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Mr. Sam Roberto <br />Project BBXX -01 -1 19A <br />May 24, 2001 <br />Page 8 <br />D.3.b. Selection, Placement and Compaction of Backfill <br />Unless a drainage composite is placed against the backs of the retaining walls, we recommend <br />that backfill placed within 2 horizontal feet of the walls consist of sand having less than 50 <br />percent of the particles by weight passing a 40 sieve and less than 5 percent of the particles by <br />weight passing a 200 sieve (Mn/DOT Specification 3149.2J, Fine Filter Aggregate). <br />Backfill placed more than 2 feet away from retaining walls may consist of suitable existing fill or <br />imported material selected, placed and compacted in accordance with the recommendations <br />provided above in Section D.I. <br />We recommend a walk behind compactor be used to compact the backfill placed within about 5 <br />feet of the retaining walls. Further away than that, a self - propelled compactor can be used. <br />D.3.c. Configuring and Resisting Lateral Loads <br />We recommend designing basement walls to support an equivalent fluid pressure of 60 pounds <br />per square foot per foot of depth (pct). This value assumes at -rest earth pressure conditions. <br />Resistance to lateral earth pressures will be provided by passive resistance against the retaining <br />wall footings, and by sliding resistance along the bottoms of the wall footings. We recommend <br />assuming a passive pressure equal to 250 pcf and a sliding coefficient equal to 0.4. These values <br />include a safety factor of 1.5. <br />D.4. Interior Slabs <br />D.4.a. Subgrade Modulus <br />Assuming implementation of the recommendations provided above in Section D.1., slab <br />subgrades will likely consist of compacted granular backfill. This being the case, we <br />recommend using a modulus of subgrade reaction, k, of 200 pounds per square inch per inch of <br />deflection (pci) to design the slabs. <br />