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Robert Rimstad April 26, 2011 AET No. 03 -03944 Page 3 of 6 3.2 Ground Water As noted on the logs, no ground water entered the boreholes during the short time period of the drilling operations. Because the soils are slow draining, it was not possible to establish whether or not a hydrostatic ground -water condition exists within the bored depths. 4.0 Pavement Recommendations 4.1 Definitions Italicized words used in the following sections have a specific definition or are a Mn/DOT reference. The definitions are presented on the attached standard sheet entitled "Definitions Relating to Pavement Construction ", or are defined in an ASTM standard dr a Mn/DOT specification. 4.2 Subgrade Preparation Most of the site soils are slow draining materials. In addition, most of the soils are moderately to highly frost susceptible. In these conditions, it is preferred to place a uniform thickness ,sand subbase of Select Granular Borrow or Modified Select Granular Borrow beneath the pavement section in order to improve the drainage characteristics of the pavement system and to moderate frost effects. Depending on final grades, soil conditions within the critical subgrade zone could range from sands to silts to clays. Demolition -type fill is also possible. It is anticipated that the naturally occurring soils will be relatively stable, and in this case, a 1 -foot thick sand subbase layer should suffice. If grades are such that fill still remains within the critical subgrade zone, a thicker sand subbase may be appropriate; although we suspect demolition -fist type fill may require removal for environmental reasons such that controlled replacement fill may be more favorable. Based on the above, it is reasonable to assume that the project can plan for a I -foot thick sand subbase throughout, with soil correction/stabilization performed beneath this zone as needed. The final subgrade prior to sand subbase placement should have proper stability within the critical subgrade zone. Its clayey soil subgrade conditions, stability should be evaluated using the test roll procedure. In granular subgrade conditions, the soils should be surface compacted with a vibratory roller compactor, and the ground surface response under the compaction process can point out buried areas where unstable soils may be present. Instability will likely be a result of wetter clayey or high silt content soils. More widespread instability can be anticipated during wetter seasons, Where unstable soils are found under the test roll or compaction process, the soils should be improved by means of scarification, drying, and re- compaction; or by subeutting and replacement. We recommend the final soils remaining in place be capable of passing a test roll prior to placing the sand subbase layer. 51