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RESUB1 BLD2021-1100+GEOTECH REPORT+10.20.2021_11.57.25_AM+2473703BLD2021-1100 COBALT GE0SCIENCES October 15, 2021 Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, Washington 98028 RR Foundation Specialist Attn: Mr. Cory Myron R E S U B cory(@rrspecialist.net Oct 20 2021 RE: Limited Geotechnical Evaluation CITY OF EDMONDS Foundation Mitigation ENT DEVE OPARTMENRVICES 814 Dayton Street Edmonds, Washington In accordance with your authorization, Cobalt Geosciences, LLC has prepared this letter to discuss the results of our limited geotechnical evaluation at the above -referenced location. Site and Project Description The site is located at 814 Dayton Street in Edmonds, Washington. The site consists of one rectangular parcel (No. 004342076oO300) with a total area of o.19 acres. The northern portion of the site is developed with a residence and driveway. The remainder of the property is vegetated with grasses, bushes, and trees. The site slopes downward to the west at magnitudes of less than 15 percent and relief of about 10 feet. The site is bordered to the east, west, and south by residential properties, and to the north by Dayton Street. We understand that portions of the residence have settled up to about 2.5 inches over time, likely beginning after construction and potentially continuing today. The project includes installation of at least 5 helical anchors with angle iron and mechanical connections. Anchors are anticipated to be driven to refusal below foundation elements. Lifting will be performed. 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 Washington — Northwest Quadrant, indicates that the site is underlain by Vashon Advance Outwash. These materials include loose to medium dense fine to medium grained sand. These materials become denser with depth. www.cobaltgeo.com (2o6) 331-1097 October 15, 2021 Page 2 of 3 Limited Geotechnical Evaluation Soil & Groundwater Conditions As part of our evaluation, we excavated one hand boring to 6 feet below grade. The hand boring encountered approximately 6 inches of topsoil and vegetation underlain by approximately 4.5 feet of loose, fine to medium grained sand trace to some gravel (Weathered Outwash). This layer was underlain by medium dense to dense, silty -fine to medium grained sand trace gravel (Advance Outwash) which continued to the termination depth of the hand boring. Groundwater was not encountered in the hand boring. Conclusions and Recommendation Based on our observations, it appears likely that the settlement is likely the result of insufficient removal of loose weathered soils prior to foundation placement. There may be areas of fill below some foundation elements, that could contribute to the settlement. The proposed mitigation utilizing helical anchors and driven piers with steel connections appears suitable to reduce vertical settlement associated with soft soils only and support the affected portions of the residence. Based on our observations, we anticipate that helical anchors and piers may extend 7 to 20 feet below grade with an average depth of 12 feet. Helical Piers@ Helical Piers® may be used to support the residence. The Helical Piers® could be installed using portable rotary tools, truck mounted rotary tools, backhoe mounted rotary tools, caisson drills, or skid -steer loaders. It is important that the torque output, rotational speed, down pressure capability, and angle control of the installation equipment is compatible with the required foundation system. The pile installation equipment should have adequate torque capacity to prevent refusal conditions at relatively shallower depths that are well above recommended bearing depths or layers. A Helical Pier® consists of an anchor (lead section) with 1, 2, 3 or more helical flights on a shaft. The number and diameter of the helices on the anchor are dependent on the soil characteristics of the site and the design loads to be applied to the pier. Based on these parameters the anchor helix configuration is chosen to best fit the site conditions. As the anchor is advanced into the soil extension sections (shaft) are placed on the lead section. The shaft configuration is based on the design loads and anticipated installation torque. The static compression load capacity of a