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Geotechnical Report Arden Oaks 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 sandy lean clay soils should be anticipated during construction and a partial removal and replacement maybe necessary. Also, the soils within the pavement subgrade consist mainly of lean clay 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 chiefly of sandy lean clay. If the clayey soils are soft 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. Underground utilities are expected to be installed by backhoes completing the excavations and placing pipe and backfills. Soil compactors 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. Any wet soils excavated would need to be dried before reuse as an engineered fill. 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 10 inches depending on 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 subgrade Within 3 foot perimeter of structures such as manholes 100 Pavement: Greater than 3 feet below top of subgrade 95 Utility Trench (unless within 3 feet of pavement subgrade) 95 Landscaping (non-structural) 90 4.4 Dewatering Based on the results of our soil borings and the proposed construction, dewatering is not anticipated. 4.5 Pavement Areas After removing the existing pavement sections and backfilling the utilities, the final subgrade should have proper stability within three vertical feet of grading grade (grade which contacts the bottom of the aggregate base). This will generally be achieved in fill areas with proper compaction of embankment materials and in cut areas through proper subgrade preparation. The stability of the pavement subgrade should be evaluated using the test roll procedure (MnDOT 2111), except a fully loaded tandem axle dump truck or a full water truck should be utilized for the proof roll. If unstable soils are found under the test roll,