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Geotechnical Report 2021 Street Improvements Hugo, Minnesota WSB Project No. 015887-000 Page 6 4. ENGINEERING ANALYSIS AND RECOMMENDATIONS 4.1 Discussion No information was provided to us regarding density tests or excavation observations for the existing fills encountered at the boring locations. Generally, fills that are not documented are recommended for removal and replacement with an engineered fill. However, our borings did not encounter deleterious materials within or beneath the fill. It appears that these fills were placed as utility backfill or for the pavement subgrade area. Based on the soil borings and our understanding of these fills, it is our opinion that the risk of construction upon them is relatively low. Organic soils and vegetated root zones are not suitable for structural support and should be removed from the roadway fill areas. Based on the results of our borings, the glacially deposited soils generally appear capable of supporting the pavements and utilities. It is our opinion that groundwater could be encountered by excavations at this site and could affect construction of utilities. Dewatering may be required during utility placement. 4.2 Backfill and Fill Selection and Compaction The on-site 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 than 5 percent passing the #200 sieve when backfilling the bottom of a wet excavation. Backfills with cobbles larger than six inches (6”) should not be placed within 3 feet of grading grade or in contact with utilities. We recommend that clayey soils be moisture conditioned to within +/-2 percent of the optimum moisture content as determined from their standard Proctor tests (ASTM D-698). Fill should be spread in lifts of 8 to 12 inches, depending on the size and type of compaction equipment used. Table 4 provides the recommended compaction levels. Table 4: Recommended Level of Compaction for Backfill and Fill Area Percent of Standard Proctor Maximum Dry Density Pavement: Within 3 feet of bottom of aggregate base Within 3-foot radius of vertical utility structure 100 Pavement: Greater than 3 feet below aggregate base 95 Utility Trench 95 Landscaping (non-structural) 90 4.3 Pavement Subgrade Preparation and Stability The soils at the bottom of the excavation should be prepared in accordance with MnDOT Specification 2112, Subgrade Preparation. If the subgrade preparation operations encounter unstable soils, we recommend removing these unsuitable materials and replacing them with Select Grading Material (MnDOT 2105.1.A.6). If the on-site soils cannot be moisture conditioned and compacted as recommended, additional measures, such as placement of additional sand subbase or a section of coarse breaker run aggregate, can be completed to provide a firm base for subsequent fill placement. Before placement of the sand subbase, 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