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REVIEWED PLN_RESUB1 BLD2022-0502+GEO REPORT+7.18.2022_3.45.25_PM+2996531BLD2022-0502 RESUB COBALT GE0SCIENCES October 19, 2021 Updated June 29, 2022 Tim and Teresa Healy tihealy((@frontier.com RE: Limited Geologic Evaluation Proposed Deck 9o1 Bell Street Edmonds, Washington Jul 19 2022 CITY OF EDMONDS DEVELOPMENT SERVICES DEPARTMENT Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, Washington 98028 ------- - - - - - - Reviewed by City of Edmonds Planning Division - - - - - - - - - - - - - -' In accordance with your authorization, Cobalt Geosciences, LLC has performed a limited geotechnical evaluation related to the proposed deck at the referenced property. Site & Project Description The site is located at Sol Bell Street in Edmonds, Washington. The site consists of one rectangular shaped parcel (No. 004342o6603800) with a total area of about 0.23 acres. The southern portion of the property is developed with a residence with daylight basement and driveway. There are deck areas, patios, and short rock and modular block walls within the property, primarily just north of the residence. The property is vegetated with grasses, bushes, trees, ivy, ferns, blackberry vines, and other understory. The property slopes downward from south to north at magnitudes of 10 to 8o percent and relief of about 28 feet. The steeper slopes are within the north half of the property, and most have been re -graded as part of the site development. The property is bordered to the north and west by residences, to the south by Bell Street, and to the west by 9th Avenue North. The project includes a new deck along the north side of the residence to replace the existing deck. New footings will likely be incorporated into the design. Site Geology The Geologic Map of the Edmonds East and West Quadrangles indicates that the site is underlain by Vashon Advance Outwash. Vashon Advance Outwash consists of glacially consolidated sand and gravel with variable amounts of silt deposited in front of the advancing glaciers approximately 15,000 years ago. Soil & Groundwater Conditions As part of our evaluation, we excavated and sampled a hand boring near the existing deck footings. The hand boring encountered approximately 12 inches of topsoil and vegetation underlain by approximately 4.5 feet of loose to medium dense, silty -fine to medium grained sand trace gravel (Weathered Advance Outwash). This layer was underlain by medium dense to dense, fine to www.cobaltgeo.com (2o6) 331-1097 October 20, 2021 Updated June 29, 2o22 Page 2 of 6 Limited Geotechnical Evaluation medium grained sand trace gravel (Advance Outwash), which continued to the termination depths of the hand boring. The slope appears to be underlain by weathered and unweathered Vashon Advance Outwash, consistent with the geologic mapping in this area. Geologic Hazards & Code Information The site contains steep slope and erosion hazards based on soil composition and slope magnitudes. The proposed construction includes replacement of existing foundation elements with possible new footings for the replacement deck. This work will occur in areas that are currently graded and near the top of the steep slope areas. We performed a reconnaissance of the steep slope areas and the subject property. Overall, the site slopes are stable at this time with no evidence of historic or recent landslide activity, emergent groundwater, or erosion. The risk of erosion and shallow sloughing will remain low provided the slope continues to be fully vegetated and not modified. The slope system will not be affected by the proposed development and similarly, the proposed development will not be affected by the steep slope area. This conclusion is based on utilization of proper erosion control measures where the limited excavation work will occur and limiting disruption of steep slope areas. Municipal Code Information 23.80.o6o Development standards — General requirements. A. Alterations of geologically hazardous areas or associated buffers may only occur for activities that: i. 