A dense granular material (GM) is composed of dense random packing of solid particles and posses disorder structures which has been a long-standing mystery. On the one hand, GMs are amorphous materials with no long-range structural order; on the other hand, both topological and mechanical short range order are pronounced in these materials, due to their high granular packing density. The unique internal structure of GMs underlies their unique mechanical ad physical properties, which render GMs are useful for various applications in more and more engineering and industries in recent years, such as rock-filled dams, granular type debris flows, and the pebble-bed High Temperature Reactor. However, fundamental knowledge on the structural aspect of GMs remains seriously lacking. For example, how the granular mesoscopic structures are formed on the short-to-medium range, how the structure differs in different GMs and changes with granular composition, external loadings, and processing history and methods, and more importantly, how the structure influences the properties of GMS, are still well answered questions. Granular materials are currently the most actively studied materials. In this paper, we review the tremendous efforts over the past decades devoted to unraveling the particle-level structure of GMs and the structural origin of their unique mechanical behaviors, including the simplified inter-particle contact models and the structure measurement techniques and analysis methods. The failure of granular materials play a vital role in transition from solid-to fluid-like states. The related jamming transition, the concepts of "soft spots" and the micro-displacement measurement techniques are then analyzed. Outstanding questions in this research area will be outlined. Suggestions are proposed as well. The studies on the fundamental problems of granular materials should employ the cutting edge studies of the condensed physics and mathematics to establish new concepts. Because of extensive uses in engineering and industries, the fundamental studies are suggested to closely connect with engineering and industries by focusing on the core technologies, which would greatly prompt the development of this research area. © 2017, Chinese Academy of Mechanics. All right reserved.
