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r 1 <br />Mr. Dak Gustafson <br />October 11, 1994 <br />Page 4 <br />partkle size of 2" and no more than 50% material finer than a #4 sieve or 3% finer tfun a #200 <br />sieve. This type of material is free draining, exhibits very little shifting when frozen, and is <br />easier to compact in confined spaces siKih as expected at this project site. We recommend <br />placmg tfK granular fill in lifts no more than 6’m thickness with vibratory compactic,i of each <br />lift. Care should be taken m choosing compacuen equ.pmem which has sufficient energy for <br />effective compaction yet does not have e.xcessive energ>' or weight to cause significant shiftmg <br />of the wall facing stones dunng the gnvcl compaction process. A porous gcotcxulc separauon <br />fabric is recommended to separate the pravel fill from fine gramed native soils at surfaces where <br />seepage from the soil iiuo the gravel can be c.xpectcd. This includes beneath the topsoil layer <br />placed over the gravel. <br />Our computations iiKlicatc the critical, or governing, factor for wall stability is lateral sliding <br />of the retaining wall base. In order to attain the minimum recommended factor of safet>’, we <br />recommend the following minimum depth (distance back from the back of the tacing stone) of <br />g**'grid-reinforced gravel. <br />WaU Hdglit (ft.) <br />Depth of Reinforced Backfill Zone (ft.) <br />Level Slo|X! <br />Behind Wall <br />2.5:1 .Ma.\imum <br />Slope Behind Wall <br />•>2.6 3 4 <br />4 4.5 6.8 <br />6 6.8 10 I <br />S S.9 - <br />For wall heights between those listed above, you f\ interpolate in direct proportion to arrive <br />at minimum reinforced backfill zone depths. <br />We recommend using a geogrid at least equal ■*' the specifications of Tensar UX1500 (SR-2) <br />uniaxial material for slope reinforcement. For at least some fastemng of the face stone to the <br />reinforced backfill, we recommend extending tlie geoprid 8" to 9" into the stone course joints <br />at the geogrid layer level. You should realize this connection relies only on the frictional <br />resistance of movement between the stone and geogrid material. Thus, care should be taken <br />when moving equipment or heavy objects near the facing stone, so the stoiK is not shifted or <br />damaged.