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

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20-01 Variolite Street Reconstruction Ramsey, Minnesota NTI Project No. 19. MSP09109.000 UTILITIES The fill and native glacial deposit soils observed at the soil boring locations were generally suitable for utility support. Replacement backfill required in utility trenches should consist of non -organic material similar to the surrounding soil. All import fill should be approved by NTI or the City's representative. Due to the observed groundwater levels and depending on the installation depth of the utilities, temporary dewatering would likely be required during some of the utility trench excavations. In addition, the granular nature of the on -site soils will likely result in significant volumes of water entering the excavations unless proper dewatering measures are implemented. If dewatering is needed during construction, we recommend that the groundwater be maintained a minimum of 2 feet below the bottom of the excavation. Stabilization of the trench subgrade may also be required in order to provide a stable platform for construction. Stabilization could consist of a one half to one foot layer of crushed rock or sand with a maximum 5 percent material passing the No. 200 sieve and 50 percent passing the No. 40 sieve. As noted previously, swamp deposit soils were observed at Borings SB-23, SB-24, SB-35, SB-37, SB-39 through SB-44, and SB-46 through SB-49. The peat (Pt) soils are generally not suitable for utility support and utility backfill. NTI does not recommend the founding of utilities in or above swamp deposited soils. However, if the design team elects to take the risk of doing so, appropriate measures should be taken. If the utilities are to be founded above or within the organic soils, the least risky options for support of utilities would be to perform a soil correction below the utilities down to competent, non -organic native soils, or to support the utilities on a deep foundation system with bents for support at the joints. Another alternative would include the placement of additional bedding to provide a more stable platform to construct the utilities upon. However, the team should anticipate that utilities supported in this manner will likely undergo significant differential movement and/or sagging over time. This will be particularly evident at the connections to any proposed structures. At a minimum, flexible connections should be utilized within the pipe alignment where possible and at all connections to structures. The Geotechnical Engineer of Record or their designated representative should observe the project excavations to determine that unsuitable materials have been properly removed and adequate bearing support is provided by the exposed soils. The exposed soil at the base should be compacted to 95 percent standard Proctor dry density (ASTM D698). Such observations and testing should be performed prior to backfilling. It is especially important that trench backfill for utility construction within paved areas be thoroughly compacted to minimize future pavement damage. We recommend that such soils be compacted in accordance with the recommendations noted in the "Placement and Compaction of Engineered Fill" section in Appendix B of this report. The stability of embankments along utility excavations is dependent on soil strength, site geometry, moisture content, and any surcharge load for excavated soils and equipment. We present cautionary remarks concerning stability of excavation sideslopes in the "Excavation Stability" section of this report. The Contractor is solely responsible for assessing the stability of and executing underground utility and project excavations using safe methods. The contractor is also responsible for naming the "competent individual" as per Subpart P of 29 CFR 1926.6 (Federal Register - OSHA). Page 7 of 10