CONDUIT AND DUCT BANK TRENCHING: Trenches for conduits and duct banks shall be carefully dug to the width and lines indicated on the drawings or at other locations acceptable to the City representative, and to the exact depth required for the proper grade of the conduits and ducts with encasement. Wherever possible, the trenches shall be excavated to permit the conduit bedding or duct bank to rest on undisturbed earth. Where it is necessary to trench through backfill, the earth shall be well compacted before the conduits and duct banks are installed. Subgrade soil for conduits and duct banks shall be firm and compact. Should the subsoil in any area be mucky or should it work into mud under the feet of the workmen, the contractor shall reinforce it. The subgrade shall then be brought to proper level by means of a thin layer of sand tamped or rolled into the reinforced subsoil. h) CONDUIT AND DUCT BANK BACKFILL: Concrete duct banks, direct burial conduits and manholes backfilled with 1-inch clean rock, except where water lines are placed above the electric and telephone conduits. Where water lines occupy the same trench as electric and telephone, backfill shall be made with well tamped crushed stone aggregate with fines, 1" and smaller. These materials shall be provided by the City. i) CONCRETE: Concrete shall consist of a mixture of portland cement, fine aggregate, and water combined in the proportions as required for the various applications. Admixture for the purpose of entraining air, regarding or accelerating the set, or other purposes may be added as specifically required or permitted. The concrete shall be provided by the City. All materials used in the concrete mix shall conform to the specifications of the City, Public Works Department, for portland cement concrete. The design mix for all concrete used shall consist of 564 lbs. (6 sacks) of portland cement, 1835 lbs. of fine aggregate, and 1535 lbs. of coarse aggregate per cubic yard. The maximum coarse aggregate size shall be 1-inch, except concrete used for encasement of duct banks shall have a maximum aggregate size of 3/4 inch. Concrete shall have a total air content of 5 percent, plus or minus one percent, and shall be thoroughly mixed in a mechanical mixer for not less than two minutes. The consistency, when placed, shall be workable without segregation and with a slump of not more than 4- inches, except concrete used in duct banks shall have a slump of 6-inches.