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. B. Critical Facilities Prohibited. Critical facilities shall not be sited within geologically hazardous areas unless there is no other practical alternative. [Ord. 4026 § 1 (Att. A), 2016; Ord. 3527 § 2, 20041. The proposed work includes replacement and slight expansion of an existing deck that may be close to, or slightly within a steep slope hazard area. Provided the construction is performed in accordance with our design recommendations (pin pile foundation support), geotechnical oversight, and per the approved plans, the proposed project can be completed without increasing the threat of geologic hazards on the prope , or adjacent properties, adversely affecting other critical areas, and can be certified to be safe as designed. The use of pin piles to support deck elements removes the current deck foundation surcharges from the slope: thereby providing a net increase in slope stabilithegeologic hazard risks will be mitigated to a level equal to or less than the pre -development conditions. www.cobaltgeo.com (2o6) 331-1097 October 20, 2021 Updated June 29, 2022 Page 3 of 6 Limited Geotechnical Evaluation 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 2.2.80.oft 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; We recommend a minimum effective or traditional foundation setback of to feet from slopes with magnitudes of 40 percent or more. The use of pin piles to support deck elements will result in an effective setback greater than 10 feet. The effective setback is the horizontal distance from the base of the foundation (in this case, pin piles) to the adjacent slope face. 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 warranted provided the work occurs within the planned locations only. There is existing development in these areas. Any disturbed vegetation outside of the deck location should be replaced. The risk of soil movements within or near the project location and buffer is very low and will remain low provided the work is completed per plan and our recommendations. 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; All of these conditions will be met provided the work is performed in accordance with the approved plans, with periodic geotechnical oversight, and pipe piles are utilized to support new foundation elements. Overall, there will be minimal if any alteration to or within erosion and landslide hazard areas. Most of the work will occur within existing developed areas. 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 www.cobaltgeo.com (2o6) 331-1097 October 20, 2021 Updated June 29, 2022 Page 4 of 6 Limited Geotechnical Evaluation 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 use of pin piles will result in factors of safety slightly higher than what is present currently in this area. The factors of safety currently present are higher than the minimum required values. This is based on the moderate topographic relief, marginally steep slopes, and presence of relatively dense, coarse -grained glacially consolidated soils. The deck is situated near the residence and in an area that is currently developed. The location minimizes alterations to hazard areas a much as feasible and will not result in a greater risk to neighboring properties. 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; We concur. 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; It is our opinion that this type of work could occur anytime during a typical year. The work does not include significant grading, vegetation removal, or heM equipment. 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; www.cobaltgeo.com (2o6) 331-1097 October 20, 2021 Updated June 29, 2022 Page 5 of 6 Limited Geotechnical Evaluation 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 8. Prohibited Development. On -site sewage disposal systems, including drain fields, shall be prohibited within erosion and landslide hazard areas and related buffers. We are not aware of any modifications to drainage systems. Conclusions and Recommendations Due to the presence of loose native soils within sloped areas, probable fill, and the presence of numerous short landscaping walls, deck footings should be supported on isolated concrete footings bearing on driven pipe piles. The pipe piles must extend to refusal in dense native soils. We anticipate a pile depth of 15 to 25 feet below grade. It is our opinion that the proposed foundation work will not adversely affect slope stability on the property or adjacent areas provided the new footings are supported as recommended in this report and earthwork activities are periodically monitored by the geotechnical engineer. Deck Foundation Design The proposed deck north of the residence may be supported