1815 HOLBROOK AVE 2025-03-06 - Laserfiche WebLink

June 9,2022 Page ii of 15 Geotechnical Evaluation Stormwater Management Feasibility The site is underlain by fill and at depth by advance outwash sands. The outwash sands are typically permeable and have some suitability for infiltration of stormwater runoff. The site contains and is close to local steep slopes created through prior grading (at least in part). Our deeper boring encountered groundwater about 24 feet below grade. We did not observe spring activity or emergent groundwater in downslope areas east and northeast of the site. It appears that groundwater likely has a downward gradient similar to the existing topography, ultimately daylighting near the base of, or within the ravine feature located about 70o feet east of the site. We performed a small scale pilot infiltration test(PIT) in TP-i.The test was performed in general accordance with the Washington State Department of Ecology stormwater manual. The area was excavated to a testing depth of approximately 3 feet below the ground surface. The design infiltration rate was determined by applying correction factors to the observed infiltration rate as prescribed in Volume III, Section 3.3.6 of the DOE. The observed rate must be reduced through appropriate correction factors for site variability(CFv),uncertainty of test method(CFT), and degree of influent control(CFM)to prevent siltation and bio-buildup. It should be noted that construction traffic or other disturbance to the target infiltration area could compact the soil,which may decrease the effective infiltration rates. The correction factors and resulting design infiltration rate are also shown in the table below. Test Test Measured Correction Factors Design Number Depth(ft) Infiltration Infiltration Rate(in/hr) Rate CFv CFT CFM (in/hr) TP-i 3.0 3.7 0.33 0.5 0.9 0.55 We used a more conservative factor of safety for site variability to account for the presence of nearby slopes. The use of this value would help distribute runoff over a large area. This will help the infiltration to be more widespread and not be focused in any one area below the property. We recommend utilizing infiltration drywells in the northwest portion of the property,at least io feet from the north property line and new residence. We recommend drywells be set at least to an elevation of 194 feet. This will require extensive excavation work. We must verify soil conditions during system placement. Drywells may be sized using the rate above or if allowed, the Medium Sand designation per the USDA Textural Triangle. Systems should be spaced as much as feasible. We recommend that foundation drains for the new residence be routed to a drywell located in the northeast portion of the site,near elevation 193 feet. The drywell should have a depth of at least 6 feet and diameter of at least 3 feet. Permeable pavements are generally feasible in the outwash sands provided they are located at least 20 feet from the steeper slope areas. We must be on site to verify soil conditions at pavement areas during construction. 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. www.cobaltgeo.com (206)331-1097