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HomeMy WebLinkAboutBLD2023-00891 Drainage Memo - BLD Engineering / Geo-tech Reports - 12/28/2023 Mud Bey Geat dwkel Swvlam LLC 1001 Cooper Point Rd SW,Suite 140,PMB#108 1 Olympia,WA 98502 1360.481.9784 CHeathman@MudBayGeotech.com December 28, 2023 Job: 2567-MAS Subject: Drainage Improvement Memo 30 SE Fuchsia Ave, Shelton, WA 98584 Dear Derek Johnson, This email provides a summary of our geotechnical site reconnaissance completed at 30 SE Fuchsia Ave, Shelton, Washington on the date of December 20, 2023. The purpose for the site visit was to assess the on-site drainage conditions and provide recommendations for site drainage. Parcel #319045400015 comprises an area of approximately 0.28 acres within the Fawn Lake Community in Shelton, Washington. The site is accessed via SE Fuchsia Ave, directly southeast of the subject parcel. The site slopes gently northwest from SE Fuchsia Ave, descending roughly 15-20 feet in elevation across the length of the property. The southeastern majority of the property has been cleared and grubbed for the development of a new single-family residence. The western margin of the property features undisturbed vegetation, primarily in the form of big leaf maple, alder, and sword fern. A building pad of imported crushed rock fill material is situated near the center of the parcel. The thickness of fill material is greatest along the northwestern corner of the building pad and decreases moving southwest into slope. We understand that excavation along the southeastern corner of the building pad resulted in groundwater seepage onto the building pad. A curtain drain was installed along the upslope margins of the building pad to capture and reroute the water. The installation of the curtain drain improved drainage across the pad, though further surface and groundwater improvements are necessary. Below is a brief summary of our review of available online resources: • Mason County WA GIS: o No mapped wetlands directly on-site. Nearest mapped wetlands positioned approximately 225 feet northeast of project site, near shoreline of Fawn Lake. Nearest upslope wetlands situated roughly 1250 feet south-southwest of site. 0 • WA DNR Surface Geology: o Upslope, southeastern half of the property is mapped as Pleistocene continental glacial drift. o Downslope, northwestern half of the property is mapped as Pleistocene continental glacial till. .w USDA NRCS Soil Map: Sf— Shelton gravelly sandy loam • o Sediment consists of very gravelly medial sandy loam sourced from basal till and intermixed with volcanic ash. o Moderately well-draining. o Depth to water table about 24 to 36 inches. o Approximately 25 to 36 inches to densic material. o Hydric Soil Rating: No. Geotechnical Recommendations • We recommend installing a second curtain drain upslope of the development pad to capture additional groundwater and reroute downslope and away from structures and developments. No curtain drains should be constructed within 10 feet of the upslope edge or 30 feet of the downslope edge of the drainfield and reserve area. The curtain drains should have a width of 18 inches and extend to a minimum depth of 2 to 3 feet. Perforated drainpipe with a diameter of 4-inches should be placed at the bottom of the excavation and backfilled with material meeting the requirements of Gravel Backfill for Drains in Section 9-03.12(4) of the WSDOT Standard Specifications. Prior to placing the perforated drainpipe and backfilling the excavation, we recommend placing a construction geotextile for underground drainage in the bottom and sidewalls of the trench. The construction geotextile used should meet the requirements of a WSDOT Standard Specification Construction Geosynthetic for Underground Drainage, Class A, Moderate Survivability. The subgrade for the curtain drains should be graded slightly to allow for the natural flow of intercepted water toward a low point for collection. • We recommend the installation of a perimeter footing drain around the foundation of the structure. • To reduce surface erosion and runoff. it is recommended to vegetated the bare slopes on- site. • Once drainage improvements are installed, the plastic sheeting covering the development pad should be removed to aid in evaporation and drying of the building pad. We appreciate the opportunity to serve your geotechnical needs on this project and look forward to working with you in the future. Please contact us at your earliest convenience if you have any questions or would like to discuss any of the contents of this report. Sincerely, ,gyp a0riNF 12/28/2023 Chris Heathman, P.E. Mud BayGeotechnical Services, LLC �s3ss �v c ,���Ss�Uf 1 At.