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BLD2020-0894+STORMWATER FEASIBILITYCOBALT G E 0 S C I E N C E S March 30, 2020 David Harris Harris.davidm(&hotmail.com RE: Stormwater Feasibility Evaluation Proposed Duplex 21022 74th Avenue West Edmonds, Washington Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, Washington 98028 In accordance with your authorization, Cobalt Geosciences, LLC has prepared this letter to discuss the results of our stormwater feasibility evaluation at the referenced site. Site and Project Description The site is located at 21022 — 74th Avenue West in Edmonds, Washington. The site consists of one rectangular shaped parcel (No. 00566900400201) with a total area of 0.17 acres. The central portion of the property is developed with a residence. The remainder of the property is vegetated with grasses, bushes, shrubs, and deciduous and evergreen trees. The site slopes gently downward from north to south at magnitudes of less than 5 percent and relief of about 4 feet. The property is bordered to the north, south, and west by residential properties, and to the east by 74th Avenue West. The project includes construction of a new duplex, garages, and access driveway. Stormwater runoff will be managed through infiltration devices, if feasible. Area Geology The Geologic Map of Washington — Northwest Quadrant, indicates that the site is near the contacts between Vashon Glacial Till and Vashon Advance Outwash. 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. Vashon Advance Outwash includes fine to medium grained sand trace gravel. These materials are typically permeable and relatively dense. Soil & Groundwater Conditions As part of our analysis, we excavated a test pit up to 10 feet below grade, where accessible. We encountered approximately 6 inches of grass and topsoil underlain by approximately 3.5 feet of loose to medium dense, silty -fine to medium grained sand trace gravel (Weathered Advance Outwash). This layer was underlain by dense, fine to medium grained sand trace to some gravel (Advance Outwash), which continued to the termination depth of the test pit. Groundwater was not encountered in the exploration. www.cobaltgeo.com (2o6) 331-1097 March 30, 2020 Page 2 of 4 Stormwater Feasibility Evaluation Stormwater Management Feasibility We conducted in situ infiltration test in TP-1 at a depth of 4 feet below grade to confirm suitability of infiltration systems in the area. Following saturation and application of correction factors for site variability (0.33), testing (0.5), and influent control (o.9), the design infiltration rate was determined to be 1.6 inches per hour. Groundwater was not encountered in the test pit up to a depth of io feet below grade. We recommend that the bottom of the trenches or drywells be situated within native soils consistent with those described above (clean medium grained sand). We should be on site to confirm soil conditions and provide additional recommendations if necessary. The site is near the contacts with glacial till, which is generally not permeable. If till -like soils are encountered at system locations and depths, it will be necessary to remove and replace the soils with washed or angular rock. We should be provided with final plans for review to determine if the intent of our recommendations has been incorporated or if additional modifications are needed. Verification testing of infiltration systems should be performed during construction. Foundation Design The proposed duplex may be supported on a shallow spread footing foundation system bearing on undisturbed medium dense or firmer native soils or on properly compacted structural fill placed on the suitable native soils. Any undocumented fill should be removed and replaced with structural fill below foundation elements. Structural fill below footings should consist of clean angular rock 5/8 to 2 inches in size. 