Helical Pier@ is the sum of all individual helix capacities below liquefiable soils and in bearing layer. Individual helix static compression capacity is the result of the projected area of the helix, and its bearing pressure. It is recommended that the piers penetrate into relatively dense native soils a minimum of 5 feet, or until refusal whichever is shallower. The bearing layer will be at variable depths below the existing ground surface due to previously natural slope conditions (anticipated to be 7 to 20 feet). Increased capacity can be obtained with increased penetration, and additional helical flights on the lead section. Helical Pier@ installation should be monitored to verify installation torque, and proper embedment into the presumed bearing layer. The Helical Pier@ lengths may need to be modified during construction if it is determined that the depth to the bearing layer varies. Helical Pier@ anchors are well suited to field adjustments as length can be varied by merely adding or deleting extension sections (shafts) during installation. www.cobaltgeo.com (2o6) 331-1097 October 15, 2021 Page 3 of 3 Limited Geotechnical Evaluation Monitoring installation torque in the field is used to estimate the anchor compression capacity, and also as a quality control during anchor installation, provided that the anchor is bearing in dense or hard soils. Dependent on the pile size and the equipment used to install the anchors, an empirical factor is multiplied by the average torque over the final 3 feet of installation to estimate ultimate capacity. Allowable Helical Pier Compression Capacity Pa may be estimated from the following equation provided that the pier is in the recommended bearing soils: Pa = Kt x T/FoS, Where T is the applied torque, Kt is the empirical ratio factor. The following industry standards apply to shafts with blades spaced along the shaft at 2.5 to 3.5 times the average blade diameter on -center and meeting the manufacturer's specifications. 1.5" and 1.75" Square Shafts - Kt = 9 ft-1 2.875" O.D. Round Shafts - Kt = 9 ft-1 3.0" O.D. Round Shafts - Kt = 8 ft-1 3.5" O.D. Round Shafts - Kt = 7 ft-1 Proof testing of at least twenty percent of the helical piers in eight equal increments up to 200 percent of the design load, if required by the permitting authority. Each load increment up to the 200 percent of design load should be held for five (5) minutes and the vertical strain monitored. If the total strain between 1 and 5 minutes is less than 0.04 inches, the helical pier may be considered acceptable. If the recorded strain exceeds 0.04 inches, the helical pier should either be deepened and retested or abandoned and a new helical pier shall be installed and tested. 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 o� WAS4 54896 1014L 10/15/2021 Phil Haberman, PE, LG, LEG Principal Figure 1; Site Plan Figure 2; Repair Plan Figure 3; Hand Boring Log www.cobaltgeo.com (2o6) 331-1097 Approximate Hand N xB-1 Boring Location A Foundation Mitigation SITE PLAN 814 Dayton Street Edmonds, Washington FIGURE i Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 www.cobaltgeo.com cobaltgeoogmail.com Cobalt Geosciences, LLC Foundation Mitigation REPAIR P.O. Box 82243 81 Dayton Street Kenmore, WA 98028 4 reePLAN (206) 331-1097 Edmonds, Washington FIGURE 2 www.cobaltgeo.com cobaltgeoPgmail.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: SEPTEMBER 2021 Depth: 6' Initial Groundwater: None Contractor: Elevation: N/A Sample Type: Grab Method: Hand Auger Logged By: PH Checked By: SC Final Groundwater: N/A o Moisture Content (%) -0 Plastic Liquid u- O U E 3 Limit Limit oa � Material Description o SPT N-Value C) 0 10 20 30 40 5 Vegetation/Topsoil — — — — — — — — -- ;; --- SP -------------------------------------------- Loose, silty -fine to medium grained sand with gravel, - 1 dark yellowish brown to yellowish brown, • moist. (Weathered Outwash) — 2 :M1 —3 — 4 % SP --------------------------------------------- Medium dense to dense, fine to medium grained sand with gravel, yellowish brown to grayish brown, moist. (Outwash) End of Hand Boring 6' —7 —8 —9 — 10 Cobalt Geosciences, LLC P.O. Box 82243 Foundation Mitigation Hand Kenmore, WA 98028 (2o6) 331-1097 814 Dayton Street Boring _ www.cobaltgeo.com Edmonds, Washington Log cobaltgeo(&gmail.com