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Robert Rimstad <br />April 26, 2011 <br />AET No. 03 -03944 <br />Page 4 of 6 <br />If organic soils are tbund to he present, we recommend removing these soils where present <br />within the critical subgrade zone. <br />We caution that instability of soils present beneath the soils being reworked and compacted may <br />limit the ability to compact the upper soils. In this case, greater depths of subcutting and stability <br />improvement may be needed. <br />4.3 Fill PlacemenWompaction <br />Following subcutting and preparation of existing soils, fill can be placed as needed to reattain top <br />of subgrade. Fill should be placed per the requirements of Mn/DOT Specification 2105.3F1 <br />(Specified Density Method). In ASTM terms, this specification requires soils placed within the <br />critical subgrade zone be compacted to a minimum of 100% of the standard maximum dry unit <br />weight defined in ASTM: D698 (Standard Proctor test), at a water content 65% to 102% of the <br />standard optimum water content. A reduced minimum compaction level of 95% of the standard <br />maximum dry unit weight can be used below the critical subgrade zone. <br />4.4 Subsurface Drainage <br />The sand subbase layer should be provided with proper subsurface drainage to prevent build -up <br />of water within the sand. This can be accomplished by placing properly engineered drainage <br />lines which are connected to catch basins or outfall into ditches. <br />5.0 Pavement Thickness <br />In our opinion, the recommended subgrade preparation approach can be assigned a design R- <br />value of 30, which is based on the placement of a 1 -foot thick sand subbase placed over clayey <br />soils having an AASHTO Soil Croup classification of A -6. <br />The recommended pavement section for light -duty pavement areas can consist of 3 inches of <br />bituminous (placed in two lifts) over 5 inches of Class 5 aggregate base. Light -duty pavements <br />would be those areas limited to automobile and passenger truck/van areas, which exclude buses <br />and commercial trucks. <br />The recommended pavement section for heavy -duty pavement areas can consist of 4% inches of <br />bituminous (placed in two lifts) over 8 inches of Class 5 aggregate base. Heavy -duty pavements <br />would be those areas of bus traffic. <br />If you wish to place a concrete pad at bus loading areas, we recommend a concrete thickness of 6 <br />inches over 4 inches of aggregate base. <br />52 <br />