1311 W MUKILTEO BLVD 2022-06-29 - Laserfiche WebLink

Geotechnical Engineering Evalua. • NGA File No. 1295621 Tsang Slope Restoration October 21,2021 Everett,Washington Page 6 ( 8 For planning purposes, we recommend that temporary cuts in the on-site material be no steeper than two units horizontal to one unit vertical (2H:1V). If groundwater seepage is encountered, we would expect that significantly flatter inclinations would be necessary. We should be retained to specifically review proposed geometry for significant cuts planned on this site. We recommend that cut slopes be protected from erosion. Erosion control measures may include covering cut slopes with plastic sheeting and diverting surface water runoff away from the top of cut slopes. We do not recommend vertical slopes for cuts deeper than four feet if worker access is necessary. We recommend that cut slope heights and inclinations conform to appropriate OSHA/WISHA regulations. Permanent cut and fill slopes should also be no steeper than 2H:1V. However,flatter inclinations may be required in areas where loose soils are encountered. Permanent slopes should be covered with erosion control matting and vegetated promptly. The vegetative cover should be maintained until established. Slope Improvements We understand erosion control netting has been installed on the slope and tacked with landscaping staples. Since the restoration and regrading activities did not expose dense, undisturbed soils at the surface, we expect the erosion hazard to be significant, and for surficial raveling to occur during the wet season. The risk of raveling and sheet erosion can be reduced by utilizing high quality erosion control covers in addition to properly securing them to the slope. Degradable,all-natural coconut coir matting or jute netting or geosynthetic erosion control matting such as Tensar Turf should be installed on restoration areas prior to planting, and this erosion control surfacing staked with 18-to 24-inch long, wedge-shaped stakes made by cutting untreated 2x4s diagonally. Stakes should be placed in regular intervals no greater than three feet on-center, and erosion control fabric should incorporate an overlap of at least a foot in restoration areas. Metal stakes consisting of 2-foot long steel rebar with a welded 'T'to the top may also be used for tacking erosion control fabrics to the slope. 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 a/bus), Nootka rose (Rosa nutkana), Ocean Spray (Holodiscus discolor), and Oregon- grape(Manhonia nervosa).Alternatively,the slope should be hydroseeded if treatment is scheduled for an appropriate for the time of year for grass seed growth. NELSON GEOTECHNICAL ASSOCIATES, INC.