Section 2109. Natural Subgrade.
Construction of natural subgrade shall consist of the shaping and consolidation of a prescribed portion of all subgrades in preparation for the placement of a pavement, pavement widening, base course, or subbase.
NATURAL SUBGRADE FOR PAVEMENT, PAVEMENT WIDENING, BASE COURSE, OR SUBBASE.
The subgrade shall be constructed to have uniform stability for a width at least equal to that of the proposed pavement or base, plus 2 feet (1 m) on each side. It shall be brought to an elevation and cross section such that, after being rolled, the surface shall be at the required elevation. At the time the subgrade is prepared, the roadbed shall have been constructed to the full width and to at least the elevation of the finished subgrade. In cases where the materials present in the 6 inches (150 mm) next below the elevation of the subgrade are not reasonably uniform as to composition or stability for the full width of the subgrade, these materials shall be scarified, mixed, and recompacted, or otherwise treated, to produce a uniform condition. Recompaction shall meet requirements of Article 2107.05 on Primary projects and Article 2107.06 on Secondary projects. Stones over 4 inches (100 mm) in size shall be removed from the loosened portion of the subgrade and removed from the project as directed by the Engineer.
Depressions that develop during rolling shall be filled with suitable material meeting the requirements of subgrade treatment as specified in the contract documents and rolling shall continue until the subgrade is uniformly firm, properly shaped, and substantially true to grade and cross section. It shall be maintained as constructed until the pavement is placed. Material, other than sand, which will not compact readily under the roller shall be removed and replaced with material which will compact readily, and that portion of the subgrade shall be rolled again. The roller used shall meet requirements of Article 2001.05, B, except for work involving widening of such width as to make use of a conventional roller impractical, where a trench type roller meeting requirements of Article 2001.05, E, shall be used.
Rolling of the subgrade shall extend for at least 12 inches (0.3 m) outside each edge of the proposed pavement. Piles or ridges of earth or material that would seriously interfere with the operations of finishing pavement shall not be left on the shoulders.
During the process of constructing subgrade, the soil shall be maintained in a condition sufficiently moist to facilitate compaction and produce a firm, compact surface. Sprinkling or wetting of the finished subgrade will be required, as necessary, to insure a reasonable moisture content at the time pavement or base is placed upon it.
If, in preparation of subgrade, it becomes necessary to excavate below the elevation of the earth shoulders, ditches or drains shall be provided at frequent intervals to permit ready drainage of surface water from subgrade to side ditches.
The Contractor is responsible for maintenance of the completed subgrade during subsequent construction activities. Before allowing hauling equipment to use the completed subgrade, the Contractor must be satisfied as to the effect this hauling equipment may have on the partially completed work. In order to minimize damages to the subgrade the Contractor may adjust the equipment or methods or limit the size of the construction loads allowed thereon. Loads in excess of the legal axle load will not be allowed on the completed subgrade. If rutting or any other damage occurs to the subgrade as a result of the hauling operations, the Contractor shall immediately repair the subgrade. This repair will include, if necessary, scarifying to a depth of 6 inches (150 mm), aerating, and recompacting at the Contractor's expense. Recompaction shall meet requirements of Article 2107.05 on Primary projects and Article 2107.06 on Secondary projects.
Should traffic by others authorized to do work on the project be specifically permitted by the Engineer to exceed the Contractor's self imposed limit, the Contracting Authority will pay a share of repair costs set by the Engineer representing an increase in cost of repair of damage, if any, caused by such traffic.
Subgrade preparation shall be completed sufficiently in advance of pavement or base work so that normal progress can be maintained, considering the capacity of mixing and placing equipment.
Before the final template shape is made, the subgrade shall be proof rolled with equipment meeting the requirements of Article 2001.05. Depressions that develop shall be corrected as described above. If the subgrade is to be cut to the final grade elevation with an automatically controlled subgrade machine, the prepared subgrade shall be graded to an elevation that will permit the machine to accomplish the final cut in one continuous forward pass. The elevation of the subgrade surface will be indicated by grade stakes. The surface shall be corrected in both profile and cross section to within 0.05 foot (15 mm) of the desired elevation. In irregular or short sections, the subgrade shall be checked by the most accurate practical method, subject to approval of the Engineer.
TREATMENT OF SUBGRADE FOR CONCRETE PAVEMENT.
Unless otherwise ordered by the Engineer, the subgrade, at time of placing concrete for Concrete Pavement, Section 2301, or Concrete Base, Section 2201, shall be in a uniformly moist but not muddy condition to a depth of not less than 1 inch (25 mm) or shall be covered with a single layer of plastic film meeting requirements of Section 4107. Adjacent strips shall lap not less than 12 inches (0.3 m). Plastic film shall not be stretched to the extent that its width is noticeably reduced. Plastic film which has been used for curing concrete, salvaged in usable condition, may be used for subgrade treatment.
SPECIAL COMPACTION OF SUBGRADE.
When special compaction of subgrade is required in the contract documents, the following shall apply:
That portion of the roadbed to be covered by the pavement or base course, plus 3 feet (1 m) beyond the outer limits of the pavement or base course, shall be constructed in the following manner:
A plane surface 6 inches (150 mm) below the finished grade line shall be exposed. Soil removed in the operation may be placed along the sides of the roadbed and shall be used for backfilling the excavation. The soil underlying this exposed surface shall be scarified for a depth of 6 inches (150 mm).
The scarified material shall be pulverized to the extent that, when tested, no soil particles will remain on the 2 inch (50 mm) sieve. The scarified material shall be uniformly dried or wetted to a moisture condition which will permit obtaining the required compaction without subsequent rutting from the batch trucks or other paving equipment in the paving area. If rutting occurs, to the extent that the thickness of the flexible or rigid pavement being spread does not conform to the design dimensions, construction of the pavement shall be stopped immediately and shall not be started again until the rutted subgrade has been reworked.
The material at the time of compaction shall not be drier than 6 percentage points below its optimum moisture. The density shall not be less than 95% of maximum density as determined by Iowa DOT Materials Laboratory Test Method 103.
The material used to bring the subgrade to the required finished profile and cross section shall be placed in accordance with the above requirements for pulverization, moisture content, density, and stability.
METHOD OF MEASUREMENT.
Unless otherwise provided, work connected with construction of natural subgrade for pavement, base course, pavement widening, or subbase will not be measured for payment.
The quantity of Special Compaction of Subgrade, in stations (meters), will be the quantity shown on the contract documents. The quantity of Special Compaction of Subgrade will be determined along the center line of the roadbed.
2109.07 BASIS OF PAYMENT.
Unless otherwise provided, work connected with construction of natural subgrade for pavement, base course, pavement widening, or subbase will not be paid for directly but shall be considered as associated work included in the contract unit price for construction of the pavement, base course, or widening.
The Contractor will be paid the contract unit price for Special Compaction of Subgrade per station (meter). This payment shall be full compensation for excavating, manipulating, replacing, and compacting the material; and for furnishing all water required for the work.
Excavation or filling in excess of 3 inches (75 mm) for either elevation adjustment or subgrade correction at locations other than structures or existing pavement will be paid for according to Article 2102.14, or if no contract unit price is provided, as extra work, except when both grading and paving are the responsibility of the same Contractor.