Internal Stability of Granular Materials by William S. Housel, presented at the thirty-ninth Annual Meeting of the American Society for Testing Materials from June 29 to July 3, 1936, in Atlantic City, New Jersey. The paper addresses inconsistencies in internal friction theories and presents test results on five granular materials. It shows no direct relation between shearing resistance or friction and principal pressures, which express internal stability in a granular mass. Shearing resistance is measured by direct shear tests, while principal pressures are measured by a specially designed stabilometer test. The study introduces an "arch theory" of internal stability, emphasizing the mechanical property that produces resistance to displacement via mutual support of particles, involving static forces and reactions too large to be affected by molecular forces. The paper derives general equations for ultimate bearing capacity and pressure distribution at sub-ultimate loads. Results from bearing-capacity tests correlate with internal-stability theory. The study offers a comprehensive view of how granular materials behave under stress through a system of elementary soil arches where vertical pressure is sustained by horizontal thrusts from adjacent particles. The "arch action" concept is applied to load distribution, principal pressure relations, and bearing capacity, demonstrating consistent logic. Tests conducted by the University of Michigan in collaboration with other organizations led to these findings, highlighting the complexity of static resistance, the shortcomings of current friction-based theories, and the need for an internal-stability-focused approach.