Building Better Roads from the Ground Up

A strong foundation is the key to a strong pavement structure

By Tom Kuennen, Contributing Editor

A pavement is only as strong as its foundation. Without an adequate base or foundation, a road simply cannot stand up to long-term traffic volumes, increasing vehicle weights and speeds, and the assault of the elements.

Strong subbases bolster the base and pavement layers above.

While the concept of a subbase is simple, the reality is that in today’s world of advanced technology, a subbase design and construction can be complex and demanding. Subbase design may require analysis of existing, virgin and reclaimed materials, application and mixing of stabilization chemicals, installation of stabilization fabrics, and measurement of compaction using “smart” technology built into dirt rollers. Subbases must also be drained and protected from frost.

For new alignments, stabilize the subgrade prior to work on subbase, base and pavement layers.

And the new philosophy of mechanistic-empirical design, as articulated by the American Association of State Highway & Transportation Officials (AASHTO) with the National Cooperative Highway Research Program (NCHRP), is bringing a new rigor to the design and construction of subbases.

The result will be better-performing pavement structures.

Damage from Inadequate Subbases

Subbases inadequate for the traffic loads they carry will manifest their shortcomings in a variety of ways.

A new Cat 160M2 motor grader preps the subbase prior to base placement.

The most common clue to base failure-related pavement woes in is fatigue cracking. Fatigue, or bottom-up, cracking results when traffic load stresses propagated to asphalt pavement foundations cause foundation cracks to work their way upward through the pavement.

In asphalt pavements, it’s manifested as a series of interconnected cracks resembling an alligator hide, hence its popular name alligator cracking. It develops into many-sided, sharp-angled pieces, usually less than 12 inches on the longest side.

If not stabilized, expansive subbase and base layers will heave and destroy pavement.

Low-severity fatigue cracking characterizes an area of cracks with no or only a few connecting cracks. The cracks are not spalled nor sealed, and pumping of base materials out the cracks is not evident. In moderate fatigue cracking, the interconnected cracks form a complete pattern, cracks may be slightly spalled and may be sealed, and pumping is not evident. High-severity fatigue cracking is an area of moderately or severely spalled interconnected cracks forming a complete pattern, the pieces of which may move when subjected to traffic loads. Cracks may be sealed, and pumping may be evident.

In portland cement concrete (PCC) pavements, longitudinal cracking describes cracks that are mostly parallel to the pavement centerline, and are attributed to subgrade heaving that pushes upward against the rigid slab and cracks it.

“Daylighted” permeable base – exposed here at the shoulder – is a lower-cost PCC pavement drainable design that doesn’t require separation fabric or drainage systems.

Base and subbase layers that are composed of expansive soils with an abundance of clay must be stabilized, frequently done with cement. This is particularly true of soils in Louisiana, Texas and the American Southwest. Expansion of these base and subbase layers will cause heaving in the pavement, forcing it upward, causing it to fissure and break. The pavement likely will have to be completely reconstructed.