1215 MADRONA AVE 2016-08-24 - Laserfiche WebLink

Gustafson Residence Retaining Wall NGA File No 924115 1215 Madrona Avenue June 12, 2015 Everett, Washington Page 7 wall along with a schematic detail of the new wall are shown on the Schematic Retaining Wall Site Plan and Schematic Pin Pile Wall Detail in Figure 7. Detailed recommendations regarding the proposed retaining wall design and installation have been included in the following subsections of this letter. The surficial soils encountered on tl�is site are considered moisture-sensitive and inay disturb easily when wet. To lessen the potential impacts of construction and to reduce cost overruns and delays, we recominend that construction take place during the drier suinmer months if possible. If construction takes place during the rainy months, additional expenses and delays should be expected. Tllese extra expenses could include additional erosion control and temporary drainage ineasures to protect the proposed developinent area including placement of a blanket of rock spalls to protect exposed subgrades, and the need for importing all-weather materials for structural fill. All construction operations and drainage iinproveinents planned as part of this project should be planned and completed in a manner that enhances the stability of the steep slope, not reduces it. Any excavation spoils generated during site improvements should not be stockpiled on site but rather promptly hauled away. Also, all current and future runoff generated within the site should be collected and routed to a permanent discharge location at the bottom of the slope, or to an approved drainage system. Under no circumstances should water be allowed to concentrate or flow uncontrollably over the slope. The vegetation cover on tl�e slope should be evaluated for compatibility with desired slope stability conditions, and a vegetation management plan should be devised to enhance slope stability. The proposed retaining wall and the slope should be monitored on an on-going basis after construction especially during the wet season for any signs of instability, and corrective actions proinptly taken should any signs of instability be observed. Lawn clippings and any other household trash or debris should never be cast over the slope. Any future plans for structures near the slope or inodifications to the slope should be specifically evaluated by NGA and approved by the City of Everett. Retaining Wall Design and Construction The proposed new retaining wall should consist of 3-inch square driven steel tubing or 2-inch pin piles bridged using pressure-treated timber lagging. The proposed retaining wall will likely incorporate tieback anchors consisting of grouted or mechanical anchors. The new retaining wall should generally be located along the top of the steep slope in the approximate location of the existing tiinber wall. The final extent and heights of the wall should be detennined by your structural engineer. However, we recommend that NELSON GEOTECHNICAL ASSOC/ATES, INC.