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REVIEWED-RESUB 1-BLD2023-0183-Getotech ReportRESUB Mar 16 2023 .,.,.,.,.,REVIEWED,.,.,.,.,., BY CITY OF EDMONDS BUILDING DEPARTMENT' COBALT G E 0 S C I E N C E S December 9, 2022 Updated March 3, 2023 RR Foundation Specialist Attn: Alesha Stickles RE: Limited Geotechnical Evaluation Foundation Mitigation 16131 75th Place West Edmonds, Washington BLD2023-0183 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 limited geotechnical evaluation at the above -referenced location. Site and Project Description The site is located at 16131 .75th Place West in Edmonds, Washington. The site consists of one irregularly shaped parcel (No. 005131058o11oo) with a total area of about o.16 acres. The western portion of the property is developed with a residence and parking areas. The remainder of the property is vegetated with grasses, bushes, shrubs, and sparse trees. The site slopes gently downward from east to west with relief of about 15 feet. The site is bordered to the north and east by residential properties, to the west by .75th Place West, and to the south by 162nd Street SW. We understand that portions of the building have settled up to 1.5 inches over time, likely beginning after construction and potentially continuing today. The deck east of the residence has settled more in places. The settlement is widespread. The project includes installation of at least 8 helical anchors and 8 driven piers below the affected foundation elements. Anchors will be attached with steel brackets and angle iron if necessary. Anchors are anticipated to be driven to refusal below foundation elements. Lifting will not 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 the Edmonds East Quadrangle indicates that the site is underlain by Whidbey Formation and Pre -Olympia Deposits. www.cobaltgeo.com (2o6) 331-1097 December 9, 2022 Updated March 3, 2023 Page 2 of 4 Limited Geotechnical Evaluation These materials can include silty -sands, silts, and gravelly materials which are typically heterogenous and can be layered. These materials generally become denser with depth below a weathered zone. The site is within an older landslide feature that may have areas of reactivation. We understand that the City requires new development to have extensive geotechnical analysis, including borings and risk assessments. Soil & Groundwater Conditions We advanced several shallow probes and a hand boring near foundation elements, where accessible. The soils consisted of loose mixtures of silt, sand, and gravel which became medium dense approximately 6 feet below grade. Groundwater could be present at shallow depths, perched on denser and/or finer grained materials. We reviewed numerous nearby boring and test pit logs. In general, the soils became denser with depth and varied widely in composition. 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 fill and foundation placement. Additional factors that could contribute to the settlement include saturation of foundation soils due to downspout drain leakage, age of foundation systems, and/or inadequately designed foundation systems for the current loads. The proposed mitigation utilizing helical anchors and driven piers with steel connections appears suitable to reduce settlement 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. This estimate is based on our experience with soil conditions at similar sites in this area. Depths may vary if higher loads are required. Statement of Risk We understand that the site is within a known landslide feature and that the City of Edmonds requires extensive geotechnical analysis for any new development and other specific activities. It is our opinion that this project does not require peer review related to these hazards since the proposed work is limited to pier installation (underpinning). This work typically takes less than a week and requires very small excavations close to the foundation elements which are immediately backfilled. The pier installation work will not adversely affect any geologic hazards at the site and adjacent areas and will not increase or decrease slope stability in the area. These piers merely provide additional support to the foundation, transferring the loads into more competent soil deposits at depth. Helical Piers@ Helical Piers® may be used to support the structure, where proposed and required. 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 www.cobaltgeo.com (2o6) 331-1097 December 9, 2022 Updated March 3, 2023 Page 3 of 4 Limited Geotechnical Evaluation 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. 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 3 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. www.cobaltgeo.com (206) 331-1097 December 9, 2022 Updated March 3, 2023 Page 4 of 4 Limited Geotechnical Evaluation 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. Driven Piles We understand that 2-7/8 inch piles may be used to support portions of the structure. These piles will likely be advanced using a double -cylinder hydraulic system. Piles should be advanced until working pressures are achieved that provide the specific minimum capacities with a factor of safety of at least 2. Work will be halted if the structure experiences uplift flexure. The working pressure for a 3.5 inch cylinder bore is 2,500 pounds per square inch (psi), and 2,000 psi for a 4 inch diameter cylinder bore. The required pile length in order to develop the recommended pile capacity is expected to vary depending on the depth of dense to very dense soils below the residence. We estimate pile lengths Of 7 to 20 feet below foundation grades, with a likely average of about 12 feet. Deeper piers may be observed if higher loads are required/observed. A total of 3 percent of the piles (one pile minimum) should be load tested to verify the design capacities. All load tests shall be performed in accordance with the procedure outlined in ASTM D1143. The maximum test load shall be 2 times the design load. Load testing may need to be terminated if additional lifting of the structure occurs. A representative of the geotechnical engineer shall provide full time observation of pile installation and testing to verify the driving refusal criteria. 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 scope pertains to foundation mitigation only and not any assessment of slope stability at the site or for the structure. Drilled borings and other analyses would be required for this type of assessment. Sincerely, Cobalt Geosciences, LLC Pa'(HONY q d� WAS, �r CID 5489 G �4 � FCi��a U\< ss�o��C1ST 3/3/2023 Phil Haberman, PE, LG, LEG Principal www.cobaltgeo.com (2o6) 331-1097 Sno. Co. GIS Image qN Approximate Hand W Hg_1 Boring Location Foundation Mitigation SITE PLAN 16131 75th Place West 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 16131 th Place West Kenmore, WA 98028 3 75 PLAN (206) 331-1097 Edmonds, Washington FIGURE 2 www.cobaltgeo.com cobaltgeoogmail.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: November 2022 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 4 Moisture Content (%) Plastic 1 Liquid u- O U E 3 Limit Limit oMaterial Description o SPT N-Value C 0 10 20 30 40 5 Vegetation/Topsoil --- ---- -- --- SM -------------------------------------------- Very loose to medium dense, silty -fine to medium grained sand with gravel, dark yellowish brown to yellowish brown, moist. (Fill? and Weathered Whidbey Formation?) 2 3 4 5 End of Hand Boring 6' 7 8 9 10 Cobalt Geosciences, LLC P.O. Box 82243 Foundation Mitigation Hand Kenmore, WA 98028 331-iog7 3-. 161 1 th Place West 3 75 Borin(2o6) g www.cobaltgeo.com Edmonds, Washington Log cobaltgeo Cd) gmail. corn