State-of-Knowledge in Modulus of Subgrade Reaction Testing with Associated Pavement Design Implications

The paper addresses the 1) state-of-knowledge of the plate load test and modulus of subgrade reaction ( k ), 2) industry’s desire to simplify the plate load test, and 3) k -value’s effect on rigid pavement design. The theory behind the k -value was traced back over 150 years to evaluate the current state-of-knowledge. A suite of 144 plate load tests were executed on three subgrade materials under varying base course thicknesses. Plate load testing was designed to evaluate various plate sizes and testing standards (e.g., the military standard—US Army Corps of Engineers (USACE) Concrete Research Division-Concrete (CRD-C) 655-96 and the American Society for Testing and Materials [ASTM] standard—ASTM D1196-21). By obtaining plate load tests on varying base course thicknesses, field measured effective k curves were developed. Overall, results show k ASTM was approximately 50% higher than k CRD . Although a 24, 18, and 12 in. diameter plate setup produced similar results to the 30, 24, 18 in. diameter plate setup, additional variability was introduced when using smaller plates. Setups using plates smaller than 24, 18, 12 in. diameters more than double the k -value when comparing with the traditional 30, 24, 18 in. diameter plate setup. If the Department of Defense’s (DoD’s) design procedures continue to use the k -value, then it is of extreme importance to safeguard the integrity of the test and its results produced, given k -value’s impacts to layer thickness. The results of the field data imply the current effective k curves underestimate the global stiffness contribution provided by the base layer.