Secrets of the Plate Load Test (1941-1955)

The plate load test has been required for concrete pavement design since 1941, yet it is still not universally acceptable, particularly on account of its cumbersome nature, not to mention the scatter in its results. On the other hand, suggestions for its replacement have not been well received due to the well-deserved reverence for its original developers. This study reopens the archives of primary sources to show that the 30-in. (760-mm) plate diameter was first stipulated in haste by the US Army Coprs of Engineers, in anticipation of World War II efforts, despite serious lingering disagreements and concerns about emerging geotechnical considerations. Antecedents in studies of soil bearing capacity illustrate the enormity of the challenges faced by the Corps after the Pearl Harbor attack and historical documents reveal the mobilization of almost super-human efforts that ensued during the Emergency that followed. Subsequent elaboration of the procedural specifications for this test were similarly driven by expediency and organizational policy, sometimes in contradiction to time-honored engineering principles. The 1955 seminal paper on the topic by Terzaghi is recognized as a belated critique of the concepts engenderd by the War era, and as a roadmap for doing pavement engineering in peace times that has been largely ignored. Opinions of other key pioneers during the period considered are reviewed and are found to be consistent with Terzaghi’s conclusions. The urgency for a more robust and efficient method of determining the subgrade modulus, k, is highlighted, and practical modern tools for its development are identified.