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Report of Geotechnical Exploration and Review Fox Street Improvements, Orono, Minnesota AMERICAN July 29, 2016 ENGINEERING Report No. 20-14448 TESTING, INC. Page 7 of 12 Granular Borrow specification upon visual classification. These soils would be considered to be moderate to high frost susceptibility. For “improved” performance, our primary recommendation would be to place a uniform thickness sand subbase of Modified Select Granular Borrow as top of subgrade. It is our opinion that a 1-foot thick sand subbase layer would suffice for this project. We recognize that the City may not wish to import Modified Select Granular Borrow for cost reasons. It would be possible to eliminate the imported sand subbase layer, although some frost- related distress may be more pronounced. Without a sand subbase layer, it is reasonable to assume that future performance (from a frost standpoint) would be similar to that experienced in the past. If you elect to place a sand subbase and there is a need to vary the thickness of the sand subbase, we recommend the thickness have a taper of no steeper than 10H:1V. The subcut and sand layer placement should extend slightly beyond the back of the curb to maintain frost uniformity. The sand subbase should be provided with proper subsurface drainage to prevent build-up of water within the sand. This can be accomplished by placing properly engineered drainage lines along the length of the roadway which are connected to storm catch basins or other outlet structures. 6.3.2 Stability Improvement The final subgrade should have proper stability within the critical subgrade zone. The existing soils in the upper 4 feet of the subgrade consist of variable soil types primarily including lean clay, sandy lean clay, and clayey sand. Some continued subgrade settlement will likely occur, with or without a sand subbase layer, in areas where the buried organic soils are present. In our opinion, it would not be economically warranted to remove all of the existing fill soils in the critical subgrade zone and recompact or replace them for this project. Where sandier soils are exposed in the subcut bottom (whether at bottom of aggregate base or bottom of sand subbase grade), the soils should be surface compacted with a vibratory roller compactor. This process should be observed to evaluate whether unstable soils may exist within the subgrade which are buried. If deflections are noted under the compaction process, then there may be a need to subcut unstable clayey/silty soils.