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HomeMy WebLinkAboutRS_Geotechnical_Report_190911_V1 Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, Washington 98028 www.cobaltgeo.com (206) 331-1097 January 8, 2019 Inderjit Mangat aqhomes@yahoo.com RE: Stormwater Feasibility Evaluation Proposed Two-Lot Development 2732 Aberdeen Avenue Renton, Washington In accordance with your authorization, Cobalt Geosciences, LLC has prepared this letter to discuss the results of our stormwater feasibility evaluation at the above-referenced location. Site and Project Description The site is located at 2732 Aberdeen Avenue Northeast in Renton, Washington (Figure 1). The site consists of one rectangular tax parcel (No. 3343901400) with a total area of 15,180 square feet. The site slopes gently toward the northwest at magnitudes ranging from 0 to 10 percent. The overall topographic relief across the property is approximately 3 feet. The south half of the property is developed with a single family residence and accessory structures. The north half is undeveloped. The property is vegetated with sparse evergreen and deciduous trees and grasses. The site is bordered to the east and south by single family residences, to the west by Aberdeen Avenue NE, and to the north by NE 28th Street. The project includes construction of two new residences in the east and west portions of the property. Stormwater runoff will be infiltrated into the near-surface soils if feasible. 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 King County, indicates that the site is near the contacts between Vashon Glacial Till and Vashon Recessional Outwash. January 8, 2019 Page 2 of 4 Stormwater Feasibility Evaluation www.cobaltgeo.com (206) 331-1097 Vashon Glacial Till is typically characterized by an unsorted, non-stratified mixture of clay, silt, sand, gravel, cobbles and boulders in variable quantities. These materials are typically dense and relatively impermeable. The poor sorting reflects the mixing of the materials as these sediments were overridden and incorporated by the glacial ice. Vashon Glacial Till is commonly overlain by Vashon Recessional Outwash on till planes. Recessional outwash includes normally consolidated sands with local interbeds of silt and clay. These deposits are typically less than 20 feet thick and groundwater is often found near the contact of the outwash and underlying till. Site Explorations We advanced four hand borings to 8 feet below grade to evaluate the shallow soil conditions. All of the hand borings encountered 4 to 6 inches of grass and topsoil underlain by approximately 1 to 1.5 feet of loose fill. This layer was underlain by approximately 1 to 3 feet of loose, silty-fine to medium grained sand with trace gravel (Vashon Recessional Outwash). This layer was underlain by loose to medium dense, fine to medium grained sand with trace gravel (Vashon Recessional Outwash), which continued to the termination depths of the hand borings. Groundwater and soil mottling were not observed in any of the explorations at the time of the investigation. Conclusions & Recommendations The hand borings encountered medium grained sand at relatively shallow depths. Shallow infiltration trenches appear to be suitable for use as stormwater management BMPs. We recommend a minimum bottom of trench depth of 4 feet below grade. If soils with significant silt content (weathered soils, fill, till-like materials) are encountered at this depth, they should be removed and replaced with clean rock. It is possible for pieces of glacial till to be found within recessional outwash. We should be on site to observe the soil conditions in the trenches. We recommend that any system be set back at least 5 feet from any structures and property lines. We should be provided with the final plans to verify that our recommendations have been implemented. Infiltration Infiltration of surface water runoff is feasible based on the results of our subsurface investigation. The soil classifications at potential infiltration system depths are presented in the following table: Exploration Number Sample Elevation (Feet Below Grade) Organic Content Cation Exchange Capacity Soil Classification (USDA/USCS) HB-1 4 2.9% 5.3 meq Medium Sand/SP HB-3 4 3.3% 6.0 meq Medium Sand/SP January 8, 2019 Page 3 of 4 Stormwater Feasibility Evaluation www.cobaltgeo.com (206) 331-1097 We performed a small-scale pilot infiltration test to confirm infiltration suitability. The test was performed in HB-3 at a depth of 3 feet below grade. Following an approximate 6-hour saturation period, we performed the falling head portion of the test. After application of correction factors for site variability (0.33), testing method (0.5), and influent control (0.9), the final design infiltration rate of 2.6 inches per hour was obtained. We would classify the upper weathered soils (0-4 feet below grade) as Loamy Sand (USDA) and silty-sand to poorly graded sand (SM-SP per USCS). Soils below this level consist of Sand (USDA) and poorly graded sand (SP per USCS). The shallow soils that underlie the site consist of medium sand as described in the 2016 Surface Water Design Manual (King County). From the SWDM, infiltration trenches or drywell in medium sand may accept runoff from 1,000 square feet of impervious surface. Each trench (per 1,000 square feet of runoff) should be at least 2 feet wide and 30 feet long. As noted above, we recommend a bottom of trench elevation of about 4 feet below existing site elevations. More information can be found in the SWDM on pages C-50 and C-51 (and others). 