Agenda - Council - 04/14/2020 - Laserfiche WebLink

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20-01 Variolite Street Reconstruction Ramsey, Minnesota NTI Project No. 19. MSP09109.000 PAVEMENT RECOMMENDATIONS The Contractor should be aware that the subgrade soils will be moisture -sensitive and protecting them from inclement weather will aid in maintaining stability. The stripping of the existing pavement and aggregate base course should occur immediately prior to subgrade preparation and base aggregate installation to minimize weather -induced instability. The subgrade and aggregate base should be compacted and sloped to drain to help prevent ponding in the roadway during construction. Undocumented fill soils were encountered within the roadway corridor. The most conservative method of construction would be to remove and replace all the existing undocumented fill from beneath the proposed roadway areas. If the City were willing to accept some risk of future settlement and maintenance, the existing fill may remain in place provided that it can be sufficiently compacted to provide a stable platform for construction of the proposed pavements. As noted previously, organic (peat, Pt) soils were encountered within the roadway. We recommend the removal of, at a minimum, soils with high organic content (3 percent or greater by dry weight) from beneath pavement areas. Slightly organic soils, those with an organic content less than 3 percent, can remain in place provided that they are at least 3 feet below the bottom of the pavement section and can be sufficiently compacted to provide a stable platform for construction of the proposed pavements. It should be noted that pavements constructed over organic soils will not generally perform over the long-term the same as pavements constructed over inorganic soils, and some movement and long-term pavement maintenance should be anticipated. Backfill should consist of inorganic soil similar to the onsite soils. Prior to installing the new pavement section, the existing subgrade should be scarified to a depth of at least 12 inches and re -compacted. A proof roll test should then be performed to determine soft or unstable subgrade areas. The proof roll should be performed with a tandem axle dump truck loaded to gross capacity (at least 20 tons). Acceptance criteria of the proof roll shall be limited to rut formation no more than one inch depth (front or rear axles) and no pumping (rolling) observed during the visual inspection. Proof roll tests should be observed by an experienced technician or geotechnical engineer prior to placement of the aggregate base course to verify the subgrade will provide adequate pavement support. If rutting or localized unstable subgrade areas are observed, those areas should be subcut, moisture - conditioned, and re -compacted or removed to a stable depth. If imported fill is required in paved areas, it should consist of debris -free, non -organic, mineral soil similar in composition to the subgrade soils encountered in the surrounding areas. If sand is imported into areas that are underlain by relatively impervious fine grained soils, the sand layer must be drained with drain tile in order to prevent frost heave from water trapped within the imported sand layer during freezing temperatures. Individual lifts of engineered fill should be tempered for moisture content, placed, and compacted as noted in the "Placement and Compaction of Engineered Fill" section in Appendix B of this report. Page 8 of 10