301 - Subbase
Granular Material Class II will be used for constructing subbase.
The subbase will be placed on an approved subgrade surface that meets density and cross section requirements.
Verify that subgrade has been examined for possible undercut areas prior to placing subbase material.
Care should be taken not to place any subbase material on unstable, saturated or rutted subgrade and that placement methods or equipment do not distort (rut) the existing subgrade.
Subbase should be placed, compacted and trimmed to the proposed cross section.
Difficulty in obtaining density may be attributed to the lack of sufficient moisture content.
The Contractor may have to rewater the grade to maintain density.
The completed work should be checked by string line or other methods to ensure that the proper slope and elevation have been obtained. This information needs to be recorded in a field book or other documentation.
The subbase must be free from ruts and contamination.
Distorted areas in the subbase should be investigated for rutted or saturated subgrade.
Distorted areas from rutted or saturated subgrade will be removed, repaired and replaced at the Contractor’s expense. A work order may have to be used to get this accomplished.
Subbase transition to an existing subgrade or subbase will be a minimum of 12 inches (300 mm) of depth per 50 feet (16 m) of horizontal length. Existing subbase left in place will be transitioned with new sand subbase material to avoid differential frost heave.
Subbase is not to be placed on a frozen subgrade without a written work order from the Engineer. If it is, it will be tested for density in the spring and recompacted if necessary.
Plan quantity plus or minus any known changes, if agreed to by the Engineer and Contractor, as a basis for final quantity for subbase, is encouraged whenever possible. Subbase measured by the cubic yard (cubic meter) (CIP) will be documented and paid for by the “staked section” method which is explained in Section 104 of this manual. Sometimes actual measurements will be used for small volumes. When the pay unit is cubic yards (cubic meters) - loose measure, then the volume of the hauling units will need to be determined. The Contractor will need to provide load tickets for each load.
If the pay item Grading and Compacting Existing Material is included in the contract, it will be paid by the square yard (square meter) which will be determined by the actual dimensions of the work done. It will include any additional water needed to achieve compaction.