on small diameter pipe piles driven to refusal below isolated concrete footings. Two-inch diameter Schedule 8o galvanized steel pipes may be used as the pipe piles. An axial capacity of 3 tons per pile may be used in the design. Pipe piles should be driven with a 14o-pound pneumatic hammer with refusal criteria consisting of 3 cycles of 6o seconds per inch of penetration. Pile depths will likely be 15 to 25 feet. Lateral resistance for footings can 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). Closure The information presented herein is based upon professional interpretation utilizing standard practices and a degree of conservatism deemed proper for this project. We emphasize that this report is valid for this project as outlined above and for the current site conditions and should not be used for any other site. Our work is limited to the deck project and does not assess stability of the residence or any walls. www.cobaltgeo.com (2o6) 331-1097 October 20, 2021 Updated June 29, 2022 Page 6 of 6 Limited Geotechnical Evaluation Sincerely, Cobalt Geosciences, LLC 6/29/2022 Phil Haberman, PE, LG, LEG Principal HOH o. 54896 4 -ISTEAR��\ ` S�aNAt �- www.cobaltgeo.com (2o6) 331-1097 HB-1 Approximate N �0 Hand Boring Location A Proposed Deck goi Bell Street Edmonds, Washington AERIAL PHOTO FIGURE 1 Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 (2o6) 331-1097 www.cobaltgeo.com cobaltgeoPgmail.com q d' Cy a fj New Deck 2Q. � 2B' 5, 1 O / k a ILA, Existing Residence W 0 N .Q ,ob Cobalt Geosciences, LLC Proposed Deck SITE P.O. Box 82243 COBALT got Bell Street PLAN Kenmore, 8 (2o6) 33i-1097io97 • Edmonds, Washington FIGURE 2 www.cobaltgeo.com cobaltgeo ftmail.com Unified Soil Classification System (USCS) MAJOR DIVISIONS SYMBOL TYPICAL DESCRIPTION GW Clean Gravels Well -graded gravels, gravels, gravel -sand mixtures, little or no fines Gravels (more than 50% (less than 5% _ GP fines) Poorly graded gravels, gravel -sand mixtures, little or no fines COARSE GRAINED SOILS of coarse fraction retained on No. 4 sieve) GM Gravels with Fines (more than 12% GC fines) Silty gravels, gravel -sand -silt mixtures Clayey gravels, gravel -sand -clay mixtures (more than 50% retained on ;°e sw Clean Sands Well -graded sands, gravelly sands, little or no fines No. 200 sieve) Sands (50% or more of coarse fraction (less than 5% SP fines) Poorly graded sand, gravelly sands, little or no fines passes the No. 4 sieve) sM Sands with Fines Silty sands, sand -silt mixtures (more than 12% sc fines) Clayey sands, sand -clay mixtures ML Inorganic silts of low to medium plasticity, sandy silts, gravelly silts, FINE GRAINED SOILS (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 oL Organic 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 CH elastic silt Inorganic clays of medium to high plasticity, sandy fat clay, or gravelly fat clay OH Organic Organic clays of medium to high plasticity, organic silts HIGHLY ORGANIC SOILS primarily organic matter, dark in color, PT and organic odor Peat, humus, swamp soils with high organic content (ASTM D4427) 1 Classification of Soil Constituents 1 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 (0.08 mm) Sand -Fine #200 to #40 (0.08 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) Moisture Content Definitions 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, WA98028 Soil Classification Chart Figure C1 (206) 331-1097 _ www.cobalt eg o.com cobaltgeo(d-) gmail.com Log of Hand Boring HB-1 Date: OCTOBER 2021 Depth: 8' Initial Groundwater: None Contractor: Elevation: N/A Sample Type: Grab Method: Hand Auger Logged By: PH Checked By: SC Final Groundwater: None o-a Q Moisture Content (�) Plastic 1 Liquid U Q O zo 3 a N Material Description 3 Limit Limit DCP Equivalent N-Value o m ? o' O 0 10 20 30 40 501 Vegetation/Topsoil �— ---- -- Illi SM -- -- -- — ------------------ Loose, silty fine to medium grained sand trace to some gravel, yellowish brown to grayish brown, 2 moist. (Outwash?/Fill?) 3 4 5 --- ---- -- ' -- SM/ --------------------------------------------- Medium dense, silty -fine to medium grained sand trace gravel, — b Sp yellowish brown to grayish brown, moist. (Weathered Outwash) — 7 :.. End of Hand Boring 8' —9 — 10 Cobalt Geosciences, LLC Proposed Deck Hand P.O. Box 82243 COBALT gol Bell Street BoringKenmore, 1097WA 8 ( ) 331-io97 G E • S C I E N C k S Edmonds, Washington Log www.cobaltgeo.com cobaltgeoPgmail.com