5704 9TH DR W 2025-02-28 - Laserfiche WebLink

April 11,2024 Page 2 of 3 Reference: Wall Design Recommendations The keyways should be excavated in undisturbed medium dense or firmer native soils or structural fill compacted to at least 95 percent of the modified proctor (ASTM D1557 Test Method) and verified by the geotechnical engineer. Note:it will be necessary to over-excavate and replace any loose or soft soils to achieve suitable soil conditions. All backfill in the grid zone must be compacted as above. Note that significant drying of on site soils will be necessary to achieve compaction requirements. This is likely only possible during summer months. A minimum 6-inch-thick gravel base should be placed and compacted prior to block placement(1- 1/4 to 1-1/2-inch minus or clean crushed rock). A minimum 12-inch-wide drainage zone should extend behind the back of the walls to the suitable native soils or compacted structural fill. The drain zone should extend up to the top of the wall. The drainage zone should consist of clean angular rock, 5/8 to 2 inches in size (note: 5/8" clean angular rock or 2"ballast are acceptable). A minimum 4 inch diameter perforated PVC pipe(Schedule 40)should be installed behind the wall at their base. The wall drain should outfall through the base or ends of the wall and discharge into an approved stormwater conveyance system. If necessary, drains could extend below the face of some of the walls. We can provide additional drainage information if necessary. Runoff may drain in areas that are heavily forested with slope magnitudes of 25 percent or less. A Cobalt representative should be on site during all wall construction activities,including keyway excavation work,grid placement,soil compaction,drainage placement,block and rock placement, and finish grading. It is critical that Cobalt is on site during fill placement during construction of the geogrid reinforced wall system to verify rock placement, drainage, and design parameters. If Cobalt geotechnical personnel are not on site to verify the above aspects of construction, the suitability,stability,and long term performance of the walls cannot be determined or confirmed. Design Information The following soil parameters were used in our designs: Soil Tie Friction Angle Cohesion Unit Weight Retained and Reinforced Soil 36 degrees o psf 130 pcf psf=pounds per square foot pcf=pounds per cubic foot We have attached design files with this letter. We note that all backfill and/or native soils behind the walls must have the soil parameters noted above. This could require significant soil removal, possibly drying, and re-compaction if fill or loose soils are present. We should be on site during construction to verify soil conditions and any compaction. Our design is based on creation of a 2 foot level bench in front of the wall with 2.5 feet of embedment in medium dense native soils and a variable fore slope up to 3o degrees. We included taller wall sections in case a slightly taller wall is required. Note that the base must be medium dense native soils prior to rock pad placement.The upper tier may have 12 inches of embedment. Cobalt should be on site to verify all aspects of wall construction. These include keyways,drainage placement, boulder placement, geogrid and soil compaction of fills, temporary excavations, and cribbing of lock and load walls. www.cobaltgeo.com (206)331-1097