10930 HOLLY DR Geotech Report 2025-08-08

Helix Design Group,Inc.-Greenwood Heating&Air Conditioning,Everett,WA April 22,2022 Geotechnical Engineering Report Z0219 4.4 Drainage Systems In our opinion,the proposed structures should be provided with permanent drainage systems to reduce the risk of future moisture problems. We offer the following recommendations and comments for drainage design and construction purposes. Perimeter Drains: We recommend that structures be encircled with a perimeter drain system to collect seepage water. This drain should consist of a 6-inch-diameter perforated pipe within an envelope of pea gravel or washed rock,extending at least 6 inches on all sides of the pipe,and the gravel envelope should be wrapped with filter fabric to reduce the migration of fines from the surrounding soils. Ideally,the drain invert would be installed no more than 8 inches above the base of the perimeter footings. Subfloor Drains: Based on the groundwater conditions observed in our site explorations,we do not infer a need for subfloor drains. Discharge Considerations: If possible, all perimeter drains should discharge to a storm sewer system or other suitable location by gravity flow. Check valves should be installed along any drainpipes that discharge to a sewer system,to prevent sewage backflow into the drain system. If gravity flow is not feasible,a pump system is recommended to discharge any water that enters the drainage system. Runoff Water: Roof-runoff and surface-runoff water should not discharge into the perimeter drain system. Instead,these sources should discharge into separate tightline pipes and be routed away from the building to a storm drain or other appropriate location. Grading and Capping: Final site grades should slope downward away from the buildings so that runoff water will flow by gravity to suitable collection points, rather than ponding near the building. Ideally,the area surrounding the building would be capped with concrete, asphalt, or low-permeability (silty)soils to minimize or preclude surface-water infiltration. 4_5 Subgrade and Retaining Walls We understand that subgrade and/or retaining walls up to 4 feet high may be needed to achieve design elevations during site grading. The following general recommendations should be applied to the design of retaining walls. Wall Foundations: Subgrade and/or retaining wall foundations should be designed using the recommendations for spread footings presented in Section 4.2 Wall Drainage: Drainage should be provided behind subgrade and retaining walls by placing a zone of drain rock containing less than 3 percent fines(material passing No.200 sieve)against the wall. This drainage zone should be at least 24 inches wide(measured horizontally) and extend from the base of the wall to within 1 foot of the finished grade behind the wall. Smooth-walled perforated PVC drainpipe having a minimum diameter of 4 inches should be embedded within the sand and gravel at the base of the wall along its entire length. This drainpipe should discharge into a tightline leading to an appropriate collection and disposal system. Migizi Group,Inc. Page 12 of 16