Performance of Flexible Pavements over Two Subgrades with Similar CBR but Different Soil Types (Silty Clay and Clay) at the FAA's National Airport Pavement Test Facility

The National Airport Pavement Test Facility (NAPTF) is located at the FAA William J. Hughes Technical Center, Atlantic City International Airport, New Jersey. It is used to generate full-scale pavement response and performance data for development and verification of airport pavement design criteria. During the Construction Cycle 5 — Test Strip, four flexible pavement test items were subjected to accelerated traffic tests using representative aircraft landing gear configurations. Pavements were constructed over two subgrades with similar CBR (approximately 3) but different soil types (silty clay and clay). Two test items (LFC1-N and LFC1-S) were designed to fail in approximately 100 passes (4-wheel gear and 55,000 lbs wheel load) and the pavement structure consisted of 2.5-inch P401 HMA surface, 8-inch P209 crushed stone base, and 16-inch P154 subbase. The other two test items (LFC2-N and LFC2-S) were designed to fail in approximately 2000 passes and the pavement structure consisted of 2.5-inch P401 HMA surface, 8-inch P209 crushed stone base, and 24-inch P154 subbase. The test items exhibited permanent deformations at the surface (evidenced by rut depths) of over 4 inches and upheaval at the sides of the ruts in excess of 1 inch. This paper presents the results from Heavy Weight Deflectometer (HWD) tests, trafficking tests (rut depth measurements) and posttraffic tests (trenching study). The trenching involved removal of the P-401 AC layer, the P-209 base, and the P-154 subbase layer to reveal the subgrade interface and subsequent subgrade layers below. Tests conducted on the pavement component layers included CBRs, in situ densities and moisture contents. Layer interface profile measurements from trench walls clearly show shear flow in the subgrade, with vertical movement of the subgrade material in the upheaval areas. This information will be used for developing thickness design procedures and load evaluation of airport pavements.