5402 S 1ST AVE 2022-05-23 - Laserfiche WebLink

Stormwater Infiltration Letter NGA Project No. 1040718 Zillah Street Residential Development Infiltration June 20,2018. Everett,Washington Page 5 respectively. A total correction factor of 0.315 was applied to the measured field infiltration rate obtained from the falling head portion of the tests to determine the long-term design infiltration rate. Using the above correction factor,we calculated a long-term design infiltration rate of approximately 0.04 inches per hour.for the southern property and 0.63 inches per hour for the northern property. In our opinion, a long-term design infiltration rate of 0.04 inches per hour could be utilized to design the on-site shallow infiltration systems within the native silty fine to medium sand with gravel found on the southern property, and 0.63 inches per hour can be utilized to design the on-site infiltration systems within the native silty fine to medium sand with gravel soils found within the northern property. We recommend that any infiltration systems within the properties be sized and design in accordance with the Department of Ecology 2014 Stormwater Management Manual for Western Washington. If shallow infiltration systems are deemed feasible within the southern property, we recommend that a suitable overflow component be incorporated into the overall system design that is connected to flow into an approved system. If permeable pavements are utilized within the properties, we recommend that the permeable pavement section be underlain by a minimum of 12-inches of clean crushed rock. We recommend that any proposed infiltration systems be placed as to not negatively impact any proposed or existing nearby structures and also meet all required setbacks from existing property lines, structures, and sensitive areas as discussed in the drainage manual. In general, infiltration systems should not be located within proposed fill areas within the site associated with site grading or retaining wall backfill as such condition could lead to failures of the placed fills and/or retaining structures. We should be retained to evaluate the infiltration system design and installation during construction. USE OF THIS LETTER This letter was prepared for Mr. Melvin Gay and his agents, for use in planning and budgeting the above- referenced project only. Our services included an evaluation of the infiltration capability of the site soils at specific locations, and should not be considered as an in-depth geotechnical study of the site or an evaluation of the overall site stability. This letter may be used for bidding and estimating purposes, but our letter, conclusions, and interpretations should not be construed as a warranty of the subsurface conditions. The subsurface conditions between explorations may vary. A contingency for varying conditions should be incorporated into the project plans. We recommend that NGA be retained to review the design and provide monitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether or not earthwork activities comply NELSON GEOTECHNICAL ASSOCIATES, INC.