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Geotechnical Report 2020 PMP Street Improvements Arden Hills, Minnesota WSB Project No. 014297-000 Page 5 4. ENGINEERING ANALYSIS AND RECOMMENDATIONS 4.1 Discussion Based on our borings, it is our opinion that the proposed watermain and pavement can generally be supported on the soils encountered in the borings. However, soft, wet lean clay soils should be anticipated during construction of the watermain and a partial removal and replacement will likely be necessary. Also, the soils within the pavement subgrade consist mainly of silty sand soils, which are frost susceptible. Consideration should be given to partially subcutting these soils and replacing with a non-frost susceptible granular fill to reduce the potential frost heave below the pavement section. 4.2 Watermain Utilities Invert elevations are anticipated to be within 8 feet of existing grades and we anticipate the subgrade soils for the utilities will consist of granular sandy soils and lean clay. Where the clayey soils are soft and wet or become soft it may be necessary to perform a partial subcut and replacement. Where soft unstable soils are encountered at invert grade, subexcavation of these materials to a depth of 1 to 2 feet and replacement with a coarse sand or gravel is recommended. In addition to providing a more stable uniform platform to construct the utilities, the sand backfill can act as a platform from which to place a sump and pump for dewatering. Underground utilities are expected to be installed by backhoes completing the excavations and placing pipe and backfills. Soil compacto rs should be used to compact the fill in thin even lifts to the specified densities. 4.3 Backfill and Fill Selection and Compaction It is our opinion the onsite non-organic soils may be reused as backfill and fill provided they are moisture conditioned and can be compacted to their specified densities. Wet soils that are excavated would need to be dried before reuse as an engineered fill. We recommend use of a minimum of 2 feet of clean coarse sand with less than 50 percent passing the #40 sieve and less tha n 5 percent passing the #200 sieve when backfilling the bottom of a wet excavation. Backfills with cobbles larger than six inches (6”) should not come in contact with utilities. We recommend that soils be moisture conditioned to meet compaction specifications as determined from their standard Proctor tests (ASTM D-698). The fill should be spread in thin lifts (8 to 12 inches depending on material and compaction equipment) to allow for full depth compaction. Table 2 indicates the recommended compaction levels. Table 2: Recommended Level of Compaction for Backfill and Fill Area Percent of Standard Proctor Maximum Dry Density Pavement: Within 3 feet of top of aggregate base Within 3 foot perimeter of structures such as manholes 100 Pavement: Greater than 3 feet below aggregate base 95 Utility Trench (unless within 3 feet of aggregate base) 95 Landscaping (non-structural) 90 4.4 Dewatering Based on the results of our soil borings and the proposed construction, water seepage into the utility trenches is anticipated. The seepage is anticipated where excavations extend into wet clay soils. It is our opinion that sumps and pumps placed in low points of the excavations should be adequate to control most infiltrating water situations.