Before placing the subbase material, the subgrade or foundation must be properly prepared, the Wisconsin DOT says. “It should be smooth, shaped to conform to required crown and grade, and be compacted to the required density.”

Where travel of the placing equipment ruts or disturbs the foundation, means must be employed to correct these conditions ahead of placing the subbase material, the Wisconsin DOT warns. “If the subbase is constructed on a rutted foundation, the roadbed will not drain properly and areas of weakness may develop in the pavement structure. Placing, shaping and compacting the subbase material to conform for its full width to the required grade, section and density is necessary for satisfactory construction of the proposed base course. The inspector should frequently check the subbase course for correct depth and spread.”

Draining the Subbase

Wisconsin DOT warns of the danger of water in pavement structures. It’s commonly said that “water is the enemy of pavements.” Therefore, whatever can be done to keep water out of the pavement structure is effort well spent.

As noted below, saturated pavement structures will actually pump water and base fines out of HMA fatigue cracks or along the sides of PCC slabs, indicating subbase and base layers in dire straits.

Pavement structures will contain water in its free state, as capillary water between the granular material, bound moisture, or water vapor. Free water is the form of most concern, engineers say, because it can do the most harm and is the only form of water that can be significantly removed by gravity drainage.

The subgrade, granular subbase and other pavement layers always are constructed with cross slope to facilitate drainage. Rain or melt water will enter pavement through cracks and joints in the driving surface. A properly designed pavement will use gravity to encourage water to find its way through voids in the granular base and subbase following the slope, to either exit the structure into side ditches, or into a built-in pavement drain that will take it to ditches and ultimately to a creek, wetland or bioswale.

Permeable road bases are made of an open-graded granular material that allows free flow of water through the subbase or base layer, and then out to a drainage appurtenance. The permeable base may be unbound or bound, as in the case of the cement-treated permeable base – which adds structural strength – and may be separated from the subgrade by an impermeable drainage fabric that keeps fines from migrating from the subgrade into the subbase.

‘Daylighted’ Permeable Bases

Optimal use of fabric requires a drainage system, but a lower-cost design for PCC pavements — the “daylighted permeable base” — allows free draining of water to roadside.

“Daylighted permeable bases are well-suited for roadways with flat grades (1 percent or less) and shallow ditches, where it is difficult to outlet drainage pipes at an adequate height above the ditch,” says the FHWA in its 2009 Tech Brief publication, Daylighted Permeable Bases.

“Daylighted permeable bases have been used for more than 20 years in the United States to remove infiltrated water from pavement structures,” FHWA writes. “[W]hen appropriately used, designed, constructed and maintained, daylighted permeable bases have the potential to perform just as well as edge-drained permeable bases, for about the same or even lower cost.”

Two types of materials have been used for daylighted permeable bases, FHWA says. The first is an unstabilized large-sized stone, also called a rock base, typically constructed about 18 to 24 inches thick. The second type of material is a permeable base gradation such as would be used for an edge-drain system, either untreated or treated with asphalt or portland cement, and typically constructed about 4 to 6 inches thick. The permeability requirements and asphalt or cement content required to maintain long-term stability are the same for daylighted permeable bases as for edge-drained permeable bases, FHWA says.

A permeable daylighted base needs a suitable separator layer beneath it to prevent subgrade fines from migrating up into and clogging the base, but not necessarily a fabric, FHWA reports. “This may be an appropriately graded untreated aggregate subbase, an appropriate geotextile fabric, or a layer of subgrade soil treated with sufficient lime or cement to achieve good long-term stability and resist erosion,” the agency says.