The latest in transportation pavement

From the foundation on up, there are a number of growing trends in pavement design for transportation projects. Focusing on three of the many trends, these are the latest techniques and cost-saving measures being used to construct roadways and other transportation pavement projects today.

Water please
Expansive soils, also known as swelling soils, exist across the United States, with some states having higher concentrations than others. Regardless of where a map shows its presence, this particular type of dirt should be tested for if a project is west of the Mississippi, where expansive soils are more likely to be a problem.

Despite their potential to cause millions of dollars in damage and remediation efforts, expansive soils are just recently gaining the respect required to integrate testing into the design process for pavement projects. There are, however, a number of mitigation techniques, primarily falling into two categories—moisture treatment and replacement. The latter is more expensive and requires expansive soils to be dug out, hauled off site, and filled with another, non-expansive material. Most parts of the country do not have the resources to replace the soil without incurring high costs to have it brought in from elsewhere.

Expansive soils were untreated along this stretch of pavement in Carrollton, Texas.

From an economic and materials standpoint, moisture treatment is the preferred method for mitigating expansive soils. Appropriate treatment simply needs to satisfy the soil’s need for moisture, thereby killing the swell. Treatments are typically made from 3 feet to 8 feet deep and require nothing more than water and some degree of stabilization. When done correctly, pavement should require no more than routine maintenance after moisture treatment. Such is the case at Denver International Airport, where 5.5 million square yards of pavement on the airfield side were treated with moisture to address the presence of expansive soils. For 10-plus years, the treatment of existing expansive soils has allowed for damage-free runways, aprons, and taxiways. The story is different on the road leading to the airport, which was not treated for expansive soils and offers a bumpy ride and high maintenance costs.

There currently are no existing federal regulations addressing expansive soils, but a number of municipalities are taking the issue to heart. Colorado and Texas stand out in these developments largely because these states have some of the highest concentrations of expansive soils in the country, but it is worth repeating that any pavement project in any state should be tested for the presence of expansive soil.

In the late 1980s, Jefferson County, Colo., created expansive soil mandates after swelling soils led to an extremely uneven road; it had 2 inches of asphalt on one side and 14 inches on the other because of differential movement. Similarly in the early 1990s, Denver enacted measures requiring developers to test for and address expansive soils. Last year, the city of Frisco, Texas, created The Engineering Design and Construction Protocol for pavements within the Eagle Ford shale formation, requiring subgrade investigations and pavement design that considers the need for moisture treatment of expansive soils to mitigate heave.

Denver’s Metropolitan Government Pavement Engineers Council and the North Texas Tollway Authority have enacted policies requiring moisture treatment be used to address expansive soils. Irving, Texas, and several cities in the Dallas/Fort Worth metro area are considering protocols for pavement design that would specifically address expansive soils.

On stable ground
With approximately 4 million miles of paved road and more than 236 million registered public and commercial vehicles in the United States, there is good reason to address the increasing amount of traffic and heavier vehicles on our roads through pavement design.

One trend being seen more than others is the use of stabilized subgrades. The popularity is largely because of its effective and inexpensive methodology, which provides structural support for pavement. In 2000, there was a slight increase nationwide addressing structural support through stabilized subgrades, but recently this method has been more widely used.

Tying back to expansive soils, a stabilized subgrade can improve the support for pavement over moisture-treated soil (which may produce a soft-yielding subgrade). This procedure also improves the support characteristics of the subgrade, providing such a large increase in strength that it becomes a structural component of the pavement system.

Looking at the economic benefits of this technique, the biggest savings come from the minimal need for additional materials. Instead of replacing the subgrade with aggregate base or other fill, such as sand or gravel, stabilizing the existing subgrade preserves natural resources, while also reducing the budget needed for doing so. The materials and delivery cost of bringing in an additive, which would amount to a relatively small percentage of the total subgrade soil volume, is clearly a less expensive option to bringing in 100 percent of the needed aggregate base or other fill—especially considering heightened transportation costs resulting from higher fuel prices. On occasion, the cost of transportation can cost more than the materials alone.

