7.2. HRSR 08-02-2021 - Laserfiche WebLink

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Preliminary Report of Geotechnical Exploration Main & Gates Redevelopment; Elk River, Minnesota AMERICAN July 9, 2021 ENGINEERING Report No. P-0001445 TESTING, INC. Page 8 of 10 drain tiles be placed below the slab in a free draining sand subbase (below the Class 5 base) at a spacing of 20 feet on center connected to the sump pump. The sand thickness should be at least 6 inches thick. These drain tiles would relieve uplift hydrostatic pressure below the slab in the event of a significant rise in the ground water level. We recommend that the basement wall backfill be a free draining sand, with no more than 5% (by weight) passing the number 200 sieve and no more than 50% (by weight) passing the number 40 sieve. This type of sand would also be considered non-frost susceptible (NFS). The basement walls should be designed for an at-rest pressure of 60 pcf (equivalent fluid density), since they are fixed at the top and bottom. Any loads adjacent to the basement walls should be considered surcharges with an at-rest earth pressure coefficient of 0.5 used for lateral load application to the basement walls. The foundations can be designed for a passive resistance of 360 pcf (equivalent fluid density) for a compacted sand backfill against the footings. The design should include an appropriate safety factor of at least 2 as movements on the order of 1 inch or more would be required to mobilize the full passive resistance. The coefficient of friction for sliding resistance for concrete on granular soil can be taken as 0.45. A safety factor of at least 1.5 should be applied to this sliding friction value. 6.6 Foundation and Exterior Building Backfilling All backfill placed around the foundations should be compacted to at least 95% of their respective Standard Proctor maximum dry densities. Fill soils should be placed and compacted at water contents within ±3% of the respective optimum water contents, based on the respective Standard Proctor tests. The excavated sand can be used as backfill that is placed in lifts of 8 inches in thickness or less. Some of the silty on-site fill soils are frost-susceptible and may cause some freeze-thaw movements of exterior slabs, sidewalks, and stoops above them. Stoops placed over the basement backfill would be supported by non-frost susceptible as described in section 6.5. If the fill placement is not uniform below and around the surrounding pavements, there is a risk that damage to the pavement approaches, curbs, and other exterior site features may occur. Draft