“What soil are we paving on, what density does that soil need to be compacted to, is it properly graded?” asked Wayne Jones, senior regional engineer with the Asphalt Institute, during his presentation “Introduction to Thickness Design” at the 2024 National Pavement Expo in Tampa.
Read more of Jones’ insight in this article, How Thick Should Your Pavement Be?.
The subgrade is an important factor to consider when determining pavement thickness. There are different ways to classify the soil you will be paving on, including the unified soil classification system and the AASHTO soil classification system.
The most common test to evaluate the subgrade strength of roads and pavements is the California Bearing Ratio (CBR), which determines how many pounds of pressure it would require for a 1-inch diameter cylinder to penetrate two inches into the material. “A good [material] would require 17 pounds of pressure; medium, 8 pounds; poor, 3 pounds,” Jones said.
Although subgrade can be tested with various nuclear and proctor tests, Jones recommends simply proof-rolling it. “Proof rolling with a roller or heavily loaded truck will identify soft spots for corrective action,” he said. “If you see any flexing, it’s not stable enough. You need to strengthen that subgrade before you put the aggregate or asphalt down to make sure it’s strong enough to hold up the pavement and the loads that will be applied.”
Jones stressed the importance of achieving the correct moisture content of the sub-base before compaction, usually 3 to 6%. “Some contractors have a portable pugmill that they put their aggregate through to add the proper amount of water,” Jones said. “Then they run it up a conveyor or put it in a stockpile from which to load trucks, and then put that aggregate in an old asphalt paver to be laid. Then, it’s got perfect water content, it’s perfectly on line and grade, and it has some initial compaction to it.”
Sometimes, further stabilization may be required. A particularly challenging material in Jones’ experience is clay. He recommended the use of powdered lime. “Years ago, I was stabilizing the subgrade of a parking lot for a warehouse with a lot of trucking in and out of it, and I could not get the clay to dry out and stiffen up that late in the year,” Jones said. “I brought out a subcontractor who pulverized lime and water into the clay.” The result was a material that set up quickly and solidly.
“If you’re facing a similar situation, check out the Asphalt Recycling and Reclaiming Association to find contractors in your area who can help you with that, usually for just a couple dollars per square yard,” Jones suggested.