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. Verification testing of infiltration systems should be performed during construction. 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. 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. January 8, 2019 Page 4 of 4 Stormwater Feasibility Evaluation www.cobaltgeo.com (206) 331-1097 Sincerely, Cobalt Geosciences, LLC Phil Haberman, PE, LG, LEG Principal Figure 1; Site Plan Figure 2; Hand Boring Logs N Stormwater Feasibility Evaluation 2732 Aberdeen Avenue Renton, Washington Cobalt Geosciences, LLCP.O. Box 82243Kenmore, WA 98028 (206) 331-1097 www.cobaltgeo.com cobaltgeo@gmail.com SITE PLAN FIGURE 1 Subject Property HB-3 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 lessthan 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 Soft4 - 8 Medium stiff8 - 15 Stiff15 - 30 Very stiff Over 30 Hard Cobalt Geosciences, LLCP.O. Box 82243Kenmore, WA 98028(206) 331-1097 www.cobaltgeo.com cobaltgeo@gmail.com Soil Classification Chart Figure C1 Log of Hand Boring HB-1 Date: January 4, 2019 Contractor: Method: Hand Auger Depth: 8’ 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 (%)PlasticLimit LiquidLimit 10 20 30 400 50 1 2 3 4 5 6 7 8 9 10 End of Hand Boring 8’ Vegetation/Topsoil Loose, silty-fine to medium grained sand with to trace gravel, dark yellowish brown to yellowish brown, moist. (Fill)SM- SP Medium dense, fine to medium grained sand trace gravel,grayish brown, moist. (Recessional Outwash)SP Cobalt Geosciences, LLCP.O. Box 82243Kenmore, WA 98028(206) 331-1097 www.cobaltgeo.com cobaltgeo@gmail.com Stormwater Feasibility Evaluation 2732 Aberdeen Avenue Renton, Washington Hand Boring Log SM- SP Loose to medium dense, fine to medium grained sand trace to some silt, yellowish brown to grayish brown, moist. (Weathered Recessional Outwash) Log of Hand Boring HB-2 Date: January 4, 2019 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 (%)PlasticLimit LiquidLimit 10 20 30 400 50 1 2 3 4 5 6 7 8 9 10 End of Hand Boring 6’ Vegetation/Topsoil Loose, silty-fine to medium grained sand with to trace gravel, dark yellowish brown to yellowish brown, moist. (Fill)SM- SP Medium dense, fine to medium grained sand trace gravel, grayish brown, moist. (Recessional Outwash) SP Cobalt Geosciences, LLCP.O. Box 82243Kenmore, WA 98028(206) 331-1097 www.cobaltgeo.com cobaltgeo@gmail.com Stormwater Feasibility Evaluation 2732 Aberdeen Avenue Renton, Washington Hand Boring Log SM- SP Loose to medium dense, fine to medium grained sand trace to somesilt, yellowish brown to grayish brown, moist. (Weathered Recessional Outwash) Log of Hand Boring HB-3 Date: January 4, 2019 Contractor: Method: Hand Auger Depth: 8’ 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 (%)PlasticLimit LiquidLimit 10 20 30 400 50 1 2 3 4 5 6 7 8 9 10 End of Hand Boring 8’ Vegetation/Topsoil Loose, silty-fine to medium grained sand with to trace gravel, dark yellowish brown to yellowish brown, moist. (Fill)SM- SP Medium dense, fine to medium grained sand trace gravel, grayish brown, moist. (Recessional Outwash) SP Cobalt Geosciences, LLCP.O. Box 82243Kenmore, WA 98028(206) 331-1097 www.cobaltgeo.com cobaltgeo@gmail.com Stormwater Feasibility Evaluation 2732 Aberdeen Avenue Renton, Washington Hand Boring Log SM- SP Loose to medium dense, fine to medium grained sand trace to somesilt, yellowish brown to grayish brown, moist. (Weathered Recessional Outwash) Log of Hand Boring HB-4 Date: January 4, 2019 Contractor: Method: Hand Auger Depth: 8’ 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 (%)PlasticLimit LiquidLimit 10 20 30 400 50 1 2 3 4 5 6 7 8 9 10 End of Hand Boring 8’ Vegetation/Topsoil Loose, silty-fine to medium grained sand with to trace gravel, dark yellowish brown to yellowish brown, moist. (Fill)SM- SP Medium dense, fine to medium grained sand trace gravel,grayish brown, moist. (Recessional Outwash)SP Cobalt Geosciences, LLCP.O. Box 82243Kenmore, WA 98028(206) 331-1097 www.cobaltgeo.com cobaltgeo@gmail.com Stormwater Feasibility Evaluation 2732 Aberdeen Avenue Renton, Washington Hand Boring Log SM-SP Loose to medium dense, fine to medium grained sand trace to somesilt, yellowish brown to grayish brown, moist. (Weathered Recessional Outwash)