4.1 DESCRIPTION: Asphalt pavement structures for subdivision streets shall be designed in conformance with the requirements of this specification. Thickness of the total pavement, and of component layers, shall be determined on the basis of street classification, subgrade support, and relative strength of component materials in compliance with the procedures hereinafter set forth. Two design procedures are provided and either may be used. The first provides minimum thickness standards for each street classification and soil classification. The second provides for adjustment of pavement design thickness for each street classification to reflect the measured subgrade strength.
4.2 STREET CLASSIFICATION: Subdivision streets are classified into three groups - local, collector, and arterials. These street classifications are defined in Article II of these regulations.
4.3 SUBGRADE SUPPORT: Soil support shall be determined from one of the following procedures:
4.3.1 CALIFORNIA BEARING RATIO: Representative samples of soil, or soils, to be used in construction of the subgrade, or from prepared subgrade, shall be tested in accord with the current requirements of the Kentucky Department of Highways Test Procedure for California Bearing Ratio (CBR) except that the CBR-value at 0.1 inch penetration shall be used.
4.3.2 SOIL CLASSIFICATION: Soil to be used, or that has been used, for subgrade construction shall be sampled and classified in accordance with AASHO Designation M-147. Soil ratings to express their relative effectiveness as subgrade for design purposes are designated as soil groups A, B, or C in terms of the referred to street classification system. Where prior testing has established typical classification for frequently encountered soils, the Planning Commission, or its duly authorized representative, may approve designation of a soil group without additional tests. Soil groups are defined as follows:
Soil Group A - Granular soils that drain well; sand, gravel, or combinations of sand and gravel. This soil group includes AASHO Soil Classifications A-la, A-1-b, A-2-4, A-2-5, and A-3.
Soil Group B - Soils, including silty-clay, sandy-clay and sands or gravels containing considerable silt and clay, which retain considerable strength when wet. This soil group includes AASHO Soil Classifications A-2-6, A-4, and some A-6 soils. A-6 soils included are those having a group index of ten or less.
Soil Group C - Heavy clay soils, that lose most of their strength when wet, and highly resilient silts. This soil group includes AASHO Soil Classifications A-5 and A-6 soils having a group index greater than ten, A-7 and A-2-7.
4.4 PAVEMENT THICKNESS REQUIREMENTS: Thickness of component layers of the pavement for streets and driveways and of the total pavement structure shall be determined by one of the following methods:
4.4.1 Method A: Method A shall apply when subgrade strength tests (CBR) are not available. Design thickness of both the total pavement and its components shall be determined from Table 6 for the appropriate soil classification and street classification.
4.4.2 Method B: Method B shall apply when subgrade soil has been tested to determine its representative California Bearing Ratio (CBR). Required design thickness for the appropriate street classification shall be determined from Figure 4 for asphalt pavement thickness. Thickness determined from this Figure, for any street classification and CBR-value, will be in inches of asphalt concrete surface and base and the design thickness.
Base types, herein specified, other than asphalt concrete base type I or type II may be used. These other base types are asphalt base type III or type IV, granular base and/or subbase as herein specified. When bases other than asphalt concrete are used, the minimum thickness of asphalt concrete surface course, type I or type II for the base type selected, as specified in Table 6, shall be used.
(Ord. 1971-2, passed 4-6-71)