1901 W MUKILTEO BLVD Geotech Report 2022-10-25

, Scriven Residence Improvements-REVISED NGA File No 974316 1901 West Mukilteo Boulevard November 29,2016 Everett,Washington Page 14 dependent on the conditions encountered during construction. We should work with your contractor to determine the best course of action at the time of construction. If any irrigation systems are located near or within the steep slopes they should also be abandoned and removed. Areas of exposed soils should be covered with erosion control matting, and then planted with deep-rooted plants. For the sloping areas directly below the new wall,we recommend that the exposed soil be covered with heavy duty erosion control matting such as Tensar C350 Turf Reinforcement Mat, or equivalent. The erosion control matting should be staked to the surface at a minimum of five feet on center using 2- to 3-foot long metal stakes with a "T" welded to the end. After the matting is placed, we recommended that deep-rooted vegetation be planted on the slope and grass seed be placed to re-establish vegetation growth. The vegetation should be maintained until established. We recommend a mixture of 25% each of the following vegetation: Snowberry (Symphoricarpos albus), Nootka rose (Rosa nutkana), Ocean Spray (Holodiscus discolor), and Oregon-grape (Manhonia nervosa). We should be retained to review and comment on final slope improvement plans and observe the slope repairs. The actual improvement methods for these areas will highly depend on the conditions encountered during construction. We should work with your contractor to determine the best course of action at the time of construction We also recommend that all residence downspouts and yard drains be investigated to understand where they are directed. We observed numerous PVC, and corrugated pipes placed along the steep slope to the north of the residence. We anticipated that these pipes likely originated from the roof down spouts and yard drains associated with the residence, but we were unable to confirm this during our site visit. We observed that these pipes were likely daylighting on the steep slopes below the top of slope area. We recommend that all of the existing roof, footing, yard, and driveway drains associated with the residence be tightlined into a new Type 1 catch basin to be setback a minimum of 20 feet from the top of the steep slope. From this main catch basin, the outfall pipe should be connected and constructed to maintain positive water flow. We recommend a minimum 6-inch HDPE tightline pipe be installed into the catch basin and directed to flow through the retaining wall and down the surface of the steep slope to a dissipater tee at the end of the HDPE pipe that outfalls onto a rock spall diffuser pad that is located under the entire length of the dissipater tee. The outfall of this pipe should be located within the lower gentler slope within the northern portion of the property. The dissipater tee could consist of the same HDPE pipe as the main pipe with '/2-inch diameter holes drilled in it at 6-inch on center to further dissipate the flow and should extend a minimum of two feet on each side of the main HDPE pipe. The diffuser pad should consist of a layer of 4- to 8-inch rock spalls placed to further disperse the flow from the dissipater tee, armor the slope, further minimize erosion effects, and to hold the diffuser tee in place. The pipe should NELSON GEOTECHNICAL ASSOCIATES, INC.