Special Issue on Horizons in Granular Mechanics: The Legacy of Dr. Masao Satake

This special issue of the Journal of Engineering Mechanics is dedicated to the memory of Dr. Masao Satake (1927–2013), Professor of Civil Engineering at Tohoku University and Tohoku Gakuin University. Dr. Satake was a pioneer in the fields of geomechanics and granular mechanics, making significant contributions to both fields and exemplifying their intimate connections. His ideas continue to have a profound influence on our understanding of granular materials, as he had advanced the view of these materials as discrete systems of individual particles, which behave as a continuum when considered in abundance. As the organizing partner of a series of collaborative conferences between colleagues in Japan and the United States, he was also instrumental in establishing and fostering a community of engineering researchers and in developing granular mechanics as a subfield within engineering mechanics.

In his earliest papers, one finds Dr. Satake making connections between discrete and continuum models of granular materials, viewing their bulk behavior as originating at the contacts between particles. His recognition of the importance of interparticle contacts, and in particular their arrangement and orientation, was expressed as a fabric tensor in a 1982 paper: a characteristic tensor that conveys a material’s microstructural anisotropy. The fabric tensor is now recognized as a mathematical foundation of granular mechanics, and it is now widely included in both discrete and continuum models of a wide range of granular, cellular, and fibrous materials. As early as 1971, he developed a generalized micropolar constitutive model based upon discrete mechanics. Among the first to recognize the importance of the intermediate principal stress on the strength and stiffness of soils, Dr. Satake proposed a general yield criterion for three-dimensional loading. Such criteria, refined by others over the following decades, are now the basis of most advanced continuum models of large-scale geotechnical problems.

Dr. Satake’s magnum opus was his microscale definitions of the stress and strain in granular materials, culminating in a 2004 paper (at the age of 77) in the International Journal of Solids and Structures. True to his advice to students to confront difficult problems rather than avoid them, a series of works led to this paper, which precisely describes an appropriate microscale tessellation of space and the tensor definitions of stress and strain within these elemental units. To this day, researchers continue to investigate and mine these fundamental concepts, and applications continue to surface within the granular mechanics community, a community that Dr. Satake had strived to build.

Beginning with the 1978 US-Japan Symposium on Continuum-Mechanical and Statistical Approaches in the Mechanics of Granular Materials, Dr. Satake jointly organized a series of symposia that brought together a devoted community of researchers, building the foundation of what has become a vibrant area of research that spans the disciplines of engineering, physics, mathematics, and materials research. Dr. Satake also served as President of the Tohoku Branch of the Japanese Society of Civil Engineers, as Director of the Japanese Society of Soil Mechanics and Foundation Engineering, and as the international Chairman of the Technical Committee of Mechanics of Granular Materials of the International Society for Soil Mechanics and Foundation Engineers. These and other efforts were recognized with the 2002 Distinguished Service Prize of the Japan Society of Civil Engineers.

In the spirit of the Japan-US symposia, the Granular Materials Committee of the Engineering Mechanics Institute of ASCE organized the Dr. Masao Satake Memorial Symposium on Granular Mechanics, held at the Institute’s 2015 conference at Stanford University. The symposium began with presentations on the life of Dr. Satake, given by his colleagues, Drs. Yoshio Tobita, Fusao Oka, Teruo Nakai, Hayley H. Shen, and James T. Jenkins. Following the symposium, participants have offered the papers in this special issue. The papers span discrete, continuum, and multiscale behaviors of granular materials, these special materials to which Dr. Masao Satake had made such important and lasting contributions.