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2006 Streets, Gem Lake, Minnesota - Geotechnical Recommendations January 18, 2006 Page 3 Compact the select granular borrow to a minimum of 100 percent of Standard Proctor maximum density, according to ASTM D698. Use the Specified Density Method in Mn/DOT 2105 to verify the density, and run a minimum of 3 density tests (sand cone or nuclear gauge) per street in the select granular borrow. Utility Construction Trench excavation should be sloped in accordance with OSHA regulations. The soils encountered vary from sand to sandy lean clay, which corresponds to Type A, B, and C soils according to OSHA. Therefore, for planning purposes, use the worst case soils (Type C) to determine the steepest allowable trench side slopes. This is 1.5(H):l(V) for excavations 20 feet deep or less. OSHA requires that excavations in excess of 20 feet be designed by a licensed Professional Engineer. OSHA regulations state that trench and excavation shoring are solely the responsibility of the contractor. Trenches can be excavated in the on -site soils using conventional methods and equipment (i.e., backhoes). Excavate trenches to the bottom elevation of the pipe, then scarify and re -compact the bottom 6 inches of the trench. Place the pipe on the compacted soil at the bottom of the trench. The backfill around the bottom and sides of the pipe, up to 6 inches above the top of the pipe, must not consist of clay (CL), clayey sand (SC), or silt (ML), or silty sand (SM). Where the bottom of the trench is in these soils, use on -site soils consisting of sand (SP) or sand with silt (SP-SM) for backfill around the pipe. If there is not enough of this type of sand on -site, granular bedding (Mn/DOT 3149.217) will need to be imported. We expect that imported granular bedding will be necessary in the southern 2/3 of Big Fox Road, and in sporadic, isolated locations on the other streets. When the utility cross sections and profiles are available, we recommend that an SEH geotechnical engineer review them to more precisely define where imported granular bedding will be necessary. Place the soils (either suitable on -site soils or imported granular bedding) from the bottom of the trench to 6 inches above the top of the pipe in lifts of not more than 6 inches and compact to a minimum of 95 percent of Standard Proctor maximum density according to ASTM D698. More than 6 inches above the pipe, any on -site soils may be used as backfill in the trench up to the select granular backfill subgrade zone. The soils placed above the pipe must be placed in lifts of not more than 12 inches and compacted to 100 percent of Standard Proctor maximum density (ASTM D698). The on -site clay (CL), clayey sand (SC), and silty sand (SM) tend to be sensitive to fluctuations in moisture content. If these soils are allowed to become wet, achieving the required density for compaction will be very difficult. If this happens, sand will need to be imported from off -site to backfill the trench. This is more likely to happen in the fall, when the weather tends to be relatively cool, cloudy, and rainy. We expect these types of soils to be encountered in the middle and south portions of Big Fox Road, and they occur intermittently on the other streets in this project. In order to achieve proper compaction of the trench backfill, a self-propelled vibrating sheepsfoot roller will be necessary. If utility service lines cross the upper portion of the trench on regular intervals (gas and telephone lines, for example), these services will need to be relocated prior to excavating the trench. There must be enough space to allow proper compaction equipment into the trench, and to allow that equipment to operate without damaging the services. Utilities installed as described above correspond to a Type 5 bedding in AWWA C600-99 and AWWA C605-94. Concrete pipe installed as described above would have Class B bedding according to the Concrete Pipe Design Manual from the American Concrete Pipe Association.