Pavement design may become more efficient and cost-effective based on enhanced data from subgrade layers obtained by an automated plate load test. The APLT system is starting to gain interest from transportation and power agencies throughout North America.

The Illinois State Toll Highway Authority last year saw a demonstration of the system and agreed to use it in a geotechnical exploration program for its planned rehabilitation of the Central Tri-State Tollway corridor. At more than 90 locations, the rolling electronic-hydraulic control system of loading plates, computers and a scraper blade either tested or sampled subbase and subgrades under existing mainline pavement along a stretch of Interstate 294 during night-time closures, says Brendan FitzPatrick, director of Global Services for Ingios Geotechnics Inc., the technology provider.

The APLT technology “provides us with accurate readings of [pavement] stiffness, not just a best guess based on sample borings,” says Steve Gillen, the tollway authority’s materials manager. “We now have actual measurements of how well the subgrade will support the pavement, so we can avoid over-design.”

At each tollway site, the system performed three to nine tests, with each 1,000-cycle test taking about 20 minutes, says McMahon. The system is designed to measure and develop resilient modulus values in accordance with new pavement guidelines from the American Association of State Highway and Transportation Officials.

Unlike deflectometer testing, APLT can directly test the foundation layer and measure peak, recoverable and permanent deflections—rather than just peak deflection—of foundations, embankments, stabilized materials and compacted fill, according to the company.

The APLT system is faster and more accurate than static plate load testing, says Bill Vavrik, chief engineer with ARA, the tollway’s pavement designer. “In old-school plate load testing, you could do only one test a night. You put a plate down, loaded it up and looked at the

deflection. Now, we’re doing a repeated load test—we run a load pulse through the soil every second, followed by a rest. We can do that thousands of times at different levels of stress.”

This approach more accurately reflects the pavement’s experience of the actual stresses of real-time traffic, Vavrik notes. “In the lab, you put a soil sample together and break it, but it’s not the same. With APLT, we can optimize pavement design using real information, knowing how the subgrade will behave at certain stress levels.”’

For the Chicago-area corridor that will be rehabilitated, the improved data is especially crucial because the corridor has lanes built in the 1950s, 1970s and 1990s in cut-and-fill, embankment and below-ground elevations, adds Vavrik. “We have a checkerboard of soils and aggregates and characteristics, and this is the highest-traffic segment on the network. So, we can’t afford to get it wrong and go back out there” after initial repaving, he notes.

David White, president and chief engineer of Ingios, says he started developing the concept four years ago, after frustration with the slowness and generality of traditional tests. “We are learning from APLT new information about pavement foundations—like how non-uniform they are from point to point,” he says. Some P3 teams and a powerplant agency have also used the APLT system, he says.