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HomeMy WebLinkAboutRS_Geotechnical_Letter_08202020_v1Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, Washington 98028 www.cobaltgeo.com (206) 331-1097 August 20, 2020 Sukhjinder Mangat Sukhjinder.mangat@hotmail.com RE: Limited Geotechnical Evaluation Proposed Residence 1415 N. 24th Street Renton, Washington In accordance with your authorization, Cobalt Geosciences, LLC has prepared this letter to discuss the results of our limited geotechnical evaluation at the referenced site. The purpose of our evaluation was to determine the feasibility of utilizing infiltration devices for stormwater runoff management along with providing recommendations for foundation and retaining wall design. Site and Project Description The site is located at 1415 N. 24th Street in Renton, Washington. The site consists of one rectangular parcel (No. 3344500234) with a total area of about 23,595 square feet. The property is developed with a single-family residence and driveway. The remainder of the property is undeveloped and vegetated with grasses, blackberry vines, ivy, ferns, and variable diameter trees. The site slopes gently downward from northeast to southwest at magnitudes of 4 to 8 percent and relief of about 10 feet. There are locally steeper slopes further southwest and downgradient of the property. These slopes have magnitudes of 15 to 30 percent. The property is bordered to the east, west, and south by residential properties and to the north by N. 24th Street. The project includes construction of a new multi-story residence with garage and crawlspace areas. Stormwater management may include dispersion, detention, or infiltration facilities depending on feasibility. Area Geology The Geologic Map of King County indicates that the site is underlain by Vashon Recessional Outwash and at depth by Vashon Glacial Till. Vashon Recessional Outwash consists of sand, gravel and lacustrine deposited silt and clay. These soils are variably permeable and highly erosive. Recessional outwash has not been consolidated by glacial activity and can be permeable. Vashon Glacial Till includes dense to very dense mixtures of silt, sand, clay, and gravel. These deposits are typically impermeable. August 20, 2020 Page 2 of 5 Limited Geotechnical Evaluation www.cobaltgeo.com (206) 331-1097 Soil & Groundwater Conditions As part of our evaluation, we excavated two hand borings within the property to determine the shallow soil and groundwater conditions, where accessible. The hand borings encountered about 6 inches of topsoil and grass underlain by 3 to 4 feet of loose to medium dense, silty-fine to fine grained sand with gravel (Weathered Glacial Till?). This layer was underlain by medium dense to dense, silty-fine to fine grained sand with gravel (Glacial Till), which continued to the termination depths of the hand borings. Groundwater was not encountered in the explorations during the field work; however, mottled soils were present at shallow depths. There is a chance that perched groundwater may develop between the weathered and unweathered glacial till. The depth to groundwater would likely be 3 to 8 feet or more. Stormwater Management Feasibility The site is underlain by weathered and unweathered glacial till. These soils have a low permeability which decreases with depth as soil density increases. The upper soils are mottled which indicates that shallow groundwater could be present during the wet season. Furthermore, the proposed development is adjacent to properties with basements and/or crawlspaces that are downgradient of this property, as well as moderately steep slope areas to the southwest. There is a high probability that if infiltration systems were utilized, the groundwater would migrate laterally, eventually daylighting in a crawlspace or basement downslope of the property, or as spring activity on downslope areas. We performed an in-situ infiltration test (Small Scale PIT) in an excavation near HB-1 at a depth of approximately 4 feet below grade. After testing and application of correction factors for site variability (0.5), testing (0.5), and influent control (0.9), the infiltration rate was 0.22 inches per hour. This is lower than what is considered to be feasible. We excavated below this area and found the water perched on fine grained mottled soils. We recommend direct connection of runoff devices to City infrastructure. 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. Foundation Design The proposed residence may be supported on a shallow spread footing foundation system bearing on undisturbed 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 2,000 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. August 20, 2020 Page 3 of 5 Limited Geotechnical Evaluation www.cobaltgeo.com (206) 331-1097 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 ½ 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. 