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REVIEWED BLD2023-1212+Geotech Report+9.28.2023_2.15.21_PM+3810847L�2 COBALT GEOSCIENCES September 27, 2023 REVIEWED BY CITY OF EDMONDS RR Foundation Specialist Attn: Alesha, Ty -Lynn, Jordan, or Amanda RE: Limited Geotechnical Evaluation Foundation Mitigation 7300 176th Street SW Edmonds, Washington RECEIVED Oct 03 2023 CITY OF EDMONDS DEVELOPMENT SERVICES DEPARTMENT BLD2023-1212 Cobalt Geosciences, LLC P.O. Box 1792 North Bend, WA 98045 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 7300 176th Street SW in Edmonds, Washington. The site consists of one irregularly shaped parcel (No. 00513600000902) with a total area of about 0.25 acres. The northern portion of the site is developed with a residence with a daylight basement and driveway. The remainder of the property is vegetated with grasses, shrubs, bushes, and sparse trees. The southern portion of the site slopes downwards to the southwest at approximately 20 percent with a relief of about 25 feet. This slope terminates on the neighboring property to the south. The site is bordered to the south, east, and west by residential properties, and to the east by 176th Street SW. There is a 1 to approximately 4.5 feet tall concrete wall along the east margin of the driveway that leans to the west and is locally cracked. The project includes installation of at least 4 helical tie -backs extending to refusal in relatively dense native soils behind the concrete retaining wall. Channel will be used between tiebacks. Additional carbon fiber will be used to repair cracks on the concrete retaining wall. 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 part of the Edmonds West quadrangles, indicates that the site is underlain by a contact between both Vashon Advance Outwash and Vashon Glacial Till. Vashon Advance Outwash consists of fine to medium grained sand with minor silt and gravel and the Vashon Glacial till consist of mixtures of silt, sand, gravel, and clay in a non -sorted matrix or diamict. These deposits become denser with depth below a weathered zone. www.cobaltgeo.com (2o6) 331-1097 September 27, 2023 Page 2 of 3 Limited Geotechnical Evaluation Soil & Groundwater Conditions As part of our evaluation, we excavated one hand boring to 8 feet below grade. The hand boring encountered approximately 6 inches of topsoil and vegetation underlain by loose to medium dense, silty -fine to medium grained sand with gravel (Fill over Weathered Glacial Till) which continued to the termination depth. The soils became medium dense about 6 feet below grade. Conclusions and Recommendation Based on our observations, it appears likely that the wall is beyond the intended design life and may lack typical wall design controls. Contributing factors could be loose backfill that becomes cyclically wet and dry. The increase in weight could contribute to wall movements. If poor drainage is present (likely), hydrostatic pressures will slowly push the wall over. The proposed mitigation utilizing helical tieback anchors 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 15 feet (or more) behind the wall with an average of 10 feet. Helical Piers@ Helical Piers® may be used to support the wall as tiebacks. 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. A triple helix is typically used for tiebacks, although it may depend on the loads required. 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 15 feet (estimated only)). 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 September 27, 2023 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 three 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 o.o4 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 PadHONYI, vQ OF WA B �10 54896 -IST FSS�ONAL��� 9/27/2023 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 HB-1 Approximate Hand Boring Location Snohomish County PDS Image Foundation Mitigation 1300 176th St SW Edmonds, Washington Site Image Figure 1 Nw, Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 www.cobaltgeo.com cobaltge @gmail.com 22.N, CF F co h0 20 C10 3 Stairs L' 16' .................. : M = Crack : 4.5' = Tie Back 0 = Core Drill N 2 N � 1 Cobalt Geosciences, LLC Foundation Mitigation Repair Plan P.O. Box 82243 00 1 6th St SW Kenmore, WA 98028 COBALT 73 % (206) 331-1097 �7 Edmonds, Washington Figure 2 www.cobaltgeo.com cobal�eopo(�gmail.com 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 (2o6) 331-1097 _ www.cobaltgeo.com cobaltgeo(&gmail.com Log of Hand Boring HB-1 Date: August 2023 Depth: 8' Initial Groundwater: None Contractor: Elevation: N/A Sample Type: Grab Method: Hand Auger Logged By: PH Checked By: SC Final Groundwater: N/A oo 4 Moisture Content (%) Plastic Liquid L _ > OU - U a Limit Limit t o t LO U Material Description o m ? o SPT N-Value 0 10 20 30 40 5 ---- — ---- -- --- SM Vegetation/Topsoil -------------------------------- ------------ Loose, silty -fine to fine grained sand with gravel, .: dark yellowish brown to yellowish brown, moist. (-4 feet of fill over likely Till) —2 Becomes medium dense at 6' —3 —4 —5 —6 —7 TIN End of Hand Boring 8' —9 — 10 Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 www.cobaltgeo.com cobaltgeopgmail.com Foundation Mitigation 7300 176th Street SW Edmonds, Washington Hand Boring Log