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 2,500 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 frictional and passive resistance of the soil may be combined without reduction in determining the total lateral resistance. www.cobaltgeo.com (2o6) 331-1097 March 30, 2020 Page 3 of 4 Stormwater Feasibility Evaluation 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. Concrete Retaining Walls The following table, titled Wall Design Criteria, presents the recommended soil related design parameters for retaining walls with a level backslope. Contact Cobalt if an alternate retaining wall system is used. This has been included if retaining walls are proposed. Wall Design Criteria "At -rest" Conditions (Lateral Earth Pressure — 55 pcf (Equivalent Fluid Density) EFD+) "Active" Conditions (Lateral Earth Pressure — 35 pcf (Equivalent Fluid Density) EFD+) Seismic Increase for "At -rest" Conditions 11H* (Uniform Distribution) (Lateral Earth Pressure) Seismic Increase for "Active" Conditions 6H* (Uniform Distribution) (Lateral Earth Pressure) Passive Earth Pressure on Low Side of Wall Neglect upper 2 feet, then 275 pcf EFD+ (Allowable, includes F.S. = 1.5) Soil -Footing Coefficient of Sliding Friction 0.40 (Allowable; includes F.S. = 1.5) *H is the height of the wall; Increase based on one in 500 year seismic event (10 percent probability of being exceeded in 50 years), +EFD — Equivalent Fluid Density The stated lateral earth pressures do not include the effects of hydrostatic pressure generated by water accumulation behind the retaining walls. Uniform horizontal lateral active and at -rest pressures on the retaining walls from vertical surcharges behind the wall may be calculated using active and at -rest lateral earth pressure coefficients of 0.3 and 0.5, respectively. A soil unit weight Of 125 pcf may be used to calculate vertical earth surcharges. To reduce the potential for the buildup of water pressure against the walls, continuous footing drains (with cleanouts) should be provided at the bases of the walls. The footing drains should consist of a minimum 4-inch diameter perforated pipe, sloped to drain, with perforations placed down and enveloped by a minimum 6 inches of pea gravel in all directions. The backfill adjacent to and extending a lateral distance behind the walls at least 2 feet should consist of free -draining granular material. All free draining backfill should contain less than 3 percent fines (passing the U.S. Standard No. 200 Sieve) based upon the fraction passing the U.S. Standard No. 4 Sieve with at least 30 percent of the material being retained on the U.S. Standard No. 4 Sieve. The primary purpose of the free -draining material is the reduction of hydrostatic pressure. Some potential for the moisture to contact the back face of the wall may exist, even with www.cobaltgeo.com (2o6) 331-1097 March 30, 2020 Page 4 of 4 Stormwater Feasibility Evaluation treatment, which may require that more extensive waterproofing be specified for walls, which require interior moisture sensitive finishes. We recommend that the backfill be compacted to at least go percent of the maximum dry density based on ASTM Test Method D1557. In place density tests should be performed to verify adequate compaction. Soil compactors place transient surcharges on the backfill. Consequently, only light hand operated equipment is recommended within 3 feet of walls so that excessive stress is not imposed on the walls. 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. 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. Sincerely, Cobalt Geosciences, LLC WA -off 54896 <� 'P,FCIST �SS�ONAL��� Exp 6/26/2020 Phil Haberman, PE, LG, LEG Principal www.cobaltgeo.com (2o6) 331-1097 FO= 1/Y RMAR t OF LA", AT a21 X a E OF ovab Caw MW21WT _ E7.az' �lop aloof TP-1 1itTi1M" Ii4iL --� .Afn.g7T1,fYJfAlT7��A .�' 611' L4 i d ' . 1 ° Subject Property e sFrrt 21022 � / d, • d Approximate Test Pit Location //mil i;,w, 14 e e NAO''21'b0't' e d RYA 4aQ1, AT OMU, OMt / < OF atc'b Corn 13 of L8� E 6- I�C{Njrl02ir' Proposed Duplex 21022 - 74th Avenue West Edmonds, Washington SITE PLAN FIGURE i j I t ----------- X JR01' t ao.ol�----- Ex 1w EC Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 www.cobaltgeo.com cobaltgeopgmail.com II[ 71 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: March 24, 2020 Depth: 10' Groundwater: None Contractor: Jim Elevation: Logged By: PH Checked By: SC N 0) o Moisture Content (%) U JO U Q E Plastic Limit I Liquid Limit U L N Material Description DCP Equivalent N-Value ? o o O 0 10 20 30 40 50 Cobalt Geosciences, LLC ®, Proposed Duplex P.O. Box 82243 \ COBALT 21022 74th Avenue West Test Pit Kenmore, W09 98028 Logs � ) 33 97 GEOSCIENCES Edmonds, Washington www.cobaltgeo.com cobaltgeoCRigmail.com Topsoil/Vegetation SM/ Loose to medium dense, silty -fine to medium grained sand with gravel • ••`e SP yellowish brown, moist. (Weathered Advance Outwash) 2 , L: .r 3 Sp Medium dense to dense, fine to medium grained sand with gravel, 5 grayish brown, moist. (Advance Outwash) 6 7 8 End of Test Pit 10'