grade and road preparation, temporary excavations, erosion control, and drainage systems. Sincerely, Cobalt Geosciences, LLC HONry9 W Asyy 06 r •1 y`. �� 54896 ��FssCISTER��\�� ZONAL�- 11/28/2022 Phil Haberman, PE, LG, LEG Principal www.cobaltgeo.com (2o6) 331-1097