An interesting aspect about this method is that it is consistent with current mandates from state departments of transportation and the Federal Highway Administration. These mandates require optimization of resources, particularly aggregates.

Lime slurry is added on top of moisture-treated clay soil. This process stabilizes the soil by drying it out.

The alternative to bringing in 100 percent new fill is to stabilize the existing subgrade with lime, fly ash, and/or portland cement—the only approved stabilizers for regular use. These materials are relatively available and the total cost is usually considerably less than import of aggregate fill or base. Typically, a project will require 4 percent to 10 percent of the total subgrade volume in additives.

Lime is a calcium-carbonate or limestone that has been cooked at high temperatures to boil off the water in the stone, thereby calcining (burning) it and creating elemental calcium (C++). The use of lime to stabilize soils dates back to before the creation of Rome. In fact, it was used on The Appian Way, the renowned road built in 312 B.C., connecting Rome to Brindisi in southeast Italy.

Fly ash is a byproduct of coal-fired power plants, and portland cement is a mix of sedimentary minerals that are combined and crushed into fine powder. Portland cement acts as the glue in concrete.

Predicting the future
Recently, there has been a revitalized interest in managed pavement systems. In most cases, data is analyzed to assess the current pavement condition. In managed pavement systems, the same information is used to map out future needs. The advantage to analyzing the data one step further is the ability to forecast trends and issues that may arise with any given pavement section, allowing for better management of the budget and manpower that will be needed in the future.

From general maintenance needs to remediation of larger issues, managed pavement systems can significantly extend the life of a pavement system beyond the typical 20-year design. When done properly, pavement systems can be maintained at a higher level for about half the cost typically incurred, remaining at a state of nearly 80 percent new.

An asphalt pavement system can be well managed by implementing preventative maintenance measures, usually in the form of seal coats. Much like painting a house, the seal coat acts as a sacrificial layer so that the pavement can continue to wear. Periodically, an overlay will also be needed for a well-managed pavement system. This additional thickness of asphalt mix put over the top of a roadway is typically added in 12- to 15-year intervals.

It boils down to proper design, which more and more is being valued and paid for in initial construction, resulting in an extended service life.

Dollars and sense
A few project examples help illustrate the value these new techniques bring to transportation pavement design. In one instance, a subdivision in Texas that was less than two years old needed to have its roads completely rebuilt as a result of damage from expansive soils. The initial pavement construction cost, paid by the developer, was around $2 million. Damage from expansive soils required complete replacement of the pavement at a cost of more than $5 million at taxpayer expense, since the roadways had become public property. This amount does not include the $20,000 to $30,000 per year being spent to "Band-aid" the roads before full reconstruction could be done.

In Colorado, half of a subdivision was constructed conventionally with aggregate base course and asphalt pavement on compacted subgrade. The other half used moisture treatment and stabilized subgrade. One year after the project was completed, the cost of repair to bring the traditionally constructed side of the subdivision up to county standards was $85,000. On the side where stabilized subgrade was used, a mere $700 was spent to replace curbstones that had been broken by snowplows. These costs are in 1979 dollars—imagine the cost today.

Areas including Dallas and Austin, Texas, and along the Front Range in Colorado are taking notice of these trends, which address issues that are not as well known in other parts of the country. But everyone should take notice as these trends speak to considerable savings as a result of better performing pavement systems.

A significant number of municipalities and owners have come to realize that their investment in pavement systems is one of the most valuable assets they have. More and more, steps are being taken to ensure this investment is not only well maintained, but designed with cost- and time-saving efforts from the foundation up.

Darrel Holmquist, P.E., is a senior principal consultant for CTL|Thompson Texas. He may be reached at 512-832-8883 or via e-mail at dholmquist@ctlthompson.com.