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Home My WebLink AboutRS_Geotech Letter_180719_v1.pdf Cobalt Geosciences P.O. Box 82243 Kenmore, Washington 98028 (206) 331-1097 PO Box 82243 Kenmore, Washington 98028 cobaltgeo@gmail.com 206-331-1097 March 28, 2018 Mr. Paul Miller prmiller@sumsales.com 206-954-1137 RE: Stormwater Feasibility Evaluation Proposed Single-Family Residence 3731 Park Avenue North Renton, Washington Dear Mr. Miller, In accordance with your authorization, Cobalt Geosciences, LLC has prepared this letter to discuss the results of our stormwater feasibility evaluation at the referenced site. The purpose of our evaluation was to determine the classification of shallow soils at potential infiltration trench locations and whether groundwater was present at system depths. Site and Project Description The site is located at 3731 Park Avenue North in Renton, Washington. The site consists of one rectangular parcel with an area of approximately 15,000 square feet. The site is currently undeveloped and we understand that the previous residence was recently demolished. The property is vegetated with grasses and sparse trees/shrubs near the property margins. The site slopes gently downward toward the northwest at magnitudes less than 10 percent and topographic relief of about 5 feet. The site is bordered on all sides by single-family residences. The access driveway extends along the south property line. The proposed development includes a new single-family residence located in the central portion of the property. We anticipate that the new residence will have a crawlspace area. Runoff from impervious surfaces will be directed into dispersion or infiltration trenches depending on feasibility. Area Geology The Geologic Map of King County, indicates that the site is underlain by Vashon Recessional Outwash. Vashon Recessional Outwash typically consists of normally consolidated sand with variable amounts of silt and gravel and local interbeds of silt and clay. These deposits are relatively permeable and are often underlain by Vashon Glacial Till. RECEIVED 07/19/2018 amorganroth PLANNING DIVISION March 28, 2018 Page 2 of 4 Stormwater Feasibility Evaluation PO Box 82243 Kenmore, Washington 98028 cobaltgeo@gmail.com 206-331-1097 Site Exploration We excavated one test pit in the east-central portion of the site to evaluate the shallow subsurface soil conditions. The test pit encountered approximately 12 inches of loose, silty-fine to medium grained sand with gravel (Fill) underlain by approximately 3 feet of loose to medium dense, silty- fine to fine grained sand (Weathered Recessional Outwash). This layer was underlain by approximately 2 feet of loose to medium dense, silty-fine to fine grained sand (Recessional Outwash), which continued to the termination depth of the test pit. Groundwater was not observed in the test pit at the time of the investigation. We observed mottled soils from 4 to 6 feet below grade indicating that perched groundwater may develop periodically below the site. Conclusions & Recommendations The near surface soils consist of fine grained sand with variable amounts of silt. Shallow infiltration trenches or dispersion trenches are suitable for use as stormwater management BMPs. We recommend a minimum bottom of infiltration trench of 1.5 to 2.5 feet below grade. If soils with significant silt content are encountered at this depth, they should be removed and replaced with clean rock. We recommend that any system be set back at least 15 feet from the north property line at 5 feet from the east and west property lines. Systems should be located at least 5 feet north of the proposed residence. If necessary, trenches could be located near the east or south property lines; however, we should be provided with the final plans to verify that our recommendations have been implemented. Infiltration Limited infiltration of surface water runoff is feasible