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REVIEWED RESUB1 BLD2021-1101+GEO report+9.7.2021_10.55.24_AM+2394999RESUB BLD2021-1101 Sep 07 2021 CITY OF EDMONDS DEVELOPMENT SERVICES DEPARTMENT COBALT Cobalt Geosciences, LLC GE0SCIENCES P.O. Box82243 Kenmore, Washington 98028 August 11, 2021 RR Foundation Specialist Attn: Mr. Cory Myron cory(&rrspecialist.net RE: Limited Geotechnical Evaluation Foundation Mitigation 178o8 73rd Place West 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 178o8 73rd Place West in Edmonds, Washington. The site consists of one irregularly shaped parcel (No. 00473700000900) with a total area of about 0.39 acres. The south central portion of the site is developed with a residence and driveway. The remainder of the property is vegetated with grasses, bushes, and sparse trees. The site slopes downward from east to west at low magnitudes with minimal relief. There are local short slopes up to 50 percent in magnitude near the east and west margins of the site, indicating that the property has been previously graded. The site is bordered to the east and west by residential properties, and to the south by 73=d Place West, and to the north by undeveloped land. We understand that portions of the residence have settled about 4 inches over time, likely beginning after construction and potentially continuing today. The most significant settlement is in the southwestern portion of the residence. The project includes installation of at least 11 helical anchors. Anchors will be attached with steel brackets and angle iron if necessary. Anchors are anticipated to be driven to refusal below foundation elements. Lifting may be performed as allowed by the structure. Polylift foam will be used below local slab areas. 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 West Quadrangles, indicates that the site is underlain by Vashon Glacial Till. www.cobaltgeo.com (2o6) 331-1097 August 11, 2021 Page 2 of 4 Limited Geotechnical Evaluation The till consists of a nonsorted mixture of silt, sand, clay, gravel, and cobbles. These deposits typically become dense within the upper 15 feet, below a weathered zone. 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 3.5 feet of loose, silty -fine to medium grained sand trace to some gravel (Weathered Glacial Till). This layer was underlain by medium dense to dense, silty -fine to medium grained sand with gravel (Glacial Till) 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 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 below grade with an average depth of 10 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 15 feet). Increased capacity can be obtained with increased penetration, and additional helical flights on the lead section. www.cobaltgeo.com (2o6) 331-1097 August 11, 2021 Page 3 of 4 Limited Geotechnical Evaluation 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 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. www.cobaltgeo.com (2o6) 331-1097 August 11, 2021 Page 4 of 4 Limited Geotechnical Evaluation Sincerely, Cobalt Geosciences, LLC r"0NY� MV Wa�-4 � / J v 54896 NAL Phil Haberman, PE, LG, LEG 8/11/2021 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 17808 73rd Place West Edmonds, Washington FIGURE i Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 www.cobaltgeo.com cobaltgeopgmail.com Cobalt Geosciences, LLC Foundation Mitigation REPAIR P.O. Box 82243 17808 73rd Place West PLAN Kenmore, WA 98028(206) 331-1097 Edmonds, Washington FIGURE 2 www.cobaltgeo.com cobaltgeo(&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: July 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 oo 4 Moisture Content (%) Plastic Liquid L _ > OU - U a Limit Limit t o t LO U Material Description o E 9 m 0 ? o SPT N-Value G 0 10 20 30 40 5 Vegetation/Topsoil ---- _ ---- -- --- SM -------------------------------------------- Loose, silty -fine to medium grained sand with gravel, dark yellowish brown to yellowish brown, :. • dry to moist. —2 —3 SM Medium dense, silty -fine to medium grained sand with gravel, yellowish brown to grayish brown, moist. (Glacial Till) —5 End of Hand Boring 6' —7 —8 —9 — 10 Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 (2o6) 331-1097 www.cobaltgeo.com cobaltgeopgmail.com Foundation Mitigation 178o8 73rd Place West Edmonds, Washington Hand Boring Log