Category:350 Design for Bases and Aggregate Surfaces

Bases are provided for several reasons, including to provide drainage, to reduce or eliminate pumping (ejection of fine-grained soil and water from beneath the pavement), to provide a working platform for overlying pavement placement, for improvement of subbase characteristics, or as a separator layer.

The Construction and Materials Division will provide a recommendation for base type with the pavement thickness determination, along with a recommendation for modified subgrade if necessary.

Pavement drainage considerations are discussed for each material type below. Where appropriate, pavement drainage should be provided either by daylighting the material or by providing longitudinal edge drains. Although not always appropriate, daylighting material eliminates the future maintenance requirements associated with a geocomposite or pipe aggregate edge drains.

Pay limits for base are shown on the A2 Shoulder Standard Plan.

350.1 Stabilized Permeable Base

Stabilized permeable base, sometimes referred to as open graded treated base, provides rapid positive drainage. The components are coarse-graded crushed stone with virtually no fines stabilized with asphalt cement or Portland cement. The layer thickness is typically 4 in. (100 mm). Type 1 aggregate base is used as a separator layer under stabilized permeable base to prevent infiltration of fines.

350.2 Rock Base

The preferred base for all new rigid and flexible pavements is a 12 in. or 18 in. rock base whenever available in suitable quantities on or in close proximity to the job. Rock base consists of 12 in. or 18 in. of Class C excavation. It is placed full roadbed width and daylighted to the in-slope or fill-slope, except on the high side of superelevated curves and when not economically feasible under light traffic pavements. In this case the rock base is not daylighted to the ditch and soil is used as a fill material. Undergrading in rock cuts is performed to allow placement of the full specified lift thickness. A 4 ft. (1.2 m) deep ditch is preferred. Determination of rock base availability must be made early in the project through consultation with the District Geologist and materials personnel, as elevations, pavement structure design and other considerations may be affected when used.

350.3 Type 5 Aggregate Base

Type 5 base is used to provide moderate drainage, reduce or eliminate pumping and provide a working platform. Drainage capability is much lower than permeable base, but still enough to keep water from contacting the pavement bottom when properly sloped. Typical thickness is 4 in. (100 mm). Longitudinal pipe aggregate edge drains have typically been used in conjunction with the Type 5 base, however, consideration should be given to daylighting the material if possible.

350.4 Type 1 Aggregate Base

Type 1 aggregate base is used to provide a working platform or a separator layer between pavement or permeable base and the soil subgrade. It is a more stable material with higher fines content. Although pavement edge drains are not typically provided for Type 1 aggregate base, consideration should be given to daylighting the material when possible.

350.5 Modified Subgrade

Subgrade soils are modified with lime, flyash, lime kiln dust, or other material to decrease the plasticity index (PI) and increase stiffness. The dosage rates of modifying material vary depending on the soil type to provide a working platform and/or pavement structure, but generally no drainage. Typical minimum thickness is 6 in. (150 mm), but may range up to 12 in. (300 mm) or greater. District or Central Office Soils and Geology personnel may identify the need for this item during design; however, the need for modified subgrade is usually determined during construction operations.

350.6 Shoulders

Aggregate stabilized shoulders are appropriate on Minor Roads as explained in Shoulder Surface.