4702 W GLENHAVEN DR 2018-05-16 - Laserfiche WebLink

, � 4702�Y Gtenhaven Dri��e CAO,Snohomish County,Washingfcin iVl�terials Testing& Consulting,Inc. Jul�� 3 I. 2017 1 GB030 pervious surfaces should be employed to limit the ef�ect of development on e�isting site conditions (such as for access drives and walkways). New impervious surfaces shall control and direct 5torm water to appropriate drainage locations. It is presumed that if the drainage recommendations given in this report are followed, added storm water from the development wi11 not adversely impact the landslide hazard area. MTC does not recommend dispersion or infiltration of collected stormwater between the proposed residence and slope crest, or on the slope itself, as increased runoff or localized stormwater inundation can be detrimental to long-term erosional and global slope stability. Roof and footing water sources shall be tightlined a«�ay from the building to an existing catch basin, or down the slope to be released past the base using appropriate energy-reducing features at the exit point to limit the potential of point erosion. Allowance of downslope dispersal into pre-existing drainage ditches or channels may be subject to local regulations. Roof and footing drains should be tightlined separately, or should be gathered in an appropriately sized catch basin structure and redistributed collectively. If storm drains are incorporated for impervious flatwor.ks (driveways, patios, etc.), collected waters shall also be disposed according to the above recommendations. All drainage tightlines shall be composed of appropriately sturdy material (such as rigid PVC), sized adequately according to anticipated volume, and anchored sufficiently. MTC recommends slope tightlines be inspected by the owner periodically to look for signs of damage or displacement that could result in leakage or catastrophic failure and slope erosion or failure, and be re-anchored or replaced if required. If perched water becomes a common issue on site during the rainy season, an additional system of perimeter drains and/or a curtain drain may be very helpful in maintaining dry upland conditions and reducing water influx to the building pad and adjacent slope. Curtain drains can be particularly effective in low gradient directional upland environments where permeable soils overlie relatively impermeable conditions. MTC recommends that we be contacted �o provide supplemental recommendations for site preparation and installation of curtain drains if the need arises. Erosion Protection Erosion is one of the most common driving forces leading to slope instability. In addition to the above commentary, the following general recommendations should be implemented to reduce long-term erosion potential of the slope below the project site and maintain the present slope stability: 1. The ground surface adjacent to the building should be sloped to drain away from the building pad and sloped at a 5% minimum to prevent ponding of water adjacent to the house. Footing drains and yard drains should be incorporated as needed for the Uuilding and site design to help maintain a dry building area. �