Agenda - Council - 01/13/2015 - Laserfiche WebLink

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Staff is therefore recommending to correctall poor subgradesoilsby excavating 3 feet of existing poor soils and replacing it with clean imported select granular borrow. This will provide a uniform subgrade section that will not be prone to differential settling over time and will therefore requireless maintenance over timeto reduce the City’s long-term maintenance costs, and will maximize the life of the street. Stormwater Improvements: Stormwater improvements are proposed to include new concrete curb and gutter and new storm sewer infrastructure including concrete catch basins and pipe sewers.Some 6” pipe drain will also be added along the outside edges of subgrade corrections at the low point to ensure adequate subgrade drainage. No stormwater ponding improvements are currently proposedwith the project but the opinion of probable costs do include a contingency cost of $40,000 for stormwater ponding if the Lower Rum River Watershed Management Organization requires some level of ponding with the project. 4.1.3Geotechnical Considerations This project involves reconstructing two street segmentsthat have been in service for over 26years. Based on NTI’s Geotechnical Evaluation Report, the existing poor subgrade soils likely contributed to the pavementsectionsdeteriorationand must be corrected to some extent before constructinga new bituminous pavement section on top of the soils. Subgrade soils can be corrected in numerous ways. The easiest and most effectivemethod to correct poor subgrade soils is to excavateand replace the poor soils with clean fill material, such as select granular borrow(sand). The clean fill material is then compacted to provide a stableand uniform subgrade.This method canbe the most expensive means to correct subgrade soils, but it also provides an extremely stable subgrade section that will help ensure that the maximum pavement life can be achieved. Another way to correct poor subgrade soils is to “scarify” or “farm” the existing subgrade soils. This simply consists of excavating the subgrade soils to a depth determined by the engineer, mixing the poor subgrade soilstogether and allowing them to dry as needed, then replacing and re-compacting the soils.This method is relatively inexpensive, but the results are also not as stable as the “excavate and replace” method previously discussed. Geotextile fabric can also be placed between different layers of soils to separate poor soils from clean soils and prevent them from mixing. The City has used this method before with some success, although installing such fabric is not much cheaper than installing 2 or 3 feet of clean fill, plus the City runs the risk of having someone, typically a private utility contractor, cut through the fabric which then removes any benefit it was intended to provide. Appendix D. NTI’s Geotechnical Evaluation Report is included in Feasibility Report th Garnet Street and 168Avenue Reconstruction City of RamseyProject No. 13-10 Page 11