based on the results of our subsurface investigation. The soil classification 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) TP-1 2 5.6% 7.8 meq Loamy Sand (SP-SM) 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 consisted of finer grained materials that may cause infiltrating runoff to become perched. Therefore, we suggest a bottom of trench depth between 1.5 and 2.5 feet below existing grades. We performed an in-situ small-scale Pilot Infiltration Test (PIT) in TP-1 at a depth of 3 feet below grade. Following a saturation period, the average infiltration rate (constant head) was determined to be 0.45 inches per hour. March 28, 2018 Page 3 of 4 Stormwater Feasibility Evaluation PO Box 82243 Kenmore, Washington 98028 cobaltgeo@gmail.com 206-331-1097 It is our opinion that limited shallow infiltration or limited dispersion using trenches are both suitable for use at the site provided guidelines and requirements of the 2016 Surface Water Design Manual (King County SWDM) are followed. Per the SWDM, infiltration trenches in loamy sand must have a length of 21 feet inside the UGA (Seatac 1.0 and less), 48.3 feet inside the UGA (greater than Seatac 1.0), and 70 feet outside of the UGA (see Page C-58 of the SWDM). Each trench is meant to serve 1,000 square feet of impervious surface. 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 gener al 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 exca vation 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 March 28, 2018 Page 4 of 4 Stormwater Feasibility Evaluation PO Box 82243 Kenmore, Washington 98028 cobaltgeo@gmail.com 206-331-1097 Sincerely, Cobalt Geosciences, LLC Phil Haberman, PE, LG, LEG Principal Attachments Figure 1; Site Plan Test Pit Log Sieve Analysis SITE PLAN FIGURE 1 P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 cobaltgeo@gmail.com Cobalt Geosciences Proposed Single-Family Residence 3731 Park Avenue North Renton, Washington TP-1 N TP-1 TEST PIT LOGS P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 cobaltgeo@gmail.com Cobalt Geosciences Proposed Single-Family Residence 3731 Park Avenue North Renton, Washington LLC Test Pit TP-1 0-1’ Silty-Sand with Gravel (SM) Loose, silty-fine to medium grained sand with gravel, yellowish brown to grayish brown, moist (Fill) 1-4’ Poorly Graded Sand to Silty Sand (SP-SM) Loose to medium dense, silty-fine to fine grained sand trace gravel, yellowish brown to grayish brown, moist. (Weathered Recessional Outwash) 4-9’ Silty Sand (SM) Medium dense, silty-fine to fine grained sand trace cobbles, mottled yellowish brown to grayish brown, moist. (Recessional Outwash) End of Test Pit 9’ No Groundwater No Caving SM SP-SM Recessional Outwash Recessional Outwash 1’ 4’ USCS Graphic SM Fill Project Name:Tested By:PH Date:3/28/2018 Location:Checked By:SC Date:3/28/2018 Boring No:Sample No.:1 Sample Depth:Gnd Elev.: USCS Soil Classification: AASHTO Soil Classification: Weight of Container (g):10.0 Weight of Container & Soil (g):73.2 Weight of Dry Sample (g):58.4 Sieve Number Diameter (mm) Mass of Sieve (g) Mass of Sieve & Soil (g) Soil Retained (g) Soil Retained (%) Soil Passing (%) #4 4.75 10 13.1 3.1 5.3 94.7 #10 2.00 10 14.8 4.8 8.2 86.5 #20 0.85 10 16.5 6.5 11.1 75.3 #40 0.43 10 25.6 15.6 26.7 48.6 #60 0.25 10 26.3 16.3 27.9 20.7 #200 0.075 10 17.1 7.1 12.2 8.6 Pan 10 14.9 4.9 8.4 0.0 TOTAL:58.3 99.8 Sieve Diameter (mm)% Passing % Passing 4 4.75 100 10 2 100 40 0.425 100 200 0.075 100 4 4.75 0 10 2 0 40 0.425 0 200 0.075 0 % Gravel:5 D10:Cu: % Sand:86 D30:Cc: % Fines:9 D60: 11% moisture SP-SM Grain Size Distribution Curve Results: Sieve Analysis Data Sheet ASTM D422-63(2007) Proposed Residence 3731 Park Avenue North TP-1 2' 0 10 20 30 40 50 60 70 80 90 100 0.010.101.0010.00% Passing Particle Diameter (mm) #4 #10 #40 #200 GRAVEL Coarse SAND Medium SAND Fine SAND SILT/CLAY Cobalt Geosciences PO Box 82243, Kenmore WA 98028 cobaltgeo@gmail.com (206) 331-1097