Use of Heavy Vehicle Simulator-Aircraft in the Development of the CBR-Beta Procedure

Abstract

The original California Bearing Ratio (CBR) procedure was developed in the 1940s for the design of flexible pavements. The U.S. Army Corps of Engineers modified this equation in the 1950s for heavy aircraft with multiple wheel gears using a thickness adjustment (α) factor. This “CBR-Alpha” performance model is based on a Boussinesq theory of stress distribution and employs an equivalent single wheel load computation. It has gained international recognition since its implementation in the Aircraft Classification Number computation by the International Civil Aviation Organization in 1983. In recent years, however, criticisms regarding the CBR-Alpha procedure’s inability to accurately predict performance for heavier modern aircraft has led efforts towards its improvement. In the late 2000s, the U.S. Army Engineer Research and Development Center (ERDC) conducted a testing program using the Heavy Vehicle Simulator-Aircraft (HVS-A), where the test data was used for the validation of an improved mechanistic-empirical design procedure. A single wheel gear with high tire pressure and single and dual wheel gears with low tire pressures were programmed into the HVS-A to simulate F-15 and C-17 aircraft traffic, respectively. The HVS-A test data, along with earlier test data from the Federal Aviation Administration (FAA) and ERDC, were used in the development of the “CBR-Beta” procedure. The resulting “CBR-Beta” procedure eliminates the need for the α-factor computation by using Frohlich’s equation. This presentation shows the results of the HVS-A testing program and the reformulation of the CBR procedure, highlighting the contribution of accelerated testing in this effort.