1617 EDGEMOOR LN 2016-11-10 - Laserfiche WebLink

Retaining Wall Installation Final Letter NGA File No. 9106B 15 Mckinnon Residence Retaining Wall June 15, 2016 Everett, Washington Page 3 daylighted through the retaining wall. The 6-inch HDPE pipe was anchored to the steep slope below the retaining wall with 2-inch diameter steel stakes and collars. The HDPE pipe outlet is located along the toe of the upper steep slope along the southern side of an existing old logging road. The outlet of the pipe consists of a 6-inch HDPE perforated tee section of pipe. During our site visit, we observed that the perforated tee section of the pipe had been partially filled with concrete sludge that was apparently washed into the drainage pipe from the driveway above. We also did not observe a rock spall splash pad below the pipe. In our opinion, it appears that the drainage systems have been generally installed in accordance with our recommendations and the approved plans.. However, we recommend that the concrete sludge found within the perforated tee be removed and a 2 to 4-inch rock spall splash pad be placed beneath the entire length of the perforated tee. We also recommend that the tee be perfectly level. We recommend that all drainage pipes be inspected periodically to verify they are working properly and any repairs, if needed, be performed promptly. We also recommend that the area above the wall and the sloping area below the wall be continually monitored through the. wetter winter months for any indications of instability and corrective actions promptly implemented should any signs of distress be observed. We observed that the area directly below the wall has covered with jute netting and plants. This area appears to be performing well. Closure Based on our understanding of the site conditions and our part-time observations during the implementation of the recommended repairs, it is our opinion that the retaining wall and stabilization measures have been installed successfully and in accordance with our recommendations. If any signs of distress are observed, we should be contacted immediately. At that time, we would provide recommendations keep any repairs to the slope, as warranted. We should stress that the steep slope below the wall will always be prone to sloughing and shallow sliding events due to the inclination of the slope, the nature of the near surface soils, and the wet weather conditions during the winter months. Such events, if they do occur, should not immediately impact the stability of the wall system. However, erosion and sloughing events on the slope below the wall, if not addressed, can ultimately impact the wall. ••• NELSON GEOTECHNICAL ASSOCIATES, INC.