2000 TOWER ST INDEX HALL 2023-12-29

J&*#*hh •R ENGINEERING Capillary Break To retard the upward wicking of groundwater beneath the floor slab, we recommend that a capillary break be placed over the subgrade. Ideally, this capillary break would consist of a 4-inch-thick layer of pea gravel or other clean, uniform, gravel, such as "Gravel Backfill for Drains' per 2021 WSDOT Standard Specification 9-03.12(4). Vapor Retarder We recommend that a layer of plastic sheeting (such as Visqueen or Moistop) be placed directly between the capillary break and the floor slab to prevent ground moisture vapors from migrating upward through the slab. During subsequent casting of the concrete slab, the contractor should exercise care to avoid puncturing this vapor barrier. 4.7 DRAINAGE SYSTEMS In our opinion, the proposed structure should be provided with permanent drainage systems to minimize the risk of future moisture problems. We offer the following recommendations and comments for drainage design and construction purposes. Grading and Capping Final site grades should slope downward away from the structure so that runoff water will flow by gravity to suitable collection points, rather than ponding near the structure. Ideally, the area surrounding the structure would be capped with concrete, asphalt, or low -permeability (silty) soils to reduce surface - water infiltration. Perimeter Drains We recommend that the structure be encircled with a perimeter drain system to collect seepage water. This drain should consist of a 4-inch diameter PVC 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. The perimeter drain should be located near the base of the footings. Runoff Water Roof -runoff and surface -runoff water should not be allowed to flow into the perimeter foundation drainage systems. Instead, these sources should flow into separate tightline pipes and be routed away from the structure to a storm drain or other appropriate location. MP ENGINEERING, PLLC MPGEOTECH.COM