6.6. SR 08-19-2002 - Laserfiche WebLink

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AET #20-02044 - Page 14 After final grades are established, and prior to the placement of the aggregate base layer, the subgrade should be test-rolled to delineate any areas of unstable soils. If unstable soils are found, they should be subcut and replaced with drier fill or they should be aerated, dried and recompacted back into place if weather conditions permit. For subgrades prepared as discussed above, and assuming the limiting subgrade soil consists of silty sands, clayey sand, and sandy lean clay which successfully pass a test roll, we recommend the pavement be designed based on an assumed R-value of 20. Sand Subbase We strongly recommend that a sand subbase layer be used at this site. It is our opinion a sand subbase layer (typically 1' thick or more) can be used to improve the long term performance of the pavements (see attached sheet for details). The use of a sand subbase may increase initial construction costs; however, these higher costs can be offset due to the improved long term performance, reduced maintenance costs, improved constructability and the use of thinner pavement sections. The sand should be placed over a stable subgrade (prepared as described above). The subbase should be included as part of the upper 3' zone where 100% compaction is required. We recommend using sand which has no more than 12% of the particles (by weight) finer than the #200 sieve, meeting the requirements of Mn/DOT Specification 3149.2B2 for Select Granular Borrow. If a 1' thick sand subbase is used (and silty sands are the limiting soils), the pavement can be designed based on an assumed R-value of 35. If a 2' thick sand subbase is used, the design can be based on an assumed R-value of 55. Subsurface drainage is important for long-term pavement performance. If the sand layer is used for the upper subgrade layer, you should consider providing subsurface drainage methods such that water cannot build-up within the granular soils above the poorer draining soils. This can be handled by strategically placing draintile lines or french drains which can then be directed to