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 basement areas are proposed. Wall Design Criteria “At-rest” Conditions (Lateral Earth Pressure – EFD+) 55 pcf (Equivalent Fluid Density) “Active” Conditions (Lateral Earth Pressure – EFD+) 35 pcf (Equivalent Fluid Density) Seismic Increase for “At-rest” Conditions (Lateral Earth Pressure) 11H* (Uniform Distribution) Seismic Increase for “Active” Conditions (Lateral Earth Pressure) 6H* (Uniform Distribution) Passive Earth Pressure on Low Side of Wall (Allowable, includes F.S. = 1.5) Neglect upper 2 feet, then 250 pcf EFD+ Soil-Footing Coefficient of Sliding Friction (Allowable; includes F.S. = 1.5) 0.40 *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 August 20, 2020 Page 4 of 5 Limited Geotechnical Evaluation www.cobaltgeo.com (206) 331-1097 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 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 90 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. August 20, 2020 Page 5 of 5 Limited Geotechnical Evaluation www.cobaltgeo.com (206) 331-1097 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 8/20/2020 Phil Haberman, PE, LG, LEG Principal PH/sc Proposed Residence 1415 N. 24th Street Renton, Washington SITE PLAN FIGURE 1 Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 www.cobaltgeo.com cobaltgeo@gmail.com HB-2 Subject Property HB-1 N PT Well-graded gravels, gravels, gravel-sand mixtures, little or no fines Poorly graded gravels, gravel-sand mixtures, little or no fines Silty gravels, gravel-sand-silt mixtures Clayey gravels, gravel-sand-clay mixtures Well-graded sands, gravelly sands, little or no fines COARSE GRAINED SOILS (more than 50% retained on No. 200 sieve) Primarily organic matter, dark in color, and organic odor Peat, humus, swamp soils with high organic content (ASTM D4427)HIGHLY ORGANIC SOILS FINE GRAINED SOILS (50% or more passes the No. 200 sieve) MAJOR DIVISIONS SYMBOL TYPICAL DESCRIPTION Gravels (more than 50% of coarse fraction retained on No. 4 sieve) Sands (50% or more of coarse fraction passes the No. 4 sieve) Silts and Clays (liquid limit less than 50) Silts and Clays (liquid limit 50 or more) Organic Inorganic Organic Inorganic Sands with Fines (more than 12% fines) Clean Sands (less than 5% fines) Gravels with Fines (more than 12% fines) Clean Gravels (less than 5% fines) Unified Soil Classification System (USCS) Poorly graded sand, gravelly sands, little or no fines Silty sands, sand-silt mixtures Clayey sands, sand-clay mixtures Inorganic silts of low to medium plasticity, sandy silts, gravelly silts, or clayey silts with slight plasticity Inorganic clays of low to medium plasticity, gravelly clays, sandy clays, silty clays, lean clays Organic silts and organic silty clays of low plasticity Inorganic silts, micaceous or diatomaceous fine sands or silty soils, elastic silt Inorganic clays of medium to high plasticity, sandy fat clay, or gravelly fat clay Organic clays of medium to high plasticity, organic silts Moisture Content Definitions Grain Size Definitions Dry Absence of moisture, dusty, dry to the touch Moist Damp but no visible water Wet Visible free water, from below water table Grain Size Definitions Description Sieve Number and/or Size Fines <#200 (0.08 mm) Sand -Fine -Medium -Coarse Gravel -Fine -Coarse Cobbles Boulders #200 to #40 (0.08 to 0.4 mm) #40 to #10 (0.4 to 2 mm) #10 to #4 (2 to 5 mm) #4 to 3/4 inch (5 to 19 mm) 3/4 to 3 inches (19 to 76 mm) 3 to 12 inches (75 to 305 mm) >12 inches (305 mm) 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 0 to 5 percent of the soil (i.e., slightly silty SAND, trace gravel). Relative Density Consistency (Coarse Grained Soils) (Fine Grained Soils) N, SPT, Relative Blows/FT Density 0 - 4 Very loose 4 - 10 Loose 10 - 30 Medium dense 30 - 50 Dense Over 50 Very dense N, SPT, Relative Blows/FT Consistency Under 2 Very soft 2 - 4 Soft 4 - 8 Medium stiff 8 - 15 Stiff 15 - 30 Very stiff Over 30 Hard Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 www.cobaltgeo.com cobaltgeo@gmail.com Soil Classification Chart Figure C1 Log of Hand Boring HB-1 Date: August 12, 2020 Contractor: Method: Hand Auger Depth: 6’ Elevation: N/A Logged By: PH Checked By: SC Initial Groundwater: None Sample Type: Grab Final Groundwater: N/A Material Description SPT N-Value Moisture Content (%)Plastic Limit Liquid Limit 10 20 30 400 50 1 2 3 4 5 6 7 8 9 10 End of Hand Boring 6’ Vegetation/Topsoil SM Dense, silty-fine to medium grained sand trace gravel, yellowish brown to grayish brown, moist. (Glacial Till) SM Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 www.cobaltgeo.com cobaltgeo@gmail.com Proposed Residence 1415 N. 24th Street Renton, Washington Hand Boring Log Loose to medium dense, silty-fine to medium grained sand with gravel and cobbles, mottled dark yellowish brown to yellowish brown, moist. (Weathered Glacial Till) Log of Hand Boring HB-2 Date: August 12, 2020 Contractor: Method: Hand Auger Depth: 6’ Elevation: N/A Logged By: PH Checked By: SC Initial Groundwater: None Sample Type: Grab Final Groundwater: N/A Material Description SPT N-Value Moisture Content (%)Plastic Limit Liquid Limit 10 20 30 400 50 1 2 3 4 5 6 7 8 9 10 End of Hand Boring 6’ Vegetation/Topsoil SM Dense, silty-fine to medium grained sand trace gravel, yellowish brown to grayish brown, moist. (Glacial Till) SM Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 www.cobaltgeo.com cobaltgeo@gmail.com Hand Boring Log Loose to medium dense, silty-fine to medium grained sand with gravel and cobbles, mottled dark yellowish brown to yellowish brown,moist. (Weathered Glacial Till) Proposed Residence 1415 N. 24th Street Renton, Washington